Symposium Organizers
Bernard Bewlay General Electric Company
Yue-hui He Central South University
Martin Palm Max-Planck Institut fuer Esenfoschung GmbH
Masao Takeyama Tokyo Institute of Technology
Joerg Wiezorek University of Pittsburgh
U1: Intermetallics for Hydrogen Storage and Thermoelectric Applications
Session Chairs
Haruyuki Inui
Jörg Wiezorek
Monday PM, December 01, 2008
Constitution A (Sheraton)
9:45 AM - **U1.1
Crystal Structures and Hydrogenation Behaviors of the RMn (3≤n<5) (R=La, Mg; M=Ni, Co) ``Superlattice" Alloys.
Etsuo Akiba 1 , Jin Nakamura 1 , Kenji Iwase 2 , Yujun Chai 1 , Hirotoshi Enoki 1 , Kouji Sakaki 1 , Kohta Asano 1 , Yumiko Nakamura 1
1 ETRI, AIST, Tsukuba, Ibraki, Japan, 2 , Ibaraki University, Tokai, Naka-gun, Ibaraki, Japan
Show Abstract10:15 AM - U1.2
Hydrogen Storage in Zr-Ni Intermetallic Alloys.
Sesha Srinivasan 1 , Diego Escobar 1 , Yogi Goswami 1 , Elias Stefanakos 1
1 , University of South Florida, Tampa, Florida, United States
Show AbstractThe Zr-Ni intermetallic alloys namely, (i) ZrNi 70/30 and (ii) ZrNi 30/70 have been investigated for the reversible hydrogenation behavior. These composites show Zr-Ni intermetallic multi-phase formation as explored by the X-ray diffraction studies. The sorption kinetics of ZrNi 70/30 seems much faster (~3-4 times) than that of ZrNi 30/70 alloys. Again the initial desorption rate increases with increase of temperature. A well defined plateau region was obtained for the ZrNi 70/30 with an equilibrium pressure ranges from 〈1 bar (300 ○C) to 10 bars (390 ○C). For ZrNi 30/70, the sloppy plateau region extends to higher equilibrium pressures. It is estimated that the total effective hydrogen concentration for ZrNi 70/30 (~ 1.0 wt.%) is at least 2 times that of ZrNi 30/70 (~0.5 wt.%) composites. From the PCT isotherms, the enthalpy of reaction (ΔH) has been calculated to be ~39 kJ/mol H2 for the ZrNi 70/30. The surface morphologies of the hydrogenated materials exhibit the presence of cracks and particle size pulverization in comparison to the pristine alloys.
10:30 AM - U1.3
Hydrogen Permeation in Anisotropic Nb-TiNi Alloy Prepared by Forging and Rolling.
Kazuhiro Ishikawa 1 , Jun-ichi Matsuoka 1 , Sho Tokui 1 , Kiyoshi Aoki 1
1 Materials Science and Engineering, Kitami Institute of Technology, Kitami, Hokkaido, Japan
Show AbstractThe present authors have proposed Nb-TiNi alloys consisted of the primary Nb solid solution and the eutectic TiNi+Nb phases as new non-Pd hydrogen permeation alloys. Recently, it was found that an anisotropic Nb phase was formed in the Nb40Ti30Ni30 alloy after heavy forging and rolling, i.e., the Nb phase was elongated and contracted along the rolling direction (RD) and normal direction (ND), respectively. In this anisotropic alloy, hydrogen permeability strongly depends on the direction. For example, the values of hydrogen permeability along RD (ΦRD) and ND (ΦND) are about 8 times larger and 1/8 times lower than that of the original one, respectively. In the present study, hydrogen permeation in the anisotropic Nb-TiNi alloy is discussed on the basis of the experimental results and the composite law, and it is demonstrated that high hydrogen flux can be obtained in the anisotropic alloy with a good resistance to hydrogen embrittlement.The Nb40Ti30Ni30 alloy is prepared by an induction melting in an Ar atmosphere, then the alloy ingot was forged and rolled at 1173 K. The alloy samples were cut from the ingot for XRD analysis, microstructural observation and hydrogen permeation measurement. Both surfaces of the alloy disk were coated by Pd to avoid oxidation and to enhance hydrogen dissociation. Hydrogen permeation measurement was performed in 0.1-0.9 MPa H2 at 553-673 K using a mass flow meter.When the angle of the direction of hydrogen permeation against RD is defined as θ, hydrogen permeability of this alloy is decreased with increasing θ. The maximum and minimum hydrogen permeability is obtained at θ=0° and θ=90°, respectively. The value of ΦRD is independent to the thickness of the alloy membrane above 100μm. On the contrary, hydrogen flux through the alloy membrane is increased with decreasing the thickness. Large hydrogen flux of 73 ccH2cm-2min-1 is obtained at 673 K in the pressure difference of 0.8 MPa without hydrogen embrittlement.
10:45 AM - U1.4
Hydrogen Storage Properties, Metallographic Structures and Phase Transitions of Mg-based Alloys Prepared by Super Lamination Technique.
Nobuhiko Takeichi 1 , Koji Tanaka 1 , Hideaki Tanaka 1 , Nobuhiro Kuriyama 1 , Hiroshi Miyamura 2 , Shiomi Kikuchi 2
1 , AIST, Ikeda, OSAKA, Japan, 2 Department of Materials Science, University of Shiga Prefecture, Hikone, SHIGA, Japan
Show AbstractMagnesium is a one of promising materials for hydrogen storage media because it can absorb a large amount of hydrogen as MgH2 and relatively low cost. However, the hydrogen absorption/desorption kinetics is too slow for practical use and needs high temperatures such as 573K. To improve the reaction kinetics and diffusion properties, a reduction of the grain size and an addition of various catalysis have been investigated. Recently, we reported Mg-based alloys, prepared by super lamination technique that is a kind of such mechanical processes, reversibly can absorb and desorb hydrogen at 473K, which was lower than reaction temperatures of conventional Mg-based materials. In the present study, we applied the super lamination technique to form Mg-Pd and Mg-Al binary composites. The formation behavior of intermetallic compounds, metallographic structures and their hydrogen storage properties of Mg/Al and Mg/Pd laminate composites were investigated.The Mg/Al laminate composites with (Mg/Al)=17/12 and Mg/Pd laminate composites with (Mg/Pd)=6,3 and 2.5, where (Mg/Al) and (Mg/Pd) mean the ratio of the numbers of Mg to Al or Pd atoms, were prepared by repetitive-rolling method using conventional two-high rolling mill. After several stack and rolling process, the laminate composites were subjected to appropreate heat treatmens and the intermetallic compounds were uniformly formed. The growth of the intermetallic compounds and its concentration profiles were studied using a scanning electron microscope. Hydrogen storage properties (reaction kinetics and equilibrium sorption properties) of the laminate composites were evaluated by using a Sieverts’ apparatus.The phase transformations of laminate composites during hydrogen absorption and desorption was analyzed by in-situ XRD measurement.By heat treatment at 673K, intermetallic compounds of Mg17Al12 and Mg2Al3 formed between the initial Mg-Al boundaries. Also, intermetallic compounds of Mg6Pd, Mg3Pd and Mg5Pd2 were formed at the interface between Mg and Pd by interdiffusion. Those intermetallic compounds In Mg/Al composite, Mg17Al12 , was formed by heat treatment of laminate composite, can reversibly absorb and desorb hydrogen, up to 0.67 H/M, at 673K. PC-isotherms of the Mg/Pd laminate composites show single plateau at 1 MPa. On the other hand, the Mg/Pd laminate composites can reversibly absorb and desorb a large amount of hydrogen, up to 1.46~0.9 H/M, at 573K. Except Mg/Pd laminate composites with (Mg/Pd)=2.5, PC-isotherms of the Mg/Pd laminate composites show two plateaux, PL and PH. In case of Mg/Pd laminate composite with (Mg/Pd)=6, PL and PH at 573 K were 0.02 and 2 MPa, respectively. PC-isotherms for the Mg/Pd laminate composite with (Mg/Pd)=2.5 at 573K show single plateau at 2 MPa.Those intermetallic compounds can absorb and desorb hydrogen reversible through complex multistage disproportionation reaction and recombination of Mg and Al or Pd.
11:00 AM - U1.5
Thermoelectric Devices Based on Nanocomposites with Ultra High Seebeck Coefficients.
Otto Gregory 1 , Ximing Chen 1 , Gustave Fralick 2
1 Chemical Engineering, Univ. of Rhode Island, Kingston, Rhode Island, United States, 2 , NASA Glenn Research Center, Cleveland, Ohio, United States
Show AbstractThermoelectric devices based on nanocomposites comprised of wide bandgap oxide semiconductors or insulators and refractory metals, are being considered for high temperature thermoelectric devices. Very large and repeatable Seebeck coefficients have been observed using oxide semiconductor-NiCoCrAlY nanocomposites. Specifically, (n-type) ITO-NiCoCrAlY nanocomposites, (p-type) ITO-NiCoCrAlY nanocomposites, Al2O3-NiCoCrAlY nanocomposites and ZnO-NiCoCrAlY nanocomposites have been prepared by co-sputtering techniques and the thermoelectric response of these materials has been measured relative to platinum and other nanocomposites. Combinatorial chemistry techniques were used to screen the thermoelectric response of the candidate oxide semiconductor-refractory metal nanocomposites. Combinatorial nanocomposite libraries were patterned over platinum reference electrodes using microlithography techniques, and a temperature gradient was applied across the hot and cold junctions of the resulting small footprint thermocouples, generating an electromotive force. The chemical composition of the most promising combinatorial nanocomposite libraries (in terms of thermoelectric response) was analyzed by SEM/EDS. A sign change was observed in the screening experiments indicating a transition from an "n-type" to a "p-type" material with increasing NiCoCrAlY content in the ITO based nanocomposite. When a "p-type" ITO-NiCoCrAlY nanocomposite was combined with platinum to form a metal-ceramic junction, a Seebeck coefficient on the order of 1100 μV/°C was obtained (maximum electromotive force of 350 mV was achieved at a hot junction temperature of 1210 °C). When the "p-type" ITO-NiCoCrAlY nanocomposite was combined with n-type ITO, an electromotive force of 2000 mV was achieved for a hot junction temperature of 1140 °C and when combined with an n-type ITO-NiCoCrAlY nanocomposite, an electromotive force of 3400 mV was achieved for a hot junction temperature of 1160 °C. Other bi-ceramic junctions based on ITO and ZnO, also exhibited a very large thermoelectric response (1100 mV) at 1160 °C. In all cases the cold junction was maintained at room temperature, ~25°C. Seebeck coefficients of 10mV/°C were achieved for these junctions after repeated thermal cycling to 1200 °C. Therefore, ZnO-NiCoCrAlY nanocomposites have been prepared by the same co-sputtering techniques and the thermoelectric response relative to "n-type" ITO-NiCoCrAlY nanocomposites was investigated. The electrical and thermal properties of these materials are also being investigated, but the thermoelectric response of these selected metal-oxide semiconductor nanocomposites measured to date are large enough to warrant their investigation for use in energy harvesting devices.
11:45 AM - U1.7
Change in the Thermoelectric Properties with the Variation in the Defect Structure of ReSi1.75.
Shunta Harada 1 , Katsushi Tanaka 1 , Kyosuke Kishida 1 , Norihiko Okamoto 1 , Haruyuki Inui 1
1 Department of Materials Science & Engineering, Kyoto University, Kyoto Japan
Show AbstractSemiconducting rhenium silicide, ReSi1.75, is of interest owing to its potentials as a promising candidate material for thermoelectric applications. The dimensionless figure of merit (ZT) for binary rhenium silicide is as high as 0.7 at 1073K along [001]C11b. This value is higher than silicon-germanium alloy, which is practically used. The crystal structure and the stoichiometry of rhenium silicide are very controversial. The crystal structure of rhenium silicide has been reported as a tetragonal C11b (MoSi2-type, space group I4/mmm) structure and the stoichiometry was thought to be Re:Si=1:2. And other kinds of crystal structures and stoichiometry have been reported by several researchers. However, according to our recent analysis by electron diffraction by transmission electron microscopy (TEM) and high-angular annual dark field scanning transmission electron microscopy (HAADF-STEM), the crystal structure is monoclinic with the space group of Cm due to an ordered arrangement of vacancies in silicon sites in the underlying C11b structure. The reason rhenium silicide possesses such complex crystal structure with a lot of vacancies in silicon sites is that rhenium silicide belongs to the group of semiconductor with the 14 valence electron count (VEC) per metal atom. Therefore the concentration of vacancies in silicon sites is expected to change by substituting an element of rhenium and/or silicon with other elements having more or less number of valence electron. In the present study, by using transmission electron microscopy and high resolution electron microscopy (HREM), we investigate the crystal structure variations of rhenium silicide by substituting rhenium and/or silicon with ternary elements. At the same time, we investigate the effect on the thermoelectric properties by substituting rhenium and/or silicon with ternary elements. In addition we found out that the crystal structure and thermoelectric properties of binary rhenium silicide also changes by the heat treatment. Through these experiments, we figured out the significant relationship between defect structure and thermoelectric properties of rhenium silicide.
12:00 PM - U1.8
Thermoelectric Energy Recycling for Green IT Applications.
Sang Mock Lee 1 , Jong Soo Rhyee 1 , Sang Il Kim 1 , Kyu Hyoung Lee 1
1 Materials lab, Sansung Advanced Institute of Technology, YongIn Korea (the Republic of)
Show Abstract12:15 PM - U1.9
Phase Stability and Thermoelectric Properties ofHalf-Heusler (Ma0.5, Mb0.5)NiSn (Ma, Mb = Hf, Zr, Ti).
Yoshisato Kimura 1 , Hazuki Ueno 1 , Takahiro Kenjo 1 , Chihiro Asami 1 , Yoshinao Mishima 1
1 Materials Science and Engineering, Tokyo Institute of Technology, Yokohama Japan
Show AbstractThermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. We are focusing on half-Heusler compounds MNiSn (M = Hf, Zr, Ti), well-known n-type thermoelectric materials, which can be used at around 1000 K to directly convert waste heat into clean electrical power. Half-Heusler compounds have excellent electrical properties, i.e., high absolute values of Seebeck coefficient and low electrical resistivity. On the other hand, relatively high thermal conduction is a disadvantage of half-Heusler compounds. Many research groups, including ours, have succeeded to reduce the lattice thermal conductivity by substituting M site elements between the group 4 elements Ti, Zr and Hf. It is called as the solid solution effect since the differences in atomic mass and atomic size in a solid solution effectively lower the lattice thermal conduction through enhancing phonon scattering. In the present work, to evaluate actual effects of the M site substitution on thermal conduction, nearly single-phase (Ma0.5, Mb0.5)NiSn alloys were fabricated by directional solidification using the floating zone melting method, where Ma and Mb are any two of Hf, Zr and Ti. Seebeck coefficient, electrical resistivity, thermal conductivity and carrier concentration were measured in a temperature range from 300 to 1073 K. The lattice thermal conductivity can most effectively be reduced in (Ti0.5, Hf0.5)NiSn alloys most probably by the solid solution effect since the differences in atomic mass and atomic radius are maximized in the case between Ti and Hf. Moreover, we have found a tendency of the two-phase separation between Ti-rich and Ti-poor half-Heusler (Ti, Hf)NiSn phases in alloys with a nominal composition of (Ti0.5, Hf0.5)NiSn through microstructure observation and chemical concentration measurement using electron probe microanalysis. A similar tendency of the two-phase separation has also been observed in (Ti0.5, Zr0.5)NiSn alloys. We have confirmed the two-phase separation between half-Heusler phases using the powder X-ray diffractometry conducted on (Tix, Zr1-x)NiSn and (Tix, Hf1-x)NiSn alloys (x = 0 to 1). Broadening or splitting of diffraction peaks were clearly observed depending on alloy compositions. On the other hand, we previously reported that Zr and Hf are all-proportion miscible in (Zr, Hf)NiSn as a continuous solid solution phase. Related phase equilibria were also considered by the calculation of phase stability. It should be noted that the lattice thermal conduction can be reduced by microstructure factors due to the two-phase separation, such as interfaces between Ti-rich and Ti-poor (Ti, Hf)NiSn phases. Regarding the performance of thermoelectric properties, The (Zr0.5, Hf0.5)NiSn alloy has the best dimensionless figure of merit of all the alloys investigated because it has the best balance of the highest electrical power factor together with a reasonably low thermal conductivity.
12:30 PM - U1.10
Fabrication of CoTiSb, NbNiSn and FeMoSb Half-Heusler Phases for Thermoelectric Applications.
Wilfried Wunderlich 1 , Yuichiro Motoyama 1
1 Dep. Mat. Sci, Tokai University, Faculty of Engineering, Hiratsuka-shi, Kanagawa-ken, Japan
Show AbstractHalf-Heusler phases have gained recently much interest as thermoelectric materials. Screening of possible systems was performed by ab-initio simulation using VASP-software. The energy-versus-Volume (E(V)) curves were calculated and calibrated. For CoTiSb, NbNiSn, FeMoSb and others, the stability of Half-Heusler phase against concurrent crystal structures like TiNiSi, ZrCoAl, ZrBeSi, FeSiV and Full Heusler was confirmed. However, the thermo-dynamical driving force as calculated from the difference in lattice energies is less than 0.1eV/atom. Hence, the fabrication of Half Heusler phases is a challenge and requires three steps, surface activation of the raw material by ball milling, arc-melting of pressed pellets and finally long-term annealing treatment in a vacuum furnace. For the CoTiSb system, diffusion couple experiments clarified the complicated diffusion mechanism, which can lead to constitutional vacancies. The FeMoSb system lacks in slow Mo-diffusion. The specimens were characterized by SEM-EDS chemical mapping and Seebeck voltage with open and closed electric circuit were measured in a self-built device at temperature differences up to 500K. On doped CoTiSb specimens, Seebeck coefficients up to 0.1 mV/K, on NiNbSn 0.3 mV/K were measured, although the microstructure was not yet optimized.
12:45 PM - U1.11
Mechanical and Thermal Properties of Single Crystals of Some Thermoelectric Clathrate Compounds.
Norihiko Okamoto 1 , Takahiro Nakano 1 , Kyosuke Kishida 1 , Katsushi Tanaka 1 , Haruyuki Inui 1
1 Materials Science and Engineering, Kyoto University, Kyoto Japan
Show AbstractClathrate compounds have stimulated a great deal of interest as promising candidates for practical thermoelectric materials because they exhibit relatively high electrical conductivity and very low lattice thermal conductivity. These properties are considered to originate from their particular crystal structures consisting of polyhedral cages filled with a guest atom. The cages, which consist of tetrahedrally-bonded group IV/III elements, are considered to play an import role in electrical conduction while the guest atoms, which are typically alkali metals or alkaline-earth metals, are weakly bonded to the cages and vibrating inside the oversized cages to efficiently scatter the heat-carrying phonons. Among a number of clathrate compounds with various structure types, the “type-I” clathrate compounds with the cubic space group Pm-3n have been investigated the most intensively. Knowledge on the mechanical properties of these type-I clathrate compounds is indispensable when their practical application in thermoelectric devices is considered, since these clathrate compounds will inherently be subjected to thermal stresses arising mostly from their own thermal expansion and contraction within thermoelectric devices. To the best of our knowledge, however, almost nothing is known about the mechanical properties of many candidates for thermoelectric materials including these type-I clathrate compounds Ba8Ga16Ge30 and Sr8Ga16Ge30, except for low-temperature elastic constants for a few clathrate compounds. In this study, the mechanical and thermal properties of single crystals of the type-I clathrate compounds Ba8Ga16Ge30 and Sr8Ga16Ge30 have been investigated by measuring the elastic constants, coefficients of thermal expansion (CTE) and plastic deformation behavior in compression. The values of bulk moduli extrapolated to 0 K for Ba8Ga16Ge30 and Sr8Ga16Ge30 are almost identical to each other, whereas the values of Young and shear moduli for Ba8Ga16Ge30 are larger than those for Sr8Ga16Ge30. The values of CTE for Ba8Ga16Ge30 and Sr8Ga16Ge30 are virtually identical with each other. The feasibility of these two clathrate compounds as a thermoelectric material in terms of mechanical stability under possible thermal stresses is evaluated by calculating thermal stresses that are expected to develop within these compounds when used as a thermoelectric material in thermoelectric devises.
U2: Iron Aluminides - Physical Metallurgy, Processing, and Properties
Session Chairs
Ian Baker
Kyosuke Yoshimi
Monday PM, December 01, 2008
Constitution A (Sheraton)
2:30 PM - **U2.1
An Overview of the Properties of Iron Aluminides.
Ian Baker 1
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States
Show AbstractAn overview of the properties of iron aluminides such as Fe3Al, Fe2MnAl and FeAl will be presented. These compounds exist over a wide range of composition and, thus, alloy stoichiometry is a key variable in their behavior. Several key phenomena will be discussed in detail, viz, the so-called yield anomaly, environmental embrittlement, and strain-induced ferromagnetism, in terms of dislocation behavior and the role of anti-site atoms and vacancies. Recent work on developing these materials for commercial applications will also be discussed.Research sponsored by NSF grant DMR 0552380 and DOE contract DE-FG02-07ER46392.
3:00 PM - U2.2
Forging of Steam Turbine Blades with an Fe3Al-based Alloy.
Peter Janschek 2 , Knut Bauer-Partenheimer 2 , Ronny Krein 1 , Martin Palm 1
2 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 1 , Leistritz Turbinenkomponenten Remscheid GmbH, Remscheid Germany
Show AbstractForging of steam turbine blades with an Fe3Al-based alloyP. Janschek1, K. Bauer-Partenheimer1, R. Krein2, Palm21 Leistritz Turbinenkomponenten Remscheid GmbH, Lempstrasse 24, D-42859 Remscheid, Germany2 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Strasse 1, D-40237 Düsseldorf, GermanyFe-Al-based alloys are light weight materials with outstanding corrosion behaviour. Recent advances in strengthening these alloys at high temperatures now make them suitable for applications in extreme environments such as steam turbines for power generation.In a preliminary study cylinders of 25 mm diameter and 28 mm height of two different Fe3Al-based alloys were forged between 900 and 1150 °C. The alloys behaved quite differently. Fe-25Al-20Ti-4Cr [1] (all compositions in at.%) did hardly deform even at the highest temperatures and showed numerous cracks after forging. In contrast, Fe-25Al-xTa [2,3] could easily be forged without any formation of cracks and the forged pancakes were regular shaped.From the latter alloy precursors 660 mm long each weighing about 12 kg have been produced by investment casting. The castings showed good form filling, were free of pipes and pores and had smooth surfaces. Because no measures for grain refinement were undertaken columnar grains were found in the castings which made them rather brittle.The precursors were heated up to 1100 °C and then forged on a 630 kJ counterblow hammer with 10 blows each. The dies were standard tooling used for commercially produced steam turbine blades.The forged steam turbine blades again showed very good form filling and smooth surfaces. The blades were finished by cutting off the burrs on a band saw and then grinding the edges by standard procedures. No heat treatment has been applied.Samples were cut out of certain parts of the forged turbine blades and the microstructures were examined by metallography. Basic mechanical properties such as hardness, yield stress and brittle-to-ductile transition temperatures were evaluated and are compared with those of the as-cast material.[1] R. Krein, M. Palm, Acta Mater. 56 (2008) 2400.[2] D.D. Risanti, G. Sauthoff, Intermetallics 13 (2005) 1313.[3] D.D. Risanti, G. Sauthoff, Mater. Sci. Forum 475-479 (2005) 865.
3:15 PM - U2.3
Grain Refinement for Strengthening in Fe3Al-based Alloys through Thermomechanical Process.
Satoru Kobayashi 1 , Akira Takei 2 , Takayuki Takasugi 2 1
1 , Tohoku University, Sakai Japan, 2 , Osaka Prefecture University, Sakai Japan
Show AbstractA thermomechanical process to form deformed/recovered grain structure is known to be useful in improving room temperature strength and ductility of Fe3Al-based alloys. Grain refinement, on the other hand, is also expected to improve those properties. Recently we achieved in reducing grain size to ~20μm through warm rolling and recrystallization process in Fe3Al-based alloys containing ~0.1 volume fraction of κ-Fe3AlC precipitates. Effects of grain refinement and microstructure on tensile properties were examined between room temperature and 600°C in such alloys. Three grain-size levels were prepared for a recrystallized grain structure and a deformed/recovered grain structure by changing warm rolling and/or annealing conditions. Ductility and UTS increased with decreasing grain size in each grain structure at room temperature. The properties obtained in the deformed/recovered structure were higher than those in the recrystallized structure, and the former structure with an averaged grain size of ~20μm showed more than 1200MPa tensile strength and 8% tensile ductility at room temperature in air. The strength of the structure was higher than that of conventional wrought Fe3Al alloys below 500°C and the specific tensile strength was as high as that of Ti-6Al-4V alloy at temperatures around 400°C. Correlation between microstructure, texture and tensile properties will be discussed.
3:30 PM - U2.4
Ab Initio Study on Elastic Properties of Fe3Al-based Alloys.
Martin Friak 1 , Johannes Deges 1 , Ronny Krein 1 , Frank Stein 1 , Martin Palm 1 , Georg Frommeyer 1 , Joerg Neugebauer 1
1 , Max-Planck-Institut fuer Eisenforschung, GmbH, Duesseldorf Germany
Show AbstractFe3Al-based alloys constitute a very promising class of intermetallics with great potential for substituting austenitic- and martensitic steels at elevated temperatures. A wider use of these materials is partly hampered by their moderate ductility at ambient temperatures. Theoretical ab initio based calculations are becoming increasingly useful to materials scientists interested in designing new alloys. Such calculations are nowadays able to accurately predict basic material properties by providing only the atomic composition of the material. We have therefore employed this approach to explore (i) the relation between chemical composition and elastic constants, as well as (ii) the effect transition-metal substituents (Ti, V, W, Cr, Mn) have on this relation. Using a scale-bridging approach we model the integral elastic response of Fe3Al-based polycrystals employing a combination of (i) single crystal elastic stiffness data determined by parameter-free first-principles calculations in combination with (ii) Hershey’s homogenization model. The ab initio calculations employ density-functional theory (DFT) and the generalized gradient approximation (GGA). The thus determined elastic constants have been used to calculate the ratio between the bulk and shear moduli as an indication of brittle/ductile behavior. Based on this approach we have explored chemical trends and identified optimal composition to achieve tailored mechanical properties. Using this information we have cast a selected set of Fe3Al-based ternaries, obtained for these the elastic constants by performing impulse excitation measurements at room as well as liquid nitrogen temperature and compared them with our theoretical results
3:45 PM - U2.5
Mapping the Phase Diagram Al-Fe-Ni-Ti using the Diffusion Multiples Technique.
Liliana Duarte 1 2 , Christian Leinenbach 1 , Ulrich Klotz 3 , Joerg Loeffler 2
1 Department Advanced Materials and Surfaces, Laboratory for Joining and Interface Technology , EMPA, Swiss Federal Laboratories for Materials Testing and Research, Duebendorf Switzerland, 2 Department of Materials, Laboratory of Metal Physics and Technology, ETH Zurich, Zurich Switzerland, 3 Department of Physical Metallurgy, FEM, Research Institute for Precious Metals and Metals Chemistry, Schwaebisch Gmuend Germany
Show Abstract4:30 PM - U2.6
Microstructure and Mechanical Behavior in High Strength Nanostructured Spinodal FeNiMnAl Alloys.
