The fracture and fatigue crack growth characteristics of niobium silicide/Nb(ss) in-situ composites as well as a variety of Nb alloys were determined over a range of temperatures and loading rates. It is shown that the in-situ composites exhibited nominally rate-independent and temperatureindependent values for the toughness with an average toughness exceeding 24 MPa.m0.5. The fatigue behavior of both the Nb alloys and in-situ composites revealed that both the Paris Law exponents and fatigue threshold values were similar to that exhibited by metallic materials.

Single crystals of [001]-oriented NiAI single crystals were subjected to thermal fatigue by a method which employs induction heating of disk-shaped specimens heated in an argon atmosphere. Several time-temperature heating and cooling profiles were used to produce different thermal strain histories in specimens cycled between 973 K and 1473 K. After thermal cycling, pronounced shape changes in the form of diametrical elongations along <100> directions with accompanying increases in thickness at and near the <100> specimen axes were observed. The deformations were analyzed in terms of operative slip systems in tension and compression, ratchetting (cyclic strain accumulation), and the elastic properties of NiAl. The experimental results correlate best with thermal stresses associated with the large elastic anisotropy of NiAl.

Properties of point defects such as antisites and vacancies in Ni3A1 are studied by means of ab initio calculations for supercells. Temperature dependent quantitities such as defect formation energies are derived by means of a grandcanonical ensemble. Stimulated by experiments of residual resistivities suggesting an outstandingly large activation energy of 4.6 eV due to Al vacancies, several models for point like defects are treated in combination with calculated migration barriers for nearest neighbor jumps and also the six-jump model.

Crystals of γ-TiAl, containing ∼54.5 at% Al, with various orientations were deformed at different temperatures and examined by transmission electron microscopy (TEM). It was found that while in many cases slip and climb of ordinary dislocations tend to predominate, slip by [001] dislocations can be important also. The occurrence of [001] slip has not been reported before. The values of critical resolved shear stress were determined and strong locks on [001] dislocations have been identified as local segments of ½ ⟨112] dislocations arising from interactions between [001] and ½⟨110] dislocations.

Self-diffusion of57Co and95Nb in the cubic Laves phase NbCo2 has been studied using the radiotracer method. The temperature dependences of the self-diffusivities are described for Nb31Co69 by

,

and for Nb29Co71 by

,

The diffusivity of 95Nb in Nb31Co69 is considerably lower than that of 57Co. The latter increases significantly with increasing Co content.

The results are discussed in terms of vacancy mediated diffusion in connection with structural aspects of the Laves phase together with the scarce information available for defects in NbCo2. The results are compared with interdiffusion data from our laboratory and with self-diffusion in the also close-packed L12 intermetallics.

A transmission and scanning electron microscope investigation is undertaken to study microstructural development during transient liquid phase (TLP) bonding of NiA1 to Ni-base substrates. The bonds were produced through a conventional technique employing a Cu foil interlayer or a widegap technique using a composite preform containing powders of NiAl and Cu. The time required for completion of isothermal solidification was greatly reduced in wide-gap bonds as compared to conventional bonds. Microstructural features of conventional TLP bonds of polycrystalline-NiAl/Ni were controlled by the ratio of Al: Cu across the joint. The precipitation of the a phase encountered in polycrystalline- NiAl/Martin Marietta 247 superalloy (MM247) bonds was suppressed in wide-gap bonds of single crystal-NiAl(Hf) and MM247. In general, the extent of second phase precipitation, in the asbonded condition, was greatly reduced by the use of the wide-gap technique. However, extensive precipitation of HfC and W-rich phases was observed after post-bond heat treatments.

The temperature as well as orientation dependence in anomalous hardening occurs in single crystal Ti-56AI between 673K and 1073K under single slip of ordinary dislocations. The ordinary dislocations (1/2<110]) are gliding not only on (111) plane but also on (110) plane in the temperature range where the anomalous hardening occurs in single crystal Ti-56A1. The TEM study shows that the (110) cross-slip of ordinary dislocations is a double cross-slip in nature in which first, the dislocations cross-slip from the primary (111) slip plane to (110) plane followed by cross-slipping again onto another primary slip plane. This double cross-slip leaves a pair of edge segments 'superjogs' in (110) planes. It appears that these superjogs are immobile in the forward direction and act as pinning points. Furthermore, these pinning points would act as a Frank-Read source for the double cross-slipped dislocations, which generate dislocation loops as well as dislocation dipoles. The pinning structure, multiplane dislocation loops, and dipoles of double cross-slip origin all contribute to anomalous hardening at high temperatures in this material.

