The present paper will describe the science and technology of titanium aluminide (TiAl) alloys and the engineering development of TiAl for commercial aircraft engine applications. The GEnx is the first commercial aircraft engine flying titanium aluminide (alloy 4822) and it represents a major advance in propulsion efficiency, realizing a 20% reduction in fuel consumption, a 50% reduction in noise, and an 80% reduction in NOx emissions compared to prior engines in its class. The GEnx uses the latest materials and design processes to reduce weight, improve performance, and reduce maintenance costs. General Electric TiAl low pressure turbine blade production status will be presented along with the history of implementation. In the second half of 2006, GE began to explore near net shape casting as an alternative to the overstock conventional gravity casting plus machining approach. To date more than 30,000 TiAl low pressure turbine blades have been manufactured for the GEnx 1B (Boeing 787 series) and the GEnx 2B (Boeing 747-8) applications. The present state of the art of a range of casting techniques will be discussed with consideration for net shape casting technologies. The implementation of TiAl in other GE and non-GE engines will also be discussed.

The CALPHAD (calculation of phase diagrams) method is widely recognized as a powerful tool in both scientific and industrial development of new materials and processes. For the implementation of consistent databases, where each phase is described separately, models are used which are based on physical principles and parameters assessed from experimental data. Such a database makes it possible to perform realistic calculations of thermodynamic properties of interesting multi-component systems. However, a commercial available TiAl database can be applied for thermodynamic calculations to both conventional Ti-base alloys and complex intermetallic TiAl alloys to describe experimentally evaluated phase fractions as a function of temperature. In the present study calculations were done for a β-solidifying TiAl alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), termed TNMtrade; alloy. This alloy consists of ordered γ-TiAl, α2-Ti3Al and βo-TiAl phases at room temperature. At a certain temperature α2 and βo disorder to α and β, respectively. Using the commercial database the thermodynamic calculations reflected only qualitative trends of phase fractions as a function of temperature. For more exact quantitative calculations the commercial available thermodynamic database had to be improved for TiAl alloys with high Nb (and Mo) contents, as recently reported for Nb-rich γ-TiAl alloys. Therefore the database was modified by experimentally evaluated phase fractions obtained from quantitative microstructure analysis carried out by light-optical and scanning electron micrographs as well as conventional X-ray diffraction after long-term heat treatments and by means of in-situ high-energy X-ray diffraction experiments. Based on the CALPHAD-conform thermodynamic assessment, the optimized database could now be used to correctly predict the phase equilibria of this multi-component alloying system, which is of great interest for applications in automotive and aircraft engine industry.

The effect of stress on the deformation and crack nucleation and propagation mechanisms of a γ-TiAl intermetallic alloy (Ti-45Al-2Nb-2Mn (at.%) - 0.8v.%TiB2) was studied by means of in situ tensile (constant strain rate) and tensile-creep (constant load) experiments performed at 973 K inside a scanning electron microscope. The alloy was processed by centrifugal casting in the form of parallelepipedic specimens of 44 x 20 x 2.5 mm3, which were subsequently hot isostatically pressed at 1458 K and 1700 bar for four hours to remove any remnant porosity. It presented a nearly-lamellar polycrystalline microstructure, with a small fraction (0.18) of equiaxed α2 and γ grains located at colony boundaries. The average colony size was 126 ± 52 µm, while the average α2 and γ lamella thickness were, respectively, 329 and 422 nm. The evolution of the microstructure and the nucleation and propagation of cracks were tracked during the high temperature mechanical tests in the SEM. Colony boundary crack nucleation was found to be activated during the secondary stage in creep tests at 300 MPa and 400 MPa and during the tertiary stage of the creep tests performed at higher stresses and at constant strain rate. Interlamellar ledges were only observed during the high stress tensile-creep tests (σ>400 MPa) and during the constant strain rate test. Quantitative measurements of the nature of the crack propagation path along secondary cracks and along the primary crack were carried out. It was found that colony boundaries were preferential sites for crack propagation under all the conditions investigated. The frequency of interlamellar cracking increased with increasing stress, but this crack propagation mechanism was always of secondary importance. Translamellar cracking was only observed along the main crack.

The development of suitable hot forming processes, e.g. forging, is an important step towards the mass production of TiAl parts. Due to their improved formability at elevated temperatures, research activities are focused on TiAl alloys containing additional amounts of ductile bcc high-temperature beta phase. During hot forming several microstructure parameters e.g. phase fractions, grain size or crystallographic texture, change. However, conventional analysis methods can only infer the changing mechanisms from post-process metallographic studies. We used a conventional deformation dilatometer (DIL 805A/D) modified for working in the HZG synchrotron beamlines at DESY for hot deformation experiments. This setup enables the continuous monitoring of the interaction and evolution of several microstructure parameters in situ, i.e. during processing, whereas conventional analysis methods can only infer from post-process metallographic studies. In particular, we observed the evolution of phase fractions, grain size and crystallographic texture during deformation while simultaneously recording the process parameters (temperature, force and length change). Here we present the hot compressive deformation behaviour of an as-cast and hot-isostatically pressed Ti-43Al-4Nb-1Mo-0.1B (in at.%) alloy. Several specimens were deformed at three temperatures (1100, 1200 and 1230°C) each with two strain rates (1eminus;3/s and 3eminus;3/s) up to a total strain of phi=minus;0.6 (~45% reduction). Immediately after the deformation (within 1s) the specimens were quenched with a quenching rate greater than minus;50K/s in order to study the deformed microstructure. During the experiments the diffraction rings were continuously recorded on a mar555 flat panel detector with a frame rate of 0.3Hz. The obtained results allow a more detailed optimization of process parameters regarding final alloy properties, such as grain size or texture. It is for example possible to separate the effect of deformation and recrystallization on the texture evolution during the in situ experiment, which is often difficult to determine from post mortem investigations.

Intermetallic titanium aluminides, with their superior properties at higher temperatures while having a low density, have evolved into a lightweight alternative to nickel-based superalloys. Potential fields of application include aircraft and automobile engines. However, the fundamental problem in implementing this promising class of materials is their insufficient hot-workability, which makes conventional processing routes, like forging, difficult. The addition of certain alloying elements, such as molybdenum, stabilizes the disordered body-centered cubic β-phase at elevated temperatures. It offers indeed the opportunity to suppress the formation of α2-phase and is used to generate the so-called β/γ-alloys. Unfortunately, the effect of molybdenum on the individual phases and their distribution during slow heating and rapid cooling in the course of isothermal heat treatments is not completely understood. Microstructure strongly influences a material's mechanical properties. Although conventional 2D image analysis provides statistically secure information about the microstructure, a complete understanding of the 3D arrangement (e.g. phases connectivity) is not accessible. In this study, FIB/SEM tomography has been employed to analyze the 3D microstructure of a β/γ-alloy with a nominal composition of Ti-44at%Al-7at%Mo. A region of interest was consecutively sliced with a focused ion beam (FIB) revealing cross sections that were imaged with a scanning electron microscope (SEM). Thereby, a region of interest of approx. 40 µm x 40 µm has been removed in 250 slices 200 nm thick and 20 µm deep. γ-TiAl and β-TiAl provide a good contrast in SEM, leading to reliable segmentation results. The voxel size of the reconstruction, 50nm x 63nm x 200nm, results from the resolution of the SEM images and the slicing thickness. After shading correction, the 2D images were binarized using a global gray value threshold. Noise and artifacts in segmentation were removed using morphological operations such as erosion and dilation, and manual corrections. The region of interest has been chosen to contain a former grain boundary of the γ-TiAl phase, indicated by elongated γ-TiAl structures. The analysis of said structures has shown that the γ-TiAl as well as the β-TiAl are not separated by these former interfaces. For both phases, nearly the whole volume (> 99.9%) forms an interconnected network. No isolated particles of considerable volume have been found inside the studied volume. Some smaller isolated particles at the borders of the volume likely had external connections. Furthermore, a quantitative analysis of the 3D reconstruction was done in order to discern the connectivity and to obtain the phase fraction using the MAVI software package. Finally local texture analysis by means of electron backscatter diffraction (EBSD) showed that β-TiAl as well as γ-TiAl are highly textured with {110} β-TiAl parallel to {111} γ-TiAl.

Above 750 - 800°C oxidation becomes a serious life time issue for the new group of interme-tallic light-weight high temperature alloys based on TiAl. Fast growing titanium oxide competes with protective alumina as a surface scale in the oxidation reaction by which the formation of a slow-growing protective oxide scale is prevented. The key to the development of alloys with sufficient oxidation resistance is the understanding of the thermodynamic and kinetic situation during the oxidation process. The latter is influenced by the type of alloying elements, by the Al und Ti activities in the alloy, by the oxidation temperature and by the environment (e.g. dry or humid air, sulfur species, etc.). In the paper a comprehensive summary of the oxidation me-chanisms and of the parameters influencing oxide scale formation will be given. Besides the role of metallic alloying elements also the surprising halogen effect will be discussed in some detail. The paper finishes with some very recent results concerning the prevention of the oxida-tion induced embrittlement of titanium aluminides.

Intermetallic alloys based on γ-TiAl are considered as promising candidates for high temperature applications in automotive and gas turbine engines. However, their oxidation resistance is insufficient at temperature above 800°C due to the formation of fast growing non-protective mixed oxide scales of titania and alumina. The use of protective coatings is a suitable method to improve the oxidation resistance of γ-TiAl components. Metallic and ceramic overlay coatings have been investigated. Ternary Ti-Al-Cr alloys proved to be potential coating materials for protection of titanium aluminides. Their high oxidation resistance was associated with the Ti(Cr,Al)2 Laves phase exhibiting low oxygen permeability and being capable of forming a protective alumina scale. It could be further increased by additions of small amounts of reactive elements. In the present work, the oxidation protection capability of an intermetallic Ti-Al-Cr-Zr coating deposited on the Ti-45Al-8Nb alloy was studied performing cyclic oxidation tests and cross-sectional analyses of oxidised samples. Intermetallic Ti-Al-Cr-Zr layers were deposited on disc-shaped γ-TiAl specimens using a combined technique of high power impulse magnetron sputtering and unbalanced magnetron sputtering operating simultaneously at the Nanotechnology Centre for PVD Research, Sheffield Hallam University in UK. The coating thickness was 11 µm, and the chemical composition of the as-sputtered amorphous layer was Ti-49Al-34Cr-4Zr (at.%). It exhibited a dense layered structure and excellent adhesion to the substrate. The oxidation behaviour of the coated samples was determined at 1000°C performing cyclic oxidation tests in laboratory air. Post-oxidation analysis of thermally cycled specimens was carried out using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The microstructure of the protective layer was also investigated by transmissions electron microscopy and X-ray diffraction measurements. The Ti-Al-Cr-Zr coating exhibited excellent oxidation resistance at 1000°C associated with the formation of a thin continuous alumina scale for dwell times exceeding 1000 hrs. The α-Al2O3 scale contained pores and fine precipitates of a Zr-rich phase. During the high temperature exposure, the coating depleted in aluminium and chromium and was enriched in titanium due to interdiffusion. After 1000 cycles of 1 h dwell time at 1000°C, the coatings consisted of an outer layer of the hexagonal C14 Laves phase and an inner layer of a further phase being possible a Laves phase whose structure was not yet determined. In both layers, pores and small precipitates rich in Zr and Y were found.

Intermetallic titanium aluminides are potential materials for application in high temperature components. In particular, alloys solidifying via the beta-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic beta-phase at elevated temperatures ensuring good processing characteristics during hot-working. Additionally, for the practical use the oxidation resistance, especially in the temperature range up to 800°C, is a growing demand. Therefore, the fluorine effect on the oxidation resistance of multi-phase TiAl-based alloys in the cast/hot-isostatically pressed condition was investigated. In the first part of this study the so-called TNM alloy which exhibits a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at %) was examined at 800°C in air. The oxidation behavior of the fluorine treated samples compared to those samples without any treatment was remarkable. In the second part of this work two model alloys without any Nb but higher Mo-contents were studied. During the isothermal thermogravimetric analysis tests the TiAl-Mo alloys show already faster oxidation kinetics in the untreated conditions than the untreated TNM alloy. However, after fluorine treatment all alloys exhibit slow alumina kinetics indicating a positive fluorine effect. This effect worked also under thermocyclic conditions. The results of isothermal and thermocyclic oxidation tests at 800°C in air will be presented and discussed in the view of composition and microstructure of the TiAl alloys investigated, along with the impact of the fluorine effect.

Due to their high strength-to-weight ratio, low thermal creep and temperature independent yield strength and Young's modulus, gamma-TiAl has been considered as candidate refractory structural material for a number of demanding engineering applications such as power engineering and aerospace. Despite a noticeable progress in the development of gamma-TiAl based alloys and their manufacturing techniques, there are a number of solutions still required, specifically, to improve the oxidation resistance of the intermetallics. Empirical dynamic calculations were used to observe a distinct increase in aluminium interstitial defects compared to titanium interstitial species after a displacement cascade. Thermodynamic data from static ab-initio models support this result even though the Frenkel defect formation energies are predicted to be similar. Migration calculations were then used to determine whether these interstitials are free to move through the gamma-TiAl bulk lattice and therefore have the possibility of migrating to the surface where a passive alumina layer can be formed.

Because of their outstanding oxidation-corrosion resistance, low density, and good mechanical properties, industrial application of titanium-based intemetallics includes aircraft, medical instruments, and mechanical damping. For electronic applications, titanium-tin intermetallics exhibit a range of electronic and magnetic properties. Recent studies of the intermetallic compound β-Ti6Sn5, for example, indicate ferromagnetic (FM) ordering after rare-earth doping. To take full advantage of these mechanical and electronic properties, complete understanding of the behavior requires knowledge of the local atomic structure at boundaries and defects. Moreover, exploration of oxide scales is essential in these systems to understand their influence on mechanical properties. In this presentation, we report direct observations made by Z-contrast aberration-corrected scanning transmission electron microscopy (AC-STEM) of twin boundaries, dislocations, and stacking faults in Ti-Sn intermetallic compounds. The sub-angstrom resolution of AC-STEM reveals the detailed nature of displacements and chemistry at defects. We will demonstrate that the atomic scale chemical analysis enabled by aberration-corrected STEM provides the investigation of chemical segregation to planar faults and within the oxide scale. The knowledge gained from the atomic defects in these compounds improves the understanding of deformation mechanisms in complex structures.

The pin-on-disc wear behavior of nanostructured two-phase Fe₃₀Ni₂₀Mn₂₀Al₃₀ and eutectic lamellar-structured Fe₃₀Ni₂₀Mn₃₅Al₁₅ is compared, emphasizing the influence of the microstructure and mechanical properties of alloys, as well as the effect of test environments. Although the hardness of eutectic Fe₃₀Ni₂₀Mn₃₅Al₁₅ (~ 300 HV) was lower than that of nanostructured Fe₃₀Ni₂₀Mn₂₀Al₃₀ ( ~ 510 HV for the as-cast alloy and ~ 540 HV for the annealed alloy), the former alloy showed better wear resistance than the latter, which is probably due to the better ductility of eutectic Fe₃₀Ni₂₀Mn₃₅Al₁₅. For nanostructured Fe₃₀Ni₂₀Mn₂₀Al₃₀ itself, the as-cast alloy has more wear loss than the annealed alloy. The reason could be the annealed alloy has both a higher hardness and better ductility. It is also shown that the wear was reduced by the removal of the oxygen from the test environment. Moreover, the wear of nanostructured Fe₃₀Ni₂₀Mn₂₀Al₃₀ is more sensitive to oxygen than that of eutectic Fe₃₀Ni₂₀Mn₃₅Al₁₅. In order to elucidate wear mechanisms, the wear pins and debris were examined using a combination of X-ray diffractometry, scanning electron microscopy and transmission electron microscopy equipped energy dispersive X-ray spectroscopy, the latter using specimens produced by focused ion beam milling. Both two-body and three-body abrasive wear controlled the wear process of the two alloys, while plastic flow mechanism was more evident during the wear of the eutectic Fe₃₀Ni₂₀Mn₃₅Al₁₅.

Dry sliding pin-on-disk wear tests were conducted in air on the nanostructured alloy Fe30Ni20Mn25Al25, which consists of alternating b.c.c. and B2 phases with interfaces aligned along <100>, at both room temperature (298K) and elevated temperature (673K) against a stainless steel 347 counterface. The surfaces of the worn pins were examined using a combination of scanning electron microscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy. The wear tracks on the disks were analyzed using both optical microscopy and optical profilometery. It was found that the pins showed lower wear rates at elevated temperature than at room temperature. Debris collected during the wear tests consisted of materials from the pin and the disk. The pins undergoing elevated temperature wear tests showed a porous sublayer due to debris compaction consisting of Fe, Cr, Mn, Al and Ni, from both pin and disk, according to X-ray elemental maps. By comparison, the surfaces of the worn pins undergoing room-temperature wear tests had a heavily deformed sublayer and there were obvious gapping places between the sublayer and the original homogeneous pin material beneath. Wear occurred by both two-body and three-body abrasion at both temperatures.

The effects of alloy stoichiometry on the tribological properties of B2-structured FeAl alloys were investigated using four different FeAl alloys (40, 43, 48, and 50 at. % Al). Room temperature pin-on-disk tribotests were performed against a yttria-stabilized zirconia counterface in four different environments: air, argon, 4% hydrogen/nitrogen and oxygen. Except for Fe-43Al, it was found that the alloys had much lower wear rates in oxygen-free environments where the wear rates decreased with increasing aluminum. Interestingly, the wear rates were not affected by the presence of hydrogen. The tips of the wear pins were examined using both scanning electron microscopy and transmission electron microscopy, the latter using specimens produced by focused ion beam milling. The results indicated that two-body and three-body abrasive wear, as well as plastic deformation and delamination were the main wear mechanisms. The abrasive particles consisted of the counterface material and oxidized FeAl particles. Zirconia particles were also found to be embedded in the tribolayer on the worn tips of the pins.

B2-type intermetallic compounds generally have low ductility at room temperature. There are several B2-type intermetallic compounds with high ductility such as Ni-Al and Ti-Ni based alloys. In the former, the γ-phase decomposed at the grain boundaries plays an important role in the enhancement of ductility[1]. On the other hand, the ductility of Ti-Ni based alloys is due to the increment in the effective number of slip systems with the formation of deformation twins[2]. We have recently developed B2-type Zr-Co based alloys with enhanced ductility by partial replacement of Co with Ni[3]. We concluded that the marked enhancement of ductility is the result of transformation-induced plasticity, which is attributed to the presence of deformation-induced martensite. In the present study, we focused on an Fe-Co alloy with an α'-phase having a B2 structure at room temperature. We investigated the relationship between the microstructures and the mechanical properties of B2-type Fe-Co-Ni ternary alloys, since the substitution of Ni for Fe or Co in the Fe-Co alloys produces a γ-region at room temperature. Fe-40at%Co-10at%Ni and Co-40at%Fe-10at%Ni alloys annealed at 873K consisted of dual phases with an α' matrix and a γ-phase at the grain boundaries. The volume fraction of the γ-phase in those two alloys was about 12.6 % and 16.0 %, respectively. Grain refinement was also achieved in both these alloys. The yield strength, tensile strength, and total elongation increased by the substitution of Ni. It is noteworthy that Fe-40at%Co-10at%Ni and Co-40at%Fe-10at%Ni alloys have a high total elongation of more than 10%, while the Fe-50at%Co alloy has a very low total elongation of 1.4 %. In the tensile fractured Co-40at%Fe-10at%Ni alloy, (111)γ deformation twins were observed in the γ-phase. Notably, deformation-induced martensite with L10 structure was formed near the fractured edge in the Fe-40at%Co-10at%Ni alloy. Therefore, high ductility is not only considered to be due to only grain refinement and γ-phase formation, but also deformation induced L10 martensite. [1] K. Ishida, R. Kainuma, N. Ueno and T. Nishizawa : Metall. Trans., 1991, Vol. 22A, pp. 441-446. [2] M. Nishida, K. Tanaka, S. Li, M. Kohshima, S. Miura and M. Asai : J. Phys. IV, 2003, Vol. 112, pp. 803-806. [3] M. Matsuda, K. Hayashi and M. Nishida : Mater. Trans., 2009, Vol. 50, No. 9, pp. 2335-2340.

Metal thermal protection system (TPS) is one of the key techniques of the reusable launch vehicle (RLV).To satisfy the need for lightweight materials used on panel of metal thermal protection system, Ti-Al intermetallic compound and NiCoCrAl alloy have been selected to be as parent material and flexibility-enhanced material respectively. Large sized TiAl/ NiCoCrAl micro laminated sheet was deposited by electron beam physical vapor deposition (EBPVD) technology with dual-target in this work, and then the microstructure was studied by using XRD and SEM, and the mechanical properties of deposited micro-laminated sheet were characterized by using nano-indentation and tensile test at room or high temperature, after that, the toughening mechanism was analyzed according to fractography and structural characteristics of micro-laminates. The results show that TiAl/NiCoCrAl micro-laminated sheet is alternating overlaid by NiCoCrAl of 11 layers and TiAl of 10 layers with flat and well combined interface. Thereinto, TiAl layers are composed of γ\α2 and tau; phase with small component fluctuations. NiCoCrAl layers are composed of Ni3Al\γ-Ni and NiCrCo phase with the microstructure of flat columnar crystal. The mechanical properties of TiAl-based micro-laminated sheet are much better than that of the TiAl-based alloys sheet at room and high temperature. Especially at about 1023K, the toughening effect of TiAl/NiCoCrAl micro-laminated sheet is the best, and its elongation percentage reaches 72.2%. the presence of NiCoCrAl layers will lead to the cracks stagger along the inter-laminar interface or the layer due to which micro laminate expresses a good characteristic of delayed fracture. The toughening mechanisms are that the crack deflection and micro-bridge connection caused by the toughening layers increases crack propagation resistance.