Xiaolan Wu 1 , Ian Baker 1 , Yifeng Liao 1 , Michael Miller 2
1 , Thayer School of Engineering, Dartmouth College, , Hanover, New Hampshire, United States, 2 , Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractIn order to understand the microstructure and mechanical behavior of some recently-discovered, very high strength FeNiMnAl spinodal alloys, an ingot of Fe35Ni15Mn25Al25 was drop cast and directionally solidified under Ar using a modified Bridgman furnace. TEM studies showed that the as-cast alloy had a periodic coherent microstructure consisting of alternating B2 (ordered BCC) and BCC phases. EDS showed that the BCC phase was rich in Fe and Mn, while the B2 phase was rich in Ni and Al, features confirmed by analysis using a Local Electrode Atom Probe. Hardness measurements were performed on the alloy as a function of annealing time at 550°C. The directionally solidified alloy showed a steady increase in hardness from 437 HV with annealing time, but the as-cast alloy, which was initially harder at 523 HV, showed more complex behavior. The final hardness after 72 h anneals was very similar at 676 HV for the two initial conditions. The paper will attempt to relate the mechanical properties to the changes in microstructure.Research was supported by DOE Award #DE-FG02-07ER46392
4:45 PM - U2.7
Microstructure and Mechanical Properties of a Eutectoid FeNiMnAl Alloy.
Yifeng Liao 1 , Ian Baker 1
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States
Show AbstractThe microstructure and mechanical properties of an as-cast eutectoid alloy with nominal composition Fe-20Ni-35Mn-15Al (at. %) have been investigated. The eutectoid transformation was shown by DTA to occur at 1502 K. The microstructure consisted of alternating 200-300 nm wide B2 (ordered body-centered cubic) and ~500 nm wide FCC lamellae, and showed good stability at temperatures up to 1033 K when heated in a TEM. The alloy exhibited a yield stress of ~750 MPa and an elongation of ~8% under tension at room temperature. Both ductile dimpled and brittle cleavage regions were observed on the resulting fracture surface. TEM in-situ straining experiments showed that deformation occurred by the glide of <110> dislocations in the FCC phase, which piled up at the interfaces with the B2 phase. Few dislocations were observed in the B2 lamellae both during TEM in-situ straining and in post-mortem TEM examinations of tensile-tested specimens. Determination of the yield stress as a function of temperature showed that it dropped only slightly to ~730 MPa at 600K but fell to ~208 MPa at 900K. The effects of different heat treatments on the mechanical properties will also be discussed.
5:00 PM - U2.8
Effect of Transition Metal Additions on B2↔A2 Order-Disorder Phase Transition Temperatures in B2 Type FeAl Intermetallic Compounds.
Mehmet Yildirim 1 , M.Vedat Akdeniz 1 , Amdulla O. Mekhrabov 1
1 Department of Material Science and Metallurgical Engineering, Middle East Technical University, Ankara Turkey
Show AbstractFe-Al intermetallic compounds are the most suitable candidates for high temperature applications due to their excellent physical, mechanical and oxidation properties in addition to their low material cost and low density. These unique properties are attributed to the long-range ordered (LRO) superlattices and strongly depend on deviation from alloy stochiometry, content and type of alloying element additions. The effect of content and type of the ternary alloying element additions on the order-disorder phase transition temperature of B2-type ordered Fe0.5(Al1-nXn)0.5 intermetallics have been determined by experimental methods. In this study B2-type ordered Fe0.5(Al1-nXn)0.5 intermetallics, where X= Cr, Ni, Nb, Hf, Mo, Ta, W, Mn and Zr are considered at n=0.01, 0.03 and 0.05 at.% concentrations, were prepared by arc melting technique and characterized by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). The order-disorder phase transition temperatures of both as-cast and heat-treated samples were determined by differential scanning calorimetry (DSC) measurements at different cooling and heating rates. Comparison of B2↔A2 order-disorder phase transition temperatures with theoretical predictions that were presented in our previous study shows good qualitative agreement with most X ternary alloying elements
5:15 PM - U2.9
Magnetism of Mechanically Deformed Powder Fe-Al Alloys.
Estibaliz Apinaniz 1 , Estibaliz Legarra 2 , Fernando Plazaola 2 , Damian Martin-Rodriguez 4 , Jose Javier Saiz Garitaonandia 3
1 Fisika Aplikatua I Saila, University of the Basque Country (UPV-EHU), Bilbao Spain, 2 Elektrizitate eta Elektronika Saila, University of the Basque Country (UPV-EHU), Bilbao Spain, 4 , ANSTO, Menai, Australian Capital Territory, Australia, 3 Fisika Aplikatua II Saila, University of the Basque Country (UPV-EHU), Bilbao Spain
Show Abstract5:30 PM - U2.10
Cracking in the Cast Fe-40Al-1C Alloys.
Jaromir Kopecek 1 , Jan Drahokoupil 1 , Karel Jurek 2 , Marketa Jarosova 2 , Petr Hausild 3 , Vladimir Sima 4 , Pavel Lejcek 1
1 Department of Metals, Institute of Physics of the AS CR, Prague Czechia, 2 Department of Structure Analysis , Institute of Physics of the AS CR, Praha Czechia, 3 Department of Materials, Faculty of Nuclear Sciences and Physical Engineering of the Czech Technical University, Prague Czechia, 4 Department of Physics of Materials, Faculty of Mathematics and Physics of the Charles University, Prague Czechia
Show Abstract5:45 PM - U2.11
Ultrasonic Vacuum Chill Casting and Hot Rolling of FeAl Based Alloys.
Vladimir Sima 1 , Premysl Malek 1 , Petr Kozelsky 2 , Ivo Schindler 2 , Petr Hana 3
1 Department of Physics of Materials, Charles University in Prague, Prague Czechia, 2 Faculty of Metallurgy and Material Engineering, VŠB - Technical University of Ostrava, Ostrava Czechia, 3 Department of Physics, Technical University of Liberec, Liberec Czechia
Show Abstract
Symposium Organizers
Bernard Bewlay General Electric Company
Yue-hui He Central South University
Martin Palm Max-Planck Institut fuer Esenfoschung GmbH
Masao Takeyama Tokyo Institute of Technology
Joerg Wiezorek University of Pittsburgh
U3: Titanium Aluminides I - Physical Metallurgy, Processing and Properties
Session Chairs
Bernard Bewlay
Masao Takeyama
Tuesday AM, December 02, 2008
Constitution A (Sheraton)
9:15 AM - **U3.1
Solidification of TiAl-based Alloys.
Ulrike Hecht 1 , Dominique Daloz 2 , Anne Drevermann 1 , Juraj Lapin 3 , Victor Witusiewicz 1 , Julien Zollinger 1
1 , ACCESS e.V., Aachen Germany, 2 LSG2M, Ecole des Mines de Nancy, Nancy France, 3 Institute of Materials and Machine Mechanics, Slovak Institute of Sciences, Bratislava Slovakia
Show AbstractNear net shape casting of TiAl-based alloys proved to be an attractive processing route for small aero-engine and automotive parts. Worldwide efforts are being made to increase not only the reliability of casting processes and the quality of cast parts, but also the size of the castings: large aero-engine blades and also industrial gas turbine blades are at the heart of today’s developments. Within the European integrated project IMPRESS*, the R&D activities related to TiAl processing equally push in this direction. The main research topics of IMPRESS will be briefly outlined. Naturally, one of the central topics relates to microstructure formation during solidification. Knowledge about the solidification behavior of TiAl-based alloys as function of alloy composition and cooling conditions is essential. Several questions emerge: they regard (i) the sequence of phase formation along the solidification path, (ii) the microsegregation that develops due to element partitioning between theses phases and diffusion inside these phases and (iii) the role of microsegregation during subsequent phase transformations in the solid state.In this presentation we give an overview of solidification characteristics of niobium containing TiAl-based alloys: in the first part of the presentation we present unidirectional solidification experiments that allowed freezing-in the sample region formed upon solidification and we show the results of microsegregation analysis for alloys with and without peritectic α(Ti) formation. The experimental results are compared to thermodynamic calculations of the solidification path, using recent thermodynamic descriptions elaborated for the ternary Ti-Al-Nb and the quaternary Ti-Al-Nb-B system, respectively.In the second part of the presentation we discuss the influence of the solidification path and of the associated microsegregation on the subsequent phase transformation, e.g. on growth of hcp α(Ti) from the parent bcc β(Ti) phase. This high temperature transformation is important for alloys with primary β(Ti) solidification, because it determines the final grain size of the material. * IMPRESS – Acronym of the European Integrated Project “Intermetallic Materials Processing in Relation to Earth and Space Solidification”
9:45 AM - U3.2
Grain Refinement of γ-TiAl Based Alloys by Inoculation.
Daniel-Hendrik Gosslar 1 , Robert Guenther 1 , Christian Hartig 1 , Ruediger Bormann 1 , Julien Zollinger 2 , Ingo Steinbach 3
1 Institute of Materials Science and Technology, Hamburg University of Technology, Hamburg Germany, 2 , Access e.V., Aachen Germany, 3 Interdisciplinary Centre for Advanced Materials Simulation, Ruhr-University Bochum, Bochum Germany
Show AbstractCasting of γ-TiAl based alloys in the composition range of 43 to 46 at.% Al would greatly benefit from the possibility of refining the primary β(Ti) phase by an appropriate inoculation of the melt. At present however, no efficient grain refinement for β(Ti) is available. In this presentation we describe first results, meant to identify and test potential grain refiners for both, the primary β(Ti) phase and the peritectic α(Ti) phase.Model calculations based on heterogeneous nucleation enable the simulation of grain size evolution. The inoculant particle size distribution, the cooling rate and the alloy constitution are the main input quantities for this model calculation.The incorporation of inoculant particles into the melt can be accomplished by a refiner master alloy. Hereby, the issue of concern is regarded to the particle stability inside the melt with respect to both, chemical stability and clustering or sedimentation processes. This may alter the particle distribution and affect strongly the model calculations. High energy ball milling has proven to be a favorable preparation method for master alloys, leading to a homogenous distribution of inoculants within a metallic matrix. Potential inoculants for the β(Ti) and / or the α(Ti) phase can be characterized based on lattice mismatch calculations. A high potency is indicated, among other physical and chemical factors, by a low lattice mismatch between substrate and nucleus. Therefore TiB2 could act as a potent substrate for heterogeneous nucleation of α(Ti) and β(Ti), if the volumetric content in the melt is sufficiently high. Adjustment of the TiB2 particle size distribution is obtained by varying milling parameters such as milling time and intensity. For model calculations the needed particle size distribution has been evaluated by SEM pictures. First results on the behavior of the master alloy dissolved in a TiAl-melt will be discussed.
10:00 AM - U3.3
Unidirectional Solidification and Single Crystal Growth of Al-rich Ti-Al Alloys.
Anne Drevermann 1 , Georg Schmitz 1 , Günter Behr 2 , Christo Guguschev 2 , Nico Engberding 3 , Martin Palm 3 , Frank Stein 3 , Martin Heilmaier 4 , Daniel Sturm 4
1 , ACCESS e.V., Aachen Germany, 2 , Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden Germany, 3 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 4 , Otto-von-Guericke-Universität Magdeburg, Magdeburg Germany
Show AbstractTi-Al alloys with an aluminium content of 60 ± 2 at.% Al are considered as promising candidates for new materials and components. Compared to today’s gamma-Titaniumaluminides they offer increased oxidation resistance at high temperatures and simultaneously decreased weight.To investigate the basic mechanical and thermomechanical properties of these new materials by tensile tests, bending tests etc. sufficiently large single crystalline domains are required. To generate these sufficiently large single crystalline domains with diameters of a few millimetres and a few centimetres of length we applied the methods of unidirectional solidification of a Ti - 60 at.% Al alloy in a Bridgman Stockbarger furnace and in an optical floating zone furnace equipped with a 5 kW Xenon lamp.Several kilograms of master alloy were produced by Vacuum Arc Remelting and precursor rods for the directional solidification/ floating zone experiments were produced by re-melting the alloy in a cold-wall induction crucible and subsequent centrifugal casting into a permanent Niob mould.Experiments in the Bridgman-Stockbarger furnace were performed to optimise grain selection and minimise oxidation during directional solidification. Variable parameters were solidification velocity, temperature gradient and crucible material. Using a seed-crystal originating from one of these experiments turned out to be beneficial to generate large domains. Parallel to these experiments single crystal material was grown in an optical floating zone furnace.The focus of this article is on samples grown by unidirectional solidification with a Bridgman-Stockbarger furnace describing the process development in detail with respect to grain selection, nucleation on seed crystals, oxidation of the samples during processing, effects of different crucibles and pre-treatments. Respective results are compared to samples processed by an optical floating zone process.
10:15 AM - **U3.4
Development of TiAl Alloys by Spark Plasma Sintering.
Alan Couret 1 , Houria Jabbar 1 , Jean-Philippe Monchoux 1 , Marc Thomas 2
1 CEMES, CNRS, Toulouse France, 2 DMSN, ONERA, Chatillon France
Show Abstract10:45 AM - U3.5
Hot-die Forging of a β-stabilized γ-TiAl Based Alloy.
Wilfried Wallgram 1 , Sascha Kremmer 1 , Helmut Clemens 2 , Andreas Otto 3 , Volker Guether 3
1 , Bohler Schmiedetechnik GmbH&CoKG, Kapfenberg Austria, 2 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 3 , GfE Metalle und Materialien GmbH, Nuremberg Germany
Show Abstract11:00 AM - U3.6
Experimental Studies and Thermodynamic Simulations of Phase Transformations in Ti-(41-45)Al-4Nb-1Mo-0.1B Alloys.
Helmut Clemens 1 , Barbara Boeck 1 , Wilfried Wallgram 2 , Laura Droessler 1 , Gerald Zickler 3 , Harald Leitner 3 , Sascha Kremmer 2 , Andreas Otto 4
1 Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 2 , Bohler Schmiedetechnik GmbH&CoKG, Kapfenberg Austria, 3 , Christian Doppler Laboratory for Early Stages of Precipitation, Leoben Austria, 4 , GfE Metalle und Materialien GmbH, Nuremberg Germany
Show Abstract11:30 AM - **U3.7
The Prospects of TiAl Application for the Aerospace and the Automotive Industries.
Sadao Nishikiori 1
1 Production Technology Development Dept, IHI, Yokohama Japan
Show Abstract Gamma titanium aluminides, TiAl, have been considered as alternate material, replacing the existing Ni-base alloys. In order for TiAl alloys to be widely employed in structural materials, we have investigated the possibility of using. In 1995, ground engine test was performed with LPT #5 blades cast from the TiAl alloys, GE48-2-2 developed by GE, along with IHI alloy. We have developed a manufacturing process of both alloys which can supply sound blades on actual operation level. A lot of production technologies have shown great growth through this experience for the decade of the 90’s. With increased demands from the automotive industry, subsequently, other alloys were developed, including TiAl alloys which has a fully lamellar structure to improve creep strength up to approximately 1100K-1200K range. We have fabricated passenger vehicle turbochargers using TiAl turbine wheels since 2004. As of today, a total number of this product is well over 100,000 units. New production methods which add to the applicability of TiAl are indispensable in actual use, such as the blades of aero-engines and the impellers of automotive turbochargers and must be economically manufacturable. The present status of TiAl application for the aerospace and the automotive industries is reviewed.
12:00 PM - U3.8
Microstructure Stability and Tensile Ductility of a Ti-43Al-4Nb-1Mo-0.1B Alloy.
Laura Droessler 1 , Limei Cha 1 , Thomas Schmoelzer 1 , Wilfried Wallgram 2 , Gopal Das 3 , Helmut Clemens 1
1 Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 2 , Bohler Schmiedetechnik GmbH&CoKG, Kapfenberg Austria, 3 , Pratt & Whitney, East Hartford, Connecticut, United States
Show Abstract12:15 PM - U3.9
Unique Plastic Deformation Behavior in TiAl-PST Crystals with Aligned Elongated Pores.
Takayoshi Nakano 1 , Takahiro Tachibana 1 , Koji Hagihara 1 , Yukichi Umakoshi 1 , Patrick Veyssiere 2
1 Course of Materials Science & Engineering, Division of Materials & Manufacturing Science, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan, 2 Laboratorie d’Etude des Microstructures, CNRS-ONERA, Chatillon cedex France
Show Abstract12:30 PM - U3.10
Effect of Microstructural Parameters on Toughness of Ti-Al-Nb-Cr Alloys.
Michael Kesler 1 , Sonalika Goyel 1 , Orlando Rios 1 , Hans Seifert 2 1 , Fereshteh Ebrahimi 1
1 Materials Science and Engineering, University of Florida, Gainesville , Florida, United States, 2 , Technische Universitaet Bergakademie Freiberg, Freiberg Germany
Show Abstract Titanium Aluminides are candidate materials for aerospace turbine engine applications due to their high strength to weight ratio and strength retention at high temperatures. Currently, high Nb γ+α2 alloys are being considered, but their applications are limited to relatively low temperatures because of loss of creep resistance. It has been shown that alloys consisting of γ-TiAl and σ-Nb2Al phases render excellent high temperature creep resistance; however, these alloys have unacceptable room temperature toughness and tensile ductility. These properties are anticipated to be strongly dependent on the volume fraction and connectivity of the hard and brittle σ-phase. In this study, alloys based on Ti-Al-Nb-Cr were designed and prepared by arc melting. The compositions of these alloys were selected such that different volume fractions of the σ-phase were obtained during the aging process. These alloys solidified as a single β-phase, wherein they could be solutionized and retained upon quenching. DTA was conducted to determine phase transformation temperatures. Based on these results temperatures for solutionizing and aging heat treatments were identified. The scale and morphology of the microstructure was controlled by varying the aging temperature. The results revealed that the morphology of the γ-phase is sensitive to whether this phase precipitates before, simultaneously or after σ-phase formation. Furthermore, the heating rate during aging significantly affected the nucleation rate and hence the size of the phases. It was also found that the σ-phase does not dissolve easily upon heating to the β+γ phase region, a quality which is desirable for creep resistance. Apparent toughness of the different microstructures was evaluated by indentation technique. In this presentation, the dependence of toughness on the volume fraction and particle size of σ as well as its dependence on the γ-phase morphology will be elucidated.The supported by NSF/AFOSR under grant number DMR-0605702 is greatly appreciated.
12:45 PM - U3.11
An Overview of the ESA-ESRF-ILL Collaboration in the Framework of the IMPRESS Integrated Project.
Guillaume Reinhart 1 2 3 , Gail Iles 1 2 3 , Garry McIntyre 1 , Andrew Fitch 2 , David Jarvis 3
1 ILL diffraction group, ILL (Institut Laue-Langevin), Grenoble France, 2 Material science group, ESRF (European Synchrotron Radiation Facility), Grenoble France, 3 Physical Science Unit, Directorate of Human spaceflight, Microgravity and exploration, ESA/ESTEC, Noordwijk Netherlands
Show AbstractThree intergovernmental research organisations from the EIROforum collaboration : the European Space Agency (ESA), the European Synchrotron Radiation Facility (ESRF) and the Institut Laue-Langevin (ILL), are cooperating to perform advanced experimental characterisation in the field of materials science within the framework of the IMPRESS integrated Project. IMPRESS is an acronym for Intermetallic Materials Processing in Relation to Earth and Space Solidification. This project is coordinated by ESA within the European Commission’s 6th Framework Programme (FP6). The main scientific objective of IMPRESS is to gain a better understanding of the links between material processing routes, structures, and final properties of intermetallic alloys. The project aims to develop and test two distinct prototype-based intermetallic materials : (i) γ-TiAl turbine blades for aero-engines and stationary gas turbines, and (ii) Raney-type Ni-Al catalytic powder for use in hydrogen fuel cell electrodes and hydrogenation reactions.The opportunity to carry out investigations combining the use of both synchrotron radiation at ESRF and neutrons at ILL provides unique experimental data to complement other benchmark experiments performed on the ground and in microgravity. We present an overview of the different synchrotron X-ray and neutron characterisation techniques implemented at ESRF and ILL which are used to understand the way in which these materials form during solidification processes. Preliminary qualitative and quantitative analyses are reported. They show that precise and critical information on the studied systems can be collected, which can eventually be used as references to validate models and numerical simulations developed within the IMPRESS collaboration.
U4: Titanium Aluminides II - Structure, Properties, and Coatings
Session Chairs
Helmut Clemens
Tresa Pollock
Tuesday PM, December 02, 2008
Constitution A (Sheraton)
2:45 PM - U4.2
Precipitation Reactions in Ti40Al60 and Ti38Al62 Alloys and Symmetry Relations of the Phases Involved.
Klemens Kelm 1 2 , Stephan Irsen 2 , Anne Drevermann 3 , Julio Aguilar 3 , Georg Schmitz 3 , Martin Palm 4 , Frank Stein 4 , Nico Engberding 4 , Martin Heilmaier 5 , Daniel Sturm 5 , Holger Saage 5
1 Center for Materials Analysis, Technische Fakultät, Christian-Albrechts-Universität Kiel, 24143 Kiel Germany, 2 , Stiftung caesar, 53175 Bonn Germany, 3 , ACCESS e.V, 52072 Aachen Germany, 4 , Max-Planck-Institut für Eisenforschung GmbH, 40237 Düsseldorf Germany, 5 Institut für Werkstoff- und Fügetechnik, Otto-von-Guericke-Universität Magdeburg, 39016 Magdeburg Germany
Show AbstractIn the system Al-Ti, several phases occur at compositions between 50 at.-% Al and 66 at.-% Al. While at room temperature g-TiAl is the stable phase near 50 al.-% Al, r-Al2Ti is the stable phase near 66 at.-% Al up to 1215_C. Above this temperature, g-AlTi becomes the stable phase even at 66 at.-% Al. Usually, the metastable phases h-Al2Ti and Al5Ti3 first precipitate in the compositions. Furthermore, the metastable Phase is frequently observed in specimens with depicted compositional range. [1]Despite the fact that the crystallographic structures of all the mentioned compounds are known, the descriptions of their structural interrelationships are limited. While the transformation from h-Al2Ti to r-Al2Ti has been attributed to the elimination of non-conservative antiphase boundaries [2], the interrelations of Al5Ti3 and h-Al2Ti to g-AlTi have only been stated to be “superstructures” and the topological arrangement of building blocks of Al5Ti3 and h-Al2Ti has been analyzed [3].We studied the phase evolution of two alloys with compositions of 60 at.-% Al and 62 at.-% by in situ transmission electron microscopy in the range between room temperature and 900_C. Only the three phases g-TiAl, h-Al2Ti and Al5Ti3 have been observed besides some spurious oxidation effects. While in the sample containing 62 at.-% the h-Al2Ti phase persisted besides g-AlTi, in the 60 at.-% sample Al5Ti3 and h-Al2Ti precipitated from the g-AlTi matrix and dissolved within the matrix in dependency from sample temperature within minutes. By these experiments using transmission electron microscopy, we were able directly to follow the solution and re-precipitation steps of the involved superstructure phases for the first time. We attribute the appearance of the metastable phases to the kinetic effects of the ordering of the crystallographic structures. While h-Al2Ti and Al5Ti3 can be related to g-AlTi by a group-subgroup-relationship, the same is not possible for the r-Al2Ti phase. A relation by a group-subgroup-relationship means that the crystallographic positions of the atoms split due to symmetry reduction. Thus, diffusion only has to take place among these splitting positions, which in the case of Al5Ti3 and h-Al2Ti are confined to a single crystallographic plane. This means that the atomic motion necessary to achieve this ordering can be confined to the length of the unit cell dimensions. I contrast, the formation of the stable phase r-Al2Ti needs the coordinated three dimensional movement half of the atoms of every unit cell and as well as of half of the atoms of every crystal upon the transformation from h-Al2Ti, resulting in the long transformation times of several hours observed.[1] J. C. Schuster, M. Palm, Journal of Phase Equilibria and Diffusion 27, 255 (2006).[2] C. Loiseau, C. Vanuffel, phys. stat. sol. a, 107, 655 (1988).[3] U. D. Kulkarni, Acta mater. 46, 1193 (1998).
3:15 PM - U4.4
Observation of <2c+a> Dislocations Glide in Duplex Ti-48Al After Room Temperature Tensile Deformation.
Andreas Kulovits 1 , Jorg Wiezorek 1
1 MEMS, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractTiAl based engineering alloys offer attractive combinations of density-specific properties rendering them attractive for selection in structural components of advanced transportation systems to realize energy savings during operation. TiAl based alloys typically comprise at least two phases, namely the tetragonal ordered phase gamma-TiAl and a minority fraction of the hexagonal ordered phase alpha-2 Ti3Al. During general shape changes at room temperature both constituent phases must accommodate plastic deformation and the deformation behavior of the alpha-2 phase has important implications for the mechanical behavior of TiAl based alloys. Ti3Al exhibits a yield stress anomaly for the <2c+a> dislocation slip systems, the activation of which is required for general shape change. Furthermore, transmission electron microscopy (TEM) after deformation of single crystals of Ti3Al revealed an interesting tension-compression anomaly for the glide activation of <2c+a> dislocation; at room temperature <2c+a> dislocations glide on {11-21} type pyramidal planes under compression loading and on {2-201} type pyramidal planes under tensile loading. We previously reported the choice of pyramidal glide plane critically influences details of the transfer of plastic deformation induced shears associated with dislocation glide and mechanical twinning activity from the gamma-phase to the alpha-2 phase in lamellar grains of TiAl alloys during room temperature compression. Here we used TEM to investigate the dislocation glide activity in the alpha-2-phase constituent in polycrystalline two-phase Ti-48at.%Al with a duplex microstructure after room temperature tensile deformation to failure. The results are discussed in relation to the mechanical behavior of Ti3Al, including the choice of <2c+a> dislocation glide plane, intraphase interface shear transfer and mechanical properties of two-phase TiAl. We gratefully acknowledge support from the National Science Foundation, Division of Materials Research, and helpful discussions with Drs. H.L. Fraser and X.D. Zhang of The Ohio State University.
3:30 PM - U4.5
TEM of C-component Dislocations Associated with Pyramidal Slip Activity in Hexagonal Alpha-2-Ti3Al.
