Thermomechanically processed TiAl-based intermetallic alloys with various alloy compositions and microstructures were tensile tested in various environmental media including air, water vapor and a mixture gas of 5vol.%H2+Ar as a function of temperature. All the TiAl-based intermetallic alloys showed reduced tensile fracture stress (or elongation) in air, water vapor and a mixture gas of 5vol.%H2+Ar not only at ambient temperature (RT∼600K) but also at high temperature mostly from 600K to 1000K (sometimes higher temperature than 1000K). The high-temperature environmental embrittlement of TiAl-based intermetallic alloy depended upon the microstructure. The possible species causing the high-temperature environmental embrittlement are hydrogen atoms decomposed from water vapor (H2O) or hydrogen gas (H2), similar to those causing the low-temperature environmental embrittlement.

PST TiAl samples of different orientations were prepared and deformed by compression at room temperature. The deformation structures on the free surfaces were scanned using an AFM. It was found that when the angle between the lamellar interfaces and the loading axis is between 20 degree and 80 degree, PST samples deform primarily by shear in slip planes parallel to the lamellar interfaces. When the angle is below 20 degree, both the gamma phase and the alpha 2 phase deform by shear in slip planes inclined with the lamellar interfaces, but the shear vectors lie in the interface. When the angle is close to 90 degree, complex deformation behavior occurs. Shear in planes parallel to the lamellar interfaces contributes more to the overall strain in the directions perpendicular to the loading axis and the out-of-plane shear contributes to the strain in the compression direction. The characteristic U-shape curve of the yield stress versus the angle between the loading axis and the lamellar interfaces can be explained quite well using different C.R.S.S. for the three different deformation modes.

In this paper the deformation behavior of a fully lamellar microstructure, which is usually present in cast γ-TiAl based alloys, is studied by numerical modeling. After large compressive deformation at elevated temperatures the lamellar colonies are often bent or buckled depending on their orientation, which is representative for a deformation instability characterized by a large wave length. Such a deformation behavior is triggered by both, structural defects of the lamellae and their somewhat irregular arrangement. In addition, a shear band-type deformation mode occurs according to an instability mode exhibiting a short wave length. These two deformation modes interact in a rather subtle way, which leads to a very inhomogeneous deformation pattern.

Phase equilibria in Fe-Ni-Nb ternary system at elevated temperatures have been examined, in order to identify the two-phase region of γ-Fe (austenite) and ε-Fe2Nb (C14). The ε single phase region exists in the range of 27.5 to 35.5 at.% Nb in the Fe-Nb binary system, and it extends toward the equi-niobium concentration direction up to 44 at.% Ni in the ternary system at 1473 K, indicating that more than half of the Fe atoms in Fe2Nb can be replaced with Ni. Thus, the γ+ε two-phase region exists extensively, and the solubility of Nb in γ phase increases from 1.5 to 6.0 at.% with increase in Ni content. The lattice parameters of a and c in the C14 Laves phase decrease with increasing Ni content. The change in a axis is in good agreement with calculation based on Vegard's law, whereas that of c axis is much larger than the calculated value. The result suggests that atomic size effect is responsible for a-axis change and the binding energy is dominant factor for the c-axis change. To extend these findings to development of new class of austenitic steels strengthened by Laves phase, an attempt has been made to control the c/a ratio by alloying. The addition of Cr is effective to make the c/a ratio close to the cubic symmetry value (1.633).

The crystal structures and thermoelectric properties of Ba-Ge type-I clathrate compounds (Ba8GaXGe46-X and Ba8Ga16-YInYGe30) have been investigated as a function of Ga and In contents. Ba8GaXGe46-X alloys have a crystal structure that contains an ordered arrangement of Ge vacancies, forming a superstructure based on the normal type-I structure until X reaches 3, whereas they have the normal type-I structure when X exceeds 3. The dimensionless thermoelectric figure of merit (ZT) increases with the increase in the Ga content, exhibiting the highest value of 0.49 for Ba8Ga16Ge30. The power factor for Ba8Ga10In6Ge30 is 1.5 times that for Ba8Ga16Ge30 so that the In containing alloy exhibits a ZT value as high as 1.03 at 700°C.