Scanning Transmission Electron Microscope (STEM) Energy Dispersive X-Ray analysis (EDX) linescans and mapping have been used to examine the large angle grain boundary chemistry of Ni-rich Ni3AI both with and without boron. The results show that the Al content is reduced while the Ni content is unchanged at the grain boundaries in all these alloys, with the percentage of reduction of Al at the grain boundaries decreasing as boron concentration increases. This is ascribed to differential surface sputtering of the lighter Al atoms.

The microstructure evolution involving Mo-B-Si as-solidified alloys with compositions in the Mo solid-solution(ss) + T2 two-phase field has been examined following arc-melting and rapid solidification processing (RSP). Several solidification pathways in the arc-melted alloys have been identified. Compositional segregation during conventional solidification results in the formation of additional phases such as borides in the arc-melted alloys which require a prolonged solid-state annealing to obtain equilibrated two-phase microstructures. The RSP methods employed, splat-quenching (SQ) and powder drop tube processing (DTP), allow for significant microstructural modifications that facilitate the attainment of uniform dispersions of Mo(ss) phase in a T2 phase matrix.

Polycrystals of binary Ni3Al with different compositions are deformed in compression between 300K and 1300K. They exhibit a strength anomaly with peak temperatures between 800K and 950K. Below the peak, the aluminium rich compounds are the strongest ones while the nickel rich compounds are the weakest ones. As the aluminium content increases, the complex stacking fault energy, measured through weak beam experiments, increases as well. This confirms the cross slip mechanism of mobile dislocation exhaustion through Kear-Wilsdorf lock formation. A ‘ductile-to-brittle transition’ is observed at 1000K. Fracture occurs along grain boundaries, the strength of which decreases as environmental effects are enhanced at high temperatures.

The microstructure and plastic deformation behavior in Ti-62.5at.%Al single crystals were investigated focusing on effect of the formation of h- and r-AI2Ti precipitates. Crystal structure, morphology and volume fraction of the precipitates depended strongly on the annealing temperature. The r-A12Ti phase precipitated as a plate in the Li0 matrix containing A15Ti3 superstructure below 1200°C, while the h-Al2Ti phase appeared at the higher temperature. The r-A12Ti had six-fold periodicity along the c-axis of the L10 structure and formed a semi-coherent boundary with the matrix maintaining an orientation relationship with {101)r-A12Ti//{101)L10 In contrast, small particles of h-A12Ti with three-fold periodicity along the b-axis were homogeneously distributed in the matrix forming an obscure coherent phase boundary.

Yield stress for specimens with a [201] loading axis anomaly increased showing a maximum peak around 800°C and then rapidly decreased. [111] <110] slip was dominant in the Li0 matrix and research-article-Al2Ti precipitates at temperatures below the peak, while slip occurred on (001) above the peak temperature. The anomalous strengthening and the change in operative slip system were discussed focusing on the effect of anisotoropy of APB energy and formation of Al2Ti precipitates.

Diffusion bonded bi-PST crystals of three different series were prepared and they were deformed in tension at room temperature. Yield stress and elongation exhibited by bi-PST crystals consisting of component crystals with the lamellar microstructure aligned along the tensile axis do not significantly differ from those of component crystals. Plastic strain incompatibility at the interface exerts a strong influence on the deformation behavior of bi-PST crystals when the incompatibility activates additional deformation modes which are much harder than deformation modes operative in each component crystal.

Gamma titanium aluminides class of materials possess several unique physical and mechanical properties. These characteristics can be attractive for specific industrial applications. By applying different heat treatment schedules one can change the microstructural features of this class of materials. In the present investigation, two heat treatment schedules were used to produce two different microstructures, duplex (D) and nearly lamellar (NL) in the cast and HIP'ed Ti-47Al-2W- 0.5Si alloy.

The tensile strength and creep behaviour, in the 700–850°C temperature range, of this alloy have been determined and correlated to the corresponding microstructures. In addition, the fatigue crack propagation behaviour in this alloy has been studied at different temperatures. The results on the creep behaviour showed that the alloy with nearly lamellar microstructure has a strongly improved creep strength as compared with that of the duplex microstructure.

The fragmentation and spheroidization of α2 laths in a fully-lamellar TiAl alloy during creep were examined. Three possible mechanisms, Rayleigh's perturbation model, subgrain boundary groove mechanism and intersection of deformation twins with α2 lamellae were presented and discussed. During creep deformation, the pile-up of interfacial dislocations leads to a change of planar interface, which, in turn, causes a difference in local chemical potential, and further results in the spheroidization of α2 lamellae. On the other hand, the deformation of the α2 phase is expected to be induced by the high local stress concentration introduced by the pile up of interfacial dislocations. The dynamic recovery process may lead to the formation of subgrain boundaries in the α2 lamellae, which results in the spheroidization and termination of α2 lamellae with the aid of diffusion during creep.