Jorg Wiezorek 1 , Andreas Kulovits 1
1 MEMS, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractThe hexagonal ordered phase Ti3Al is a major constituent in super-alpha-2 alloys and a minor constituent in TiAl-based alloys, which are currently considered for high temperature structural applications in advanced airborne and ground transportation system. The lamellar grains in morphologically fully-lamellar and duplex microstructures of polycrystalline TiAl-based alloys consist of thin slabs of gamma-TiAl and alpha-2-Ti3Al and exhibit anisotropic mechanical properties. Single crystals of Ti3Al are also mechanically strongly anisotropic. The (0001)<11-20> and {1-100}<11-20> slip systems can be activated with relative ease in Ti3Al, whereas c-component dislocation slip, {2-201}<11-26> and {11-21}<11-26>, operates only for loading close to the c-axis. The latter slip system is associated with anomalous yielding and the deformation behavior of lamellar grains in two-phase TiAl may be affected by the behavior of the minority phase to a significant degree. Thus, here we used binary Ti-48at%Al model alloys to study the fine structure of c-component dislocations activated during c-axis loading of lamellar grains in TiAl-alloys by diffraction contrast and high-resolution transmission electron microscopy. In addition to the large amounts of debris characteristically associated with pyramidal slip in Ti3Al non-planar configurations of c-component dislocations have been observed. The results are discussed in relation to the mechanical behavior of Ti3Al and two-phase TiAl. We gratefully acknowledge support from the National Science Foundation, Division of Materials Research, and helpful discussions with Drs. H.L. Fraser and M.J. Mills of The Ohio State University.
3:45 PM - U4.6
High Carbon Solubility in a Gamma-TiAl Based Ti-45Al-5Nb-0.5C Alloy and its Effect on Hardening.
Christina Scheu 2 1 , Erich Stergar 2 , Michael Schober 2 , Limei Cha 2 , Helmut Clemens 2 , Arno Bartels 3 , Frank-Peter Schimansky 4 , Alfred Cerezo 5
2 Department of Physical Metallurgy and Materials Testing , University of Leoben, Leoben Austria, 1 Department of Chemistry and Biochemistry, University of Munich, Munich Germany, 3 Materials Science and Technology , Technical University Hamburg Harburg , Hamburg Germany, 4 GKSS Research Center, Institute of Materials Research, Hamburg Germany, 5 Department of Materials, Oxford University, Oxford United Kingdom
Show AbstractThe C distribution within the gamma-TiAl-phase of a Ti-45Al-5Nb-0.5C alloy with near-gamma microstructure has been studied by atom probe measurements. Volumes of up to 60x60x750nm3 were analysed and revealed that in most areas the C atoms are homogenously distributed. Only a few C enriched features were detected which are most likely Cottrell atmospheres surrounding dislocation cores. The C concentration within the gamma-phase is about 0.25 at.%, which is a factor of ten higher than the solubility limit reported for other TiAl alloys. The reason for this unusual high C solubility is explained with the help of an existing model which relates the number of octahedral sites consisting of six Ti atoms to the solubility limit of interstitials. Nb is known to locate exclusively at Ti sites and induces Ti antisites, i.e. Ti on Al sites, which in turn increases the number of octahedral sites surrounded by Ti atoms. The large amount of C in solid-solution can explain the 30% increase in yield strength for the C-containing sheet in comparison to a C-free sheet containing the same Ti, Al and Nb concentration.
4:30 PM - U4.7
Nano-scaled γ-TiAl: New Insights.
Limei Cha 1 , Christina Scheu 2 , Helmut Clemens 1 , Gerhard Dehm 3
1 Metallkunde und Werkstoffprüfung, Montanuniversität Leoben, Leoben Austria, 2 Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, München Germany, 3 , Erich Schmid Institut für Materialwissenschaft, Leoben Austria
Show AbstractIntermetallic titanium aluminides are promising candidates sustaining high mechanical and severe thermal conditions as e.g. occurring in aerospace, spacecraft and automotive applications. Especially γ-TiAl based alloys provide excellent strength up to temperatures of 800°C. In addition, they possess a high melting point, good corrosion resistance and low density. The mechanical properties are often determined by the presence of a lamellar microstructure which can be substantially varied by modifying the length scale between the internal γ-TiAl/α2-Ti3Al heterophase as well as γ-TiAl/γ-TiAl homophase boundaries. In this fundamental study, nanometer-scaled lamellar microstructures were adjusted in Ti-45Al-7.5Nb-(0-0.5)C alloys (in at.%) and analyzed mainly by means of transmission electron microscopy. Different annealing temperatures subsequent to oil quenching from the single α-phase region were used to obtain well-defined lamellar microstructures, which display ultrafine widths of a few tens of nanometers to a few nanometers.Surprisingly, the hardness of these alloys is enhanced by increasing the volume fraction of γ-TiAl laths. This behavior is different from earlier studies and is a consequence of the occurrence of a size effect: The nanometer-sized width of the γ laths prevents dislocation plasticity and deformation is carried by the wider α2-Ti3Al laths.
4:45 PM - U4.8
Ductility of Gamma-TiAl-Based Microstructures in the Light of Deformation Mode Interaction—Crystal Plasticity Modeling and Micro-Mechanical Experiments.
Claudio Zambaldi 1 , Franz Roters 1 , Stefan Zaefferer 1 , Dierk Raabe 1
1 Microstructure Physics and Metal Forming, Max-Planck-Institute for Iron Research, Düsseldorf Germany
Show AbstractThe deformation behavior of a gamma-TiAl based alloy (Ti-46Al-8Nb, at.-pct.) is incorporated into an elastic-viscoplastic crystal plasticity formulation coupled with the finite-element method. The deformation by ordinary dislocation glide, super dislocation glide as well as unidirectional deformation on four twinning systems are assigned different strength and hardening characteristics. For the hexagonal alpha2 phase prismatic, basal and pyramidal deformation modes are taken into account. The interaction of deformation mechanisms during deformation is captured by the formulation. The single phase constitutive behavior is calibrated by nano-indentation experiments in differently oriented single phase regions. Nano-indentation experiments are evaluated by means of a three-dimensional crystal-plasticity finite-element model of the deformation during nano-indentation. Load-displacement curves as well as the material pile-up around the indents are matched with the experimental data. The simultaneous activation of deformation mechanisms during nano-indentation is used to clarify their relative strengths and cross-hardening behavior. Two kinds of microstructures are investigated. The lamellar microstructure, resulting from slow cooling, is analyzed in terms of kinematic constraints imposed by the lamellar interfaces. The strong anisotropy of lamellar material is analyzed by calculating two-phase representative volume elements as well as material-point simulations for different sets of constitutive parameters. Secondly, the mechanical behavior of massively transformed and annealed microstructures is modeled. Microstructures obtained via a massive transformation step, exhibit a lower degree of kinematic constraints in terms of parallel gamma/alpha2 interfaces. On a grain-scale, this results in a less anisotropic behavior for plastic deformation and possibly improved ductility. Additionally, an attempt is described to include into the model the internal stresses that are known to reach significant values in TiAl-based alloys. The influence of the internal stress state on the activation of deformation modes is analyzed.The modeling work is complemented by mechanical characterization through nano-indentation and small-scale tensile tests. The microstructural characterization is greatly enhanced by the development of a novel orientation mapping technique. It makes possible the reliable determination of all six orientational variants of pseudo-cubic gamma-TiAl in a scanning electron microscope setup for electron backscatter diffraction.
5:00 PM - U4.9
Surface Treatment of TiAl with Fluorine for Improved Performance at Elevated Temperatures.
Alexander Donchev 1 , Pattrick Masset 1 , Michael Schuetze 1 , Hans-Eberhard Zschau 1
1 Karl-Winnacker-Institut, Dechema, Frankfurt/Main, Hessen, Germany
Show AbstractAlloys based on aluminium and titanium are potential materials for several high temperature applications. The use of TiAl would increase the efficiency of e.g. aero turbines, automotive engines and others due to their properties, among others low specific weight and good high temperature strength. The oxidation resistance is low at temperatures above approximately 800°C so that no long term use of TiAl-components is possible without improvement of the oxidation behaviour. Several ways are cited in the literature e.g. alloying with Nb. To produce TiAl-components specially designed for their later use surface treatment procedures are better because they do not interfere with the bulk properties. Small amounts of halogens in the surface zone of TiAl-samples lead to a dramatic improvement of the oxidation resistance at temperatures up to 1100°C for more than 8000 hours in air. In this paper results of the work on the halogen effect over the last years are presented. The results of thermogravimetric measurements, thermocyclic oxidation tests of small coupons and thermodynamic calculations for different atmospheres (e.g. air, H2O, SO2) are shown and the halogen effect mechanism is discussed. The postulated mechanism is in good agreement with the results of the oxidation tests. The limits of the halogen effect will be also mentioned. Predictions for the halogenation of TiAl-components can be given so that the processing can be planned in advance.
5:15 PM - U4.10
The Role of Fundamental Material Parameters for the Fluorine Effect in the Oxidation Protection of Titanium Aluminides.
Hans-Eberhard Zschau 1 , Michael Schuetze 1 , Alexander Donchev 1
1 Karl-Winnacker-Institut, DECHEMA e. V., Frankfurt am Main Germany
Show Abstract5:30 PM - U4.11
Analysis of the Liquid, Gamma, Sigma and eta Phase Equilibrium in the Ti-Al-Nb Alloy System.
Orlando Rios 1 , Damian Cupid 2 , Hans Seifert 2 , Fereshteh Ebrahimi 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Institute of Materials Science , Freiberg University of Mining and Technology , Freiberg, Saxony, Germany
Show AbstractTi-Al-Nb alloys have shown promising properties for high temperature applications. This alloy system is thermodynamically complex with multiple invariant reactions involving the liquid phase, solid state transformations and strongly temperature dependent solubility limits. An accurate knowledge of the equilibrium phase diagram is essential to the design of Ti-Al-Nb alloys and the development of effective thermo-mechanical processing and heat treatment schemes. Although the phase diagram has been well researched several controversies exist in the high temperature phase equilibria and the resulting liquidus projection. Extensive thermodynamic optimization incorporating experimental data available in the literature has indicated that a clear description of the invariant reaction involving L, γ, σ, η phases is not fully developed, yet characterizing this reaction is important to the understanding of high temperature stability in the Ti-Al-Nb system. Based on the existing assessments this invariant reaction can be either a ternary eutectic or a transition reaction.To address the above controversy, three alloy compositions were selected in the vicinity of different assessments of the invariant point, such that each of which crossed through the isothermal four phase field. DTA analysis was conducted at different rates for these alloys. The curves for all three alloys exhibited two main groups of peaks. Different heat treatments were designed to distinguish the phase transformations associated with these peaks. The microstructure and the phase components of the alloys in as-arc-melted slowly solidified and variously heat treated samples were examined using XRD, OP, SEM and TEM. Based on these analyses, the temperature of the invariant reaction was found to be close to 1527 οC. Samples were heat treated slightly below this temperature and then water quenched. EPMA analysis showed that the compositions of the three phases in each alloy were similar, which placed them in the three phase field resulting from the invariant reaction. In this presentation the phase transformation paths in the three alloys and the nature of the invariant reaction will be discussed.This work has been supported by NSF/AFOSR under grant number DMR-0605702.
5:45 PM - U4.12
The Effect of Cr Addition on Phase Stability of Ti-Al-Nb Alloys.
Sonalika Goyel 1 , Michael Kesler 1 , Orlando Rios 1 , Hans Seifert 2 1 , Fereshteh Ebrahimi 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 , Technische Universitaet Bergakademie Freiberg, Institut fuer Werkstoffwissenschaft, Freiberg Germany
Show AbstractHigh Nb TiAl-based alloys show promising high temperature strength and oxidation resistance. Previous studies have demonstrated that alloys with a γ+σ microstructure exhibit excellent creep properties at high temperatures. To impart good high temperature workability and microstructural control upon aging, it is necessary for the alloy to solidify as the single β-phase and be able to retain this phase upon quenching to room temperature. We have substituted Nb with Cr in selected Ti-Al-Nb alloys and studied the effect of Cr addition. The thermodynamics and kinetics of phase transformation in these alloys were evaluated using various techniques including DTA, high temperature and room temperature XRD, TEM and SEM. The analysis of the results has revealed that Cr addition affects thermodynamics as well as kinetics of β to γ and β to γ+σ phase transformations. In general, these transformation temperatures were reduced however the degree of influence was dependent on the alloy composition. Microstructural studies showed that the γ phase nucleates preferentially at the β grain boundaries and grows with Widmanstätten morphology even at very high temperatures. This observation suggests that the growth of this phase is mainly interfacial controlled. The addition of Cr was found to somewhat decrease this tendency. Compositional analysis of individual phases using EPMA revealed that, consistent with previously reported investigations, Cr partitions into the β-phase. This presentation discusses the possible mechanisms responsible for the kinetics and morphological effects of Cr addition on the β to γ phase transformation.This work has been supported by NSF/AFOSR under grant number DMR-0605702.
U5: Poster Session:
Iron Aluminides, Titanium Aluminides, Nickel Aluminides, and Silicides
Session Chairs
Bernard Bewlay
Yuehui He
Martin Palm
Tresa Pollock
Masao Takeyama
Jörg Wiezorek
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - U5.1
Crystal Structure and Thermoelectric Properties of Mn-Substituted Ru2Si3 with the Chimney-Ladder Structure.
Tatsuya Koyama 1 , Norihiko Okamoto 1 , Kyosuke Kishida 1 , Katsushi Tanaka 1 , Haruyuki Inui 1
1 Kyoto University, Materials Science and Engineering, Kyoto Japan
Show AbstractChimney-ladder compounds with the general chemical formula of MnX2n-m (n, m: integers) possess tetragonal crystal structures which consist of two types of subcells; one composed of transition metal atoms (M) with the β-Sn structure and the other composed of group 13 or 14 atoms (X) with a helical arrangement along the tetragonal c-axis. Since the chimney-ladder compounds generally exhibit very low thermal conductivity, presumably due to its long periodicity along the c-axis, they have been extensively investigated as promising thermoelectric materials. The high-temperature (HT) phase of Ru2Si3 is one of the chimney-ladder compounds with n=2 and m=1. Recently we have found that the HT-Ru2Si3 phase is stabilized by substituting Ru with Re so as to exist even at low temperatures in a wide compositional range of the Re content (Re: 14 to 73%), and that the thermoelectric power factor for alloys with high Re contents increases with the Re content and the highest value was obtained for the alloy with the highest Re content (73%), which is the solubility limit of Re in the chimney-ladder phase. In order to further enhance the thermoelectric properties, another ternary element which extends the solid solubility region of the HT-Ru2Si3 phase is favorable. We have chosen Mn as the ternary element because Mn4Si7 with the chimney-ladder structure exists as a counterpart of HT-Ru2Si3 in the Ru2Si3-Mn4Si7 pseudo-binary system so that the solid solubility region of the chimney-ladder phase is anticipated to extend in a wider composition range than the Re case. Our study, in fact, shows that the Mn-substitution stabilizes the HT-Ru2Si3 phase in a wide compositional range of the Mn content; 12 to 100%. Compositional analyses indicate that the Si/M ratio gradually increases as the Mn content increases. This is considered to be due to the addition of Si atoms in the Si subcell in order to compensate the decrease in the valence electron concentrations (VEC) per M atom by the substitution of Ru (group 8) with Mn (group 7) with fewer valence electrons. The Seebeck coefficient and electrical resistivity of the Mn-substituted Ru2Si3 are explained in terms of the VEC deviation from the idealized value, 14, which is expected for intrinsic semiconductors with the chimney-ladder structure. The highest dimensionless thermoelectric figure of merit (ZT=0.76) is obtained for 90%Mn-substituted alloy. The relationships between the microstructure and thermoelectric properties will be discussed.
9:00 PM - U5.10
High Temperature Oxidation of Fe-28Al-W Alloys Containing 0.1 % Y.
Remzi Gurler 1 , Nese Korpe 2 , Ibrahim Celikyurek 1
1 , Metallurgical Institute, Eskisehir Turkey, 2 Metallurgy and materials, Engineering Faculty, Eskisehir Turkey
Show Abstract9:00 PM - U5.11
Effects of Initial Grain Size on Recrystallization and Tensile Properties in Rolled Fe3Al-based Alloys Containing κ-Fe3AlC Particles.
Akira Takei 1 , Satoru Kobayashi 2 , Takayuki Takasugi 1
1 Materials science, Graduate School of Engineering,Osaka Prefecture University, Sakai Japan, 2 , Osaka Center for Industrial Materials Research,Insititute for Materials, Tohoku University, Sakai Japan
Show Abstract9:00 PM - U5.12
Grain–boundary Precipitation Strengthening in Creep of Fe-20Cr-30Ni-2Nb Steel Strengthened by Intermetallic Phases.
Keiichi Kurata 1 , Naoya Hashizume 1 , Naoki Takata 1 2 , Takashi Matsuo 1 2 , Masao Takeyama 1 2
1 Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan, 2 , Consortium of the Japan Research and Development Center for Metals(JRCM), Tokyo Japan
Show AbstractAdvanced ultra-super critical (A-USC) power plants require wrought materials with 105 h creep rupture strength more than 100 MPa at 973 K. Nickel-base alloys strengthened by GCP phases meet the condition, whereas none of the conventional austenitic heat resistant steels strengthened by transition metal carbides do not. Thus, a new class of austenitic steels strengthened by intermetallic phases of carbon free Fe-20Cr-30Ni-2Nb (at%) was proposed. In this steel only Fe2Nb Laves phase (C14) finely precipitates at grain boundaries and within grain interiors above 1073 K, whereas very fine metastable Ni3Nb-γ" phase (D022) precipitates coherently within grains, in addition to the grain boundary precipitation of Laves phase, at 973 K. In this study, the creep properties of the steel have been examined at 973 K under stresses above and below the yield strength (150 MPa). Regardless of the stress, the creep rate significantly reduced more than a few orders of magnitude during transient creep region and reaches minimum of about 10-6 h after 100 h. However, following creep behavior in accelerated stage is quite different, depending on the stress level. The creep rate increases aggressively to time to rupture of 340 h at 200 MPa. The acceleration in creep slows down with decreasing stress, and it obviously ceases for a certain period of time before reaching fracture (4594 h) at 120 MPa. The reduction in creep rate during the transient stage is caused by homogeneous precipitation of γ"phase in grain interiors. However, this precipitation strengthening is effective only for short-term creep since the γ"phase transforms soon to stable Ni3Nb-δ (D0a) phase. The microstructure observation of the ruptured specimen at 120 MPa clearly revealed the decoration of grain boundaries with stable Laves phase particles. It was also found that a pre-aged specimen at 1073 K having many Laves phase particles at grain boundaries exhibits more than 50 % creep elongation at 973 K / 140 MPa, indicating that the grain boundary Laves phase is not the factor to embrittle the sample. The projection of the creep rupture strength at 973K for 105 h is about 80 MPa, much superior to conventional 347 steels strengthened by carbides (40 MPa). Thus, the grain-boundary precipitation strengthening by stable Laves phase is responsible for the superior long-term creep strength of this steel. The detailed strengthening mechanism will be discussed in terms of the quantitative analysis of the area fraction of Laves phase at grain boundaries.
9:00 PM - U5.13
Consolidation of Mechanical Alloyed Ti-Al Intermetallic Compound by Electro Discharge Sintering(EDS).
Jang Hyoung-Soon 1 , Lee Won-Hee 1 , Kang Tae-Ju 1 , Cho Yoo-Jung 1
1 Sejong University, Department of Advanced Materials Engineering, Seoul 143-747 Korea (the Republic of)
Show Abstract9:00 PM - U5.14
Microscale Fracture Toughness Testing of TiAl PST Crystals.
Daisuke Miyaguchi 1 , Masaaki Otsu 1 , Kazuki Takashima 1 , Masao Takeyama 2
1 Dept. Materials Science & Engineering, Kumamoto University, Kumamoto Japan, 2 , Tokyo Institute of Technology, Tokyo Japan
Show AbstractTiAl based alloys with a fully lamellar structure exhibit superior fracture properties compared to those with other microstructures. This is mainly due to the activation of extrinsic toughening mechanisms including crack deflection and shear ligament bridging. These extrinsic toughening mechanisms are controlled by the mechanical properties of lamellae, in particular the lamellar interface fracture strength. It is, therefore, extremely important to evaluate the fracture properties of lamellar to improve the fracture toughness of such TiAl based alloys. In this investigation, a microscale fracture testing technique is applied to examine the fracture properties of lamellar in TiAl PST crystals. Micro-sized cantilever specimens with a size ≈ 10 × 15 × 50 μm3 were prepared from Ti-48Al two-phase single crystals (PST) lamellar by focused ion beam (FIB) machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into the specimens by FIB. Two types of notch directions (interlamellar and translamellar) were selected when introducing the notches. Fracture tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness (KQ) values of the interlamellar type specimens were obtained in the range 1.2 − 3.6 MPam1/2, while those of the translamellar specimens were 5.0 − 8.1 MPam1/2. These fracture toughness values are lower than those having been previously reported in conventional TiAl PST samples. For macro-sized specimens, extrinsic toughening mechanisms, including shear ligament bridging, act in the crack wake, and the crack growth resistance increases rapidly with increasing length of crack wake for lamellar structured TiAl alloys. In contrast, the crack length in microsized specimens is only 2 − 3 μm. This indicates that extrinsic toughening mechanisms are not activated in micro-sized specimens. This also indicates that intrinsic fracture toughness can be evaluated using microscale fracture toughness testing.
9:00 PM - U5.15
Microstructure and Compression Behavior of Ti3Al Based Ti-Al-V Ternary Alloys.
Tohru Takahashi 1 , Ayumu Kiyohara 1 , Daisuke Masujima 1 , Jun Nagakita 1
1 Dept. Mechanical Systems Engg., Tokyo Univ. Agric. & Tech., Koganei, Tokyo, Japan
Show AbstractOrdered alloy phase of Ti3Al shows a rather wide solid solubility range in aluminum and also in vanadium. Several Ti-Al-V ternary alloys have been prepared to investigate the alloy composition effect upon the microstructure, crystallography, and mechanical characteristics. The materials containing 75, 70 or 65 mol% titanium, and 0 or 5 mol% vanadium were prepared by arc melting. Metallographic observation has revealed that the binary Ti-Al alloys contained somewhat coarse grains with about 100 μm grain diameters. In contrast to this, ternary alloys containing 5 mol% vanadium showed smaller grained microstructures with grain diameters around 15 μm. The grain size could not be normalized to a unified value in the present study. X-ray diffraction study and microanalysis showed that the alloys contained single phase α2. Not every possible peak of the D019 ordered structure has been observed in the XRD patterns. The lattice parameters, a and c, were observed to become a little smaller as the aluminum content increased and also when vanadium was added. Compression tests have been performed at various temperatures ranging from ambient temperature up to 1200K on rectangular parallelepiped specimens with 2mm×2mm×3mm dimensions. Alloys containing more aluminum showed higher strength, and vanadium addition enhanced the strengths of the alloys. Deformability and strength are both enhanced by vanadium addition in some alloys. Temperature dependence of strength showed a little variation upon chemical compositions.
9:00 PM - U5.17
Identification of Ordering Domains in Gamma-TiAl by Crystal Orientation Mapping.
Claudio Zambaldi 1 , Stefan Zaefferer 1 , Stuart Wright 2
1 Microstructure Physics and Metal Forming, Max-Planck-Institute for Iron Research, Duesseldorf Germany, 2 , EDAX/TSL, Draper, Utah, United States
Show AbstractTetragonal gamma-TiAl exhibits cubic pseudo-symmetry due to its c over a ratio being just about 2% larger than unity. Therefore, the identification of the gamma-TiAl ordering domains by electron backscatter diffraction (EBSD) is made difficult and can result in frequent misindexing. In the past this problem was either overcome by identifying order domains by transmission electron microscopy with the known limitations such as small covered sample volumes and difficult sample preparation. Or the order domain structure was ignored in EBSD measurements by using a generic face centered cubic structure to solve for the crystal orientations. The latter method results in a significant loss of microstructural information as only the two twin-related domain families will be discriminated instead of all six orientational variants. A novel high accuracy approach to the crystal orientation mapping based on EBSD in a scanning electron microscope (SEM) is presented. An accurate calibration procedure for the pattern center is combined with high exposure times for pattern acquisition. Additionally, a precise measurement of the position of the diffraction bands using a high EBSD camera resolution and a highly resolved Hough transform are employed. Together with a newly developed indexing algorithm, the order domains can be identified in a reliable manner. The correct orientation is determined based on the angular deviation of the recalculated band positions from the experimental ones. Manual identification of superlattice reflections in the Kikuchi patterns was used to successfully validate the presented method. The developed method makes possible an accurate mapping of the order domains in gamma-TiAl-based alloys. Crystal orientation maps with a clearly revealed domain microstructure are presented. The domain structure is discussed with respect to the solid state phase transformations occurring during cooling of solidified material or during different heat treatment routes.Size distribution and spatial arrangement of ordered domains in gamma-TiAl is known to influence the mechanical performance of this high-temperature structural material. The different kinds of interfaces between the orientational variants possess different boundary energies. This can lead to rearrangement of boundaries and degradation processes during service of gamma-TiAl-based structural components. The developed method is of importance for the characterization of these processes. Furthermore, the full, tetragonal microtexture of the material can be determined.
9:00 PM - U5.18
Influence of Carbon on Phase Stability Among β, α and γ Phases and Microstructure Evolution in Ti-Al-Nb Alloys.
Masafumi Kurashige 1 , Shun Oinuma 1 , Takashi Matsuo 1 , Masao Takeyama 1
1 Metallurgy and Ceramics Science, Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan
Show Abstract9:00 PM - U5.19
Ion Implantation of Fluorine: a Promising Technique for Enhancing the High-temperature Oxidation Resistance of TiAl Alloys.
Rossen Yankov 1 , Andreas Kolitsch 1 , Frans Munnik 1 , Alexander Donchev 2 , Michael Schuetze 2
1 FWII, Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden Germany, 2 High Temperature Materials, Karl-Winnacker-Institut der DECHEMA e.V, Frankfurt am Main Germany
Show Abstract9:00 PM - U5.2
Electric Structure and Thermoelectric Properties in AgTeTl-Systems using ab Initio Calculation.
Hiroki Funashima 1
1 Department of Physics, Faculty of Engineering and Technology, Tokyo University of Science, Noda Japan
Show Abstract9:00 PM - U5.20
The Efficiency of Oxidation Resistance of TiAl Turbocharger Rotors Aluminized by Reduced Pressure MOCVD.
