The discovery of an ordered L1₂ Co₃(Al,W) phase in the ternary Co-Al-W system in 2006 has led to the development of precipitation hardened Co-base superalloys which build a new class of high temperature materials. Similar to Ni-base superalloys these Co-Al-W alloys possess a face-centered cubic γ Cobalt matrix which is strengthened by a high volume fraction of coherently embedded γ' Co₃(Al,W) precipitates. Although they share many common characteristics with Ni-base superalloys distinct differences also exist. The solidus temperature is relatively high compared to Ni-base superalloys and Tungsten segregates only slightly on the dendrite scale which is beneficial for castability and homogenization heat treatments. Due to the large temperature range between solidus and solvus temperature of the intermetallic γ' phase they can be easily heat treated and they are interesting as potential cast and wrought alloys, however, a γ'-solvus temperature below 1000°C is too low for single crystal applications at very high temperatures. In order to increase the γ'-solvus temperature and to improve other properties of γ/γ'-Co-base superalloys further elements need to be added.
In this work the effect of alloying elements on different properties such as γ'-volume fraction, γ'-solvus temperature and lattice misfit between γ and γ' phase will be shown. Investigations on the γ/γ&’ partitioning behavior show that Tungsten, for example, partitions much stronger to the γ&’ precipitate phase in Co-base superalloys compared to Ni-base superalloys. As a consequence Co-base superalloys possess a positive lattice misfit which causes tensile stresses in the γ matrix channels and a different deformation and rafting behavior compared to Ni-base superalloys with a negative lattice misfit. In addition to the investigations on the microstructure the yield and creep strength at different temperatures of several γ/γ&’ Co-base superalloys will be presented and discussed.

Recently discovered cobalt base fcc / L1₂ (γ / γ&’) two phase alloys have been attracted as a candidate of a new class of superalloys for high temperature structural materials. Though the alloys have a potential for exhibiting a superior creep resistance comparable to modern Ni-based superalloys, the alloys have a poor oxidation resistance and improvements in high temperature oxidation resistance is required for practical applications. There are two ways to improve in oxidation resistance, one is alloying and the other is coating the surfaces with some oxidation resistant materials. The former is more important because some oxidation resistance of the base alloy is required even when coating layers prevent its oxidation. Among the alloying elements of aluminum, silicon and chromium which are well known as elements improving oxidation resistance of alloys, we choose chromium for the improvement, because reported phase-diagrams of Co-Al-W ternary system indicate a difficulty for alloying with aluminum without changing γ / γ&’ two phase microstructure. The oxidation resistance is significantly improved upon alloying with 10mol% of chromium when the oxidation test is carried out at 900 °C in air. However the improvement is less significant at 1000 °C, and the lattice misfit between γ and γ&’ phases is reduced upon alloying with chromium resulting in a rounded shape of the γ&’ precipitates which degrades the mechanical properties of the alloy at elevated temperature.

Phase stability of intermetallic compound of γ&’ with L1₂ structure in Fe, Ni, Co and Cu-based alloys is briefly reviewed focusing on stable and metastable γ (Al) / γ&’ phase equilibria. The γ / γ&’ phase equilibria of Ni-Al-X alloy can be classified into six categories where Ta, Nb and Ti are the strong γ&’ (Ni₃Al) stablilizing elements⁽¹⁾. The solvus temperatures of the metastable γ&’ phase in Fe-based alloys are increased by the additions of Ta, Nb and Ti. This trend can also be seen in the ternary compound of the γ&’ Co₃ (Al, W) phase in Co-based alloys⁽²⁾⁽³⁾. Based on the phase stability of the γ&’ phase, the following advanced alloys have been designed and developed.
New ferrous shape memory alloys: Fe-Ni-Co-Al-based alloys with the γ + γ&’ two-phase structure exhibit an excellent superelasticity and high transformation stress strengthened by Ta⁽⁴⁾ and Nb⁽⁵⁾.
Co-based superalloys: Cast Co-based superalloys have been applied to FSW (Friction Stir Welding) tool strengthened by the γ&’ Co₃ (Al, W) compound⁽⁶⁾.
Cu-based alloys: Cu-Ni-Al based alloys with high strength and high electronic conductivity strengthened by the γ&’ (Ni, Cu)₃Al phase have been designed using the thermodynamic database of Cu-based alloys⁽⁷⁾. This alloy has been commercially produced and used as suspension wire in a DVD drive.
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The effects of varying the Al content in the range 11-15 at. % on the microstructure and room temperature mechanical properties of lamellar-structured FeNiMnAl alloys consisting of B2 and f.c.c. phases have been studied, and the temperature dependence of the yield strength of one composition, i.e. Fe₃₆Ni₁₈Mn₃₃Al₁₃, has been investigated. Decreasing the Al content initially (13 and 14 at. % Al) led to marked increases in both the f.c.c. phase fraction and f.c.c. lamellar spacing, lambda;, but for further reductions in Al content (11 and 12 at. % Al) the lamellar structure was no longer present. The elongation to fracture of the FeNiMnAl alloys increased with decreasing Al concentration from 6.5% at 15 at. % Al to 31.1% at 11 at. % Al. For the lamellar-structured alloys, the yield stress, σ_y, obeyed a Hall-Petch-type relationship with lambda;, i.e. σ_y = σ₀ + klambda;^-1, where σ₀ is the lattice resistance and k is a constant. The compressive yield stress of Fe₃₆Ni₁₈Mn₃₃Al₁₃ was found to be independent of temperature up to 700 K, after which it decreased dramatically due to the softening of the B2 phase. In-situ straining in transmission electron microscope was used to investigate the fracture mechanisms in as-cast Fe₃₆Ni₁₈Mn₃₃Al₁₃.
This research was supported by DOE Award DE-FG02-10ER46392.

The microstructure and mechanical properties of Fe₂₈Ni₁₈Mn₃₃Al₂₁, which consists in the as-cast state of (Ni, Al)-rich B2 and (Fe, Mn)-rich f.c.c. phases aligned along <100>, were studied both before and after various heat treatments using transmission electron microscopy and scanning electron microscopy. It was found that the aligned two-phase microstructure forms by a eutectoid transformation. While the finely-structured (~50 nm) as-cast Fe₂₈Ni₁₈Mn₃₃Al₂₁ was brittle under tension (plastic yield was observed in compression), the alloy showed reasonable ductility (6-9 % elongation) after the phases were coarsened (to several microns) by annealing for times up 250h at 1173K, with a concomitant reduction in yield strength from 1200 MPa to ~550 MPa. Anneals at temperatures up to 1073K produced beta-Mn structured precipitates, which led to fracture under tension before yielding. Interestingly, anneals at 1173K, where the beta-Mn structured precipitates were not observed to form, delayed their precipitation when anneals were subsequently performed at lower temperatures.
This research was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences grant DE-FG02-07ER46392.

For many binary and ternary, transition-metal-based systems, it is known that hexagonal Laves phases of the C14 structure type occur as equilibrium phases having very extended homogeneity ranges. Good examples are the X(Y,Al)₂ Laves phases, where X is an early transition metal as, e.g., Ti, Zr, Nb, or Ta, and Y stands for a late transition metal, e.g., Fe, Co or Ni. In these cases, Al can replace large amounts of the late transition metal leading to a continuous, approximately linear increase of the hexagonal lattice parameters a and c in dependence on the Al content. Interestingly, this is not true for the c/a ratio, which behaves clearly non-linear and in many cases seems to have a minimum value at intermediate Al contents. Various examples will be given and are discussed in relation to details of the crystal structure.

Ferritic heat resistant steels are widely used as high temperature components in power plants etc. The carbonitride and carbide phases such as the M₂₃C₆, VN and NbC are the main strengthening phases in conventional creep enhanced ferritic steels such as ASTM/ASME Grade 91 and Grade 92. These strengthening phases are, however, coarsened and/or decomposed after long-term creep deformation and thereby losing their strengthening effects.
Laves phases (Fe₂Mo and Fe₂W) are formed in a coarsened shape in the conventional ferritic steels, and are therefore not considered as effective strengthening species. Coarsening kinetics of Laves phases is, however, reported to be very slow in Grade 92. Thus, Laves phase might be used as an effective strengthening phase for the long-term creep deformation of 9-12% chromium ferritic steels, if the phase is finely precipitated.
We have recently found that periodically arrayed rows of very fine Fe₂Hf Laves phase particles are formed in 9 % chromium ferritic matrix by interphase precipitation along a eutectoid type reaction of δ ferrite → austenite + Fe₂Hf and a subsequent phase transformation of the austenite into α ferrite. In this presentation, the formation process of the arrayed Laves phase particles and the coarsening behaviors of the particles will be explained. A new strategy for controlling precipitation of Lave phase will be shown for further improving the long term creep resistance of ferritic steels.

In order to achieve energy conversion efficiencies of >50% for steam turbines/boilers in power generation systems, materials are required that are both strong and corrosion-resistant at temperatures greater than 973K, and are economically viable. Austenitic steels strengthened with Laves phase, B2 NiAl and L1₂ Ni₃(Al,Ti) precipitates, and alloyed with aluminum to improve oxidation resistance, are potential candidate materials for these applications.
The microstructure and microchemistry of recently developed alumina-forming austenitic (AFA) stainless steels (Fe-14Cr-32Ni-3Nb-3Al-2Ti (wt.%)) have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). Two series of thermo-mechanical treatments were performed on these steels to improve their mechanical performance. During the treatments, materials were cold rolled to a 90% thickness reduction and then heat treated at 1073K for different times. These thermo-mechanical treatments reduced the grain sizes to the nanoscale (~100 nm) and enhanced the room temperature yield strength to greater than 1000 MPa.
The microstructure and microchemistry of these thermo-mechanically treated AFA stainless steels were characterized and analyzed with SEM and TEM. It was found that a solutionizing anneal at 1473K before cold rolling is effective for uniformly redistributing the large size Laves phase precipitates that formed upon casting. Thermo-mechanical treatments can produce a finer-scale and more uniform distribution of Fe₂Nb Laves phase and B2 NiAl precipitates, both in the f.c.c. matrix and at the grain boundaries. Synchrotron X-ray diffraction results show that the lattice misfit between the Ni₃Al(Ti) precipitates and the iron-based f.c.c. matrix are less than 0.28% after different thermo-mechanical treatments. Both the nanoscale grains (grain boundary strengthening) and the high volume fraction of the L1₂ Ni₃(Al,Ti) precipitates (precipitate strengthening) contribute to the high yield strength, which can be up to 1289 MPa for the thermo-mechanically treated alloys.
This research was supported by the U.S. Department of Energy under NETL Award DEFG2612FE0008857. Atom probe tomography research performed as part of a user project supported by ORNL's CNMS, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE (MKM and LY). Argonne National Laboratory's work was supported under U.S. Department of Energy contract DE-AC02-06CH11357. The authors would like to acknowledge Dr. Yukinori Yamamoto and Dr. Michael P. Brady of ORNL both for providing the AFA stainless steels and for insightful advice.

Understanding of the precipitation behavior of two different phases consisting of the same solute element in certain systems is essential for microstructure / property control, since prior precipitation of one phase would affect the precipitation kinetics as well as the morphology of the other phase. The novel austenitic heat resistant steels of Fe-20Cr-30Ni-2Nb (at.%) developed for A-USC power plant have two different types of intermetallic phases in equilibrium state: one Fe₂Nb Laves (TCP: topologically close packed) phase with C14 crystal structure and the other Ni₃Nb (GCP: geometrically close packed) phase with D0₂₂ / D0_a crystal structure. In our previous study, the former tends to nucleate at grain boundaries whereas the latter in grain interiors. It should be noted that the precipitation of Laves phase at grain boundaries covering more than 80% of the total grain boundary area increases the creep rupture strength. In this study, thus, the effect of Nb supersaturation on the precipitation kinetics of Fe₂Nb TCP phase and Ni₃Nb GCP phase in the novel austenitic heat resistant steels has been investigated, in order to control the precipitation of each phase independently. The Nb supersaturation for the formation of the two phases is controlled by Ni and Nb content of the steels, and the compositions of the steels studied are Fe-20Cr-(34, 35, 36)Ni-(2.5, 3)Nb (at.%). Steels were made by arc melting into button ingots of 35 g. They were solution treated at different temperatures from 12000C to 12700C, and then aged at temperatures ranging from 7000C to 10000C to determine the TTP diagram. Microstructures were examined by FE-SEM and TEM. In the case of Fe-20Cr-35Ni-2.5Nb, Fe₂Nb phase precipitates at grain boundaries, followed by grain interior in any temperature range, and the nose temperature of the Fe₂Nb phase exists at around 9500C, whereas that of the Ni₃Nb phase as meta-stable form of D0₂₂ is around 8000C. At this temperature, Fe₂Nb phase nucleates at grain boundaries first after 120 s, followed by its precipitation within grain interior after 900 s. And then, the meta-stable Ni₃Nb phase precipitates semi-coherently within grain interior after 3 h and transforms to the stable form with D0_a structure after 10 h. Regarding the nucleation, Fe₂Nb phase at grain boundaries is not affected by Ni and Nb content, whereas Ni₃Nb phase is enhanced by increase in both Ni and Nb content. In growth, however, The Fe₂Nb phase is enhanced by increase in Nb content but suppressed by increase in Ni content; On the other hand, the Ni₃Nb phase is enhanced by increase in both elements. From these results, increase in Nb content with decrease in Ni content is effective to enhance the area fraction of Laves phase at grain boundaries. The nucleation mechanism of TCP phase at grain boundaries as well as different decomposition pathways of fcc-matrix to (fcc+Fe₂Nb+Ni₃Nb) three phases due to the different Nb supersaturation will be discussed.

A new family of alumina-forming austenitic (AFA) stainless steels with good oxidation resistance and mechanical strength, shows potential for use in the fossil fuel industry. The high thermal stability of intermetallic Laves phase precipitates makes them promising candidates for improving the high-temperature creep strength of AFAs. The precipitation of the Fe₂Nb Laves phase has been studied in an AFA-type type alloy, Fe-20Cr-30Ni-2Nb-5Al (at. %). The alloy was cold worked in order to introduce a defect structure that would facilitate the precipitation of a finer and denser dispersion of Fe₂Nb. Fe₂Nb Laves size and area fraction both with and without cold work, were determined using an image analysis routine that allowed the comparison of both Laves and NiAl precipitates. Image analysis has yielded a better understanding of the precipitation process of the main target of this work, the Laves phase, as well as providing insight into its unique precipitation relationship with NiAl. Despite particle growth at 700°C and 800°C, for each aging condition, samples with 90% cold work had the highest total volume fraction of Laves and NiAl precipitates as compared to the simply aged state. Having a quantitative assessment of precipitate sizes and growth trends furthers the objective of reconciling how mechanical trends observed relate to the underlying microstructure and provides a new understanding of the effect of Laves phase precipitation in AFAs.
This research was sponsored by the National Science Foundation Grant DMR 1206240.

The study of yield of super alloys has traditionally focussed on polycrystalline alloys, as it is here that the yield properties are most critical for high stress applications such as turbine discs. However it is clear that as the volume fraction of γprime; of these alloys is increased to improve strength, the same mechanisms are seen in both types. The strength of superalloys is associated with coherent ordered precipitates of γprime;: in polycrystalline alloys the emphasis is on the role of the anti-phase boundaries through weak and strongly coupled dislocations but in single crystal alloys Kear Wilsdorf locking is quoted as the source of the increasing strength. Although not often a limiting factor in single crystal alloy design, yield is nevertheless important and is particularly so where stress concentrations result in low cycle fatigue challenges. There are many observations of dislocation pairs in two-phase alloys and a number of cases where the slip plane has been determined and the APB energy calculated. However less attention has been given to as to how the mechanisms put forward to determine the locking and unlocking mechanisms identified in single-phase intermetallics can be transferred to two-phase superalloys. Indeed there are important differences between the two, particularly in the peak temperature at which the effect disappear. In single phase intermetallics the yield stress drops as the material undergoes cubeslip: in two-phase alloys the peak temperature appears to be linked the reduction in the volume fraction of the alloy, the implication being that the two phase structure inhibits cubeslip in the intermetallic phase.
We present observations on yield in the single crystal superalloy CMSX-4®* showing closely paired dislocations forming dipoles in tests interrupted shortly after the yield point and discus the role these play in determining the strength of the alloy. At higher temperature we also see creep-like mechanisms involving stacking faults at the yield point. Both structures are seen in samples fatigued around the yield point. We also show dislocations in polycrystalline alloys and discus how dislocation pairs in complex microstructures experience both weak and strong coupling with the populations of secondary and tertiary precipitates and also how they interact with grain boundaries.
The implications of these observations in modelling strength are discussed, in particular whether it is appropriate to add the separate effects from the different populations of precipitates, grain boundaries and the heavily solution strengthened matrix.
* CMSX-4 is a registered trademark of Cannon Muskegon Corporation
This work was supported by Rolls-Royce plc and the EPSRC under Grant Numbers EP/H022309/1 and EP/H500375/1.

The oxidation behavior of ternary intermetallic subsystems of superalloys, e.g. Ni-Al-Cr was studied by high-throughput methods in a temperature range from ambient to 700°C in air. The composition spread was deposited in the form of thin film materials libraries containing the complete ternary and the three binaries on a single 100 mm diameter substrate. These libraries were then heat treated in vacuum to induce phase formation. Subsequently the libraries were exposed to air at different temperatures and for different durations. The composition-dependent oxidation was observed over the whole materials library by optical and electrical screening methods. Along with the composition-dependent oxidation, also a composition-dependent delamination of the films from the substrate was observed. The study reveals the compositional regions of the binary and ternary thin films which withstand the longest in this harsh conditions.
Funding of DFG within SFB TR 103 is acknowledged.

Lattice misfit is a critical factor in designing superalloys for high temperature applications as it influences the precipitate morphology and stability. The misfit results from a small lattice parameter difference between ordered γ&’ precipitates and the disordered γ matrix. Typically, these lattice misfits are determined using diffraction techniques. Diffraction based characterization methods, such as X-ray or neutron diffraction are volume averaging approaches that preclude real space analysis with local spatial sensitivity, i.e. across interfaces or at defects. For improved spatial resolution, convergent beam electron diffraction (CBED) has been successful but requires prior knowledge of the specimen crystallography and electron energy. In contrast to these methods, real-space analysis using atomic resolution scanning transmission electron microscopy (STEM) enables direct imaging of the crystal structure, but accurate or precise measurements have been prevented by sample drift during imaging. The recent introduction of revolving STEM (RevSTEM) by the LeBeau group, however, corrects this drift distortion to enable picometer precision with sub-picometer accuracy and direct access to lattice misfit.
In this presentation, we demonstrate absolute lattice parameter measurements and γ&’/γ misfit in a NiAlCr superalloy at ambient temperature. STEM images were acquired using a probe-corrected scanning transmission electron microscope (FEI Titan 60-300 S/TEM) operated at 200 kV. Real space analysis of the RevSTEM images enabled simultaneous lattice parameter measurements along two perpendicular directions across the γ/γ&’ interface. We report that the lattice parameter deviates systematically as a function of distance across the γ/γ&’ interface. The observed tetragonality is indicative of lattice distortion due to thermal expansion mismatch. Furthermore, we demonstrate that this technique enables the direct mapping of the strain field in the superalloy system without the need for a reference area within the same image. The authors acknowledge funding for this project from the Air Force Office of Scientific Research (Grant No. FA9550-12-1-0456).

Deformation of these L1₂ compounds is usually carried by <110> dislocations gliding on (111) octahedral or (001) cube slip planes. <110> dislocations are known to dissociate into partials, and these are two different dissociation schemes so far reported. The first one is of the APB-type, in which <110> dislocations dissociate into two collinear superpartials, 1/2<110>, separated by an APB. Further sub-dissociation of each of superpartials is possible, involving a CSF in between. Another is of the SISF type, in which <110> dislocations dissociate into two 1/3<112> type partials separated by a SISF. Which of the two is preferred is considered to be determined by the relative energy of APB and SISF on the octahedral plane. More importantly, dislocation dissociation schemes are believed to determine the temperature dependence of yield stress through the core effects. While the core of 1/2[110] dislocations dragging APB on octahedral plane is believed to be planar, the core of 1/2[110] dislocations dragging APB on cube plane as well as that of 1/3[121] dislocations dragging SISF on octahedral plane is believed to be non-planar, resulting in a rapidly decreasing yield stress with the increase in temperature. However, our recent study clearly indicates that the SISF dissociation does not exist even the SISF energy is significantly low (as in Ir₃Nb). While the APB energy anisotropy and elastic anisotropy determines the presence or absence of yield stress anomaly, the CSF energy controls deformation of L1₂ compounds carried by APB-coupled superpartials. The classification of L1₂ compounds in terms of the temperature dependence of yield stress is discussed in#12288;relation to the CSF energy and the APB energy anisotropy and elastic anisotropy.

Ni based dual two-phase intermetallic alloys, which are composed of L1₂-structured Ni₃Al and D0₂₂-structrued Ni₃V phases, possess excellent mechanical properties and good phase stability at high temperature, compared with those of commercial Ni based superalloys. Because of these desirable features, they are promising for usage as novel high-temperature materials for turbine blades, thermal engine, FSW tools, and so on. For these applications, it is also important to understand the variation of thermal conductivity and heat capacity as a function of temperature. In this study, we investigated the influence of composition and microstructure on the thermal conductivity for Ni₃Al-Ni₃V pseudo binary alloy system. Alloy ingots with nominal compositions of Ni-x mol% Al- (25- x) mol% V (x = 0 to 25) were prepared by arc-melting in an argon atmosphere, and then heat-treated at 1553 K in vacuum for homogenization. The alloys with Al content of x = 0 to 2.5 mol% have a single phase of Ni₃V, and the alloys with x = 15 to 25 mol% a single phase of Ni₃Al. The alloys containing 7 to 12.5 mol% Al have two phases of Ni₃Al and Ni₃V compounds. In the two-phase alloys, the Al content of constituent Ni₃Al and Ni₃V phases was 12 mol% and 2.2 mol%, respectively, which was analyzed by TEM-EDS. The volume fraction of Ni₃Al phase increased with increase of the Al content. The thermal conductivity for the Ni₃Al and Ni₃V alloys with a stoichiometric composition was 37 W/mK and 34 W/mK, respectively. In the single-phase alloys, the conductivities decreased as the content of Al in Ni₃Al phase (or V in Ni₃V phase) was substituted with V (or Al). For the two-phase alloys, the conductivity decreased with increase of Al content. This can be explained in terms of the volume fraction and the conductivity of constituent Ni₃Al and Ni₃V phases: the conductivity of Ni₃Al phase containing 12 mol% Al was analyzed to be 13 W/mK, which was lower than that of the Ni₃V phase containing 2.2 mol% Al (32 W/mK). Therefore, the thermal conductivity of the two-phase alloys decreased with increasing Al content of the two-phase alloys, because the volume fraction of Ni₃Al phase increased with increasing Al content. It was also found that the conductivity of the two-phase alloys increased with increasing temperature. The thermal conductivity at 1073 K decreases as the Al content increases, similarly with the variation of thermal conductivity at room temperature. This is also attributable to increase of the volume fraction of the constituent Ni₃Al phase.

The effect of impurities on the strengthening of Ni₃Al is investigated via ab initio calculations. For each candidate impurity (e.g., Ti, Co, Hf), cluster expansion is performed to predict the total energies of large supercells sampled in Monte Carlo, obtaining antiphase boundary energies as a function of impurity concentration and temperature, from which the optimum conditions are identified. The approach can be readily applied to other systems and has been added to the Alloy Theoretic Automated Toolkit (ATAT) software package.