Recently, Nb-Si based alloys have attracted lots of attentions as potential candidate materials for ultra-high temperature application, because of the low density and high melting point. However, it is still very difficult to obtain materials with the good balance of high temperature strength and room temperature toughness. To solve this issue, microstructure-control is considered to be a very good method. During the microstructure-control of Nb-Si based alloys with eutectic reaction (L->Nb + Nb₃Si) and eutectoid reaction (Nb₃Si -> Nb + Nb₅Si₃), the key is the control of Nb₃Si phase stability. In previous reports, it was revealed that elements had different effects on the stability of Nb₃Si, i.e., Mo, and W (>3 at%) destabilize Nb₃Si phase. In contrast, Ti and Ta stabilize Nb₃Si phase. Hereby, the effect of the combination additions of stabilizing, destabilizing, and an accelerating element (Zr) of Nb₃Si phase were investigated, such as Cr and W, Cr and Ta, Cr and Zr. According to the SEM observation, different microstructures were obtained with different combination of additives, and fracture toughness at room temperature and strength at high temperatures of these samples were also evaluated to reveal the effects of the microstructure on mechanical properties of Nb-Si based alloys.

We developed a novel austenitic heat resistant steel, Fe-20Cr-30Ni-2Nb (at.%), strengthened by two intermetallic phases, Fe₂Nb (TCP) and Ni₃Nb (GCP), based on our phase diagram study on Fe-Ni-Nb ternary system at elevated temperatures. In this study, influence of Mo in the phase equilibria among the γ-Fe/TCP/GCP phases has been investigated in order to understand the Mo effect on phase stability of the intermetallic phases with respect to that of the γ phase. The alloys with compositions of Fe-(30~60)Ni-(5~15)Nb-(5~15)Mo (at.%) were prepared by arc melting, and then equilibrated at 1373 or 1473 K, followed by aging at 1073 K for up to 480 h. The two-phase region of γ+ε and γ+δ exists in Fe-rich and Ni-rich side, respectively, in the quaternary system. However, the liquid and hP24 (GCP) phases existing in the region of 60 at.% in Fe-Ni-Nb ternary system at 1473 K disappears by addition of 10 at.% Mo. Instead, in addition to a small amount of Fe₅Mo₃-R phase, Fe₇(Nb,Mo)₆-mu; phase in equilibrium with γ-Fe phase appears. This indicates that Mo stabilizes the mu; phase and thus leads to a transition peritectoid reaction of ε+δ→γ+mu;. EPMA analysis revealed that Mo partitions more to ε (TCP) phase than γ phase, whereas it partitions more to γ phase than δ (GCP) phase. Note that the partition coefficient of Nb between ε and γ phases, k_Nb^ε/γ, and that of between δ and γ phases, k_Nb^δ/γ, remain unchanged with the value larger than 1 regardless of the presence of Mo in solution. These results clearly indicate that Mo stabilizes ε phase against γ phase, but it destabilizes δ phase against γ phase. The results at 1373 K will be presented, and the temperature dependence of the phase stability will be discussed. Part of this study was carried under the research activities of “Advanced Low Carbon Technology Research and Development Program” (ALCA) in JST (Japan Science and Technology Agency).

Previous our studies have shown the possibility of the laser control of the self-propagated high-temperature synthesis (SHS) of the TixAly intermetallic phases during the selective laser sintering (SLS) in the Ti-Al powder mixtures [1]. The main goal of this study was the titanium aluminide phase synthesis with a changing ratio of mixture powders (from 0 to 90 wt.% of Al and from 90 to 10 wt.% of Ti) during direct laser metal deposition process (DLMD) on titanium substrate. Three dimensional (3D) laser cladding technology with co-axial Ti and Al powder injection was used for the complex functional graded structure (FGS) fabrication in a single-step route. Applicability of argon and nitrogen gas environment was tested. The process operational windows and relationships between the main laser cladding parameters and the intermetallic phase structures of the built-up objects are determined. Optical microscopy, microhardness and SEM with EDX analysis of the laser-fabricated intermetallics are examined. Mechanisms of TixAly (x, y = 1..3) intermetallic transformations in exothermal reactions are discussed. [1]. Morozov Yu.G., Nefedov S.A., Shishkovsky I. V. et al. Study of selective laser sintering conditions at Al-Ti powder system. // Izvestiya Akademii Nauk. Seriya Fizicheskaya. 2002. Vol. 66. Issue 8. P. 1156-1158. Keywords: Direct laser metal deposition (DLMD); the laser control of the self-propagated high-temperature synthesis (SHS), functional graded structures (FGS), titanium aluminide phases.

Gray cast irons have been used as various kinds of sliding materials because they exhibit low friction coefficients under the oil lubricating conditions. However, the gray cast irons have low strength and low wear resistance. Therefore, the surfaces of the gray cast irons are sometimes nitrized and quenched to increase the strength and wear resistance, although they do not exhibit good corrosion resistance. The aluminizing, boronizing, chromizing and siliconizing of the gray cast irons may improve these problems. In this study, some gray cast iron specimens were coated by aluminizing, boronizing, chromizing and siliconizing, and their microstructure, microvivkers hardness and tribological properties were examined. The surfaces of the aluminized, boronized, chromized and siliconized specimens mainly consisted of FeAl, Fe2B, (Cr, Fe)23C6 and FeSi phases, respectively. Also, the surface of the boronized specimen exhibited the highest microvickers hardness of all the specimens. The aluminized, boronized and chromized specimens exhibited the friction coefficients as low as the non-coated specimens when they were slid against AISI 52100 steel balls in poly-alpha-olefin. In addition, the boronized and chromized specimens exhibited much higher wear resistance than the non-coated specimens. According to the XPS analyses, the worn surfaces of the aluminized, boronized, chromized and siliconized specimens were covered with iron oxides, Al2O3, B2O3, Cr2O3 and SiO2. It is considered that these oxides were formed by the reactions of the FeAl, Fe2B, (Cr, Fe)23C6 and FeSi layers with oxygen and water molecules in the oil and air. It is also considered that the low friction coefficients of the aluminized, boronized and chromized specimens were due to the formation of low friction and thin Al(OH)3, H3BO3 and CrO3 films by the reactions of the Al2O3, B2O3 and Cr2O3 with oxygen and water molecules during the friction and wear tests, respectively.

Refractory metal disilicides are promising candidates of ultra-high-temperature structural materials. Among them, we have recently developed NbSi2/MoSi2 composites. In the C40-NbSi2/C11b-MoSi2 composites, peculiar lamellar microstructure is developed by the appropriate preparation method [1]. Such a microstructure is confirmed to be effective to improve the mechanical properties of the duplex-phase silicides [2,3]. We demonstrated that alignment of the lamellar structure is stable in annealing up to 168 h at 1400 degreeC. For practical use of this crystal, however, the thermal stability of the C40/C11b lamellar structure should be further improved. To achieve this, in this study, the effect of fourth element additions in (Mo0.85Nb0.15)Si2 crystals was examined. The improvement of the thermal stability of microstructure was indeed achieved by the addition of several kinds of fourth-element. Among them, the effect was especially significant in Cr- and Zr-bearing crystals. It was quantitatively evaluated by the Moiré fringe analysis in TEM that the local misfit strain at the interface in those crystals becomes much smaller than others. This must lead to the suppression of growth of C11b-phase precipitates that do not have the variant orientation relationship with respect to C40 matrix phase, resulting in the improvement of thermal stability of the lamellar microstructure. The present results suggest that the atomic size ratio of additional element with respect to Mo, which is the main constituent element in the C11b-phase, must be one of the important factors to control the misfit strain at the lamellar interface. However, the existence of other controlling factors was also supposed since the addition of Zr found to exhibit the large improvement in thermal stability of the lamellar microstructure, nevertheless it has much larger atomic radius than Mo. [1] T. Nakano, Y. Nakai, S. Maeda, Y. Umakoshi, Acta Mater. 50 (2002) 1781. [2] T. Nakano, K. Hagihara, Y. Nakai, Y. Umakoshi, Intermetallics 14 (2006) 1345. [3] K. Hagihara, T. Nakano, Acta Mater. 59 (2011) 4168.

Earlier work (Zeng et al., J Appl. Phys., 99 (2006) E08902-1) has shown that producing a nanocrystalline microstructure can have beneficial effects on the magnetic properties of L1o-structured, metastable, tau;-phase MnAl powders. One way to obtain such nanocrystalline powders is by high-energy mechanical milling. In the present work we focus on Mn- 46 at. % Al. gas atomized to 325-mesh powder in the presence of argon and then mechanically milled using a water-cooled union process attritor. X-ray diffraction patterns of the gas-atomized powders showed mostly the presence of the unstable high-temperature ε-phase with significant amounts of the equilibrium γ2 and β phases that increased with decreasing particle size. Using X-ray line broadening analysis, a grain size of 12nm was determined for powders milled for 20 hours. Annealing at a variety of times and temperatures was performed to obtain the ferromagnetic tau;-phase. Magnetic measurements show a coercivity of up to 4.2 kOe and a saturation magnetization of up to 108 emu/g for the powder annealed at 500°C for 10 mins. The results provide insights into the possibilities of commercialization of rare earth-free MnAl permanent magnets.

Recently, TiSi2 is used as contact and interconnect in industry. However, due to the low resistivity, low lattice mismatch with silicon, comparably low annealing temperature for silicidation, and high eutectic point, CoSi2 came into the spotlight. Various methods are widely used for applying the silicides in ULSI industry; titanium interlayer mediated epitaxy (TIME), oxide-mediated epitaxy (OME), and nitride-mediated epitaxy. We fabricated the vertically aligned CoSi2 NWs using aqueous electroless etching (AEE) method, atomic layer deposition (ALD), and rapid thermal annealing (RTA) process in this paper. Furthermore, we compared the difference of CoSi2 formation according to the two deposition methods of Co layer on silicon substrate; PVD and ALD. Vertically aligned SiNWs were grown by immersing the Si wafer sample into the aqueous solution of HF and AgNO3. Then, using PE-ALD process, the 20-nm-thickness Co layer was deposited on SiNWs and then, Ru capping layer of 20-nm-thickness was also deposited on the Co/SiNW sample with in-situ method. Deposition of Co layer and Ti capping layer on Si substrate by magnetion sputtering was also carried out for comparing the different aspects of silicidation between ALD and PVD. Co film of ~20 nm-thickness was deposited on silicon substrate followed by in-situ deposition of Ti layer with thickness of ~12nm on Co/Si(001). The two-layer stacked samples were annealed by RTA process. Before the formation of silicide NWs, we fabricated planar CoSi2. We measured the x-ray diffraction (XRD) spectra of PVD-Ti/PVD-Co/Si (100) annealed by RTA. Before RTA treatment, α-Ti (002) and β-Co (111) peaks appeared at 38.1 ° and 44.4 °, respectively. After the RTA treatment, the diffraction peaks are observed at 28.7 °, 29.96 °, 33.2 °, and 47.96 °, which can be indexed as diffraction peaks of the CoSi2 (111), CoxTiyNz, CoSi2 (200) and CoSi2 (220), respectively. CoxTiyOz phase was formed during RTA process. With the diffractions peaks, we demonstrated that the polycrystalline CoSi2 phase was formed. In the XRD spectra of ALD Ru/ALD Co/SiNW annealed by RTA. adiffraction peak was observed at 34.86 °, which can be indexed as the diffraction peak of the CoSi2 (200). Comparing with the CoSi2 formed by PVD, the CoSi2 (220) peak was not appeared. We suggested that the reason is that the surface characteristic of silicon wafer was different. For example, the silicon substrate could be damaged more during the sputter process compared to the PE-ALD for deposition of Co layer. Therefore, due to the difference of interface conditions between Co layer and silicon substrate, the orientations of CoSi2 were different. For fabricating CoSi2 nanowires, Co and Ru capping layer were deposited on SiNWs grown by AEE using PE-ALD. After RTA treatment, the CoSi2 (200) peak was observed by XRD measurement and we suggested that the orientation of CoSi2 depends on the interface conditions between Co layer and silicon substrate.

Most intermetallic compounds exhibit very low levels of ductility at room temperature. Despite their relatively simple crystal structures, even many of B2-type intermetallic compounds are not exceptional. However, a polycrystalline B2-type equiatomic ZrCo compound has an exceptionally high tensile elongation of 7% [1]. It has been reported that the martensitic transformation from the B2 to the B33 structures takes place by a substitution of Ni for Co in ternary Zr-Co-Ni alloys [2]. We therefore expect that the tensile ductility at room temperature of ternary Zr-Co-Ni alloys is further enhanced by the martensitic transformation. In the present study, we have investigated the relationship between the microstructures and mechanical properties in the ternary Zr50Co50-xNix alloys substituting Ni for Co. The mechanism of the ductility enhancement is also discussed on the basis of the scanning and transmission electron microscope observations of the tensile specimens. The tensile strength and elongation increased remarkably by substituting Ni for Co. The Zr50Co39Ni11 alloy had the extremely high total elongation of 23%. In the Zr50Co39Ni11 alloy deformed at room temperature, deformation-induced martensite with the lenticular morphology was observed in the B2 parent phase near the fractured edge, in addition to dislocation with the <100>B2-type Burgers vectors. The deformation-induced martensite was dominantly formed and grown at the stress-concentrated area. It was also observed that many microcracks existed perpendicular to the tensile direction on the surface near the fractured edge in the Zr50Co39Ni11 alloy. The occurrence of microcracks increased with increasing the residual strain. The microcracks were formed along the interface between the lenticular martensite variant and B2 parent phase, and between the martensite variants. Consequently, we conclude that the remarkable enhancement of the ductility in the B2-type Zr-Co-Ni alloy is due to the transformation-induced plasticity associated with the deformation-induced martensite and the microcrack formation. [1] T. Yamaguchi, Y. Kaneno and T. Takasugi: Scr. Mater. 52 (2005) 39. [2] E. M. Carvalho and I. R. Harris: J. Less-Common Metals 106 (1985) 143.

Abstract It has been known that the intermetallic compound, with CaF2 structure has interesting properties such as physical and optical property. The color of the intermetallics PtAl2 is gold in cleavage plane and mirror finished surface [1]. This surface is the texture formed by plastic working such as polishing and porphyrization. In this work, we have investigated the relationship between the texture and the optical properties by X-ray diffraction (XRD) and electron back scattering diffraction (EBSD). Samples of PtAl2 ingot were prepared by arc melting appropriate amounts of the components under argon atmosphere followed by annealing for homogenization. Samples were mechanically polished with alumina powders down into 0.05 mu;m diameter. These polished samples were characterized by XRD and EBSD. As a result of polishing, PtAl2 surface has anisotropic texture. Detailed relationships between the texture and the optical properties of the bulk PtAl2 will be discussed at the poster session. Reference [1] Liang-Yao Chen and D.W Lynch, phys.stat.sol.(b) 148, 387 (1988)

The high temperature compressive deformation characteristics of a series of ternary Cr-Cr₂Ta based alloys with Si additions have been studied. Alloys of composition Cr_90-xTa₁₀Si_x, with x=3, 5 10 and 15 at.%, were prepared by vacuum arc-meltingbefore being heat treated at 1000 ^oC for 1000 hours. The microstructure of the alloys following the heat treatment was found to comprise primarily of a Cr-rich A2-solid solution and a C14-Cr₂Ta Laves phase. Only at higher concentrations of Si, was the occurrence of a third Cr₃Si phase observed. These samples were subjected to compression testing at 1000 and 1100 ^oC to determine the high temperature yield strength of the alloys as a function of the Si content. Compression of selected alloys was also carried out at other temperatures and a monotonic decrease in the yield strengths of the alloy was observed with increasing temperature. The alloys were found to retain significant strenght at elevated temperatures. Post-deformation studies of the microstructure of the alloys showed a high dislocation density in the metallic solid solution phase with significantly lower dislocation density in the intermetallic phase. The Burgers vectors of dislocations in the hard Laves phase were characterized and found to lie on the basal plane of the structure.

A Ni3(Si,Ti) intermetalic alloy with an L12 crystal structure shows a unique high temperature hardness property; the reduction of hardness with increasing temperture for this intermetallic alloy is considerably small compared with that for conventional metallic materials. Therefore, the Ni3(Si,Ti) alloy is expected as new wear resistant materials at high tempareture. However, the hardness of this intermetallic alloy at room temperature is inferior to that of existing wear resistant materials. It is well known that dispersion strengthening by hard particles such as carbide, niteride and boride is an effective method enhancing the hardness of metallic materials. These composite material can not be made by melting and casting method. Alternatively, they are fabricated by powder metallurgical method. In this study, the Ni3(Si,Ti) intermetalic alloy was synthesized by the powder metallurgy method using elemental powders and atomized (alloyed) powders. The raw powder mixtures with various compositions were sintered and densificated by a spark plasma sintering apparatus and then homogenized at high temperatures. The effects of the chemical composition of powder mixtures, sintering condition such as temperature and heating rate, and homogenization temperature on the microstructure and the mechanical properties were examined.

The dual two-phase Ni3Al-Ni3V intermetallic alloy is a new type of high temperature structural materials which have been developed in our laboratory. A feature of this alloy is its unique microstructure: the dual two-phase microstructure is comprised of primary cuboidal precipitates of Ni3Al (L12) phase and channel regions of Ni3V (D022) and Ni3Al (L12) phases. Owing to this microstructure, the mechanical properties of this intermetallic alloy are superior to those of many conventional superalloys. However, it is a critical issue to reduce the content of constituent element V that is expensive and harmful particularly for oxidization resistance at high temperature. Therefore, the objective of this study is to establish alloy designing which can reduce the amount of V, without degenerating the dual two-phase microstructure, i.e., maintaining their superior mechanical and chemical properties. Nb and Cr were chosen as additive elements because they have a large partition coefficient to the channel region and result in the valence electron almost the same value as V, thereby maintaining the dual two-phase microstructure, via keeping the phase stability of D022. A nominal composition of Ni75Al10V15 doped with 50 wt.ppm boron (expressed by at.% except for boron) was used as a base alloy composition. Nb and Cr were substituted for V and for V, Ni/V and Ni, respectively. The alloy ingots prepared by arc melting in argon gas atmosphere were annealed at 1553 K for 5 h in a dynamic vacuum, followed by furnace cooling to room temperature. It was found that the dual two-phase microstructure was maintained even though reducing the amount of V by 7 at.%, 7 at.%, and 10at.% by substituting of Cr for V, Cr for Ni/V, and Cr for Ni, respectively. The alloys with reduced V content showed a similar to or higher levels of hardness (and strength), and also an improved oxidation resistance than the base alloy.

The Ni base dual two-phase intermetallic alloys composed of Ni3Al (L12) and Ni3V (D022) phases have been developed by the present authors. These intermetallic alloys show excellent mechanical properties and high phase stabilities at high temperature compared with commercial nickel base superalloys. So far it has been demonstrated that solid solution hardening by the addition of Ti and Nb is very effective for improving mechanical properties of these intermetallic alloys. In the present study, to further improve the mechanical properties, the effect of Re addition on microstructure and hardness of the dual two-phase intermetallic alloys was investigated as functions of alloying (substituting) method of Re as well as aging condition (temperature and time). Re was added to the base alloy composition by three methods: Re was substituted for Ni, Al and V, respectively. Alloy ingots prepared by arc-melting were solution-treated at 1280 °C and then aged at lower temperatures of 850 - 975 °C. The alloy where Re was substituted for Ni showed higher hardness than the base alloy in short aging time. On the contrary, the alloys where Re was substituted for Al or V did not show age hardening behavior. These results were discussed on the effects of the phase separation and ordering in the channel region, and hardening due to Re-rich phase precipitation.

Ferromagnetic shape memory alloy NiMnGa is a promising candidate for a new actuator material showing large magnetic-field-induced strain (MFIS) and fast response. Although the single crystal of NiMnGa with 5M martensite generates about 6% MFIS by applying magnetic field, the shape change does not recover automatically by removing magnetic field. Thus, we have proposed and developed NiMnGa/polymer composite materials which are composed of NiMnGa particles and polymer matrix. Expected MFIS of the composites is around 1% in 50vol%NiMnGa/silicone composite. In previous works single-crystal-like NiMnGa particles were fabricated by mechanical crash utilizing intrinsic intergranular brittleness of NiMnGa polycrystal ingots. However, MFIS of the composites containing mechanical-crushed NiMnGa Particles was much smaller than theoretically calculated values. One of the possible reasons why the experimental MFIS was smaller than expected is that the reorientation of martensite variants is prevented by lattice defect introduced by transgranular deformation during mechanical crash. Therefore, in this study, in order to reduce the lattice defects introduced during mechanical crash, Bi addition was attempted to promote intergranular embrittlement of NiMnGa polycrystal ingots. Bi addition was expected to enhance the intergranular embrittlement by grain boundary segregation in a similar manner to that of Ni ingots. Ni-28at%Mn-22at%Ga alloys with 0-0.3at%Bi (0-1wt%Bi) were fabricated by Ar arc melting method. After the homogenization at 1273K for 86.4ks in vacuum, phase transformation temperatures were determined by differential scanning calorimetry (DSC) measurement. Compression tests were performed at temperatures below 573K and chemical analysis and microstructural observation were carried out by scanning electron microscopy (SEM) equipped with an X-ray energy dispersive spectroscopy (EDS). From the results of DSC measurements, it was confirmed that the martensitic transformation temperatures were not changed by Bi addition. By SEM observation it was found that the intergranular embrittlement was promoted by grain boundary segregation of Bi, and that the intergranular embrittlement was remarkably enhanced when NiMnGa was at the austenite state than at martensite state. This must be due to the stress relaxation happened by twin deformation in the martensite phase.