Takakazu Suzuki 1
1 , AIST, Tsukuba Japan
Show AbstractThe temperatures of exhaust gas from a gasoline turbine reach up to 1000K, which is 100-200K higher than that from a diesel turbine. Some improvement, therefore, for the oxidation resistance of titanium aluminide for the use of turbocharger rotors required. By using the thermal decomposition of Tri-iso-butyl aluminum, nonporous and adherent aluminum films can be deposited. The pre-heating for several hours before aluminizing has been more effective to enhance the uniformity of the coating for complex-shaped turbocharger rotors. Experimentally two kinds of coupons that have different surface conditions have been prepared. type1 (Rmax=14 micron m) is one that has been mechanically polished with an emery paper of #1200, type2 (Rmax=3.5 micron m) is one which has been pickled by a 40% aqueous solution of hydrochloric acid. An oxidation testing has been carried out at 1173K for 760ks in static air. The oxidation resistances of titanium aluminides have been evaluated from the mass gains. The mass gains after oxidation test of TiAl which are aluminized and subsequently treated at 1123K for 4.5ks have been evaluated. Though the oxidation resistance of both types (mass gain:85g/m2, 116.7g/m2) has been more improved than those of no aluminized specimens(199.6g/m2), it has clarified that the oxidation resistance of type1 has much better result than that of type2. From the result of EPMA line analysis, the outside layer has been assigned as Al2O3 and the next layer has been assigned as a mixture of TiO2 and Al2O3, and the inside layer has been assigned as TiAl3. The result of type2 is almost same as type1. Since the roughness of type1 has been larger than that of type2 those surface areas touching aluminum have been wider than that of type2. Thus, the formation of TiAl3 must be enhanced by increasing the surface area. The only difference in the diffusion layers of type1 and type2 is the thickness of TiAl3 layer. Therefore, the difference on both the TiAl3 layers' thickness must be a major reason why the oxidation resistance of type1 is better than that of type2. For the results of oxidation test of TiAl turbocharger rotors (type2) the ratio of mass gain has been introduced because turbocharger rotors have such a complicated shape that is impossible to determine the surface area. These results suggest that the efficiency of aluminizing by a MOCVD (Ratio 0f Mass Gain:0.13%) is comparable to a PACK method(Ratio of Mass Gain:0.09%), which is a powder processing. TiAl turbocharger rotors of complex shape are aluminized uniformly by MOCVD. The TiAl3 layer, by which the oxidation resistance can be improved, is formed with subsequent heat treatment above 933K. The surface roughness has been important factor for aluminizing for improving the oxidation resistance of titanium aluminide., because by the increasing of the surface roughness of titanium aluminide, the formation ability of TiAl3 layer is enhanced.
9:00 PM - U5.21
Protective CrAlYN Coatings for γ-TiAl Based Alloys.
Florian Rovere 1 , Martin Moser 1 , Reinhold Braun 2 , Paul Mayrhofer 1 , Helmut Clemens 1
1 Physical Metallurgy and Materials Testing, Montanuniverität Leoben, Leoben Austria, 2 Institute of Materials Research, DLR-German Aerospace Centre, Cologne Germany
Show AbstractUsing γ-TiAl alloys in high temperature applications requires coatings that effectively protect the material from oxidation. Here, the coating system Cr-Al-Y-N deposited by physical vapour deposition (PVD) is investigated with respect to their influence on mechanical properties and oxidation resistance of γ-TiAl alloys. Ti-47Al-2Cr-0.2Si sheet material was selected as substrate as this alloy exhibits high ductility at room temperature but low oxidation resistance. Thermal exposure in air reveals that the coatings significantly improve the oxidation resistance of the base material. The influence of the coatings on the mechanical properties of γ-TiAl was investigated by indentation experiments and 4-point-bending tests at room temperature. The coatings exhibit hardnesses of 35 to 40 GPa. The plastic strain in the γ-TiAl outer fiber during four-point-bending tests at room temperature is only slightly reduced by the deposition with CrAlYN. After 168 h oxidation at 800 °C uncoated γ-TiAl samples crack without plastic deformation due to the oxide layers and interdiffusion zones formed. Contrary, the CrAlYN protected γ-TiAl four-point bending specimen exhibit a plastic strain of 0.122 % after oxidation at 800 °C for 672 h, as only a thin oxide layer forms. We conclude that CrAlYN is effectively retarding oxidation and interdiffusion of γ-TiAl. The commonly observed deterioration of the mechanical properties by oxidation and interdiffusion is significantly reduced.
9:00 PM - U5.22
Application of the Fluorine Effect to TiAl-components.
Alexander Donchev 1 , Pattrick Masset 1 , Michael Schuetze 1 , Raluca Pflumm 1
1 Karl-Winnacker-Institut, Dechema, Frankfurt/Main, Hessen, Germany
Show AbstractThe oxidation resistance of TiAl-alloys can be improved by several orders of magnitude by fluorine doping of the surface zone of the material. The oxidation mechanism changes from the formation of a thick mixed oxide scale to a thin protective alumina scale. This fluorine treatment influences only the surface region of the components so that the bulk properties are not affected. Recent results achieved with TiAl-components showed the potential of a fluorine treatment for the use of TiAl in several high temperature applications. Turbine blades for aero engines or automotive turbocharger rotors made of TiAl were treated with fluorine by different methods and their performance during high temperature oxidation tests in wet and dry air and SO2-containing atmospheres is shown. Further on by selective local fluorination of TiAl a self organizing structure develops on the surface because of the different oxidation behaviour of the fluorinated and not fluorinated domains above 800°C. This means that a simple high temperature activation of such a sample leads to the formation of areas covered by a thin protective alumina scale alternating with a thick mixed oxide layer where no fluorine was applied. The aim of part of the research work on TiAl at Dechema is also to reproduce a shark-skin pattern (tiny parallel ridges) on the surface in order to minimise the aero dynamic resistance of turbine blades rotating in a gas flow. Different methods can be used for this attempt. Results of such selectively treated samples are also presented.
9:00 PM - U5.23
Phase Equilibria of Ti-Al-Ru Ternary Alloy System at 1200C.
Qizheng Liu 1 , Philip Nash 1
1 Mechanical, Materials Science and Aerospace, Illinois Institute of Technology, Chicago, Illinois, United States
Show Abstract9:00 PM - U5.24
Effect of Additional Elements on the Microstructural Stability of Fcc / L1_2 Two Phase Co-Al-W Base Alloys.
Masahiro Ooshima 1 , Katsushi Tanaka 1 , Kyosuke Kishida 1 , Norihiko Okamoto 1 , Haruyuki Inui 1
1 Materials Science and Engineering, Kyoto university, Kyoto, Kyoto, Japan
Show AbstractThe efficiency of aircraft engines and gas turbine generators is improved by increase in the gas temperature and consequently the operating temperature of the turbine blades equipped. In order to develop higher efficiency engines, considerable efforts have been accumulated to enhance the temperature capabilities of turbine blades. Nowadays, nickel base superalloys are widely used for turbine blades operated at the temperature up to about 1350 K because they exhibit superior performance at high temperature. Though the temperature capabilities of nickel base superalloys gradually increase year by year, there is the limitation of the melting point. Recently, a new cobalt base fcc / L1_2 (γ / γ') two phase alloys having the same microstructure to nickel base superalloy (cuboidal γ' coherently precipitates in the γ matrix) have been reported. Since cobalt has higher melting point than that of nickel by 40 K and its alloys have superior hot corrosion, oxidation and wear resistance compared with those of nickel base alloys, the cobalt base γ / γ' two phase alloys is expected as a new class of heat-resistance alloys. Ternary Co-Al-W two phase alloys exhibit excellent mechanical properties up to about 1050 K, however, the strength rapidly fall above the temperature resulting from that temperature closed to the γ' solvus temperature. This indicates that the additional elements have to be alloyed to improve the mechanical properties at higher temperatures. Recent reports indicate that some additional elements especially for tantalum improve the stability of the γ' phase and hence the mechanical properties at higher temperatures are improved, however, the effect of other additional elements is not known yet. In the present study, we have systematically examined the effect of additional elements on the change in the stability of the γ / γ' two phase microstructure. The change in lattice constants of the γ and γ' phases that is an important parameter related to a morphology of the γ' precipitates has also been examined.
9:00 PM - U5.25
Processing of High Temperature Ru-Al-Cr B2 Alloys.
Yutaka Hashimoto 1 , Norihiko Okamoto 1 , Manuel Acosta 2 , David Johnson 2 , Haruyuki Inui 1
1 Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto Japan, 2 Materials Engineering, Purdue University, West Lafayette, Indiana, United States
Show Abstract9:00 PM - U5.26
Microstructure and Mechanical Properties of Co3(Al,W) with the L12 Structure.
Takashi Oohashi 1 , Norihiko Okamoto 1 , Kyousuke Kishida 1 , Katushi Tanaka 1 , Haruyuki Inui 1
1 Materials Science and Engineering, Kyoto University, Kyoto Japan
Show Abstract Since the ternary intermetallic compound Co3(Al,W) with the L12 structure was discovered, two-phase Co-base alloys composed of the γ-Co solid-solution phase and the γ’-Co3(Al,W) phase as a strengthening phase have been investigated as promising high-temperature materials. Some Co-base alloys have been reported to exhibit high-temperature strength greater than those of conventional Ni-base superalloys. Although the excellent high-temperature physical properties of the Co-based alloys are considered to result from the phase stability and strength of Co3(Al,W), the pristine physical properties of Co3(Al,W) have not been fully understood, supposedly due to the difficulties in obtaining single-phase Co3(Al,W). In the present study, we examine the effect of heat treatment on the microstructure of alloys with compositions close to single-phase Co3(Al,W) as well as their mechanical properties, e.g. elastic modulus, thermal expansion, etc., in hope of deriving the pristine properties of the Co3(Al,W) phase. A single crystal with the composition of Co-10Al-11W grown by floating-zone melting exhibits a thermal expansion coefficient of 10×10-6 K-1 at room temperature, which is virtually identical to those of the commercial Ni-base superalloys. However, it increases with increasing temperature followed by a discontinuity at around 1000°C, inferring the phase transformation from γ’ to γ. The investigated thermal expansion behavior indicates that the lattice mismatch between the γ’ and γ phases is reversed from positive at room temperature to negative at high temperatures above around 500°C. The results of elastic property measurement and environmental embrittlement investigation of polycrystalline Co3(Al,W) will also be presented.
9:00 PM - U5.27
Influence of Elastic Properties on the Morphology of the γ’ Precipitates in Ni-base Superalloys.
Wataro Hashimoto 1 , Katsushi Tanaka 1 , Kyosuke Kishida 1 , Norihiko Okamoto 1 , Haruyuki Inui 1
1 , Kyoto University, Kyoto Japan
Show AbstractNi-base single-crystal superalloys are widely used for turbine blades of aircraft engines and gas turbine generators, because the alloys exhibit superior performance at high temperatures. The excellent mechanical properties at high temperatures are significantly owing to the γ’ precipitates distributed in the γ matrix. Since γ / γ’ interfaces efficiently block the motion of creep dislocations, the morphology of the γ’ precipitates is an important factor for strengthening superalloys. When the alloys are crept under a low tensile stress at a high temperature, a directional coarsening of the γ’ precipitates occurs and forms the lamellar structure, so-called “raft structure”, consisting of γ and γ’ plates alternately stacked along the tensile stress direction. It is known that the raft structure efficiently suppresses creep deformation because creep dislocations are hard to penetrate into the γ’ phase. The raft structure gradually collapses by further creep deformation and the collapse governs the lifetime of superalloys. Our elastic analysis of the raft structure previously performed implies that the well aligned raft structure (γ / γ’ interfaces are flat and lie on the (001) crystallographic plane) is expected to have a long lifetime under the creep condition. The initial shape and arrangement of γ’ precipitates before creep deformation significantly affect to the morphology of the raft structure, and are closely related to elastic properties of both γ and γ’ phases. Some assessments have been reported but most of them have made qualitative analysis with 2-dimensional models or a small volume fraction of the γ’ precipitates. The purpose of this study is to understand relationships between the elastic properties of the γ and γ’ phases and the morphologies of the γ’ precipitates distributed in the γ phase. We have calculated the elastic energies with variations in the shape, volume fraction and elastic moduli of precipitates. This calculation reveals that large elastic anisotropy makes the shape of the precipitates more angular. This is a similar result to the previous reports. We have also found that the difference in the elastic properties between the matrix and the precipitates also has a significantly effect on the shape of the precipitates.
9:00 PM - U5.28
New Pt-based Superalloy System Designed from First Principles.
Vsevolod Razumovskiy 1 2 , Eyvaz Isaev 3 2 , Andrey Ruban 1 , Pavel Korzhavyi 1
1 Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm Sweden, 2 Department of Theoretical Physics, Moscow Institute of Steel and Alloys, Moscow Russian Federation, 3 Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping Sweden
Show Abstract
Platinum-based alloys find applications as high-temperature materials in glassmaking and aerospace industries[1-3]. In particular, crucibles for glass melting and tool parts for glass-fiber drawing are made of platinum or Pt-based alloys. As these alloys typically have melting temperatures far exceeding those of Ni-based superalloys, they have been discussed as candidate structural materials for ultra-high temperature applications. Today’s commercial Pt-based alloys mostly have a homogeneous microstructure, quite similar to that of the first generation Ni-based superalloys (nimonics). Nowadays Ni-based superalloys have the well known two-phase γ-γ′ microstructure. Here γ denotes the fcc matrix phase (Ni-based solid solution) and γ′ the L12 - ordered precipitate phase (Ni3Al-based solid solution). During the long application period of Ni-based superalloys, this microstructure has demonstrated its exceptional structural stability and the best characteristics of high temperature strengthening. This kind of microstructure may be expected to similarly increase the service lifetime and the strength of Pt-based alloys. The starting point for the present investigation is provided by the work of Johannesson et al. [4]. Their search pointed to several L12 Pt-based compounds, including Pt3Sc. The latter compound has a large negative enthalpy of formation and remains ordered up to the melting point (1850 °C). The lattice misfit between Pt and Pt3Sc is estimated to be less than 1%.
Elastic properties of pure Pt, disordered Pt-Sc binary alloys, and ordered intermetallic compound Pt3Sc have been calculated using the EMTO [5] method. Good agreement with experiment is found for pure Pt. We have also calculated defect formation energies for Pt3Sc and phonon spectra for PtSc and Pt3Sc intermetallic compounds. Our calculations predict that Pt-Sc alloys with the γ–γ ′ microstructure (where γ is a substitutional solid solution of Sc in Pt, and γ′ is the Pt3Sc based phase) should combine good strength with exceptional ductility [6]. Moreover, this system has a very high melting temperature (Pt3Sc is stable up to 1850°C), which means that these alloys could be used as a material for ultrahigh temperature applications, where the ductility offered by Pt-Sc alloys is an advantage in comparison with the brittleness of Ir-based alloys.
[1] D. Lupton, Adv. Mater. 5, 29 (1990).
[2] M. V. Whalen, Platinum Met. Rev. 32(1), 2 (1988).
[3] E. I. Rytvin, High-temperature strenth of platinum-based alloys (Metallurgy press, Moscow, 1987).
[4] G. H. Johannesson, T. Bligaard, A. V. Ruban, H. L. Skriver, K. W. Jscobsen, J. K. Norskov, Phys. Rev. Lett. 88, 255506 (2002).
[5] L. Vitos, Phys. Rev. B 64, 014107, (2001).
[6] Razumovskiy VI et al., Intermetallics (2008), doi:10.1016/j.intermet.2008.04.016
9:00 PM - U5.29
Phase Equilibria and Microstructure on γ’ Phase in Co-Ni-Al-W System.
Shinagawa Kazuya 1 , Omori Toshihiro 1 , Oikawa Katsunari 1 , Ohnuma Ikuo 1 , Kainuma Ryosuke 2 , Ishida Kiyohito 1
1 Department of Materials Science, Tohoku University, Sendai Japan, 2 Institute of multidisciplinary research for advanced materials, Tohoku University, Sendai Japan
Show AbstractNi-base superalloys strengthened by precipitation of the γ’ (Ni3Al) phase with the L12 structure in the γ matrix (A1) have been widely utilized for high-temperature applications, including aircraft engines and power generation systems, where the γ’ phase shows positive temperature dependence of strength. On the other hand, GCP (geometrically closed packed) phase has not been successfully applied in Co-base superalloys. Recently, our research group has found a new stable γ’ (Co3 (Al, W)) phase with the L12 strucutre in the Co-Al-W ternary system. It is expected that the Ni substitution for Co stabilizes of γ’ phase greatly increases by Ni substitution for Co because Ni3Al with the L12 structure is very stable and the γ’ phase exists in the Ni-Co-Al ternary system. A detailed phase diagram of Co-Ni-Al-W system is thus important for the design of not only Co-base but also Ni-base superalloys. In the present study, the phase equilibria, with a focus on the γ’ phase in the Co-Ni-Al-W system, were determined by electron probe microanalysis (EPMA) and X-ray diffractometry (XRD). The γ’ solvus and solidus temperatures were measured by differential scanning calorimetry (DSC) and the microstructure was observed using FE-SEM in Co-(10-70)Ni-Al-W alloys. The phase equilibria in the Co-Ni rich portion at 900°C was determined in the Co-(10-70)Ni-Al-W system. The composition range of the stable γ’ is small in the Co-Al-W ternary system, while the γ/γ’ two-phase region expands and shifts to lower W and higher Al content regions with increasing Ni content in the Co-Ni-Al-W quaternary system. Results of the partition coefficient Kxγ'/γ (=Cxγ’/Cxγ) of each element in Co-xNi-10Al-7.5W at 900°C revealed that Al preferentially concentrates in the γ’ phase rather than in the γ phase and that the partition coefficient of Al increases with increasing Ni content. On the other hand, the partition of W, which is high in the 10Ni alloy, decreases with increasing Ni content and is below KWγ'/γ=1.0 at about 50% Ni, which means that W changes from a γ’ former to a γ former due to the increase of Ni composition. The γ’ solvus temperature increases with increasing Ni content while the solidus temperature is hardly affected by Ni content. Also, the γ’ solvus temperature increases with increasing W content. Morphology of the γ’ phase changes from cuboidal to spherical with increasing Ni content, which is due to the decrease of the lattice mismatch from 0.4184% to 0.1958% , originating from the partition change of Al and W.
9:00 PM - U5.3
Characterization of Natural Nanostructured Thermoelectric (Ga,In)2Te3.
Ken Kurosaki 1 , Manabu Ishimaru 2 , Anek Charoenphakdee 1 3 , Hideaki Matsumoto 1 , Hiroaki Muta 1 , Shinsuke Yamanaka 1
1 Graduate School of Engineering, Osaka University, Suita Japan, 2 Institute of Scientific and Industrial Research, Osaka University, Ibaraki Japan, 3 Faculty of Science and Technology, Rajamangala University of Technology Suvarnabhumi, Ayutthaya Thailand
Show AbstractWe are currently exploring high-performance bulk thermoelectric materials with extremely low thermal conductivity. Ga2Te3 would be one of the candidates. The thermal conductivity of Ga2Te3 is lower than that of In2Te3, although the Debye temperature of Ga2Te3 is higher than that of In2Te3. The thermal conductivity values of Ga2Te3 are extremely low (around 0.5 Wm-1K-1), which is only approximately 1.5 times of the minimum thermal conductivity. The structures of Ga2Te3 and In2Te3 are based on the cubic zinc-blende structure. Due to the valence mismatch between cation and anion, one-third of the cation sites are vacancies. The vacancies in In2Te3 regularly exist in atomic scale, while those in Ga2Te3 form two-dimensional vacancy planes existing periodically in nanoscale. The naturally-occurring vacancy planes in Ga2Te3 would scatter phonons efficiently, leading to the significant reduction of the thermal conductivity. Ga2Te3 and its related materials can be an ideal phonon glass electron crystal (PGEC) thermoelectric material.
9:00 PM - U5.30
Thermodynamic Assessment of the Co-Nb and Al-Co-Nb Systems.
Cuiyun He 1 , O. Dovbenko 2 , F. Stein 1 , M. Palm 1 , D. Raabe 1
1 , Max-Planck-Institute für Eisenforschung GmbH, Duesseldorf Germany, 2 , MSI, Materials Science International Services GmbH, Stuttgart Germany
Show AbstractThe thermodynamic assessments of the Co-Nb binary and Al-Co-Nb ternary system were carried out on the basis of new experimental data including the thermodynamic properties and phase equilibria by using the CALPHAD method.Phase equilibria and temperatures of invariant reactions in the Co-Nb system have been reinvestigated and will be the topic of a separate presentation. In the Al-Co-Nb system, phase equilibria at 800, 1000, 1150, 1200 and 1250 °C were investigated with special emphasis on the three Laves phases. Samples produced by levitation melting were annealed to attain equilibrium. The microstructures were examined by light-optical and scanning-electron microscopy, the compositions of the coexisting phases were determined by electron probe microanalysis, and the crystallographic structures were established by X-ray powder diffraction. In addition invariant reactions between the isothermal sections and melting temperatures were determined by differential thermal analysis.For the thermodynamic assessments the Gibbs energies of three Laves phases C14, C15 and C36 are described by the sublattice model. The thermodynamic parameters of the Co-Nb binary and Al-Co-Nb ternary systems have been optimized for reproducing the experimental results in each system, respectively. Agreement between the calculated results and experimental data is obtained.This work has been performed within the Inter-Institutional Research Initiative “The Nature of Laves Phases” funded by the Max Planck Society.
9:00 PM - U5.31
Alloying Effect on Mechanical and Chemical Properties of Cold-rolled Ni3(Si,Ti) Foils.
Y. Fujimoto 1 , Y. Kaneno 1 , T. Takasugi 1
1 Materials science , Graduate School of Engineering, Osaka Prefecture University, Sakai Japan
Show AbstractL12-type Ni3(Si,Ti) intermetallic thin foil can be fabricated by thermomechanical processing and subsequent heavy cold rolling. Low-temperature tensile strength of the cold-rolled foil is extremely superior to that of conventional alloys such as nickel-based alloys and stainless steels. However, for high-temperature tensile strength and elongation as well as oxidation resistance, further improvement is desired. In this study, microstructure, tensile properties and oxidation resistance of the cold-rolled Ni3(Si,Ti) foils which were alloyed with some elements (i.e., Al, Cr, Co, Mo) were investigated. All the alloyed Ni3(Si,Ti) ingots were successfully cold-rolled to thin foils with a thickness of 200μm, indicating that these alloying elements did not spoil the cold-rolling workability of the Ni3(Si,Ti) alloy. The Al-added Ni3(Si,Ti) alloy showed a L12 single-phase microstructure, while other alloys, i.e., the Cr-, Co- and Mo-added Ni3(Si,Ti) alloys, exhibited a two-phase microstructure consisting of L12 phase and fcc Ni solid solution phase. Similar to the unalloyed Ni3(Si,Ti) foil, all the cold-rolled foils showed high tensile strength of over 2GPa and the fully-recrystallized foils exhibited high tensile elongation of 30-40% at room temperature. The alloying elements generally enhanced high-temperature tensile properties of the Ni3(Si,Ti) foils. Among these alloying elements, Mo and Co are particularly effective in enhancement of high-temperature strength. On the other hand, high-temperature tensile elongation was improved by all the elements investigated. The observed high tensile ductility at high temperature is assumed to be attributed to suppression of the propensity to intergranular fracture which is frequently observed in the unalloyed Ni3(Si,Ti) foil. Oxidation resistance at 1173K for the alloyed Ni3(Si,Ti) foils was improved compared with the unalloyed Ni3(Si,Ti) foil. In this case, Al and Cr were more effective in improving the oxidation resistance than Co and Mo. Consequently, the present intermetallic foils were expected to be used as a new type of a nickel-based high-temperature structural material which possessed superior mechanical and chemical properties to the conventional alloys such as Inconel and Hastelloy alloys.
9:00 PM - U5.32
Plasma-Assisted Surface Modification of Dual Two-Phase Intermetallic Alloys Composed of Ni3X Type Structures.
Yasuyuki Kaneno 1 , Naoki Matsumoto 1 , Noriyoshi Tsuji 2 , Shin-ichi Tanaka 3 , Takayuki Takasugi 1
1 Department of Materials Science, Osaka Prefecture University, Sakai, Osaka Japan, 2 , Tanaka Ltd., Osaka Japan, 3 , SDC Inc., Sakai, Osaka Japan
Show AbstractDual two-phase intermetallic alloys composed of two kinds of geometrically close packed (GCP) Ni3X phases, i.e., Ni3Al (L12) and Ni3V (D022), show a coherent microstructure not only at the micron scale but also at the sub-micron scale, and also a high microstructural stability at high temperatures. The dual two-phase intermetallic alloys exhibit excellent mechanical and creep properties especially at high temperatures due to their peculiar microstructures, and thereby are expected to be used as high-temperature structural materials such as jet engine and gas turbine parts. However, wear resistance property of the dual two-phase intermetallic alloys is insufficient for application of conventional heat resistance parts because surface hardness and chemical stability are not always high enough. In this study, the dual two-phase intermetallic alloys composed of Ni3Al and Ni3V containing Nb, Co, Cr and so on were plasma-nitrided or carburized as functions of temperature and time. The surface layer of the plasma-treated materials was characterized by micro-hardness measurement, scanning electron microscopy, electron probe micro analysis and X-ray diffraction analysis. It was found that the hardness of the surface layer of the dual two-phase intermetallic alloys was enhanced by the plasma-treatment, and also primarily depended on treating temperature. The surface hardness of the nitrided and carburized materials showed a peak around at 850K and at 1025K, respectively. Also, the surface hardness was higher in the nitrided material than in the carburized material whereas the hardened layer was deeper in the carburized material than in the nitrided material. The XRD analysis revealed that and vanadium nitrides (VN) and vanadium carbides (VC) were formed in the surface of the nitrided and carburized materials, respectively, suggesting that the enhanced surface hardness was attributed to these nitrides or carbides. Also, the attempts of the application of the plasma-treated two-phase intermetallic alloys to e.g. friction stirring welding tool for high melting point materials are presented.
9:00 PM - U5.33
Spontaneous Catalytic Activation of Ni3(Si,Ti) Intermetallic Foils in Methanol Decomposition.
Yasuyuki Kaneno 1 , Takayuki Takasugi 1 , Hiroshi Tsuda 1 , Ya Xu 2 , Masahiko Demura 2 , Toshiyuki Hirano 2 , Hideo Iwai 2
1 Department of Materials Science, Osaka Prefecture University, Sakai, Osaka, Japan, 2 , National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractNi3(Si,Ti) intermetallic alloy with L12 structure which has been developed by Ti addition to Ni3Si exhibits excellent mechanical and chemical properties in a wide temperature range, and also shows a fairly good plastic deformability. We have succeeded in fabricating cold-rolled thin foils of the Ni3(Si,Ti) alloy from conventional polycrystalline ingot. The cold-rolled Ni3(Si,Ti) foils annealed at intermediate temperature show extremely high tensile strength of over 2GPa with reasonable tensile ductility. It has been recently demonstrated by some of the present authors that cold-rolled Ni3Al foil with the same crystal structure as Ni3(Si,Ti) showed a catalytic ability for methanol decomposition by spontaneously activating during reaction. Therefore, we have performed methanol decomposition test over the cold rolled Ni3(Si,Ti) foil. It was found that the catalytic ability of the cold-rolled Ni3(Si,Ti) foil for methanol decomposition reaction (CH3OH → 2H2 + CO) was significant at temperatures above 713K. The onset temperature of the catalytic activation was basically identical with that in the Ni3Al foil. The catalytic activity of the Ni3(Si,Ti) foil showed an incubation period accompanied with small peak at the beginning of reaction unlike that of the Ni3Al foil. Then, it rapidly increased with increasing reaction time and reached a maximum followed by a slight decrease. Surface analysis revealed that fine Ni particles supported on carbon fibers, and SiO2 and TiO2 thin layers were spontaneously formed on the surface of the foil during reaction. Consequently, it was suggested that the selective oxidization of Si and Ti led to the formation of fine Ni particles during reaction and these Ni particles acted as catalyst for the methanol decomposition. Similarity and discrepancy between Ni3(Si,Ti) and Ni3Al were discussed on the mechanisms of spontaneous catalytic activation.