Nb-Si “in-situ” metal matrix composites consist of Nb3Si and Nb5Si3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. Cr and Ti have been shown to increase the oxidation resistance and metal loss rate at elevated temperatures compared to the binary Nb-Si system. In this study, the LENS™ (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr-Si alloy system from elemental powder blends. Fast cooling rates associated with LENS™ processing yield a reduction in microstructural scale over conventional alloy processes such as directional solidification. Other advantages of LENS™ processing include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. Processing parameters can also be varied, resulting in distinct microstructural differences. Deposits were made with varying compositions of Nb, Ti, Cr and Si. The as-deposited as well as heat treated microstructures were examined using SEM and TEM techniques. The influence of composition and subsequent heat treatment on microstructure will be discussed.

A binary Fe3Al alloy is investigated with respect to hot and warm rolling behavior and microstructural as well as microtextural modifications. Rolling has been performed in the A2 and B2 order regimes. The differences in microstructure are investigated. The performed texture analysis reveals the differences in hot and warm rolling textures depending on the hot rolling temperature. On the basis of microtexture investigations by means of electron backscatter diffraction (EBSD) differences concerning orientation gradients and sub-grain structures are found. A model of combined order-related and non-order related effects is proposed explaining the observed material behavior. The results are used for process modification.

Planar defects produced in L10-ordered FePd during annealing after cold-deformation in the disordered cubic state have been characterized by transmission electron microscopy (TEM). The defects evolving during annealing include arrays of overlapping stacking faults (SF's), {111}-conjugated microtwins (μT's) and thermal antiphase boundaries (APB's). The defect formation mechanisms proposed here are similar to twinning mechanism reported for FCC-metals during annealing. Thus, SF arrays and faulted μT's in the L10-ordered FePd appear to form during the early stages of annealing by atomic attachment faulting on {111}-facets of the transformation interfaces. During later stages of annealing the reduced amount and the change in nature of the driving forces for the microstructural rearrangement result in changes in the predominant defect formation mechanism. The features of the defect genesis in L10-FePd are discussed with respect to solid-state transformations during processing of these ferromagnetic intermetallics.

The growth of the D03-type antiphase domain (APD) in Fe3Al was investigated focusing on the effect of excess vacancies that were introduced during the quenching process from the disordered state. The variation in the APD size exhibited considerable deviation from the conventional “parabolic growth law” in the early stage of APD growth. This variation was numerically calculated on the assumption that the migration of the APD boundaries was enhanced by non-equilibrium excess vacancies and the vacancy concentration decreased during the isothermal annealing for the APD growth. The calculated variations in the APD size could be successfully fitted to the experimental results in cases with quenching temperatures (Tq) of 873 K or 1073 K, but not when Tq was 1273 K. The APD growth in the latter case was much slower than the expected growth derived from the calculation. This discrepancy was attributed to the rapid decrease in the vacancy concentration due to vacancy clustering since a significant amount of dotted contrasts were observed in TEM image of only the specimen quenched from 1273K.

We report on microstructure, mechanical properties and wear resistance of Fe-Al based alloys with various alloying elements. The microstructures were examined using optical and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscope (EDS). Two types of alloys were prepared using vacuum arc melting; one is Fe-28Al based alloys (D03 structured) with and without alloying elements such as Mo and Zr. The other one is Fe-35Al based alloys (B2 structured) produced with same manner. For both types of alloys, equiaxed microstructures were observed by the addition of Mo, while dendritic structures were observed by the Zr addition. These microstructural features were more evinced with increasing the content of alloying elements. Concerning the mechanical properties and wear resistance, Fe-35Al based alloys with or without Mo addition superior to Fe-28Al based alloys especially in the high temperature region.

Most of the studies aimed at the development of creep-resisting Fe-Al intermetallics have been oriented at application temperatures of the order of 500–650°C, where these materials may compete with conventional stainless steels. The Fe-Al intermetallics are, however, particularly excellent in their oxidation and corrosion resistances at temperatures of the order of 1000°C, where Chromium-Nickel steels are no longer able to withstand the aggressive environments. This presentation is part of a study aimed at the development of good creep resistance at such high temperatures.