Intermetallic systems have an impressive interest for engineering and medical applications, as 3D and more and more as 2D materials for micromechanical and microelectronic parts/devices. Intermetallic thin films can be formed by heat treatment of alternate nanolayers of metallic materials that react exothermically. The aim of this work is to compare Ni aluminides and titanides prepared by heat treatment of Ni/Me (Me = Al and Ti) metallic nanomultilayers. Among the five stable compounds of the Ni-Al system, the nickel aluminides NiAl and Ni₃Al are the most promising for high-temperature applications. NiAl with a high melting temperature is very attractive due to its low density, good corrosion resistance and high thermal conductivity. Regarding the Ni titanides, NiTi-based alloys are the most popular due to their superior properties concerning shape memory effect (SME) and superelasticity. NiTi shape memory thin films are of increasing interest in the field of microengineering.
Ni/Me multilayer thin films were magnetron sputtered from pure Ni and Me targets. The substrates were on a thick copper block to avoid heating and consequently intermixing and reaction during the deposition process. The multilayer thin films were designed in order to have an overall atomic chemical composition close to 50Ni:50Me and modulation periods, Λ, ranging from 4 to 140 nm. The overall chemical composition can be easily controlled by adjusting the sputtering power applied to each target, while the modulation period is adjusted by varying the substrates&’ rotation speed. The structural evolution towards the intermetallic phases was investigated in real-time by in-situ x-ray diffraction using cobalt and synchrotron radiation.
In the as-deposited short period multilayers, it is possible to detect traces of intermetallic or amorphous phases, which could influence the phase formation during heat treatment. Heating the Ni/Me thin films leads to disruption of the layered structure by interdiffusion followed by chemical reaction to form intermetallic compounds. In agreement with the overall chemical composition, the desired equiatomic NiMe intermetallic phases were manufactured. Nevertheless, for Ni/Al thin films with periods above 20 nm intermediate trialuminide phases are formed (NiAl₃ and Ni₂Al₃). On the contrary, the formation of B2-NiTi from Ni/Ti multilayers occurs without the formation of intermediate phases whatever the modulation period. For each system the reaction temperature slightly depends on the multilayer period. In addition, it should be noted that Ni and Al react at temperatures below 200 0C, while the reaction temperatures observed for the Ni/Ti multilayer thin films are between 300 and 400 0C.

The intermetallic compound NiAl (B2 Phase) is a potential candidate for high-temperature applications due to its lower density (5.7 g/cm³), higher melting point (T_m = 1638 °C) and heat conductivity (87 W K^-1 m^-1) as compared to Ni-based superalloys. Due to formation of alumina it possesses excellent oxidation resistance. However, its low creep resistance above 600 °C and poor room temperature fracture toughness have prevented so far the use in structural high temperature applications. The introduction of well-aligned and finely distributed ductile fibers respectively lamellae, e.g. by directional solidification (DS) of a eutectic, has been shown to increase both fracture toughness and high-temperature strength, while the density is increased by only 10 %. [1-2]
Nickel aluminide forms quasi-binary eutectics with different high melting point refractory metals such as Cr, Mo, Re, and W. In this study B2-NiAl with different alloy additions (nominal compositions of NiAl-34Cr/ NiAl-33Cr-0.5Mo/ NiAl-31Cr-3Mo/ NiAl-10Mo) has been directionally solidified with growth rates varying from 10 to 100 mm/h. During directional solidification a distinct crystallographic relation between matrix and fibers is established, yielding a <100> orientation for the Cr-fibers as well as for the NiAl-matrix parallel to the growth direction. Two different contactless techniques have been used to produce the material, an optical and an induction float zone process. Hence, microstructural evolution has been characterized as function of the growth rate. Then, the influence on the creep behavior is discussed with particular emphasis placed on the role of fiber orientation on the mechanical response. Specifically, this was evaluated in form of compression creep tests between 900 and 1100 °C with constant true stresses ranging from 100 and 300 MPa. The creep controlling mechanism is examined for chromium-rich nickel aluminide alloys and compared to NiAl-Mo, where the stress exponent has been estimated to lie between 5 to 7 suggesting dislocation climb [3].
Detailed TEM analyses of as-DS and subsequently creep-deformed samples have been carried out. The microstructure of the crept samples shows first plastic deformation taking place in the NiAl matrix, while Cr-fibers are deformed only elastically. After reaching the minimum strain rate, however, the dislocations penetrate the fibers, hence, leading to the observed accelerated creep deformation [1].
1. Dudová, M., et al., Creep in directionally solidified NiAl-Mo eutectics. Scripta Materialia, 2011. 65: p. 699-702.
2. Haenschke, T., et al., Synthesis and characterization of lamellar and fibre-reinforced NiAl-Mo and NiAl-Cr. Journal of Physics,
2010. 240
3. Seemueller, C., et al., Influence of fiber alignment on creep in directionally solidified NiAl-10Mo in-situ composites. Intermetallics, 2013. 35: p. 110-115.

Nanoscaled intermetallic systems like reactive multilayers release energy during rapid self-propagating reactions on a small scale. Hence, bonding with reactive multilayers radically reduces heat-affected zones in the components and opens up the way to reliable and efficient joining in smallest dimensions. This requires high-energy materials systems producing ductile intermetallics. However, this combination has not been achieved so far since intermetallics are usually highly brittle at room temperature. Here, we present a study on the Ru/Al system offering this very beneficial combination of high energy and ductility. Measured propagation velocities of about 10m/s and reaction temperatures up to 1950°C indicate a high density of stored energy. In situ experiments prove the synthesis of pure B2-RuAl guaranteeing the ductility. Our molecular dynamics simulations are in notably good agreement, and reveal the non-equilibrium RuAl formation mechanism as well as the high potential for applications of such systems with tailored reduced dimensions.

The B2 compound ruthenium aluminide has been reported to be ductile at room temperature. Combined with its other beneficial properties like high melting point (>2300 K) and good oxidation and corrosion resistance, this compound has the potential to be the basis for novel high-temperature structural alloys. To achieve this, RuAl must further be studied, focusing on processing, additional alloying elements and mechanical properties.
It is the aim of the present work to investigate the role of iron as a ternary element in B2-RuAl. Different processing routes like hot-pressing of elemental powders and arc melting, as well as the diffusion couple method were used to prepare samples. These were investigated by a combination of X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy and atom probe tomography.
The insights gained from these experiments are manifold. No miscibility gap was found between B2- RuAl and B2-FeAl. Furthermore, only B2-phase was found in alloys with aluminium contents well below 50 at.% which is not the case in binary RuAl. The most interesting result, however, is that already small amounts of iron alloyed to ruthenium aluminide can prevent the formation of hexagonal ruthenium as a detrimental second phase during arc melting. This opens the possibility to produce ductile and thermally stable alloys via ingot metallurgy, circumventing the largest drawback of RuAl.
The site occupancy of Fe atoms in RuAl is discussed, as well as the influence of the amount of iron on the lattice parameter and the mechanical properties of the intermetallic compound.

Iron aluminides based on FeAl or Fe3Al are promising materials for structural applications. As compared to steels, they have low density and potentially low cost combined with high melting point and excellent high-temperature oxidation resistance. The room temperature strength of FeAl alloys can be increased significantly by freezing in the high thermal vacancy concentrations present at elevated temperatures. The vacancy concentrations tend to increase with increasing temperature and increasing Al concentration. In contrast, because of their lower thermal vacancy concentrations, vacancy strengthening in quenched Fe3Al alloys is believed to be much smaller and has not received much attention to date. In the present work, the influence of annealing time and quench temperature on the room temperature strength of extruded and recrystallized Fe3Al alloys is evaluated. For aluminum concentrations between 28 and 32 at% and quench temperatures between 400 and 900°C both the magnitude and the kinetics of strengthening are found to be consistent with reported values for the thermal vacancy concentrations and vacancy migration rates. The mechanical properties were carried out using compression tests. Possible reasons for the observed mechanical behavior are discussed and evaluated in light of information available in the literature. Since the plastic yield of Fe3Al depends strongly on composition, temperature and heat treatment, Fe3Al may be strengthened by one or more of several mechanisms based on aluminum dissolved in iron, precipitation of second phases, antiphase boundary (APB) formation by the movement of uncoupled superpartial dislocations, APBs acting as barriers to dislocation motion, anti-site defects associated with non-perfect long range order (LRO), and vacancies. Their relative contributions to strength may depend on the temperature, duration and cooling rate of prior heat treatments. Consequently, unraveling the dominant strengthening mechanisms in iron aluminides containing less than ~40 at% Al is a rather complex process. To assess the potential contributions of other strengthening mechanisms, appropriate heat treatments will need to be designed in follow-on studies that alter microstructural features relevant to those mechanisms while maintaining a constant vacancy concentration.

Among low ductility the poor creep resistance is limiting the use of iron aluminides for high temperature structural applications. Centrifugally cast iron aluminides containing 27, 33 and 39 at.% aluminum with low amounts (<0.5at.%) of Zr, Nb, C and B have been investigated with respect to the interplay between microstructural evolution and creep behavior. Within the investigated compositional range the Fe-Al matrix phase may consist of different states of order (disordered solid solution, D0₃, and B2) and of precipitates containing the micro alloying elements. The evolution of creep rate at various test conditions (up to 1200 h creep tests at temperatures ranging from 625 to 700°C) together with subsequent microstructural investigation (SEM, TEM, DSC, and EBSD) 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 low plastic strains (< 1%) with a subsequent substantial acceleration towards a steady-state region over an extended region of strain. 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. The presence of D0₃-ordered domains in Fe₃Al alloys beyond the phase transition temperature (D0₃-->B2) was already confirmed by Morris in in-situ TEM studies, however, without showing the influence on creep [1]. By contrast, in the strictly B2-ordered alloys, while the shape of the creep curve is similar to the former one, the creep rate increase is attributed to continuous depletion of micro-alloying elements in the FeAl matrix and subsequent formation of coarse and unevenly distributed precipitates. Our interpretation is supported by the observed differences in the activation energy for creep being twice as high for D0₃-order in Fe-27Al as compared to B2-ordered alloys. Finally, our investigations confirm that the observed alteration in creep rate is only time-dependent and neither stress nor strain driven.
[1] D.G. Morris, M. Leboeuf, S. Gunther, M. Nazmy, Philosophical Magazine A 70 (1994) 1067.

Iron aluminides are investigated since long as a possible alternative for expensive stainless steels or superalloys. Their low density, outstanding corrosion and wear resistance meet the requirements for structural and functional applications up to service temperatures of 1000 °C. Limited ductility at ambient temperature and a lack of strength above 600 °C have limited their use up to now. Additive layer manufacturing (ALM) of these alloys has been hardly investigated, which is characterized by high cooling rates and the possibility of near net-shape production of parts.
To gain first basic experience of processing iron aluminides by ALM, samples of the binary iron aluminide Fe₃Al (Fe-28at.-%Al) have been produced out of pre-alloyed powders by selective laser melting (SLM) and laser metal deposition (LMD). During the SLM process, powder is deposited layer by layer on a building platform and selectively molten according to CAD data. Subsequent lowering of the building platform, followed by the deposition of a new powder layer, paves the way for a consecutive melting step. For LMD, the powder material is fed along the axis of the laser beam through a powder feed nozzle. The powder gas flow and the laser radiation are focused on the substrate surface from which a thin layer is molten. By moving the sample or the optics, a track is built, by overlapping tracks a layer, by multilayers bulk parts are formed.
For the samples produced by both processes, a density of > 99.5 % was achieved and cracks were avoided by preheating the substrate at 200 °C. Due to the strongly directed heat transfer, large elongated grains grew in the building direction, more pronounced for SLM than for LMD. These grains showed a continuous misorientation of 10-14° for LMD samples. Due to higher temperature gradients, higher misorientations of up to 20° are observed for SLM samples.
The anisotropic microstructure of the ALM samples leads to anisotropic mechanical properties. The samples tested perpendicular to the building direction showed markedly higher flow stresses than the samples tested in building direction.
In a second step, the ternary iron aluminide Fe-Al-Ti is investigated. To achieve better mechanical properties, titanium is added for stabilization of the D0₃ structure at higher temperatures and for grain refinement. However, the ductility is decreased by titanium due to solid solution hardening. First results prove the grain refinement effect of titanium. In comparison to the binary alloy, the grains are not directional solidified and have not grown across several layers. The grains have an average grain size of 10 to 20 µm, while in samples of the binary Fe-Al alloy elongated grains of several hundred-micron length formed. The brittleness of the alloy requires pre-heating temperatures of at least 700 °C to build crack free volumes. The hardness of the samples is increased to about 400 HV0.1, compared to approx. 300 HV0.1 for the binary alloy.

Fe-23Al-6Ni (at%) alloys are composed of the Fe-Al matrix and the B2-type NiAl precipitates. Addition of small amount of Ti to the alloys led to the formation of the L2₁-type Ni₂AlTi (H) phase in the Fe-Al matrix. Fe-Al based single crystals containing the H precipitates were prepared by a floating zone method and the deformation behavior was examined. The single crystals furnace cooled from 1373 K had coarse H precipitates with large misfit strain. The precipitates were surrounded by misfit dislocations and were stable below 1130 K. The yield stress of the single crystals remained almost constant at 1.1 GPa up to 823 K and decreased gradually. Moreover, the single crystals containing the coarse H precipitates exhibited higher strength than those without Ti. In the single crystals, 1/2<111> dislocations bypassed the coarse precipitates, resulting in high strength. In contrast, fine precipitates about 10 nm in diameter could be seen in the crystals solutionized and annealed at 823 K. Since the possible slip system of the H single phase is known to be {110}<110>, the precipitates became a strong obstacle to <111> slip in the Fe-Al matrix, which led to high yield stress above 1.2 GPa up to 823 K.

The ternary system Fe - 25 at% Co - 9 at% Mo shows an age hardening behavior similar to aluminum alloys. After solution annealing, followed by rapid quenching the Fe-Co matrix is hardened during aging due to the precipitation of the intermetallic µ-phase (Fe,Co)₇Mo₆. In the overaged condition the entire Mo content is in the coarse µ-phase and, therefore, the matrix consists only of 71 at% Fe and 29 at% Co. The binary Fe-Co system shows a transformation from the disordered bcc-structure to the ordered B2-structure between 25 and 72 at% Co at a critical ordering temperature and, therefore, the remaining overaged matrix in the Fe - 25 at% Co - 9 at% Mo system should also show this transition. However, an ordered phase is brittle and hence unwanted. Better mechanical properties in terms of ductility can be achieved with a partially or fully disordered phase. Such a microstructure can be obtained by rapid quenching of the material from temperatures above the critical ordering temperature. This approach was implemented on the ternary Fe - 25 at% Co - 9 at% Mo alloy. The effect of different cooling rates on the mechanical properties was investigated with hardness and tensile tests. The ordering in the Fe-Co matrix was examined with high resolution transmission electron microscopy, atom probe tomography and neutron diffraction.

B2-type FeAl has attracted considerable scientific attention these decades for its interesting physical and mechanical properties. From the phase diagram, the B2 phase can exist in a large temperature and compositional range but only stable in the Fe-rich side. In a classical model, excess Fe atoms in the Fe-rich FeAl form an anti-site atom on the Al-sites, and thermal defects usually generate in the form of triple defects (TRDs), consisting of two Fe-vacancies and one anti-site Fe atom. This study is aiming at analyzing the long range ordering behavior of Fe-rich FeAl through high-temperature XRD and HAADF-STEM techniques. In this study, single-crystal ingots with the composition of Fe-40 at.% Al were prepared by the optical floating zone technique in an Ar atmosphere. Homogenization and vacancy-elimination heat-treatment were then performed on the ingots. [011]-oriented thin foils cut from the single-crystal ingots and electro-polished were observed by STEM. Meanwhile, high-temperature XRD measurements were done using powder sample. According to the XRD profiles, the long-range order parameter of Fe-rich FeAl was found to increase slightly with increasing temperature in an intermediate temperature range. In the HAADF images of samples quenched from the temperatures, the brightness of Al atoms slightly changes at some atomic columns. These images suggest that re-ordering locally occurred at the intermediate temperature.

Fe-Al alloys with about 55 to 65 at.% Al undergo a rapid eutectoid transformation at 1095 °C: ε (Fe₅Al₈) harr;FeAl + FeAl₂. Hence, the as-cast Fe-Al alloy with about 60.5 at.% Al shows very fine-scaled lamellar microstructure, with average lamellar spacing below 0.5 mu;m, consisting of FeAl and FeAl₂ phase. The microstructure is similar to the α₂ + γ lamellar microstructure of Ti-Al based alloys, which is known for having well-balanced properties in terms of creep, ductility and strength. However, there is limited knowledge about the properties of Fe-Al based alloys in this composition range. In this study, a series of as-cast as well as heat-treated Fe-Al alloys with composition from 55 to 65 at.% Al are investigated. Wet chemical analysis and electron probe microanalysis (EPMA) are applied to obtain the alloy- and phase- composition. The microstructure and crystal structure are studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Differential thermal analysis (DTA) is used to determine all transition temperatures and to establish the homogeneity range of the high-temperature ε phase. The Fe-60.5 at.% alloy is especially analysed by focused ion beam (FIB) for studying the evolution of the lamellar microstructure in three dimensions. Moreover, the first results on compressive creep behaviour of these alloys also will be presented and discussed.

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 (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 particle formation by intracoating failure (powdering), especially when the Γ and Γ₁ layers are thick. Thus, the Γ (Γ/Γ₁) phase has been believed to be relatively brittle [1,2]. However, we have recently revealed that the Γ and zeta; phases exhibits compression plasticity whereas the other three (Γ₁, δ_1k, δ_1p) phases do not exhibit any appreciable plastic strain prior to failure by employing a method based on compression testing of single-phase polycrystalline micropillars prepared from the coating layer of GA steels by the focused ion beam (FIB) method [3]. Nevertheless, nothing is known about operative slip systems and their critical resolved shear stresses (CRSSs) in the deformable Γ and zeta; phases. Micropillar compression testing of single crystals is clearly needed. In the present study, we prepare single crystals of the Γ and zeta; phases by a solution growth method and investigate the plastic deformation behavior of single crystals of the Γ and zeta; phases by compression tests of micropillar specimens made as a function of crystal orientation and specimen size at room temperature, in order to deduce operative slip systems and their CRSS values in bulk [4].
[1] J. Mackowiak and N. R. Short, Inter. Metals Rev. 24, 1 (1979).
[2] A. R. Marder, Prog. Mater Sci. 45, 191 (2000).
[3] N. L. Okamoto, D. Kashioka, M. Inomoto, H. Inui, H. Takebayashi, and S. Yamaguchi, Scripta Mater. 69, 307 (2013).
[4] N. L. Okamoto, M. Inomoto, H. Adachi, H. Takebayashi, and H. Inui, Acta Mater. 65, 229 (2014).

Mo-Si-B-based alloys are considered to be one of promising ultra-high temperature materials capable of operating at temperatures higher than those of Ni-based SX superalloys. However, the high density and low room-temperature fracture toughness of the Mo-Si-B alloys hinder their practical application. 4th and 5th element additions have been investigated to improve the properties of the Mo-Si-B alloys. Quite recently, We have newly developed MoSiBTiC alloys by adding TiC to Mo-Si-B alloys. The MoSiBTiC alloys have excellent properties such as low density, excellent high-temperature (creep) strength and good room-temperature toughness. The MoSiBTiC alloys are stably composed of Mo_ss, T₂, (Mo,Ti)C, and (Mo,Ti)₂C phases. Controlling interfaces between them is essential to further improve the mechanical properties. Therefore, the purpose of this study is to observe and analyze interfaces in the MoSiBTiC alloys including interfacial peeling, micro-cracking and orientation relationship between different phases. The interfaces were observed by high-angle-annular dark-field (HAADF) scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). It was found that Mo_ss and (Mo,Ti)C formed a semi-coherent interface with the specific orientation relationship of {220}_(Mo,Ti)C//{200}_Moss, <001>_(Mo,Ti)C//<010>_Moss, indicating relatively strong cohesion force at the boundaries. Furthermore, it was often observed that (Mo,Ti)₂C near the Mo_ss/(Mo,Ti)₂C interfaces decomposed to Mo_ss and (Mo,Ti)C during heat-treatment at 1,800 °C for 24 h and/or high temperature deformation. The phase transformation may accommodate thermal stress induced at the interface.

We have recently developed NbSi₂/MoSi₂ duplex silicide crystals as a promising candidate for ultra-high-temperature structural material aiming at a use over 1673 K [1]. The fabrication of Mo-oversaturated C40-single-phase single crystal and the following appropriate heat-treatment enable to develop the duplex silicide with oriented lamellar microstructure composed of C11_b-MoSi₂ and C40-NbSi₂. The lamellar structure satisfies the crystallographic orientation relationship of (0 0 0 1)_C40//(1 1 0)_C11b and <1 -2 1 0>_C40//[1 -1 0]_C11b resulting in formation of three variants of the C11_b phase in the 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 high temperature strength and the low temperature fracture toughness. For a practical use of this crystal, thermal stability of the C40/C11_b lamellar microstructure should be further improved. We believe that one possible approach to achieve better thermal stability of the lamellar structure is the addition of minute amount of another elements to induce the segregation of them on the C40/C11_b lamellar interface. Based on the above viewpoint we examined the microstructure, and recently we found that the additions of Cr and Zr effectively improve the thermal stability of fine lamellar microstructure composed of C11_b/C40 phases [2]. In this presentation, the influence of such control of lamellar microstructure to the mechanical properties is discussed. The results of three-point bending test and high-temperature compression test is focused, and the controlling factors of the mechanical properties will be discussed. Reference [1] T. Nakano, Y. Nakai, S. Maeda, Y. Umakohsi: Acta Mater 50 (2002) 1781-1795. [2] K. Hagihara, Y. Hama, K. Yuge, T. Nakano: Acta Mater 61 (2013) 3432-3444.

Semiconducting iron disilicide (β-FeSi₂) is considered to be an environmentally friendly semiconductor since both Si and Fe are nontoxic and occurs abundantly in the Earth&’s crust. This material has been investigated not only as a thermoelectric application but also as a photo-detector and Si-based light emitter operating at wavelengths suitable for optical fiber communications (1.3-1.5 mu;m). This is because β-FeSi₂ has a band gap of approximately 0.80 eV, and a very large optical absorption coefficient of over 10⁵ cm^minus;1 at 1 eV. Recently, photoresponsivity of β-FeSi₂ single crystals is reported to reach to 58 mA/W at 0.91 eV. These optical properties of this semiconducting material have been expected for a development of photovoltanics devices or monolithic optical devices on Si substrates. For the application of this material to those devices, β-FeSi₂ crystal with low defect density is essential. However, the existence of metallic phase of α-FeSi₂, which is thermal equilibrium phase above 1210 K, makes difficult to fabricate high-crystal quality β-FeSi₂ from the melt.
In this paper, we report on β-FeSi₂ growth via vapor-liquid-solid (VLS) method using liquidus phase obtained by Au-Si eutectic reaction, and report on a dramatic reduction of the defect density in β-FeSi₂.
Gold (Au) layers with 40-nm, 20-nm, 12-nm and 3-nm in thickness were deposited on Czochralski (CZ) Si(100) wafers at room temperature by evaporation method (<5×10^-6 Torr). β-FeSi₂ was deposited on these Au-coated Si wafers by using metal organic chemical vapor deposition (MOCVD) method for 1 hour. Iron pentacarbonyl [Fe(CO)₅] and monosilane (SiH₄) were used as sources. The substrate temperature was changed between 773 and 1023 K, and the deposition rate was 1.6 nm/min.
From the SEM and TEM observations, square, trapezoid, and rod-like single crystalline grains with sizes from several hundred nanometers to tens micrometers were observed on the Si substrate surface. The growth of thee coarse island crystal grains indicates heterogeneous silicide-crystal nuclear generation and the growth of those nuclei. A clear photoluminescence (PL) spectrum for β-FeSi₂ was observed up to 220K. These results indicate the formation of high-crystal-quality β-FeSi₂ with a low-level non-radiative center. Moreover, the temperature dependence and excitation power dependence of the PL peak positions and PL intensities indicate that the radiative recombination process is modulated by Au atoms in β-FeSi₂ crystals. This high quality β-FeSi₂ with low defect density opens the door for the real applications.

Significant performance improvement of aircraft engines and land-based gas turbine engines requests the development of a new class of heat resistant materials. Alloys based on Nb silicides are considered as promising candidates because of their good combination of high temperature strength and room temperature toughness. However, one major drawback of Nb silicide based alloys for practical application is the lack in the oxidation resistance at high temperature. Thus some kind of surface coating is required besides significant improvement in oxidation resistance of the substrate materials. For this, long-term stability of coating layer is a matter of consideration. Previous studies have demonstrated that Cr is one of the inevitable alloying elements for improving the oxidation resistance of the alloy. This knowledge yields that the coating for Nb silicide based alloys should be based on NbSi₂ alloyed with Cr, at least. Examination of the phase equilibria between NbSi₂ and CrSi₂ revealed that although both disilicides have C40 structure, they are immiscible below the melting temperature. Vertical section that connects NbSi₂ with CrSi₂ was experimentally determined. Oxidation behavior of Cr doped NbSi₂ shows better oxidation resistance against monolithic NbSi₂, which is believed to be derived from the formantion of Cr₂O₃ layer on the surface.

