Directionally solidified Nb-xMo-22Ti-18Si (x = 10, 20 and 30 mol%) and Nb-10Mo-yW-10Ti-18Si (y = 0, 5, 10 and 15 mol%) alloys were prepared. All of the alloys consisted of Nb solid solution and (Nb, Mo, (W,) Ti)5Si3 silicide. In compression tests performed on the Nb-xMo-22Ti-18Si alloys, a sample with x = 10 had the highest maximum stress at 1670 K, but it showed a sharp stress drop after reaching the maximum stress. Samples with x = 20 and 30 showed lower yield and lower maximum stress than that with x = 10, and they showed a small stress drop and an almost constant flow stress of 330 and 400 MPa, respectively. The sample with x = 20 showed a minimum creep rate (εm) of 9.6 × 10-7 s-1 at 1670 K at a stress of 200 MPa. The yield stress of the Nb-10Mo-yW-10Ti-18Si alloys increased with increasing W content, although the stress decreased gradually after reaching the maximum stress and showed no steady state deformation. The εm of the sample with y = 15 was 1.4 × 10-7 s-1 at 1670 K at a stress of 200 MPa, which was much slower than that of Nb-20Mo-22Ti-18Si.

A novel method to develop new quaternary alloys with an fcc/L12 coherent structure is proposed. This paper reviews the development of quaternary Ir-Nb-Ni-Al alloys. The microstructure, lattice misfit, and compressive 0.2% flow stress of 15 kinds of alloys were investigated systematically. Two kinds of coherent structures, fcc/L12-Ir3Nb and fcc/ L12-Ni3Al, were observed in most alloys. Two two-phase structures, fcc+L12-Ir3Nb and fcc+L12-Ni3Al, were observed in Ir-rich and Ni-rich regions, respectively. The lattice misfits of quaternary Ir-Nb-Ni-Al alloys were higher than those of Ni- or Ir-base binary alloys. The compressive 0.2% flow stresses of quaternary alloys increased dramatically compared with those of Ni-base superalloys. The quaternary alloys located in the Ir-rich region were not only had higher strength but also better ductility than Ir-base binary alloys. The potential use of quaternary alloys is discussed.

The effect of surface defects on the performance of TiAl-base alloys is an issue of importance in contemplating their application into engine components. Due to the relatively low ductility and low impact resistance of gamma alloys the validation of models for estimating economic life and for safe-life approaches employed for components becomes of great importance. Surface defects can be attributed to various sources during the manufacturing or handling of the components. In fact, little is known about the detrimental effects of surface defects on gamma alloys. In the present study, the effect of artificially introduced surface defects, on the high cycle fatigue behavior of the Ti-47Al-2W-0.5Si, will be investigated and correlated with the crack growth behavior at 700°C. The results are reported in the form of the Kitagawa diagram in which the safe and unsafe zones for crack advance and fracture are defined.

The energetics of the interaction of the <100>{010} edge dislocation in NiAl with early 3d transition metal (TM) impurities was studied using the ab initio real-space tight-binding LMTO-recursion method with 20,000 atom clusters and up to 1,000 non-equivalent atoms in the dislocation core. The coordinates of the atoms in the core were determined within the Peierls-Nabarro (PN) model with restoring forces determined from full-potential LMTO total energy calculations. TM impurities were then placed in different substitutional positions near the dislocation core. For most positions studied, the interaction between impurities and the dislocation is found to be repulsive (dislocation friction). However, when the impurity is in the position close to the central atom of the dislocation core, the interaction becomes strongly attractive, thus causing dislocation locking. Since the size misfit between the Al atom and the substituting TM atom is very small, this locking cannot be explained by elastic (or size misfit) mechanisms; it has an electronic nature and is caused by the formation of the preferred bonding between the electronic states of the impurity atom and the localized electronic states appearing on the central atom of the dislocation core. The calculated results are then discussed in the scope of experimental data on solid solution hardening in NiAl.