Hardened steels such as tool steels and bearing steels are widely used as wear resistant materials. The hardness of these steels shows high hardness at room temperature but significantly decreases at high temperature due to microstructural change, resulting in limitation of working temperature. Recently, our research group has developed two kinds of wear resistant materials based on Ni3X type intermetallic phases. One is a Ta/Re-added Ni3Al (L12)/Ni3V (D022) two-phase intermetallic alloys. Another one is a Ta (or Nb)-added Ni3(Si,Ti) (L12) intermetallic alloys (NST alloys). Both intermetallic alloys show favorable hardness property as high temperature wear resistant materials: the reduction of hardness with increasing temperature is very small compared with conventional metallic materials such as hardened steels. Taking advantage of their hardness property, the Ta/Re-added two-phase intermetallic alloy has been applied to FSW (Friction Stir Welding) tools used to join type 430 stainless steel plates with 1.0 mm thickness. It was found that the intermetallic FSW tool has successfully joined type 430 steel plates up to a total welding length of 20,000 mm (20 m) without apparent decrease in weight and length. On the other hand, the NST alloy has been applied to inner and outer rings of ball bearing. After rotation test at 600°C for 100h, wear loss of the ball bearing using the NST alloy rings is considerably small in caparison to that using the type 440C stainless steel rings. The estimated life time at 600°C for the ball bearing using the NST alloy rings was longer by eighty times than that for the ball bearing using type 440C stainless steel rings. Consequently, these results clearly indicates that the developed intermetallic alloys are promising wear resistant materials which can be used at high temperature.

The Nd-Fe-B hard magnet which is used in hybrid cars needs the Dy addition in order to improve its high temperature performance. Reduction in the Dy usage has been an urgent issue since the supply of the Dy metal is heavily influenced by political circumstance. One of the promising magnets is Sm2Fe17Nx which possesses good high temperature characteristics. Its theoretical magnetization is 1.54T, slightly lesser to 1.61T of the Nd-Fe-B magnet. However, its anisotropy field reaches 11MA/m, higher than that(6.1MA / m) of the Nd-Fe-B magnet by a factor of two. Furthermore, its Curie temperature is 373K, much higher than that of the Nd-Fe-B magnets. Therefore, the Sm2Fe17Nx magnet has been expected to be a promising candidate as an alternative material of the Nd-Fe-B magnet. However, the fabrication of the Sm2Fe17Nx magnet in the bulk form is difficult since it is likely to decompose into a soft magnetic phase α-Fe and non-magnetic phase SmN above 873K. For this reason, the Sm2Fe17Nx magnet has currently been produced as resin bonded magnets. In this work, we have adapted high pressure sintering method in order to produce a high density bulk magnet and investigated the effect of the pressure on the magnetic properties as well as on the microstructure. We have prepared Sm2Fe17Nx magnets sintered under various pressure at 623K. The Sm2Fe17Nx that was sintered under 400MPa show no decomposition, and the relative density is 95.7%. In the presentation, we will discuss the effect of the pressure effect on the magnetic properties and the microstructure.

MoSi₂ alloys are leading candidates for ultra-high temperature structural materials for applications of gas turbine power generation systems because of their high melting point, lightweight and good resistance to oxidation. The diploidization of C11_b type structure of MoSi₂ and C40 type structure of NbSi₂ forms a lamellar structure. This lamellar structure improves C40-NbSi₂/C11_b-MoSi₂ duplex silicide room-temperature toughness and creep strength. The thermal stability of the lamellar structure should be improved for practical applications of this material. In a previous study, Cr addition into NbSi₂/MoSi₂ duplex silicide was found to be effective to improve the thermal stability of the lamellar structure in which Cr atoms segregated to the lamellar interface [1]. But it is unclear whether Cr is the most optimum additive element that stabilizes lamellae, and the segregation mechanism is difficult to clarify only by experiment. On the other hand, we can simulate segregation phenomena to various interfaces using phase-field simulation based on thermodynamics and elastic theory [2]. The main objective of this study is to examine the segregation behavior of various alloying elements. In this study, we considered not only the bulk chemical free energy but also the segregation energy evaluated by first principles calculation at the interface to reflect interaction between solute and interface. The simulation suggested that segregation behaviors greatly differ depending on the additive elements. In the case of Cr-addition, C11_b and C40 bulk phase became equilibrium state by the distribution of solute atoms. The C40 phase became enriched with Nb and Cr, the C11_b phase became enriched with Mo, which agreed with the equilibrium phase diagram. At the interface, slight segregation of Cr atoms was observed, whereas Nb and Mo concentrations monotonically changed across the diffuse interface between C11_b and C40 phase. In another simulation of growth of C11_b phase from C40 matrix phase, Cr atoms significantly piled up in front of the interface during the interfacial migration. Significant segregations of Zr and Hf to the static interface were formed, which were attributed to the chemical interaction between solute atoms and static interface. This study is supported by the advanced low carbon technology Research and Development Program of Japan Science and Technology Agency. [1] K.Hagihara, T.Nakano, S.Hata, O.Zhu and Y.Umakoshi, Scripta Mater., 63 (2010) 613. [2] Y.Koizumi, T.Nukaya, S.Suzuki, S.Kurosu, Y.Li, H.Matsumoto, K.Sato, Y.Tanaka, A.Chiba, Acta Mater., 60 (2012) 2901.

Physical and structural properties can influence the magnetic properties of a system, including hysteretic behavior. We will present the results of X-ray scattering performed on six Co/Pt multi-layer thin films with perpendicular magnetic anisotropy. The films were grown by 50 alternating layers of 0.7nm Pt and 0.4nm Co with chamber pressures between 3 and 20 mTorr. Measurements of the crystallinity, from the bulk lattice peaks of the Pt, provide an additional metric for determining the structural ordering of the films. We will compare the results of our current efforts to results obtained from earlier x-ray reflectivity, atomic force microscopy, and electron microscopy experiments from similar samples, as well as the magnetic properties obtained from magnetometry and magnetic force microscopy measurements. Understanding the correlation between the crystallinity and the hysteretic behavior and magnetic properties for these samples may help improve designs for magnetic media and increase the storage capacity of hard disk drives.

Recently Mg-TM (transition metal)-RE(rare earth) ternary alloys have received a considerable amount of attention as new light-weight structural materials since they exhibit high strength and high ductility simultaneously. Their excellent mechanical properties have been considered to be related closely to the existence of precipitates with long period stacking ordered (LPSO) structures of 18R or 14H types, both of which are characterized by periodic arrangements of stacking faults within the hcp stacking of parent Mg and also by enrichment of TM and RE atoms in atomic layers adjacent to the stacking fault. However, inherent characteristics of the LPSO phases including crystal structure and deformation mechanism have not been fully understood yet. As for the deformation behavior of single-phase LPSO phases, only limited studies using directionally solidified (DS) ingots composed of plate-shape grains with their (0001) being nearly perpendicular to the growth direction are available so far and studies using single crystalline ingots have never been carried out since it is quite difficult to fabricate single crystals of the Mg-Zn-Y LPSO phases. In the present study, micropillars of single crystalline Mg-Zn-Y LPSO phase with various loading axis orientations being parallel or inclined to (0001) and of varying dimensions were prepared from the DS ingot of the 18R-type LPSO phase by focused ion beam (FIB) technique. These single crystalline micropillars of the Mg-Zn-Y LPSO phase were deformed in compression using a micro hardness testing machine equipped with a flat diamond tip under a constant loading rate at room temperature (RT). When the loading axis is inclined to (0001), the basal slip is the dominant deformation mode irrespective of the specimen dimensions and the yield stress increases with decreasing the specimen dimensions. When the loading axis is parallel to (0001), kink-like deformation bands are observed to be formed in relatively large micro-pillars, whereas deformation marking totally different from the deformation bands is seen in the relatively small micro-pillars.

Hot-dipped galvannealed (GA) steels are widely used for the chassis of automobiles and building materials because of their high corrosion resistance, weldability, and paintability. The coating layer usually consists of a series of intermetallic phases in the Fe-Zn system, stacked on the steel substrate in the order of Γ(Fe₃Zn₁₀), Γ₁(Fe₅Zn₂₁), δ_1k(FeZn₇), δ_1p(FeZn₁₀) and zeta;(FeZn₁₃). When the GA steels are deformed under severe conditions such as press forming operation, the coating layer occasionally fails (powdering, flaking), resulting in reduced corrosion resistance and paintability. The coating failure has been understood only phenomenologically, and almost nothing is known about the mechanical properties of each of the intermetallic phases. In our preliminary compression experiments of polycrystalline micropillars of each phase prepared from the thin coating layer (~10 mu;m) of the GA steels, we found that the zeta; phase with a base-centered monocilinic lattice (Pearson symbol: mC28) exhibits plastic deformation to some extent followed by fracture. This fracture is considered to occur either because (1) the number of independent slip systems in the zeta; phase is insufficient although at least five independent slip systems are required for polycrystals to deform without crack generation, or because (2) possible presence of hcp-Zn phase between the columnar zeta; grains induces cracks to initiate from the Zn/zeta; interface. In the present study, we investigated the deformation modes of the zeta; phase via compression tests of micropillar specimens prepared from small strip-shaped single crystals (~100×300×5000 mu;m) by using the focused ion beam (FIB) method with varying loading axis orientations. The compression tests of the micropillars were performed with a micro hardness testing machine equipped with a flat diamond tip at room temperature. For most orientations, slip on the {110} plane was confirmed to operate by slip trace observations on two orthogonal surfaces of specimens. The slip direction for the {110} slip was found to be <1-12> judging from the change in the specimen shape before and after compression. The (100)[001] slip system also can operate though it was confirmed only for a very narrow region of orientations for which the schmid factor of {110}<1-12> is close to zero. The value of the critical resolved shear stress (CRSS) for the (100)[001] slip system is approximately four times that of the {110}<1-12> slip system. If any other slip systems were observed, their CRSS values would be extremely higher. The deformation of the zeta; phase in a polycrystalline form is difficult because the number of the independent slip systems is insufficient.

The development of a new high temperature structural material is recently required in various field. As one of the potential materials, Nb-Si alloys have attracted attention due to their high melting point and low density. In a previous study, Miura et al.[1] aimed to strike a balance between high temperature strength and fracture toughness at room temperature, and it was revealed that they could obtain a suitable microstructure composed of ductile Nb matrix containing finely dispersed spheroidizing Nb5Si3 phase by the addition of ternary elements such as Au. Moreover, introducing Nb dendrite is effective for improving fracture toughness. Thus, the main purpose of the present study is evaluating fracture toughness of Nb-Si-X(X:Au,Os,Re) alloys using small specimen and investigating effects of the microstructure and minor elements on the fracture toughness. Alloy ingots of Nb-15at%Si-3 at%X(X:Au,Os,Re) are obtained by arc-melting under Ar atmosphere. Heat-treatments were conducted at up to 1500 degree C for 100 hours. The specimen size was a dimension of about 1.0×2.0×10mm and a Chevron notch with angle of 90 degree was cut. Four-point bending tests were conducted under a laser confocal microscope for in-situ observation of crack propagation. The fracture toughness at room temperature was determined from notch depth and fracture load for all specimens, and the effect of the microstructure and minor elements is investigated. [1] Seiji Miura, Toru Hatabata, Tetsuo Mohri, Plasticity 2012 Proceedings, p118-120(2012)

To understand the fundamental mechanism of segregation and ordering of solutes in the Mg-based alloys with long period stacking-ordered (LPSO) structures, it is important to elucidate the solute-trapping behavior at the stacking faults (SFs) and also the effect of solutes on the stability of the SFs. In this study, we investigate the interaction energies and trapping effects of various solute atoms in the vicinity of the SFs in Mg using first-principles calculations based on density functional theory (DFT). We examine the solid solution properties of Mg-TM (TM=Zn, Al, Co)-RE (RE=Y, Gd) single crystals containing SFs using the Vienna Ab-initio Simulations Package (VASP). Total-energy minimization calculations are performed using the projector-augmented-wave method within the Perdew-Wang generalized-gradient approximation. The calculations employ the supercells of Mg containing 96 atoms (6 atomic layers along the [0001] direction) with different stacking sequence, i.e., hcp, fcc, type I₁ and I₂ intrinsic SFs, etc., in which one or two Mg atoms are replaced by Zn, Al, Co, Y and Gd atoms, respectively. A plane-wave basis set with 262.6~345.9 eV kinetic energy cutoff is adopted. We also employ a 10×12×10 Monkhorst-Pack k-mesh for carrying out the Brillouin-zone integration. The solute trapping by SFs in Mg can be quantitatively characterized by the difference between the dissolution energies of the solute atom in the hcp lattice and in the faulted lattice plane (ΔE). In the case of Zn, there is no significant difference among the dissolution energies in hcp-, fcc- and faulted Mg lattices, and thus Zn-trapping behavior by SFs is not clearly observed. On the other hand, we found that Y is more easily dissolved in fcc-Mg than in hcp-Mg (ΔE = 0.10 eV), so that Y-trapping energies are evaluated as 0.04 eV at the type I₁ (hch stacking) SF plane and 0.10 eV at the type I₂ (hcch stacking) SF plane, respectively. Also, in the case of Y, it is noteworthy that the weak trapping of Y is clearly observed even at the normal hcp layers neighboring to SFs, i.e., ΔE = 0.04 eV at the “hhc” stacking and ΔE = 0.06 eV at the “chc” stacking, respectively. This indicates that the energetics of the dissolution and trapping of Y in Mg is certainly affected by the stacking sequence of at least quintuple (five neighboring) layers along the [0001] direction. In addition, the above results correspond to the recent experimental results on the formation of TM₆RE₈ cluster in Mg-TM-RE LPSO phases, which demonstrate that RE enrichment occurs in four consecutive close-packed planes (hcch stacking), but the level of RE enrichment is higher in the inner two layers than in the outer layers of the quadruple RE-enriched layers. This study was supported by Grant-in-Aid for Scientific Research on Innovative Area, “Synchronized LPSO Structure,” No. 23109004.

Excess thermal vacancy hardening and positive temperature dependence of yield stress (strength anomaly) are two major topics in B2 FeAl. “Solid solution” hardening by thermal vacancies is one of the mechanisms for explaining the strength anomaly. In this theory, the newly generated thermal vacancies in the strength anomaly temperature range impede dislocation motion by the “solid-solution” hardening mechanism. However, several aspects still remain unclear in this mechanism. 1) The extent of hardening is so strong that beyond the ability of point defects. 2) The strength anomaly significantly depends on crystal orientation. 3) Some other B2 intermetallic compounds with lower vacancy concentration also show strength anomaly. Since solid-solution hardening is due to a strain field caused by the size and modulus effects between solution and matrix atoms, the temperature dependence of the lattice parameter and elastic moduli of FeAl were investigated in this research. The size and modulus effects of thermal vacancies on the strength anomaly will be discussed. Alloys with the compositions of Fe-40, -43, and -46Al (at.%) were prepared by arc-melting. Homogenization heat treatment at 1100°C for 24h was conducted to the ingots, and then they were slowly cooled at 18°C /h and aged at 400°C for 100h to eliminate supersaturated vacancies. The hardening of excess thermal vacancies was measured by Vickers hardness tests. The temperature dependence of lattice parameter was measured by high temperature XRD using powder samples. The temperature dependence of elastic moduli was measured by both the Electro-Magnetic Acoustic Resonance (EMAR) and Free Resonance Methods. The hardness increases with increasing quenching temperature above 400°C, indicating an increase in quenched-in vacancy concentration. However, both the lattice parameter and the elastic moduli have a linear relationship with temperature. In other words, the size and modulus effects of thermal vacancies were not found in the strength anomaly temperature range. Thus, the “solid-solution” hardening by thermal vacancies may not be the main reason for the strength anomaly in FeAl.

In most L1₂ compounds such as Ni₃Al, Ni₃Si and Co₃Ti, anomalous positive temperature dependence of yield stress is observed at high temperatures. However, there are some other L1₂ compounds that do not exhibit the anomaly. Many of them are known instead to exhibit a large negative temperature dependence of yield stress at low temperatures. Pt₃Al is usually cited as the representative L1₂ compound for this second category. The difference in the temperature dependence of yield stress of the L1₂ compounds of the two categories has been understood in terms of the dissociation scheme of the dislocation with b=<110> and planarity of the core structure of the resultant partial dislocations. For those L1₂ compounds that exhibit the yield stress anomaly, the dislocation with b=<110> is believed to dissociate into two collinear superpartials with b=1/2<110> separated by an APB (anti-phase boundary). The core of each superpartial is believed to be planar. For those L1₂ compounds that exhibit a large negative temperature dependence of yield stress at low temperatures, on the other hand, the dislocation with b=<110> is believed to dissociate into two superpartials with b=1/3<112> separated by an SISF (superlattice intrinsic stacking fault). The core of the SISF-coupled superpartials is believed to be non-planar. However, no TEM study was made on the dislocation structures for Pt₃Al and nothing is known about the dislocation mechanism including the dissociation schemes for the large negative temperature dependence for yield stress at low temperatures. Therefore, the purpose of this study is to reveal the mechanism of the negative temperature dependence of yiels stress at low temperatures for Pt₃Al through the observation of the deformation microstructure using a transmission electron microscope (TEM). Master alloys with the compositions of Pt-27 at.%Al and Pt-29 at.%Al were prepared by arc-melting in an argon atmosphere. Single crystals were grown in an alumina crucible by the Bridgman method. The loading axes were [-1 2 12], [-1 2 25] and [-234]. Compression tests were carried out in the temperature range from 77 to 1273 K. Operative slip planes were determined by slip trace observations on two orthogonal surfaces of specimens. Slip trace analysis demonstrated that {001} slip system was operative for most crystal orientations while {111} slip system was operative for a narrow orientation region close to [001]. Superlattice dislocations on the (111) plane were dissociated into two collinear superpartials separated by APB, which is different from the dislocation dissociation scheme that have been believed previously. Furthermore, dislocations on (111) have a strong tendency to align along their screw orientation, forming many cusps and prismatic loops. This indicates that screw dislocations experience a high Peierls stress and that cross-slip occurs very frequently.

(RE,Mg)₂Ni₇-based hydrogen-absorbing alloys (RE: Rare Earth) have attracted a great deal of interest as negative electrode materials in nickel-metal-hydride (Ni-MH) batteries with very low self-discharge property and high capacity. Such attractive properties are closely related to the crystal structure of (RE,Mg)₂Ni₇ phases. For further development of new class of hydrogen-absorbing materials for negative electrodes, it is essential to understand the relationship between the crystal structure evolution and the hydrogen absorption/desorption properties. However, the structural change of ternary (RE,Mg)₂Ni₇ phases and also their parental RE₂Ni₇ phases during hydrogenation has not been fully clarified yet. RE₂Ni₇ is composed of block layers, each of which consists of two RENi₅ unit layers and one RE₂Ni₄ unit layer. In the case of La₂Ni₇, it has been reported that hydrogen is preferentially absorbed in the La₂Ni₄ unit layer at low pressure and the hydride formed at this stage is stable after desorption. The crystal structure of the hydride has been proposed to be characterized by the preferential and anisotropic expansion of the La₂Ni₄ unit layers along the stacking direction based on the powder X-ray and neutron diffraction experiments. However, the details of the crystal structure of the La₂Ni₇ hydride is still controversial. In this study, the structural change of La₂Ni₇ phase during hydrogenation was investigated by atomic-scale scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM). Atomic scale characterization of the La₂Ni₇ hydride by high-angle annular dark-filed (HAADF) STEM revealed that not only the anisotropic expansion of the La₂Ni₄ unit layer previously reported but also the atomic displacement of La (and possibly Ni) in the La₂Ni₄ unit layer occur during one-cycle of hydrogen absorption/desorption up to 1.2H/M, resulting in the structural change from hexagonal to orthorhombic or lower symmetries. Based on the HAADF-STEM observations and TEM diffraction analysis, a new model of the crystal structure evolution of La₂Ni₇ during hydrogenation will be proposed.