Today, Ni-base superalloys have been used as a typical high temperature material. Especially, high temperature properties of Ni-base superalloys have been developed enormously by improving the manufacturing process such as directional solidification and single crystal growth methods. As a result, the current maximum operating temperature is about 1150 #8451;. However, considering the melting point (T_m) of Ni-base superalloys (1350 ~ 1450 #8451;) and the maximum operating temperature of the high temperature materials (generally 0.85 T_m), there has been a demand to replace the Ni-base superalloys for further application at higher temperature.
In recent years, refractory metals with excellent thermal properties have attracted attention as a next generation&’s high temperature material. Among these refractory materials, Mo-Si-B alloy showed an improved performance in terms of thermal properties compared to Ni-base superalloys. Also, it has a superior high temperature oxidation resistance. However, Mo-Si-B alloy has low fracture toughness due to formation of intermetallic compounds.
It has been reported that Mo-Si-B alloy which satisfies the high temperature oxidation resistance and mechanical properties including fracture toughness was composed of two intermetallic compound phases (Mo₅SiB₂, Mo₃Si) and ductile α-Mo phase. In particular, for the Mo-Si-B alloy to possess an excellent fracture toughness, α-Mo phase should be formed as a continuous phase with the uniformly distributed intermetallic compound phases.
In this study, in order to obtain a microstructure with a continuous α-Mo phase, the core-shell intermetallic compound powder consisting Mo₅SiB₂ and Mo₃Si phase as the core and nano-sized Mo particles surrounding the core was fabricated. First, intermetallic compound powder was prepared via Mechano-chemical process and then core-shell intermetallic compound powder was fabricated through the Chemical vapor transport of Mo phase which was generated during hydrogen reduction of Mo oxide powder. Pressureless sintering was carried out at 1400#8451; for 10 hours. Sintered Mo-Si-B alloy showed a relative density of about 95% and uniformly distributed intermetallic compound phases. As a result of three point bending test (ASTM E 1820-01), fracture toughness was 12.6 MPamiddot;m^1/2 which showed about 40% improvement compared to other researches.
It is concluded that a Mo-Si-B alloy with an enhanced fracture toughness was successfully fabricated at a relatively low sintering temperature from using core-shell intermetallic compound powder.
Acknowledgment
This work was supported by a research fund of the Agency for Defense Development (ADD) (Project No. 211155-911059011).

In order to increase the energy efficiency, the light weight and the high temperature and pressure of the working gas are required for jet-engines and gas-turbines. Currently, Mo-Si-B alloy which has excellent high temperature strength has attracted great attention to replace the Ni-base superalloys as a new ultra-high temperature material. However, its low fracture toughness and high density are drawbacks. Quite recently, our research group has developed MoSiBTiC alloy by adding TiC to the Mo-Si-B alloys in order to improve mechanical performance. For further improvement, the optimization of its microstructure consisting of Mo solid solution (Mo_ss), the intermetallic and carbide phases is necessary. In addition, the reduction in the density would be more attractive. In this study, the effect of Ti addition on the density and microstructure development of MoSiBTiC alloy was investigated. MoSiBTiC alloy with the compositions of Mo_(65-x)Si₅B₁₀Ti_(10+x)C₁₀ (x = 0, 5)(at.%) were prepared by conventional arc-melting. Homogenization heat-treatment was conducted at 2073 K for 24h in an Ar atmosphere. The primary phase in both the alloys was TiC and had two eutectic phases of Mo_ss+TiC and Mo_ss+Mo₅SiB₂+TiC. With 5 at. %Ti addition, the formation of Mo₂C was suppressed during solidification, and thus there was no eutectic phase of Mo_ss+Mo₅SiB₂+ Mo₂C in the Ti-added MoSiBTiC alloy. The density was reduced to approx. 8.3 g/cm³ from 8.8 g/cm³ by the Ti addition. Therefore, it is found that Ti is an effective element for the development of the MoSiBTiC alloy.

There is an increasing demand for new structural materials which can be used at very high temperatures above the upper limit of Ni-base superalloys. MoSi₂-Mo₅Si₃ eutectic composite is one of the candidates because of its high eutectic temperature of 1900 0C and very fine microstructure of script-lamellar type formed simply by directional solidification. However, it is critical to further improve its poor fracture toughness at room temperature for its practical application. The purpose of this study is to investigate the effects of ternary additions on the microstructure and room temperature fracture behavior of MoSi₂/Mo₅Si₃ eutectic composites. Directionally-solidified ingots of MoSi₂/Mo₅Si₃ eutectic alloys containing twelve different elements (Ti, V, Cr, Fe, Co, Ni, Nb, Ta, W, Ir, B and C) were grown by optical floating zone method at various growth rates ranging from 5 to 50mm/h. Strong segregation on the interphase boundary is noted for alloying elements (Fe, Co, Ni, Ir, B, C) whose solubility in MoSi₂ and Mo₅Si₃ is negligibly small. As a result, microstructure refinement and change in the interface morphology occur upon alloying with these elements at a very small level of alloying additions (less than about 0.1 ~ 0.16 at.%). The observed change in the interface morphology is considered to occur due to the fact that the interface energy is reduced by segregation of these alloying elements. Among six elements (Ti, V, Cr, Nb, Ta and W) with a relatively high solubility in MoSi₂ and Mo₅Si₃, microstructure refinement and increase in the lattice misfits are achieved by the ternary additions of Ta and W. Although the morphology of the interfaces in the homogeneous script lamellar structure is not significantly altered upon alloying these elements, the interface energy is considered to increase upon alloying Ta and W through the increase in lattice misfits. In fact, the Ta-alloyed DS ingots exhibit the highest propensity for delamination along the interfaces when fracture behavior was investigated by an indentation fracture method. This result suggests the possibility that the fracture toughness of MoSi₂/Mo₅Si₃ eutectic composites can be improved by increasing the interfacial energy through lattice misfit control by alloying elements such as Ta and W.

Among various refractory materials, many investigations have been conducted on Nb-silicide based alloys because of their high melting point and low density. However, these alloys show poor ductility at low temperature. Microstructural control based on the “Ductile-phase toughening”, where a fine scaled brittle phase is dispersed in a ductile phase matrix, is an effective approach to improving their poor ductility. In this microstructure, the small brittle phase tends to be a single crystal and the fracture toughness of single crystalline alpha-Nb₅Si₃ is needed to understand -the fracture behavior of Nb/alpha-Nb₅Si₃ two-phase alloys. However, it is difficult to prepare single crystalline or coarse grained alpha-Nb₅Si₃ with size enough for conventional fracture testing methods, such as bending test for macro specimen or indentation fracture method, because alpha-Nb₅Si₃ is formed from the high temperature phase of beta-Nb₅Si₃ through a polymorphic phase transformation. In this study, we attempt to establish a micro-sized fracture test for single crystal alpha-Nb₅Si₃ by preparing the micro-sized specimen from alpha-Nb₅Si₃ polycrystals with the average grain size of about 80 micrometer. Alloy ingots of Nb-37.5 at.% Si were arc-melted under an Ar atmosphere, followed by a heat-treatment at temperature up to 1650 degree C for 4 hours. The specimens were milled by focused ion beam technique. The dimensions of the alpha-Nb₅Si₃ cantilever beams are 3 x 3 x 15 micrometer, and a chevron notch with the notch angle of 90 degree was introduced near the fixed end of each beam. The beams were loaded by using a nano-indentation equipment at room temperature. The fracture toughness at room temperature was evaluated to be about 3 MPa m^1/2, which is higher than that previously reported.

MoSi₂ has been considered as a promising base material for structural applications at very high temperatures above 1500 °C. Extensive studies on the mechanical properties have revealed that MoSi₂ single crystals with the loading axis orientations away from [001] exhibit significant plastic flow at temperatures as low as room temperature, whereas those close to [001] can be plastically deformed only above 900°C. Five slip systems on {1-10}<111>, {011}<100>, {010}<100>, {023}<100> and {01-3}<331> have been confirmed to be operative depending on crystal orientation and test temperature. Among these five deformation modes, CRSS for {01-3}<331> slip system exhibits strong anisotropy with the highest values obtained for the loading axis orientation of [001] (hard orientation), although the underlying mechanism has not been fully clarified yet. Recently, mechanical tests using micropillars of single crystals have been shown to be useful for studying fundamental deformation behavior of brittle materials. In the present study, micropillars of single crystalline MoSi₂ with various loading axis orientations and sizes have been prepared and tested at room temperature to obtain more details on various slip systems in MoSi₂. Five different slip systems were successfully activated by controlling the loading axis orientation. The {01-3}<331> slip were successfully activated for micro-pillar specimens not only with the soft orientation (near [010]) but also with the hard orientation of [001]. CRSS values for all slip systems exhibit size-scale effect, i.e., ‘smaller is stronger&’ phenomena, following power-law relationships with different power-law exponents depending on the slip systems.

Intermetallic titanium aluminides solidifying as primary via the β-phase are of great interest for several high-temperature applications. Besides the common α/α₂- and γ-phase they possess a significant volume fraction of the disordered body-centered cubic β-phase at elevated temperatures. This phase ensures good hot-working behavior. Nevertheless, the practical use of such multiphase alloys at a temperature as high as 800°C or above requires sufficient oxidation protection. Therefore, the thermocyclic oxidation behavior of three b-solidified γ-TiAl-based alloys at 800°C in air, with and without fluorine treatment, is reported in this presentation. The behavior of the known TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B, in at.%) is compared with that of two Nb-free model alloys which contain different amounts of Mo (Ti-44Al-3Mo-0.1B and Ti-44Al-7Mo-0.1B, in at.%). During thermocyclic high-temperature exposure in air a mixed oxide scale develops on all three untreated alloys. This non-protective behavior can be improved via the so-called fluorine effect, as demonstrated for previously investigated TiAl alloys with a two-phase microstructure (α₂ + γ). Small additions of fluorine in the subsurface region of the alloys change the oxidation mechanism from mixed oxide scale formation to alumina. The above mentioned alloys were treated with fluorine prior to high-temperature exposure. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. Due to the fluorine treatment all alloys exhibit slow alumina kinetics. The results of thermocyclic oxidation tests at 800°C in air are presented and discussed with focus regarding the fluorine effect on the oxidation resistance of this new kind of multiphase TiAl-alloys.

Intermetallic β-solidifying TNM alloys with a composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%) exhibit excellent processing characteristics due to the high amount of disordered β-phase present at hot-working temperatures. Balanced mechanical properties can be tailored by adjusting the material&’s microstructure during a post-forging heat treatment consisting of homogenisation, high-temperature and stabilisation annealing. In the present work, a TNM alloy with an increased content of β-stabilising alloying elements (Nb, Mo) was investigated by means of in-situ high-energy X-ray diffraction experiments. In a dilatometer setup, forged and homogenised specimens were annealed at temperatures below the γ-solvus temperature and subsequently subjected to various technologically relevant cooling rates ranging from 35 to 1200 K min^-1. The correlation between these cooling rates and the resulting microstructures was investigated with respect to the evolution of phase fractions as a function of time and temperature. Thereby, the focus was laid on the evolution of the γ-TiAl phase, for which a continuous cooling transformation diagram was derived. Heating experiments close to thermodynamic equilibrium conditions and quantitative metallography on heat-treated and water-quenched specimens gave additional information needed for temperature calibration. The microstructural evolution of the specimens during processing as well as after different cooling stages from the high-temperature treatment was investigated by means of scanning electron microscopy. The performed in-situ diffraction experiments with synchrotron radiation allowed gaining a deeper insight into the phase transformation behaviour of this type of alloy, which is not accessible with conventional characterisation techniques.

Ti₃Al with the D0₁₉ superlattice structure based on the h.c.p. (hexagonal close-packed) lattice has attracted a great deal of interest as a new class of light-weight high-temperature structural material since it is not only the major constituent phase of the so-called super-α₂ alloys but also the minor constituent phase of the lamellar structure in two-phase TiAl-based alloys. Previous studies have revealed that slip on {1-100}<11-20> (prism a-slip), (0001)<11-20> (basal a-slip), {0-221}<11-26> (type I pyramidal 2a+c-slip) and {11-21}<11-26> (type II pyramidal 2a+c-slip) are the operative slip systems and their CRSS strongly dependent on chemical composition. In the lamellar TiAl-based alloys, Ti₃Al lamellar has a thickness in the submicron to micron range. Therefore, information on the deformation behavior at the submicron/micron scale is important to understand further the fundamental deformation mechanisms of the lamellar TiAl-based alloys. Although the deformation behavior of Ti₃Al has been extensively studied using bulk single crystals, the deformation behavior of Ti₃Al at submicron/micron scale has not been explicitly investigated yet. Recently, mechanical tests using micropillar specimens have received a considerable amount of attention as a new method to investigate size-scale effects on deformation behavior of materials at the sub-micron/micron scale. In the present study, compression tests of micropillars of Ti₃Al single crystals were carried out as a function of loading axis orientation, specimen size and chemical composition in order to obtain fundamentals of the deformation behavior of Ti₃Al at the submicron/micron scale. When the loading axis is parallel to c-axis, {11-21}<11-26> pyramidal slip was confirmed to be activated in all tested micropillar specimens of Ti₃Al with Al-rich composition of Ti-36.5at.%Al. In the stress-strain curves, strain-burst behavior was observed and the amount of burst strain was found to depend on the specimen size. CRSS values estimated from the stress for the first strain-burst are higher than that obtained for bulk single crystals and increase with decreasing specimen size approximately following a power-law relationship. Details of the results including size-scale effect and compositional dependence for various deformation modes in Ti₃Al will be presented.

Effect of lamellar colony boundary β-Ti phase on crack propagation behavior in Ti-42Al-8V and Ti-43Al-5V-4Nb (at.%) has been examined in terms of in-situ bending and tensile test devices in SEM. These alloys were selected based on our design principle for wrough TiAl alloy with a unique transformation pathway of β-Ti+α-Ti→α→α+γ-TiAl→β+α+γ Fully lamellar microstructures with and without colony boundary β phase were propduced in the folloiwng heat treatment using the phase transforamtion: firstly in β+α two-phase region, just above the α shingle phase region, and held there for 1-2 hours, followed by controlled cooling down to α+γ two-phase region though the α single phase region. In both microstructures, crack mainly initiates and propagates along lamellar interfaces having low angle with respect to the loading axis (theta;). The crack stops at the colony boundaries in case that the lamellar angle (theta;) of the neighboring grain in front of the tip is high, whereas it comes easily through the boundary when its angle is low. However, in the specimen with colony bounadary β phase, main crack stops at the β phase even through the angle is low, and many slip lines occur in the β phase. The abosrption energy calulated from the load-displacement curves is 20 mJ for fully lamellar microstructure where that with the β phase becomes 35 mJ even though the lamellar volume fraction is 75 %. This clealy demonstrats that the β phase is effective in crack propagation resistance due to the plastic deformation, leading to higher toughness. The in-situ tensile test rerults with the same micorsturctures will be presented, and the effect of strain rate on the resistanct to cracking of the β phase will be discussed.

A dual two-phase microstructure composed of geometrically close packed (GCP) Ni₃Al (L1₂) and Ni₃V (D0₂₂) phases is formed at high temperature by a eutectoid-type reaction from A1 (fcc) phase to L1₂ and D0₂₂ phases. The intermetallic alloy with the dual two-phase microstructure shows attractive mechanical properties as high-temperature structural materials [1]. However, further increasing in mechanical properties (strength, elongation, and so on) is required to be used as advanced high-temperature structural materials. For example, it has been studied that solid solution strengthening by addition of Ti, Nb [2] and Ta is effective for improving strength as well as hardness properties [3]. In this study, the effect of Si addition on the microstructure and mechanical properties of the dual two-phase intermetallic alloys is investigated. It is expected that Si addition increases oxidation-resistance, and reduces density and cost. Si was added to the base alloy composition Ni₇₅Al₉V₁₃Nb₃+50 wt.ppm B by three substitution methods for Ni, Al and V, respectively. Alloy button ingots prepared by arc-melting were solution-treated at 1280 0C for 5 h in a vacuum. Microstructure observation was carried out by scanning electron microscopy (SEM), electron probe micro analysis (EPMA), and X-ray diffraction (XRD). Mechanical properties were evaluated by tensile test (at room temperature and 1073 K) and Vickers hardness test (at room temperature). The 1 at.% Si-added alloy maintained the dual two-phase microstructure for either substitution method. On the other hand, the 2 at.% added-Si alloys did not maintain the dual two-phase microstructure, and contained third-phase dispersions, irrespective of substitution method. The third-phase dispersion was identified to be Ni₁₇Si₇Nb₆ (T phase). Accordingly, solid solubility limit of Si to the present dual two-phase intermetallic alloy composition was shown to be under 2 at.%. Regarding mechanical properties, yield and tensile strength of the 1 at.% Si-added alloys were similar to or higher than those of the base alloy while elongation was lower than that of the base alloy, for either substitution method. Strength property as well as elongation of the 2 at.% Si-added alloys were little improved. Consequently, it is revealed from this study that Si soluble in the dual two-phase microstructures is a useful element for improving the chemical, physical and mechanical properties as advanced high-temperature structural materials.
[1] Y. Nunomura, Y. Kaneno, H. Tsuda and T. Takasugi, Acta Materialia, 54 (2006) 851-860.
[2] K. Kawahara, T. Moronaga, Y. Kaneno, A. Kakitsuji and T. Takasugi, Materials Transaction, 51 (2010) 1395-1403.
[3] T. Moronaga, S. Ishii, Y. Kaneno, H. Tsuda and T. Takasugi, Materials Science and Engineering A, 539 (2012) 30-37.

Ni base superalloys are known as a high temperature structural material. In order to further improve their high-temperature strength, a Ni base dual two-phase intermetallic alloy has been developed [1]. The alloy shows a characteristic microstructure composed of Ni₃Al (L1₂) and Ni₃V (D0₂₂) phases, and has higher strength than the conventional Ni base superalloys at elevated temperatures. We have reported that the mechanical properties of the dual two-phase intermetallic alloys can be enhanced by alloying: for instance, Nb and Ti enhance the strength while Ta and Re increase the hardness. [2, 3] In this study, the effect of W addition on microstructure and mechanical properties of the dual two-phase intermetallic alloys has been investigated.
W was added to the base alloy in place of either Ni, Al or V. Alloy ingots were made by argon arc-melting, and were solution-treated at 1553K for 5h. Microstructures were examined and characterized by FE-SEM, EPMA and XRD. Mechanical properties were evaluated by Vickers hardness and tensile tests.
The microstructural observations revealed that the dual two-phase microstructure comprised of the primary Ni₃Al precipitates and the channel regions was maintained irrespective of the manner of W addition, although the cuboidal shape of the primary Ni₃Al precipitates was somewhat changed. Also, it was found that needle-like dispersions were formed in the primary Ni₃Al precipitates in the alloy in which W was substituted for Ni. The addition of W yielded a remarkable increase in hardness in all the alloys. The yield stress at room temperature was significantly enhanced by W addition, whereas the enhancement was moderate at 1073K. The observed increase in hardness and strength is possibly due to solid-solution strengthening, and/or increase in the flow stress for mechanical twinning, particularly at high temperatures.
[1] Y. Nunomura, Y. Kaneno, H. Tsuda and T. Takasugi, Acta Materialia, 54,851-860 (2006)
[2]K. Kawahara, T. Moronaga, Y. Kaneno, A. Kakitsuji and T. Takasugi, Materials Transaction, 51(2010), 1395-1403, No.8.
[3]T. Moronaga, S. Ishii, Y. Kaneno, H. Tsuda and T. Takasugi, Mat Sci Eng. A, 539(2012) 30-37.

Hereby the additive manufacturing process of electron beam melting was considered with a purpose to investigate the possibility to produce a final functional thin-wall product of intermetallic NiAl powders by this method. After preparing the intermetallic powder, samples from this powder were melted using various selected parameters: preheating, power and scan velocity of electron beam. The process window was optimized. Microstructure, phase structures, microhardness, density and surface roughness were studied depending on process parameters. Reasons of several process difficulties such as melt ball formation and layer delamination were discussed.
Keywords: electron beam melting (EBM), NiAl intermetallics, thin-wall process optimization

Ni₃Nb (δ) with D0_a crystal structure is one of the GCP (Geometrically close-packed) phase observed in wrought Ni-base superalloys containing a certain amount of Nb, such as IN718, the most common alloy for jet engine component. However, this phase is not expected as strengthener but rather considered as detrimental phase due to the Widmanstatten morphology in fcc matrix after long-term exposure. The main strengthener of the alloy is the metastable form of Ni₃Nb (γ”) with D0₂₂ crystal structure. Thus, the temperature capability of the alloys are limited up to 973 K below the nose temperature where the formation kinetics of the transformation from γ” to δ becomes sluggish. Thus, the δ phase is mainly used as obstacles for suppression of grain growth in hot-forging process at sub-solvus temperatures. However, recently several attempts have been made to increase the temperature capability of the alloy, because of demand for efficiency improvement by increase in combustion temperature. One of the promising methods to increase the creep strength is to decorate the grain boundaries by stable intermetallic phase as “Grain-Boundary Precipitation Strengthening (GBPS)” mechanism. The higher the area fraction ρ (fraction of grain boundaries covered by the precipitates against total grain boundaries) above 80%, the creep resistance and creep rupture strength increase. In this study, thus, precipitation behavior of the GCP δ phase at grain boundaries has been examined using the alloy IN718 and some model alloys. The TTT diagram of the precipitation of δ phase in the alloys was constructed after the complete homogenization above the solvus temperature, and the heat treatment was conducted above and below the nose temperature of 1198 K. At the higher temperature, the δ phase nucleates at grain boundaries with spherical shape, but it tends to grow to one of the grains with orientation relationship of {111}γ//(010)δ, <110>γ//[100]δ, leading to the ρ as high as 65% even after the long-term aging. Below the nose temperature, the δ phase precipitaes within grain interiors with less ρ at grain boundaries. Effect of prestrain and minor alloying effects on the nucleation of δ phase using the model alloys will be presented in conjunction with the interfacial energy and supersaturation of the Nb in the alloy matrix.

New or optimized multifunctional 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 intermetallics to be investigated are deposited as thin film materials libraries by special magnetron sputter deposition methods. These libraries are subsequently processed and characterized using high-throughput experimentation methods such as 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 intermetallic materials for thermoelectric (Ti-Ni-Sn), magnetocaloric (Co-Mn-Ge) and permanent magnet (Fe-Co-Nb) applications. The obtained results are visualized in the form of composition-function diagrams.
Funding of DFG within the projects LU1175/9, SPP 1599 and from the EU within the project REFREEPERMAG is acknowledged.

Ternary subsystems of Ni- and Co-based superalloys are explored by combinatorial and high-throughput methods. The materials systems to be investigated (Ni-Al-Cr, Cr-Ni-Re, Co-Ti-W, Co-Al-W) are deposited in the form of materials libraries by special magnetron sputter deposition methods. These materials libraries are subsequently processed at high temperatures (< 1100°C) and characterized by high-throughput experimentation methods (automated EDX, XRD, temperature-dependent resistance screening) in order to relate compositional information with structural properties. The talk will cover examples of the combinatorial exploration of ternary sub-systems of superalloys with regard to identifying unknown phases as well as to explore the compositional ranges of the different phases. Examples of up-scaling from thin film findings to the bulk are discussed.
Funding of DFG within SFB TR 103 is acknowledged.

Thermal spraying is widely used as a coating process, in which melted (or heated) materials are sprayed onto a surface of the substrate. Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites. There is increasing demand for the thermal spraying materials with high wear resistance particularly at high temperatures. Recently, we have developed a Ni base dual two-phase intermetallic alloy which shows a characteristic microstructure composed of primary Ni₃Al (L1₂) precipitates with a cuboidal shape and channel regions of the Ni₃V (D0₂₂) and Ni₃Al (L1₂) eutectoid phases [1]. This intermetallic alloy shows high hardness as high-temperature wear resistant materials: the reduction in hardness with increasing temperature is very small compared with conventional metallic materials such as hardened steels. In this study, fabrication of the Ni₃Al/Ni₃V dual two-phase intermetallic alloy coatings by a low-pressure plasma spraying method was attempted.
Atomized alloy powders of the Ni base dual two-phase intermetallic alloy were sprayed on a stainless steel (type 304) plate. Microstructural observations revealed that the as-sprayed coating layers showed a solidified microstructure consisting of fine columnar grains. The as-sprayed coating layers were composed of fcc disordered Ni solid solution. This is possibly due to rapid cooling from the molten (semi-molten) powders. When the coating layers were heated up to high temperature (~1280°C), the ordered Ni₃Al/Ni₃V dual two-phase microstructure was formed, and thereby the hardness of the coating layer was significantly enhanced from 384 HV for the as-sprayed state to 536 HV for the heat-treated state. It was concluded that the Ni base Ni₃Al/Ni₃V dual two-phase intermetallic coating with superior hardness property can be fabricated via the low-pressure plasma spraying process followed by the post heat-treatment.
[1] Y. Nunomura, Y. Kaneno, H. Tsuda and T. Takasugi, Acta Mater., 54 (2006) 851-860.