Studies of a variety of cast Fe3Al-base alloys, strengthened by solution or precipitate/dispersoid-forming alloying additions, are reported. The alloys show good strength from room temperature to about 500°C, but thereafter strength falls rapidly as thermally-activated deformation processes become operative. Solution additions are capable of producing good low temperature strength, but do not contribute significantly to creep strength at very high temperatures (above 700°C). Precipitation hardening has been examined in Nb-containing alloys, where Fe2Nb Laves precipitates form at intermediate temperatures. These materials show good strength up to about 700°C, but at higher temperatures the fine precipitates coarsen excessively. Strengthening in the intermediate temperature range varies depending on whether the solute is precipitated prior to high temperature testing or concurrent with this.

The microstructural characteristics and creep behavior of two fine-grained XD TiAl alloys, Ti-45Al and 47Al–2Nb–2Mn+0.8vol%TiB2 (at%), were investigated. A nearly lamellar structure (NL) and two kinds of fully lamellar (FL) structures in both alloys were prepared by selected heat treatments. The results of microstructural examination and tensile creep tests indicate that the 45XD alloy with a NL structure possesses an inferior creep resistance due to its coarse lamellar spacing and larger amount of equiaxed γ grains at the grain boundaries, whereas the same alloy in a FL condition with fine lamellar spacing lowers the minimum creep rates. Contrary to 45XD, the 47XD alloy with a NL structure exhibits the best creep resistance. However, 47XD with a FL structure with finer lamellar spacing shows inferior creep resistance. On the basis of microstructural deformation characteristics, it is suggested that the well-interlocked grain boundary and relatively coarse colony size in FL and NL 47XD inhibit sliding and microstructural degradation at the grain boundaries during creep deformation, resulting in better creep resistance. Therefore, good microstructural stability is essential for improving the creep resistance of these alloys.

A new CBED method is proposed for the identification of the chirality of enantiomorphic crystals, in which asymmetry in the intensity of the reflections of Bijvoet pairs in an experimental symmetrical zone-axis CBED pattern is compared with that of a computer-simulated CBED pattern. The intensity difference for reflections of these Bijvoet pairs results from multiple scattering among relevant Bijvoet pairs of reflections, each pair of which has identical amplitude and different phase angles. With the method, a single CBED pattern is sufficient and chiral identification can be made for all possible enantiomorphic crystals that are allowed to exist in crystallography. The method is successfully applied to some chiral intermetallic compounds.

Alloys of RuAl-Ru were processed using various solidification methods, and the fracture behavior was examined. The fracture toughness values for RuAl-hcp(Ru, Mo) and RuAl-hcp(Ru, Cr) alloys ranged from 23 to 38 MPa√m, while the volume fraction of RuAl ranged from 22 to 56 percent. Increasing the volume fraction of RuAl resulted in a decrease in fracture toughness. The hcp solid solution was shown to be the more ductile phase with a fracture toughness approaching 68 MPa?m, while the B2 solid solution (RuAl) was found to have a fracture toughness less than 13 MPa√m. An alloy of Ru-7Al-38Cr (at.%) that consisted of a hcp matrix with RuAl precipitates had the highest room temperature toughness and the greatest hardness.

Phase transformations and phase transition temperatures in several Nb-rich γ-TiAl based alloys were investigated experimentally and compared to thermodynamic simulations. The present study combines light-optical and scanning electron microscopy, X-ray diffraction and differential-scanning-calorimetry for the characterization of the prevailing phases and phase transformations. Thermodynamic simulation based on the CALPHAD method was used for predict phase stabilities. The results from experiments on a variety of γ-TiAl based alloys are compared to thermodynamic calculations. Finally, the influence of carbon on the transition temperatures is presented.

A combination of ab initio total energy calculations with Calphad approach is applied to model the site occupancy and thermodynamic properties of the Fe-Cr, Co-Cr, Fe-V, and Fe-Mo binary sigma-phases as a function of composition and temperature. For each binary sigma-phase the parameters of the model are the ab initio calculated total energies of so-called end-member compounds, which represent all the 25=32 variants of complete occupancy of each of the five crystallographically inequivalent sites by one or the other alloy component. The paramagnetic state of the sigma-phases has been taken into account within the disordered local moment approach. The Fe and Co atoms are found to retain high spin moments when they occupy high-coordination-number sites in the structure. Using our model we were able to reproduce the experimentally observed site occupancy in the FeCr sigma-phase. The calculated site occupancies in the Co-Cr, Fe-V, and Fe-Mo sigma-phases are also presented and discussed.