Perturbed angular correlation of gamma rays was applied to determine properties of equilibrium defects in B2 NiAl near the stoichiometric composition. Point defects were detected through quadrupole interactions they induce at In probe atoms on the Al sublattice. Well-resolved signals were observed for probe atoms having zero, one or two Ni-vacancies (VNi) in the first neighbor shell. The fractions of probes in different sites are analyzed using a thermodynamic model to determine defect properties as follows. The equilibrium high-temperature defect is determined to be the triple defect combination (two VNi and one Ni-antisite atom) through the variation of the vacancy concentration with composition and not, for example, the Schottky vacancy pair. The binding enthalpy of VNi with a probe atom was determined to be in the range 0.18–0.24 eV. Site fractions were measured for three samples having 50.03, 50.14 and 50.91 at.% Ni at temperatures up to 1300 C. Vacancy concentrations were deduced from the site fractions and binding enthalpy. The equilibrium constant for formation of the triple defect was determined as a function of temperature from the vacancy concentrations and sample compositions. The formation enthalpy was found to be in the range 1.65–1.83 eV, depending on the binding enthalpy. The formation entropy was found to be -3.2(4) kB. The large, negative value of the formation entropy probably cannot be explained in terms of a binding entropy, and we speculate that triple defects harden the B2 lattice, perhaps by disrupting the well-known 1/3 <111> ‘soft mode’ lattice instability in B2 and bcc materials.

In-situ composites based on Nb-Si alloys are potential candidates for application as structural materials in future aircraft engines. In order to achieve the balance of properties that is required, additional alloying species, such as Ti, Hf, Mo, Cr, Al, are employed. The present paper describes the effect of Hf additions on phase stability of Nb-Si alloys. Liquid-solid and solid-state phase equilibria have been investigated. The Nb-Hf-Si liquidus surface and the isothermal section at 1500°C will be described. A broad range of Nb and Hf compositions were investigated for Si concentrations up to 35.0%.

Multiphase Mo silicide alloys containing T2 (Mo5SiB2), Mo3Si and Mo phases were prepared by both melting & casting (M&C) and powder metallurgical (PM) processes. Glassy phases are observed in PM materials but not in M&C materials. Microstructural studies indicate that the primary phase is Mo-rich solid solution in alloys containing ≤(9.4Si+13.8B, at. %) and T2 in alloys with ≥(9.8Si+14.6B). An eutectic composition is estimated to be close to Mo–9.6Si–14.2B. The mechanical properties of multiphase silicide alloys were determined by hardness, tensile and bending tests at room temperature. The multiphase alloy MSB-18 (Mo–9.4Si–13.8B) possesses a flexure strength distinctly higher than that of MoSi2 and other Mo5Si3 silicide alloys containing no Mo particles. Also, MSB-18 is tougher than MoSi2 by a factor of 4.

Stacking faults at order interfaces in TiAl have been examined by HREM. In addition to very extensive OCISF formation, C-core dislocation complexes spread over two parallel { 111 } planes have been observed at order interfaces lying predominantly parallel to the original (0002)α, planes. Such C-cores, corresponding to 1/2<211] and 1/2<112] interfacial dislocations, have been observed with localised regions of both extrinsic and hexagonal stacking.

The <1126>(1121) pyramidal slip in Ti3Al shows the so-called yield stress anomaly. A marked change in dislocation configuration is observed in the temperature range concerned; dislocations observed at the lowest temperature of the range, −70°C, are ordinary pairs of edge c+a/2 superpartial dislocations coupled by antiphase boundaries (APBs) while at the other extreme temperature 700°C, pairs of unlike superpartials coupled by APBs are observed. The change in dislocation configurations is interpreted as resulting from immobilization of edge c+a/2 superpartial segments by climb dissociation onto the basal plane.

The defect character of steps on lamellar γ/α2 interfaces in a quinternary TiAl-based alloy has been studied using high resolution transmission electron microscopy. The interfaces consisted of atomically flat coherent terraces separated by interfacial steps across equal even numbers of {111}γ and (0002)α2 planes. Circuit mapping was used to identify the Burgers vectors of these steps from lattice images obtained at [101]γ and [110]γ zone axes. It was found that the Burgers vectors exhibited by the two-layer steps are different from those reported previously and are consistent with those expected for perfect disconnections as described by Pond's topological theory of interfacial defects, and not with the usual partial dislocation model.