Fe₃₀Ni₂₀Mn₂₅Al₂₅ is a newly discovered nanostructured alloy, which consists of alternating b.c.c. and B2 phases with the interfaces aligned along <100>. Dry sliding pin-on-disk wear tests on this alloy were conducted at a sliding velocity of 1 m/s in room temperature air against a stainless steel 347 counterface. In order to investigate the influence of the debris produced during the wear process and the mechanisms of three-body wear, a brush was used to clean the debris throughout the wear process. The results were compared with those of wear tests at the same conditions without using a brush. The wear tracks were observed using optical microscope and the depth profiles were measured using optical profilometry after wear tests. The debris produced during the wear process was analyzed using X-ray diffraction. Cross-sectional specimens of the tips of the worn pins were prepared using focused ion beam and were examined using transmission electron microscopy to determine if a mechanically mixed layer was formed in the wear process. By comparison, it was found that the pins showed higher wear rates with debris being cleaned throughout the wear tests.

Zinc-coated (galvanized) steel is widely used in applications in automotive and building industries. The galvanized steel is sometimes 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. Because deformation and fracture properties of these intermetallic compounds determine the press formability response of the galvannealed steel, we have been investigating deformation behavior of each intermetallic phase through compression tests of polycrystalline/single-crystal micropillar specimens in order to elucidate the optimum microstructure of the coating layer [1,2]. However, the crystal structural information about the intermetallic phases is very limited although it is indispensable for understanding their plastic deformability. It is only recently that the crystal structure of the δ_1p phase was determined by Belin et al. [3]. Moreover, the crystal structure of the δ_1k phase has not been determined yet. In the present study, we have refined the crystal structure of the δ_1p phase by single-crystal synchrotron X-ray diffraction combined with ultra-high resolution scanning transmission electron microscopy (STEM), revealing that the structure can be described to build up with Fe-centered Zn₁₂ icosahedra (Fe@Zn₁₂) which are linked to one another to form basal slabs [4]. Further STEM observations of the δ_1k phase have indicated that the δ_1k phase has a superlattice structure based on the δ_1p phase having a tripled periodicity along the a-axis direction of the δ_1p phase, accompanied by one-dimensional stacking disorder of structural blocks along the c-axis direction. On the other hand, the zeta; phase (FeZn₁₃) has been known to possess a unit cell consisting of Fe@Zn₁₂ and one Zn atom which is gluing Fe@Zn₁₂ icosahedra [2,5]. We have demonstrated that each individual Fe@Zn₁₂ icosahedron behaves as if it is a large-sized atom during slip deformation [2].
[1] N. L. Okamoto, D. Kashioka, M. Inomoto, H. Inui, H. Takebayashi, and S. Yamaguchi, Scripta Mater. 69, 307 (2013).
[2] N. L. Okamoto, M. Inomoto, H. Adachi, H. Takebayashi, and H. Inui, Acta Mater. 65, 229 (2014).
[3] C. H. E. Belin and R. C. H. Belin, J. Solid State Chem. 151, 85 (2000).
[4] N. L. Okamoto, K. Tanaka, A. Yasuhara, and H. Inui, Acta Crystallogr. B 70, 275 (2014).
[5] R. Belin, M. Tillard, and L. Monconduit, Acta Crystallogr. C 56, 267 (2000).

Ferritic creep-resistant steels are the most important materials for heavy wall pipe components of boiler section in advanced ultra-super-critical coal-fired electricity-generating plants. The creep strength of the ferritic steels is strengthened by several carbides and nitrides. Quite recently, Laves phases also have attracted increasing attention as one of effective strengtheners in new creep-resistant steels. The objective of this study is to investigate composition-dependent microstructural change of Laves phase in Al containing ferritic creep-resistant steels.
Fe-Cr-Al-W-Nb steels at various compositions were prepared by a conventional arc-melting technique. Aging heat treatment was carried out for the prepared steels after solution heat treatment. After heat treatment, microstructure observations were performed by optical microscopy and scanning electron microscopy. Constituent phases in the steels were identified by X-ray diffractometry. The compositions of constituent phases and precipitates were analyzed by electron probe microanalysis.
Fe-15Al-10Nb (at.%) has a eutectic phase consisting of the ferritic phase and Fe₂Nb Laves phase. In steels that Nb was partly substituted by W was composed of the primary ferritic phase and the eutectic phase consisted of the ferritic phase and Fe₂Nb Laves phase. Fine Fe₂W Laves phase and Fe₇W₆mu; phase precipitated in the ferritic phase of the W added steels after aging heat treatment. These microstructural differences between the ferritic steels with and without W would be caused by the difference in the solubility of Nb and W in the ferritic phase.

A pseudobinary system of NiTi-PdTi is attractive as high temperature shape memory alloys due to the wide temperature range of actuation and good shape memory behavior. In order to deeply understand the nature of shape memory behavior of (Ni, Pd)Ti alloys, this study focus on the morphology of a self-accommodation microstructure (SAM) of an NiPdTi alloy having a martensitic microstructure at room temperature. The SAM is formed by habit plane variants (HVs) connecting each other. In general case, HV has internal twin that is introduced to make habit plane (interface between HV and parent phase) invariant plane. This internal twin is called lattice invariant deformation (LID) twin. We have found that a Ti-Nb-Al alloy causes a cubic-orthorhombic martensitic transformation in the same way as NiPdTi, and that two types of HV pairs are formed as the preferential SAM with no LID twin. The incompatibility at the junction plane (JP) between HVs is expressed by rigid body rotation that is required by the kinematic compatibility (KC) condition at JP. The preferential HV pairs had a smaller incompatibility at JP in the Ti-Nb-Al alloy that has no LID twin. Therefore, in order to characterize the general property of SAM with LID twin, the other purpose of this study is to reveal the relation between the incompatibility and the preferential microstructure experimentally and theoretically using NiPdTi in which the LID twin forms.
An Ni₂₅Pd₂₅Ti₅₀ (at%) alloy was fabricated by Ar arc melting method. The ingot was homogenized at 1373K for 21.6ks. For transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) observation, 3mm disks were cut by electro-discharge machining and electro-polished by a twin-jet method with an electrolyte of 80% CH₃OH + 20% H₂SO₄. The lattice parameters were determined by theta;-2theta; X-ray diffractometry (XRD). The incompatibility at the JP between HV_i and HV_j were using the KC condition: Q_ijU_j - U_i = a.n^t, where a and n are column vectors. U_i is the deformation gradient of HV_i that is calculated by the lattice parameters. The rotation Q_ij is the additional rotation of HV_j that is required to cancel out the incompatibility at the JP with respect to HV_i..
The analysis based on the KC condition in Ni₂₅Pd₂₅Ti₅₀ revealed that the incompatibility at the JP between HVs can be eliminated by the rotation (realistic solution of Q_ij exists) among a certain 6 pairs of HV. In these HV pairs, the HV pair whose JP has <211> type II twin orientation relationship corresponds to “herring bone” microstructure and the rotation angle of Q_ij is minimum. TEM and EBSD observation showed that each HV has LID twin of {111}type I twin as predicted by the theoretical analysis. SAM exhibits a herringbone microstructure in which the habit plane and the JP is almost parallel. The relation between the KC condition at JP and the preferential morphology of SAM will be discussed.

We studied about the Ni₂MnGa-type magnetic Heusler shape memory alloys. Thermal strain, permeability, and magnetization measurements of the ferromagnetic shape memory alloys Ni_50+xMn_27minus;xGa₂₃ and Ni₂MnGa_1-xFe_x were performed. Ni_50+xMn_27minus;xGa₂₃;For x = 2.7, in which the martensite transition and the ferromagnetic transition occur at the same temperature, the martensite transition starting temperature T_M shift in magnetic fields around a zero magnetic field was estimated to be dT_M/dB = 1.1 ± 0.2 K/T, thus indicating that magnetic fields influence martensite transition. The dT_M/dB values determined from the thermal strain results are approximately the same as the values calculated by the Clausius-Clapeyron relation. Itinerant magnetism is also discussed by means of our magnetization results, based upon the results of the magnetic field dependence of the magnetization and compared with predictions of Takahashi&’s theory[1]. As for x = 2.5, the M⁴ vs. B/M plot crosses the origin of the coordinate axis at the Curie temperature, and the plot indicates a good linear relation behavior around the Curie temperature. The result is in agreement with the Takahashi's theory. Ni₂MnGa_1-xFe_x;The characteristics of the determined phase diagram are similar to those of the phase diagrams of Ni_2+xMn_1-xGa, and also Ni_50+xMn_27minus;xGa₂₃ systems, where the magneto-structural transition between the paramagnetic austenite phase and ferromagnetic martensite phase occurs[2]. The magneto-structural transformation and the associated magnetic entropy change were studied by using magnetization measurements performed around the martensite transition temperature T_M. As for x = 0.300, in which the martensite transition and ferromagnetic transition occurs at the same temperature (T_M = T_C = 410 K), the magnetic entropy change -ΔS_M was 7.8 J/kgK in the magnetic field of 2 T, which is equal to that of Ni₂MnGa_0.88Cu_0.12[3]. The value is comparable to that of Ni_2.18Mn_0.82Ga. The refrigeration capacity of x = 0.300 is 37 J/kg. This value is of the same order as those for the Ni₂MnGa_0.88Cu_0.12[3], and Ni_2.15Mn_0.85-xCuxGa (x=0.5 and 0.7) alloys[4]. The thermal strain of x = 0.275 and x = 0.300 under atmospheric pressure indicated large thermal hysteresis. When the heating process from the room temperature to the temperature above T_R (the reverse martensite transition temperature), a contraction was observed in a narrow temperature range at T_R. On the contrary, there was not sharp and clear jump when cooling from the austenite phase. The stress-strain measurements of Ni₂MnGa_1-xFe_x alloys show super elasticity. These results indicate that Ni₂MnGa_1-xFe_x alloys are good candidates for smart shape memory alloys with ductility and flexibility.
[1]Y. Takahashi, J. Phys. Condens. Matter 13 (2001) 6323. [2]M. Kataoka et al., Phys. Rev. B 82 (2010) 214423. [3]T. Sakon et al., J. Alloys. Compds. 577S (2013) S376. [4]J. F. Duan et al., J. Appl. Phys. 103 (2008) 063911.

Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. The present authors&’ group is focusing on Half-Heusler MNiSn (M=Ti, Zr) as high potential thermoelectric materials for elevated temperature applications, and has proposed and conducted thermoelectric materials design based on lattice defects of Half-Heusler ordered structure and interfaces related to Heusler MNi₂Sn phase. The ordered structures of Half-Heusler MNiSn and Heusler MNi₂Sn are quite similar in the point that a half of Ni-site in Heusler is replaced by vacancies in Half-Heusler. In M-Ni-Sn alloy systems, MNiSn and MNi₂Sn exist as individual stable phases equilibrating with each other, where semiconductor MiNiSn is well-known excellent n-type thermoelectric materials while MNi₂Sn are metallic phase. In the present work, our interests were placed on lattice defects, nano-structures and interfaces that can be introduced based on the close relationship between Half-Heusler and Heusler in terms of phase stability and phase equilibrium. Objective of the present work is to understand how these distinctive characteristic lattice defects and interfaces are formed and introduced in Half-Heusler MNiSn-base alloys, and to evaluate their effects on thermoelectric properties. Several types of Half-Heusler MNiSn-base alloys were fabricated by conventional arc-melting and the directional solidification using optical floating zone melting method (OFZ-DS). Microstructure observation and crystallographic analyses were mainly conducted by transmission electron microscopy (TEM) and X-ray diffractometry. Thermoelectric properties were evaluated by measurements of Seebeck coefficient, electrical resistivity and thermal conductivity. Interfaces introduced by Heusler ZrNi₂Sn precipitation in Half-Heusler ZrNiSn matrix were examined in detail using TEM analyses. Traveling solvent OFZ-DS is necessary to fabricate ZrNiSn single-phase alloys, and Ni-rich ZrNiSn phase typically tends to be grown. The Ni content is controllable depending on the OFZ-DS condition. Clustering of Heusler like nano-structure was formed in ZrNiSn even close to the stoichiometric composition. Similar clustering was also observed in the state prior to the early stage of precipitation of ZrNi₂Sn in ZrNiSn matrix. Seebeck coefficient is sensitive to this microstructure change. The vacancy-site occupation in Half-Heusler TiNiSn is a distinctive lattice defect we focused on for the evaluation in this work. We reported that vacancy-site occupation of Co in ZrNiSn results in the drastic conversion of n-type thermoelectric property to p-type. The solubility of Co in the vacancy-site of TiNiSn+Co is much larger, and its effect on Seebeck coefficient is smaller than in the case of ZrNiSn+Co.

The SmCo₅ has the highest anisotropy field of 28.0 MA/m and high Curie point of 1020 K. Therefore, the SmCo₅ magnets have excellent performance under high temperature conditions, i.e. 450 K~500 K. The drawback is, however, that the saturation magnetization of the SmCo₅ magnets is lower than that of the Nd₂Fe₁₄B magnets. Therefore, improvement in the coercivity is required to compensate for intrinsic low saturation magnetization value in the SmCo₅ magnets. According to the previous report [1], the Ta addition is found to be very effective in achieving high coercivity in SmCo₇. In this work, we studied the effect of the Ta addition on the magnetic properties of SmCo₅ in order to obtain the condition to achieve high coercivity. As a result, we obtained high coercivity of more than 1.6 MA/m. Results of farther improvement will be reported in detail in the poster presentation. [1] Z.H.Guo, C.C.Hsieh, H.W.Chang, M.G.Zhu, W.Pan et al., J. Appl. Phys., 107, 09A705 (2010)

Mg - transition metal (TM) - rare earth (RE) ternary alloys containing precipitates with long-period stacking ordered (LPSO) structures have received a considerable amount of attention as new light-weight structural materials since they simultaneously exhibit high strength and high ductility after hot extrusion process. There is no doubt that their excellent mechanical properties are closely related to the existence of the LPSO phase, however, underlying mechanisms endowing them attractive properties have been remained unsolved. This is mostly because their inherent characteristics of the LPSO phases including crystal structure, formation process and deformation mechanism have not been fully clarified yet. Recently, we have revealed the crystal structures of highly-ordered LPSO phases with the 18R and 14H-type stacking in the Mg-Al-Gd system by TEM and HAADF-STEM. Main characteristics are summarized as follows. First, Gd and Al are enriched in four consecutive close-packed atomic layers. Second, Gd and Al atoms form Al₆Gd₈ atomic clusters with the L1₂-type atomic arrangement in Gd and Al-enriched atomic layers and the clusters take a long-range in-plane ordered arrangement. Third, although each structural block of the Mg-Al-Gd LPSO phases is perfectly ordered, the Mg-Al-Gd LPSO phases generally exhibit one-dimensionally disordered nature along the stacking direction of the structural blocks and consequently, their crystal structures are best described with the crystallographic concept of the order-disordered (OD) structure. On the other hand, no apparent long-range in-plane ordering has been confirmed so far for the most intensively studied Zn,Y- poor Mg-Zn-Y LPSO phases coexisting with hcp Mg although the formation of the L1₂-type atomic clusters in the Zn,Y-poor Mg-Zn-Y LPSO phase was confirmed. These previous results suggest that the difference in the in-plane ordering nature should be closely related to the difference in the number density of the Zn₆Y₈ atomic clusters in the structural block and that the Mg-Zn-Y LPSO phases with the same in-plane ordering as those developed in the Mg-Al-Gd LPSO phase would be obtained by increasing the Zn, Y concentration. In the present study, some Mg-Zn-Y ternary alloys containing highly-ordered Mg-Zn-Y LPSO phases were successfully prepared and the crystal structure variations of various Mg-Zn-Y LPSO phases were investigated as a function of chemical composition as well as heat treatment conditions. HAADF-STEM analysis revealed that the in-plane long-range ordering of the Zn₆Y₈ atomic clusters in the structural block is identical to that of the Al6Gd8 in the Mg-Al-Gd LPSO phases and the most stable polytypes for these three types of highly-ordered LPSO phases were determined based on the crystallographic concept of the OD structure. Details of the ordered structure as well as crystal structure variation by the heat treatment for the three types of the Mg-Zn-Y LPSO phases will be presented.

Mg-based alloys containing yttrium (Y) have received much attention owing to their improved mechanical properties for their use as lightweight structural materials. In Mg-Y alloys, nanoclusters composed of Y-Y atomic pairs play an important role in the age hardening behavior and the resultant precipitation of the βprime; phase (Mg₇Y) contributes to the peak hardness during the aging process. Recent scanning transmission electron microscopy revealed that the βprime; structure comprises multiple two-dimensional zigzag-sharped Y clusters, which are aligned at a regular intercluster spacing of 2radic;3a_Mg (asymp; 1.1 nm) with a one-dimensional long-range periodic order. Such a characteristic superlattice of Y clusters realized in the βprime; phase is believed to result from the nature of segregation and ordering of Y atoms in the hexagonal close-packed (hcp) Mg matrix; however, the detailed mechanism for the formation of ordered nanostructures in the Mg-Y system has not yet been established.
In this study, we evaluated the stability of the various arrangements of zigzag-sharped Y clusters to elucidate the ordering principles of a Y cluster superlattice in hcp Mg, by considering the intercluster interaction energy as a function of intercluster distance (d) and zigzag phase shift, which is derived from first-principles calculations based on density functional theory (DFT). We found that the intercluster interaction between zigzag-shaped Y clusters acts over a long range up to ~3 nm and becomes steadily larger as the intercluster distance decreases. The effect of zigzag phase shift becomes apparent in the arrangements at an intercluster distance below approximately 4radic;3a_Mg, so that it can influence the stability of the cluster arrangements even with the same intercluster distance. In particular, it is noted that the arrangement with the lowest value of the intercluster interaction energy at d = 2radic;3a_Mg was found to correspond to the experimentally observed structure of the βprime; phase. To analyze the principal factors of the intercluster interactions, we attempted to decompose the intercluster interaction energy into two parts: contribution mainly from the chemical interaction and that mainly from the elastic interaction between the clusters. The DFT results suggested that the interplay between the “attractive” chemical interaction and “repulsive” elastic interaction plays a significant role in determining the stability of ordered arrangements of Y clusters in the Mg matrix, and provides the controlling mechanism for the formation of long-range periodic order in the Y cluster superlattice.

The long-period stacking-ordered (LPSO) structure phase in Mg-Zn-Y alloys is considered to play an important role in alloy strengthening. However, the effect of the LPSO phase on the mechanical properties has not yet been clarified, as the size of the LPSO phase is usually in the range of micrometers and it is rather difficult to measure the mechanical properties of only the LPSO phase by using a conventional mechanical testing technique. In this investigation, the deformation behavior of LPSO phase was examined using microbending tests. The material used was a directionally solidified Mg₈₅Zn₆Y₉ alloy with an LPSO (18R) single phase. The orientation of the LPSO-phase grains was determined by electron back-scatter diffraction (EBSD) analysis, and microsized cantilever-beam specimens with dimensions of 50 µm (L)×10 µm (W)×20 µm (B) were fabricated by focused ion beam machining. Two types of specimens with different beam orientation were prepared. For the specimen, the beam direction was nearly [11-20] and the side surface was almost (0001), whereas for the specimen, the beam direction was parallel to [0001] and the side surface was (10-10). Microbending tests were carried out using a mechanical testing machine for micro-sized specimens. The specimen was found to exhibit ductile deformation behavior. From scanning electron microscope (SEM) and EBSD observations, prismatic slip traces were seen on the surface under tension near the fixed end of the bending specimen. The critical resolved shear stress (CRSS) for the prismatic slip was calculated to be 440 MPa. In contrast, several load drops were repeatedly observed in the load-displacement curve for the specimen. SEM and EBSD observations revealed the activation of a basal slip on the surface under tension near the fixed end of the specimen. The CRSS of the basal slip was calculated to be 18 MPa, and this value is close to those obtained from a previous investigation. In addition, the activation of a {11-29} twin is also suggested to occur on the side surface near the fixed end. The above results thus suggest that the deformation in the LPSO phase in Mg-Zn-Y alloy exhibits large anisotropy.

Selective oxidation of alcohols is one of the important industrial reactions for fine chemicals production; it serves key intermediates of aldehydes, ketones and carboxylic acids. Conventionally, stoichiometric oxidation processes using homogeneous oxidants such as KMnO₄, K₂Cr₂O₇ and TEMPO were used for the oxidation reaction, however, these classical processes have big disadvantages derived from salts formation during reaction. For building more environmentally-friendly chemical process, aerobic alcohol oxidation over heterogeneous catalyst has been studied by several groups; it provides only water as co-product after reaction in theory, and the catalyst was easily separated from the reaction mixture.
Heterogeneous monometallic and bimetallic nano-catalysts composed by precious metals of Au, Pd, Pt and/or Ru are studied well for the reaction. In this study, we focused on the transition metal Cu incorporated Pd catalyst. The PdCu nano-catalyst has been applied for hydration of acrylonitrile to acrylamide, hydrogenation of 1,3-cyclooctadiene, Sonogashira cross-coupling, CO oxidation, NO reduction and so on. Here, we found that CuPd bimetallic catalyst also exhibited good activity for selective oxidation of alcohols using molecular oxygen as an oxidant.
The PVP-protected bimetallic palladium/copper nanoparticles supported onto solid base hydrotalcite with different Pd/Cu ratio (Pd_xCu_y-PVP/HT) were prepared with 2-ethoxyethanol reducing method as follows. Cu(OAc)₂bull;H₂O, Pd(OAc)₂ and PVP (K = 30) were resolved in 2-ethoxyethanol, then refluxed for 2 h serving Cu_xPd_y-PVP nanoparticles. Thereafter, solid base hydrotalcite support was added into the suspension, and further refluxing for 1 h, filtering and washing with water, and drying in vacuum to obtain the Pd_xCu_y-PVP/HT catalyst. Characterizations were performed by XRD, TEM, XPS, XAFS, ICP-AES and so on.
First of all, as-prepared Pd_xCu_y-PVP/HTs with various Pd/Cu molecular ratios (Pd+Cu = 0.10 mmol/g (const.)) were applied for benzyl alcohol oxidation using O₂ gas as oxidant at 313 K. It was observed that the yield of benzaldehyde was strongly influenced by the Pd/Cu ratio, and the Pd₈₀Cu₂₀-PVP/HT showed the highest activity (44% yield). Additionally, Pd₁₀₀-PVP/HT was less active (36% yield) whereas Cu₁₀₀-PVP/HT was completely inactive (0% yield) under the same reaction condition. The mean diameter of 2.2 nm, 3.1 nm, and 2.8 nm was determined for Cu₁₀₀-PVP/HT, Pd₈₀Cu₂₀-PVP/HT, and Pd₁₀₀-PVP/HT, respectively, by using TEM images. Thus, it is likely that the particle size of PdCu-PVP NPs scarcely contributed to the activity. The Pd₈₀Cu₂₀-PVP/HT was also capable for selective aerobic oxidation of cinnamyl alcohol towards cinnamaldehyde (54.5% yield and 80.5% selectivity). We will focus on the electronic state of Pd active center with XPS and XAFS study for reveal the promotion effect of Cu additives into Pd atoms.

Pt-Cu alloys have been of great interest over the past few years due to their potential applications as catalysts, an alternative to pure-Pt and Pt-group catalysts. The formation of Pt-Cu intermetallic compounds is important for the alloy designing of Pt-Cu alloy system. The existence of Pt₇Cu intermetallic compounds was investigated by the prediction method using ab-initio calculations and the High Voltage Electron Microscopy (HVEM) for the irradiation induced ordering. The Pt₇Cu intermetallic compound whose proto-type was Ca₇Ge was predicted at 0 K by first principle calculations using VASP. The ordering temperature of fcc-based Pt₇Cu intermetallic compound was estimated above room temperature by cluster expansion method and grand canonical monte-calro simulation using CASM code. The formation of Pt₇Cu intermetallic compounds was confirmed by the long-time annealing for 1.2x10⁶ s at 703 K and short-time irradiation for 3.6x10³ s at 600 K. MeV electron irradiation is effective for the reduction of the annealing time for the ordering in Pt-Cu alloy system.