Low rare-earth nanocomposite magnets are expected to be a candidate for the next generation permanent magnets due to strong ferromagnetic coupling between the hard and the soft magnetic phases.The fabrication of nanocomposite magnet in a bulk form is particularly of importance for industrial applications.Hydrogenation-Decomposition-Desorption-Recombination (HDDR) reaction by mechanical milling under hydrogen atmosphere (rMM) with subsequent heat treatment is known as a method of synthesizing homogeneous nanostructure. In this study ,we aim to investigate the phase changes which take place during the rMM process in order to obtain insight for the synthesis of SmCo/α-Fe nano-crystalline magnet and SmCo/α-Fe bulk nanocomposite magnets also utilizing Spark-Plasma-Sintering (SPS) of the reactively milled powders.As the starting materials, we used Sm2Co17/α-Fe alloys which exhibit high magnetic performance.This SmCo/Fe alloys were pulverized by mechanical milling under hydrogen atmosphere.The particle diameter and form after the pulverizing process, were probed by Scanning-Electron-Microscope(SEM),and the phase changes which take place during the rMM or during the pulverizing were studied with X-ray diffraction (XRD). The magnetic properties were investigated by using a Vibrating-Sample-Magnetometer (VSM) to study the effect of the sintering conditions by the SPS. As a result of the rMM process on the Sm2Co17 powders, the Sm2Co17 grains transform into a mixture of Sm2Co7,Sm-H nano-grains. The XRD patterns,show that FWHM of the Sm2Co17 and Sm-H peaks is independent of the milling conditions. Detailed relationships between the microstructure and magnetic properties of the SmCo/α-Fe nanocomposite magnets will be discussed in the symposium

MoSi₂ is a promising material for structural applications at very high temperatures due to its very high melting point (2020°C) and excellent oxidation resistance. However the poor fracture toughness at low temperatures and inadequate strength at high temperatures are drawbacks to its practical applications. One promising way to solve these drawbacks is to form an in-situ composite with a second phase. The MoSi₂/Mo₅Si₃ eutectic alloy is one of the candidates because of its high eutectic temperature (1900°C) and fine script-lamellar microstructure. Mechanical properties and thermal stability of the microstructure of two-phase composites are generally influenced by interfaces between two phases. It is thus quite important to establish ways to optimize interface properties of the MoSi₂/Mo₅Si₃ eutectic alloys. In the present study, we investigate the effects of ternary additions on the microstructure and its thermal stability of MoSi₂/Mo₅Si₃ eutectic composites in hope that the interfacial segregation of ternary elements or controlling lattice misfit between the two phases. Directionally-solidified (DS) ingots of MoSi₂/Mo₅Si₃ eutectic composites with various ternary additions (Ti, V, Cr, Fe, Co, Ni, Cu, Nb, Ta and W) were grown by the optical floating zone method at various growth rates of 5 to 200 mm/h. As-solidified ingots were heat-treated at 1500°C for 500h in vacuum. In the cases that the ternary elements Ti, V, Cr, Nb, Ta and W, which are known to be partitioned into both phases, homogeneous two-phase script-lamellar microstructure can be obtained in DS ingots grown at relatively slow growth rates, but the conditions for obtaining the homogenous microstructure depend strongly on the amount and type of the ternary elements. The lamellar spacing in the script-lamellar structure of the DS ingots is mainly controlled by the growth rate, i.e. the narrower lamellar spacing is obtained for the faster growth rate. Detailed analysis of the lattice parameter, microstructure and segregation behavior of the ternary element suggested that morphology of one type of the interfaces can be modified from flat to ragged ones mainly by decreasing the corresponding lattice misfits. In the case of the DS ingots with Fe, Co and Ni that are not portioned into both phases, the ternary element are tend to segregate strongly at the interfaces resulting in their apparent morphology change similar to the case described above but with very slight ternary additions less than 0.5%. Thus, the small ternary additions of Fe, Co and Ni are found to be quite effective in obtaining the ragged interface, which is expected to be beneficial to improve the mechanical properties of the MoSi₂/Mo₅Si₃ eutectic composites.

MoSi₂ has been recognized as a promising material for structural applications at very high temperatures because of its high melting point (2020°C) and excellent oxidation resistance. For the practical application of this material, it is necessary to improve its low-temperature fracture toughness and high-temperature strength. One possible way is to form an in-situ composite with a secondary phase. Mo₅Si₃ is one of the candidates because it has superior creep strength at elevated temperatures and also because directionally-solidified MoSi₂/Mo₅Si₃ eutectic alloys possess very high eutectic temperature (1900°C) and fine script-lamellar microstructure composed of a continuous MoSi₂ matrix and interconnected network of Mo₅Si₃. The purpose of this study is to investigate the effects of the lamellar spacing of the script-lamellar structure and ternary additions on the high-temperature strength of the MoSi₂/Mo₅Si₃ eutectic composites. Directionally-solidified ingots of binary and some ternary MoSi₂/Mo₅Si₃ eutectic alloys were grown by optical floating zone method at a various growth rates ranging from 5 to 200mm/h. Microstructure of the DS ingots changes from homogeneous script-lamellar structure to cellular structures with increasing growth rate. Very fine script-lamellar structure is observed in the central part of each cell elongated along growth direction, while serious coarsening occurs at the interface regions of the cellular structure. Orientation relationship between matrix MoSi₂ and secondary Mo₅Si₃ were confirmed to be [1-10] _MoSi2 // [001] _Mo5Si3 (approximately parallel to the growth direction), (110) _MoSi2 // (110) _Mo5Si3, and (001) _MoSi2 // (1-10) _Mo5Si3 in the script-lamellar structure irrespective of the growth rate and ternary addition. The average lamellar spacing of the script-lamellar structure was confirmed to be affected primarily by the growth rate. The average lamellar spacing slightly decreases with increasing the growth rate from 5 to 200mm/h. Compression tests were carried out along [1-10]MoSi₂ close to the growth direction in the temperature range from 1000°C to 1500°C. Yield stresses of the binary alloys decrease drastically with increasing temperature and are found to be 3~7 times as high as that of corresponding single crystals of binary MoSi₂.

Fe-50 wt% Co alloy powders with average particle size of 10 µm were compacted by spark plasma sintering (SPS) at 700, 800, 900 and 950°C by applying 40, 80, 100 MPa uniaxial pressures for 2, 5, 10 minutes. The microstructures of the cross-section of the sintered samples were examined by optical microscopy and scanning electron microscopy. The DC magnetic characteristics of the sintered samples were studied using a fully automatic universal measurement system developed in the laboratory [1]. The densities of the samples were found to increase with temperature from 700 to 900°C for constant sintering pressure and time and to decrease for the material sintered at 950°C. The effects of sintering time on density were more significant in samples sintered at 700°C and 800°C than those densified at 900°C. The consequences of small increases in pressure values on density values were significant for samples sintered at 700°C. The coercivity (H_c) of the compacts decreased with increasing sintering temperature, pressure and times irrespective of the density values. The sample sintered at 950°C, which contains the largest grains among the prepared samples, exhibited the minimum coercivity. Unlike H_c, the remanence (B_r) and saturation induction (B_sat) values were strongly affected by the specimen density and least affected by grain size. B_r and B_sat values were found to vary linearly with sintering temperature and pressure owing to density enhancements. An increase in processing time at 800 and 900 °C, although enabling higher density, exhibited contradicting effects on structure insensitive B_sat values. The SPS parameters to realise maximum density and optimum magnetic properties for Fe-50% Co alloy were found to be 900°C, 80 MPa and 5 minutes.

Micro-compression testing as a technique to investigate the deformation behavior of materials is still in its nascent stages. The point to point variations in mechanical properties and their combined effect on overall properties with decreasing sample size can be investigated using micro-compression testing. A lot of studies have been conducted on the size-scale effects (smaller-is-stronger) and the mechanical response of micro-crystals of pure metals and some two-phase alloys, under the application of uni-axial compressive stress. Ni-base superalloys possess a unique γ /γ&’ microstructure which imparts them high temperature strength. With the varying lattice parameter of both γ and γ&’, the internal stress conditions may also vary due to the change in the magnitude and sense of the lattice mismatch δ. At size-scales comparable to the size of γ&’ precipitates (of the order of 1mu;m and smaller) the effect due to the magnitude and sense of internal stresses may become rather significant. Only a handful of studies have been done so far on micro-compression effects on Ni-base superalloy micro-pillars but nothing has been reported on these effects for sample sizes smaller than 2mu;m. In the present study, we have studied the micro-compression behavior of Ni-base superalloy micro-pillars from larger (16mu;m) to smaller (<1mu;m) especially focusing on effects of internal stresses. Micro-pillars with loading axis direction parallel to <001> were fabricated up to the sub-micron levels using Focused Ion Beam Method (FIB). Compression testing was carried out using a micro-indentation system equipped with a diamond flat punch tip. Microstructures were investigated by Scanning Electron Microscope (SEM). Size-scale effect was observed at yield stress (0.2% proof stress), as well as at different values of strain offsets. The yield stress followed power law dependence until 2mu;m as has been reported earlier. Below 2mu;m, the variation in stress levels seemed to exhibit an additional increase. Shearing of both the γ channels and the γ&’ precipitates by the slip lines was observed indicating that stress levels were significant enough to cause the dislocations to cut into the γ&’ precipitates at room temperatures. The role played by the unique γ/γ&’ microstructure and the variations in internal stresses is further discussed.

Effect of lamellar orientation on cracking behavior of the α₂/γ lamellar single crystals made by directionally solidified has been examined, by means of in-situ three-point bend test in SEM. The single crystals were grown by optical floating zone furnace. As-grown crystal exhibits fully lamellar microstructure, with 20 % of volume fraction of α₂ phase. The average thickness of α₂ plate is about 200 nm, whereas that of the γ plate shows two peaks: one about 100 nm and the other 1000 nm. Rectangular single notched specimens with a size of 30 mm in length, 4 mm in height, 2 mm in thickness were cut from the crystals. The notch was made by electro discharged machining to a half of the thickness. The lamellar orientation was defined by two-angle combination (theta;, lambda;) with respect to the notch direction, a parallel kink angle (theta;) and a twist angle (lambda;). The load-displacement curve revealed that the specimen with (0, 0) orientation deforms elastically, followed by catastrophic failure by cracking along an α₂/γ interface at maximum load of 16 N. The stress required to delaminate the interface made the “crack opening mode”(Mode I) can be calculated astau;_c(0,0) = 51 MPa. With increasing the theta; up to 45°, the cracking behavior of the specimen with (theta;, 0) orientation is similar to that of specimen with (0, 0) orientation, although the maximum load increases. In contract, with increasing the lambda; up to 45°, the specimen with (0, lambda;) orientation shows work hardening behavior even after initiation of the primary crack along an α₂/γ lamellar interface, and many slip lines across the lamellae become visible. After reaching maximum, the load drops gently to failure with secondary crack bridging. In case of a specimen with (0, 30) orientation, tau;_c(0,30) becomes 513 MPa, an order of magnitude higher than that of the (0, 0) specimen. The big increase intau;_c is caused by plastic deformation due to an introduction of “parallel shear mode”(Mode III). The specimen with (theta;, lambda;) orientation shows the work hardening behavior, but the maximum load decreases with increasing theta; due to the activation of “forward shear mode”(Mode II). Thus, within 45°, increase in the twist angle lambda; with respect to the loading axis is effect in toughening. The mechanism of cracking behavior as well as that of the specimens with angle larger than 45° will be presented. A Part of this study was carried under the grants-in-aid for scientific research (No.23360301) of “Japan Society for the Program of Science”(JSPS).

MnAl alloys have gained much attention as high-performance magnets due to their inexpensive and widely available constituent elements in contrast to their rare-earth based counterparts such as SmCo, NdFeB etc. Their excellent corrosion resistance also makes them more practical in some commercial applications [1], [2]. Carbon doping of MnAl alloys not only enhances the stability of the metastable magnetic tau; phase, but also improves the machinability, mechanical and magnetic properties [3-5]. While various techniques exist for the fabrication of permanent magnet alloys, mechanical alloying (MA), which is capable of producing micro- and nanocrystalline alloy powders, has the added advantage that it involves fewer processing steps and does not require the use of expensive instrumentation [2], [6]. Until this date, only a few studies have been done on the magnetic properties of MA MnAlC powders. In this work, the effects of annealing conditions upon the magnetic and structural properties of MA Mn_0.53Al_0.47-xC_x (x = 0-0.015) alloy powders were systematically investigated. The alloy powders were annealed in a tube furnace at 900^oC for 1 hr under different annealing atmospheres and gas flow rates to form the ferromagnetic tau;-phase. Both magnetization M_{(H=10 kOe)} and remanent magnetization M_r show strong dependence on the amount of ferromagnetic MnAl tau;-phase in the alloys, while coercivity H_c not only depended upon the amount of tau;-phase present, but also demonstrated a dependence upon grain size of the tau;-phase. The gas flow rate produced the most significant effect on the stabilization of the magnetic tau; phase resulting in M_{(H=10 kOe)} = 72 emu/g, M_r = 37 emu/g and H_c = 1.9 kOe in optimized heat-treated MA Mn_0.53Al_0.46C_0.01 powders. This result is attributed to the fact that the increase in gas flow during annealing reduced the formation of the non-magnetic Mn₃AlC phase, while the percentage of tau;-phase in the alloys and grain size of the powders are shown to increase significantly from 3 to 65%, and 15.4 nm to ~40 nm, respectively. Importantly, under the application of a 50 kOe magnetic field, M _{(H=50 kOe)} increased to 97 emu/g which is the highest value measured to date for this system. References [1] A. J. J. Koch, P. Hokkeling, M. G. Van den Steeg, and K. J. De Vos, J. Appl. Phys. 31, 75S (1960). [2] T. Saito, J. Appl. Phys. 93, 8686 (2003). [3] J. H. Huang, and P. C. Kuo, Materials Science and Engineering B14, 75 (1992). [4] T. Saito, J. Appl. Phys. 97, 10E304 (2005). [5] Q. Zeng, I. Baker, J. B. Cui, and Z. C. Yan, J. Magn. Magn. Mater. 308, 214 (2007). [6] O. Kohmoto, N. Kageyama, Y. Kageyama, H. Haji, M. Uchida, and Y. Matsushima, Journal of Physics: Conference Series 266, 012016 (2011).

Diffusion brazing is a potential method to repair parts made from TiAl-alloys. The closure of cracks (in noncritical or not highly loaded areas) in components of aero engines is an example where this method could be established. In this work diffusion brazing is used to join TiAl-alloys in order to investigate the phase constituents, phase distribution and microstructure of the joint zone. The idea of diffusion brazing is to use a solder with a lower melting point than the substrate material. This is achieved by adding a melting point depressing element to the solder. Therefore Ni- and Fe- based braze alloys are investigated in this work. At temperatures below the melting point of the substrate material the brazing solder melts. During a holding time at brazing temperature the melting point depressing element diffuses into the substrate material and the melting point in the brazing zone is continuously increasing until the whole brazing zone is solidified. Six different brazing materials with varying contents of titanium, aluminium, iron and nickel are investigated. They are analysed twice, first in as-brazed state and second after a heat treatment at 1000°C. The microstructure is characterised by scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDX) and chemical electron backscattered diffraction (EBSD) for phase and crystallographic analysis. In addition the phases present in the brazed zone were identified by high energy X-ray diffraction (HEXRD) at the material science beamline HEMS at the PETRA III synchrotron in Hamburg, Germany. The distribution of phases over the brazing zone could be determined by scanning perpendicular to the joint. Furthermore changes in lattice parameters gave evidence for variation in the chemical composition over the analysed zone due to the diffusion during the brazing process. The braze zone itself is composed of two to three transitional layers from the substrate material to the middle of the joint. While near the substrate material the phase constitution reassembles a TiAl-alloy in the middle of the joint the microstructure is similar to α/β-titanium alloys. The phase analysis from the diffraction measurements shows, besides phases commonly encountered in TiAl-alloys such as γ, α₂ and β, additional phases, which are related to the presence of nickel or iron as melting point depressing elements. The microstructure of the brazed zone changes significantly during a heat treatment in the range of 1000°C, which shows that the as-brazed state is far from thermodynamical equilibrium.

Reverse biased ferromagnet/semiconductor Schottky contacts can be utilized to overcome the conductivity mismatch problem and to achieve spin injection/detection in semiconductor spintronics. For the wide bandgap semiconductor GaN, Lu et al. [1] showed that the AuCu type structure of Mn_xGa_1-x with x = 0.5 - 0.6 grows epitaxially on GaN with its [111] direction perpendicular to the GaN(0001) plane and with a resulting lattice mismatch of 2%. Recently, our group investigated the structural, magnetic and transport properties of MnGa(111) on GaN(0001) in a wide range of manganese compositions [2]. However, a detailed knowledge of the interface properties and their possible implications on the spin injection efficiency is still lacking. This issue is addressed in the present work, both by experimental methods and by density functional theory (DFT) calculations. Ferromagnetic Mn_xGa_1-x layers with x = 0.5 have been grown epitaxially on GaN(0001) by molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) was employed to monitor the in-plane epitaxial relationship between MnGa and GaN during growth at various substrate temperatures. The interface is very sharp as seen by cross-section Transmission Electron Microscopy (HRTEM). Based on the observed orientational relationships we explored the atomic and electronic structure of the interface using first-prinicples calculations. The calculations predict strong spin injection in the minority spin direction of MnGa. We rationalize this spin orientation of the injected electrons in terms of underlying concepts that can be generalized to a wider class of materials. Furthermore, the Schottky barrier height of the MnGa/GaN interface has been determined experimentally by electrical characterization of an epitaxial MnGa/GaN Schottky diode. References [1] E. Lu, D. Ingram, A. Smith, J. W. Knepper and F. Y. Yang, Phys. Rev. Lett. 97, 146101 (2006) [2] A. Bedoya-Pinto, C. Zube, J. Malindretos, A. Urban and A. Rizzi, Phys. Rev. B 84, 104424 (2011)

Recently discovered cobalt base fcc/L1₂ two phase alloys have been attracted as a candidate of a new class of superalloys for high temperature structural materials. The alloys exhibit two-phase microstructures consisting of the solid-solution based on Co with a face-centered cubic structure (designated the γ phase) and the L1₂-ordered intermetallic compound based on Co₃(Al,W) (designated the γ&’ phase) as similar to the nickel base superalloys. In practically used nickel base superalloys, the value of lattice misfit which is the difference in the lattice constants between γ and γ&’ phases is negative, that is, the lattice constant of the γ&’ phase is smaller than that of γ phase. On the other hand, the value is known to be positive in Co-Al-W based superalloys. A different microstructural evolution during creep at high temperatures is expected by the change in the sign of lattice misfit. In the framework of elastic-plastic models of forming the so-called raft structure in a tensile creep along the [001] direction, negative/positive lattice misfit leads the raft structure in which the γ&’ phase elongated perpendicular/parallel to the stress direction. In the present study, we have carried out creep tests of some mono-crystalline Co-Al-W based alloys with a tensile stress of 137 MPa at 1000 °C. The microstructures of crept specimen are investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). The γ&’ phase in crept specimens are not only elongated along the stress direction as expected but also elongated one of the <100> direction that is perpendicular to the stress direction. As a result the shape of the γ&’ phase is not a rod but a plate. By TEM observation, many SISFs are induced in the γ&’ phase by creep. Similar microstructure is also observed in Ni-based superalloys but under conditions of lower temperature and higher applied stresses. The observation of numerous stacking faults in the γ&’ phase is a clear indication that the γ&’ phase in the present alloy is weaker than that in many modern Ni-based superalloys.

Binary rare-earth (R) - iron intermetallic compounds with Th₂Ni₁₇ structure have been extensively studied for their potential application as permanent magnets. The main drawbacks of these materials are low Curie temperature (T_c) and magnetic anisotropies. In recent years, many efforts have been made to solve these problems including substituting smaller or bigger magnetic or non-magnetic elements for iron atoms in R₂Fe₁₇ and introducing interstitial atoms into it which in turn brings in changes in T_c and magnetocrystalline anisotropy. It was reported by Fidler et al. [1] that substitution of refractory elements such as ( Ti, V, Mo, Nb) can bring grain refinement and hence enhance magnetic properties of R₂Fe₁₇ intermetallics. This work presents the effects of substituting iron atoms in R₂Fe₁₇ with bigger non-magnetic Nb atom and reporting the structure, magnetic properties and Mössbauer spectroscopy (MS) studies of the Dy₂Fe_17-xNb_x (0le;xle;1.5) compounds. The samples Dy₂Fe_17-xNb_x (x = 0.0 to 1.50 with increase step 0.25) were prepared by the process of arc melting in high-purity argon atmosphere. The phase and structure of samples were determined using XRD with Cu K_α radiation. The magnetic properties of the samples were investigated using SQUID and MS. The T_c was determined using VSM. The X-ray powder diffraction (XRD) results show that the samples have Th₂Ni₁₇-type structure (space group, P63/mmc). XRD patterns show that samples are single phase up to x = 0.75. The XRD patterns contain some impurities of DyFe₃ and NbFe₂ at higher Nb concentration which was also detected in the MS. The linear unit-cell volume expansion was observed at lower concentration of Nb which is consistent with Rietveld analysis. But the linearity breaks at higher concentration of Nb, when they start forming impurities. The saturation magnetization (M_s) of Dy₂Fe_17-xNb_x at 5 and 300 K decreases almost linearly with increasing Nb concentration because of the dilution of magnetic moment of Fe. For Dy₂Fe₁₆Nb₁ the maximum in T_c was observed around 460 k which is 80 K higher than the Dy₂Fe₁₇ compound. In MS studies, the hyperfine fields (HF) of Dy₂Fe_17-xNb_x were increased upon Nb substitution. Increase in the HF can be attributed to the increase in positive exchange interaction and reduction in isomer shift at 12k which may have resulted from increase in s electron density at 12k site due to Nb substitution. In conclusion, Dy₂Fe_17-xNb_x compounds were observed to have pure phase up to x = 0.75, Nb. The unit-cell volume expansion observed linear at lower concentration of Nb but linearity observed to break at higher concentration. Increase in T_c (460 K, at x = 1) is reported in Dy₂Fe_17-xNb_x compounds upon Nb atom substitution. M_s was observed to decrease monotonically with Nb doping. Increased HF parameters were observed with Nb doping due to local electronic variation. Reference: [1] J. Fidler, J. Bernardi, and T. Schrefl, Scr. Metall. Mater. 33, 1781 (1995).