The brittle-to-ductile transition (BDT) and the strain-rate dependence of the brittle-to-ductile transition temperature (BDTT) have been recently investigated in single crystals of TiAl [1]. It was found that the activation energy associated with the BDTT is 1.4 eV when the slip is dominated by ordinary dislocations and 4.9 eV when it is dominated by superdislocations. Despite this difference in the activation energies, the BDTT, while varying with the strain-rate, remains in the same temperature range, viz., between 516–750C and 635–685C for ordinary and superdislocations, respectively. In this paper, we examine how the activation energy of the BDTT can vary with the type of dislocation activity and explain why it can attain values which are clearly much larger than the activation energy for dislocation motion. We describe a strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature and use it to explain the characteristics of the BDT in TiAl. We show that the activation energy associated with the BDTT is a composite value determined by two or more inter-dependent thermally activated processes and its magnitude can be much larger than the activation energy for dislocation motion in certain materials. The predictions of the model are in good agreement with observations in TiAl.

We have investigated the mechanical behavior of the following single-phase polycrystalline alloys with the MoSi2 body-center tetragonal structure: MoSi2 alloyed with ∼2.5 at.% Re, MoSi2 alloyed with 2 at.% Al, MoSi2 alloyed with 1 at.% Nb, and MoSi2 alloyed with 1 at.% Re and 2 at.% Al. Several anomalies in the mechanical behavior of alloyed materials were observed. For example, (i) addition of only ∼2.5 at. % Re results in an order of magnitude increase in compressive strength at 1600 °C, (ii) additions of Nb and Al cause solution softening at near-ambient temperatures, and (iii) quaternary MoSi2-Re-Al alloys show strengthening at elevated temperatures and reduction in flow stress with enhanced plasticity at near-ambient temperatures in compression. The mechanisms of anomalous solution hardening and softening are discussed.

Recent research [1–3] suggests that strain-induced ferromagnetism in lightly-strained FeAl arises chiefly from anti-phase boundary tubes. Magnetic and calorimetric measurements have been performed on two different orientations of B2-structured Fe-40Al single crystals that had been cold rolled to a variety of strains. The saturation magnetization and magnetic susceptibility are related to the enthalpy associated with the annealing out of antiphase boundary tubes and, hence, to the degree of deformation and crystal orientation.

Positron lifetime and Doppler broadening measurements were carried out at a low temperature, 100K, for B2-type intermetallic compounds FeAl with composition ranges from 41.2 to 50.7 at%Al systematically in order to clarify the feature of lattice defects of the alloys. Quenching temperature dependences of positron lifetimes ranging from 573 to 1173 K was examined for alloys with 41.2, 49.0 and 50.7 at%Al. Two lifetime component analyses could not be made except for a few spectra, indicating a saturation trapping of positrons at atomic vacancies. The mean positron lifetimes are in a range of 175–195 psec, which are not corresponding to vacancy clusters but to mono and/or di-vacancies. The tendency for S-parameter to increase with increase in Al content was found. This suggests that such an increase of S-parameters is attributed to change in the fraction of atoms around the vacancies, not to increase in vacancy concentration.

The silicide Mo5Si3, with the W5Si3 structure, exhibits a high anisotropy of its coefficients of thermal expansion (CTEs) in the a and c directions, namely, CTE(c)/CTE(a)=2.2. In order to determine whether the CTE anisotropy can be controlled, molybdenum was partially substituted with Nb. CTEs were determined by high temperature x-ray powder diffraction. Partial substitution with 44 at. % Nb reduced the value of CTE(c)/CTE(a) to a value near 1. For higher Nb concentrations, CTE(c)/CTE(a) increased again. When nearly all the Mo was replaced by Nb, the crystal structure changed to the Cr5B3 structure type and the CTE anisotropy decreased to a value near 1. Our thermal expansion results are interpreted in terms of the site occupation of Nb, the Nb-induced increase in the interatomic spacing of the Mo atom chains along the c-direction, and the reduction in the anisotropy of the lattice anharmonicity in the a and c directions.

Compression tests performed on both stoichiometric and cobalt-rich CoTi over a range of temperatures show a positive temperature dependence of the yield stress with increasing temperature, before a decline occurs at high temperatures. In the region of the peak yield stress, serrated yielding and a negative rate sensitivity of the yield stress were observed. Static strain-aging also occurs. These observations are consistent with strong solute-dislocation interactions. Results from quenching experiments and strain rate change tests are presented, together with transmission electron microscope observations of the dislocation structures below, at, and slightly above the peak temperature. The results suggest that the yield anomaly in CoTi can be accounted for by a classical dynamic strain aging mechanism.