Intermetallic compounds have various crystalline phases which strongly depend on the temperature. In the present study, we have chosen some intermetallic compounds (FeRh, Ni₃V, Ni₃Al and FeCo) as targets for ion irradiation, and the effect of the irradiation on the crystal structures and the mechanical property was examined. Then, we discussed the irradiation effects by considering the temperature dependence of crystal phases in the thermal-equilibrium phase diagrams. FeRh, Ni₃Al, Ni₃V and FeCo bulk specimens were irradiated with 5-16 MeV Al, I or Au ions at room temperature. The ion fluences were 1 x 10¹² - 1 x 10¹⁶ /cm². After the irradiation, their crystal#12288;structures were investigated by using a surface x-ray diffraction measurement (SXRD). We also measured the x-ray absorption fine structure (EXAFS) spectra at a synchrotron radiation facility to examine the short range order of the atomic arrangement around selective elements. The microstructure observation was carried out by means of a transmission electron microscope. The change in surface hardness by the irradiation was measured by using a Vickers hardness tester. The experimental result shows that the disordered fcc phase (A1 phase) appears at room temperature in FeRh, Ni₃Al and Ni₃V by the ion irradiation. The A1 phase is usually observed as a high-temperature phase in the thermal-equilibrium phase diagram of Ni₃V and FeRh. In the phase diagram of Ni₃Al, however, the A1 phase does not exist but the ordered L1₂ structure is kept just below the melting temperature. For FeRh, the L1₀ phase is induced by the irradiation as well as the A1 phase. Although the L1₀ phase does not exist in the phase diagram of FeRh, it is known that this phase is induced by the high speed deformation. The present result suggests that the irradiation-induced crystal phase transformation cannot be explained only as a result of the thermal spike (effective increase in local temperature), but that other effects such as sequential displacements of atoms and/or the irradiation-induced stress have to be considered. The hardness change which is accompanied by the irradiation-induced phase transformation and the thermal stability of the irradiation-induced non thermal-equilibrium phases will also be discussed in the conference.

We made the ternary films by mean of reactive sputtering where the target was a disk of cooper with gold inserts in a nitrogen atmosphere. Analyzed in situ by XPS a Cu₃Au_0.8N_1.2 composition was obtained while Rietveld refinement analysis on XRD diffractogram, points to Cu₃Au_0.6N cubic system in the Fm3m (225) space group¹.
To gain deeper insight density functional theory calculations were performed. A completely filled structure, Cu₃AuN did not fully match experiment. For a better approximation, we made a 2x2x2 supercell where we can vary the atoms ratio. The best match with the experimental results was for a Cu₃Au_0.5N. From the ab intio calculations we were able to closely reproduce XPS and XRD results and can be concluded that the crystalline system is cubic, Pm3m space group, and half, randomly filled Au-sites. Some Au atoms probably occupy Cu-sites. The unoccupied gold sites may be considered as voids in the metalloid sub-lattice. As these vacancies are randomly distributed in the “parent” metal structure, the long-range order of the Cu₃Au alloy no longer exists in the ternary alloy. The peaks position almost perfectly fit the experiment. We still have some inconsistencies that could be because the simulated XRD is for powders, whereas the actual pattern was taken from a thin film where there are preferential orientations and defects. The degree of uncertainty in XPS composition is acceptable given the method limitation for bulk samples. The formation energy calculations suggest that higher Au and N ratios could exist. The nitrogen was incorporated as N^3- taking its electronic charge from both the copper and the gold, but mostly from copper.
As a consequence of lower structure symmetry the bands degeneracy is higher than the binary alloy. Fermi level closer inspection shows that conduction and valence bands do not overlap, in fact there is a small pseudogap at ~1 eV in the valence band. However the Fermi level is intersected several times by bands coming from the conduction region. The Au atoms introduce states near the Fermi level that close the gap, decreasing the resistivity. Therefore and in agreement with the electric measurements, the material should be classified as a heavily doped n-type semiconductor with both kinds of electric charge carriers, where its conductance is dominated by electrons.
Acknowledgments. Partial support was by DGAPA-UNAM grants IN107213, IN107213, IN103711. Calculations were performed at DGCTIC-UNAM. We thank Aldo Rodriguez for technical support.
1. G. Soto, I. Ponce, M.G. Moreno, F. Yubero, W. De la Cruz , Journal of Alloys and Compounds 594 (2014) 48-51

Since Au-based shape memory and superelastic alloys (SMAs) exhibit high biocompatibility and excellent X-ray radiography, they have a large potential to exceed TiNi practical SMAs in the field of biomedical implant devices. Recently, we have found that Fe addition to AuCuAl alloys improves both strength and ductility resulting to superelasticity appeared at room temperature. However, in order to pursue further improvement of the mechanical properties, suitable thermo-mechanical treatment condition should be revealed. This is because, as for the practical biomedical applications, suitable thermo-mechanical treatment so-called shape memory treatment or superelastic treatment is desired to stabilize shape memory / superelastic properties. In the case of Ti-Ni SMAs a combination of 30-40% cold rolling in area reduction and intermediate annealing at 673-773K is used. Then, the first purpose of this study is to reveal the effect of intermediate annealing on martensitic transformation and mechanical properties of cold-rolled Fe-added Au-Cu-Al alloy. The second purpose of this study is to reveal the effect of solution treatment temperature of another Fe-added Au-Cu-Al alloy.
Au-27Cu-18Al-2Fe and Au-30Cu-18Al-2Fe (at%) alloys were prepared by Ar arc-melting using non-consumable W electrode. The ingots were hot-pressed at 873K for 21.6ks to fabricate alloy disks with 2mm thickness. In order to reveal the effect of intermediate annealing, Au-30Cu-18Al-2Fe alloy was cold-rolled with 30% reduction thickness and heat-treated at 433K to 773K for 3.6ks. In order to clarify the effect of solution treatment temperature, Au-27Cu-18Al-2Fe alloy was solution-treated at 373K to 873K for 3.6ks. Optical microscopy was used for the microstructural observation at room temperature (RT).#12288;Phase constitution and martensitic transformation temperatures were determined by theta;-2theta; X-ray diffractometry (XRD) using CuKα at RT and differential scanning calorimetry (DSC) in a temperature range between 223K to 423K, respectively. A cyclic loading-unloading tensile test with constant strain increment of 1% was performed at RT.
As for the intermediate annealing, martensitic transformation start temperature (M_s) and finish temperature (M_f) of Au-30Cu-18Al-2Fe were M_s=288K and M_f=271K after the cold rolling, but M_s=290K and M_f=282K after the intermediate annealing. M_f was lowered and transformation hysteresis became wide by cold rolling. Judging from these results, recovery must be taken place when the annealing temperature is equal to or higher than 573K. As for the solution treatment condition, Au-27Cu-18Al-2Fe became extremely brittle after the solution treatment at 873K, but exhibited good mechanical properties as well as superelasticity after the solution treatment at 673K and 773K. It is concluded in Fe-added AuCuAl alloys that, for the good mechanical properties, the suitable heat treatment temperature is around 673-773K for both thermo-mechanical and solution treatments.

Recently, a new ternary L1₂ (γprime;) phase Co₃(Al,W), which can coexist with a fcc solid-solution phase (γ) based on Co, has been discovered. We have investigated the compression deformation behavior in polycrystals of the L1₂-Co₃(Al,W) and found that Co₃(Al,W) exhibits a positive yield stress-temperature dependence (yield stress anomaly: YSA) as in the case of Ni₃Al and many other L1₂ compounds. However, comparing to Ni₃Al, the YSA in Co₃(Al,W) was observed in a very narrow temperature range from 950 to 1,100 K. Our TEM observations revealed that the YSA in Co₃(Al,W) can be ascribed to the thermally activated cross-slip of APB(anti-phase boundary)-coupled dislocations from (111) to (010) plane as in the case of many other L1₂ compounds, as well as that the 1/2[101] superpartial dislocations are further dissociated into 1/6[112] Shockley partial dislocations, indicating that the CSF (complex stacking fault) energy is very low. The very high onset temperature of the YSA (950 K) in Co₃(Al,W) may be attributed to this very low CSF energy. On the other hand, the relatively low peak temperature of the YSA (1,100 K) is due to the decomposition of the L1₂ phase into the fcc phase. Therefore, it is considered that alloying elements that can increasing the CSF energy as well as the solvus temperature of the L1₂ phase will lower the onset temperature as well as increase the peak temperature of the YSA so as to significantly improve the high-temperature strength of Co₃(Al,W). In the present study, we investigate the effects of quaternary alloying elements on the temperature dependence of yield stress in polycrystals of L1₂-Co₃(Al,W). Ingots with a nominal composition of (Co_1-xNi_x)-13at%Al-13at%W (x = 0.2, 0.4, 0.6) were prepared by arc-melting under an argon gas flow, and then annealed at 1123 K for 168 h followed by furnace cooling. Microstructure and chemical compositions of the phase were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The SEM observation indicated that the main phase was L1₂-Co₃(Al,W) confirmed that no γ (fcc) phase was included. Compression tests were conducted from 298 to 1,423 K in vacuum. It is demonstrated that the onset temperature of the YSA in Ni-alloyed Co₃(Al,W) (x = 0.2) is much lower than that in the ternary L1₂-Co₃(Al,W), exhibiting a higher strength at high termptaures. However, in contrast to the prediction, further Ni-addition does not always influence the temperature-dependence in the same way. The possible reasons for this trend will be discussed in terms of the easiness of the slip on (010) at high temperatures.

Alloys based on refractory element, Nb, are expected to replace Ni based superalloy because they have advantages such as high melting point and low density. However, it is required to improve their oxidation resistance because pure Nb rapidly oxidizes in a high temperature atmosphere. B2 aluminide is a candidate of protective coatings for Nb(bcc) matrix from oxidation. To keep good bonding between the matrix and the coating, phase equilibrium between B2 aluminide phase and bcc solid solution (bcc_ss) phase is required. According to previous works, Nb_ss(bcc_ss) phase and β-NiAl(B2) phase never coexist in equilibrium in Nb-Ni-Al system^[1]. On the other hand, the coexistence relationships of Nb_ss phase with PdAl(B2) phase in Nb-Pd-Al system^[2] and Mo_ss(bcc_ss) phase with β-NiAl phase in Mo-Ni-Al system^[3] have been reported. Therefore, by adding Mo or Pd to Nb-Ni-Al system, two-phase equilibrium between bcc_ss phase and B2 phase is expected by replacing Nb to Mo in bcc_ss phase and Ni to Pd in B2 phase. In this study, we tried to find composition range of bcc_ss - B2 two phase field. Various Nb-Mo-Ni-Pd-Al alloys heat-treated at 10000C were prepared and composition analysis of their constituent phases were conducted by using EPMA. It is found that, in the Nb-Mo-Ni-Pd-Al system higher Pd fraction in B2-(Ni,Pd)Al phase results in higher Nb fraction in coexisting bcc_ss phase, while without Pd, the composition range of bcc_ss coexisting with B2-NiAl phase is limited to be low Nb.
[1] N. Saunders, In: G. Petzow, G. E#64256;enberg, editors. Ternary alloys: MSI. VCH (1993) p. 348-57.
[2] P. Cerba et al., Journal of alloys and compounds 201 (1993), p 57-60.
[3] O. Kubaschewski, In: G. Petzow, G. E#64256;enberg, editors. Ternary alloys: MSI. VCH (1993) p. 199-218.

Quite recently, we have newly developed TiC-added Mo-Si-B alloys for ultra-high temperature structural applications. The alloys have a Mo concentration of 65 - 70 at.% and TiC of 7.5 - 10 at.%. The density of the alloys is relatively low and comparable to Ni-base superalloys (8.7 - 9.0 g/cm³) with the volume fraction of Mo solid solution (Mo_ss) of 35 - 45 %. The melting point of the alloys is lowered to < 2000 °C by the eutectic reactions of Mo_ss, Mo₅SiB₂ (T₂), (Mo,Ti)C and (Mo,Ti)₂C, and thus casting process is more practical for producing the alloys. The TiC-added Mo-Si-B alloys have good deformability and high strength at and above 1000 °C. The peak stress values of the alloys are more than double those of Mo-6.7Si-7.9B (at.%), Mo-Ti-Zr (TZM) and Mo-Hf-C (MHC). The plastic strain in the alloys at temperatures lower than the ductile-brittle transition temperature of T₂ is generated by the plastic deformation of not only Mo_ss but also (Mo,Ti)C and (Mo,Ti)₂C. Furthermore, the room-temperature fracture toughness of the TiC-added Mo-Si-B alloys is better than 13 MPa(m)^1/2. Therefore, the TiC-added Mo-Si-B alloys have a high potential as a novel Mo-based ultra-high temperature material.

Molybdenum silicides and borosilicides are promising structural materials for gas-turbine power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi₂) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo₅Si₃ (called T₁) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi₂minus;T₁ composites that combine high oxidation resistance of MoSi₂ and good mechanical properties of T₁. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo₅SiB₂ (called T₂) phase is considered as an attractive material.
In the present paper, MoSi₂minus;T₁ composites and materials based on T₂ phase are obtained by mechanically activated self-propagating high-temperature synthesis (MASHS). This process involves a self-sustained combustion of Mo/Si and Mo/Si/B mixtures due to exothermic reactions between their components. High-energy ball milling (mechanical activation) of the mixture is used to facilitate the ignition. To obtain denser products, the so-called SHS compaction (quasi-isostatic pressing) has been employed. Oxidation resistance of the obtained MoSi₂minus;T₁ composites has been studied using thermal analysis. It has been shown that SHS compaction significantly improves the oxidation resistance.
Self-sustained combustion of Mominus;Siminus;B mixtures for the formation of T₂ phase becomes possible if the composition is designed for adding a more exothermic reaction of MoB formation. These mixtures exhibit spin combustion, the characteristics of which are in good agreement with the spin combustion theory. Oxidation resistance of the obtained multi-phase Mominus;Siminus;B materials is independent on the concentration of Mo phase in the products so that the materials with a higher Mo content are preferable because of better mechanical properties. The “chemical oven” technique is used to obtain alloys with higher concentrations of T₂ phase.

Multiphase Mo-Nb-Si-B cast alloys are potential candidates for application in jet turbine engines due to their superior creep properties and adequate oxidation resistance at ultrahigh temperature above 1200 ^oC. In order to satisfy several application requirements , it is required to control multiphase solidification microstructure of (Nb,Mo)-bcc, (Mo,Nb)₅Si₃-T₁ and (Mo,Nb)₅SiB₂-T₂ in the Mo-Nb-Si-B quaternary alloys.. In the present study, in order to understand the mechanism of a three-phase eutectic reaction in the Mo-Nb-Si-B quaternary system, the crystallography of the bcc/T₁/T₂ three-phase eutectic microstructure has been examined in directionally solidified alloys. The directional solidification was conducted on a Mo-32.2Nb-19.5Si-4.7B (at.%) alloy using an optical floating zone (OFZ) furnace in a flowing Ar gas atmosphere at a constant growth rate of 10 mm/h. The microstructure was observed by SEM. The orientation of the constitute phases was analyzed by EBSD. The microstructure of the directionally solidified alloys is characterized by elongated T₂ phase regions surrounded by inclusions of the bcc and T₁ phases with an interwoven morphology. The T₂ grains in eutectic cells are faceted on the (001) planes and elongated along the [110] direction. The T₂ phase has an orientation relationship of (001)_T2 // (011)_bcc and [130]_T2 // [2-11]_bcc with the bcc phase, whereas no particular orientation relationships were detected for the T₁ phase with the bcc and T₂ phases. These crystallographic features of bcc/T₁/T₂ three-phase eutectic microstructure suggest that the primary T₂ phase crystallizes and grows along the [110] direction in liquid phase, followed by nucleation of the bcc phase on the interface between T₂ and liquid phases, resulting in a final bcc/T₁ two-phase eutectic reaction surrounding the elongated T₂ phase. The microstructure and crystallographic features have also been examined in the as-cast three-phase eutectic Mo-Nb-Si-B alloy prepared by arc-melting. Based on these results, the reaction mechanism of bcc/T₁/T₂ three-phase eutectic decomposition in the quaternary Mo-Nb-Si-B system will be discussed in terms of the reaction pathway.

MoSi₂ with the tetragonal C11_b structure has been considered as one of the promising base materials for ultra-high temperature application because of its high melting point (2020 °C), excellent oxidation resistance and relatively low density. However, the poor fracture toughness at room temperature and the insufficient strength at elevated temperature are drawbacks to be improved for its practical applications. One possible method to improve these drawbacks is to form composites with hard ceramics or intermetallics. MoSi₂ / Mo₅Si₃ eutectic composites with high eutectic temperatures around 1900 °C are expected to have higher probability of improving the insufficient properties of monolithic MoSi₂ through controlling their interface properties because of fine microstructure of so-called script lamellar type formed simply by directional solidification (DS). Recently, we have systematically investigated the effects of ternary additions on microstructures and mechanical properties of MoSi₂ / Mo₅Si₃ eutectic composites and found that about 1 at.% addition of carbon result in the formation of homogeneous MoSi₂ / Mo₅Si₃ / Mo₅Si₃C ternary eutectic microstructure. Since further improvement in the mechanical properties of MoSi2-based composites is expected for the ternary eutectic composites, it is important to clarify the microstructural characteristics and their influence on the mechanical properties. In the present study, we have studied microstructures, orientation relationship between the component phases and mechanical properties of directionally solidified ingots of the MoSi₂ / Mo₅Si₃ / Mo₅Si₃C ternary eutectic composites. The MoSi₂ / Mo₅Si₃ / Mo₅Si₃C ternary eutectic structure is found to be composed of rod-shaped Mo₅Si₃ and Mo₅Si₃C phases extending along the growth direction in the MoSi₂ matrix. Detailed EBSD analysis confirmed that the orientation relationships between MoSi₂ and Mo₅Si₃ are identical with those observed in MoSi₂ / Mo₅Si₃ two phase eutectic composites. Mo₅Si₃C rods are formed so as to maintain a certain orientation relationship with MoSi₂ and Mo₅Si₃, which is approximately described as (110)_MoSi2 // (1-10)_Mo5Si3 // (12-30)_Mo5Si3C, [1-10]_MoSi2 // [001]_Mo5Si3 // [0001]_Mo5Si3C. Detailed structural analysis by TEM/STEM suggested that the lattice misfits between the component phases play an important role in the formation of the ternary eutectic microstructure. Detailed analysis of the interface structure as well as the mechanical properties of the ternary eutectic composites will be presented.

The structures of transition-metal disilicides are constituted of different stacking of identical atomic planes at four different positions A, B, C, D: AB in the C11_b structure of e.g. MoSi₂, ABC in the C40 structure of e.g. VSi₂ and ABDC in the C54 structure of e.g. TiSi₂ disilicides. In comparison with the FCC lattice with the ABC atomic plane stacking along the <111> directions, the occurrence of the fourth position, D, essentially alters the properties of defects and consequently the mechanical properties. The structure of generalized planar defects and their impacts on the dislocation core configurations are discussed. In particular, we examine stacking faults and related partial dislocations on the basal planes in different types of disilicides as well as the related twin boundaries and dissociated dislocations. Our analysis of the stacking-fault-like defects is based on the calculations of γ-surfaces using ab initio methods. Predictions of possible metastable defects in all types of disilicides are reported.

Three phase Mo-Mo₃Si-Mo₅SiB₂ alloys possess excellent mechanical properties over a wide temperature range. The Mo solid solution phase is needed for balanced mechanical properties at room temperature. However, this phase suffers from catastrophic oxidation behavior at high temperatures caused by the formation and evaporation of MoO₃. The oxidation resistance of three phase alloys benefits from a high volume fraction of intermetallic phases. In particular Mo₅SiB₂ leads to the formation of a borosilicate protective glassy layer on the material's surface while exposed to air at elevated temperatures. Hence, it is unlikely to identify alloy compositions that will yield both optimum mechanical and oxidation performance.
Different coating systems and techniques, such as pack cementation, magnetron sputtering and plasma spraying are discussed in the literature to control the oxidation properties of Mo-based alloys. A different approach is to apply coating systems based on polymer derived ceramics (PDCs). Our present work introduces PDCs as a new type of promising and innovative oxidation-protective coatings for high temperature Mo-based alloys. After dip-coating with perhydropolysilazane (PHPS) and pyrolysis under an Ar atmosphere at 800 °C, dense and well adhered SiON ceramic layers could be achieved. These were investigated by scanning electron microscopy and ellipsometry. Cyclic oxidation tests at 800 °C and 1100 °C were performed to investigate mass changes due to the thermal treatment. Indeed, even thin pyrolyzed PHPS layers with a thickness of around 70 nm to 100 nm protected the Mo-Si-B substrate during the initial oxidation. By increasing the SiO₂ concentration at the material&’s surface a first oxidation barrier was provided and thus, the strong initial mass loss could be decreased as compared to uncoated alloys. Furthermore, first results of the ongoing optimization process on PDC-coatings applied to Mo-Si-B alloys will be presented, involving usage of filler particles or varying pyrolysis atmospheres.

C11_b-MoSi₂ forms microstructures with other silicides, such as lamellar structure with C40-NbSi₂ and labyrinth structure with D8_m-Mo₅Si₃. The stabilization of the microstructures and the modification of interface properties are the keys to the improvements of mechanical properties required for the practical use of MoSi₂-based alloys as ultrahigh-temperature gas-turbine materials. In the present study, microstructural evolution and interfacial segregation in MoSi₂/NbSi₂-based C11_b/C40 duplex disilicides have been investigated by phase-filed simulations with special focus on the roles of anisotropy of interfacial energy, lattice misfit, and the segregation energy of alloying elements, which were evaluated by first principles calculations. The simulation results revealed that the high anisotropy of interfacial energy dominates the formation of lamellar structure, and the Cr-segregation at lamellar interface, which was experimentally measured, is mainly due to the segregation energy. It was also predicted that the Cr-addition greatly retards the interface migration and stabilizes the lamellar structure. In order to find more effective alloying elements, segregation of all the transition metal elements at the lamellar interface has been investigated systematically. Zr, Hf and Ta were predicted to segregate at lamellar interface and stabilize the lamellar structure more significantly than Cr, suggesting that the additions of these elements are more effective to stabilize the lamellar structure than Cr-addition. The magnitude of segregation simulated has more strong correlation with the depth of segregation energy potential than that of elastic energy potential. The solute-interface interaction will be discussed with particular attention to the competition between the reduction of interfacial lattice mismatch and site preference of the alloying elements. [1] T. Yamazaki et al. Intermetallics 2014 (in press)

Transition metal disilicides are one of the promising candidates for the ultra-high-temperature structural materials because they exhibit high melting points and good oxidation resistance, and thus they have a potential for improving the thermal efficiency in power generation systems. We have developed the NbSi₂(C40)/MoSi₂(C11_b) duplex crystal with the oriented fine lamellar microstructure, by the appropriate annealing of a C40 structured (Mo_0.85Nb_0.15)Si₂ single crystal grown by the floating zone (FZ) method [1]. The NbSi₂/MoSi₂ lamellae was found to exhibit the specific crystallographic orientation relationship of (0001)_C40//(110)_C11b and <1210>_C40//[110]_C11b. The development of such lamellar microstructure in the duplex crystal was found to improve the high-temperature strength, and in addition an increase in low temperature fracture toughness was also reported [2, 3]. Those experimental results demonstrated that the control of microstructure play a critical role for improving the mechanical properties of the NbSi₂/MoSi₂ duplex lamellar crystals.
In this study, we have examined the effect of B addition on the lamellar microstructure in the NbSi₂/MoSi₂ duplex crystals. It was found that the microstructure was drastically varied by the small amount of B addition. The ratio of C11_b phases showing the lamellar microstructure with the flat interface was decreased, and the amount of C11_b phase grains that did not show the above-described specific crystal orientation relationship with respect to the C40 matrix phase was largely increased by the B addition, compared to that in the non-added NbSi₂/MoSi₂ crystal.
In the presentation, relation of the amount of added B concentration and the variation in microstructure is discussed. In addition, the variations in room temperature fracture toughness by the B addition will be also discussed.
References
[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.