In this work, we have studied the effect of high energy ball milling (HEBM) on light and heavy rare-earth Nb doped R₂Fe₁₆Nb₁ compounds. The focus of the work is to bring enhancement in magnetic properties of R₂Fe₁₇ (2:17) compounds with the ball milling (BM). The potential of HEBM method in material processing has been extensively explored for amorphous alloys [1], grain refinement [2], nanocomposite mixing [3] etc. The HEBM create a large amount of defects like new surfaces, dislocations, vacancies, grain boundaries etc.The defects and microstructural refinement often enhances magnetic properties of the alloys. The samples R₂Fe₁₆Nb₁ were prepared by arc melting high in purity argon atmosphere. The phase and structure of samples were determined using x-ray powder diffraction (XRD) with Cu K_α radiation. The magnetic properties and Curie temperature (T_c) of the alloys were investigated using VSM and Mössbaure spectroscopy (MS). Beside pure 2:17 phase, the starting material was observed to have small amount of α-Fe (in Er₂Fe₁₆Nb₁) and NbFe₂ (in Gd₂Fe₁₆Nb₁) phase. The XRD patterns show that the peak intensities of 2:17 and NbFe₂ phases were diminishing in R₂Fe₁₆Nb₁ whereas peak intensity of α-Fe was found to increase with the BM time. The diminishing peak intensity of 2:17 phase indicates that nanocrystalline phase is transforming into the amorphous phase. The lattice parameters were observed to be linearly increasing as a function of BM time in Gd₂Fe₁₆Nb₁ alloy. This is because the Nb atom from NbFe₂ phase is being introduced in the 2:17 phase. But lattice parameters of Er₂Fe₁₆Nb₁ alloy observed constant upon BM. The magnetic measurement of 30 min ball milled samples show great enhancement in magnetic properties as well as T_c of the alloy. The enhancement in coercivity (H_c) and remanence arises in 30 min BM samples partly from grain refinement and exchange coupling between R₂Fe₁₆Nb₁ and α-Fe. The H_c of the BM samples decreases with BM time which could be due to increased amorphization of crystalline 2:17 phase and increment of soft α-Fe phase in the composites. The maximum in T_c ~ 550 K was observed for 90 min BM Gd₂Fe₁₆Nb₁ and T_c~485 K was observed for 30 min BM Er₂Fe₁₆Nb₁. It was observed from MS that hyperfine field and isomer shift were increasing in Gd₂Fe₁₆Nb₁ upon BM.However, this effect was absent in BM Er₂Fe₁₆Nb₁ alloy. Our results indicate that BM is an effective method for promoting exchange coupling between hard-and soft phases of the composite for further enhancing magnetic properties of the substituted alloys. Upon BM the secondary phases have been utilized towards improving the overall magnetic properties of the alloy, which otherwise would have detrimental effect on the magnetic properties of alloys. [1] C. C. Koch, O. B. Cavin, C. G. Mc Kamey and J. O. Scarbrough. Appl. Phys. Lett., 43 (1983) 1017. [2] H. J. Fecht, NanoStructure Materials, 1 (1992) 125. [3] G. B. Schaffer., Scripta Metall., 27 (1992) 1.

Ni-rich NiTi alloys, when properly processed, display relatively stable shape memory and superelastic behavior. However, their potential applications are severely limited because of low transformation temperatures, which is commonly less than room temperature. Recent efforts have focused on using ternary additions to raise the transformation temperatures, while simultaneously improving stability through nanoscale precipitation. In this work, we have investigated the effect of aging on the structure and shape memory behavior of an extruded Ni-29.7Ti-20Hf (at.%) alloy. The samples were solution treated at 1050 deg.C for 0.5 hr followed by aging at 550 deg. C for various times. Upon aging, the Hf addition has been shown to form nanoscale plate-like precipitates; however, little is understood with respect to the microstructure of this fine phase from this ternary alloy composition. Once the precipitates formed, the Mf and Ms transformation temperatures were observed to be slightly higher than 100 deg. C through differential scanning calorimetry. Transmission electron microscopy and atom probe tomography have been performed to quantify the precipitates. The precipitates have a near equiatomic Ni(Ti,Hf) composition. Load bias property responses up to 700 MPa and 20 thermal cycles through the transformation temperature exhibited a narrow hysteresis; the precipitates are believed to impede dislocation motion and be a possible reason for the improved structural stability.

Creep behavior of carbon free Fe-20Cr-30Ni-2Nb (at.%) steels strengthened by Fe₂Nb Laves phase at 1073 K has been examined in air and water vapor atmosphere, in order to identify the effect of water vapor on creep properties. The steel studied has the compsition with 0.03 at.% B. Solution treated (1473 K/2 h) specimen and pre-aged (1073 K/240 h) specimen were used to intentionally change the area fraction of the Laves phase on grain boundaries (ρ) of 0% and 80%, respectively. Creep test was carried out at 1073 K/70 MPa in air and water vapor. Under air atmosphere, the specimen with ρ=0% ruptured after 293 h, whereas the specimen with ρ= 80% exhibits the rupture time of 833 h. In addition, the latter specimen shows smaller creep rate than the former in entire creep stage. This pronounced extension of rupture life is caused by the "grain boundary precipitation strengthening" (GBPS) effect by Laves phase, as proposed by authors. Under water vapor atmosphere, the specimen with ρ= 0% exhibits the minimum creep rate smaller than that of 8.0x10^-4 /h of the specimen tested in air, and the rupture life is extended to 376 h. The pre-aged specimen with ρ= 80% shows no difference in creep properties regardless of the atmoshere. These results suggest that the grain boundaries not covered by the Laves phase are somehow sensitive to the enviroment, and the grain boundary Laves phase is effective in reducing the enviromental effect. Thus, the novel steels strengthened by the GBPS are promising for use of further advanced ultra-super critical (FA-USC) power plant to be operated above 700 ^oC. Detailed oxide layer formed with and without water vapor, and effect of oxide layer on the formation and morphology of the grain boundary Laves phase will be presented. Part of this study was carried under the research activities of “Advanced Low Carbon Technology Research and Development Program” (ALCA) in JST (Japan Science and Technology Agency).

After almost three decades of intensive fundamental research and development activities intermetallic titanium aluminides based on the γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750°C. A drawback, however, is their limited ductility at room temperature, which is expressed in a low plastic fracture strain. This behavior can be attributed to both hindered dislocation movement and microstructural inhomogeneity. Advanced TiAl alloys, such as β-solidifying TNMtrade; alloys, are complex multi-phase materials which can be processed by ingot or powder metallurgy as well as precision casting methods. TNM alloys contain Nb and Mo additions in the range of 3 - 7 atomic percent as well as small additions of B and C. Each manufacture process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat-treatments. The background of all these heat-treatments is at least twofold, i.e. concurrent increase of ductility at room temperature and creep strength at elevated temperature. In order to achieve this goal the knowledge of the occurring solidification processes and the following phase transformation sequences are essential. Therefore, thermodynamic calculations were conducted to predict the phase diagram of engineering TiAl alloys. After verification with experimental methods, e.g. short and long-term heat-treatments, differential scanning calorimetry and X-ray diffraction, these phase diagrams provided the base for the development of smart heat-treatments. To account the influence of deformation and kinetic aspects sophisticated ex- and in-situ methods have been employed to investigate the evolution of the microstructure during thermo-mechanical processing and subsequent multiple heat-treatments. For example, in-situ high-energy X-ray diffraction was conducted to study dynamic recovery and recrystallization processes during hot-deformation tests. Due to the peculiar scattering behavior of TiAl alloys neutron diffraction was used for the investigation of order/disorder reactions. Summarizing all results a consistent picture regarding microstructure formation and its impact on mechanical properties in TNMtrade; alloys can be given. Finally, the development status and perspectives of these alloys as innovative automotive and aircraft engine materials are presented.

The aim of this paper is to discuss the capability of the Spark Plasma Sintering (SPS) route to process blades for aircraft engines in TiAl intermetallic alloys. SPS is a powder metallurgy technique for which the heating of the sintered sample occurs by Joule&’s effect and application of uniaxial pressure. The first part of the talk will be dedicated to the development of materials, i.e. to the obtaining of an alloy satisfying the industrial requirements. In particular, attention will be focussed on the influence of the chemical composition and of the processing parameters. The microstructures of the alloys will be described in details. Their mechanical properties are characterised by performing tensile tests at room temperature and creep tests at 700°C under 300 MPa. It will be demonstrated that: i) the alloys must contain refractory elements and a small amount of boron, ii) thermal treatments must be performed inside or outside the SPS set-up to improve the homogeneity of the microstructure, iii) the duplex microstructures are the most performing. In these conditions, a satisfying compromise is achieved between the room temperature ductility and the high temperature mechanical properties. In the second part, the method followed to sinter directly in the SPS set-up near net shape blades will be described. The basic idea is to use the graphite assembly containing the powder to give the final shape to the piece. Finite Element Modelling is used as a predictive tool to determine the processing parameters and to control the temperature. The density and the microstructure of the products will be examined.

Among the TiAl alloys, TNM alloys attract presently a great deal of interest because of an excellent hot-workability and balanced mechanical properties. TNM is a β solidified alloy, for which the phase diagram and the microstructures have been largely investigated. During the last few years, several TiAl alloys were successfully sintered by Spark Plasma Sintering (SPS). SPS is a powder metallurgy technique for which the densification is due to the simultaneous application of a pulsed direct current and of a uniaxial pressure. The aim of the present work is to elaborate a TNM alloy starting from a gas atomized Ti-43.5Al-4Nb-1Mo-0.1B powder. Samples were sintered at various temperatures increasing by steps of 25°C, from 1200°C to 1400°C. Several multi-phased microstructures were achieved depending on the sintering temperatures and the process parameters, like the pressure or the plateau duration at the dwell temperature. The formation of microstructures is explained from the analysis of the phase diagram, in consistency with previous works on cast and powder metallurgy alloys. The mechanical properties were measured by tensile tests at room temperature. Creep tests at 700°C and 300 MPa are in progress. The correlation between microstructures and mechanical properties is based on studies by Scanning and Transmission Electron Microscopy. Attention is mainly focused on near lamellar microstructures which contain either γ grains or β+γ zones situated at the colony boundaries. The microstructures are quantitatively analyzed and the effect of boron on grain refinement is investigated. For a good compromise between room temperature ductility and high temperature strength a careful adjustment of the microstructure is necessary. For example, to overcome 1% of elongation, an amount of β+γ zone at grain boundaries is found necessary.

The strong demand for higher efficiency, reduction of fuel consumption, CO₂ and NO_x emissions as well as weight reduction in aircraft engines lead to a substitution of presently used materials by novel light-weight, high-temperature alloys like γ-TiAl based alloys. Turbine blades are engine parts that are subjected to high mechanical as well as thermal loading. Thus alloys are required which provide high creep strength and fatigue properties as well as a sufficient ductility at room temperature. Presently, Ni-base alloys are state of the art. World-wide fundamental research conducted over the last two decades has clearly shown that balanced material properties can be obtained by hot-working and subsequent heat-treatments of TiAl alloys. Due to a small “deformation window” hot-working of TiAl alloys is a complex and challenging task and, therefore, isothermal forming processes are favored. In order to expand the process window a novel Nb and Mo containing γ-TiAl based alloy (TNM^TM alloy) was developed. As a result of a high volume fraction of β-phase at elevated temperatures this alloy can be hot-die forged under near conventional conditions, which entails that conventional forging equipment with minor and inexpensive modifications can be used. During subsequent heat-treatment a significant reduction of β-phase can be achieved and mechanical properties can be tailored to customer requirements. The presentation summarizes our effort to establish a “near conventional” forging route for the fabrication of TiAl components for aerospace industry. The path from lab scale compression tests to industrial scale forging trials is shown.

A robust processing route at low cost is an essential requirement for high temperature materials used in automotive engines. Because of their excellent high temperature properties, e.g. low density, high Young&’s modulus as well as high specific strength, intermetallic titanium aluminides are potential candidates for application in advanced combustion engines. So-called 3rd generation alloys such as TNMtrade; alloys with a nominal composition of Ti-43.5at%Al-4at%Nb-1at%Mo-0.1at%B are multi-phase alloys consisting of γ-TiAl, α2-Ti3Al and a low volume fraction of βo-TiAl phase. In this paper a novel hot-processing route, which is a combination of a one-shot hot-forging step and a controlled cooling treatment, leads to improved mechanical properties. The increase in strength can be attributed to a small lamellar spacing within the γ/α2 colonies. In order to analyze phase fractions and to determine mechanical properties microscopic examinations, X-ray diffraction measurements, hardness tests as well as tensile and creep tests were conducted. The obtained mechanical properties will be presented and the thermal stability of the microstructure under mechanical loading at elevated temperatures will be discussed.

Intermetallic alloys, like γ-TiAl, have attracted significant attention during the last decades in the area of aerospace and engine applications due to their excellent combination of properties at elevated service temperatures, such as high creep strength and oxidation resistance supplemented by low density. Successful applications of these alloys are also dependent on the development of suitable and cost-effective joining techniques. Diffusion brazing is one of the most important and mature technique for bonding or repairing components. During brazing the joint area is heated above the melting point of the filler metal. One or more melting point depressing elements diffuse into the substrate, leading to isothermal solidification of the filler alloy. Ge-containing systems have become of special interest in this context. The Al-Ge binary system has a low eutectic point at 420°C. Ge is also known to be an efficient melting point depressant in nickel- and titanium-based alloys. Therefore, Ge-alloys are considered as promising filler metals for diffusion brazing of (Ni)-superalloys as well as for joining of structural intermetallics like TiAl. In the present study, brazed joints of TiAl and Ni3Alpure intermetallics as well as a Ti-47Al-4(Nb,Mn,Cr,Si,B) alloy were produced. Different Ge-based alloys, with relatively low melting points, were used as filler alloys. The joints were investigated by optical microscopy and scanning electron microcopy (SEM). The phases formed at the reaction layers were characterized by electron probe microanalysis (EPMA). Mechanical characterization of the interface was performed by microhardness and shear tests. The feasibility of the joining process was clearly demonstrated.

TiAl alloys are introduced in jet engines, turbocharger etc. due to their superior high specific strength in high temperature. However manufacturing processes of TiAl parts are still more difficult than conventional Ni based alloys because of their high reactivity of liquid and/or low toughness. As for forging process of TiAl, Takeyama advocated forging process use of beta Ti phase which has high deformability. In Ti-Al binary phase, beta Ti phase exists only high temperature and low Al content region. By addition of beta stabilizer such as Nb, beta phase can exist at relatively low temperature and alpha + beta + gamma or beta + gamma phase regions appear. Therefore forging process window becomes wider and various microstructures can be obtained by heat-treatment. In this study, the effects of microstructure on mechanical properties were investigated using forged TiAl which has different microstructure. Chemistry of experimental alloy is Ti - 43Al - 5V - 4Nb (at%). Cylindrical specimens were machined from as-cast ingot manufactured by VAR. Then specimens were forged to pancake at 1200C without canning. Four heat-treatment conditions were selected in order to obtain various microstructures. Tensile specimens and CT specimens were machined from heat-treated pancake. Tensile tests were conducted at RT and 760 degree C, and crack propagation tests were also conducted at RT. Microstructure of as-forged, heat treated and fractured specimens ware observed using SEM, TEM and EBSD. Microstructure of ingot consists of elongated alpha/gamma lamella colonies and small amount of beta and gamma grains. After pancake forging, lamella colonies were deeply distorted and ultrafine recrystallized alpha2 and gamma grains were observed along colony boundaries. After 1st step heat treatment at 1200 - 1300 degree C and air cooling, microstructure was changed to equiaxed alpha2 grains and small amount of beta phase or near lamellar structure. And gamma/gamma lamella +beta, duplex and equiaxed alpha + beta + gamma structure were also obtained by various heat treatments. Equiaxed alpha 2 ( or alpha2 lamellar structure with small amounts gamma ) + beta structure has superior 0.2% yield strength than the other microstructures at both RT and 760 degree C. As for elongation, equiaxed alpha 2 + beta + gamma structure shows larger value at both temperatures. Further analysis of the microstructure and fracture surface observations will be reported.

High Nb containing TiAl alloys have the potential for high temperature applications in aerospace and automotive industry due to their high strength and good oxidation behavior at elevated temperatures. The addition of C in these alloys enhances the strength and creep properties further by solid-solution hardening and precipitation hardening. Two types of carbides are known in TiAl alloys (P-type with a cubic Perovskite structure and H-type with a hexagonal lattice), which are stable at different temperature ranges. However the exact formation and stability conditions of carbides in high Nb containing TiAl alloys are still under discussion. It is important to investigate the influence of carbon on phase transformation and microstructure, the carbon solubility and the carbide precipitation in high Nb containing TiAl alloys to comprehend the effects of carbon on the alloy behavior at service temperature. In this research the carbon solubility and carbide precipitation in Ti-45Al-5Nb-xC (x=0.5, 0.75 and 1 at. %) alloys are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high energy X-ray diffraction (HEXRD). The alloys were produced as gas atomized powders and subsequently consolidated by HIPing. The results show that the addition of carbon in Ti-45Al-5Nb alloys has a strong effect on the Ti-Al-Nb phase diagram. However carbon has no obvious grain refinement effect as reported in literature. In as-HIPed Ti-45Al-5Nb-xC (x=0.75 and 1 at. %) alloys both P-type and H-type carbides are detected. The H-type carbides in the Ti-45Al-5Nb-1.0C alloy show a high thermal stability and cannot be dissolved by heat treatment at temperatures below 1400°C. In the Ti-45Al-5Nb-0.5C alloy the precipitation of P-type carbides is observed during annealing at 800°C. P-type carbides precipitate sequentially at sites with increasing nucleation free energy: at grain boundaries, at dislocations, and in the γ matrix. At 900°C P-type carbides coarsen by Ostwald ripening and dissolve at 1000°C. No H-type carbides have been spotted in the Ti-45Al-5Nb-0.5C alloy. In comparison, in the Ti-45Al-5Nb-0.75C alloy no obvious carbides precipitation sequence is detected during annealing at 800°C. Nevertheless, the amount of P-type carbides forming at grain boundaries, dislocations and in the γ matrix increases. The P-type carbides in the matrix exhibit a needle-like shape aligned along [001] direction. After longer annealing times the morphology of carbides in γ grains starts to change from a needle-like to a plate-like shape and the P-type carbides now grow along all three <100> directions. At temperatures above 1100°C, P-type carbides disappear and H-type carbides grow fast.

Light weight in combination with high strength at elevated temperatures make intermetallic TiAl alloys promising candidates for applications in aero-engines. The alloys' microstructure can be adjusted through heat treatments and chemical composition to provide either good room temperature ductility (near gamma or duplex) or high high-temperature strength (lamellar alpha₂ + gamma). For practical applications however alloys are needed that have sufficient room temperature ductility and exhibit excellent strength at elevated temperatures. Therefore one focus of ongoing research is to optimize the alloys' mechanical properties by adjusting its microstructure. Recently a TiAlNb alloy has been developed that shows a promising combination of mechanical strength and ductility [1], [2]. The alloy's microstructure consists of lamellar (alpha₂ + gamma) colonies and regions of B19/beta₀ and gamma laths that appear pearlite-like. The modulated contrast within the B19/beta₀ laths is caused by strain between co-existing B2 and B19 phases that are also thought to provide the improvement of mechanical properties. The origin of the observed modulated laths is not understood yet. Models suggest that the decomposition of beta/beta₀ phase involving a shuffle mechanism could play a role [3]. Up to now the alloy with the modulated microstructure and the baseline composition Ti(40-44)Al-8.5Nb has been produced by hot extrusion followed by stress-relief annealing. Here we use an alternative production route via powder metallurgy (PM) and a two step heat treatment to generate the modulated microstructure constituent in TiAlNb based alloys. TEM investigations illustrate that the modulated microstructure of the final alloy evolves from a predominantly alpha₂ phase containing alloy, where alpha₂ grains are pervaded by a network of anti-phase boundaries (APS). A comparison of the mechanical properties to those measured in the original alloy shows promising behavior at elevated temperature (800°C) but poor room temperature ductility. [1] Appel, F.; Oehring, M.; Paul, J. D. H. Advanced Engineering Materials, 8 (2006) 371. [2] Appel, F.; Oehring, M.; Paul, J. D. H. Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 493 (2008) 232. [3] Nguyen-Manh, D.; Pettifor, D. G., in Gamma Titanium Aluminides.; The Minerals, Metals & Materials Society, (1999) 175.

In the last decades there has been a growing interest in developing new light-weight intermetallic alloy families, which are able to substitute the heavy superalloys at a certain temperature range. At present a new Ti-Al-Nb-Mo family, called TNM, is being optimized to fulfil the challenging requirements. The aim of the present work is to study the microscopic mechanisms of mobility of defects at high temperature in this TNM alloys in order to contribute to the understanding of their influence on the mechanical properties and so the further optimization of the alloys. Mechanical spectroscopy has been used to study the internal friction and the dynamic modulus up to 1000 C of a TNM alloy under different thermal treatments. These measurements allowed follow the microstructural evolution during in-situ thermal treatments. A relaxation process has been observed at about 780 C and characterized as a function of temperature and frequency in order to obtain the activation parameters of the responsible mechanism; in particular the activation enthalpy has been measured as H= 3 eV. These results are discussed and an atomic mechanism is proposed to explain the observed relaxation.

