The annihilation of dislocations by cross-slip is studied by numerical simulation of infinitely long dissociated screw dislocations, allowed to move in an elastically anisotropie crystal. The external load is along [123] and cross-slip is permitted both on the octahedral and on the cube plane. The latter, together with cube slip, is thermally activated. Anisotropie elasticity modifies the properties of cross-slip significantly. Under the conditions of the simulations, the processes of APB jumps (APBJs) and repeated APB jumps (RAPBJs) can be largely promoted by interactions with other dislocations, while it is much less likely to occur at an isolated dislocation submitted to the same applied stress. The encounter of dislocations of opposite signs produces dipoles which may or may not tend to annihilate by cross-slip. APB tubes may form upon annihilation under certain circumstances again largely controlled by elastic anisotropy.

The strain-rate dependence of flow stress in single crystals of binary, stoichiometric Ni3Al was studied in the temperature region of the yield stress anomaly. Below 400 K, the steady-state flow stress was found to be independent of strain rate, though it changed temporarily when the strain rate was changed. The strain-rate insensitivity can be explained by assuming that the flow stress is controlled by the multiplication/immobilization of mobile dislocations.

The notch-tip deformation field of Ni3AI single crystals doped with boron is characterized by fan-shaped sectors within which one slip system dominates. Cracks initiate and propagate along slip planes on both sides of the notch plane.

Dense NiAl intermetallic was synthesized from very fine elemental powders via solid state reactions. Homogeneous blends of micron size Ni and Al powders were consolidated to full density and heat treated in a 425–800°C temperature range. During heat treatment, formation of various intermediate intermetallics phases: NiAl3, Ni2A13, Ni3Al and NiAl was observed. The sequence and kinetics of these phase formations at different temperatures were studied employing X-ray diffraction analysis (XRD). A model for a description of synthesis reaction kinetics in Ni-Al blends was developed. Based on the obtained results, the synthesis of NiAl was performed in two stages : reactions in 425–550°C range with consumption of Al, followed by a reaction at up to 800°C. It allowed uncontrolled SHS (self propagating high temperature synthesis, resulting in the occurrence of liquid phases and in formation of reaction products in a very fast /explosive manner) to be avoid. The synthesis temperatures are considerably lower than those used currently in processing of NiAl.

Single crystals of Ni3AI (1 at%Ta) with a compression axis of [123] were subject to creep at a stress of 550 MPa and a temperature of 520 °C. Slip trace and TEM microstructural observations reveal that primary octahedral slip is responsible for the primary creep. In the secondary stage, cube cross slip (010) is operative. There is no obvious sign of inverse creep.

This paper reviewed the recent progress in the environmental embrittlement of suicide. On the surface of suicides, the Si in the suicides such as Fe3(Si,Al) alloy reacts with both oxygen and water vapor more easy than with iron. A molecular hydrogen machanism of surface reaction, i. e. Si + 2H2O = SiO2 + 2H2, can be derived. The moisture-induced embrittlement of suicides can be considered to be an embrittlement in a localized high pressure molecular hydrogen condition. It is a kinetic hydrogen gas embrittlement. Suicides may have more severely intrinsic brittleness than iron aluminides due to their special electronic structure and bonding mechanism, leading to elucidate the role of environment on ductility with difficulty. The improvement of both the intrinsic brittleness and moisture-induced embrittlement are critical for the development of suicides.

Moisture-induced embrittlement of Ll2 alloys (such as Ni3(Si,Ti) and Ni3AI) is observed by tensile test and SEM fractography. A variety of microstructures were prepared by selecting pre-deformation and heat treatment conditions. It is shown that tensile ductility and the associated fractography depend on structure as well as test atmosphere. Well-annealed specimens are susceptible to moisture-induced embrittlement while pre-deformed specimens are resistive to moisture-induced embrittlement. Also, this embrittlement is generally sensitive to the heat treatment scheme preceded by the pre-deformation. Results indicate that the embrittlement occurs when hydrogen is enriched on grain boundaries. On the other hand, the embrittlement can be suppressed when hydrogen is trapped at lattice defects such as dislocations and vacancies. These results are discussed in association with the kinetics of hydrogen in the pre-deformed microstructure.

The effects of processing on the microstructure and properties of MoSi2 are examined. A diverse array of samples, processed through a variety of means, were investigated for their microstructural features and indentation fracture behavior. Results from this study indicate that a finely dispersed secondary phase enhances tortuous crack paths, crack branching, and microcracking, thereby improving fracture toughness. The effects of substitutional alloying (Re, Al) and composite additions (SiC) have also been investigated.