Niobium silicide based alloys comprising of niobium solid solution and Nb₅Si₃ -silicide have been researched for some decades as potential candidates to replace nickel base superalloys in land based and aircraft turbine applications [1]. A high melting point, which results in good high temperature strength combined with a density well below that of nickel based alloys is promising. The poor oxidation resistance of binary Nb-Si-alloys is typically addressed by the addition of titanium, chromium, aluminum, and hafnium [2]. Derived from the well-known MASC composition (Nb-16Si-25Ti-2Al-2Cr-8Hf) an alloy with increased silicon content for a larger silicide phase fraction was chosen for this work (Nb-20Si-23Ti-6Al-Cr-4Hf).
A keystone in applying those materials is proper process and microstructure control. Microstructural homogeneity is an advantage of powder metallurgy. By combining this with near net-shape compaction processes such as net-shape HIPing or powder injection molding (PIM) components with minimum effort in post-processing could be manufactured. The powder was produced either by gas atomization (GA) of pre-alloyed rods or by mechanical alloying (MA) of elemental powders in an attritor. After solidification for all variations of HIP and PIM with GA and MA powder, a homogenous distribution of niobium solid solution, α-Nb₅Si₃, and γ-Nb₅Si₃ could be achieved. HIP material was almost dense at > 99.5 % of theoretical density, while PIM left closed porosity of up to 2 % and 4.5 % for GA and MA powder, respectively. The latter could be removed by an additional HIP step. Subsequent heat treatment between 1300 and 1500 °C was done to coarsen the grain size for improved high temperature strength and at the same time stabilize the preferred α-Nb₅Si₃ over γ-Nb₅Si₃.
Mechanical properties were assessed for the different processing conditions. Creep behavior was measured in the range 1000 to 1100 °C, 30 - 200 MPa, and generally shows a small stress exponent of 2, suggesting grain boundary sliding as a contributing deformation mechanism due to the fine-grained equiaxed microstructure, even after coarsening treatment. A ductile-to-brittle transition temperature (DBTT) as low as 750 °C could be reached, which is well below similar MoSiB-based alloys [3]. Other mechanical properties such as room temperature fracture toughness determined by the SEVNB method, and compressive and bending strength from RT to 1100 °C, are also assessed regarding different production conditions.
[1] B.P. Bewlay, M.R. Jackson, P.R. Subramanian, J.-C. Zhao, Metall. Mater. Trans. A 34 (2003) 2043.
[2] B.P. Bewlay, M.R. Jackson, H.A. Lipsitt, Metall. Mater. Trans. A 27 (1996) 3801.
[3] M. Krueger, P. Jain, K.S. Kumar, M. Heilmaier, Intermetallics 48 (2014) 10.

Niobium silicide based alloys are promising materials to replace Nickel based superalloys in high temperature structural applications due to their high melting point and relatively low density [1]. Unfortunately, good mechanical strength and oxidation resistance at elevated temperatures are difficult to be achieved simultaneously in a satisfying manner.
It is the hypothesis of this approach that this challenge may be met by directional solidification (DS) of eutectic alloy compositions because (i) It is well known that DS processing can lead to enhanced creep lifetimes by minimizing the amount of transversal grain boundaries within the material. Especially for eutectic alloy compositions it is believed to positively alter the creep properties due to the formation of a continuous and fine lamellar microstructure [2]. (ii) Also, this fine lamellar morphology is capable of improving the oxidation resistance, providing that one constituent phase forms a protective oxide quickly, which than can cover the other non-resistant phases, accordingly.
For this reason, the eutectic composition of the ternary system Nb-Si-Cr is directionally solidified employing a vertical float zone process. In this system, silicon is known to significantly improve the creep properties by formation of silicide phases (e.g. Nb₅Si₃ or Nb₃Si), whereas chromium is added to mainly improve the oxidation resistance due to the potential for Cr₂Nb Laves phase formation [3]. The influence of varying processing parameters (growth rate, rotating speed and direction of the feed and seed rods) on the resulting microstructures is comparatively assessed. Compressive creep tests in vacuo under constant stress at temperatures up to 1300 °C give first insights into acting creep deformation mechanisms. In comparison to arc-molten samples of the same composition the potential of this DS eutectic to improve mechanical properties at elevated temperatures is demonstrated. Thermo-gravimetric studies of the oxidation resistance between 800 °C and 1200 °C reveal a very low weight gain over the whole temperature range, which is in contrast to investigated non-eutectic ternary alloys in the Nb-Si-Cr system. The cause of this surprisingly good behavior in terms of formed oxides and oxidation kinetics is thoroughly assessed. In particular, it is shown that the oxidation behavior of the eutectic is not dependent on the processing route, but rather on the very fine phase arrangement and distribution, proving the high temperature capability of this new alloy.
[1] B.P. Bewlay, M.R. Jackson, P.R. Subramanian, J.-C. Zhao; Metall. Mater. Trans. A 34, no. 10, 2043-2052 (2003)
[2] H. Bei, G.M. Pharr, E.P. George; J. Mater. Sci. 39, 3975-3984 (2004)
[3] B. Bewlay, M.R. Jackson; J. Mater. Res. 11, no. 8, 1917- 1922 (1996)

Titanium Aluminides possess a range of attractive properties for aerospace applications. Depending on the processing route selected to manufacture a given component, a broad range of microstructures (e.g. equiaxed gamma, nearly fully lamellar, etc.) can be achieved. In this work the room temperature fracture toughness and fatigue-crack growth behavior of as-cast titanium aluminide was investigated. As-cast samples were tested in air in the longitudinal and transverse directions at room temperature. The effects of changes in sample orientation on the toughness and fatigue crack growth behavior were determined at load ratios, R, of 0.1, 0.3, 0.7, and 0.9. The fatigue crack growth threshold, Paris law slope, and overload toughness, Kc, were determined. Scanning Electron Microscopy was utilized to determine the effects of sample orientation and testing condition on fractography while laser confocal microscopy was conducted to quantify surface topography.

Novel manufacturing process are more and more used to produce advanced structural materials as in the case of the gamma titanium aluminide intermetallic alloys (gamma-TiAl). In this work we examine a Ti-48Al-2Cr-2Nb alloy obtained with an additive manufacturing technique by Electron Beam Melting (EBM) by conducting monotonic and cyclic loading experiments both on tension and compression samples for investigating the influence of the microstructure in strain accumulation process by fatigue loading. The residual strain maps corresponding to different applied stress levels, number of cycles and microstructures are obtained through the use of high-resolution Digital Image Correlation (DIC). The strain maps were overlaid with the images of the microstructure and detailed analyses were performed to investigate the features of the microstructure where high local strain heterogeneities arise. Such experiments, conducted ex-situ at room temperature, allow to characterize the effect of different microstructures on the strain accumulation process, providing additional information into the effect of the lamellar and equiaxed grains and also to capture the evolution of the local deformation process for TiAl.
The quantitative analysis by DIC of the strain distribution provides further information on the role of the intermetallic phases on the fatigue behavior of gamma-TiAl alloys. In particular, the results of high cycle fatigue experiments with specimens containing artificial defects show that the fatigue endurance strength of the TiAl alloy with microstructural features in the range 80-150 microns can be accurately described by a modified short crack model, incorporating the microstructural characteristic dimensions of potential source of crack initiation.
The comparison with the strain accumulation in fully lamellar microstructure with larger grain size permits to highlight the influence of the position of grain boundaries and the orientation of the lamellae for the onset of fatigue cracking. The analysis and comparison of the strain maps provide information for the selection of the microstructural parameters during material design.

Microstructural instability is a determining criterion for the development of high-temperature materials. The influence of C, Si and Mo on the microstructural stability during creep was investigated for a β-solidifying γ-TiAl based alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), named TNM. The results for the nominal TNM alloy were compared with those obtained from the TNM-0.3C-0.3Si and TN1.5Mo-0.5C alloys. Thereby, a coarse nearly lamellar γ microstructure (NL γ) after casting and hot-isostatic pressing (cast/HIP) as well as a nearly lamellar β microstructure (NL β) with fine α₂/γ-lamellae after adjacent heat treatment were investigated. For adjusting a fine lamellar microstructure with good mechanical properties a two-step heat treatment in the (α+β)-phasefield region was applied with subsequent annealing below the eutectoid temperature at 1000 °C for 1 hour. During the final stabilization heat treatment a discontinuous precipitation occurs starting from the fine lamellar colonies. Additionally, this decomposition reaction process continuous during ensuing creep testing leading to a reduced creep resistance. Creep tests were carried out at 815 °C and 150 MPa to examine the potential of C, Mo and/or Si on preventing the microstructural instability during creep. The fine lamellar NL β microstructure of TNM and TNM-0.3C-0.3Si alloy demonstrated an increased creep rate and a pronounced tertiary creep stage was observed when compared to the cast/HIP condition. This is primarily caused by the continuation of the decomposition reaction. The TN1.5Mo-0.5C alloy showed the highest creep resistance in both conditions and after the heat treatment the lowest amount of discontinuous precipitation due to higher levels of C and Mo. Additionally, further progression of the reaction front during creep was not determined. This presentation contains the identification of the mechanisms behind the observed phenomena, which is a fundamental knowledge base for the further development of high-temperature TiAl alloys.

Titanium aluminides represent an interesting alternative to established nickel-base superalloys for high temperature applications due to their high specific strength. Especially, fully lamellar titanium aluminides, consisting of two Intermetallic phases γ-TiAl and α₂-Ti₃Al, exhibit excellent creep properties due to a high fraction of interfaces that hinder dislocation motion. Due to differences in crystal structure and lattice parameters of γ-TiAl and α₂-Ti₃Al the γ/α₂ interface is semi-coherent. Alloying elements changes the lattice parameter of both phases and hence the lattice misfit and induced internal stresses which affects the creep properties of the alloy.
In order to study the influence of alloying elements on the lattice misfit and creep strength three different model alloys, Ti-45Al-5Nb, Ti-45Al-5Ta and Ti-45Al-5Zr, are investigated. The elements niobium, tantalum and zirconium have been selected due to their different γ/α₂ partition behavior, their relatively high solubility in titanium aluminides and their significant influence on the lattice parameter of both phases. Fully lamellar microstructures with similar lamellar spacing have been adjusted in all alloys and synchrotron diffraction measurements show that the alloying elements lead to different lattice misfits and coherency stresses. Creep tests were performed and the influence of the elements niobium, tantalum and zirconium is discussed.

Orthorhombic titanium based Ti₂AlNb-alloys cannot be used above a temperature limit of about 800°C due to enhanced oxidation and environmental embrittlement. This embrittlement is caused by the high oxygen solubility which deteriorates the mechanical properties. Even if these materials possess an Al-content up to ca. 25at.% no protective alumina layer is formed. Instead a non-protective fast growing mixed scale is found. Several attempts have been made to increase their operation temperature e.g. by coatings but none has proven sufficiently protective so far. One new way presented in this paper is the enrichment of Al in a narrow surface zone by the powder pack process so called aluminization followed by a fluorination step. Exposure tests at elevated temperature have shown that the aluminized specimens form an alumina layer during exposure in oxidizing environments. Due to the gradient in the Al-concentration interdiffusion with the substrate and the Al-rich diffusion zone occurs which lowers the Al-concentration in the diffusion zone. If the Al-content drops below a critical value Ti-oxides will also form which deteriorates the protection provided by the alumina scale. The subsequent fluorination gets the fluorine effect to operate which stabilizes the protective alumina layer. Untreated specimens are covered with a thick non protective scale and exhibit enhance oxygen ingress in the subsurface zone while treated specimens reveal a thin protective alumina layer and no oxygen inward diffusion. In this paper results of exposure tests of untreated and treated orthorhombic Ti₂AlNb-alloys will be presented and compared with the a₂-phase Ti₃Al.

Materials based on the half-Heusler semiconductor TiNiSn are attractive thermoelectrics, as they exhibit very large Seebeck coefficients, S, for their high electrical conductivities, σ. However, the thermal conductivity, κ, of these materials is relatively large --- ~7 W/mK, 4-5 times that of state of the art thermoelectrics --- hampering their efficiency as measured by the thermoelectric figure of merit, ZT=(S²σ/κ)T. As such, microstructural engineering is an important tool for reducing thermal conductivity and improving performance.
We present here our study of bulk, biphasic materials of the composition TiNi_1+xSn (x=0.00, 0.05,hellip;, 0.25) prepared by induction melting and annealing. In particular, we focus on the effects of Heusler TiNi₂Sn phase morphology, grain orientations, and particle size on the thermal transport properties, employing 3-D tomography to characterize the microstructure. Using a scanning electron microscope equipped with electron backscatter diffraction (EBSD) and electron dispersive x-ray spectroscopy (EDX) detectors as well as a femtosecond laser for serial sectioning, we are able to resolve phase distribution and grain orientation in three dimensions through our samples.
We find that above x = 0.15 an onset of long-range connectivity (over 100 mu;m) in the metallic Heusler phase occurs, corresponding to a large increase in thermal conductivity and consequent drop in ZT. In concert with thermal analysis, diffraction, and computational modeling of thermal transport, this 3-D microstructural data informs us to future approaches for microstructural reform and improvement of ZT in Ti--Ni--Sn thermoelectrics.

Aluminum-Manganese electrodeposits produced from non-aqueous baths have complex structures that reflect a competition among amorphous, intermetallic, nanocrystalline solid solution, and even quasicrystalline structures. In a narrow range of 8-12 at.% Mn, dual phase structures of coexisting nanocrystalline face-centered-cubic (FCC) phase and amorphous phase can be produced. The thermal stability of such Al-Mn nanocrystalline-amorphous dual phase alloy is examined using various characterization techniques such as x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The mechanism and kinetics of phase transformation of the dual phase structure are studied. Crystallization of the amorphous phase directly to orthorhombic Al₆Mn intermetallic is seen at relatively low temperatures. At higher temperatures, Mn is diffusionally ejected from the FCC phase to form Al₆Mn. The effect of Mn concentration on thermal stability is also studied to help understand the stabilization mechanism.

Hydrogen storage alloys are expected as an energy storage media since they show reversible hydrogen absorption-desorption reactions, high volumetric hydrogen density and fast kinetics. Although cyclic durability is one of paramount requirements for practical applications, in most cases, hydrogen storage capacity continuously decreases with increasing hydrogenation-dehydrogenation cycle number. In V-Ti alloys, the degradation of hydrogen storage capacity seems to be strongly linked to the accumulation of dislocations [1]. Therefore, the cyclic durability might be improved if we suppress the accumulation of dislocation.
A significant reduction in capacity by hydrogen cycling was observed in stoichiometric Mg_1.0Pr_1.0Ni₄ with C15b structure but interestingly, Mg-rich Mg_1.4Pr_0.6Ni₄ shows excellent cyclic durability. It is probable that the accumulation of dislocation is suppressed by changing the chemical composition. In order to understand the effect of chemical composition on the cyclic durability of Mg_2-xPr_xNi₄, we investigated change in local structure of Mg_2-xPr_xNi₄ by hydrogen cycling, particle size distribution after hydrogenation and mechanical properties.
As described above, reduction in the hydrogen storage capacity of Mg_1.4Pr_0.6Ni₄ during 100 cycles was negligible. On the other hand, the capacity of Mg_1.0Pr_1.0Ni₄ dramatically decreased from 0.64 H/M (the number ratio of absorbed hydrogen to metal) to 0.57 H/M by only 15 cycles. Pair distribution function (PDF) [1] of Mg_1.0Pr_1.0Ni₄ obtained from the synchrotron X-ray total scattering data showed r-dependent peak broadening and it became stronger with increasing cycle number. Such peak broadening has a strong correlation with the amount of capacity reduced. This is similar to the case of V-Ti alloys [1], hence reduction in the hydrogen storage capacity of Mg_1.0Pr_1.0Ni₄ is most likely caused by dislocations accumulated during hydrogen cycling. However, such r-dependent broadening effect was not observed in Mg_1.4Pr_0.6Ni₄ PDF. After hydrogen cycling, Mg_1.4Pr_0.6Ni₄ showed much smaller particle size with narrower size distribution than Mg_1.0Pr_1.0Ni₄ even though Mg_1.4Pr_0.6Ni₄H_~3.6 (13.3%) has relatively smaller volume expansion than Mg_1.0Pr_1.0Ni₄H_~4 (15.4%). This suggests that Mg_1.4Pr_0.6Ni₄ is more brittle than Mg_1.0Pr_1.0Ni₄. In fact, Mg_1.4Pr_0.6Ni₄ has significantly higher hardness than Mg_1.0Pr_1.0Ni₄. These results suggest that Mg substitution for Pr in Mg_2-xPr_xNi₄ increases the formation energy of the dislocations and therefore dominant mechanism for releasing the stress upon hydrogenation changes from the formation of dislocations to pulverization leading excellent cyclic durability [2].
This work was partly supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
[1] H. Kim et al., J. Phys. Chem. C, 117, 26543-26550 (2013).
[2] K. Sakaki et al., J. Phys. Chem. C, 118, 6697-6705 (2014).

The thermopower (S) and electrical conductivity (σ) in conventional semiconductors are coupled adversely through the carriers&’ density (n) making it difficult to achieve meaningful simultaneous improvements in both electronic properties through doping and/or substitutional chemistry. Here, we discuss the effectiveness of coherently embedded nanostructures in tailoring the density, mobility and effective mass of existing ensembles of carriers within the semiconducting n-type and p-type half-Heusler (HH) matrices. We observed a drastic decrease of the effective carrier density within the half-Heusler nanocomposites at 300K followed by a sharp increase with rising temperature. We proposed that the embedded nanostructures form a potential barrier at the interface with the matrix due to the offset of their conduction band minima (CBM). The energy barrier (DE) discriminates existing carriers with respect to their energy by trapping low energy (LE) carriers, while promoting the transport of high energy (HE) ones. This “carrier culling” results in surprisingly large increases in the mobility and the effective mass of HE carriers contributing to electronic conduction. The simultaneous reduction in the density and the increase in the effective mass of free carriers resulted in large enhancements of the thermopower whereas; the increase in the mobility minimizes the drop in the electrical conductivity. Using X-ray powder diffraction, electron microscopy, and electronic transports data, we will discussed the mechanism of phase formation and transformation, at the sub-ten nanometer scale, in bulk half-Heusler (HH) matrix and the mechanism by which the embedded nanostructures regulate electronic charge transport within the semiconducting HH matrices. Emphasis will be placed on the n-type Zr_0.25Hf_0.75Ni_1+xSn_1-yBi_y and Ti_0.1Zr_0.9Ni_1+xSn, and the p-type Ti_0.5Zr_0.5Co_1+xSb nanocomposites.
(1) Makongo, J. P. A. et al., J. Am. Chem. Soc.2011, 133, 18843.
(2) Liu, Y. et al., J. Am. Chem. Soc. 2013, 135, 7486.
(3) Sahoo, P. et al., Nanoscale 2013, 5, 9419-9427.

During the last decade, TiAl alloys have been successfully produced by Spark Plasma Sintering (SPS). SPS is a powder metallurgy technique, for which the heating of the sample occurs by the application of a pulsed direct electric courant. This process allows achieving original microstructures with enhanced properties due to its rapid processing cycle as well as non-textured, homogeneous structures as with all powder metallurgy techniques.
The aim of the present talk is to describe the route to obtain alloys with improved mechanical properties. The difficulty is to reach a good compromise between the creep resistance at high temperature and the ductility at room temperature. The microstructures are mastered through the optimization of the chemical composition and the control of the parameters of the SPS cycle. To explain and improve the mechanical properties, post mortem analyses of samples deformed at room temperature and of samples crept at service temperature were performed by transmission electron microscopy. They were supported by in situ straining experiments performed inside the transmission electron microscope.
In consistency with the powder metallurgy route, we decided to look for a fine microstructure with a near or a fully lamellar microstructure. Two alloys containing boron to limit the grain growth and heavy elements to improve the creep resistance were sintered by SPS: the TNM alloy provided by Professor Clemens Group (Leoben University - Austria) and the IRIS alloy patented by CEMES. In both cases, a fine near lamellar microstructure was achieved. It was constituted by lamellar grains surrounded by borders formed by γ and β₀ grains.
These alloys are found to exhibit the following interesting properties: i) a good reproducibility of these properties due to the use of the powder metallurgy route, ii) an interesting plastic elongation at room temperature (higher than 1.5% for the IRIS alloy) due to the presence of an amount of γ phase and to the limited size of lamellar grains and iii) an exceptional creep resistance at 700°C due to the predominance of the lamellar structure and the strength of the γ phase.

Titanium-aluminides present an intermetallic material with great potential as structural material in high-temperature applications, such as aircraft turbine blades, turbo-chargers, etc. The material, however, is hard to deform plastically during thermo-mechanic processing. Ample of research is undertaken world-wide, including recent high-pressure techniques, such as high-pressure torsion or near-net-shape forging in huge load frames. Constituting phases dramatically influence the plastic deformation behavior, however, the high-pressure phase diagram of these materials have not yet been established. In the present, we investigate the crystallographic structure of gamma-based TiAl intermetallics by synchrotron X-ray diffraction under high pressure up to 10 GPa and high temperature up to melting. We found that a three-phase field of alpha, beta and gamma phase appears, which can be used as an ideal deformation window in high-pressure forging processes.

An important step towards the mass production of TiAl parts is the development of suitable hot-forming processes. Thermo-mechanical treatments (TMT) as hot-rolling or forging are well-established processes to improve the mechanical properties and to homogenize the microstructure of metals and alloys as well as for near net-shape production. On the other hand, TMT can produce unwanted mechanical anisotropy due to the formation of crystallographic textures. However with conventional analysis technics the texture formation mechanisms can only be inferred from post process metallographic and diffraction studies in which the real high temperature conditions are often masked by subsequent phase transformations or recrystallization.
To overcome this problem, we used a deformation dilatometer (DIL 805A/D) modified for working in the HZG synchrotron beamline HEMS at DESY for hot-deformation experiments. In situ high-energy X ray diffraction (HEXRD) was performed during hot-forming. This setup enables an in situ observation of the interaction and evolution of several microstructure parameters during processing. In particular, we directly observed the evolution of crystallographic texture, phase fractions and grain size during deformation and simultaneously recorded the process parameters, like temperature, force and length change.
We studied the hot-deformation behavior of a Ti#8209;42Al#8209;8.5Nb (in at.%) alloy. In order to start with texture-free sample material the alloy was powder-metallurgically produced and hot isostatic pressed. By varying the process temperature we were able to adjust different phase fractions in the alloy: in equal parts α-Ti(Al) and γ-TiAl at 1100 °C, mainly α-Ti(Al) at 1200 °C, and α-Ti(Al) + β-Ti(Al) at 1300 °C. Thereby it is possible to analyze the interactions between the different phases during texture formation. Immediately after deformation most samples were quenched to retain the deformed microstructure. Additionally some samples were heat treated to study the influence of static recrystallization on the deformation texture.
This kind of in situ studies during deformation and subsequent annealing can provide new insights into the interaction of deformation, recrystallization and phase transformation and its relation to the formation of the final texture.

Intermetallic titanium aluminides represent an attractive option to replace heavier Ni-base supershy;alloys in advanced aircraft engines as well as in automobile combustion engines. An enhancement of mechanical properties of this alloy class can be achieved by alloying with C, either by solid-solution hardening or by precipitation hardening. Which effect is finally contributing to the improved mechanical properties is determined by the C concentration and the thermal history of the sample material.
Within this study a Ti-43.5Al-4Nb-1Mo-0.1B-0.75C (in at.%) alloy in the hot-forged and homogenized state was investigated. At room temperature this alloy consists of γ-, α₂- and β_o-phase, whereas no carbides were observed, thus the C concentration is lower than the overall solubility of the alloy system.
The aim of this study was to determine the distribution of the alloying elements as well as impurities such as silicon within the constituent phases by the use of atom probe tomography. Carbon was found to preferentially segregate to the α₂-phase. Lower amounts of C were observed in the γ-phase, whereas the β_o-phase was found to be completely depleted of C.
Furthermore, interfacial segregation effects of impurities were investigated. Some impurities were found to pile-up at interfaces, whereas others were found to be distributed in the constituent phases.
The gained data contribute to the understanding, solubility and precipitation behavior in advanced intermetallic γ-TiAl alloys, which in turn determine the mechanical properties.

