First-principles calculations by the use of a plane-wave pseudopotential method are performed to investigate intrinsic point defect behavior in TiNi. The results show that TiNi is an antisite type intermetallic compound. The calculated interaction energies between the point defects demonstrate that Ti antisites are attractive to each other whereas Ni antisites are mutually repulsive. The attraction between Ti antisites indicates that excess Ti in TiNi may agglomerate so that a Ti-rich phase can easily precipitate. The repulsion between Ni antisites implies that the excess Ni is of certain solubility in TiNi. This result explains well the asymmetric feature of TiNi field on the binary phase diagram. In order to understand the correlation between the composition dependent elastic modulus and martensitic transformation (MT) temperature, the elastic moduli critical to MT, i.e., c′ and c44, are calculated as a function of the composition of the off-stoichiometric TiNi and a series of ternary TiNi-X alloys, by the use of exact muffin-tin orbital method in combination with coherent potential approximation. It turns out that, generally speaking, the early transition metal (TM) alloying elements in the periodic table increase c′ but decrease c44; the middle ones increase both c′ and c44, whereas the late ones decrease c′ but increase c44. An examination of the theoretical composition dependent elastic modulus and the experimental MT temperature shows that the MT temperature is more sensitive to the variation of c44 than to that of c′.

In the present work, Nb-18.1Si-1.5Zr alloy rods are produced with a growth rate ranging from 1.5 to about 1500 mm/h using the optical floating zone (OFZ) furnace. A part of each specimen is heat-treated at 1650 oC for 100 h. The microstructure was observed using SEM and TEM and analyzed using EPMA and EBSD.

Eutectic-cells are observed in as-grown specimens with a growth rate of 150 mm/h or higher. It is found by EBSD analysis that the solidification direction of Nb is along <113> and that of Nb3Si is along <001], and {112} of Nb and {110) of Nb3Si are parallel. The present crystallographic orientation relationship between Nb and Nb3Si is different from that found in previous reports by several researchers. It was also confirmed that the heat-treated microstructure in the specimen grown by OFZ with a growth rate of 150 mm/h is similar to that in the heattreated specimen prepared by arc-melting.

Deformation behavior of binary stoichiometric Ti5Si3 single crystals was examined as a function of the loading axis orientation and temperature. Two different types of deformation modes, namely {1100}[0001] prism slip, {2112}1/3<2113> pyramidal slip were newly identified to be activated above 1300 °C depending on the loading axis orientation. Critical resolved shear stresses (CRSS) for the {1100}[0001] prism slip and {2112}1/3<2113> pyramidal slip were estimated to be about 130 MPa and 330 MPa at 1400 °C, respectively. The values of the CRSS for these two slip systems decrease monotonously with increasing the temperature.

The B2 intermetallic compound RuAl has a melting temperature above 2000 °C and is a candidate for high temperature structural applications. A large extension of the B2 phase field is found in the Ru-Al-Cr system as was documented by the characterization of arc-melted and heat treated alloys. Two compositions consisting of Ru-35Al-19Cr and Ru-20Al-38Cr (at. %) were directionally solidified in an optical floating zone furnace. Depending upon the processing conditions, single phase, polycrystalline, B2 microstructures could be produced. The coefficient of thermal expansion (CTE) was measured from room temperature to 1250 °C for the Ru-20Al-38Cr alloy, and an average value of 11×10-6 K-1 was found. Additionally, the thermal conductivity was measured as 27 W/mK at room temperature for the Ru-20Al-38Cr B2 alloy and as 89 W/mK for binary RuAl.

Half-Heusler phases have gained recently much interest as thermoelectric materials. Screening of possible systems was performed by drawing their stability region in a three-dimensional Pettifor map. The fabrication of Half-Heusler phases requires three steps, surface activation of the raw material by ball milling, arc-melting of pressed pellets and finally long-term annealing treatment in a vacuum furnace. On doped TiCoSb specimens, Seebeck coefficients of 0.1 mV/K, on NiNbSn 0.16 mV/K were measured, although the microstructure was not yet optimized.