Stacking faults on (001) in MoSi2 and WSi2 with the Cllb structure have been characterized by transmission electron microscopy (TEM), using their single crystals grown by the floating-zone method. Although WSi2 contains a high density of stacking faults, only several faults are observed in MoSi2. For both crystals, (001) faults are characterized to be of the Frank-type in which two successive (001) Si layers are removed from the lattice, giving rise to a displacement vector parallel to [001]. When the displacement vector of faults is expressed in the form of R=l/n[001], however, their n values are slightly deviated from the exact value of 3, because of dilatation of the lattice in the direction perpendicular to the fault, which is caused by the repulsive interaction between Mo (W) layers above and below the fault. Matching of experimental high-resolution TEM images with calculated ones indicates n values to be 3.12 ± 0.10 and 3.34 ± 0.10 for MoSi2 and WSi2, respectively.

The deformation behavior of single crystals of Mo(Si,Al)2 with the C40 structure has been studied as a function of crystal orientation and Al content in the temperature range from room temperature to 1500°C in compression. Plastic flow is possible only above 1100°C for orientations where slip along <1120> on (0001) is operative and no other slip systems are observed over whole temperature range investigated. The critical resolved shear stress for basal slip decreases rapidly with increasing temperature and the Schmid law is valid. Basal slip appears to occur through a synchroshear mechanism, in which a-dislocations (b=1/3<1120>) dissociate into two synchro-partial dislocations with the identical Burgers vector(b*1/6<1120>) and each synchro-partial further dissociates into two partials on two adjacent planes.

The intermetallic compound, RuAl with B2 CsCl type structure, has been shown to possess room-temperature toughness and plasticity. NiAl also forms a B2 compound and it is claimed that a pseudo-binary compound, (Ru,Ni)Al, may be formed because the difference in lattice parameter between the two binary phases is slight. In this work a study has been made of the mechanical properties of some polycrystalline compounds, across the RuAl-(Ru,Ni)Al pseudo-binary, prepared from high-purity elemental powders. Compressive yield stresses were measured between room-temperature and 900°C, and the mechanisms of plastic flow are discussed in relation to the dislocation structures observed by TEM. Hot-microhardness tests were made to provide an indication of the effect of solid-solution hardening.

Site occupancies in three C15-structured AB2(X) Laves phases have been determined with Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI). In NbCr2(V), the results are consistent with exclusive site occupancies of Nb for the A sublattice and Cr and V for the A sublattice. The B-site occupancy of V can be interpreted in terms of electronic structure. In NbCr2(Ti), the results are consistent with Ti partitioning mostly to the A sites with some anti-site defects likely. In HfV2(Nb), the results are consistent with Nb partitioning between the A and A sites. The results of the ALCHEMI analyses of these ternary C15 Laves phase materials are discussed with respect to previously determined phase diagrams and first-principles total energy and electronic structure calculations.

The single-crystal elastic constants of C15 NbCr2 have been computed by using a first-principles, self-consistent, full-potential total energy method. From these single-crystal elastic constants the isotropie elastic moduli are calculated using the Voigt and Reuss averages. The calculated values are in fair agreement with the experimental values. The implications of the results are discussed with regards to Poisson's ratio and the direction dependence of Young's modulus.

Ni3Ga is among a number of LI2 ordered intermetallic alloys whose yield stress increases with temperature. In this work we have examined the creep strength of [123] and [001] oriented Ni3Ga specimens in the temperature regime of the yield stress anomaly and confirmed that the creep strength shows the normal rapid decrease with temperature. Inverse creep occurs in the [001] specimens where slip is on the {111} planes only. [123] specimens exhibit steady-state creep and slip line and TEM observations have shown slip on the cube plane and dislocations of both <110> and <100> Burgers vectors are present. We have carried out creep tests of prestrained [123] samples and demonstrated that the storage of primary <110> {111} screw dislocations, locked in the Kear-Wilsdorf configuration, has no effect on creep.