9:00 PM - U5.34
Catalytic Property of Chemically Pretreated Ni3Al/Ni Two-phase Alloy Foils for Methane Steam Reforming.
Kamikihara Daisuke 1 2 , Xu Ya 2 , Demura Masahiko 2 , Hirano Toshiyuki 1 2
1 , Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki Japan, 2 , National Institute for Materials Science, Tsukuba, Ibaraki Japan
Show Abstract9:00 PM - U5.35
Surface Modification of Ni/Ni3Al Two-phase Foils by Electrochemically Selective Etching.
Hyeyoun Lee 1 2 , Masahiko Demura 1 , Ya Xu 1 , Dang-Moon Wee 2 , Toshiyuki Hirano 1
1 Fuel Reforming Materials Group, NIMS, Tsukuba Japan, 2 Materials Science & Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractNi3Al foils show catalytic activity for hydrogen production reaction such as methanol decomposition and methane steam reforming [1]. The catalytic activity is expected to be higher on rough and irregular surface compared with flat surface. In case of Ni(γ)/Ni3Al(γ') two-phase foils, such irregular surface can be obtained if γ phase is selectively etched. Electrochemical method is one of the attractive ways for the selective etching because one can precisely control the selective etching conditions by changing the applied current or potential. We here examined the electrochemical condition for the selective etching of γ phase using the γ/γ' single-crystalline ingots (Ni-18at.%Al) with uniform distribution of γ' precipitates. Then, we applied this technique to the γ/γ' foils, which have heterogeneous distribution of γ' precipitates in the γ matrix. Electrochemical etching was performed in the electrolyte of distilled water with 1wt.% (NH4)2SO4 and 1wt.% citric acid at a constant current of 20mA/cm2 (galvanostatic method). This condition was selected according to the previous study by Mukherji et al [2]. The surface microstructure of the γ/γ' single-crystalline ingot showed that the matrix (γ-phase) was selectively etched by the electrochemical etching, leaving the precipitates (γ' phase) behind. The EDS analysis revealed larger amount of oxygen on the surface of the γ' phase than on that of the γ phase. This indicates that a passive aluminum oxide was preferentially formed on the surface of the γ' phase during the electrochemical etching test, probably because the γ' phase has larger amount of Al than the γ phase. It is considered that this preferential formation of the passive aluminum oxide protected the γ' phase from being etched, resulting in the selective etching of the γ phase. The polarization curve obtained by potentiodynamic polarization test reasonably supported this mechanism. We carried out electrochemical etching on the γ/γ' foils under the same condition. The foils were cold-rolled and subsequently heat-treated at 1273K for 30min. Like the results of single-crystalline ingots, the foil surface showed the γ-etched microstructure after the electrochemical etching: the matrix was selectively etched, leaving the heterogeneously distributed γ' precipitates on the surface. It turned out that the rough and irregular γ' surface, which is expected to show high catalytic activity, can be formed.[1] D.H.Chun, Y.Xu, M.Demura, K. Kishida, M.H.Oh, T.Hirano and D.M.Wee, Catal. Lett. 106 (2006) 71[2] D.Mukherji, G.Pigozzi, F.Schmitz, O.Nath, J.Rosler and G.Kostorz, Nanotechnology 16 (2005) 2176
9:00 PM - U5.36
Surface Structure Modification of Ni3Al Foil Catalysts by Oxidation-reduction Treatment.
Jang Junhyuk 1 2 , Xu Ya 2 , Demura Masahiko 2 , Wee Dang Moon 1 , Hirano Toshiyuki 2
1 Materials Science, KAIST, Daejeon Korea (the Republic of), 2 , NIMS, Tsukuba Japan
Show Abstract9:00 PM - U5.37
Microstructural Factors Affecting Strength Property of Dual Two-phase Intermetallic Alloy based on Ni3Al-Ni3V Pseudo-binary System.
K. Kawahara 1 , Y. Kaneno 1 , T. Takasugi 1
1 Materials science, Graduate School of Engineering, Osaka Prefecture University, Sakai Japan
Show AbstractDual two-phase intermetallic alloys composed of geometrically closed packed (GCP) Ni3X phases (X=Al and V) show a peculiar microstructure consisting of the upper microstructure with a micron scale and the lower microstructure with a sub-micron scale. The upper microstructure is composed of Ni3Al (L12) and Ni solid solution (A1) phases at high temperature, and the Ni solid solution phase is decomposed into Ni3Al (L12) + Ni3V (D022) by a eutectoid reaction at low temperature (resulting in the lower microstructure). The dual two-phase microstructure is thus comprised of only intermetallic (ordered) phases with high coherency, leading to a high microstructural stability at high temperature. The high-temperature mechanical properties such as tensile strength and creep strength of the dual two-phase intermetallic alloys have been found to be superior to those of most conventional Ni-based superalloys. However, much less is known about relationship between microstructure and mechanical property of the dual two-phase intermetallic alloys. In this study, dual two-phase intermetallic alloys composed of Ni3Al (L12) and Ni3V (D022) phases containing Nb were investigated, focusing on the effect of the upper microstructural parameters on strength property. Alloy compositions of Ni75AlXV22.5-XNb2.5 + 100wt.ppmB (X: 0 – 22.5 at.%) were heat-treated at various temperatures where the upper microstructure is formed. The dual two-phase microstructure was formed in the Al content range between 6.5 – 11.0 at.%. In this content range, the size and the volume fraction of the L12 phase precipitates increased steadily with increasing Al content. Hardness of the alloys was primarily correlated with the volume fraction of the L12 phase precipitates; the hardness at room temperature was almost unchanged until the volume fraction of 60% and then slightly decreased. Above the volume fraction of 80%, the hardness rapidly decreased due to coarsening and coalescence (connecting) of the L12 precipitates. The effect of the volume fraction of the L12 phase precipitates on high-temperature (up to 1173K) hardness was basically the same as that on low-temperature hardness. However, the hardness of the alloys with the dual two-phase microstructure little decreased with increasing temperature. From these results, the mechanism responsible for strengthening in the dual two-phase intermetallic alloys was considered in terms of the upper microstructural parameters.
9:00 PM - U5.38
Effect of Growth Rate on Micro-structure and Micro-Structure Evolution of Directionally Solidified Nb-Si Alloys.
Yoshihito Sekito 1 , Seiji Miura 1 , Kenji Ohkubo 1 , Tetsuo Mohri 1 , Seiichi Watanabe 3 , Yoshisato Kimura 2 , Yoshinao Mishima 2
1 Division of Materials Science and Engineering, Graduate school of English, Hokkaido University, Sapporo, Hokkaido, Japan, 3 Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Sapporo, Hokkaido, Japan, 2 Interdisciplinary Graduate school of Science and Engineering, Tokyo Institute of Technology, Tokyo, Tokyouto, Japan
Show Abstract9:00 PM - U5.39
Solidification Microstructure Evolution Modeling in Nb-Si Based Intermetallics-strengthened-metal-matrix Composites.
Sujoy Kar 1 , Bernard Bewlay 2 , Ying Yang 3
1 , GE Global Research, Bangalore India, 2 , GE Global Research, Niskayuna, New York, United States, 3 , Computherm LLC, Milwaukee, Wisconsin, United States
Show AbstractFor higher fuel efficiency and greater thrust to weight ratios, there is a continuous drive for higher temperatures of turbine engine hot gas path components. Nb-silicide intermetallics, owing to their high melting point and high temperature strength, are potential candidates for high temperature applications. These intermetallics when precipitated in the metal matrix of (Nb) solid solution, result in intermetallic-strengthened metal matrix composites that have combination of room temperature toughness and high temperature strength. The microstructures of these in-situ composites can be complex and vary significantly with the addition of elements such as Ti and Hf. Hence an improved understanding of the phase stability and the microstructural evolution of these alloys is essential for optimization of these alloys. In the present paper we describe binary and ternary microstructural evolution modeling of dendritic and eutectic solidification obtained using phase-field simulations. The effect of parameters such as heat extraction rate, the ratio of the diffusivity of the solute in liquid to solid, and the interfacial energy of liquid-solid interface, on microstructural evolution during solidification is discussed in detail.
9:00 PM - U5.4
Properties of Thermoelectric Materials Synthesized by a Ball Milling Technique.
Chen-Kuo Huang 1 , Jean-Pierre Fleurial 1 , Ike Chi 1 , Caillat Thierry 1
1 , Jet Propulsion Laboratory, Pasadena, California, United States
Show AbstractINTRODUCTIONWe have synthesized a p-type Zintl compound (Yb14MnSb11) using a ball milling technique. Our synthesis is performed near room temperature and reproducibly yields homogeneous materials. The synthesis processes developed is not only simpler than traditional high temperature melting techniques, but the materials synthesized also possess superior thermoelectric properties. This encouraging result demonstrates the potential of these materials for thermoelectric applications.EXPERIMENTALThe synthesis of p-type Yb14MnSb11 powder is performed by ball milling technique. Firstly, inside an Argon-filled glove box, an appropriate amount of materials is loaded into a tungsten carbide (WC) vial containing WC balls. After the vial is taken out of the glove box, it is then clamped into a SPEX Mixer/Mill and shaken to complete the powder milling. After ball milling the vial is placed back into the glove box. The powder inside the vial is then removed and loaded into a graphite die for hot pressing. The powder is then hot-pressed into the desired shape and density. Hot-pressed pellets of Yb14MnSb11 were used to analyze the physical and thermoelectric properties of the material. Characterization of the synthesized materials included x-ray diffraction, electron microprobe, and high temperature measurements of the electrical and thermal transport properties up to 1275 K.RESULTS AND DISCUSSIONHigh-energy ball milling technique has been shown to be a convenient method of synthesis to prepare Yb14MnSb11. The material synthesis process developed is highly reproducible and economical. Initial results show that the material has superior thermal and electrical properties. The electrical resistivity was measured using the van der Pauw technique with a current of 100 mA. From the results of our measurements, the resistivity measured increases linearly with increasing temperature and reaches ∼5.65 mΩ-cm at 1275 K. The Seebeck coefficient was measured using a high temperature light pulse technique. The Seebeck coefficient for Yb14MnSb11 shows a positive increase with increasing temperature and reaches a maximum of +224 μV/K at 1275 K. The thermal conductivity of Yb14MnSb11 was measured using a flash diffusivity technique. The values obtained range between ∼7 and 9 mW/cm.K at 300 and1275 K, respectively. This low value of thermal conductivity is comparable to that of a glass, and is strongly contributing to the figure of merit of Yb14MnSb11(∼1.34 at 1275 K).CONCLUSIONS Yb14MnSb11 has demonstrated exceptional thermoelectric properties. Its high ZT (1.34 at 1275 K) measured will open up a wide interest in the research for a new generation of candidate Zintl thermoelectric compounds for power generation and conversion. ACKNOWLEDGEMENTThis work was carried out at Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Copyright 2008. California Institute of Technology. Government sponsorship acknowledged.
9:00 PM - U5.40
Metastable C15 Laves Precipitation in a Nb-Mo-Cr-Al-Si Alloy.
Yan-Ling Hu 1 , Alexandre Vasiliev 2 , Lichun Zhang 1 , Kai Song 3 , Mark Aindow 1
1 Materials Science and Engineering Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Institute of Crystallography, Russian Academy of Sciences, Leninskij, Moscow, Russian Federation, 3 Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, United States
Show Abstract In previous studies by Shah et al. a series of ductile-phase-toughened in-situ intermetallic composites was developed for very high T structural applications. An alloy with composition Nb-27Mo-27Cr-9Al-9Si (in at.%) was particularly promising. This alloy was based upon the Nb-Cr2Nb system with high levels of Mo additions to strengthen the ductile phase and lower the ductile-to-brittle transition temperature (DBTT) of the Cr2Nb Laves phase; Al and Si were added to improve the oxidation resistance and to stabilize the Laves phase. No evidence for catastrophic oxidation was found in this alloy after 48h at 1371 degree C in air. In the present paper, we present an electron microscopy study on the microstructures exhibited by this alloy. The as-cast alloy was comprised of a primary dendritic A2 solid solution surrounded by a eutectic mixture of A15 and C14 phases. The retention of these phases even after extended high-temperature heat-treatment indicates that these are the equilibrium phases for this alloy. However, Laves phase precipitates formed within the A2 dendrites at 1000 degree C adopt the C15 structure. From the defect structures and ORs exhibited by the phases, it was inferred that the C15 structure is formed due to tensile coherency stresses, and that this phase will transform to the equilibrium C14 structure by synchroshear processes on {111} when the stresses are relaxed.
9:00 PM - U5.41
Microstructure and High Temperature Oxidation Behavior of Nb-20Mo-15Si-5B-20Cr Alloy.
Julieta Ventura 1 , Shailendra Varma 1 , Rabindra Mahapatra 2
1 Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, Texas, United States, 2 Materials Laboratory, Naval AIr System Command, Patuxent River, Maryland, United States
Show Abstract9:00 PM - U5.42
Investigations of Multiphase Equilibria in the Nb-Cr-Ti-Si system Via an Approach of Integrating Thermodynamic Modeling with Key Experiments.
Ying Yang 1 , Bernard Bewlay 2 , Y. Chang 3 1
1 , CompuTherm, LLC, Madison, Wisconsin, United States, 2 , General Electric Global Research, Schenectady, New York, United States, 3 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show Abstract9:00 PM - U5.43
Substitutional Disorder and Atomic Displacements in C14 Nb1-xMn2+x.
Daniel Gruener 1 2 , Alim Ormeci 2 , Yuri Grin 2 , Guido Kreiner 2
1 Department of Physical, Inorganic and Structural Chemistry, Stockholm University, Stockholm Sweden, 2 , Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden Germany
Show Abstract9:00 PM - U5.44
Precipitation Kinetics and Morphology of Fe2Nb Laves Phase in Prestrained Fe-20Cr-30Ni-2Nb Austenitic Heat Resistant Steel.
Kazuhiro Yoshimura 1 , Naoki Takata 1 2 , Takashi Matsuo 1 2 , Masao Takeyama 1 2
1 Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan, 2 , Consortium of the Japan Research and Development Center for Metals (JRCM), Tokyo Japan
Show AbstractA new class of austenitic heat resistant steels strengthened by intermetallic phases has currently been developed for advanced ultra-super critical (A-USC) power plants. We revealed that Fe2Nb Laves phase with C14 structure precipitates finely at grain boundaries as well as grain interiors above 1073 K, whereas below 1073 K Ni3Nb-γ" phase with D022 structure also precipitates coherently within grains in a carbon free Fe-20Cr-30Ni-2Nb steel. The prolonged creep rupture strength of the steel at 973 K is much higher than conventional 347-type steels due to “grain-boundary precipitation strengthening” by the Laves phase. In this study, thus, in order to control the precipitation morphology of the Laves phase at grain boundaries, effect of prestrain on the precipitation kinetics and morphology of Fe2Nb phases in the steel has been examined by using the samples cold rolled by 16 % (CR16) and 56 % (CR56). Above 1073K where only Fe2Nb precipitates, prestrain enhances the precipitation of Laves phase, and the larger the prestrain the faster the precipitation start by at least an order of magnitude with respect to the unstrained samples. TEM observation revealed that the Laves phase preferentially nucleates at recrystallized grain boundaries and deformation bands, but not on dislocations, so that the density of introduced high-energy interfaces responsible for kinetics of the precipitation. Thus, the precipitation morphology of the Laves phase significantly varies depending on the percent cold work. In heavily deformed sample (CR56), due to the simultaneous occurrence of precipitation and recrystallization, the Laves phase is distributed homogeneously at fine recrystallized grain-boundaries and its interiors with nano-size particles. In moderately deformed sample (CR16) where only recovery occurs, the Laves phase mainly decorates the original grain boundaries and some deformation bands with aligned distribution. At 973 K, the precipitation kinetics of Laves phase in grain interiors becomes sluggish due to the decrease in chemical driving force by the prior formation Ni3Nb-γ" phase. However, kinetics of the grain boundary Laves phase is not affected and even faster than that of the γ" phase. The detailed morphology of the Laves phase after prolonged aging will be presented.
9:00 PM - U5.46
Solid-state Processing of Al-Mg Alloys.
Mira Sakaliyska 1 , Sergio Scudino 1 , Kumar Babu Surreddi 1 , Jürgen Eckert 1
1 Institut für Komplexe Materialien (IKM), IFW Dresden, Dresden Germany
Show AbstractNanostructured Al-Mg powders with compositions ranging from 10 to 90 at.% Mg have been produced by mechanical alloying (MA) of elemental powder mixtures and mechanical milling (MM) of pre-alloyed ingots. Although the solid-state transformations induced by MA and MM occur by different mechanisms due to the different starting materials used, the present results indicate that MA and MM of Al-Mg powders with the same composition lead to the formation of the same phase(s). Solid solubility extension far beyond the room temperature equilibrium value (~ 1 at.% Mg) was achieved by both MA and MM in the range 10 - 40 at.% Mg. The cell parameters of the solid solutions increase linearly with increasing Mg content with a slope that depends on the processing route used. The solid solutions are metastable and transform into the stable phases during heating through a sequence of phase transformations that involve the formation of metastable phases. The milled powders with compositions between 50 and 70 at.% Mg display the formation of the single-phase γ-Al12Mg17, therefore, solid-state processing suppresses the formation of β-Al3Mg2 and hcp-Mg in this composition range. No formation of the β-Al3Mg2 was observed during milling for the entire composition range studied. Finally, in the range 80 - 90 at.% Mg, MA as well as MM lead to the formation of two-phase nanostructured powders with a structure consisting of the equilibrium phases γ-Al12Mg17 and hcp-Mg.
9:00 PM - U5.47
Effect of Precipitate Depleted Zones on Precipitation Hardening in Mg-based Alloys.
Dmitry Shepelev 1 , Alexander Katsman 1 , Evgeniya Edelshtein 1 , Menachem Bamberger 1
1 Materials Engineering, Technion, Haifa Israel
Show AbstractThe formation of precipitate depleted zones (PDZ) near-grain boundaries in Mg-based alloys strengthened by precipitation hardening is known as detrimental to the material since wide depleted zones may affect the mechanical and corrosion properties of the alloy. Experimental investigation of PDZ evolution in Mg-Zn-Sn-alloys aged at different temperatures for different times was conducted by SEM and TEM and by measuring the microhardness of near grain boundary zones at low loads.It was found that at early stages of aging (175°C, ≤1day) the hardening is caused by formation of MgZn2 needles and T-like MgZn2/Mg2Sn particles. The near-grain boundary zone is stronger than the grain matrix, due to large round MgZn2 and Mg2Sn particles formed at grain boundaries. Increasing the aging time leads to decrease of hardening in the matrix as well as in the near-grain boundary zones due to dissolution of MgZn2 needles and the coarsening of T-like particles. Substantial microhardness decrease in the near-grain boundary zone (from 70 to ~30 HV) found at low loads (10g) was connected with formation of PDZ. It was confirmed by TEM and SEM investigations. Further aging, at 225°C for 1-8 days, leads to formation of "crusts" of enlarged T-like particles around depletion zones. As a result, microhardness of PDZs increases up to ~60HV close to one measured in the grain matrix.
9:00 PM - U5.48
Abnormal Hydrogenation Phenomena of Mg-based Film Structures on Si Substrate: the Formation of Mg2Si Alloy and Sub-Micro MgH2 Whiskers.
Yiping Zhao 1 , Yuping He 1
1 Department of Physics and Astronomy, Nanoscale Science and Engineering Center, University of Georgia, Athens, Georgia, United States
Show AbstractMagnesium hydride is one of the most promising candidates for future solid state H-storage applications, due to its lightweight, low cost, and high reversible hydrogen storage capacity of 7.6 wt% in MgH2. However, its high thermodynamic stability and sluggish reaction kinetics limit its practical applications. A great deal of research showed that the H-storage performance of MgH2 could be improved by making nanostructures with large surface-to-volume ratios to enhance surface reaction activity, and/or by adding an appropriate transition metal catalyst to accelerate the hydrogen sorption kinetics, mainly via Ball milling, which produces micrometer-sized powder with nanometer-sized grains. To obtain a fundamental understanding on how hydrogen interacts with metal nanostructure and nanocatalyst, thin film with various nanotextures provides an ideal alternative to the micro-powder since its structure and composition can be well controlled by deposition conditions. In most of the film H-storage studies, Si wafers are used as substrates for direct metal (like Mg) film depositions. Our study shows that at a moderate hydrogenation temperature, such as 200°C or lower, Mg has already started to react with the Si substrate to form a stable Mg2Si alloy with a two-layered morphology. The formation of Mg2Si reduces the amount of Mg converted to MgH2 and strongly affects the hydrogen storage reversibility and cycling performance. To solve the substrate problem, a layer of Ti film was deposited between Mg and Si as a diffusion barrier, and the results show that the formation of Mg2Si can be greatly suppressed, but a small amount of Mg2Si still can be formed progressively through the pinholes in Ti thin film. Thus, to obtain an intrinsic hydrogenation behavior of the Mg film, we designed a special diffusion barrier layer coated Si substrate for a 2 at.% Ti doped Mg film deposition by a combinational technique of multilayer, co-deposition, and dynamic shadowing growth. The diffusion barrier layer is composed of Ti nanorods and Ti film. The structure makes the Ti doped Mg film be either on top of the Ti nanorods or in the gap between the Ti nanorods, preventing the direct Mg-Si contact and thus suppressing the formation of Mg2Si alloy. When the unique film structure was hydrogenated at temperatures no higher than 300°C for ~ 120 h, tetragonal single crystal structured MgH2 sub-micro whiskers formed on the sample surface. This result reveals that the fundamental hydrogenation process of Mg could be more complicated when the silicide formation or impurities can be eliminated by an effective diffusion barrier layer.
9:00 PM - U5.49
Electron Irradiation Induced Crystal-to-amorphous-to-crystal (C-A-C) Transition in Intermetallic Compounds.
Takeshi Nagase 1 2 , Kazuya Takizawa 2 , Hirotaro Mori 1 , Akihiro Nino 3 , Yukichi Umakoshi 4
1 Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka, Japan, 2 Division of Materials and Manufacturing Science, Osaka University, Suita, Osaka, Japan, 3 Department of Materials Science and Engineering, Faculty of Engineering and Resource Science, Akita University, Akita, Akita, Japan, 4 , National Institute for Materials Science, Tsukuba, Ibaragi, Japan
Show AbstractIt is known that atomic displacement can be introduced by various processes such as severe plastic deformation, mechanical milling and electron irradiation, resulting in the crystal-to-amorphous (C-A) transition and amorphous-to-crystal (A-C) transition in some metallic materials. Recently, unique atomic displacement-induced-phase transitions containing both amorphization and crystallization was found: a crystal-to-amorphous-to-crystal (C-A-C) transition under electron irradiation[1] and a cyclic C-A transition during mechanical milling[2]. With respect to the electron-irradiation-induced C-A-C transition, it has been reported to the date that 6 intermetallic compounds undergo the unique phase transition, indicating this transition to be a phenomenon with wide generality in metallic crystals. In the present study, the origin of the electron-irradiation-induced C-A-C transition was discussed based upon the analysis of the accumulated experimental data.[1] T. Nagase and Y. Umakoshi: Scripta Mater., 48 (2003) 1237-1242.[2] M.S. El-Eskandarany, K. Aoki, K. Sumiyama and K. Suzuki: Appl. Phys. Lett., 70 (1997) 1679.
9:00 PM - U5.50
Intermetallic Precipitate Strengthened Materials for Cryogenic Applications.
Ke Han 1
1 , National High Magnetic field Laboratory, Tallahassee, Florida, United States
Show AbstractAustenitic stainless steels have been used for cryogenic applications (mainly at 4 K) for decades because of their excellent combination of the mechanical strength and toughness at cryogenic temperatures. Low carbon 316 LN (denoted as 316LLN in this paper) is one of the austenitic stainless steels. The low carbon retards the formation of the grain boundary carbides and makes the materials less vulnerable to sensitizations. However, their yield strength in annealed status is limited to a range of 1000 MPa at 4 K partially due to the low carbon content. In addition, the intermetallic compounds may form at certain temperature range and make the materials less ductile than the one heat treated at optimized heat treatment temperatures. The national high magnetic field laboratory (NHMFL) studied various other materials for cryogenic applications and selected Haynes 242 alloy as a candidate alloy for cryogenic applications. NHMFL research on the cryogenic mechanical and physical properties of Haynes 242 and 316LLN [1 ] is fruitful. The research is also expanded to evaluate the microstructure and mechanical property evolution as a result of the processing because of the phase transformations that occur during the heat treatment. The processing includes both the deformation and the heat treatment at temperatures where sensitization occurs. The materials are received at the NHMFL in round tube form and subsequently shaped into rectangular form with the superconductor cable inserted and then wound into coils. This fabrication procedure introduces deformation strains at room temperature. The estimated strains for shaping are about 10-15%. The maximum bending strain caused by coil winding is about 3%. The deformed materials are then heat treated in the temperature range of 650oC -700oC for 50-100 hours. A combination of deformation and heat treatment was also performed in order to study the stress accelerated grain boundary oxidation (SAGBO) [2 ]. The heat treatment promotes the formation of intermetallic compound precipitates in both materials. In 316LLN, the intermetallic compounds were found to form at the grain boundaries whereas in Haynes 242, they form inside the grains as an ordered phase. This work provides an insight to understand the possibility of occurrence of sensitizations, and SAGBO in our alloys and to identify the pre-stress contribution to the formation of the intermetallic compounds and to the performance of the materials at 4 K. The mechanical properties of both materials were tested at room temperatures and 4 K after the materials were heat treated under the stresses.References:[1] Lu, J.; Han, K.; Choi, E.S.; Jo, Y.; Balicas, L. and Xin, Y., J. Appl. Phys., 101, 123710 (2007)[2] Toma, L. S., Steeves, M. M., Reed, R. P., Advances in Cryogenic Engineering, 40B, edited by P. Kittel et al., Plenum, New York, 1994, pp 1291-1298.
9:00 PM - U5.51
Plastic Deformation of Intermetallic Composites Containing In Situ Ductile Dendrite Dispersions.
G. Chen 1 2 , H. Bei 2 , Y. Shi 1 , A. Gali 2 , C. Liu 2 , E. George 2
1 Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing China, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge and Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
Show Abstract9:00 PM - U5.52
Direct Modeling of the Simultaneous Flow of Compressible Atomizing Gas Jets and a Weakly Compressible Liquid Intermetallic Stream During Gas Atomization.