Urgent topics concerning energy efficiency and environmental politics require novel approaches to materials design. One recent example is the implementation of lightweight intermetallic γ-TiAl based alloys as structural materials in aerospace and automotive applications. For their use, balanced mechanical properties as well as an excellent hot-workability have to be achieved by adding alloying elements, such as Mo, which acts as a strong stabilizer of the β/β_o-phase. This phase is of great technological importance due to its beneficial effect on the solidification and hot-deformation behavior of TiAl alloys. To improve the understanding of the alloying effect of Mo, an ingot with a nominal composition of Ti-44Al-3Mo-0.1B (in at%) was produced. In the room temperature equilibrium state the phases α₂ (D0₁₉, P63/mmc), β_o (B2, Pm-3m) and γ (L1₀, P4/mmm) co-exist. At 1205°C the ordered hexagonal α₂-phase transforms to the disordered hexagonal α-phase, whereas at about 1230°C the ordered β_o-phase forms the disordered bcc β-variant. The phase constituents at different temperatures were studied by means of in-situ diffraction techniques employing synchrotron radiation and neutrons. In technological processes, however, high heating and cooling rates are used. Therefore, the material&’s behavior under non-equilibrium conditions was investigated in this study, too. In order to homogenize the alloy in the single β-phase field region, an annealing temperature of 1450°C was selected. After a holding time of 30 minutes the specimens were water quenched. During this process the majority of the β-phase transforms into α₂&’-martensite, where the remaining β_o-phase appears as a small seam around the coarse martensitic laths. Their orientation relationship determined by electron backscatter diffraction is (0001)_α2' // (110)_βo which is typical for the formation of hexagonal α₂&’-martensite. Due to the suppression of γ-phase formation, the α₂'-phase is highly supersaturated in Al. In-situ HEXRD experiments were performed continuously during heating at a rate of 2 K/min. In the diffraction patterns the emergence of shoulders within the α₂-peaks indicates the formation of a new phase possessing a B19 structure. At 645°C, this shoulder is most pronounced before it subsides again. These investigations yield information about the temperature of B19 phase formation and suggests that it acts as a transitional phase in the α₂ to γ transformation. Even though the occurrence of B19 is established in TiAl, only few details are known about the range of existence and the formation kinetics as well as on the morphology of the phase after different heat treatments. In-situ HEXRD experiments combined with TEM investigations are well suited to shed light onto this matter.

Ni-based superalloys have been the subject of intense study for many years owing to their excellent high-temperature strength and creep resistance, which is due primarily to the presence of coherent, elastically hard L12-ordered gamma-prime-precipitates. The substantial solute refractory elemental additions are used to improve the stability of gamma-prime-precipitates by decreasing the coarsening kinetics of the gamma-prime-phase, while simultaneously providing solid-solution strengthening. Molybdenum is known to modify both the morphology and coarsening rate of gamma-prime-precipitates. In this research, we investigate the temporal evolution of a Ni-6.5Al-9.8Mo at.% alloy aged at 978 K through 1024 h, employing both atom-probe tomography (APT) and lattice kinetic Monte Carlo (LKMC) simulations. APT is a three-dimensional (3-D) imaging technique that offers a high degree of analytical sensitivity with sub-nanometer spatial resolution, permitting an atom-by-atom reconstruction of a volume of material in 3-D. Thus, permitting the nucleation, growth, and coarsening behavior to be measured accurately and quantitatively. Vacancy mediated LKMC simulations are used to determine the atomic mechanisms for nucleation, growth and coarsening at early times. All the atomic pair-interaction parameters and vacancy-solute “ghost” potentials are calculated through fourth nearest-neighbor distance using first principles calculations. The Ni-Al-Mo phase diagram is calculated and compared with the experimental phase diagram. The number density of gamma-prime-precipitates, average precipitate radius, and precipitate volume fraction, for hundreds of gamma-prime-precipitates, are measured permitting the identification of three regimes: (i) concomitant nucleation and growth (t < 1/4 h); (ii) growth and coarsening (t =1/4 -4 h); and (iii) Quasi-stationary state coarsening (t >4h). The LKMC simulations assess the stability of the gamma-prime-nuclei and determine their nucleation mechanism at 978 K. With increasing time the number density decreases and the mean radius increases. The maximum value of the number density, (1.88±0.04)×1024 m-3, occurs at 1/6 h, which corresponds to the maximum percentage of gamma-prime-precipitates interconnected by necks, 21 ± 0.5%, and the smallest edge-to-edge inter-precipitate spacing, 5.2±0.8 nm. The quasi-stationary-state coarsening regime occurs for t >4 h, the volume fraction of the gamma-prime precipitates is 16.68±1.2%, in agreement with the equilibrium volume fraction, 16.86%, calculated using Thermo-Calc. In the quasi-stationary-state coarsening regime, the temporal power-law dependence for the mean radius is 0.27±0.02 and -0.83±0.02 for the number density. Lastly, a uniform distribution of spheroidal gamma-prime-precipitate is retained to t =1024 h, where the mean radius is 25.2±5.1 nm, implying that the gamma-prime-precipitates are nearly misfit free with respect to the matrix.

The ordered crystal structure of E2₁ can be regarded as the interstitially stabilized ternary L1₂ by a carbon atom at its cell center. To minimize both elastic energy and chemical energy, carbon atoms are allowed to occupy the interstice at the L1₂ cell center which is facing to all the six nickel atoms tetrahedron. Objective of the present work, aiming at the heat resistant alloys design, is to understand mechanical properties of E2₁ Ni₃AlC_1-x compounds from the viewpoint of resemblance and difference between L1₂ and E2₁ type ordered crystal structures while focusing on the site occupation behavior of interstitial carbon atoms. Single crystals of Ni₃AlC_1-x having various carbon contents from about 1 at.% to 4 at.% were prepared by the directional solidification using the optical floating zone melting method. As the deviation from the stoichiometric composition is known to increase the strength by the solid solution effect, we have tried to keep the compositional ratio of Ni:Al close to 3:1. Mechanical properties of Ni₃AlC_1-x single crystal alloys were evaluated by compression tests in a temperature range from 300 K to 1273 K. Compressive loading axes were selected to be parallel to near [123] and near [100]. Using the transmission electron microscopy and the slip trace analyses, operative slip systems of E2₁ Ni₃AlC_1-x were determined as exactly same as those of the L1₂ Ni₃Al; octahedral (111) slip at low and intermediate temperatures and cube (001) slip at higher temperatures than about 1073 K. On the other hand, mechanical anomaly, so called positive temperature dependence of the strength, cannot be observed clearly since it is completely hidden by the solid solution strengthening effect due to interstitial carbon atoms, which is remarkably large particularly at low temperature range. Critical resolved shear stress on (111) of Ni₃AlC_1-x single crystals increases with the carbon concentration at ambient temperature. Compared with CRSS measured for the binary L1₂ Ni₃Al single crystals, the increment of CRSS due to the solid solution strengthening effect as a function of carbon concentration has been estimated by subtracting value of CRSS-Ni₃Al from each value of CRSS- Ni₃AlC_1-x. It is quite interesting that a huge gap (discontinuity) appears in the increment of CRSS at around 2 at.% C. In other words, CRSS gradually increases for the first 2 at.% C, nevertheless, CRSS jumps up to a quite high value at around 2 at.% C and keep on increasing with carbon content. Note that the same propensity of carbon concentration dependence can be observed in the flow stress as a response to the strain rate dip test which was conducted during compression test.

In the ternary Co-Al-W system a L1_2 ordered intermetallic phase exists in equilibrium with an fcc Co-solid solution matrix. This phase is thermally stable to temperatures above 900 °C. Volume fractions of more than 50 % of the L1_2 ordered phase can be found in these Co-Al-W alloys. Due to the similar crystallographic structure of the L1_2-phase and the fcc matrix the interfaces between the intermetallic precipitates and the matrix are coherent except in overaged samples. Therefore similar γ/γ' microstructures as in Ni-base superalloys can be generated in this new class of Co-base superalloys. But due to the relative novelty of these γ'-hardened Co-base alloys investigations on important aspects of plastic deformation such as creep are still quite rare. In this work different Co-Al-W alloys were deformed in compressive creep and the resulting microstructural changes during creep of these materials were investigated. Since the volume fraction of the L1_2 ordered γ' precipitate phase and the lattice misfit between γ and γ' phase can be influenced by the content of W three alloys with different W-content were investigated. Depending on composition and creep conditions directional coarsening of the γ' particles occurs. The lattice mismatch was measured by diffraction with high energy X-rays at the synchrotron radiation beamline P07 at DESY, Hamburg. Unlike the majority of Ni-base superalloys the directional coarsening is oriented perpendicular to the external compressive stress axis of the creep tests in these Co-base alloys and the lattice mismatch is positive. The preferential orientation of the directional coarsening perpendicular to the external stress axis is in accordance with predictions for the case of an alloy with positive lattice mismatch at creep temperature under compressive external load. The as-crept specimens were also characterized with transmission electron microscopy. Dislocation activity was primarily observed in the γ channels. The dislocations are of ½<110>{111} type. These dislocations were deposited at the γ/γ' interfaces and stretched over a number of precipitates and γ channels. Surprisingly even at creep strains over 4 % a large number of these dislocations were straight. Only the beginning of dislocation reactions at points where two dislocations intersect were observed but no fully developed dislocation networks at the interfaces had developed yet. Additionally cutting of γ' precipitates by partial dislocations was detected by the presence of stacking faults remaining in the precipitates. The observed dislocation structure and directional coarsening of the γ' precipitates can be understood by the positive lattice mismatch of these alloys.

B2-ordered aluminides such as FeAl, CoAl and NiAl belong to the most intensively studied intermetallic compounds. Therefore, it is surprising that the central part of the Co-Al phase diagram, which is dominated by the B2 CoAl single-phase field, is not well known and is drawn with dashed lines in the still accepted and in Massalski&’s handbook reproduced version of the phase diagram [1]. No reliable data on the position of the Al-rich boundary exist and the only available information on the melting point of CoAl is from 100 years old investigations leading McAlister [1] to the statement that “there is reason to distrust available data concerning its melting behaviour”. A number of Co-Al alloys with compositions between 30 and 60 at.% Co were produced and their melting behaviour was studied by DTA. In addition, two-phase CoAl + Co2Al5 alloys were heat-treated at 900 and 1100 °C in order to determine the Al-rich phase boundary of the B2 phase. The resulting revised version of the phase diagram shows that the congruent melting point of B2 CoAl is significantly higher than previously reported. On the basis of the presented experimental results and other more recent data available in the literature, a thermodynamic re-assessment of the whole phase diagram was performed applying the CALPHAD method. Good agreement between the calculations and the experimental data is achieved.

The mechanical properties of two near-equiatomic FeNiMnAl alloys, Fe₃₀Ni₂₀Mn₃₀Al₂₀ and Fe₂₅Ni₂₅Mn₃₀Al₂₀, the microstructure of which consist of (Ni, Al)-rich B2 (ordered b.c.c.) and (Fe, Mn)-rich f.c.c. phases, were studied as function of annealing time at 823 K. The hardness of arc-melt Fe₃₀Ni₂₀Mn₃₀Al₂₀ increased continuously from 454 ± 6 VPN for the as-cast state to 664 ± 8 VPN after a 10 h anneal. For longer annealing times up to 209 h, the hardness was essentially independent of the annealing time. In contrast, the directionally solidified (DS) Fe₃₀Ni₂₀Mn₃₀Al₂₀ showed a hardness of 219 ± 16 VPN independent of annealing time. Fe₂₅Ni₂₅Mn₃₀Al₂₀ showed an as-cast hardness of 413 ± 10 VPN, followed by an initial increase to 461 ± 35 VPN after a 0.5 h anneal, after which no significant increase was observed upon further annealing. This behavior is similar to that of the other B2/L21 two-phase FeNiMnAl alloys. This paper will attempt to relate the mechanical properties to the changes in microstructure.

The objective of this study is to understand the roles of grain boundary (GB) precipitates in strengthening Fe3Al based alloys at high temperatures. In the presentation, first, we present that GB precipitates can be more effective than fine matrix precipitates in the strengthening, based on the strength levels experimentally obtained by tensile tests at 600 degree C in samples with different types of microstructures: (I) film-like kappa-Fe3AlC carbide precipitates covering GBs and fine M2C particles in the matrix, (II) only fine M2C particles in the matrix and (III) no second-phase particles in the matrix. Second, we show that Hall-Petch coefficient changes from a negative value to a positive value by introducing GB kappa precipitate films, based on the experimental results of 0.2% stresses at 600 degree C obtained from samples with different matrix grain sizes each in the presence/absence of GB kappa precipitate films.

Deformation behavior of Fe-23Al-6Ni (at%) single crystals containing the NiAl precipitates was examined focusing on the activated slip system. In the alloys slowly cooled to room temperature after homogenization at 1373 K, the NiAl phase with the B2 structure precipitated in the Fe-Al matrix satisfying the cube-on-cube relationship. The single crystals exhibited high yield stress around 1000 MPa up to 873 K. The NiAl precipitates were always cut by dislocations while no Orowan loop was observed in the deformed crystals. In addition, the activated slip system of Fe-Al-Ni single crystals depended strongly on loading axis and deformation temperature. At <149> orientation and at room temperature, the NiAl precipitates were sheared by {110} <111> slip of the Fe-Al matrix, which is a hard slip for NiAl. This resulted in strong hardening by the NiAl precipitates. On the other hand, {110} <001> slip which is primary slip system of the NiAl precipitates was activated in Fe-Al-Ni single crystals compressed with <557> orientation at room temperature. At <557> orientation, the Fe-Al matrix was sheared by {110} <001> slip, which also led to a huge increase in yield stress. Thus, the difference in primary slip system between the Fe-Al matrix and the NiAl precipitates led to high strength of the alloys. On the other hand, the yield stress of Fe-Al-Ni single crystals abruptly decreased at and above 1023 K, associated with the annihilation of the NiAl precipitates. Moreover, the primary slip system at and above 1023 K was determined to be {110} <111> at both <149> and <557> orientation suggesting that the NiAl precipitates strongly influenced the choice of the slip system.

Both the elastic and the plastic properties of iron aluminides are known to vary widely according to alloy composition and to preparation conditions. In the present study the Young&’s modulus and hardness are determined using nanoindentation techniques for alloys containing from about 20%-50%Al, some containing Cr additions, with materials in the quenched and in the well-annealed states. Modulus is confirmed to be low for Al contents near 25%, increasing at both higher and lower Al contents, but is also shown to be highly sensitive to both Cr addition and to the state of annealing. Hardness variations are equally complex, with hardness higher for Al contents near 25% and near 50%, depending sensitively on the state of annealing, but being independent of Cr additions. Techniques used for assessment of elastic and mechanical properties will be described, and the underlying reasons for changes in properties discussed, as well as the importance of these changes for general engineering application of iron aluminides.

Among low ductility the poor creep resistance is limiting the use of iron aluminides for high temperature structural applications. Besides, in the typical compositional range the Fe-Al phase diagram consists not only of different states of order (disordered solid solution, D03, and B2), but is further subdivided by a low- and high-temperature defect structure modification which may affect high temperature deformation [1]. So far unknown correlations of microstructural evolution on the creep response of centrifugally cast iron aluminides with composition Fe-27Al, Fe-33Al, and Fe-39Al (at%) containing micro-alloying additions of Nb, Zr, C, and B will be shown: The evolution of creep rate at various test conditions (up to 1000 h creep tests at temperatures ranging from 625 to 700°C) together with subsequent microstructural investigation (like TEM, DSC, and Positron Annihilation Spectroscopy PAS) allows us to reveal the influence of the kinetics of microstructural evolution on creep rate starting from the “as cast” state up to steady-state creep regime. After the primary transient all compressive creep curves exhibit a pronounced minimum creep rate at the beginning of testing with a subsequent substantial acceleration towards a steady-state region over an extended region of strain. However, the microstructural reasons for the otherwise similar shapes of creep curves are seemingly different. The minimum creep rate in Fe-27Al can be attributed to the initial presence of D03-order. Since the creep temperatures are clearly within the B2 phase field, the transformation of D03-B2 order causes the acceleration of creep. This effect is temperature- and, thus, time-dependent. Hence, higher temperatures lead to a more rapid transition towards steady-state creep and vice versa. Indications for this correlation were already found by Cahn et. al. for binary iron aluminides in 1960 with much faster kinetics [2], revealing the influence of the alloying elements in our work. By contrast, in the strictly B2-ordered alloys the course of creep rate is attributed to the change of defect structures. Our interpretation is supported by the observed differences in the activation energy for creep being twice as high for D03-order in Fe-27Al, whereas it is not affected by the change and concentration of defect structures in B2-ordered alloys. Finally, our results indicate that the observed alteration in creep rate is only time-dependent and neither driven by applied stress nor strain. [1] N. de Diego, F. Plazaola, J.A. Jiménez, J. Serna, J. del Rio, Acta Materialia 53 (2005) 163. [2] A. Lawley, J.A. Coll, R.W. Cahn, Transactions of the Metallurgical Society of AIME 218 (1960) 166.

Mo fiber reinforced NiAl in-situ composites with a nominal composition Ni-43.8Al-9.5Mo (at.%) were produced by specially controlled directional solidification (DS) using a laboratory-scale Bridgman furnace equipped with a liquid metal cooling (LMC) device. In these composites, single crystalline Mo fibers were precipitated out through eutectic reaction and aligned parallel to the growth direction of the ingot. Mechanical properties i.e. the creep resistance at high temperatures (HT) and the fracture toughness at room temperature (RT) of in-situ NiAl-Mo composites were characterized by tensile creep (along the growth direction) and flexure (four-point bending, vertical to the growth direction) tests, respectively. In the current study, a steady-state creep rate of 10-6s-1 at 1100 °C under an initial applied tensile stress of 150MPa were measured. The flexure tests sustained a fracture toughness of 14.5 MPam0.5 at room temperature. Compared to single or polycrystalline NiAl and other NiAl alloys, these results suggested a large improvement of mechanical properties, which makes this composite a potential candidate material for structural application at the temperatures above 1000 °C. The mechanisms responsible for the improvement of creep resistance at HT and fracture toughness at RT in NiAl-Mo in-situ composites were proposed and discussed based on the investigation results.

A novel eutectic alloy with nominal composition Fe₃₀Ni₂₀Mn₃₅Al₁₅, consisting of alternating B2 and f.c.c. phases, has been found to have good room-temperature strength (yield stress of ~800 MPa) and significant ductility (~10%). The fracture, both before and after annealing, was studied as a function of temperature. The dependence of fracture behavior on both the strain rate and the testing environment was also studied. Different cooling rates were used during casting to produce materials with different lamellar spacing, and, hence, the fracture behavior was also studied as a function of the lamellar spacing. In-situ straining experiments in both a transmission electron microscope and in a scanning electron microscope were used to investigate the deformation and fracture mechanisms. While dislocations were created within the f.c.c. lamellae and piled up at interphase interfaces, no dislocations were generated in the B2 phase. The deformation mechanisms controlling the strength and ductility will be discussed. This work was supported by NSF Grant DMR-0905229.

The concentration of anti-site defects and long range order for ordered B2 AgxMg1-x (0.4

Shape memory alloys exhibit characteristic thermo-mechanical coupling which is related to revertible phase transformations between austenite and martensite. Load, strain and temperature are coupled through the thermodynamic phase equilibrium criterion. The process diagramms exhibit characteristic hystereses and additionally are affected by functional fatigue properties during cyclic loading. In the thermodynamic theory, hysteresis and functional fatigue both are explained by the energetic impact of microstructural elements like phase interfaces, lattice defects and dislocations which are inevitably produced during martensitic transformations and thus dynamically interfere phase equilibrium. In this contribution we present a qualitative molecular dynamics simulation study of this situation. Our model material exhibits a non-diffusive martensitic transformation comprising square to hexagonal lattice transformations in 2D and bcc to fcc transformations in 3D, respectively. We discuss the nucleation process of martensite and show the propagation of motile transformation fronts, martensitic plate growth, the twinning process and the formation/accommodation of martensitic domain structures. During transformation processes, lattice defects are generated which affect subsequent transformations. Some of the defects persist through the transformation, providing nucleation centres for subsequent cycles. Such defects may provide a memory of previous structures, and thereby may be the basis of a revertible shape memory effect. Employing thermodynamic arguments we explain how the energetic implications of such defect structures may contribute to pronounced hysteresis and functional fatigue.

The present study investigates the effects of orientation and specimen size on the pseudoelastic response of NiTi shape memory alloy. The primary goal in this investigation is to determine the means by which the matrix accommodates the large strain associated with the martensitic transformation. This information is critical for extending the working life of components under cyclic loading/heating. It is theorized that the accommodation may take place by matrix plasticity and/or by inducing additional transformation variants, however little experimental verification exists. We demonstrate that micron-scale pillar testing can isolate individual martensite plates, allowing for the investigation of the microstructural evolution related to particular variants in the absence of interactions between competing plates. FIB-machined micropillars of various crystal orientations have been tested in compression and analyzed via mechanical response measurements and post-mortem scanning transmission electron microscopy (STEM) observations. In addition, microstructural FEM simulations that incorporate anisotropic elasticity, the crystallographic theory of martensite, and crystal plasticity as competing deformation mechanisms are conducted to determine the dominant martensite plate type based on the loading orientation. The effects of platen-specimen contact and slight loading misorientation are considered. Finally, local stress fields and plastic slip activity surrounding the internally-twinned martensite plate are analyzed through micromechanics computations and various plate morphologies are considered. The results are compared with STEM observations, and issues surrounding the martensite-induced micro-plasticity are discussed.