TiAl alloys are light-weight structural materials for applications in turbines of aero-engines. One goal of ongoing research is to overcome the inherent brittleness of these intermetallic materials while maintaining their strength at high temperatures.
With the addition of Nb to Ti-(40-44)Al alloys a good combination of ductility and strength was achieved in alloys processed by hot extrusion [1]. The alloys have a near lamellar microstructure consisting of (α₂ + γ) colonies and regions of B19/β_o and γ laths that appear pearlite-like. Structural modulations on the nm scale lead to a typical strain contrast within α₂ lamellae.
Motivated by this development a powder metallurgical version of the alloy Ti-42Al-8.5Nb is being developed with a similar combination of properties and microstructure. Starting from pre-alloyed powder the alloy is compacted by hot-isostatic pressing at 1250 °C [2]. Subsequent heat treatments at temperatures between 550 and 650 °C lead to the formation of an orthorhombic phase from the parent α₂ phase. The presence of the orthorhombic phase can be recognized in TEM images from strain contrast within α₂ lamellae.
The analysis of TEM high resolution images and diffraction patterns of samples annealed at 650 °C for hours suggest that in addition to the B19 phase that has been reported earlier in this alloy [3], there is clear evidence for the occurrence of the O-phase. The orthorhombic O-phase is known in literature for alloys with a significantly higher Nb content and is not considered in the ternary phase diagram for this composition range so far. In TEM imaging and diffraction it can be recognized from angular distortions in high resolution images and diffraction patterns recorded from α₂ lath. Imaging of α₂ laths in the hexagonal [0001] direction shows that facets form within the laths. The nature of these facets is not understood at the time. However it is observed that with prolonged annealing time the facet size seems to decrease. The alloy's microstructure in different annealing stages will be characterized by TEM.
[1] F. Appel, M. Oehring, J.D.H. Paul, Advanced Engineering Materials8 (2006) 371-376.
[2] R. Gerling, H. Clemens, F.-P. Schimansky, Advanced Engineering Materials6 (2004) 23-38.
[3] H. Gabrisch, U. Lorenz, M. Oehring, J.D.H. Paul, F. Pyczak, M. Rackel, F.-P. Schimansky, A. Stark, MRS Proceedings1516 (2013) 35-40.

New TiAl alloys with a nano scale modulated microstructure are of interest due to their balance of high specific strength and ductility. Compared to conventional titanium aluminides, they possess a laminate structure with tweed lamellae. These lamellae appear tweed like because they are composed of a modulated arrangement of at least two phases. One constituent of the crystallographic modulation in the lamellae is an orthorhombic phase, which is closely related to the hexagonal α₂-Ti₃Al and cubic B2 ordered β_o-TiAl phase. Until now no experimental information on the formation mechanism and thermal stability of this orthorhombic constituent of the modulated microstructure is available.
In this study specimens of a powder processed Ti-42Al-8.5Nb (in at. %) alloy were investigated after annealing at 600 °C and 650 °C for 0.5 to 8 hours, as well as in situ during annealing by high-energy X-ray diffraction (HEXRD).
HEXRD experiments were performed at the HZG run beam line HEMS at PETRA III at Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany. In parallel the microstructure was characterised by scanning electron microscopy (SEM), including energy dispersive X-ray (EDX) and electron backscattered diffraction (EBSD) methods.
The measurements have shown that the orthorhombic phase is structurally comparable to the O-phase (Ti₂AlNb). It forms in the temperature range of 550 °C to 700 °C from the α₂-phase by small atomic displacements and chemical reordering. The in-situ experiments yielded information about the thermal stability of the orthorhombic phase. After dissolving at temperatures above 700 °C, the phase can be re-precipitated by annealing within the formation temperature range. This behaviour indicates that this phase is a stable phase over the given temperature range in this new kind of Nb-rich TiAl alloy.

Equivalency of solute element, M₂ against M₁, k_M2/M1 is often used for alloy design of multi-component systems such as steels and Ti alloys in order to evaluate the phase stability of the interest. This idea is applied to γ-TiAl based alloys with combined addition of transition elements M (M: V, Nb, Cr, Mo). In Ti-Al-M ternary systems, the VIth group elements show stronger β-Ti stabilizing effect than the Vth group elements, and its effect becomes much larger with decreasing temperature. Thus, the addition of VIth group element as a forth elememt to one of the ternary systems is favorable from a viewpoint of wrought processing using the β phase. On the other hand, the addition of Vth gourp element is favorable, from a viewpoint of lamellar microstructure control, since they widen window between the precipitation start of γ and β phase in the a matrix phase along the transformation pathway of α→α+γ→α+γ+β. Therefore, combined addition of the Vth and VIth elements to the binary alloys is favorable, and the equivalency k_M2/M1 is often used for alloy design. This approach is acceptable if there is no interaction energy to stabilize the β phase between two elements. However, we found that the negative interaction exists in quaternary systems. In case of quaternary Ti-42Al-8Nb^eq alloys having different Nb and M concentration, using the Nb equivalency, k_Nb/V = 1.2, k_Nb/Cr = 2.1 and k_Nb/Mo = 4.7, determined from the ratio of the critical concentration of M elements for the formation of β in ternary systems, there exits a certain amount of β phase in the alloy equilibrated at 1473 K, unlike the expectation, and the amount of the β phase varies depending on the M/Nb ratio as well as the type of M element added. The more β phase exist in the alloys with the M/Nb ratio close to 1.0, whereas no β phase observed in the alloy with much higher and lower M/Nb ratio and in the alloys with the VIth gourp element. These results clearly indicate the existence of the negative interaction to stabilize β phase between the two solute elements at 1473 K. Thus, the equivalency approach does not work to estimate the amount of β phase correctly and probably bring more β phase at sevice temperatures, if it is used, leading to the early degradation of the lamellar microstructure. Therefore, alloy design using equivalency should not applicable. In this study, composition dependence M/Nb of the interaction is quantitatively analyzed, and the alloy design taking this interaction energy into account will be discussed.

Molybdenum is a strong β-stabilizer and an important alloying element in γ-TiAl based alloys, especially since a significant volume fraction of the disordered bcc β-phase improves the processing characteristics during hot-working. To increase the understanding of the alloying effect of Mo, an ingot with a nominal composition of Ti-44Al-7Mo-0.1B (in at%) was produced. By adding such a high amount of Mo, the formation of α/α₂ is suppressed, i.e. the β/β_o and γ-phases coexist in thermodynamic equilibrium. By means of in-situ diffraction techniques employing synchrotron radiation and neutrons along with electron probe micro analysis on selected sample conditions, the phase constituents at different temperatures were studied. In technological processes, however, high heating and cooling rates are used. Therefore, the material&’s behavior under non-equilibrium conditions was investigated as well. In order to homogenize the alloy, the specimens were heat-treated in the single β-phase field region of 1450°C for 30 minutes, followed by water quenching. A subsequent long-term heat-treatment at 1000°C for 168 hours led to a pronounced grain refining effect. After the individual heat-treatment steps, the precipitation behavior of γ within β_o was studied by means of scanning and transmission electron microscopy, electron backscatter diffraction, focused ion beam/SEM tomography, 2D and 3D image analysis as well as high energy X-ray diffraction and atom probe tomography. These analyses reveal nano-sized γ precipitates formed in the β_o matrix with {111} γ-TiAl parallel to {110} β_o-TiAl. The 3D morphology shows an interconnected network for both phases even across former grain boundaries, a fact which is not deduced from 2D image analysis. All the results of these investigations, combined with the results of metallographical characterization, are compared and discussed.

Titanium aluminides are a prominent class of lightweight, creep and oxidation resistant materials which are used in automotive and aircraft industry. In order to overcome some of its inferior properties, such as limited hot-workability, various concepts including alloying and sophisticated processing routes have been explored and developed. One of these alloys is the so-called TNM alloy containing Nb and Mo which stabilise the disordered body centred cubic (bcc) β-TiAl phase, thus aiding its hot-workability. Depending on the Mo content, the disordered β-phase orders during cooling to room temperature to βo with a B2 structure.
We report on first principle calculations employing Density Functional Theory to study the interplay between effects of Mo alloying and chemical disorder on the stability of bcc ordered and disordered phases. Enthalpy of formation of the above mentioned bcc phases is higher than that of the γ-phase which is thermodynamically preferred at low temperatures. However, an evaluation of elastic constants reveals that the βo-phase is also mechanically unstable at low temperatures in the binary Ti-Al binary system, hence it is ruled out as a possible metastable phase. Surprisingly, disordered β is shown to be mechanically stable. This is further confirmed by calculating the phonon dispersion curves exhibiting imaginary frequencies for the ordered βo phase, while the phonon density of states of the disordered phase contains only a negligible amount of imaginary frequencies. For instance, when Al is partially replaced by ~6at.% of Mo to create Ti-44Al-6Mo, the stability of the ordered β phase is improved, while the disordered configuration is fully mechanically as well as dynamically stable. It is therefore concluded that Mo alloying and chemical disorder act in a similar manner to stabilise the β/βo phase. Finally, by exploring the heat of formation of Ti-(50-x)Al-xMo in the whole compositional range we show that alloys with a Mo content between 6 and 20at.% exhibit the lowest heat of formation, hence these compositions of the β/βo phase are expected to appear in the microstructure of multi-phase engineering TiAl alloys.

Advanced γ-TiAl intermetallic alloys are being used as structural high-temperature materials, (for applications in automotive and aircraft engine industry), because of their low density and high specific mechanical properties. Due to the last decades of intensive development and research they are becoming competitive materials, but still alloy improvements are required to increase particular properties. In the present work the evolution of the microstructure with temperature in a Mo-rich model alloy Ti-44Al-7Mo (at%) has been studied. In order to lead to a better understanding of the defect mobility processes taking place at high temperature, internal friction (IF) and dynamic modulus measurements (DM) have been carried out by mechanical spectroscopy. The obtained results show a relaxation peak, whose activation enthalpy and time limit have been determined. The achieved results are interpreted in terms of the different kind of small changes in the microstructure and defects on atomic scale, as well as the specific motion mechanisms that they may undergone. The high-temperature background has been also analysed in relationship with the creep behaviour. Finally, measurements up to 1640 K allow crossing the phase diagram lines, in order to study the effect of phase transformations on the relaxation phenomena.

In order to enhance the high temperature capabilities of γ-TiAl based alloys, their creep properties are improved by the addition of carbon via solid solution or / and precipitation hardening. Two types of carbides were reported in TiAl alloys: perovskite (P-) Ti₃AlC phase and hexagonal (H-) Ti₂AlC phase, which form and are stable at different temperature ranges.
For efficient precipitate-strengthing the stability of the precipitates and the character of the interface between the matrix and the precipitate are very important factors. However the relevant knowledge in literature is limited and only available for a restricted number of alloy compositions. Usually the P-Ti₃AlC phase precipitates coherently with a needle-like shape and coarsens and loses coherency with the γ matrix with extending time. H-Ti₂AlC phase always appears as coarse plate at higher temperatures or at longer times. In order to obtain information on the shape development of carbides in TiAl alloys, a Ti-45Al-5Nb-0.75C alloy (in atomic percent) was produced by powder metallurgy. The pre-alloyed powders were consolidated by hot isostatic pressing. Then the material underwent different heat treatments and subsequently the microstructure was investigated by transmission electron microscopy.
P-Ti₃AlC carbides in the γ matrix have a needle-like shape after annealing at 800 °C for 24 h. This shape is changed to plate-like with extended annealing and finally they decompose into smaller sub-particles with increasing annealing time. In this splitting process the carbide plates break into small subdomains which remain coherent with the γ matrix. The splitting first occurs into an arrangement of parallel rods along the [001] direction of the matrix which later decompose further in small particles along the [100] or [010] directions of the matrix. The volume between these sub-particles is filled with the γ phase. The crystallographic orientation of this γ phase is frequently different from the surrounding γ matrix. It is proposed that the elastic interaction energy between the split subdomains may be a factor to make this decomposition into sub-particles energetically favorable.

The temperature capability of present day TiAl-alloys of about 750 °C is sufficient for their actual application as low pressure turbine blades. Other parts like turbocharger wheels in automotive engines or turbine blades in rows closer to the combustion chamber are associated with higher service temperatures. Thus, there is a huge demand to raise the application temperatures of intermetallic γ-TiAl based alloys. Precipitation of fine carbides has proven to be an effective way to achieve this. A fine distribution of perovskite P-type Ti₃AlC carbides in the γ-TiAl matrix can act as obstacles for dislocations hardening the alloy in this way. Nevertheless, investigations about the influence of temperature, carbon content and the alloy microstructure on carbide nucleation and growth are scarce in TiAl-alloys. In this work alloys of the base composition Ti-45Al-5Nb-xC (x = 0.5, 0.75 or 1.0; all at.%) were produced by powder metallurgy and consolidated by hot isostatic pressing. These materials were annealed between 800 to 1000 °C to monitor the nucleation of carbides and the development of the carbide morphology with annealing time. For this a combination of high energy X-ray diffraction (HEXRD) and transmission electron microscopy (TEM) was applied. The former was done at the Helmholtz-Zentrum Geesthacht-run beamlines at the DESY synchrotron Hamburg. In some measurements in-situ specimen environments were used to directly observe the specimens at temperature while the latter included high resolution TEM to investigate the carbide matrix interfaces.
It was found that higher carbon content increased the dissolution temperature and fraction of P-type carbides. Nevertheless, it also promoted H-type carbides which were predominant in the 1 at.% carbon alloy instead of P-type carbides. In addition the nucleation of carbides was accelerated by increased carbon content. Internal interfaces were sites of preferred carbide nucleation due to local carbon enrichment and heterogeneous nucleation. This led to preferred nucleation at grain boundaries in association with carbide free seams in their direct vicinity. P-type carbides in the γ grain interior nucleated later. To minimize the elastic distortion energy of the coherently embedded precipitates the shape of P-type carbides in the g matrix changed during growth. These shape changes already took place before the equilibrium fraction of carbides has nucleated. In the beginning carbides assumed a rod-like shape which changed to plates during coarsening. Finally splitting of these plates into smaller sub particles was observed in some cases. During extended exposure to high temperatures the fraction of larger grain boundary carbides increased at expense of the small P-type carbides in the grain interior. No mechanical tests were performed but it is assumed that the decrease of carbide density in the g grain interior is detrimental for precipitation hardening.

In the 40 years since dedicated research programs for thermoelastic shape memory alloys (SMAs) began, their commercial market has materialized to be measured in billions of dollars, spanning aerospace, automotive, consumer electronic, and medical device industries. However, when considering their potential applications, market assessors estimate the current market is one to two orders of magnitude less than the total market potential of these remarkable materials. It is generally accepted that development of physics-based constitutive models implemented in the form of engineering tools and methodologies to the same extent such means exist to design with traditional solid materials will contribute to market expansion for SMAs. To develop such models, physics-based understanding of the coupled chemical, thermal, and mechanical physics of the elastic, phase transformation, and plastic interactions within SMA microstructures that lead to shape memory behaviors is required. Afforded by in-situ diffraction, an ability to make empirical measurements to quantify relationships between microstructure and macroscopic thermo-mechanical responses of SMAs has helped elucidate such micromechanical understanding of SMA physics in recent years. In this presentation, key fundamental physical understandings and modeling capabilities of SMAs that have been enabled and accelerated by neutrons and synchrotrons will be reviewed.

Ni based magnetic shape memory alloys (MSMAs) have broad applications in actuators and MEMS devices. Two-stage stress induced martensitic phase transformation, a widely observed phenomenon in these alloys, is described conventionally as a first stage L2₁ (austenite)-to-10M/14M (M: modulated martensite) transition, followed by a second stage 14M-to-L1₀ (tetragonal martensite) transformation at higher stresses. Here we show, for the first time via in-situ nanoindentation on single crystalline Ni₅₄Fe₁₉Ga₂₇ alloy, that a reversible L2₁-to-10M/14M transformation took place at lower stress. However at higher stress, an irreversible transition from residual L2₁ to L1₀ martensite (a 2^nd type of phase transformation) occurred [Y. Liu, et al, (2014) Advanced Materials, 26: 3893-3898]. Furthermore phase fronts propagate gradually during the L2₁-to-10M/14M transformation, whereas L1₀ is abruptly emitted in a jerky style during the L2₁-to-L1₀ transformation. Detailed examination of crystal structure suggests that a direct transition from 14M to observed L1₀ is crystallographically forbidden in the current loading condition. This study provides new perspective for understanding stress induced various types of phase transformations in MSMAs.

An emerging class of Ni-based high temperature shape memory alloys (HTSMAs) displays high reliability, light weight and increased capability while lowering space and power consumption for many energy and transportation applications. This research focuses on developing a fundamental understanding of the inherent microstructure-property relationship of Ni-rich HTSMAs, of which very little is currently known. Ni-Ti-X alloys where X=Hf,Pt,Au can exhibit high transformation temperatures, large transformation strains and small permanent strains. These systems are investigated in order to determine beneficial properties, which are strongly influenced by the formation of nanoscale precipitates. Advanced electron characterization techniques are used to explore the martensitic interactions of these precipitates at low temperature, and dislocation activity at higher temperature. These insights are incorporated into microstructural modeling frameworks to understand how phase transformations, crystal plasticity, and time-dependent creep interact under isothermal and load biased thermal cycling conditions.

Nowadays there is an increasing interest in developing high-temperature shape memory alloys (SMA) able to be applied as sensor and actuators up to 2200C, so 1000C above the limited capabilities of the classical TiNi SMA. Among the different families of SMA, Cu-Al-Ni alloys are firm candidates to work in this temperature range because of their higher thermal stability than other Cu-based SMA.
However a careful compositional control is required to match the transformation temperature, Ms, between -1500C and 2000C, due to the strong dependence of Ms on Al composition (1700C/1%Al). In addition, the high temperature beta phase in Cu-based SMA has a lower yield point than the TiNi SMA and consequently this represent a clear limitation to the force to be used in actuators.
In the present work we present a new design of compositional and micro-structural control in Cu-Al-Ni SMA, oriented to solve both commented problems. A slightly different approach for the composition of the alloy and a four steps thermal treatment allow us to develop a particular microstructure of nano precipitates of NiAl. These treatments allow a precise control of the Al concentration in solid solution, and consequently of the transformation temperatures. In addition the nano-precipitates strengthen the matrix without preventing the martensitic transformation.
The microstructure obtained with different treatments will be presented, as well as the evolution of the size and distribution of the precipitates as a function of the parameters of the thermal treatments. Scanning and transmission electron microscopy have been used to do such characterization, with the support of EBSD and EDX microanalysis.
Finally, the thermo-mechanical properties, such as superelasticity and shape memory, have been studied for different degrees of precipitation and are discussed in comparison with the microstructure.

PtTi is a high temperature shape memory alloy (HTSMA) with a high martensite transformation temperature around 1300K. In order to apply PtTi-based HTSMA for various applications, the actuation temperature should be widely and precisely decreased. In our previous works, it has been found that the transformation temperatures were monotonously decreased by addition of ternary elements in the NiTi-PtTi system as well as AuTi-CoTi system. By taking into account the physical and chemical similarities, it was expected that PtTi and CoTi form a pseudobinary system of (Pt,Co)Ti and that the transformation temperature can be decreased monotonously with Co concentration. In order to confirm this idea, the phase constituent and phase transformation temperature were investigated using a diffusion couple method with a compositional gradient, and the results obtained were compared to the data obtained using bulk materials. As for the diffusion couple method, at first, near-stoichiometric PtTi and CoTi plates were fabricated by Ar arc-melting method followed by a homogenization treatment. Then, they were stacked and hot-pressed at 1373K for 3hrs under 100MPa in Ar. In order to make the compositional gradient, the PtTi/CoTi couple was heat-treated at 1373K for 168hrs in vacuum. The bonding interfaces were characterized a scanning electron microscopy and an energy-dispersive spectroscopy (EDS) analysis. Then, the diffusion couple was again heat-treated at 473, 573, 673 and 773 K for 3min in air, and the surfaces were observed by optical microscope. It was revealed that PtTi and CoTi forms a complete solid solution of (Pt, Co)Ti above 1173K, but that a new intermediate phase was formed at a composition range around 10-20at%Co at 1173K. Based on the microstructural observation, surface relief was formed during heating due to the austenite transformation. Judging from the formation of surface patterns and chemical composition obtained by EDS analysis, the reverse martensitic transformation start temperature (As) was obtained and it decreased with increasing Co content with a rate of -70K/mol%Co. The decreasing rate obtained was slightly larger in compassion with 60K/mol%Co in bulk (Pt,Co)Ti alloys.

Microelectromechanical systems (MEMS) switches or other actuators, while having the potential to function under ultra low power constraints, still require on-board power sources. Any radio frequency (RF) wireless actuation approach requires the use of transceiver elements which may be much larger than the MEMS component itself. Optical actuators circumvent both requirements for on-board power and larger, wireless RF transceiver elements while maintaining the ability to be triggered remotely. We have designed and fabricated prototype MEMS switches that can be optically triggered to close using the heat generated from optical absorption of laser light. The switches were fabricated on a silicon wafer using standard micro fabrication techniques. A thin film of residually stressed Cr and Au acted as the actuation layer, allowing the switch to close when heated by an external laser.
We now aim to extend the technology to a reversible, optically actuated switch through a combination of residually stressed Cr and a patterned thin film cantilever of NiTiCu shape memory alloy (SMA). Again, the Cr layer serves as the biasing mechanism to pull the switch open after dry etching of a sacrificial layer underneath. Upon heating beyond the SMA transition temperature, the SMA undergoes a diffusionless phase change and returns to its as-deposited, annealed position, closing the switch. We will report on the development of our fabrication process, specifically aimed at optimizing the co-sputtering process parameters to yield a SMA film with a consistent and practical transformation temperature relative to ambient conditions. In addition, we will demonstrate wavelength specific actuation based on photo-selective resist with potential applications in passive sensing, micro robotic locomotion, or even initiation of on-chip energetic materials.

Ti-Ni alloys have been extensively developed to expand their application fields by adding new characteristics, shapes and dimensions, e.g., small transformation temperature or stress hysteresis, high transformation temperatures, porous structures, tubes, fine wires and thin films. Among them, some high temperature shape memory alloys have been developed: e.g., Ti-Ni-X (X=Au, Pd, Pt, Hf, Zr). The transformation temperatures of these alloys reach up to 1273K. However, it is not easy to make fine wires or thin plates with these alloys because of their lack of ductility. Recently, deformable high temperature shape memory alloys such as Ti-Ni-Zr(Hf)-Nb, Ti-Ta-Al have been developed, and they can reveal SME at temperatures between 373K and 473K, where commercial Ti-Ni alloys cannot reveal SME. They can be cold-rolled with reductions higher than 60% and 90%, respectively, without revealing final fracture. In this lecture, the development of the Ti-Ni-Zr(Hf)-Nb and Ti-Ta-Al alloys is to be presented. Another important topic for developing high temperature shape memory alloys is to create stable microstructures at temperatures above the recrystallization temperature, because such stable microstructures usually consist of a high density of dislocations which were formed during cold working prior to annealing. Ti-Ni-Pd-Cu alloys can form unique two types of precipitates, TiPdCu and Ti2Pd, which can be formed only by annealing in cold worked samples. These precipitates are stable even at temperatures above the recrystallization temperature. This Ti-Ni-Pd-Cu alloy is also included in this presentation.

Numerous experimental studies have shown that the transformation from an austenite to martensite microstructure in binary Ni-Ti shape memory alloys (SMA) can generate plastic deformation. This can cause functional fatigue, whereby a shape memory alloy plastically deforms in an incremental fashion during repeated thermal cycling under stress. Transformation-induced plastic deformation can also cause structural fatigue, whereby repeated cycling initiates and grows fatigue cracks. Conventional strategies to increase flow strength may improve functional but not fatigue life. Thus, a fundamental understanding of the interaction between phase transformation and plasticity is needed.
This interaction is studied using a unique phase field-finite element (PF-FE) approach that incorporates finite deformation. It models the austenite-to-martensite phase transformation at the finest (nm) scale and includes the kinetics of nucleation and growth of correspondence variants. Like spectral methods, it includes the driving forces from chemical, mechanical, and interfacial energies. However, the finite deformation approach also captures the rotation and deformation of material elements during transformation and plasticity. The latter is modeled using a rate-dependent crystal plasticity flow law in the austenite phase. The formalism also includes anisotropic elasticity and thermal expansion, thus capturing texture development and local internal stress during arbitrary thermo-mechanical histories.
We examine three hypotheses associated with transformation and plasticity. The first is that plasticity can affect the microstructure of thermally-induced martensite and the critical temperature at which it forms. This is explored by varying the critical resolved shear stress for plasticity, thus moderating the amount of rate-dependent plasticity during transformation. The second hypothesis is that plasticity can affect the microstructure of stress-induced martensite and the critical applied stress at which it forms. This is explored by simulating uniaxial compression tests of single austenite crystals of varying orientation. The third hypothesis is that plasticity can affect the microstructure of stress-induced martensite at crack tips. This is explored through Mode I loading in cracked austenite single crystals.
The results of micropillar compression testing and post-mortem TEM thermal cycling of a binary Ni-Ti shape memory alloy reveal dislocation substructures. The simulations and analytic modeling indicate these substructures are generated by the local stress at austenite-martensite interfaces. The results suggest that plasticity can be mitigated by reducing stress at these interfaces. In principle, this coupled experiment-simulation approach can accelerate model validation as well as guide and interpret experimental observations.
This work is supported by the National Science Foundation, Division of Materials Research (DMR-1207494).