Mingming Tong 1 , David Browne 1
1 Engineering & Materials Science Centre, University College Dublin, Dublin Ireland
Show AbstractProduction of Raney nickel powders for catalytic applications can be achieved by atomization of molten NiAl intermetallic alloys into small droplets which solidify in flight to form spherical powders, which are subsequently activated by chemical leaching. Development of an understanding of the two-fluid interaction is necessary to gain insight into the phenomena controlling the fragmentation of the liquid stream by the impinging gas jets. Based a new model of compressible flow and a front-tracking formulation to follow the evolving liquid/gas interface, the authors have developed a unified atomization model enabling direct numerical process simulation. This unified model can deal with the simultaneous flow of compressible atomizing gas jets and the weakly compressible liquid metal stream during gas atomization. It has been used to simulate the atomization of a Ni-50wt.%Al melt stream by argon gas jets in a closed-coupled atomizer in 2D. The requisite thermophysical properties have been measured by project partners of the authors. The simulation results show that the presence of the liquid intermetallic stream has a significant effect on the gas flow, particularly the aspiration pressure. Although the aspiration pressure remains sub-ambient, the presence of the liquid stream increases it when compared with the value computed for the gas-only case study. At the plenum pressure used, which is typical of industrial applications, the gas nozzles are choked and hence the status of gas flow upstream of the tip of the liquid delivery tube is not very influenced by the presence of the liquid intermetallic stream, while the downstream gas flow is significantly affected. The basis of the new model is presented, along with simulation results using geometric and operational data supplied by industry partners. Besides the atomization of a liquid intermetallic stream by argon gas, this unified atomization model is available for use of simulating a variety of different twin-fluid atomization processes.
9:00 PM - U5.53
Role of Microstructure on Deformation Behavior in Mg12ZnY with Long-Period Stacking Ordered Structure.
Koji Hagihara 1 , Naoyuki Yokotani 1 , Akihito Kinoshita 1 , Yuya Sugino 1 , Michiaki Yamasaki 2 , Yoshihito Kawamura 2 , Yukichi Umakoshi 1
1 Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita, Osaka Japan, 2 Department of Materials Science and Engineering, Kumamoto University, Kurokami, Kumamoto Japan
Show AbstractDevelopment of light-weight magnesium alloys with high strength has recently received much attention for application in the wider field such as in the automobile industry. One of the candidates of them, the alloys containing the Mg12ZnY , called X-phase, with long-period stacking ordered (LPSO) structure of close-packed basal plane is focused. Some models and ideas to explain the strengthening mechanism with the LPSO phase have been reported on the basis of observation results of deformation substructure, but the detail has not been enough clarified yet. This is because the plastic behavior and the other mechanical properties of X-phase, such as the operative deformation mode and the critical resolved shear stress (CRSS) and so on, have not been elucidated. In this study, we investigated the plastic deformation behavior of Mg12ZnY with 18R LPSO structure by using the directionally solidified (DS) single-phase crystals grown by Bridgeman method. In addition, the influence of change in microstructure on plastic behavior was investigated by using the extruded alloys. In the gained DS crystals, the grains of X-phase show plate-like shapes with the wide surface parallel to (0001), and they exhibit preferential growth as the (0001) plane in them is aligned approximately parallel to the growth direction. By compression tests, (0001)<11-20> basal slip was identified to be the dominantly operative deformation mode in X-phase, whose CRSS is roughly estimated to be somewhat higher than that of pure Mg. When the stress is loaded perpendicular to the (0001) plane, where Schmid factor of basal slip is negligible, the deformation kinks, which are formed by a progressive rotation of the lattice due to the avalanche generation of pairs of dislocations on the basal plane, are initiated in specimens and accommodate the stress concentration to some extent. The results indicate that the LPSO X-phase is plastically deformable, but it exhibits strong plastic anisotropy. Indeed, the deformation behavior of X-phase is strongly affected by microstructure. For example, the yield stress in the extruded crystals shows much higher value compared to that of DS crystals. These results suggest that a suitable processing for the accomplishment of the homogeneous distribution and refinement of LPSO phase in alloys must be required in order to bring out the superior mechanical properties of LPSO phases in high-strength Mg-based alloys.
9:00 PM - U5.54
Crystallography of B19 Martensite in AuTi Shape Memory Alloy.
Tomonari Inamura 1 , Kenji Wakashima 1 , Hideki Hosoda 1
1 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama Japan
Show AbstractTwin structure, habit plane and morphology of B19 (orthorhombic) martensite in AuTi were examined by transmission electron microscopy (TEM) observation and phenomenological theory of martensite crystallography (PTMC). The equi-atomic AuTi alloy has the B2-B19 thermoelastic martensitic transformation at about 900K. The lattice parameters of the B19 phase were 0.294nm, 0.490nm and 0.463nm at RT. The lattice parameter of B2 phase was extrapolated to be 0.322/δ nm at RT where δ is the volume change during the B2-B19 martensitic transformation. PTMC analysis showed that {111} type I twin or <211>type II twin are the possible twinning as the lattice invariant deformation. TEM observation revealed that no long period structure exists unlike PtTi. Almost all the martensite plates had internal twin of {111} typeI. Adjacent plates of martensite were bounded by common habit planes. Such configuration of martensite plates gave rise to a herring bone like morphology. <211>type II twin was hardly observed as the internal twin whereas it was held between the adjacent martensite plates with the common habit plane. By a single trace analysis among 10 plates of martensite, habit plane orientation was determined to be (34 -50 19) of B19 lattice which corresponds to (-34 34 15) of B2 lattice. This habit plane orientation is in good agreement with the PTMC with δ=0.997 and matches to the twinning plane of the <211>type II twin. It was found that the habit plane corresponding to the twinning plane of the <211>type II twin enables the formation of the typical configuration of martensite plates in equi-atomic AuTi.
9:00 PM - U5.55
Mechanical Properties of Mg-TM-Y (TM=Ni or Cu) Alloy with Long Period Ordered Phase.
Itoi Takaomi 1
1 , Department of Mechanics Engineering, Faculty of Engineering, Chiba university, Chiba Japan
Show AbstractMagnesium alloys have high potential as structural components in aerospace and automotive applications, because of their low density and high specific strength. In 2001, it has been developed that high strength RS P/M (Rapidly Solidified Powder/Metallurgy) Mg97Zn1Y2 (at%) alloy with yield strength above 600MPa and elongation of 5% at room temperature. The high strength of the RS P/M Mg alloy due to the not only a grain refinement about 200 nm but also a long period ordered (LPO) structure formed in each Mg grains. The LPO structure with 18R-type observed in the RS P/M Mg alloy is confirmed as equilibrium phasein ternary Mg-Zn-Y diagram, and several studies have been carried out to develop high strength Mg alloy. Thereafter, Mg96Zn2Y2 (at%) alloy extrusion with σy of 390MPa at room temperature have been developed. Recently, the LPO structure was also recognized in Mg-Ni-Y and Mg-Cu-Y systems. However, little is known about microstructure and mechanical properties of the Mg-TM-Y alloys.We have developed the high strength Mg-TM-Y alloys with the LPO structure. Ternary Mg90.5Cu3.25Y6.25 (at%) cast alloy consist of the LPO - and α-Mg phases, and exhibited the yield stress and elongation of 275MPa and 5%, respectively at room temperature. The Mg-TM-Y alloys sheets were prepared by hot-rolling at 623K, and subsequently annealed at 673K for 6h. The Mg90.5Cu3.25Y6.25 (at%) alloy sheet exhibited yield stress, ultimate tensile strength and elongation of 448MPa, 512MPa and 6%, respectively, at room temperature, and 342MPa, 375MPa and 25%, respectively, at 523K. Specific yield strength of the alloy sheet at R.T defined by the ratio of σy toρis 214 MPa/Mg m-3, which is much higher than that of super extra duralumin (about 180 MPa/Mg m-3). It was concluded that the Mg-TM-Y alloy sheet combining high strength and reasonable elongation was achieved by hot-rolling and subsequent appropriate thermal annealing, using the LPO phase as a strength factor.
9:00 PM - U5.56
The Simulation of the Diagram of Order-Disorder Transitions in Cu-Au Alloys.
Mikhail Starostenkov 1 , Vladimir Myasnichenko 1 , Vladimir Dudnik 1
1 General Physics, Altay State Technical University, Barnaul Russian Federation
Show Abstract9:00 PM - U5.57
Pseudo Fine Particle Effect in Unstable Phase Transition of Gd2Al.
S. Zarini 1 , Ahmad Yazdani 1 , M. Zarei 1 , M. Hesani 1 , M. Ghazanfari 1
1 Physics, Tarbiat Modares University, Tehran Iran (the Islamic Republic of)
Show Abstract9:00 PM - U5.58
Fabrication and Interdiffusion of TiAu-(Fe, Co and Ni) Composites for High-Temperature Magnetostrictive Actuators.
Yuta Okimori 1 , Tomonari Inamura 1 , Kenji Wakashima 1 , Hideki Hosoda 1 , Shuichi Miyazaki 2
1 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama Japan, 2 Institute of Material Science, University of Tsukuba, Tsukuba Japan
Show Abstract We have proposed a new high-temperature (HT) magnetostrictive shape-memory-alloy (SMA) based actuator material. This actuator material is composed of SMA with a high martensitic transformation temperature (Ms) and ferromagnetic materials with high Curie temperature (TC) . This actuator is a laminated composite with a stacking structure as HTSMA/ferromagnet, and actuated by applying a graded magnetic field for the ferromagnetic force resulting to a large motion of SMA layer. In previous study, we fabricated TiAu/Co composites by a diffusion bonding method through hot pressing, and observed the formation of intermetallics near the interface between TiAu and Co. In this work, TiAu HTSMA (Ms=880K) and Fe, Co and Ni ferromagnetic metals were selected as the constituent materials. Then, the purposes of this work are (1) to fabricate the TiAu/ferromagnet composite materials by the similar diffusion-bonding method, (2) to characterize the microstructures near the bonding interfaces between TiAu and the ferromagnetic metals, and (3) to clarify the diffusion behavior, diffusion paths and the formation of intermetallics. A stoichiometric TiAu was made by an Ar arc melting method followed by hot pressing at 1423K for 24hrs. The TiAu plates and ferromagnetic metal plates were stacked as TiAu/Fe, TiAu/Co and TiAu/Ni, and bonded at various temperatures for various durations in a vacuum environment. The cross-section bonding interfaces were observed by a field-emission type scanning electron microscopy (FE-SEM) and the concentration profiles were measured by an energy-dispersive spectroscopy (EDS) analysis equipped with the FE-SEM. In order to evaluate the growth of diffusion zone, the TiAu/ferromagnet composites were aged under several thermal conditions. After the aging, similar observation was done by FE-SEM and EDS. As for the TiAu/Co composites hot-pressed at 1073, 1173 and 1273K for 10hrs, it was found that two reactant intermetallic phases were formed near the interface between TiAu and Co layers. Based on the EDS results and the Ti-Au-Co ternary phase equilibrium, the intermetallic phases formed were C11b Ti(Au, Co)2 and C36 Co2(Ti, Au). The diffusion path near the TiAu/Co interface is TiAu / TiAu(Co) / (Au, Co)2Ti / Co2(Ti, Au) / Co(Ti, Au) / Co. The apparent activation energy for diffusion was estimated to be 300kJ/mol. The other results for the TiAu/Fe and TiAu/Ni composites will be presented.
9:00 PM - U5.59
Phase Constitution of the Au-Cu-Ga-Mn Quaternary System.
Satoshi Tsutsumi 1 , Tomonari Inamura 1 , Kenji Wakashima 1 , Hideki Hosoda 1
1 Precision and Intelligence Laboratory , Tokyo Institute of Technology, Yokohama Japan
Show AbstractIn order to develop new gold base shape memory alloys for biomedical applications, we have focused on two intermetallic compounds, AuMn and Cu2MnGa, both of which have martensitic transformation. Kushima and co-workers have reported that Cu2MnGa has the shape memory effect and an order-disorder transformation from L21 to B2 (J. Jpn Inst. Metals, 70 (2006) 849). AuMn also has B2 high temperature phase. Based on the chemical similarity between Au and Cu, these two intermetallic compounds are expected to form a continuous solid solution of B2 at high temperature, or, to form a pseudobinary system, and a new superior shape memory alloy composed of AuMn and Cu2MnGa may be developed. However, little information exists about the phase diagram of the Au-Cu-Ga-Mn quaternary system, especially at the potion of AuMn-Cu2MnGa. Therefore, for the first step, the phase constitution between near-stoichiometric AuMn and Cu2MnGa was investigated in this study. Alloys with nominal chemical compositions of (AuMn)x-(Cu2MnGa)1-x were systematically prepared by a high frequency induction melting method in a vacuum environment. A homogenization treatment was done at 873K for 3hrs followed by quenching into water. Then, microstructural characterization and chemical analysis were done by a field-emission gun-type scanning electron microscope combined with an energy dispersive X-ray spectrometer. Besides, θ-2θ X-ray diffraction analysis was done for phase identification. In this study, the following phases were identified. Laves phase of MgZn2 structure, γ phase of Cu9Al4 structure, β phase of CsCl structure, β” phase of CuTi structure and Au2Mn phase of MoSi2 structure. Spenser and co-workers have reported that β phase decomposes into both Laves phase and γ phase near the composition of Cu2MnGa (J. Inst. Met., 84 (1955-1956) 46). And also, β” phase is phase-transformed from β phase, that is, from a cubic to a tetragonal. In all the alloys, β phase or β-related phases, which were transformed or decomposed from β phase, were confirmed regardless of the alloy composition. These results strongly suggest that all the alloys are mainly composed of β phase at the homogenized temperature. Thus, we thought that a continuous solid solution of β phase exists in the pseudobinary system between AuMn and Cu2MnGa.
9:00 PM - U5.6
Thermoelectric Properties of p-type Half-Heusler Compounds: Er(Ni,Pd)Sb.
Kenta Kawano 1 , Ken Kurosaki 1 , Hiroaki Muta 1 , Shinsuke Yamanaka 1
1 Graduate School of Engineering, Osaka University, Suita Japan
Show AbstractHalf-Heusler type intermetallic compounds with 18 valence electrons per unit cell exhibit large Seebeck coefficient because of the existence of the narrow gap and sharp slope of the density of states around the Fermi level. Almost all half-Heusler compounds studied in the thermoelectric research field are n-type materials. While the high thermoelectric figure of merit (ZT) has been achieved in the n-type compounds, the p-type ones should also be investigated. Here we show the promising p-type thermoelectric intermetallics: Er(Ni,Pd)Sb. We prepared polycrystalline samples of Er(Ni,Pd)Sb related intermetallics and performed thermoelectric characterizations above room temperature.
9:00 PM - U5.60
High Temperature Relaxation Processes in Structural Intermetallics.
Jose San Juan 1 , Pablo Simas 1 , Maria No 2
1 Fisica Materia Condensada, Universidad del Pais Vasco, Bilbao Spain, 2 Fisica Aplicada II, Universidad del Pais Vasco, Bilbao Spain
Show Abstract9:00 PM - U5.7
Thermal Conductivity Analysis of the Ag-Ga-Te and Ag-In-Te Ternary Compounds.
Anek Charoenphakdee 1 2 , Ken Kurosaki 1 , Hiroaki Muta 1 , Masayoshi Uno 1 , Shinsuke Yamanaka 1
1 Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, Osaka Japan, 2 Faculty of Science and Technology, Rajamangala University of Technology Suvarnabhumi, Ayutthaya Thailand
Show AbstractThe effectiveness of a material for thermoelectric applications is determined by the dimensionless figure of merit, ZT= (S2σ)T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. Therefore, in order to obtain high performance thermoelectric materials, the large S and the high σ as well as the low κ are required. We focus attention on Ag-Ga-Te and Ag-In-Te ternary compounds as new class of thermoelectric materials, because these compounds would be considered to exhibit low κ. Here we show the thermal conductivity characterization results on polycrystalline samples of AgGaTe2, AgInTe2, AgGa5Te8, and AgIn5Te8. The κ was evaluated from room temperature to around 870 K. The room temperature κ values are 1.88, 2.05, 0.49, and 1.08 Wm-1K-1 for AgGaTe2, AgInTe2, AgGa5Te8, and AgIn5Te8, respectively. We will discuss the magnitude relations of the κ based on the crystal structure and the Debye temperature.
9:00 PM - U5.8
Phase Equilibria and Diffusion Path for the Formation of Half-Heusler Type Thermoelectric Compound TiNiSn.
Chihiro Asami 1 , Yoshisato Kimura 1 , Takuji Kita 2 , Yoshinao Mishima 1
1 , Tokyo Institute of Technology, Yokohama Japan, 2 , Toyota Motor Corporation, Susono Japan
Show Abstract9:00 PM - U5.9
On the Effect of Strain Rate and Temperature on the Yield Strength Anomaly in L21-structured Fe2AlMn.
Markus Wittmann 1 , Janelle Chang 1 , Yifeng Liao 1 , Ian Baker 1
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States
Show AbstractAs-cast L21-structured Fe2AlMn contains ∼1.5 μm spaced a/4<111> anti-phase boundaries (APBs) enclosing ∼300 nm spaced a/2<100> APBs and deforms at room temperature by the glide of two APB-coupled pairs of a/4<111> dislocations. This Heusler phase shows a peak in the yield stress near 800 K and disorders to the B2 structure at ∼895K. In an attempt to determine whether the vacancy-hardening model [E.P. George and I. Baker, Philosophical Magazine, 77 (1998) 737], or the order-strengthening model [N.S. Stoloff and R.G. Davies, Progress in Materials Science, 1 (1996) 3] could accurately describe this yield anomaly, the effects of strain rate and temperature on the yield strength of near-stoichiometric Fe2AlMn single crystals were investigated. It was found that at temperatures below 750 K the yield stress is largely strain-rate insensitive, while at higher temperatures it shows positive strain rate sensitivity. The magnitude of the yield strength peak was insensitive to strain rate. It appears that neither model can fully account for the observed yield stress behavior, suggesting that an alternative mechanism is operative. Research was supported by NIST grant 60NANB2D0120 and NSF grant DMR0314209.
Symposium Organizers
Bernard Bewlay General Electric Company
Yue-hui He Central South University
Martin Palm Max-Planck Institut fuer Esenfoschung GmbH
Masao Takeyama Tokyo Institute of Technology
Joerg Wiezorek University of Pittsburgh
U6: Nickel/Cobalt Superalloys, and Nickel Aluminides
Session Chairs
John Lewandowski
Seiji Miura
Wednesday AM, December 03, 2008
Constitution A (Sheraton)
9:30 AM - **U6.1
Intermetallic Bond Coats: Systems Compatibility and Platinum-Group Metal Additions.
Tresa Pollock 1 , Dan Widrevitz 1 , Jason Van Sluytman 1 , Brian Tryon 1
1 MSE Dept., University of Michigan, Ann Arbor, Michigan, United States
Show Abstract10:00 AM - U6.2
Isothermal Oxidation Behavior of Pt-Based Superalloys in the System Pt-Al-Cr-Ru.
Karin Pruessner 1 3 , Jamiu Odusote 1 3 , Rainer Suess 2 3
1 School of Physics, University of the Witwatersrand, Johannesburg, Gauteng, South Africa, 3 , DST/NRF Centre of Excellence in Strong Materials, Johannesburg, Gauteng, South Africa, 2 Advanced Materials Division, Mintek, Johannesburg, Gauteng, South Africa
Show Abstract10:15 AM - U6.3
Phase Stability of γ’ Phase in Ir-Al-W Ternary System.
Toshihiro Omori 1 , Komei Makino 1 , Katsunari Oikawa 1 , Ikuo Ohnuma 1 , Ryosuke Kainuma 2 , Kiyohito Ishida 1
1 Department of Materials Science, Tohoku University, Sendai Japan, 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai Japan
Show AbstractPlatinum-group-metal based alloys are attractive for next-generation high-temperature materials due to their high melting temperatures and excellent oxidation resistance. Ir alloys have L12 intermetallic compounds (γ’ phase) such as Ir3Ti and Ir3Nb, and therefore, the Ir-based alloys with the γ’ phase have been investigated for high-temperature materials. Recently, the present authors have discovered Co3(Al,W) L12 phase and it has been reported that the Co-Al-W-based alloys with the γ (A1) + γ’ microstructure exhibit an excellent high temperature strength. The Co3(Al,W) L12 phase, which is a ternary compound, is stabilized by the combination of Al and W. In Ir-Al binary system, the L12 phase does not exist, however, it is expected to be formed by the addition of W because Ir belongs to the same group as Co in the periodic table. In this paper, the phase stability of the L12 compound phase in Ir-Al-W system will be reported. Ir-Al-W ternary alloys were prepared by arc melting. The specimens were homogenized at 1900°C and subsequently annealed at 1300°C or 1400°C. Composition of the phase constituent was measured by electron probe microanalysis (EPMA) and the phase identification was carried out by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Room- and high-temperature hardness was evaluated using a Vickers hardness test. It was found from TEM observation that Ir-10Al-10W alloy annealed at 1300°C for 72 hours shows fine γ’ precipitates with the L12 structure in the γ matrix. It is considered that the stability of γ’ phase remarkably increases by the combination of Al and W, which is similar to Co-Al-W system, and a new ternary compound Ir3(Al,W) was stably obtained. The isothermal section diagram at 1300°C and 1400°C in the Ir-rich portion of Ir-Al-W system was determined. The γ’ phase exists even at 1400°C and the phase boundaries of γ, D019 and B2 phase was also determined. The hardness is over 700Hv at room temperature in Ir-Al-W alloys strengthened by the γ’ phase and a high hardness around 500Hv is maintained even at 1000°C. Therefore, the Ir-Al-W alloys are of interest as next-generation high-temperature materials.
10:30 AM - U6.4
Alloy Design and Microstructural Control for Developing Dual Two-Phase Intermetallic Alloys Composed of Geometrically Close Packed Ni3X(X:Al and V) Structures.
Takayuki Takasugi 1 , Yasuyuki Kaneno 1
1 Department of Materials Science, Osaka Prefecture University, Sakai, Osaka Japan
Show AbstractThe so-called dual two-phase intermetallic alloy, which is composed of Ni3Al(L12) and Ni solid solution (A1) phases at high temperature annealing and is additionally refined by a eutectoid reaction at low temperature aging, according to which the Al phase is decomposed into the Ni3Al(L12)+ Ni3V(D022) phases, was recently developed. The dual two-phase intermetallic alloys exhibit highly coherent interface structure between the constituent phases of Ni3Al(L12) and Ni3V(D022) not only at the micron scale but also at the sub-micron scale, and also display high microstructural and phase stability at high temperature. The dual two-phase intermetallic alloys showed high tensile strength, accompanied with high tensile elongation and fracture toughness over a broad temperature range, and also superior oxidation/corrosion resistance. The creep test showed the extremely low creep rate and long creep rupture time when compared with conventional Ni-based superalloys. The obtained results are thus promising for the development of the dual two-phase intermetallic alloy as a new-type of high-temperature structural material.In the present article, the attempts of the compositional selection and microstructural control to develop several applications (e.g. jet-engine and gas turbine blade, high-temperature ball bearing, friction stirring welding tool for high melting point materials, and bolt and nut used in high-temperature corrosive liquids) based on the dual two-phase intermetallic alloys are presented.
10:45 AM - U6.5
Phase Stability of E21 (Fe, Co, Ni)3AlC1-x Intermetallic Carbides.
Hideki Hosoda 1 , Tomonari Inamura 1 , Kenji Wakashima 1
1 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan
Show Abstract11:30 AM - **U6.6
Intermetallic Compounds in Co-base Alloys– Phase Stability and Application to Superalloys –.
Kiyohito Ishida 1
1 Department of Materials Science, Tohoku University, Sendai Japan
Show AbstractPhase stability of intermetallic compounds in Co-base alloys are reviewed compared with that in Ni-base alloys focusing on the GCP (geometrically closed-packed) phase. Although only Co3Ti is known to be a stable γ’(L12) compound, there are some metastable γ’ phases in the Co-X binary system such as Co3Al, Co3W, Co3Ta, etc. Therefore, the phase stability of these γ’ phases is also interesting. Accordingly, a stable ternary γ’ phase has been found in Co3(Al, W), Co3(W, Ge) and Co3(W, Ga). In particular, the γ’ phase of Co3(Al, W) is playing a key role in the development of new-type of Co-base superalloys since this compound shows the positive dependence on temperature and high temperature strength. The effect of alloying elements on the phase stability of the γ’ phase of Co3(Al, W) is shown in comparison with that in Ni-base superalloys.
12:00 PM - U6.7
Physical and Mechanical Properties of Single Crystals of Co-Al-W Based Alloys with L12 Single-Phase and L12/fcc Two-Phase Microstructures.
Haruyuki Inui 1 , Katsushi Tanaka 1 , Kyosuke Kishida 1 , Norihiko Okamoto 1 , Takashi Ohashi 1
1 Department of Materials Science and Engineering, Kyoto University, Kyoto Japan
Show AbstractThe recent discovery of the stable L12-ordered intermetallic compound, Co3(Al,W) coexisting with the solid-solution based on Co with a fcc structure has opened up a pathway to the development of a new class of high-temperature structural material based on cobalt, ‘Co-base superalloys’. However, almost nothing is known about mechanical properties of these Co-based two-phase alloys, especially of the constituent L12 phase, Co3(Al,W). We have investigated some physical and mechanical properties (elastic constants, thermal conduction, thermal conductivity and plastic deformation behavior) of single crystals of Co-Al-W based alloys with with L12 single-phase and L12/fcc two-phase microstructures. The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined are considerably smaller than those previously calculated by others. When judged from the values of Poisson ratio, Cauchy pressure and Gh/Bh, the ductility of Co3(Al,W) is expected to be sufficiently high so that Co3(Al,W) can be practically used as the constituent phase of ‘Co-base superalloys’. Two-phase microstructures with cuboidal L12 precipitates being well aligned parallel to <100> and well faceted parallel to {100} are expected to be very easily formed in Co-base superalloys because of the large value of E111/E100 and cij of Co3(Al,W). The values of thermal conductivity for Co3(Al,W) (10-15 Wm-1K-1) are as low as about half those for Ni3Al with a stoichiometric composition. This can be understood easily when referring to the large composition dependence of thermal conductivity for many L12 compounds. Indeed, The values of thermal conductivity for many Ni-base superalloys are comparable to those for Co3(Al,W). The value of CTE for Co3(Al,W) (10-15 x 10-6 K-1) is also comparable to those for Ni-base superalloys in practical use. The results of compression tests at room temperature to 1000°C and creep tests at 900°C will also be presented.
12:15 PM - U6.8
Phase Stability of L12 Compound and Microstructural Change in Co-(W or Mo)-Ta Ternary Alloys.