A 3D mesoscopic simulation of dislocation behaviour is adapted to the case of γ-TiAl. It shows that, in the temperature range of the stress anomaly, ordinary dislocation motion essentially proceeds through a series of pinning and unzipping processes on screw dislocations. Pinning points are cross-slip generated jogs, whose density increases with temperature. Unzipping of cusps restores the screw character of the dislocation. The balance between pinning and unzipping becomes increasingly difficult as temperature raises, and results in dislocation exhaustion. All these features agree with the so-called “Local Pinning Unzipping” mechanism.

The tensile test in transition metal disilicides with C113 structure is simulated by ab initio electronic structure calculations using full potential linearized augmented plane wave method (FLAPW). Full relaxation of both external and internal parameters is performed. The theoretical tensile strength of MOS12 and WSi2 for [001] loading is determined and compared with those of other materials.

Pulverization behavior and microstructure evolution with hydrogenation in hydrogen absorbing Ta-Ni intermetallic-based alloys, such as Ta2Ni with Ta solid solution (Tass), TiMn2 with TiMn and Nb3Al with Nb solid solution (Nbss), are investigated to elucidate the mechanism of the hydrogen pulverization. Ta-10at.%Ni consisting of Ta solid solution (Tass) and Ta2Ni Laves phase is pulverized to coarse powder over 100 μm in hydrogenation. Crack propagation occurs preferentially in the brittle Ta2Ni phase rather than in the ductile Tass phase. When the volume fraction of brittle Ta2Ni increases with increasing Ni content, hydrogen pulverization is enhanced. The lattice parameter of Tass increases by hydrogenation, while it does not change in Ta2Ni. In addition, nano-sized regions with Moiré patterns are produced in Tass and Debye rings corresponding to tantalum hydride β-TaH appear in the diffraction pattern. These features are very similar to those of TiMn2 based alloy and Nb3 Al based alloys in the literature. Based on the present results along with those in the literature it is concluded that the hydrogen pulverization is attributable to (1) the absorption of a large amount of hydrogen in constituent phase(s), (2) the large strain introduced by lattice expansion and the hydride formation, and (3) the ease of crack nucleation and propagation in brittle constituent phase(s).

Crystals of CoSb3 were grown using the vertical Bridgman method at growth rates that varied from 0.4 to 2.8 mm/h. Thermoelectric properties were analyzed for both as-grown and post-annealed samples. Polycrystalline CoSb3 surrounded by Sb was obtained. Samples grown at the rate of 0.4 mm/h had larger CoSb3 grains than samples grown at the 2.8 mm/h rate. For the as-grown samples, the Seebeck coefficient was smaller than 200 μ/K, which is a nominal value [1–3]. The presence of residual Sb resulted in a decrease in the Seebeck coefficient and an increase in the samples' electrical conductivity. A subsequent heat treatment at 800 °C for 20 h eliminated the residual Sb, resulting in a significant increase in the Seebeck coefficients (ranging from > 200 μV/K) in the annealed samples, as compared with the as-grown samples. The samples with a higher growth rate had larger Seebeck coefficients of ∼500 μ/K after annealing.

“Order-order” relaxations in γ-Ni3Al previously extensively studied by means of resistometry, are simulated within a model of vacancy mechanism of atomic migration in a superstructure implemented with Monte Carlo technique and the Glauber algorithm. The observed operation of two simultaneous relaxation processes showing different rates, as well as the theoretically predicted effect of vacancy ordering have been definitely reproduced and analysed in detail in terms of the dynamics of particular kinds of atomic jumps. The proposed model scenario for the creation and elimination of antisite atoms in the relaxing L12-type superstructure shows that the experimentally observed features of the “order-order” processes in Ni3Al follow from an interplay between two dominating and coupled modes of long- and short-range ordering: the creation/elimination of nn pairs of antisites (SRO) and the change of the “overall” number of antisites (LRO). High profile of the first process results in a high contribution of the fast component of LRO kinetics.