Compressive deformations of L10-ordered single crystals of FePd have been investigated from room temperature to 873 K. The critical resolved shear stress for superlattice dislocations is hard to determine resulting from buckling that occurs after a small amount of conventional plastic deformation. The CRSS for superlattice dislocations determined from yield stress is significantly larger than that of ordinary dislocations. The CRSS for octahedral glide of ordinary and superlattice dislocations are virtually independent of the temperature, and the positive temperature dependence of the yield stress is not observed for both, ordinary and superlattice dislocations, by the present experiments.

Because of the small “deformation window” hot-working of γ-TiAl alloys is a complex and difficult task and, therefore, isothermal forming processes are favoured. In order to increase the deformation window a novel Nb and Mo containing γ-TiAl based alloy (TNM™ alloy) was developed. Due to a high volume fraction of β-phase at elevated temperatures the alloy can be hot-die forged under near conventional conditions, which means that conventional forging equipment with minor and inexpensive modifications can be used. With subsequent heattreatments balanced mechanical properties can be achieved. This paper summarizes our progress in establishing a “near conventional” forging route for the fabrication of γ-TiAl components. The results of lab scale compression tests and forging trials on an industrial scale are included. In addition, the mechanical properties of forged and heat-treated TNM™ material are presented.

The effects of strain rate and temperature on the yield strength of near-stoichiometric Fe2AlMn single crystals were investigated. In the temperature range 600-800K the yield stress increased with increasing temperature, a response commonly referred to as a yield strength anomaly. No strain rate sensitivity was observed below 750K, but at higher temperatures the yield stress increased with increasing strain rate. Possible mechanisms to explaining the effects of temperature and strain rate are discussed.

Refractory Metal Intermetallic Composites (RMICs) based on the Nb-Si system are considered as candidates of next-generation high temperature materials (i.e. >1200°C). Ti and Cr have been shown to have beneficial effects on the oxidation resistance and mechanical properties of Nb-Si alloys. Phase equilibria in the Nb-Si-Ti system have been studied in detail. The present study has investigated multiphase equilibria in the Nb-Si-Ti alloys with Cr additions via an approach of integrating thermodynamic modeling with designed experiments. The alloying effects of Cr on the microstructure of the Nb-Si-Ti alloys are described using both phase equilibria and solidification paths that were calculated from the thermodynamic description of the Nb-Cr-Si-Ti system developed in the present study.

Nanostructured Al-Mg bulk samples with compositions in the range of 10 – 40 at.% Mg have been produced by consolidation of mechanical alloyed powders. Powders with composition Al90Mg10 and Al80Mg20 were consolidated into highly dense specimens by hot extrusion. Room temperature compression tests for the Al90Mg10 specimen reveal interesting mechanical properties, namely, a high strength of 630 MPa combined with a plastic strain of about 4 %. The increase of the Mg content to 20 at.% increases the strength by about 100 MPa but it suppresses plastic deformation. The Al60Mg40 powder was consolidated at different temperatures by spark plasma sintering and the effect of the sintering temperature on microstructure, density and hardness have been studied. The results reveal that both density and hardness of the consolidated samples increase with increasing sintering temperature, while retaining a nanocrystalline structure. These results indicate that powder metallurgy is a suitable processing route for the production of nanocrystalline Al-Mg alloys with promising mechanical properties.

The lattice structure of the C14 Fe2Nb Laves phase with Ni in solution in Fe-Nb-Ni ternary alloys was examined by transmission electron microscopy. Binary stoichiometric Fe2Nb (Fe-33.3 at.% Nb) exhibits a featureless morphology with a low dislocation density. A similar morphology was observed in stoichiometric Fe2Nb containing 20 at.% Ni and in binary Fe-rich Fe2Nb (Fe-27.5 at.% Nb). In contrast, many planar faults parallel to the basal plane of the C14 structure were observed in Fe-rich Fe2Nb with Ni in solution, and the fault density increases with increasing Ni content up to 33.1 at.%. The high resolution transmission electron microscope (HRTEM) analysis revealed that the planar faults are related to the local change in the stacking sequence of the three 36-nets (triple layer) of the C14 structure. These results suggest that the presence of both, the point defects (Fe sublattice sites occupied by Ni atoms) and the anti-site defects (Nb sublattice sites occupied by excess Fe atoms), facilitate the formation of the planar faults.