The intermetallic compound MoSi2, which adopts the C11b crystal structure, and related alloys exhibit an excellent corrosion resistance at high temperatures but tend to be brittle at room and even relatively high temperatures. The limited ductility of MoSi2 in ambient conditions along with the anomalous temperature dependence of the critical resolved shear stress (CRSS) of the {110)<111], {011)<100] and {010)<100] slip systems and departure from Schmid law behavior of the {013)<331] slip system can all be attributed to complex dislocation core structures. We have therefore developed a Bond-Order Potential (BOP) for MoSi2 for use in the atomistic simulation of dislocations and other extended defects. BOPs are a real-space, O(N), two-center orthogonal tight-binding formalism that are naturally able to describe systems with mixed metallic and covalent bonding. In this development novel analytic screening functions have been adopted to properly describe the environmental dependence of bond integrals in the open, bcc-based C11b crystal structure. A many-body repulsive term is included in the model that allows us to fit the elastic constants and negative Cauchy pressures of MoSi2. Due to the internal degree of freedom in the position of the Si atoms in the C11b structure which is a function of volume, it was necessary to adopt a self-consistent procedure in the fitting of the BOP. The constructed BOP is found to be an excellent description of cohesion in C11b MoSi2 and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C 40, C 49 and C 54 MoSi2, A15 and D03 Mo3Si and D8m Mo5Si3 by comparing with ab initio structural optimizations.

Phase equilibria in ternary Hf-Ti-Si alloys were studied in the as-solidified and heat treated conditions using scanning electron microscopy, x-ray diffraction, and electron beam microprobe analysis. Selected solid-solid phase equilibria at 1350°C and a partial liquidus projection of the Hf-Ti-Si system at the metal rich end of the phase diagram were established. These data were then used to develop a thermodynamic description of the Hf-Ti-Si system using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated isothermal section at 1350°C and the liquidus projection can satisfactorily account for the available experimental phase equilibria data and solidification paths. Both the calculations and the experimental data suggested that the metal-rich end of the ternary phase diagram possesses one transition reaction: L + (Hf, Ti)5Si3 → Hf(Ti)2Si + β(Hf, Ti, Si).

The deformation structure of B2 type TiNi compound around 573 K has been investigated by transmission electron microscopy (TEM). Serrations are seen in stress-strain curve, which corresponds to the formation of various planar defects with twin relation. The dominant planar defect found in the specimens showing serration is {114} compound twin. The other defects are in mirror symmetry with respect to {113}, {115}, {447} planes and so on. These defects are considered to be <110> symmetric tilt boundaries in bcc structure by ignoring the atomic arrangement of B2 structure and are characterized with Σ value based on coincide site lattice (CSL) model. For instance, the Σ value of {114}, {113}, {115} and {447} boundaries are Σ9, Σ11, Σ27 and Σ81, respectively. Numerous (114) defects initially form at grain boundary and grow into grain interior. Some of those deflect to (-1–14) defects. In such case, the {447} Σ81defect is always observed at the interface of (114) Σ9 and (-1–14) Σ9 defects. This fact indicates that the sigma combination rule of the CSL model can be applied to the triple junction of defects. Similarly, {7710} Σ99 boundary forms at the interface of {114} Σ9 and {113} Σ11 defects. It can be concluded that the ductility of B2 type TiNi compound around 573 K is attributable to the formation of various planar defects with large shear strain and the increment of independent slip system due to the formation of planar defects, and that the arrangement of some planar defects conforms to the sigma combination rule of CSL model.

The microstructure, defect structure and thermoelectric properties of Al-containing ReSi1.75 based silicides have been investigated. All the Al-containing alloys investigated contain four differently oriented domains accompanied by the twinned microstructure, as the binary alloy does. However, thin defect layers containing a kind of shear structure are locally and sporadically formed at some of twin boundaries. In the defect layer, shear occurs by the vector of [100] on either (1 09) or (107) planes. Binary ReSi1.75 exhibits nice thermoelectric properties as exemplified by the high value of dimensionless figure of merit (ZT) of 0.70 at 800 °C when measured along [001], although the ZT value along [100] is just moderately high. Al-containing Re silicides considerably increase the ZT value along [100] so that the maximum value of 0.95 is achieved at 150 °C for the ReSi1.75Al0.02 alloy. The temperature dependence of electrical resistivity changes from of semiconductor for the binary alloy to of metal for the Al-added alloys and the value of electrical resistivity is significantly reduced when compared to the binary counterpart.