Hibiki Chinen 1 , Toshihiro Omori 1 , Katsunari Oikawa 1 , Ikuo Ohnuma 1 , Ryosuke Kainuma 2 , Kiyohito Ishida 1
1 Department of materials science, Tohoku university, Sendai Japan, 2 Institute of multidisciplinary research for advanced materials, Tohoku university, Sendai Japan
Show Abstract Co-base alloys strengthened by solid solution hardening and carbide precipitation have been used as high-temperature materials. However, Ni-base superalloys strengthened by γ'-Ni3Al precipitates with the L12 crystal structure are superior in high-temperature strength. Although there have been some reports on the precipitation of geometrically close-packed (GCP) Co3X phases, including the γ' phase in Co-base systems such as Co3Ti and Co3Ta, the γ' phases are unstable in Co-base systems and, thus, effective precipitation hardening due to these phases has not been applied in Co-base commercial alloys. Recently, new ternary L12 compound phases, Co3(W,Al) [1] and Co3(W,Ge) [2], have been found in Co-W-Al and Co-W-Ge ternary systems, respectively, suggesting the possibility of the existence of new L12-type ternary compounds in other Co-W-base systems, and also in Co-Mo-base alloy because Mo belongs to the same group of W. In the present study, the phase stability and the microstructure of Co-Mo-Ta and Co-W-Ta ternary alloys were investigated. It was shown by TEM observation that fine cuboidal precipitates 10-20 nm in size and with the L12 structure (γ’ phase) formed in the γ (A1) phase of Co-5W-2.5Ta alloy annealed at 800°C for 2 hours. It was also observed that the γ’ phase precipitates in the Co-4Mo-4.5Ta alloy annealed at 800°C for 2 hours. In these samples, discontinuous precipitation of the D019 phase occurred at 800°C at the same time and the D019 compound grew by further annealing. The γ/γ’ microstructure was completely replaced by the γ + D019 two-phase structure by aging at 800°C for a longer time in the Co-5W-2.5Ta and Co-4Mo-4.5Ta alloys. Therefore, the γ’ phase observed in these alloy systems at 800°C is a metastable phase. [1] J. Sato, T. Omori, K. Oikawa, I. Ohnuma, R. Kainuma, K. Ishida, Science 312 (2006) 90.[2] H. Chinen, J. Sato, T. Omori, K. Oikawa, I. Ohnuma, R. Kainuma, K. Ishida, Scripta Mater. 312 (2006) 90.
12:30 PM - U6.9
Catalytic properties of Atomized Ni3Al Powder for Methane Steam Reforming.
Yan Ma 2 1 , Ya Xu 1 , Nasahiko Demura 1 , Toshiyuki Hirano 1 2
2 , University of Tsukuba, Tsukuba, Ibaraki, Japan, 1 , National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractLow-cost and efficient hydrogen production is one of the key issues in the fuel cell technology. New high-performance catalysts are highly needed for solving the issue. We will present Ni3Al as a promising catalyst. The catalytic properties of Ni3Al have never been investigated, though it is known as an excellent high-temperature structural material. We for the first time found a high catalytic activity for methanol decomposition in Ni3Al: both powder and foil forms of Ni3 Al effectively decompose methanol into hydrogen and carbon monoxide [1,2].This finding suggests a possibility of catalytic activity for another hydrogen production reaction, i.e., methane steam reforming which is now widely used in large scale chemical plants. In this study we examined the catalytic properties of Ni3Al for this reaction using its atomized powder with stoichiometric composition (Ni-25 at% Al).Since the atomized powder used was rapidly solidified, it included a lamellar second phase (L1o-type β’NiAl) in the Ni3Al matrix. Taking advantage of this duplex structure, it was found that the powder surface became highly active by the following combined pretreatment of acid and alkali leaching. The first acid leaching preferentially dissolved the beta-prime NiAl phase, leaving a lamellar porous structure on the surface. The second alkali leaching selectively dissolved aluminum from the porous surface, resulting in the Ni-enriched surface. The BET surface area measurement revealed that the first process contributed to the increase in the surface area of the powder. The TEM observation revealed that the second process contributed to the formation of fine Ni particles on the porous surface. The pretreated powder showed high catalytic activity for methane steam reforming associated with water-gas shift reaction. The reason was attributed to the fine Ni particles formed on the porous surface.[1] Y. Xu et al., Intermetallics, 13(2005),151.[2] D.H. Chun et al., Catal. Lett., 106(2006), 71: J. Catal., 243(2006),99.
12:45 PM - U6.10
High-Temperature Behavior of B2-RuAl - A New Material with Exceptional Properties for Structural Applications.
Karsten Woll 1 , Frank Muecklich 1
1 Materials Science and Engineering, Functional Materials, Saarland University, Saarbrücken Germany
Show AbstractU7: Niobium and Molybdenum Silicide-Based Alloys
Session Chairs
Martin Heilmaier
Sharvan Kumar
Wednesday PM, December 03, 2008
Constitution A (Sheraton)
2:30 PM - **U7.1
Fracture and Fatigue of Nb-Si Alloys.
John Lewandowski 1
1 , Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractThe fracture and fatigue behavior of a variety of Nb-Si alloys will be reviewed. Fracture experiments have been conducted on both notched and fatigue-precracked samples, while the fatigue crack growth behavior has also been characterized over a range of test temperatures. Microstructures and fracture paths have been characterized via conventional metallography, scanning electron microscopy, and laser confocal microscopy. The mechanical behavior of these advanced Nb alloys will be compared to data available in the literature for a range of high temperature materials.
3:00 PM - U7.2
Modeling of Microstructure Evolution in Nb-Si Eutectic Alloy Using Cellular Automaton Method.
Kenichi Ohsasa 1 , Seiji Miura 1
1 Materials Science, Hokkaido University, Sapporo, Hokkaido, Japan
Show AbstractA numerical model was developed for the simulation of microstructure evolution during the solidification of Nb-18at%Si eutectic alloy. In this model, the cellular automaton method was used to simulate the eutectic growth of Nb solid solution and Nb3Si intermetallics. Diffusion in liquid and solid, mass conservation at the solid/liquid interface and local equilibrium at the solid/liquid interface with consideration of curvature undercooling were solved to determine the positions of the Nb/liquid and Nb3Si/liquid interfaces. The growth morphology of regular and irregular eutectics was studied using the developed model. In the simulation, coupling growth was observed and the growth rate for the coupling growth increased with increase in the degree of undercooling of melt. Lamellar spacing of coupled eutectics decreased with increase in the degree of undercooling. In the case of higher degree of undercooling, irregular, non-coupling eutectic morphology was formed. Developed model demonstrated the availability of numerical simulation for analyzing the solidification mechanism of eutectic alloys.
3:15 PM - U7.3
Effect of Microstructure of Nb-Si Alloy on Compressive Deformation Behavior at Various Temperatures.
Seiji Miura 1 , Yuki Murasato 1 3 , Yukiyoshi Tsutsumi 1 , Kenji Ohkubo 1 , Tetsuo Mohri 1 , Yoshisato Kimura 2 , Yoshinao Mishima 2
1 Graduate School of Engineering, Hokkaido University, Sapporo Japan, 3 , Hitachi.Ltd, Chigasaki Japan, 2 Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama Japan
Show AbstractA Nb-Si eutectic alloy containing 1.5%Zr and 100 ppm Mg shows an accelerated eutectoid decomposition of one of the constituent phases Nb3Si to Nb and Nb5Si3. It enables ones to obtain large Nb grains with finely dispersed Nb5Si3 microstructure only through a heat-treatment. Although compression test specimen subjected to a heat treatment at 1650 oC show a limited ductility of about 1.5% at room temperature, it is found that a pre-straining of about 20% plastic strain at 1200 oC modifies the room temperature ductility up to 2.5 %. This modification is attributed to the breaking of Nb5Si3 into finer dispersoids during high temperature pre-straining. The room temperature toughness of Nb5Si3 is also estimated to be about 2 MPa m^1/2 by applying Niihara’s equation for the cracking near micro-Vickers indents.
3:30 PM - U7.4
Deformation Behavior of Nb-Si-Mo Alloys at Room and Elevated Temperatures.
Kausik Chattopadhyay 1 , Rahul Mitra 2 , Kalyan Ray 2
1 Metallurgical Engineering, Institute of Technology, Varanasi, Uttar Pradesh, India, 2 Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, West Bengal, India
Show Abstract3:45 PM - U7.5
Deformation Behavior of Niobium Silicides during High Temperature Compression.
Nobuaki Sekido 1 , Seiji Miura 2 , Yoko Yamabe-Mitarai 1 , Yoshisato Kimura 3 , Yoshinao Mishima 3
1 , National Institute of Materials Science, Tsukuba Japan, 2 , Hokkaido University, Sapporo Japan, 3 , Tokyo Institute of Technology, Yokohama Japan
Show AbstractIn spite of the remarkable progress it is generally recognized that any further improvement in operating temperature for Ni base superalloys would be marginal, and development of a new class of heat resistant materials is required to meet demands for increased performance in gas turbine engines. For metallic systems, alloys based on Mo and Nb silicides are considered as promising candidates since they have high melting points, low densities, and ability to form protective oxide layers upon high temperature exposure. Previous studies suggest that Mo silicides seem to have an advantage in oxidation resistance; however, the mechanical properties of Nb-silicide based alloys are very attractive. The present authors have been pursuing effective alloy design for Nb-Si alloys to show balanced properties of high temperature strength and room temperature toughness. These alloys consist of Nb solid solution and Nb silicides, Nb3Si and/or Nb5Si3. Obviously the toughening mechanism in the alloys is extrinsic; microcracks formed in the brittle silicides are trapped by the ductile Nb phase, so that more work of fracture is required before catastrophic failure. Within such mechanism, microstructure is the key parameter to ensure effective inhibition of microcrack propagation. From this view point, we performed unidirectional solidification on Nb-Nb3Si eutectic alloys, by which continuous Nb rods are homogeneously dispersed within the Nb3Si matrix. In addition, deformation behavior of the alloys was examined by high temperature compression tests, followed by TEM examination to characterize the defect structure developed during deformation. Although the growth of the Nb phase is discontinuous in as-arc-melt condition, continuous Nb rods are found to form in slow solidification rates. The Nb phase in the unidirectionally solidified alloys can effectively trap and bridge microcracks, which yields higher room temperature toughness. At the same time, a texture microstructure has developed during unidirectional solidification, which results in excellent compressive strength as high as 600 MPa at 1673 K. TEM observations have revealed that recrystallized grains are present in the Nb phase. This indicates that dynamic recrystallization may have occurred during high temperature compression test, which would be partly responsible for the pronounced yield drop phenomenon observed in high temperature compression tests. It is also found that many dislocations are formed in the silicide phases. In the Nb3Si phase, planar faults, as well as dislocations, are present on {100) and {110) planes. On the other hand, planar faults are not typically observed in the Nb5Si3 phase. The Burgers vectors of dislocations formed in Nb5Si3 are identified as <100] and 1/2<111].
4:00 PM - U7.6
Study of the Effect of Al, Cr, Mo and Ta Additionson the Microstructure and Properties of Nb Silicide Based Alloys.
Panos Tsakiropoulos 1 , Kostas Zelenitsas 2 , Jie Geng 2 , T. Thandorn 1
1 Department of Engineering Materials, The University of Sheffield, Sheffield United Kingdom, 2 School of Engineering, University of Surrey, Guildford United Kingdom
Show Abstract4:30 PM - **U7.7
Current Status of Mo-Si-B Silicide Alloys for Ultra-high Temperature Applications.
Martin Heilmaier 1 , Holger Saage 1 , Manja Krueger 1 , Pascal Jehanno 2 , Mike Boening 2 , Heinrich Kestler 2
1 Institute for Materials and Joining Technology, University Magdeburg, Magdeburg Germany, 2 Technology Centre, Plansee SE, Reutte Austria
Show Abstract5:00 PM - U7.8
Effect of Particle Size of Mo solid solution on Fracture Toughness of Eutectic Mo5SiB2/Mo In-situ Composites.
Kyosuke Yoshimi 1 , Yusuke Kondo 1 , Kouichi Maruyama 1
1 , Tohoku University, Sendai Japan
Show AbstractMo5SiB2 has attracted great attention as an ultra-high temperature structural material because of its melting temperature over 2100°C, extremely high strength in the ultra-high temperature range, relatively low density close to Ni and so on. Mo5SiB2 coexists with Mo solid solution in the eutectic microstructure, so that its fracture toughness is improved by the incorporation of Mo solid solution. The eutectic Mo5SiB2/Mo in-situ composites also show excellent ultra-high temperature strength, and thus they are one of the most promising material systems for ultra-high temperature structural use beyond Ni-base superalloys. However, the concrete idea to enhance the fracture toughness of the Mo5SiB2/Mo in-situ composites has not been proposed yet. In this work, the fracture toughness of the eutectic Mo5SiB2/Mo in-situ composites is studied by the indentation method, and the importance of the particle size of Mo solid solution in the eutectic microstructures is highlighted to enhance its fracture toughness. Three kinds of Mo-Si-B ternary alloys and a Mo-Si-B containing 1at%Al alloy were produced around the Mo5SiB2/Mo eutectic composition by Ar arc-melting and the heat treatment at 1800°C for 24h. In the eutectic microstructures, the volume fraction of Mo solid solution ranges from approx. 26 to 30% and its average particle size do from approx. 3.0 to 6.4. The values of Vickers hardness do not show the dependence of the volume fraction, but clearly depend on the average particle size; i.e., it monotonously decreases with increasing the average particle size. The values of fracture toughness measured by the indentation method also do not show the dependence of the volume fraction, but it rapidly increases beyond the average particle size of 6μm, which attains in the Al-added alloy. On the other hand, the microhardness of the constituent phases was measured by the nanoindentation method, and figured out that the 1at%Al addition does not work on the solid solution hardening in both the Mo5SiB2 and Mo solid solution phases. Therefore, it is concluded that the increase in fracture toughness of the eutectic Mo5SiB2/Mo in-situ composites examined results from that the average particle size of Mo solid solution surpasses the critical one for the effective ductile phase toughening.
5:15 PM - U7.9
Tensile Creep Behavior of Two- and Three-phase Mo-Si-B Alloys.
Padam Jain 1 , Sharvan Kumar 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractMultiphase Mo-Si-B alloys are possible candidates to meet the increasing demand for structural materials suitable for ultra-high temperature applications in the aerospace industries and in advanced energy systems. Creep and oxidation resistance are prerequisites for such applications. These Mo-Si-B alloys consist primarily of a Mo solid solution matrix, and brittle Mo3Si and Mo5SiB2 (T2) phases, the latter typically constituting about fifty percent of the microstructure. Constant load tensile creep tests, were conducted on two- and three-phase Mo-Si-B alloys at different loads between 1000°C and 1300°C, and the results will be presented and will be compared to previously generated compressive creep data. Similar tests were additionally performed on a single-phase Mo-Si-B solid solution alloy and commercial Mo in order to understand the underlying creep mechanism and the role of different phases in influencing the creep response of the multiphase alloys.
5:30 PM - U7.10
Stacking Faults and Dislocation Dissociation in MoSi2.
Miroslav Cak 1 3 , Mojmir Sob 3 2 , Vaclav Paidar 4 , Vaclav Vitek 1
1 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Department of Chemistry, Faculty of Science, Masaryk University, CZ-611 37 Brno Czechia, 2 Institute of Physics of Materials, Academy of Sciences of the Czech Republic, CZ-616 62 Brno Czechia, 4 Institute of Physics, Academy of Sciences of the Czech Republic, 18221 Praha 8 Czechia
Show AbstractThe intermetallic compound MoSi2 has been of technological interest as a possible very high-temperature structural material. It crystallizes in the body-centered-tetragonal C11b structure and while it is brittle when loaded in tension, it deforms plastically in compression even at and below the room temperature. The slip systems operating at this temperature are {013)<331], {110)<111], {101)<010]. The {013)<331] slip system is unexpected since the 1/2<331] Burgers vector is not the shortest in between the lattice vectors in the {013) plane. Hence, operation of this slip system is one of the unexplained conundrums of the deformation behavior of MoSi2. Moreover, for the <001] orientation of the compressive axis the {013)<331] system is the only one that can produce plastic flow. However, the critical resolved shear stress for this system is strongly orientation dependent and it is the highest for compression along the <001] axis. For this orientation of the compressive axis no plasticity occurs at room temperature. Consequently, the room temperature ductility of MoSi2 is controlled by the mobility of 1/2<331] dislocations on {013) planes and its orientation and temperature dependence is another daunting question. Such deformation behavior suggests that the dislocation core is controlling the {013)<331] system. When analyzing dislocation cores the most important aspect is possible dissociation into partial dislocations the Burgers vectors of which are displacement vectors of metastable stacking fault-like defects. The existence of such faults is best ascertained via so-called γ-surfaces that are obtained by cutting the crystal along a chosen crystal plane, displacing the upper part with respect to the lower and evaluating the energy as a function of the displacement vector. In this paper we present the γ-surfaces for the (013) and (110) planes calculated using the density functional theory based method as implemented in the VASP code. While there is only one local minimum on the (110) γ-surface, three distinct local minima have been found on the (013) γ-surface. These minima, which determine three types of possible stacking faults, are not symmetry dictated and thus the fault vectors are principally controlled by the details of the interatomic bonding in MoSi2. Interestingly, the energies of these three stacking faults are very similar and thus they all can play role in the dislocation dissociation. This leads to a broad variety of possible dislocation splittings and the energetics of these dissociations have been studied in the framework of the anisotropic elastic theory of dislocations. The results of this analysis, which suggest the most likely dislocation splittings, are presented in this paper.This research was supported by the U.S. DOE-BES Grant no. DE-PG02-98ER45702 (MC, VV) and by the Ministry of Education and the Academy of Sciences of the Czech Republic, grants no. MSM0021622410 and AV0Z20410507, respectively (MC, MS).
5:45 PM - U7.11
The European ULTMAT Project: Properties of New Mo- and Nb-Silicide Based Materials.
Stefan Drawin 1
1 DMSM, ONERA, Châtillon France
Show AbstractIncreasing the temperature capability of turbine airfoil materials has been identified as a major requirement to develop efficient and clean aircraft engines. The development of new alloys offering increases of metal surface temperatures by at least 150°C over the presently used nickel-base single-crystal superalloys capability is of strategic importance.The ULTMAT (ULtra high Temperature MAterials for Turbines) European Framework Programme FP6 project, coordinated by ONERA, has started in January 2004 and has eneded in June 2008.Expected achievement of the project is a thorough evaluation of the capability of refractory metal (here: niobium and molybdenum) silicide based multiphase materials to withstand future enhanced temperature turbine service conditions (up to 1300°C), relying on microstructural, mechanical, physical and environmental investigations in close relation with industrial scale material processing and component fabrication technologies.The paper will present the main results currently obtained. Base materials for the alloy development are the metal/intermetallic ductile/brittle composite systems, more precisely the (generally) two-phase Mss-M5Si3 system (where Mss is a niobium solid solution and M is niobium substituted by various alloying elements: hafnium, titanium, refractory metal, etc) and the three-phase MoMo5SiB2 -Mo3Si system.Improvements in high temperature creep resistance (in the range 1000°C-1300°C) as well as medium and high temperature (700°C to 1300°C) oxidation resistance have been obtained. Processing routes have been developed (both ingot metallurgy and powder metallurgy) that have allowed the manufacture of complex shaped parts (turbine blades).
Symposium Organizers
Bernard Bewlay General Electric Company
Yue-hui He Central South University
Martin Palm Max-Planck Institut fuer Esenfoschung GmbH
Masao Takeyama Tokyo Institute of Technology
Joerg Wiezorek University of Pittsburgh
U8: Laves Phases - Structure and Properties
Session Chairs
Takayoshi Nakano
Martin Palm
Thursday AM, December 04, 2008
Constitution A (Sheraton)
9:30 AM - **U8.1
Chemical Bonding in Laves Phases Revisited.
Yuri Grin 1 , Arndt Simon 2 , Alim Ormeci 1
1 , Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden Germany, 2 , Max-Planck-Institut für Festkörperforschung, Stuttgart Germany
Show AbstractLaves phases comprise a large group of intermetallic compounds with general composition MN2 and multi component derivatives. Empirical geometric rules for stability as well as electronic rules for the occurrence of certain structural variations, e.g. cubic vs. hexagonal variants, have been developed. However, discovering of the new representatives of Laves phases revealed the limited applicability of these rules [1,2].In order to visualize features of bonding in Laves phases, several representatives were analyzed in terms of electron localizability. The chemical bonding analysis based on first-principles electronic structure methods was performed with electron localizability indicator (ELI) formalism [3]. In this study the full-potential non-orthogonal local orbital method (FPLO) with recently implemented ELI module [4] is used. The bonding situations for different classes of Laves phases range from exclusively multi-centre bonds between N atoms via an intermediate situation of two-centre bonds in the N partial structure to the formation of Nx- anions. No apparent connection is found between the bonding situation common to the members of a class and the location of these members in the near neighbour diagrams except for the salt-like phases.This project was realized within the inter-institutional initiative of the Max-Planck Society ‘Nature of Laves Phases’.[1] A. Simon. Angewandte Chem. Int. Ed. Engl. 22 (1983) 95.[2] F. Stein, M. Palm, G. Sauthoff. Intermetallics 12 (2004) 713.[3] M. Kohout, Int. J. Quantum Chem. 97 (2004) 651.[4] A. Ormeci, H. Rosner, F.R. Wagner, M. Kohout, Yu. Grin, J. Phys. Chem. A 110 (2006) 1100.
10:00 AM - U8.2
Phase-Transformation Kinetics in the TiCr2 Laves Phase.
Wolfgang Baumann 1 , Andreas Leineweber 1 , Frank Stein 2 , Shigehiro Ishikawa 2 3 , Masao Takeyama 4 , Eric Mittemeijer 1
1 , Max-Planck-Institut für Metallforschung, Stuttgart Germany, 2 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 3 , on leave from Tokyo Institute of Technology, Tokio Japan, 4 , Tokyo Institute of Technology, Tokio Japan
Show AbstractAbstract. The Ti-Cr system is known to comprise the three most prominent Laves-phase polytypes: cubic C15-TiCr2 (“ABC” stacking) as low-temperature phase, hexagonal C36-TiCr2 (“ABAC”) as intermediate-temperature phase and hexagonal C14-TiCr2 (“AB”) as high temperature phase, where the transition temperatures vary considerably as a function of composition. On the basis of a redetermined phase diagram of the Ti-Cr system, a systematic investigation of the C36 ↔ C14 phase (shear) transformation occurring at about 1550 K was performed. By X-ray powder diffraction analysis in combination with high resolution transmission electron microscopy it was shown that the phase (shear) transformation is driven by partial dislocation dipoles, leaving behind a characteristic fault structure in the C36 phase. A recently developed calibration and desmearing procedure for a differential thermal analysis (DTA) apparatus enables the investigation of the progress of the solid-solid phase transformation in the high temperature region by isochronal DTA measurements, both upon heating and cooling.An experimentally based phase transformation model was developed on basis of the modular approach, thereby the course of the transformation process was subdivided into three overlapping mechanisms (nucleation, growth and impingement) and modeled separately, to extract kinetic parameters. The developed model provides a quantitative description of the high temperature phase (shear) transformation in the TiCr2 Laves phase.Furthermore it was shown that plastic deformation induced by pressing considerably influences the phase-transformation kinetics. Obviously the dislocations formed upon deformation strongly influence the formation and mobility of the partial dislocation dipoles during the transformation. The influence of the formation and mobility of the dislocation dipoles on the phase transformation kinetics varies with the degree of plastic deformation, i.e. with the dislocation density, as indicated by X-ray powder diffractometry.This work has been performed within the Inter-Institutional Research Initiative “The Nature of Laves Phases” funded by the Max Planck Society.
10:15 AM - U8.3
Structure and Unusual Mechanical Behaviour of the Intermetallic Compound Nb2Co7.
Frank Stein 1 , Martin Palm 1 , Georg Frommeyer 1 , Lisa Siggelkow 1 2 3 , Daniel Gruener 2 4 , Guido Kreiner 2 , Marek Mihalkovic 5 , Andreas Leineweber 6
1 Materials Technology, Max-Planck-Institut fuer Eisenforschung GmbH, Duesseldorf Germany, 2 , Max-Planck-Institut fuer Chemische Physik fester Stoffe, Dresden Germany, 3 , now at Technische Universitaet Muenchen, Munich Germany, 4 , now at Stockholm University, Stockholm Sweden, 5 , Institute of Physics, Slovak Academy of Sciences, Bratislava Slovakia, 6 , Max-Planck-Institut fuer Metallforschung, Stuttgart Germany
Show Abstract10:30 AM - U8.4
Site Occupation and Defect Structure of Fe2Nb Laves Phase in Fe-Nb-M Ternay Systems at Elevated Temperatures.
Shigehiro Ishikawa 1 , Naoki Takata 1 2 , Takashi Matsuo 1 2 , Masao Takeyama 1 2
1 Dept. of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan, 2 , Consortium of the Japan Research and Development Center for Metals (JRCM), Tokyo Japan
Show AbstractComposition homogeneity range of Fe2Nb Laves phase with C14 structure in Fe-Nb-M (M : transition metals) systems and the site occupation of M in the phase have been examined. In any Fe-Nb-M ternary systems where M is Cr, Mn, Co or Ni, the C14 Laves phase region largely extends along equi-Nb concentration direction. In Fe-Nb-Mn system, a continuous solid solution is formed between Fe2Nb and Mn2Nb since Mn forms Mn2Nb with C14 structure. In case of Cr and Co, which form M2Nb Laves phase with cubic C15 structure, a fraction of 0.6 and 0.75 of Fe in Fe2Nb can be replaced with Cr and Co, respectively. Ni can dissolve up to a fraction of 0.66 of Fe in Fe2Nb, although it forms no Laves phase with Nb. From these results, these elements would substitute Fe sublattice sites of Fe2Nb. However, due to its crystal structure consisting of the stacking of kagome-net (single layer: s) and three 36-net (triple layer: t), Fe sublattice site in C14 structure should be separated into two types: one is 36-net (Fe-1) and the other is kagome-net (Fe-2) with the fraction of 0.25 and 0.75, respectively. Rietveld analysis reveals that Cr and Mn with atomic size larger than Fe tends to occupy Fe-1 sublattice site when the amount in solution is less than 0.25 fraction of Fe in Fe2Nb, but the preferential site changes to Fe-2 sublattice when the amount in solution increases beyond 0.25. In contrast, Co and Ni with atomic size smaller than Fe preferentially occupy the Fe-2 site, regardless of the amount. The c/a ratio of the C14 Laves phase increases with the increase in M content when M substitutes for Fe-1 sublattice site, whereas the ratio decreases when M occupies Fe-2 site. Note that C15 structure such as Co2Nb and Cr2Nb has only one sublattice site for the smaller atom. Based on the knowledge, together with the results on M occupying Nb sublattice site, the phase stability and crystal structure change between C14 and C15 structures will be discussed.
10:45 AM - U8.5
Composition Dependence of the Mechanical Properties of Laves Phasesin the Binary Nb-Fe and the Ternary Nb-Fe-Al System.
Simon Voss 1 , Frank Stein 1 , Martin Palm 1 , Daniel Gruener 2 3 , Guido Kreiner 2 , Georg Frommeyer 1 , Dierk Raabe 1
1 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 2 , Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden Germany, 3 , Stockholm University, Stockholm Sweden
Show Abstract11:00 AM - U8.6
Transmission Electron Microscopy of Fe2Nb Laves Phase with C14 structure in Fe-Nb-Ni Alloys.