The unusual duality of magnetic phase transition (especially high transition temperature, remarkable shoulder) is thought to be due to second phase impurities of pure Gd or Gd3Al2 or even both. The temperature dependence and field dependence of magnetization have been studied in the temperature range of 4.2-350 K in three different strengths of magnetic field to investigate the cause or/and the source of metamagnetic character which is a puzzle on Gdintermetallic compounds, IMC, in order to find the critical field needed to overcome the fluctuation field, above which the magnetic behavior of the system is stabilized, and a critical turning field is defined.

However for the clearance of intrinsic instability behavior and the nature (and source) of anisotropy in the Gd2Al system, the following experimental works has been prepared.

The study of the effect of increasing the magnetic field up to the critical “threshold“ field which is exactly the same as decreasing the conduction electron concentration “c.e.c” from Gd2Al to Gd2Au; The contribution of short range order as well as thermal fluctuation of short range exchange “Jsh” through the long – range interaction “Jl” which can be induced by localized s-f (or d-f) exchange. This effect can be the cause (or the source) of internal magnetic field Hin=<Si.Sj> and fluctuation field Hf=Hin-Hext.

A new thermodynamic assessment of the Co-Nb system is presented. All experimental phase diagram data available from the literature have been critically reviewed and assessed using thermodynamic models for the Gibbs energies of the individual phases (Thermo-Calc). Compared to previous assessments more elaborate models for the description of the C14 and C36 Laves phases and for the μ phase were employed. Thereby a calculated phase diagram is obtained which satisfactorily agrees with the experimental data.

Aiming to improve thermoelectric properties of half-Heusler (Ma,Mb)NiSn alloys (Ma,Mb = Hf, Zr, Ti), phase equilibria in the (Ma,Mb)NiSn systems were investigated focusing on the phase separation of TiNiSn from ZrNiSn and HfNiSn while (Zr,Hf)NiSn forms all proportion miscible solid solution. Diffusion couples consisting of liquid Sn and solid (Ti,Zr)Ni were used to examine the partitioning behavior which is associated with T-rich and Ti-poor half-Heusler phase separation during the reaction at the interface. Thermal conductivity can be reduced in (Ma0.5,Mb0.5)NiSn and (Ti0.13,Zr0.87)NiSn alloys due to the solid solution effect of M-site substitution. (Ti0.13,Zr0.87)NiSn alloy has high potential as a ecological thermoelectric material.

The oxidation resistance of TiAl-alloys can be improved by several orders of magnitude by fluorine doping of the surface zone of the material. The oxidation mechanism changes from the formation of a thick mixed oxide scale to a protective alumina layer. This fluorine treatment influences only the surface region of the components so that the bulk properties are not affected. Recent results achieved with TiAl-components showed the potential of a fluorine treatment for the use of TiAl in several high temperature applications. Turbine blades for aero engines made of TiAl were treated with fluorine by different methods and their performance during high temperature oxidation tests in air is shown. Further on by selective local fluorination a structured oxide scale develops on TiAl above 800°C. A simple high temperature activation cause the formation of areas covered by a thin alumina layer alternating with a thick mixed oxide scale where no fluorine was applied before oxidation. The aim is to reproduce a shark-skin pattern (tiny parallel ridges) on the surface in order to minimize the aero dynamic resistance of turbine blades rotating in a gas flow. Different methods used for this attempt and the corresponding results are also presented.

We review the current development status of molybdenum borosilicide (Mo-Si-B) alloys for ultra-high temperature applications in air. The assessment of several ingot and powder metallurgy approaches revealed that (i) the presence of a continuous Mo solid solution matrix is mandatory for adequate low temperature toughness and (ii) wrought processing of such alloys at temperatures established for refractory metals requires the presence of an ultrafine (sub-micron) microstructure. Both prerequisites could be fulfilled using mechanical alloying (MA) as the crucial processing step , however, values for the ductile-to-brittle transition temperature (DBTT) below 800°C could not be obtained due to grain boundary embrittlement by Si segregation. First results on the effect of different microalloying additions on a reduction of this segregation exemplified for the Mo solid solution will be presented and discussed.