Intermetallic titanium aluminides are one of the few classes of emerging materials that have the potential to be used in demanding high-temperature structural applications whenever specific strength and stiffness are of major concern. However, in order to effectively replace the heavier nickel-base superalloys currently in use, titanium aluminides must combine a wide range of mechanical property capabilities. Advanced alloy designs are tailored for strength, toughness, creep resistance, and environmental stability. Some of these concerns are addressed in the present paper through specific comments on the physical metallurgy and technology of gamma TiAl-base alloys. Particular emphasis is placed on recent developments of TiAl alloys with enhanced high-temperature capability.

Different techniques to produce single crystals of RuAl were investigated. Processing from the melt is problematic due to the rapid loss of Al. However, very large grained RuAl specimens can be produced from the solid-state by arc-zone melting. Directional solidification of eutectic alloys is less problematic due to the lower melting temperature. Alloys of RuAl-Mo were found to be eutectic, and the eutectic composition was determined to be RuAl-51 wt% Mo.

MoSi2 has a great potential for very high temperature structural applications. Plastic deformation of MoSi2 single crystals with the C11b structure is extremely anisotropic. It is caused by non-Schmid behavior of slip on {013}<331> with the higher CRSS values for orientations closer to [001]. In order to provide better understanding of key factors on such non-Schmid behavior in MoSi2 (c/a=2.45), we chose PdZr2 with a c/a axial ratio higher than 3 (c/a=3.30) and characterized the plastic deformation. Compression tests were conducted at various temperatures along [001], [010] and [110] axes. Slip on {013}<100> has the shortest Burgers vector and the largest interplanar spacing in PdZr2 and was observed to be activated for [110] with the lowest CRSS. While slip on {013}<331> can be activated even at -196°C for [001]. Although {013}<331> slip has the same Schmid factors for [001] and [010], the yield stress of the [010]-oriented crystals is about twice higher than that of the [001]-oriented crystals. Thus non-Schmid behavior of slip on {013}<331> is also observed in PdZr2, and the manner is opposite to that in MoSi2. Plastic anisotropy in the C11b structure will be discussed in terms of the c/a axial ratio.

The mechanical properties and dislocation microstructure of single crystals with a range of compositions within the Fex-Ni60–x-Al40 pseudobinary system have been investigated, with the purpose of bridging the behavior from FeAl to NiAl. Experiments are focused on the compression testing of <001> oriented single crystals with compositions where x = 10, 20, 30, 40, and 50 (in atomic percent). Observations of a<111> dislocation morphologies at room temperature and both a<111> and non-a<111> dislocation activity at elevated temperatures are reported and discussed. Measurements of the yield strength, elastic modulus and strain hardening rates are reported, and the variation of strength with composition is correlated with dislocation dissociation and overall dislocation morphology.

Microstructure and plastic deformation behavior in duplex-phase silicides composed of the C40 and C11b phases were examined using pseudo-binary (Nb,Mo)Si2 crystals with a single set of lamellae. Single crystals of the C40 single-phase were grown from the master ingot with a composition of (Nb0.15Mo0.85)Si2 by a floating zone method and duplex-phase microstructure containing a single set of lamellae was obtained by subsequent heat treatment at 1400°C for no less than 6h. During the heat treatment, the C11b phase was precipitated from the C40 matrix by satisfying the crystallographic relationship of (0001)C40//(110)C11b, <1210]C40//[110]C11b and < [1010]C40//[001]C11b at the lamellar boundary, while randomly oriented C11b grains also appeared at further annealing. As a result, the duplex-phase silicides with a single set of lamellae contained the C40 phase with a single orientation and three variants of the C11b phase. The lamellar spacing and the volume fraction of their phases depended strongly on annealing time.

In compression tests, yield stress and fracture strain of the duplex-phase silicides depended strongly on angle (φ) between the loading axis and lamellar boundaries, similar to TiAl-PST crystals. At φ=0°, specimens fractured just after showing high fracture stress even at 1400°C. In contrast, at φ=45° where shear deformation in the C11b phase of lamellae occurred parallel to lamellar boundaries, low yield stress and significant plastic strain were achieved at 1400°C.