Naoki Takata 1 2 , Takashi Matsuo 1 2 , Masao Takeyama 1 2
1 Dept. Metallurgy and Ceramics Science, TOKYO INSTITUTE OF TECHNOLOGY, Tokyo Japan, 2 , Consortium of the Japan Research and Development Center for Metals (JRCM), Tokyo Japan
Show AbstractOur phase diagram study on Fe-Nb-Ni ternary system revealed that the Fe2Nb Laves phase region extends toward the equi-Nb concentration direction up to 44 at.% Ni at 1473 K. XRD measurement analysis also revealed that most of the Ni atoms occupy Fe-2 sublattice site (kamoge-nets) rather than Fe-1 sublattice site (36-nets) in C14 structure of Laves phase. The hardness of Fe2Nb decreases with increasing the amount of Ni in solution and this trend is more obvious in Fe-rich Fe2Nb in equilibrium with γ-Fe (fcc) phase, that is, from 9.1 GPa to 6.3 GPa. These results clearly suggest that defect structure in C14 Laves phase gives rise to the softening effect. In this study, micro and lattice structures of the C14 Fe2Nb Laves phase with and without Ni atoms in solution were examined in detail by transmission electron microscope (TEM). Regardless of the Nb contents, featureless morphology with very few dislocations was observed in binary Fe2Nb Laves phase. In contrast, many defects of planer faults lying on the basal plane of C14 structure and dislocations were observed in Fe2Nb containing Ni in solution, especially, in Fe-rich Fe2Nb, more planer faults in Laves phase containing more Ni in solution. High resolution TEM revealed that the planer fault corresponds to the difference in stacking sequence in the three 36-nets (triple layer) of C14 structure. The faulted region with the different stacking sequence become cubic C15 structure, this is responsible for the softening of Laves phase. The mechanism to enhance the planer faults by Ni in solution will be discussed in conjunction with the hard sphere model.
11:30 AM - U8.7
The (Meta)Stability of Hexagonal NbCr2.
Jochen Aufrecht 1 , Andreas Leineweber 1 , Eric Mittemeijer 1 , Viola Duppel 2 , Anatolij Senyshyn 3
1 , Max Planck Institute for Metals Research, Stuttgart Germany, 2 , Max Planck Institute for Solid State Research, Stuttgart Germany, 3 Institute for Materials Science, Darmstadt University of Technology, Darmstadt Germany
Show AbstractThe NbCr2-Laves phase is of great interest because it is a candidate for high strength/high temperature applications and it can serve as a model system to study the complex polymorphic transformations occurring in some Laves phases. The existence of a thermodynamically stable hexagonal C14-type high-temperature modification of the binary Laves phase NbCr2 above around 1600°C, as included in the phase diagram, has been derived in the past on the basis of high-temperature differential thermal analysis, phase analysis of rapidly solidified samples and extrapolation from ternary alloy data to the binary Nb-Cr system.The present investigations on NbCr2 Laves phase alloys were performed employing electron backscatter diffraction (EBSD), high-resolution electron microscopy (HRTEM) and (high-quality) X-ray powder diffraction (XRPD) for the (micro-)structural characterization of rapidly solidified NbCr2 samples, together with high-temperature neutron diffraction and differential thermal analysis (DTA) of equilibrated NbCr2 samples. These investigations cast new light on C14-NbCr2.As-cast, rapidly solidified ingots produced by arc-melting were found to consist of the cubic C15-room temperature modification, of a heavily faulted dihexagonal C36 modification as well as of a small amount of C14-type crystals. Surprisingly, the C14 and C36 phases are found at the top of the arc-melted ingot, where the lowest cooling rate is expected. It could be shown by HRTEM and line profile analysis of the XRPD patterns that the types of stacking faults found in the C36 phases are consistent with a mechanism for the formation of C36 from C14 involving synchro-shockley partial dislocation dipoles. Annealing of the ingots at 1500°C yields pure C15 Laves phase – in agreement with the phase diagram.However, in contradiction to the established phase diagram, high-temperature neutron diffraction data up to 1700°C did not lead to a transformation of the cubic C15-room temperature structure into a hexagonal modification. It is further demonstrated that sample contamination by oxygen/nitrogen during high-temperature DTA measurements heavily affects the results of such experiments. Such a contamination produces an impurity-stabilized phase, which causes thermal effects similar to those which previously led to the proposal of a C14/C15 polymorphic transition at about 1600°C.Thus, although examination of the (non-equilibrium) as-cast state confirmed the existence of a C14 (and a C36) modification of NbCr2, the results from high-temperature neutron diffraction and high-temperature DTA measurements invalidate the published phase transformation temperature and even suggest that C14-NbCr2 may be a metastable phase, which just forms during solidification.This work has been performed within the Inter-Institutional Research Initiative "The Nature of Laves Phases" funded by the Max Planck Society.
11:45 AM - U8.8
Structure and Disorder of the Laves Phases in the Co-Nb System.
Guido Kreiner 1 , Alim Ormeci 1 , Yuri Grin 1 , Frank Stein 2 , Martin Palm 2 , Joachim Konrad 2 , Georg Frommeyer 2 , Dirk Raabe 2 , Andreas Leineweber 3 , Eric Mittemeijer 3 , Dieter Fischer 4 , Martin Jansen 4 , Daniel Gruener 5 , Osamu Terasaki 5
1 , Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden Germany, 2 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 3 , Max-Planck-Institut für Metallforschung, Stuttgart Germany, 4 , Max-Planck-Institut für Festkörperforschung, Stuttgart Germany, 5 Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratories, Stockholm University, Stockholm Sweden
Show Abstract12:00 PM - U8.9
An In-situ Electron Microscopy Study of Microstructural Evolution in a Co-Co2Nb Binary Alloy.
Sharvan Kumar 1 , Frank Stein 2 , Martin Palm 2
1 Division of Engineering, Brown University, Providence, Rhode Island, United States, 2 , Max-Planck Institut für Eisenforschung GmbH, Düsseldorf Germany
Show AbstractThe microstructure in a Co-rich, Co-15 at.%Nb alloy, produced by levitation-melting and solidification in a Cu-mold, was characterized in the as-cast condition. A predominantly lamellar eutectic morphology composed of a Co-Nb solid solution and the C15 Laves phase Co2Nb was confirmed by transmission electron microscopy. The C15 phase was heavily twinned, with only one variant of twins being present in the individual lamella, while the Co solid solution was confirmed to be face centered cubic at room temperature. The most recent version of the binary phase diagram predicts that this alloy should be a two-phase mixture of the Co solid solution and a Nb2Co7 intermetallic phase. Thus, the C15 phase is metastably retained in the as-cast microstructure. In-situ heating to 600°C in the microscope confirmed the decomposition of the Laves phase into a fine equiaxed, ~10-20 nm grain size microstructure, the product phase thought to be the newly discovered Nb2Co7 phase. The individual grains appear heavily faulted. The matrix solid solution contained some fine precipitates, but retained the fcc structure. No change in structure was observed on cooling to room temperature. Heating to higher temperatures is currently underway and the results from these experiments will be presented and their implications will be discussed. This work has been performed within the Inter-Institutional Research Initiative “The Nature of Laves phases” funded by the Max Planck Society.
12:15 PM - U8.10
Homogeneity Range and Substitutional Disorder in the Ternary C14 Laves Phase Nb(Cr1−xCox)2.
Alexander Kerkau 1 , Alim Ormeci 1 , Daniel Gruener 2 , Ewald Bischoff 3 , Yurii Prots 1 , Horst Borrmann 1 , Juri Grin 1 , Guido Kreiner 1
1 , Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden Germany, 2 Department of Physical, Inorganic and Structural Chemistry, Stockholm University / Arrhenius Laboratory, Stockholm Sweden, 3 , Max-Planck-Institut für Metallforschung, Düsseldorf Germany
Show Abstract12:30 PM - U8.11
Phase Equilibria in the Ternary Nb-Cr-Al System and Site Occupation in the Hexagonal C14 Laves Phase Nb(Cr,Al)2.
Oleg Prymak 1 , Frank Stein 1 , Alexander Kerkau 2 , Alim Ormeci 2 , Guido Kreiner 2 , Georg Frommeyer 1 , Dierk Raabe 1
1 , Max-Planck-Institut fuer Eisenforschung GmbH, Duesseldorf Germany, 2 , Max-Planck-Institut fuer Chemische Physik fester Stoffe, Dresden Germany
Show AbstractLaves phases have the ideal composition AB2 and form the largest group of intermetallic compounds. They crystallize in three structure types: cubic C15 (MgCu2-type), hexagonal C14 (MgZn2-type) and hexagonal C36 (MgNi2-type). Transition-metal Laves phases are promising candidates for novel structural materials for very high temperatures. A prerequisite for the development of such materials is the knowledge and understanding of the structure and stability of these phases. In the present work the ternary Nb-Cr-Al phase diagram, which exhibits extended phase fields of the cubic C15 and the hexagonal C14 Laves phases Nb(Cr,Al)2, has been studied. A number of Nb-Cr-Al alloys were prepared by levitation melting and annealed at temperatures between 1150 and 1450 °C for up to 1500 h. The alloys were studied by light-optical and scanning-electron microscopy, electron probe microanalysis, X-ray powder diffraction and differential thermal analysis. Isothermal sections of the ternary Nb-Cr-Al phase diagram were obtained for temperatures of 1150, 1300 and 1450 °C in order to study the effect of Al on the stability and structure of the Laves phases. Two structure types, the cubic C15 and the hexagonal C14 Laves phases are observed in this system. The C14 Laves phase in the Nb-Cr-Al system can dissolve up to 45 at.% Al by substituting Cr with Al on the two different crystallographic B-sites 2a and 6h of the C14 unit cell. The site occupations of the Al and Cr atoms on these two B-sites were determined by Rietveld analysis using the program FullProf. The obtained results for the preferred site occupations were compared to site occupation factors computed by a statistical mechanical approach based on first-principles electronic structure calculations.This work was performed within the Inter-Institutional Research Initiative “The Nature of Laves Phases” funded by the Max Planck Society.
12:45 PM - U8.12
Preferential Site Occupation in Ternary C14 Laves Phases.
Alim Ormeci 1 , Alexander Kerkau 1 , Daniel Gruener 2 1 , Guido Kreiner 1
1 , Max Planck Institute for Chemical Physics of Solids, Dresden, Saxony, Germany, 2 Deparment of Physical, Inorganic and Structural Chemistry, Arrhenius Lab., Stockholm University, Stockholm Sweden
Show AbstractU9: Fundamental Aspects of Intermetallics - Phase Stability, Defects, Theory
Session Chairs
Wolfgang Pfeiler
Frank Stein
Thursday PM, December 04, 2008
Constitution A (Sheraton)
2:30 PM - **U9.1
Dynamics of Atomic Ordering in Bulk and Thin Film Intermetallic Alloys: A Complementary Approach to Atomic Migration.
Wolfgang Pfeiler 1 , Wolfgang Pueschl 1 , Chaisak Issro 2 , Rafal Kozubski 3 , Veronique Pierron-Bohnes 4
1 Dynamics of Condensed Systems, Faculty of Physics, University of Vienna, Vienna Austria, 2 Department of Physics, Faculty of Science, Burapha University, Chonburi Thailand, 3 Advanced Materials Engineering, M. Smoluchowski Institute of Physics, Jagellonian University, Cracow Poland, 4 UMR 7504 CNRS-ULP, Institut de Physique et Chimie des Matériaux de Strasbourg, Strasbourg France
Show AbstractOne of the foremost challenges in today’s materials science is the design and development of materials with physical properties customized for technical application. Due to their excellent corrosion resistance and their advantageous mechanical and in many cases also magnetic properties, intermetallic alloys are among the most important materials of the 21st century.Most of their outstanding qualities are linked to long-range order, the fact that unlike atoms are preferred as neighbours, which then segregate to different sublattices. In most intermetallics atomic order persists up to rather high temperatures, if not up to melting. However, connected with the entropy gain, the degree of order depends on temperature and thereby the stability of the designed beneficial materials properties is affected.By monitoring changes in the degree of atomic order an access is gained to atom migration in intermetallic systems, which is complementary to the usual diffusion experiments, where the degree of order is not changed on average. This means that in the first case certain atom jumps are selectively observed which lead from one state of order to another one, whereas in the latter case, each jump is recorded, whatever jumps are involved, as long as the overall state of order remains unchanged.It is shown in this review on some selected examples how an adequate thermal treatment of the samples in combination with the experimental approach gives detailed information on atom jump mechanisms and structural changes, especially if experiment is combined with up-to-date kinetic Monte-Carlo simulations.
3:00 PM - U9.2
Lattice-Gas-Decomposition Model for Vacancy Formation Correlated with B2 Atomic Ordering in Intermetallics.
Rafal Kozubski 1 , Andrzej Biborski 1 , Lukasz Zosiak 1 , Veronique Pierron-Bohnes 2
1 Interdisciplinary Centre for Materials Modelling, M.Smoluchowski Institute of Physics, Jagellonian University, Krakow Poland, 2 Institut de Physique et Chimie des Matériaux de Strasbourg, CNRS/ULP, Strasbourg France
Show AbstractThermal vacancy formation correlated with atomic ordering was modelled in B2-ordering A-B binary intermetallics. Ising Hamiltonian was implemented with a specific Bragg-Williams-type thermodynamic formalism for thermal vacancy formation based on the phase equilibria in a lattice gas composed of atoms and vacancies. It has been demonstrated that for pair-interaction energetics favouring vacancy formation on A-atom sublattice, equilibrium concentrations of vacancies and antisite defects result mutually proportional in well defined temperature ranges. The effect observed both in stoichiometric and non-stoichiometric (both A-rich and B-rich) binary alloys was interpreted as a tendency for triple defect formation. In B-rich alloys vacancy concentration did not extrapolate to zero at K, which indicated the formation of constitutional vacancies. Energetic conditions for the occurrence of the effects were analysed in detail. The above themodynamic model was implied with Grand Canonical Monte Carlo simulations, whose results were in qualitative agreement with the Bragg-Williams ones. The modelled temperature dependence of vacancy concentration in the B2-ordering A-B binaries will be involved in the Kinetic Monte Carlo simulations of chemical ordering kinetics in these systems.
3:15 PM - **U9.3
Point Defects and Composition-dependent Elastic Properties of TiNi.
Jianmin Lu 1 , Qingmiao Hu 1 , Rui Yang 1
1 , Institute of Metal Research Chinese Academy of Sciences, Shenyang China
Show AbstractTiNi based alloys have potential applications in the morph of engine casings and wings of future aircraft for reducing take-off noise and increasing fuel efficiency during cruise. The martensitic transformations underlying the shape memory effect have been studied extensively, but many aspects of this functional intermetallic compound are still not well understood, among which are the asymmetry of the phase field and of the compositional dependence of martensitic transformation temperature across the stoichiometric composition. By studying point defect interactions using first principles methods we offer an explanation of the different solubility of excess Ti and Ni atoms. Using elastic instability criterion and computed elastic constants the cause of the variation of the transformation temperature with composition was traced to the Coulomb static electronic interactions of antisite atoms and their nearest neighbors. Alloying effects on the phase transformation behavior and mechanical properties of TiNi, relevant to its application to severer environment, will also be discussed.
3:45 PM - U9.4
Phase Stability of A15 Mo3Si with Nb and W Additions.
Pratik Ray 1 2 , Mufit Akinc 1 2 , Matthew Kramer 2 1
1 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States, 2 , Ames Laboratory, Ames, Iowa, United States
Show AbstractThere is increasing demand to improved Carnot efficiencies in fossil fuel fired systems to conserve resources and reduce pollution. These goals can only be achieved by greatly increasing the operating temperature. However, most coal-fired systems are already operating at the upper limit of their design. Increases in operating temperatures and pressures (i.e., ultra-supercritical systems and land-based turbines) are a significant materials challenge since this will require improved mechanical performance of the alloy as well as improvements in oxidation and corrosion resistance. Mo-Si based alloys are an attractive option for usage at such high temperatures due to their extremely high melting temperatures and good oxidation resistance. However, these alloys contain the Mo3Si based A15 phase. This phase is extremely brittle in nature and affects the mechanical response of these materials. In this study we explore the possibility of destabilizing the deleterious A15 phase by controlled Nb and W addition, thereby resulting in an alloy with relatively higher ductility. The Mo-Nb-Si and Mo-W-Si alloys have been synthesized by arc-melting followed by annealing at 1900oC, following which they have been characterized using electron microprobe and x-ray diffraction. Our studies indicate that an addition of approximately 30 atom percent of Nb or 15 atom percent of W is sufficient to destabilize the A15 phase.
4:15 PM - U9 Fundamentals
BREAK
4:30 PM - U9.6
Plastic Deformation of Ti5Si3 Single Crystals.
Kyosuke Kishida 1 , Fujiwara Masakazu 1 , Norihiko Okamoto 1 , Katsushi Tanaka 1 , Haruyuki Inui 1
1 Department of Materials Science and Engineering, Kyoto University, Kyoto Japan
Show AbstractTi5Si3 with a complex hexagonal-type D88 structure has attracted a great deal of interest as a new class of refractory materials because of its high melting temperature (2403K), low density (4.32g/cm3) and good oxidation resistance. However, details of plastic deformation mechanisms of Ti5Si3 have not been fully understood yet. In this study, we have investigated plastic deformation behavior of Ti5Si3 by compression tests of single crystals as a function of the loading axis orientation and temperature. Single crystals of Ti5Si3 were grown by optical floating-zone method at a growth rate of 6mm/s. Specimens with dimensions 1.5x 1.5x 4mm3 for compression test were cut from the single crystals. Four different loading axis orientations: [2-205], [4-3-10], [0001] and [2 -1 -1 24], were selected. Compression tests were carried out in vacuum in a temperature range from 1473K to 1773K at a nominal strain rate of 1x 10-4/s. Deformation structures were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscatter diffraction pattern (EBSD) and optical microscope (OM). Three different types of deformation modes, namely (1-100)[0001] slip, (-2112)1/3[-211-3] slip and (-2118)[-844-3] deformation twin, are newly identified to be activated above 1573K depending on the loading axis orientation. CRSS for the (1-100)[0001] slip, (-2112)1/3[-211-3] slip and (-2118)[-844-3] deformation twin are estimated to be about 130MPa, 330MPa and 180MPa at 1673K, respectively. The value of the CRSS for these three deformation modes decrease monotonously with increasing the temperature. Details of these newly identified deformation modes for Ti5Si3 will be presented.
4:45 PM - U9.7
Plastic Deformations in Single Crystals of FePd with the L10 Structure.
Katsushi Tanaka 1 , Chen Wang 1 , Kyosuke Kishida 1 , Norihiko Okamoto 1 , Haruyuki Inui 1
1 Materials Science and Engineering, Kyoto University, Kyoto Japan
Show AbstractThe intermetallic compounds with the L10 structure, such as TiAl, CoPt, FePd and FePt are of considerable commercial interests for both structural and functional applications. Owing to their technological interests, extensive investigations have been performed on the mechanical properties of the compound of TiAl. However, mechanical properties of other compounds with the L10 structure, especially for those of single crystals are still open question. In the Co-Pt, Fe-Pd and Fe-Pt systems, the ordered L10 phase forms in conjunction with a cubic (fcc) to tetragonal (L10) transformation, resulting in forming a microstructure so-called poly-twinned structure that is a lamellar structure consisting of {101} thin twin plates. This microstructure makes investigations of mechanical properties of single crystals of these compounds difficult. Recently, we have found that the single crystals of CoPt, FePd and FePt with the L10 structure form from single crystals with the fcc-disordered phase under an appropriate strength of a compressive stress or of a magnetic field. By using this technique, we have successfully made single crystal specimens of FePd with the L10 structure applicable to mechanical testing. Compressive deformation tests have been performed from room temperature to 823 K that is about 50 K below the order-disorder transition temperature. The critical resolved shear stress (CRSS) for octahedral glide of ordinary dislocations is smaller than that of super-lattice dislocations in all the temperature range investigated, that is the same sense to the case of our preliminarily investigation on the compound of CoPt but opposite to the case of Ti-56at.%Al where the CRSS for superlattice slip system is lower than that for ordinary slip system. From the observation of dislocation structure, superlattice dislocations of FePd have been trapped at anti-phase domain boundaries formed at the ordering process and the dissociation of the trapped superlattice dislocations have not been observed.
5:00 PM - U9.8
Direct Evidence of Solute-segregations at Dislocation Core in W-doped NbSi2.
Eiji Abe 1 , Yosuke Maemura 1 , Koji Hagihara 2 , Takayoshi Nakano 2 , Yukichi Umakoshi 2
1 Dept. of Materials Engineering, University of Tokyo, Tokyo Japan, 2 Dept. of Materials Science and Engineering, Osaka University, Osaka Japan
Show AbstractTransition-metal silicides are attractive candidates for structural applications at temperature higher than ~1273K, because of their good oxidation resistance and anomalous strength at high temperatures. It was shown that, by adding a small amount of W, NbSi2 compound shows significant increase of its high-temperature strength [1], and its strengthening mechanism was discussed in terms of interactions between the dislocations and solute atoms. In the NbSi2 crystal (C40-type), normal dislocations are dissociated into two partial dislocations by accompanying stacking faults. These faulted layers in the C40 structure may be energetically favored for solute W atoms, since the WSi2 forms the C11b-type structure that is simply a stacking-variant version of C40-type (C40-type: ABD… stacking, C11b-type: AB… stacking). At sufficient high-temperatures, W atoms may diffuse into the stacking faults at extended-dislocations to form a dragging atmosphere, realizing significant high-temperature strength by giving an additional barrier for dislocation motions. We here describe a direct evidence to support this picture, based on atomic-resolution Z-contrast STEM imaging of the dislocation core in the W-doped NbSi2. Preliminary Z-contrast observations of the (Nb0.97W0.03)Si2 compound has revealed a striking feature that W atoms are definitely located at a fault-layer at the extended dislocation core; W atoms seem not to distribute diffusively around dislocation cores (as one may predict according to Cottrell atmosphere) but sharply sit only at the stacking fault layer. We will describe a quantitative estimation of a local W concentration, and discuss a role of W atoms to realize high-temperature strength of the (Nb,W)Si2 compound. [1] T. Nakano, et al., Acta mater., 48, (2000) 3465-3475.
5:15 PM - U9.9
Development of Texture and Microstructure During Hot Deformation in Intermetallic Compounds with Bcc-based Superstructure.
Hiroyuki Yasuda 1 , Toshiya Sakata 2 , Hisanori Kohma 1 , Yukichi Umakoshi 3
1 Division of Materials and Manufacturing Science, Osaka University, Suita Japan, 2 , Tokyo University, Tokyo Japan, 3 , National Institute for Materials Science, Tsukuba Japan
Show AbstractThe texture and microstructure in the B2 and L21-type intermetallic compounds compressed at high temperatures were examined using an electron back scatter diffraction pattern (EBSP) technique. In FeAl and NiAl alloys with the B2 structure, strong <111> fiber texture along the compression axis developed due to the crystal rotation by slip deformation. Dynamic recrystallization was found to take place in the B2-type compounds and the microstructure depended strongly on Zener-Hollomon (Z) parameter. Formation of new grains resulted from the grain boundary serration at higher Z, while geometric dynamic recrystallization occurred at lower Z. On the other hand, the preferred orientation of the fiber texture in Ni2MnGa with the L21 structure changed from <111> to <001> with increasing temperature associated with the activated slip systems. The formation of crystallographic texture could be predicted by relaxed Taylor model.In Ni-rich NiAl alloys, Ni3Al phase with the L12 structure precipitated along the grain boundaries. NiAl and Ni3Al phases satisfied the Kurdjumov-Sachs orientation relationship; there are 24 crystallographic variants in Ni3Al phase. The formation of <111> fiber texture in hot-compressed NiAl phase strongly influenced the transformation texture in Ni3Al phase, which improved the ductility of NiAl/Ni3Al two-phase alloys.
5:30 PM - U9.10
Electrodeposited Intermetallic Hybrid Nanowire Electrodes for Advanced Energy Storage Devices.
Shaijumon Manikoth 1 , Fung Suong Ou 1 , Arava Leela Mohana Reddy 1 , Lijie Ci 1 , Pulickel Ajayan 1
1 , Rice University, Houston, Texas, United States
Show AbstractSynthesis of multifunctional materials with desired properties for high performance energy storage devices is still a big challenge.[1] There is great interest in developing high performance rechargeable lithium batteries for portable electronic applications due to their numerous advantages over other energy technologies. The performance characteristics of these energy devices are fundamentally determined by the structural and electrochemical properties of electrodes. There have been several attempts to improve the cycling stability of Lithium metal alloys. Several Tin-based intermetallics have been investigated for their improved performance in terms of battery applications. However, the morphology of the intermetallics plays a key role for achieving proper control of the stress associated with volume expansion. Here we demonstrate that high performance Li-batteries can be engineered using electrodeposited vertically aligned arrays of multi-segmented Ni/Sn – gold hybrid nanowire electrodes. Here the Ni/Sn intermetallic nanowire segments accommodate large strain without disintegration, while the gold segments provide good electronic contact and conduction, thus resulting in high performance of the Li-batteries.
5:45 PM - U9.11
Phase Field Simulation of Coarsening Kinetics in Al-Sc and Al-Sc-Zr Alloys.
Helena Zapolsky 1 , Julien Boisse 1 , Renaud Patte 1 , Nicolas Lecoq 1
1 GPM, UMR 6634, University of Rouen, Saint-Etienne du Rouvray, 76801, France
Show Abstract The addition of Sc to Al-based alloys is a very effective means of increasing their resistance to recrystallisation, via the formation of high density of Al3Sc precipitates. Morphology of these ordered particles is strongly depends from the volume fraction of precipitated phase. It was shown that for Al-0.3%Sc alloys aging at 300°C for 24h the form of Al3Sc precipitates evolves from spherical to dendrite. Addition of Zr together with Sc improves the effectiveness of Sc as an inhibitor of recrystallisation and increases the stability of the alloy during prolonged annealing at high temperatures. This interesting behaviour is believed to link to the presence of several kinds of very fine particles. The presence of Zr in ternary Al-based alloys reduces the susceptibility of the Al3Sc precipitates to coarsening. Recent experimental studies show that the Zr atoms dissolve in the Al3Sc phase by replacing of Sc atoms and decrease the lattice parameter of ordered Al3Sc1-xZrx phase. By changing the Sc/Zr ratio, it is therefore possible to tailor the lattice parameter mismatch between the L12 precipitates and the Al matrix. Zirconium diffuses three orders of magnitude slower than Sc in Al at 375°C and after aging at this temperature the equilibrium structure of precipitates is heterogeneous. The core of ordered precipitates is Sc rich as well as γ/γ’ interface is Zr rich. Zirconium shell blocs the diffusion of aluminium atoms and change the coarsening rate. The coarsening kinetics of gamma’ precipitates in binary Al-Sc and ternary Al-Zr-Sc alloys is studied by using the three-dimensional phase-field simulations. It was demonstrated that a morphology of L12 precipitates in Al-Sc alloys is detemined by two factors:1) the ration between elastic and interfacial energy, and 2) volume fraction of ordered phase. In ternary alloys, our focus is on the influence of diffusion coefficients of Sc and Zr atoms on the transformation path kinetics from disordered f.c.c. matrix to two phase equilibrium state with gamma’ precipitates and f.c.c. disordered matrix. The simulation results demonstrate that the Al3Sc particles precipitate firstly following by appearing of a Zr-rich shell around Al-rich ordered precipitates. This heterogeneous structure decreases the coarsening kinetics rate. Our simulations results are in good agreement with experimental observations.