Nickel-Titanium shape-memory alloys represent a class of materials capable of recovering a previously defined shape upon the application of heat. This deformation recovery is due to a martensitic phase transformation, i.e. a coordinated shift in atomic structure that is completely reversible for relatively large strains (up to approximately 8-12%). Through an appropriate combination of deformation processing and/or heat treatment, the shape recovery may be “trained” to elicit a two-way shape-memory effect (TWSME). This effect is characterized by the alloy&’s memorization of both a high and low temperature shape, allowing for spontaneous change between the two shapes as a function of cycling temperature.
Recent studies have shown that a TWSME surfaced can be induced in NiTi through a novel indentation method. The near equiatomic NiTi samples were indented to high strains, thermally cycled, then planarized and re-polished. When the sample was then reheated into the austenite phase, it produced bumps on the sample surface, a phenomenon found to be repeatable over multiple heating cycles. The protrusions were hypothesized to be caused by preferentially oriented dislocation structures remaining underneath and around the indentation site after planarization, which is only partially consistent with previous TWSME observations.
The purpose of this research is to expand upon the limited experimental TWSME surface studies by investigating the effect of pre-existing Ti₃Ni₄ precipitates. For slightly Ni-rich NiTi, precipitates are well known to influence the martensitic phase transformation. It has been shown that coherent Ti₃Ni₄ precipitates increase the stress needed for plastic flow as well as decrease the martensite transformation stress. Essentially the local stress fields caused by precipitates act in addition to an applied stress, and become the location of the phase transformation.
The role of pre-existing precipitates on indentation-induced TWSME surfaces has not previously been explored. Therefore our approach was to systematically vary the aging time, leading to either solutionized, semi-coherent, and incoherent Ti₃Ni₄ precipitates. TWSME surfaces were then created via Vickers indentation followed by planarization. Results show that semi-coherent precipitates were critical in for good TWSME behavior, and also show that protrusion height and two-way deformation recovery was sensitive to re-heat temperature. Transmission electron microscopy reveals dislocations and stabilized martensite beneath residual indents. Cumulatively, these results strongly indicate that stabilized martensite dictates the TWSME behavior.

We study the impact of the substrate on the martensite transformation of Ni-Mn-In thin films using density functional theory calculations. First we performed electronic structure calculations of Ni₂Mn_1+xIn_1-x alloys (x=0, 0.25, and 0.5). We find that at x=0.25 the FM order dominates in both cubic and tetragonal phase, while at x=0.5 tetragonal martensite phase is in ferrimagnetic state. There are strong displacements of Mn atoms with respect to Ni atoms in austenite phase that alters pair exchange interactions to become antiferromagnetic. This coupling between the magnetic order and lattice distortions leads to the martensitic transition in Ni₂Mn_1+xIn_1-x. We also find substantial change in the carrier mobility in the majority spin electrons upon martensitic transition leading to substantial modification on resistance in these alloys. The off-stochiometric Ni₂Mn_1.5In_0.5 alloy (x=0.5 ) shows that the cubic phase is unstable against the tetragonal distortion phase and undergoes the martensitic transformation to form tetragonal martensite in ferrimagnetic state. Ni₂Mn_1.5In_0.5 thin films (in both cubic and tetragonal phases) on MgO (001) substrates are studied. The presence of MgO substrate changes the relative stability of ferrmomagnetic (FM) austenite and ferrimagnetic (FiM) martensite states. The energetically favorable structures of the MgO-Ni₂Mn_1.5In_0.5 systems depend on the lattice parameters. Our calculations show that the energy difference between FM austenite and FiM martensite states in 12 layers of Ni₂Mn_1.5In_0.5 film on MgO (001) substrate is (#8710;E = 0.08 eV) per NiMnIn f.u, compared to (#8710;E = 0.24 eV) in the bulk at the same lattice parameters. When the lattice parameters of 12 layers of Ni₂Mn_1.5In_0.5 film have values close to those of MgO substrate, this energy difference become (#8710;E = -0.16 eV) per NiMnIn f.u. These results clearly indicate the possibility of control of martensitic transition in thin films by substrate. We compare our results with the magnetic and transport measurements performed on the thin films of Ni₅₀Mn₃₅In₁₅ grown by laser-assisted molecular beam epitaxy deposition.

Mg-based nanolamellar phases of the (h_mc_n)_k type (here, h and c represent hexagonal and cubic close-packed motifs, respectively; m, n, and k are integers) formed in ternary Mg-TM (transition metal)-RE (rare-earth metal) alloys have recently attracted significant attention owing to their use in the strengthening of Mg alloys and as promising components for designing advanced lightweight structural materials. In such phases (often called "long-period stacking ordered (LPSO) phases"), the two-dimensional stacking-fault (SF)-type interfaces containing solute atoms can play a significant role in the formation of a nanostructured state with low-energy boundaries. As a result, the phases are allowed to contain a high density of SFs and exhibit continuously regular nanolamellar structures formed by SF boundaries, which are an inherent part of their crystal lattice.
To understand the fundamental mechanism of the formation of Mg-based LPSO phases, it is important to elucidate the essential natures of heterogeneities and medium-range orders in two-dimensional solute-cluster packing at SF-type interfaces. In this study, we proposed and established a coarse-grained (CG) modeling approach based on ab initio calculations for predicting the equilibrium superlattice structures of solute nanoclusters confined in an atomically close-packed SF. The approach was used to exploit the intriguing solute-enriched layers observed in Mg-M-Y (M = Al or Zn) LPSO phases as examples of multicomponent SF complexions (i.e., interfacial phases). We investigated the energetic stability and two-dimensional ordering with varying packing density of L1₂- and E2₁-type core-shell-like M-Y clusters in the SF, depending on the temperature and composition, by considering effective intercluster interactions derived from first-principles calculations based on the density functional theory. Using the results of the ab-initio CG Monte Carlo calculations, we characterized the positional and orientational orders of the attractively or repulsively interacting solute clusters in two dimensions, in particular the transformation of the possible local ordering patterns of clusters, considering them analogous to two-dimensional colloidal hard-sphere system. The CG model indicated that the Zn-Y clusters are arranged in multiple (i.e., at least two) kinds of six-fold domain structures with intercluster distances of 2radic;3a_Mg and radic;19~radic;21a_Mg, respectively, in a manner consistent with recent scanning tunneling microscopy measurements. The increase in volume fraction of solute clusters results in close packing transition of solute clusters, which explains the steady reduction of radial correlation length between clusters that was observed in small-angle X-ray scattering measurements for Mg₈₅Zn₆Y₉ LPSO alloys during the annealing at high temperature.
This study was supported by Grant-in-Aid for Scientific Research on Innovative Area, "Synchronized LPSO Structure," No. 23109004.

Permanent magnets (PMs) are indispensable for many commercial applications including the electric, electronic and automobile industries, communications, information technologies and automatic control engineering. Worldwide demand for high performance permanent magnets has increased dramatically in the past few years driven by hybrid and electric cars, wind turbines and other power generation systems. New energy challenges in the world require devices with higher energy efficiency and minimum environmental impact. The current high performance rare earth magnets (Sm-Co, Nd-Fe-B) have reached the limits of their theoretical energy product. Furthermore, the future supply of 4f rare earth elements and in particular the Dy seems to be very uncertain.
This talk will address the major principles guiding the development of PMs and review current research efforts in developing rare earth-lean/free PMs. Recent progress in the development of nanocomposite PMs, consisting of a fine (at the scale of the magnetic exchange length) mixture of phases with high magnetization and large magnetic hardness will be discussed. Fabrication of such PMs is currently the most promising way to boost the (BH)_max, while simultaneously decreasing, at least partially, the reliance on the rare earth elements. The talk will also summarize ongoing research efforts to develop rare earth-free permanent magnets and in particular those programs that are based on Mn-Bi compounds.
Work supported by ARPA-E, NSF G8 and Siemens.

At room temperature, AlFe₂B₂ is ferromagnetic with a layered orthorhombic crystal structure.¹ Upon heating, AlFe₂B₂ undergoes a hysteretic ferromagnetic to paramagnetic phase change at a Curie temperature (T_C) of ~ 300 K and is reported to exhibit a magnetocaloric effect (MCE).^1,3 The MCE is the reversible adiabatic temperature change (ΔT_ad) of a magnetic material upon the application or removal of a magnetic field (H_app). The MCE is the mechanism underlying magnetic refrigeration - an emerging energy-efficient, environmentally-friendly alternative to vapor-compression cooling technology.^1,2,4 AlFe₂B₂ has a MCE (ΔT_ad~1.8 K at H_app=1.95 T) that exceeds that of any intermetallic boride of 1-2-2 type, and is comprised of lightweight, inexpensive, earth-abundant elements providing motivation for this research.³ At the current time, the fundamental science underlying the MCE behavior of the AlFe₂B₂ compound is not well understood. The aim of this current study is to fill this gap in the literature.
In this work, a bulk alloy of composition AlFe₂B₂ was synthesized via arc-melting the constituent elements (99.9% purity) in the stoichiometry ratio, 1.5 Al:2 Fe:2 B. The arc-melted ingot was subsequently sealed under vacuum (1 x 10^-6 Torr) in a vitreous silica tube and annealed at 1000 °C for 7 days. Attainment of the layered orthorhombic crystal structure was confirmed using standard Cu-K_α X-ray diffraction (lattice parameters, a= 2.926Å b= 11.022Å c= 2.862Å). Advanced structural probes including optical microscopy and SEM-EDS indicates that the microstructure of arc-melted AlFe₂B₂ consists of acicular crystallites in an Al₁₃Fe₄ matrix prior to annealing, implying an intrinsic shape anisotropy.
Magnetic measurements in magnetic fields up to H = 3 Tesla and temperatures in the range 50 K T le; 390 K indicate that the sample possesses a saturation magnetization value of ~63 emu/g and exhibits a T_C of ~300 K, consistent with values reported in the literature.¹ A 10 K thermal hysteresis (#8710;T_t) between the heating and cooling magnetothermal curves is obtained. At the current time, the hysteresis noted in the magnetothermal curves in AlFe₂B₂ is tentatively attributed to the first-order thermodynamic character of its magnetic phase transition. Furthermore, a shift in T_c to higher values upon application of a magnetic field (dT_c/dH = 2 K/T) has been observed. It is anticipated that correlations between crystal structure, microstructure, magnetism, and thermal behavior will provide insight into the fundamental driving forces underlying the magnetocaloric response of AlFe₂B₂, and related transition-metal borides.
References:
1 M. ElMassalami et al., J. Magn. Magn. Mater., 323 (2011) 2133-2136.
2 K. Gschneidner, V. Pecharsky, Phys. Rev. Lett., 12 (1997) 1-19.
3 X. Tan et al., J. Am. Chem. Soc., 135 (2012) 9553-9557.
4 V. Franco, Annu. Rev. Mater. Res., 42 (2012) 305-342.
5 L. Lewis, F. Jiménez-Villacorta, Metall. Mater. Trans. A. 44 (2013) 2-20.

High uniaxial magnetocrystalline anisotropy materials such as Fe-Pd, Co-Pt and Fe-Pt alloys has attracted wide attention because of microstructure control under a magnetic field. The magnetic properties of these alloys are closely related to the microstructure of the tetragonal phase, which arises from the A1/L1₀ ordering. Beginning with a disordered fcc phase, the ordered L1₀ phase is formed during a heat treatment at a temperature below the disorder-order transformation temperature. The ordered phase is composed of three lattice corresponding variants with an easy c-axis of magnetization. Selected formation of an ordered variant in Co-50at.%Pt¹⁾ and Fe-55at.%Pd²⁾ alloys has been reported by a two-step ordering heat treatment. The first step corresponds to the nucleation process, and is made under a magnetic field at a temperature below the Curie temperature of the disordered phase. The second step corresponds to the growth process, and is made below the disorder-order transformation temperature without magnetic field. In this study, conventional transmission electron microscopy (CTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) techniques have been applied to investigate the effect of magnetic field on variant formation and selection during the two-step ordering process. As a result random distribution of ordered domains with 5 nm in size is observed after the first step under the magnetic field. Fraction of the preferred variant with its c-axis parallel to the magnetic field is slightly higher than that of the other two variants. Growth of the preferred variant occurs preferentially during the second step of ordering leading to the formation of single variant. The effect of magnetic field on microstructure formation during the two-step ordering process will be discussed based on the observation.
References
1) S. Farjami, M. Yasui, T. Fukuda and T. Kakeshita, Scripta Materialia, 58 (2008) 811.
2) S. Farjami, T. Fukuda and T. Kakeshita, Materials Transactions, 49 (2008) 1970.

Permanent magnets are used in a wide variety of applications, from personal computers to cars and magnetic sensors. Rare-earth elements are extensively used in permanent magnets in order to achieve high anisotropies. Fe-Co is proposed as a possible alternative to rare-earth permanent magnet based alloys, due to its very high magnetic moment. The magnetocrystalline anisotropy can be induced via straining the FeCo unit cell as shown in theory [1] and confirmed by experimental studies on ultrathin films [add reference]. For this reason several underlayers have been employed to cause tetragonal distortion to the cubic FeCo cell [2,]. In this work we present our results on epitaxial growth of Fe₄₅Co₅₅ ultrathin films on L1₀ phase FePt thin underlayers, producing an exchange spring system [3-5] leading to high coercivities and very high energy products. We used magnetron sputtering to deposit FePt layers at 500 ^oC to favor the fct phase formation and to deposit Fe-Co at 300^oC. The thickness was varied between 5 and 7 monolayers for the FePt and up to 9 monolayers for the FeCo.
In all samples full FePt fct phase transformation was observed according to structural analysis, along with epitaxial growth of Fe-Co on FePt. Coercive field was reduced from 1.3 T down to almost 0.2 T through exchange coupling of FePt and FeCo magnetic layers with increasing thickness. The anisotropy is out of plane while the energy product is estimated in the excess of 200kJ/m³.
Acknowledgement: This work was supported by the EU-REFREEPERMAG project
References
[1] T. Burkert, L. Nordström, O. Eriksson, O. Heinonen PRL 93, 027203 (2004)
[2] S. Kauffmann-Weiss, S. Hamann, L. Reichel, A. Siegel,V. Alexandrakis, R. Heller, L. Schultz, A. Ludwig, and S. Fähler APL Materials 2, 046107 (2014)
[3] D. Kim, J. Hong Surface Science 606 (2012) 1960-1964
[4] D.Kim, Ar. Hashmi and J. Hong Journal of the Korean Physical Society, Vol. 62, No. 6, March 2013, pp. 918sim;923
[5] B. Wang, H. Oomiya, A. Arakawa, T. Hasegawa, and S. Ishio , J. Of Appl. Phys. 115, 133908 (2014)

Eutectoid decomposition of A1 Co-Pt alloys near 60% Pt can result in the nanochessboard structure - a 2+1D quasi-periodic arrangement of L1₀ and L1₂ phases. The tiling lengthscales are 15-30 nm, while in the orthogonal direction the lengthscales are of order hundreds of nanometers. Hence the nanochessboard represents a self-assembled 2D-periodic stacking of nanorods separated by coherent interfaces. We find that formation of this structure requires slow cooling through the eutectoid isotherm in order to properly develop the periodic composition modulation associated with the pseudospinodal decomposition process. We are investigating the magnetic properties of this magnetic nanocomposite, since the lengthscales and coherent interfaces should be conducive to exchange-coupling between the hard, uniaxial L1₀ and the much softer and more isotropic L1₂. This talk will discuss how the microstructure depends on the thermal processing parameters. Preliminary magnetic measurements find coercivities up to 2200 Oe, and evidence for exchange coupling via the remanence ratio and the loop shape. Recoil analysis is currently ongoing to better assess the coupling behavior. The mechanisms for magnetization reversal will also be discussed.

While rare earth permanent magnets have enabled technological improvements in areas as diverse as consumer electronics to renewable energy, raw materials availability may hinder future progress. This has fueled the search of new rare earth free permanent magnets and highlighted the need to develop design principles that exploit sources of magnetic anisotropy, such as the crystal environment. This, in turn, requires a fundamental understanding of how the crystal structure and its modification affect the material&’s magnetic properties. With this in mind, we synthesized ferromagnetic Fe₃GeTe₂ and studied its magnetic microstructure using magnetic force microscopy. The ground state shows three dimensional domain branching at the basal plane surface with c-axis magnetization, which is characteristic of high-anisotropy uniaxial ferromagnets. We will discuss the material parameters calculated from the observed domains and the evolution of the magnetic structure due to an external field.

Low- and zero-moment half-metallic ferrimagnetic semiconductors have been proposed for advanced applications, such as nonvolatile RAM memory and quantum computing.^1,2 These inverse-Heusler materials could be used to generate spin-polarized electron or hole currents without the associated harmful fringing magnetic fields. Such materials are expected to exhibit little to zero magnetic moment at room temperature, which makes them well-positioned for future spin-based devices. However, the predicted compounds have been shown to suffer from disorder.^3,4 This work focuses on the successful growth of low-moment inverse-Heusler compounds. Both Cr₂CoGa and Mn₃Al thin films were synthesized by molecular beam epitaxy in order to investigate their magnetic, structural, and transport properties. These thin films were annealed at various temperatures to study the ordering in the inverse-Heusler structure. Several bulk compounds, including V₃Al and Cr₂CoAl, have also been synthesized via arc-melting and annealed to determine the properties required for optimizing the structure. The magnetic moments of the individual atoms were measured by X-ray magnetic circular and linear dichroism performed at the National Synchrotron Light Source and compared to standard magnetization results.
¹ H. van Leuken, R.A. de Groot, Phys. Rev. Lett., 74, 1171 (1995).
² S. Skaftouros, K. Özdo#287;an, E. #350;a#351;#305;o#287;lu and I. Galanakis, Phys. Rev. B87, 024420 (2013).
³ M.E. Jamer, B.A. Assaf, T. Devakul and D. Heiman, Appl. Phys. Lett.103, 142403 (2013).
⁴ M. Meinert and M.P. Geisler, J. Magn. Magn. Mater. 341, 72-74 (2013).

One of our research objectives is to design new magnetic materials. We have been able for example to substitute Ti chains by chains of magnetically active elements (Cr, Mn, Fe, Co and Ni) to produce new itinerant magnets (antiferro-, ferri, and ferromagnets) in Ti₃Co₅B₂- and Zn₁₁Rh₁₈B₈-type structures. [1] We have recently reported on a novel compound Ti₇Rh₄Ir₂B₈, [2] containing not only the monocyclic planar B₆ ring but also one dimensional planar Ti₇-wheels. Other ternary isostructural borides, including Nb₇Ir₆B₈, were also published recently. [3] We have transferred the aforementioned synthetic strategy (using the arc-melting furnace under an argon atmosphere and annealing) to the ternary phase Nb₇Ir₆B₈ and obtain new quaternary phases, Nb₆MIr₆B₈ (M = Mn, Fe, Co), isostructural to Ti₇Rh₄Ir₂B₈ (space group P6/m, no. 175). Powder X-ray diffraction shows two different structural behaviors in the series: For M = Mn and Fe a mixture of Ir and M with M:Ir = 3:1 is found (Wyckoff-site 1a), while for M = Co a subsequent mix-occupied site (6k) is obtained with M:Ir = 1:19 (see Figure below, left). Consequently two sets of formula exist in the series: Nb₆M_0.75Ir_6.25B₈ (M = M and Fe) and Nb₆Co_1.1Ir_6.1B₈. The phases were also studied by SQUID magnetometry which indicates ferromagnetic ordering in Nb₆Fe_0.75Ir_6.25B₈ at 350 K, [4] and ferrimagnetic ordering in Nb₆Mn_0.75Ir_6.25B₈ and Nb₆Co_1.1Ir_6.1B₈ at 260 K and 300 K, respectively. Theoretical DFT investigations have confirmed the magnetic properties. It is found that the chemical bonding around the magnetically active elements play a significant role in the occurrence of these magnetic properties.
References
[1] B. P. T. Fokwa, Eur. J. Inorg. Chem. (2010) 3075-3092.
[2] B. P. T. Fokwa, M. Hermus, Angew. Chem. Int. Edit. 51 (2012) 1702-1705.
[3] Q. Zheng, M. Kohout, R. Gumeniuk, N. Abramchuk, H. Borrmann, Y. Prots, U. Burkhardt, W. Schnelle, L. Akselrud, H. Gu, A. Leithe-Jasper, Y. Grin, Inorg. Chem. 51 (2012) 7472-7483.
[4] M. Mbarki, R. St. Touzani, B. P. T. Fokwa, in preparation.

Materials which have a martensitic phase transition that is coupled to
a ferromagnetic spin ordering can display a variety of useful
properties, including the giant magnetocaloric effect and the
ferromagnetic shape-memory effect. Unfortunately, very few compounds
are known to display this combination of properties. In this work, we
use targeted high-throughput first principles calculations combined
with database searching and experimental inputs to search for new
compounds similar to GeCoMn and GeNiMn, which are known experimentally
to display the giant magnetocaloric effect. These materials are
examples of the under-explored group of hexagonal intermetallics with
ABC stoichiometry, which display a variety of structural transitions.
We use database searches and machine learning techniques to identify a
list of candidate hexagonal intermetallics, and we combine existing
experimental information with first principles calculations of
structural and magnetic properties to screen this class of materials
for functional behavior. We find several candidate materials, some of
which have previously been synthesized, which should reward future experimental investigation.

Fe-Si binary alloy has a variety of applications due to its excellent magnetic and mechanical properties. Some drastic change in mechanical properties as Si concentration increasing from 2-3 to 5-6wt.% has been intriguing since a long time ago. Our recent DFT calculations [1] re-produced a Si concentration dependence of the elastic properties in dilute Fe-Si alloy, which coincides with the experiments, and found a ductile to brittle transition as Si content crosses 4.2wt.%Si. Further calculations at finite temperature combining CVM (Cluster Variation Method) revealed that the origin of the sharp change in the mechanical properties in 4.6-5.6wt.%Si region is the interplay between magnetovolume effect and structure ordering of D0₃ in Fe-Si alloy. Detailed analysis of Density of States (DOS) indicates that the degradation of ductility at 4.2wt.%Si is resulted from an big antibonding peak at the Fermi Energy (Ef) which introduces the instability, and some particular Fe-Fe-O configurations are figured out to be the main contribution to the antibonding peak. We are inspired by these analysis and carried out various simulations by doping Ni atoms into the Fe-Si in attempt to improve the stability of the Ef. It is found that replace of one Fe atom by a Ni in the particular Fe-Fe-O configurations which influence Ef properties directly led to an obvious decreasing of the DOS at Ef, and a lower formation energy within various ways of Ni atoms doping, consequently this Ni-doped Fe-Si showed recovery of various elastic properties. Such exciting result is well agreement to the fact that the industry high Si steel indeed includes dilute Ni composition to create better mechanical properties. This experience can be taken an example of tailing band structures intentionally by creating some specific atomic configurations which varies bonding states and generates re-distribution of electrons towards enhancing the specific physical property.
[1] A. Saengdeejing, Y. Chen, K. Suzuki, H. Miura and T. Mohri, Computational Materials Science 70 (2013), 100

Al13Co4 exists in a complex orthorhombic crystal structure with 78 Al and 24 Co sub-lattice sites. Owing to the complexity of the crystal structure and high lattice resistance to dislocation movement, this phase is extremely strong at room temperature. Previous micro-compression tests on single crystals have revealed deformation to be mediated by (001)[010] dislocations at room temperature but the plastic deformation curves typically show significant serrations ¹.
In this study, micropillar compression experiments were carried out with in-situ microLaue diffraction of single crystal Al13Co4 at APS 34-ID-E and Diamond B16 beamlines. From the Laue spots recorded during loading, streaking as well as splitting were observed indicating the presence of a strain gradient in the sample. The resulting microstructure was correlated with post mortem SEM and TEM to understand the underlying deformation mechanism giving rise to serrations observed in stress-strain curves and high yield stresses.
¹. Walter et al., Acta Materialia, 61 (2013) 7189-7196.