Yield stress and work hardening coefficient of B2-stabilized NiTi alloys are investigated using compression tests. Compositions of NiTi alloys are based on Ni-49moJ.%Ti, to which Cr, Co and Al are chosen as ternary elements which reduce martensitic transformation temperatures of the B2 phase. Mechanical tests are carried out in liquid nitrogen at 77K, air at room temperature (R.T.) and in an argon atmosphere between 473K and 873K. Only at 77K, some alloys show characteristic stress-strain curves which indicate stress induced martensitic transformation (SIMT), but the others do not. Work hardening coefficient is found to be between 2 and 1 lGPa in all the test temperature range. The values are extremely high compared with Young's modulus of B2 NiTi. Yield stress and work hardening coefficient increase with test temperature between R.T. and about 650K in most alloys. The anomalous temperature dependence of mechanical properties is not related to SIMT but to precipitation hardening and/or anomalous dislocation motion similar to B2-type CoTi. Solution hardening by adding ternary elements is evaluated to be small for Cr and Co additions, and large for Al addition, depending on difference in atomic size of the ternary element with respect to Ni or Ti.

The transition from positive to negative temperature dependence of 0.2% yield stress is investigated m the model pseudo-binary Ni3Ge-Fe3Ge system. Ni3Ge and Fe3Ge, both Ll2 intermetallic alloys, show complete solid solubility as Fe is continuously substituted for Ni across the composition range. However, Ni3Ge exhibits the yield stress anomaly, whereas the yield stress of Fe3Ge shows a normal decline with temperature. Mechanical testing has verified this behavior, with the anomalous behavior gradually disappearing with increasing Fe content. It is proposed that this transition results from changes in the structure of dissociated superdislocation cores. Alloys with anomalous behavior from this system are characterized by the presence of screw superdislocations locked in the Kear-Wilsdorf (KW) configuration. Conversely, alloys with normal yield stress dependence are observed to contain curvilinear superdislocations that glide on the cube planes. Results from mechanical testing are presented and correlated with TEM observations of deformation structures. These results are discussed in light of planar fault energies determined through computer simulations of images.

To obtain insight into the effect of dopants on the bonding and cohesive energy of gram boundaries in Ll2 intermetallic compounds, the chemistry and electronic structure at grain boundaries in B-free and B-doped Ni-23 at % Si alloys were examined, with electron energy loss spectroscopy (EELS) providing information on the former and energy dispersive X-ray spectroscopy (EDX) on the latter. Ni-enrichment was seen at large angle boundaries, both in the absence and presence of B. EELS of the Ni L3 edge showed that the bonding at Ni-rich grain boundaries was similar in both undoped and doped alloys. Comparison of the Ni L3 edge recorded at the grain boundary and in the bulk suggests that reduced hybridization and weaker bonding occurs at Ni-rich grain boundaries in both doped and undoped alloys. These changes in bonding are interpreted in terms of changes in the cohesive energy of the boundaries.

The ternary Nb-40Ti-15Al (at%) intermetaltic has an ordered B2-type crystal structure and shows appreciable ductility. Deformation is mainly accommodated in slip bands by the glide of b=l/2<lll> dislocations on {110} planes. The tensile behavior of this material has been studied at room temperature and at intermediate temperatures. Static and dynamic strain aging have been observed in certain temperature regimes. A model based on dislocation pinning by precipitates is proposed to explain the serrations observed during dynamic strain aging.

The isothermal phase diagrams of ternary alloy systems Nb-Cr-V and Nb-Cr-Mo are determined by metallography, X-ray diffraction (XRD) and transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX). In two alloy systems, the C15 NbCr2 Laves phases are equilibrated directly with bcc solid solution without forming any intermediate phases. Relatively large amount of ternary elements V and Mo are soluble in the C15 NbCr2 Laves phases. It is shown that the C15 Laves phases extend along directions so that V occupies Cr site while Mo occupies Nb site. Also, characteristic structure containing micro twins and stacking faults is observed in the C15 Laves phase alloyed with Mo. Bcc phase has a wider solid solution range in the X(=V and Mo)-rich side than the Nb-rich and Cr-rich sides. A variety of duplex microstructures are observed depending on alloy system and alloy composition.

Because of its high melting point, excellent oxidation resistance and ductility at high temperatures, MoSi2 and its composites are attractive for elevated temperature applications. In this study, the compression creep behavior of hot pressed MoSi2 with various volume fraction of Si3N4 at 1200°C, 1300°C and 1400°C were investigated. Within the stress range of 115–425MPa, the stress exponent, n, was either 1 or 5 depended on the volume fraction of Si3N4. The activation energy of creep for MoSi2-50%Si3N4 composite was 750kJ/mol. At still higher volume fraction of Si3N4, the activation energy decreases to 693kJ/mol, which is the same as that for monolithic Si3N4.