Alloys based on aluminium and titanium are possible materials for several high temperature applications. The use of TiAl would increase the efficiency of e.g. aero turbines, automotive engines and others due to their properties, among others low specific weight and good high temperature strength. The oxidation resistance is low at temperatures above approximately 800°C so that no long term use of TiAl-components is possible without improvement of the oxidation behaviour. Small amounts of halogens in the surface zone of TiAl-samples lead to a dramatic improvement of the oxidation resistance at temperatures up to 1100°C for more than 8000 hours in air. In this paper results of the work on the halogen effect over the last years are presented. The results of thermogravimetric measurements, thermocyclic oxidation tests of small coupons and thermodynamic calculations for different atmospheres (e.g. air, H2O, SO2) are shown and the halogen effect mechanism is discussed. The postulated mechanism is in good agreement with the results of the oxidation tests. The limits of the halogen effect will also be mentioned. Predictions for the halogenation of TiAl-components can be given so that the processing can be planned in advance.

Half-Heusler compound TiNiSn is one of the most promising candidates of thermoelectric materials which can be used to directly convert the waste heat to clean electric energy at high temperatures (around 1000 K). Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. Half-Heusler compounds have the cubic C1b type ordered structure and show semiconducting behavior when their valence electron count (VEC) is around 18. TiNiSn is the most attractive one not only because it has excellent thermoelectric properties but also it consists of eco-friendly elements which are neither toxic nor costly. However, TiNiSn has a bothersome problem that fabrication of single phase TiNiSn alloy is quite difficult. We have found that TiNiSn phase forms by the ternary peritectic reaction. Thereby, inevitable non-equilibrium solidification results in the formation of impurity coexisting phases which tend to decrease thermoelectric properties. In the present work, to establish the basis of new fabrication processes for TiNiSn alloys, we have started from the investigation on the diffusion paths which are closely related to the formation of TiNiSn phase. The diffusion behavior was evaluated using solid/liquid diffusion couples composed of the binary Ti-Ni intermetallic compounds and Sn liquid phase, where we have selected TiNi, TiNi3 and Ti2Ni as solid phases for instance. The most interesting result is that the single-phase TiNiSn phase layer forms at the TiNi/Sn(L) interface during annealing at 1073 K for only 1 h. Moreover, faceted grains of TiNiSn single-crystal grow at the interface toward the liquid Sn phase. We have confirmed two interesting microstructural features using EBSD analyses. One is that most of these TiNiSn single-crystals have the same crystallographic orientation, and the other is that TiNiSn phase layer formed on the TiNi side of the interface consists of very fine sub-microns grains. While TiNiSn solely forms at the TiNi interface, Heusler TiNi2Sn also forms with TiNiSn at the TiNi3 interface and Ti6Sn5 tends to coexist at the Ti2Ni interface.

Intermetallic interlayers are key to the performance of thermal barrier coating systems. The role of platinum group metal (PGM) additions to B2 bond coat interlayers has been investigated with emphasis on diffusional aspects of coating structure evolution and properties. Additions of Ru and Ir to NiAl reduce interdiffusion and the resultant thickness of the coating and interdiffusion zone. Relative to Pt and Pd, Ru and Ir slow diffusion. Ru additions to NiAl increase creep resistance but degrade oxidation kinetics.

In high field magnets, most of the structural materials operate at very low temperatures (e.g. 1.8 K) and high magnetic fields (e.g. >20 T). Such an extreme environments ask for demanding properties from the materials. Austenite stainless steels have been used for such an application for decades and provide good combination of mechanical strength and toughness at cryogenic temperatures. In the National High Magnetic Field Laboratory, we have studied intermetallic precipitate strengthened alloys for high field magnet and cryogenic applications. An examination of the data from an intermetallic precipitate strengthened alloys shows that intermetallic precipitate strengthened alloys can be used at cryogenic temperatures and have some superior properties in extreme environments. Based on our experimental data, we developed a new alloy that has even better properties than the current intermetallic precipitate strengthened alloys.