Shape Memory Alloys (SMAs) are active metallic materials clasnot;sified nowadays as “smart” or “intelligent” materials. One of the main areas of interest is that of actuators. The use of shape memory alloys in actuators offers the opportunity to develop robust, simple and lightweight elements that can represent an alternative to electro-magnetic actuators commonly used in several fields of industrial applications, such as automotive, appliances, consumer electronics. NiTi SMAs demonstrated to have the best combination of properties. Due to its relatively high recovery stress and strain, actuators providing significant force and deformation can be designed. One of the most critical aspects of this material is that a long sequence of thermo-mechanical treatments are required to manufacture NiTi shape memory alloy components starting from ingots. It is well known that cold-working, annealing and training can strongly affect the shape and texture of NiTi grains, changing dislocation and grain boundary concentrations, inducing martensite twinning and precipitation of several compounds. Functional properties of the NiTi SMA components descend from the actual NiTi microstructure. Information on the microstructural and crystallographic changes associated to different processing routes may help to improve material performances, in view of its application in actuators and smart devices. In the present paper the effects of different thermo-mechanical treatments on the microstructure of fine (diameter=76mu;m) NiTi wires have been investigated by transmission electron microscopy (TEM). The functional and fatigue behavior of such wires have been analyzed by differential scanning calorimetry (DSC), tensile testing and thermo-mechanical cycling. Due to the high number of process parameters that can affect the microstructure and the functional properties, the experimentation was planned by using the Design of Experiments (DOE) method. This approach allowed minimizing the number of preparations and experiments by mixing the selected input parameters. The results were first elaborated by the Principal Component Analysis (PCA) that analyzes the possible trend and relationships between input and output parameters, both microstructural and functional. Data concerning grains size, dislocations density, in-situ TEM thermal transformation, precipitates and/or inclusions will be presented. Evidence and significance of relationships existing between process parameters, microstructure and shape memory properties will be shown and discussed. Finally, this work has led up to the optimization of the shape memory properties of fine wires used in automotive applications. A very short description of this actuator will also be given.

Two most closely associated properties of NiTi alloys are shape memory and superelasticity. As such, most NiTi research has concentrated on near equiatomic compositions. In the present study, we have investigated the Ni-rich 60NiTi (Ni55Ti45 at.%) and 57NiTi (Ni52Ti48 at.%) compositions. These compositions still possesses superelasticity but can exhibit elevated Vickers micro-hardness values. In the hot-rolled condition, these alloys contained the NiTi (B2), Ni4Ti3, and Ni3Ti phases with a Vickers micro-hardness value less than 425 Hv. The alloys were then solution annealed at 1050 deg. C for 10 hours and water quenched. This resulted in a significant increase in Vicker&’s microhardness to 645 Hv for the 60NiTi alloy. Upon aging at 400 deg. C up to 72 hours, the hardness did not change. For equivalent solution annealed and aging condition for the 57NiTi alloy resulted in a peak Vicker&’s microhardness of 450 Hv. After solution annealing, the Ni3Ti dissolved and both compositions retained the B2 matrix phase with a dispersion of nanoscale Ni4Ti3 precipitates. The increase in hardness in the 60NiTi alloy is contributed to prevalent antiphase boundaries within the matrix which was not observed in the 57NiTi alloy.

NiTiHf alloys have garnered interest as a high-temperature shape memory material due to hafnium&’s ability to increase transformation temperatures and its lower raw material cost as an alloying addition as compared to NiTiPd and NiTiPt alloys. The addition of aluminum to this system changes the thermal, microstructure, and mechanical properties of these alloys. The current study characterizes these behaviors by using differential scanning calorimetry, optical and scanning electron microscopy, X-ray diffraction, and compression testing.

Aging the high temperature shape memory alloy 50.3Ni-29.7Ni-20Hf (at.%) at 600°C introduces a new precipitate phase, termed the “H-phase”. It plays an important role in improving shape memory properties such as strength, dimensional stability, work output and superelasticity behaviors. The precipitate phase was investigated by conventional electron diffraction, high angle annular dark field scanning transmission electron microscopy (HAADF STEM), and three dimensional atom probe tomography (3D APT). An atomic structural model is proposed based on the parent B2 phase, from which the H-phase is formed. This model was subsequently relaxed by ab initio calculations, using Vienna Ab Initio Simulation Package (VASP). As a result of the relaxation, atom shuffle displacements occur. HAADF STEM image simulations were also carried out on both the B2 based and the relaxed H-phase structure, using the multi-slice method and code by Kirkland. The simulated images on the relaxed structure yield much improved agreement with the experimental HAADF STEM images. The relaxed H-phase structure can also produce the observed electron diffraction patterns and is consistent with the composition analysis from 3D APT. The relaxed H-phase structure has also been verified to be thermodymanically stable at 0 K.

Characterization of NiTiHf ternary alloys is important due to their attractive high temperature shape memory behavior, useful mechanical properties that include moderate transformation strain and very small irrecoverable strain during load-biased, thermal-cycling tests at moderate to high stresses, and lower cost than other ternary high temperature shape memory alloys. Near stoichiometric alloy compositions consisting of 20(at.%) Hf and varying Ni(49.5(at.)% to 51(at.)%) and Ti concentrations along, with several aging cycles, were analyzed by several microscopy and characterization techniques (CTEM, STEM, HRSTEM, EDS, DSC, and XRD). Additionally, the chemical and aging effects have been examined using isothermal, constant strain rate testing above the austenite finish temperature. The test temperatures were chosen to examine the pseudoelastic property of shape memory alloys. The results indicate that significant strengthening occurs upon aging due to the formation fine precipitates distributed in the sample for the Ni rich compositions. The structure of these precipitates and their interaction with the B2 matrix phase will be discussed with relationship to the desirable shape memory properties.

Ferromagnetic shape memory alloys are a highly promising novel materials class for use in actuator and sensor applications. Miniaturization as functional thin films and coatings for on-chip integration, however, remains a greatly unresolved challenge. This includes practical challenges, such as film synthesis and lift-off, and more fundamental aspects, such as impact of reduced dimensionality and surfaces on materials behavior. In the present contribution we report about successful synthesis of freestanding single crystal Fe-Pd ferromagnetic shape memory films using molecular beam epitaxy and an elaborate lift-off technique. They are subsequently characterized with respect to their structural, thermal and magnetic properties [1]. Residing in the two-phase region of austenite and the correct martensite phase with face centered tetragonal (fct) structure at room temperature, they reveal martensite transition with little hysteresis at 326 K and 320 K, respectively. Comparing substrate-attached with freestanding films, which show fundamentally different magnetic fingerprints, it is proposed that domain structure is capable of posing a bias on the austenite - martensite phase transition by favoring martensite variants with their easy axis aligned along the field - just as the substrate constitutes a mechanical constraint on the transition. This suggests thermo-magnetic actuation as an alternative actuation concept, where only moderate magnetic fields are feasible, but moderate temperature changes are possible. [1] Y. Ma, A. Setzer, J.W. Gerlach, F. Frost, P. Esquinazi and S.G. Mayr, Adv. Func. Mat. 22, 2529 (2012)

New or optimized multifunctional and structural intermetallic materials are needed, e.g. for miniaturization of technological products with improved functionality even in extreme conditions or for efficient production/storage/conversion of energy carriers. For the discovery and optimization of new materials combinatorial and high-throughput experimentation methods are very effective. The materials to be investigated are deposited in the form of materials libraries by special magnetron sputter deposition methods (co-deposition, wedge-type multilayer deposition, shadow masking). These materials libraries are subsequently processed and characterized by high-throughput experimentation methods (automated EDX, XRD, temperature-dependent resistance and stress screening) in order to relate compositional information with structural and functional properties. The talk will cover examples of the combinatorial development of Ni- and Fe-based intermetallic materials for shape memory (Ni-Ti-X-Y, Fe-Pd-X) and other applications. The obtained results are visualized in the form of composition-function diagrams. Examples of up-scaling from thin film findings to bulk applications are discussed

Now a days, aeronautic and aerospace are the more demanding sectors for shape memory alloys (SMA) after the bio-medical one. In particular the interest has been recently focused in very high temperature SMA, which would be able of working as sensors and actuators in the hot areas of the engines and exaust devices. In the present work we realized a study of the Ru-Nb SMA Intermetallics, which undergo two succesive martensitic transformations around 1050 K and 1180 K respectively, depending on concentration. This study has been focused on the measurements of the internal friction spectra and dynamic modulus variation curves up to 1700 K, which have been carried out in a sub-resonant torsion mechanical spectrometer. The internal friction and dynamic modulus have been studied as a function of the heating-cooling rate and the frequency in order to compare the experimental behaviour with the theoretical models for martensitic transformations. In addition to the internal friction peaks linked to both martensitic transformations we have also observed a complex relaxation process around 950 K, which seems to be linked to the interaction of the martensite interfaces with structural defects. An analysis and discusion of the potential microscopic mechanisms susceptibles of being responsibles of such interaction is also presented.

A continuing quest in science is the development of materials capable of operating structurally at ever-increasing temperatures. Indeed, the development of gas-turbine engines for aircraft/aerospace, which has had a seminal impact on our ability to travel, has been controlled by the availability of materials capable of withstanding the higher-temperature hostile environments encountered in these engines. Nickel-base superalloys, particularly as single crystals, represent a crowning achievement here as they can operate in the combustors at ~1100°C, with hot spots of ~1200°C. As this represents ~90% of their melting temperature, if higher-temperature engines are ever to be a reality, alternative materials must be utilized. One such class of materials is Mo-Si-B alloys; they have higher density but could operate several hundred degrees hotter. Here we describe the processing and structure vs. mechanical properties of Mo-Si-B alloys and further document ways to optimize their nano/microstructures to achieve an appropriate balance of properties to realistically compete with Ni-alloys for elevated-temperature structural applications.

Ternary Mg-TM(transition metal)-RE(rare earth) phases with long period stacking ordered (LPSO) structures have attracted a great deal of interest as a new type of strengthening phases in new Mg alloys exhibiting high strength and high ductility simultaneously. The LPSO structures are generally characterized by periodic arrangements of stacking faults within the HCP stacking of parent Mg and also by enrichment of TM and RE atoms in two layers adjacent to the stacking fault. Various polytypes expressed as 10H, 14H, 18R and 24R are reported to form assuming the absence of the in-plane long-range ordering, among which 14H and 18R polytypes are dominantly observed. However, the details of the crystal structure are still controversial. We have very recently investigated the crystal structure of the 18R-type LPSO phase newly found in the Mg-Al-Gd system by scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) and successfully determined the in-plane arrangement of the enriched layers. The 18R-type Mg-Al-Gd LPSO phase is composed of 6-layer structural blocks with fully-ordered atomic arrangement. The enrichment of Gd (and Al) atoms occurs in four consecutive close-packed atomic planes in each structural block and the long-range atomic ordering involving a periodic arrangement of Al₆Gd₈ clusters of the L1₂ type occurs in the four consecutive atomic planes. However, it should be noteworthy that the stacking sequence of the 6-layer structural blocks does not exhibit any long-range order along the stacking direction. Because of these characteristics, the LPSO phase in the Mg-Al-Gd system cannot be described as an ‘LPSO&’ phase any longer in a strict sense but as an order-disorder (OD) intermetallic phase with a so-called OD structure, which have been reported in many minerals but not in intermetallic compounds so far. In the presentation, we will present the details of the crystal structure of the LPSO/OD phases in Mg-TM-RE alloys on the basis of the OD theory and its evolution during heat treatment, especially focusing on the variation in both the ordered nature of the structural blocks and the disordered nature of the block stacking. Deformation behavior of the LPSO/OD intermetallic phases will also be presented.

Mg-alloys containing the long-period stacking ordered phase, the so-called LPSO phase, have been focused due to their superior mechanical properties. In order to clarify the strengthening mechanism of the Mg/LPSO two-phase alloy, we have examined the mechanical properties and the plastic deformation behavior of the LPSO phase itself by using the LPSO-single-phase alloys. Two deformation modes of the (0001) basal slip and the formation of deformation kink were confirmed to dominantly govern the plastic deformation behavior of the LPSO phase. The operation of non-basal slip occurs only under very high stress at room temperature, but it comes to appear at high temperature above 400 °C. In this presentation, the microstructural factors that governs the mechanical properties of the Mg12ZnY LPSO phase is discussed, particularly emphasizing the formation behavior of "deformation kink". Several alloys with largely different microstructures were fabricated by directional solidification and extrusion processes, and their mechanical properties were examined by compression tests. The refinement of the microstructure is significantly effective for strengthening LPSO phase alloys. The yield stress of LPSO phase alloys with a random texture can be increased by reducing the “length” of the plate-like LPSO phase grains, which obeys the so-called Hall-Petch relationship. On the other hand, the formation stress of the deformation kink tended to increase by decreasing the “thickness” of the plate-like LPSO phase grains. The introduction of the deformation kink band was found to effectively increases the yield stress and work-hardening rate of alloys by acting as a strong obstacle to the motion of basal dislocations. The strengthening by the deformation kink band is significant at and below 200 °C, but the effect gradually decreases above 300 °C. Based on the experimental results, the origin of the extremely high yield stress obtained in the LPSO phase extruded alloy is discussed. In addition, the nature of non-basal slip system that is another accommodation mode of deformation in LPSO phase also will be described based on the results of SEM-EBSP analysis for determination of silp plane, and the TEM observation for determination of Burgers vectors of dislocations.

Mo-Si-B alloys have long been considered as prospective candidates for ultra-high temperature applications. However, one of the major issues with these alloys has been their brittleness. The A15 Mo3Si phase, having only four active slip systems at room temperature, is one of the phases that contribute to the brittleness of this system. Besides, it sits between Moss and T1 phase in th phase diagram, preventing coexistence of oxidatively stable T1 and T2 phases with tough Moss phase. Our previous work has demonstrated that certain elements like Niobium and tungsten can destabilize this deleterious phase in the Mo-RM-Si ternary system (RM = W or Nb). In this presentation we will explore the role of W addition in destabilizing the A15 phase in case of the Mo-W-Si-B quaternary alloy system as a function of the refractory metal content and the Si:B ratio. Furthermore, a combination of electron microprobe and single crystal x-rays on the T2 (Mo,W)5SiB2 phase shall be presented in conjunction with thermodynamic calculations in order to explain the solute partitioning in this system. Acknowledgements:This work was supported by the AFOSR HTAM under the contract # FA9550-11-1-201.

Beyond the capabilities of Ni-base superalloys multiphase Mo-Si-B alloys are potential candidates for applications in the aerospace and power generation industry due to their enhanced material strength and oxidation resistance at ultra-high temperatures. It is well known that the microstructure and the resulting properties of Molybdenum based alloys are heavily influenced by the fabrication procedure. Ingot metallurgical processes like arc-casting lead to inhomogeneous and coarse grained microstructures with intermetallic matrices. These materials show a brittle behaviour at low temperatures [1,2]. On the other hand, powder metallurgical (PM) processing using mechanical alloying as the initial step results in a fine grained microstructure with homogeneous phase distribution [3]. However, the ultra-fine grained microstructure of PM materials deforms superplastically at temperatures in excess of 1300°C [4]. This limits the application on the order of 1200°C. In this study we investigate different multiphase Mo-Si-B alloys processed by zone melting (ZM) which starts from compacts of cold isostatically pressed elemental powders. In contrast to conventional casting methods, the distribution and size of the existing phases can be influenced by the growth rate during directional solidification. Microstructural characterization of consolidated alloys with various Mo-Si-B compositions based on SEM shows elongated domains parallel to the growing direction as well as homogeneously distributed phases in cross-section. Our first trials on ZM alloys demonstrated also that for a coarser grained microstructure (in contrast to PM) a comparable ductile-to-brittle-transition temperature of around 900°C could be obtained. First compression creep tests were performed at about 1100°C to get insights into the creep behaviour of the new directionally solidified structures. In comparison to the creep resistance of PM alloys the behaviour of ZM materials was found to be substantially increased. Hence, targeted application temperatures of around 1200 to 1300°C may become feasible. [1] J. H. Schneibel, R. O. Ritchie, J. J. Kruzic, P. F. Tortorelli, Metallurgical and Materials Transactions A, 36 A (2005) 525-531. [2] H. Choe, D. Chen, J. H. Schneibel, R. O. Ritchie, Intermetallics 9 (2001) 319-329. [3] M. Krüger, S. Franz, H. Saage, M. Heilmaier, J.H. Schneibel, P. Jéhanno, M. Böning, H. Kestler, Intermetallics 16 (2008) 933-941. [4] P. Jéhanno, M. Heilmaier, H. Saage, M. Böning, H. Kestler, J. Freudenberger, S. Drawin, Materials Science and Engineering A 463 (2007) 216-223.

Niobium silicide alloys are potential candidate materials to replace Nickel-based superalloys in high temperature applications such as in aircraft turbines or stationary gas turbines. Whilst these new materials possess low densities and considerable creep strength, the rather poor oxidation resistance at elevated temperatures is the most important issue to address. Investigating several alloys, Geng et al. [1] found the simultaneous addition of multiple elements to be beneficial for the oxidation resistance of Niobium silicides, the most promising composition being the multiphase alloy Nb-18Si-24Ti-5Cr-5Al-2Mo-5Hf-5Sn (at.%). However, each component may individually alter the oxidation properties by influencing chemical phase composition, phase evolution and microstructure in the bulk material as well as in the oxide scale. Determining the effect of a single third alloying element on the phases of interest in the Nb-Si system regarding microstructural evolution and oxide scale formation is essential to gain a mechanistic understanding of the oxidation properties. In this matter Ti and Cr seemed to exhibit the most distinct effect. Furthermore, V and Fe were selected because of their known effect on changing the composition of phases, such as enhancing the solubility of Si in the Nb solid solution. Hence, the isothermal oxidation behavior of single- and multiphase Nb-Si alloys at 800 °C and 1200 °C with varying amounts up to 25 at.% of above mentioned ternary alloying additions has been studied. The alloys were prepared by mechanical alloying of elemental powders to achieve homogeneous and fine microstructures. Subsequently the powders were consolidated using spark plasma sintering. The oxidation kinetics, morphology and microstructure of the oxide scales were investigated together with the phases present in the bulk material to correlate scale formation with the bulk microstructure. [1] J.Geng, P. Tsakiropoulos, G. Shao, Intermetallics 15 (2007), p. 270 - 281

We are developing MoSi2/NbSi2 duplex silicide crystals as a promising candidate for ultra-high-temperature structural material aiming at a use over 1673 K. The fabrication of Mo-oversaturated C40-single-phase single crystal and the following appropriate heat-treatment enable to produce the duplex silicide with oriented lamellar microstructure composed of C11b-MoSi2 and C40-NbSi2. The lamellar structure satisfies the crystallographic relationship of (0001)C40//(110)C11b and <1 10>C40// [1 0]C11b, resulting in formation of three variants of the C11b phase in C40 matrix phase. Such a microstructure and its precise control; the crystal orientation of the C40-matrix phase and the geometry of the lamellae, is expected to improve the ultra-high temperature strength and the low temperature fracture toughness; the bottlenecks of the silicides for practical applications. In this presentation, the influence of the control of lamellar microstructure on the high temperature strength is discussed based on the results examined by compression tests performed at 1673 K. In addition, for a practical use of this crystal, the control of lamellar microstructure including the improvement of their thermal stability must be important. We believe that one possible approach to achieve better thermal stability of the lamellar structure is the addition of another element aiming at the reduction of the residual internal strain (lattice misfit) on the C40/C11b lamellar interface. In this presentation, the mechanical properties of duplex silicide crystal depending on the orientation (with respect to the lamellar interface) is described, and the variations in microstructure depending of the ingot composition of (MoxNb1-x)Si2, and the effect of additional elements for the control of lamellar microstructure are also mentioned.

MoSi₂-based materials are promising candidates for ultrahigh-temperature structural applications of ultrahigh efficiency gas-turbine power generation system. Recently, oriented lamellar structure formed in C11_b-MoSi₂/C40-NbSi₂ duplex-silicide has been found to improve high temperature strength and room temperature toughness [1]. More recently, Hagihara et al. [2] demonstrated that Cr-addition is effective to improve the thermal stability of the lamellar structure, which is associated with Cr segregation at the lamellar interface. In practical use, however, the stability of the lamellar structure needs to be further improved. In the present study, we conducted phase-filed simulations of microstructural evolution and interfacial segregation in Mo-Nb-Si-X (X: alloying transition metal) quaternary system to examine the factors responsible for the stability of the lamellar structure on the basis of Calphad-based thermodynamics, micromechanics and first-principles calculations of interfacial energy, segregation energy of solute atoms and lattice parameters of fictitious XSi₂ disilicides for estimating the effects of alloying elements on the lattice misfit. When only the lattice misfit between the two phases was taken into account with the assumption of anisotropic interfacial energy, lamellar structure was not formed but a microstructure with {1 0 -1 x} type habit planes were formed. When the anisotropy of interfacial energy derived from the first principles calculation was taken into account, a lamellar structure similar to that observed experimentally was formed. Thus, it is suggested that the anisotropy of interfacial energy is responsible for the lamellar structure formation. The effect of alloying elements on the stability of lamellar structure will be discussed focusing on the changes in lattice misfit, interfacial energy and kinetics of interface migration. This study is supported by the advanced low carbon technology Research and Development Program of Japan Science and Technology Agency. [1] T. Nakano, Y.Nakai, S.Maeda and Y.Umakochi. Acta Materialia 50 (2002) 1781-1795. [2] K. Hagihara, T.Nakano, S.Hata, O.Zhu and Y.Umakoshi, Scripta Materialia 62 (2010) 613-616.

Niobium based intermetallic composites are candidate materials for future use in land-based as well as aircraft turbines. Their lower density, high temperature strength and oxidation resistance make them potential candidates to replace certain components currently manufactured in nickel-base superalloys. In this study different powder metallurgical processing routes, commonly used for refractory metal based materials, were evaluated on their impact on mechanical properties of a multicomponent Nb-20Si-23Ti-6Al-3Cr-4Hf (at%) alloy. Powder was produced by gas-atomisation, mechanical milling of gas-atomised powders for particle size reduction or high energy mechanical alloying of elemental powders. After that, consolidation was done either by HIPing or powder injection moulding (PIM). Because PIM needs small particle sizes, powder batches of gas-atomised powder (-25 µm), milled gas atomised powder (100 µm milled to -25 µm) and mechanically alloyed powder (-25 µm) were compacted via PIM. Small (-25 µm) and large (106-225 µm) particle fractions of gas-atomised powder were compacted via HIPing for comparison. Quantitative analysis of the resulting microstructures regarding porosity, phase formation, phase distribution and grain size was carried out in order to correlate them with the ensuing mechanical properties such as hardness and yield strength at various temperatures.

Microstructure, oxides and room temperature deformation of directionally solidified and heat treated Nb-24Ti-14Si-4Cr-2Al-2Hf-(0, 0.01, 0.15)Y (at.%) alloys were investigated. The results showed that the addition of Y influenced the element diffusion and caused the lamellar like eutectic structure with higher Ti and Hf concentrations. When Y was added into the alloy, complex oxides rather than HfO2 were formed. The collisions in the liquid seem to be promoted by Y atoms, which make Y oxides have more changes to rise up to the top of the directionally solidified rods. The fracture toughness tensile strength was significantly enhanced by Y addition. After the heat treatment, coarsening behavior of NbSS was suppressed by the addition of Y. Both the fracture toughness and tensile strength were improved for the Y-free alloy, while decreased for the alloys with Y addition after the heat treatment.

Fe-Al intermetallic alloys possess many positive properties which qualify them for application as structural materials at high temperature and in hostile environments. But the low ductility at room temperature limits their application. It has been reported [1-3] that the addition of Li affects the lattice parameter of FeAl, decreases the microhardness, increases the strength and improves the ductility of Fe-Al intermetallic alloys. In this study, Fe-Al intermetallic alloys based on FeAl (B2) with the addition of Li were investigated, in order to evaluate whether the addition of Li retards environmental induced brittleness to improve the ductility or not. The alloys were produced by induction melting under argon and were investigated with respect to their microstructure, bulk composition and mechanical behavior. Microstructural and phase analysis were carried out by light optical microscopy, scanning electron microscopy, X-ray diffraction and differential thermal analysis. Chemical compositions were obtained by wet chemical analysis and energy dispersive spectrometry. The mechanical behavior was investigated by microhardness and compression tests. According to composition analysis, there is a marked solid solubility for Li in Fe-Al. However, results of structural investigations and of mechanical tests differ from those in previous reports. REFERENCES 1. M. Salazar, R. Perez, G. Rosas, ATM Adv. Technol. Mat. Mat. Process. 3, 7 (2001) 2. M. Salazar, A. Albiter, G. Rosas and R. Pérez, Mat. Sci. Eng. A351, 154 (2003) 3. M. Salazar, R. Perez, G. Rosas, Mat. Sci. Forum 426-432, 1837 (2003)

Recycle-type Fe3Al (hereinafter designated as Re-Fe3Al) based alloys reinforced by the carbides of TiC or ZrC were processed by the high frequency induction melting method using a high-carbon Cr steel sludge, Al can scraps and the transition metals of Ti or Zr. The carbides were synthesized by in-situ reaction between the transition metal and carbon in the molten iron aluminum alloy. In our previous works, we have reported that the Re-Fe3Al based alloys have good wear-resistance comparable to a high-carbon Cr steel. Therefore, the Re-Fe3Al based alloys are expected as cutting tools substitutable for a high speed steel (HSS). In the present study, we have evaluated the feasibility of the Re-Fe3Al based alloys with TiC or ZrC for practical use as a cutting tool by cutting tests for pure-Cu (C1020) using a lathe. The cutting performance of the Re-Fe3Al baed alloys was compared with the HSS at three cutting speeds of 100, 200, or 300 m/min. Tool life limit is recognized as one of the most important properties for cutting tools. Tool life limit was estimated from frank wear length after the cutting tests of C1020 by finish machining. The tool life limit of the Re-Fe3Al/TiC was about three times as long as that of the HSS at the cutting speed of 300/min. The relationship between cutting speed and tool life limit (V-T graph) clearly indicated that the Re-Fe3Al/TiC was better than the HSS at a higher cutting speed. Therefore, it was concluded that Re-Fe3Al/TiC has excellent cutting tool performance. In general, a cemented carbide or HSS is used for cutting tools for metals, woods and so on. Therefore, we hope that the Re-Fe3Al based alloys that are rare-earth-free, are used as a substitute of the HSS tool.

The intermetallic B2 compound ruthenium aluminide (RuAl) has often been reported having an outstanding combination of properties which make it an interesting candidate for future applications. RuAl possesses a high melting temperature of about 2300 K, high chemical resistance in aggressive media and good oxidation resistance at elevated temperatures (1473 K). However, several groups have reported that ingot metallurgy results in the presence of δ-ruthenium, deteriorating its chemical and mechanical properties. To the present day, single-phase RuAl has only been successfully produced by powder metallurgy, mechanical alloying of nano powder and self propagating reactions in multilayered thin films. In this work, the potential of laser cladding for the production of single-phase RuAl coatings on technically relevant substrate materials is evaluated by adding pre-mixed elemental powder into a laser melt pool. A 500 W diode laser, a coaxial powder nozzle and a 3-axis CNC system are used for the cladding process. The phase composition of the coatings strongly depends on the process parameters and on the substrate material, due to dilution effects. The use of mild steel, for example, leads to the incorporation of iron into the coatings. Using scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, the effects of diluted substrate material and phase composition are thoroughly investigated. It is found that the clad coatings are free of δ-ruthenium. This makes laser cladding a promising method for the production of intermetallic coatings.

The B2 RuAl intermetallic compound possesses thermodynamic stability in high-temperature aqueous environments aided by a protective alumina layer. It exhibits a favourable combination of physical and chemical properties and shows simultaneously good oxidation-resistance up to at least 1200°C, good thermodynamic stability and high strength at elevated temperatures. In particular the coefficient of thermal expansion (CTE) of RuAl is nearly equal to that of Al2O3 in a large temperature range, thus increasing the adherence of the protective layer even during thermal cycling. These properties make RuAl very suitable as a protective coating material in applications that demand oxidation resistance, e.g. for working layers in moulding dies. This work describes the microstructural and physical characterization carried out on the RuAl thin films synthesized using PVD and the study on the oxidation behaviour at elevated temperatures on nanocrystalline RuAl thin films. Stainless steel (SS) was selected as substrate due to its technical relevance. Thin films were deposited by simultaneously sputtering Ru and Al using a rotating substrate holder resulting in a multilayered structure with a period of about 3 nm. Composition and morphology of the as-deposited thin films were characterized using atom probe tomography (APT) revealing the presence of both intermixed and multilayered zones. Subsequent annealing leads to the direct formation of single-phase B2 RuAl. To study the oxidation behaviour of this SS/RuAl thin film system, oxidation treatments were carried out in ambient air at 750 and 900°C for short times (up to 1h) and the subsequent analysis of the oxide scale was performed using STEM and X-ray diffraction. Oxidation tests show the formation of a dense, compact and well adhered oxide scale. Opposite to what is observed in coarse-grained RuAl bulk, no Al-depleted zone was detected beneath the oxide layer for the first oxidation stages, leading to higher oxide/coating stability.

Extreme functional phenomena are of relevance to alternative energy technologies such as the giant magnetocaloric effect for CFC-free refrigeration [1]. Such phenomena are observed in so-called “magnetostructural materials,” which exhibit simultaneous magnetic and crystallographic first-order phase changes. These transitions can be driven by temperature, magnetic field, pressure and may be tailored by nanostructuring. One example is the equiatomic compound FeRh with a B2 CsCl-type structure. In this work new routes of nanostructuring FeRh are investigated. Samples were prepared by RF sputtering, where sub-3 mm pieces of FeRh were placed in the racetrack of non-magnetic sputtering targets. Targets of either metallic copper or insulating refractory Al₂O₃ were selected to provide diverse matrices for phase formation. Samples of Al₂O₃FeRh (Sample A) and CuFeRh (Sample C) were deposited by RF sputtering at 50 W after the target assembly was conditioned for 10 min prior to deposition to remove contaminants from the target surface. In order to drive the FeRh phase formation, the samples were annealed in evacuated quartz tubes up to 800°C. After each annealing step the samples were characterized by x-ray diffraction (XRD), x-ray reflectivity, scanning electron microscopy (SEM) with energy-dispersive spectroscopy, and SQUID magnetometry. The composition of sample A is approximately 53 at% Al₂O₃ /46 at% FeRh with a thickness of 5 nm, and that of sample C is approximately 82 at% Cu /18 at% FeRh with a thickness of 28 nm. The as-deposited films are smooth and featureless. Upon annealing phase segregation occurs, with formation of micron-sized features in the Cu-based film and nanoscale features in Al₂O₃-based film as observed in SEM images. After annealing the Cu matrix material, structural characterization with XRD confirms the formation of chemically-disordered B2-type FeRh crystallites of 35 ± 10 nm in a nanocrystalline Cu matrix. Annealing the Al₂O₃matrix material produces highly (100)-textured B2-type FeRh crystallites of average grain size 20 ± 5 nm. Magnetization studies indicate a large increase in the room-temperature magnetic moment of both films upon annealing to 800 °C, consistent with formation of ferromagnetic FeRh. However, unlike in B2 bulk FeRh, this ferromagnetic behavior is retained at low temperatures. Overall, these preliminary studies show that post-deposition processing of FeRh in a metallic matrix of high atomic mobility enables formation of the equilibrium FeRh phase and may have applications where high conductivity of the overall material is required. On the other hand, the observed (100) texture of B2-type FeRh in a Al₂O₃ matrix may indicate that a refractory matrix has the potential for controlling the structural properties and hence the transition in FeRh. This work was supported by the U.S. Department of Energy under Contract No. DE-SC0005250 and by NSF-DMR-0907007. 1.Brück, E, J. Phys.D: Appl. Phys. 2005. 38:381

Co-Pt alloys near the nominal eutectoid composition (eutectoid temperature = 730 C) of Co₄₀Pt₆₀ are known to form a unique “nanochessboard” structure that is a 2+1D tiling of L1₀ and L1₂ rods sharing a common parent lattice. Given that L1₀ is a hard uniaxial ferromagnet, while L1₂ has a Curie temperature just below room temperature, the magnetic properties of this nanocomposite could be fascinating. We have processed a bulk alloy at a composition slightly Co-rich relative to the eutectoid. Annealing the FCC parent phase at either 700 C or 600 C, for different aging times, produces only L1₀ ordering according to x-ray measurements, likely related to the asymmetric shape of the two-phase region. Interestingly, we find maximum coercivity of about 4 kOe occurs in a sample annealed at 600 C for 96 hours, where the order parameter is still quite small, and there are no apparent diffraction peak splittings associated with tetragonal L1₀. By contrast, for samples annealed at 700 C, where the ordering can go to completion during extended aging, the coercivity ranges from about 0.6 - 1.0 kOe. Transmission electron microscopy (TEM) of the 700 C annealed samples reveals typical microstructures with platelets of different L1₀ spatial-ordering variants, and in some cases, polytwinned structures seen in many related L1₀ alloys. TEM of the microstructure of the high-hardness 600 C sample is not yet complete, but is currently underway. The relation between the observed coercivities and the ordering microstructure will be discussed. Support of the National Science Foundation under grant DMR-1105336 is gratefully acknowledged.

FePt nanoparticles (NPs) with intermetallic face-centered-tetragonal (fct) structure have attracted enormous research interest due to the unique structure-dependent ferromagnetism and the various magnetic applications. Recently, it is also demonstrated the intermetallic NPs are quite desirable as superior electrochemical catalyst in low-temperature fuel cells. Taking FePtAu trimetallic NPs as examples, we report the structure-induced enhancements in electro-catalytically formic acid oxidation. The Au-doping facilitates the formation of intermetallic structure in FePt, while the excellent chemical stability of fct-FePt in acidic electrolyte results in the enhanced durability of electro-catalyst. In addition, the existence of Au can also help to remove the CO-poisoning on Pt surface in catalytical process. Based exquisite structural manipulation, the optimal catalytic performance can be achieved. The STEM and STEM-EDX characterizations are used to monitor the relationship of structure and electrocatalytic performance.

Laves phase is not detrimental but promising as strengthening species, unlike common sense. For sustainable and reliable energy supply sources, advanced thermal power plants (A-USC) is receiving more attention since shutdown of nuclear power plants by an unprecedented devastating earthquake in Japan. For A-USC in reality, new materials with superior long-term creep rupture strength longer than 10⁵ hours under 100 MPa at 973 K are required. Besides some Ni base alloys, none of the conventional austenitic heat resistant steels such as SUS 347 HTB meet this requirement. Why? This is due to microstructure instability of major strengthening spices/metallic carbides during high temperature exposure. Then, we recently proposed a new type of austenitic heat resistant steels, a carbon free Fe-20Cr-30Ni-2Nb (at%), strengthened by two types of equilibrium intermetallics of GCP (geometrically close-packed) and TCP (topologically close-packed) phases, based on our phase diagram study on Fe-Ni-M (M: transition metals) ternary systems at elevated temperatures. The model steel exhibits excellent creep properties to meet the requirement for A-USC. The superior creep rupture strength is caused by Fe₂Nb (TCP) phase precipitated at the grain boundaries (GBs). The higher the fraction (ρ) of GBs covered by the Laves phase, the lower the creep rate along with the following equation: (creep rate)/(creep rate)₀=(1-ρ), where the (creep rate)₀ is the creep rate with ρ=0. We call the novel strengthening mechanism as “Grain-boundary Precipitation Strengthening (GBPS)”. This result suggests an important message that creep deformation occurs along grain boundaries. Note that the steel with ρ=90 % exhibits excellent creep elongation of 77 %, tensile ductility of 33 %, and Charpy absorption energy of 30 J/cm2 at room temperature, indicating no embrittlement effect by the GB Laves phase. It should also noted that Fe₂Nb Laves phase shows tremendous solid solution softening by Ni in solution and plastically deformed at room temperature. Thus, Laves phase is no more detrimental but promising. In this talk, we cover the nature of Fe₂Nb Laves phase and design concept of the novel austenitic heat resistant steels strengthened by the Laves phase. Part of this study was carried under the research activities of “Advanced Low Carbon Technology Research and Development Program” (ALCA) in JST (Japan Science and Technology Agency), and “Fundamental Studies on Technologies for Steel Materials with Enhanced Strength and Functions” by Consortium of JRCM (The Japan Research and Development Center for Metals) .

Intermetallic Fe₂Nb Laves-phase precipitates have been shown to improve the mechanical properties of a new class of Iron-based austenitic steels. Alloyed with Aluminum to provide corrosion resistance, alumina-forming austenitic (AFA) stainless steels have shown promise for use in power generation plants. The microstructure and tensile properties of two AFA-type alloys, Fe-20Cr-30Ni-2Nb-5Al (at. %) and Fe-14Cr-25Ni-2.5Nb-0.1Si-3.6Al (wt. %) were evaluated. These alloys were homogenized at 1250°C and aged at 800°C for different time intervals. Cold rolling after a solution treatment led to an increase in the amount of heterogeneously nucleated Fe₂Nb precipitates. SEM and TEM analysis revealed that an increase in the size of Fe₂Nb Laves phase precipitates both within the grain and along grain boundaries were accompanied by improved tensile properties after aging. The Laves-phase precipitates alternated with B2-structured NiAl precipitates along the grain boundary. Refinement of the intermetallic Laves phases could allow these materials to meet the need for increased creep life and high temperature strength. This work was supported by ACS PRF Grant #49157-ND10.

The mechanical properties of the Fe₂Nb Laves phase (C14 structure) in equilibrium with γ-Fe (fcc) changes substantially depending on the type of ternary solute atoms. Ni atoms replacing Fe atoms in Fe2 sublattice sites introduces basal planar faults and significantly softens the Fe-rich Laves phase; this suggests that Ni in solution may enhance the plastic deformability of the Laves phase. To demonstrate this possibility, we have examined mechanical properties of the Laves phases by uniaxial compression of ~2 mu;m diameter micropillars produced by focused ion beam milling from a Fe-Nb-Ni ternary alloy consisting of a two-phase microstructure of the Laves phase and γ-Fe. The Laves phase micropillars exhibit high strength of about 6 GPa (~G/30) approaching the theoretical shear strength of the material, followed by a burst of plastic strain and shear failure on the basal plane. If dislocation sources in the form of dislocation loops on a non-basal plane are introduced into the Laves phase micropillars by nanoindentation prior to compression, yielding occurs at a significantly lower stress level of about 3 GPa and plastic deformation by slip proceeds on a pyramidal plane close to (-1-122). Furthermore, if regenerative dislocation sources for basal slip are present in the micropillar, the Laves phase can be continuously plastically deformed in a stable manner to at least 5% strain with plastic deformation commencing at a significantly lower stress of 800 MPa. The corresponding critical resolve shear stress (CRSS) for moving these basal dislocations is ~310 MPa. We thus demonstrate an approach to examine/obtain fundamental plasticity parameters in traditionally brittle materials. Part of this study was carried under the research activities of “Advanced Low Carbon Technology Research and Development Program” (ALCA) in JST (Japan Science and Technology Agency).

Six alloys based on the Cr-Ta-Si system with quaternary additions of Ag, Ti, Hf, Mo, Al and Re were produced by vacuum arc-melting. Following a stress-relieving heat treatment at 1000 degC for 72 hours, the microstructures of the alloys were observed to consist predominantly of an A2 Cr-based solution phase and a C14 Cr2Ta-based Laves intermetallic. Electron probe microanalysis indicated that the elements, Si, Re and Hf partition strongly to the Laves phase, Ti and Al to the solid solution phase and Mo equally between both phases. The lattice parameters of the individual phases in the alloys determined by X-ray diffraction were rationalized on the basis of the atomic size ratios. The bulk hardness of the alloys, along with the nano-hardnesses of the constituent phases were also measured. The room temperature fracture toughnesses of all the alloys were evaluated both by three-point bend tests and indentation methods. Of the six alloys investigated, the Ag-containing alloy exhibited the best toughness. Fractography of the failed samples, nevertheless, showed a tendency for brittle, faceted failure of both phases, indicating primarily a cleavage-type fracture. Crack propagation in all the alloys showed a preference along Laves phase/Cr-solid solution interfaces, or through the Laves phase.

Zinc-coated (galvanized) steel is widely used in applications in automotive and building industries. Zinc is coated to improve the aqueous corrosion resistance of steel by a shielding mechanism called galvanic protection, in which the substrate steel is cathodically protected by the sacrificial corrosion of the zinc coating because zinc is less noble (more electronegative) than iron. The galvanized steel is sometimes further heat-treated (galvannealed) to alloy the zinc coating with the substrate iron by diffusion, resulting in improved coating adhesion and weldability. The coating layer of galvannealed steel consists of a lamellar series of intermetallic compounds in the Fe-Zn system; Γ (Fe₃Zn₁₀), Γ₁ (Fe₅Zn₂₁), δ_1k (FeZn₇), δ_1p (FeZn₁₀) and zeta; (FeZn₁₃) in decreasing order of the iron content. The deformation and fracture behavior of these intermetallic compounds influences the press formability response of the galvannealed steel. During press forming, zinc coating occasionally fails as a result of decohesion at the coating/substrate interface (flaking) and/or particle formation by intracoating failure (powdering). The coating failure occurs more significantly with both increasing Fe content and thickness of the coating layer. It has been reported in review articles that the δ₁ (δ_1k/δ_1p) phase is the most ductile and the Γ (Γ/Γ₁) and zeta; are brittle [1], whereas the zeta; phase is the most ductile and the δ₁ and Γ are brittle [2]. However, the coating failure has been only phenomenologically understood and still under discussion. No mechanical properties of each phase have been investigated except for hardness. If any, the neighboring phases (δ_1k/δ_1p as well as Γ/Γ₁) have not been always distinguished because the layers are as thin as a single micrometer. In the present paper, we investigate compression deformation behavior of each intermetallic phase through compression tests of micropillar specimens in order to elucidate the optimum microstructure of the coating layer. Micropillar specimens with an aspect ratio of 1:3 were machined from galvannealed steel by using a JEOL JIBminus;4000 focused ion beam (FIB). Uniaxial compression tests were conducted with a flat punch indenter tip in a Shimadzu MCT-211 micro compression tester. To our surprise, we found that the Γ phase is the most ductile while the Γ₁, δ_1k and δ_1p are brittle, which is completely different from what has been believed previously. Based on the results, we discuss the optimum microstructure of the coating layer of galvannealed steel for better coating properties. [1] J. Mackowiak and N. R. Short, Int. Met. Rev., 1979, 1 (1979). [2] A.R. Marder, Prog. Mater. Sci., 45, 191 (2000).