The solid solubilities of 15 common alloying elements in the L12-type intermetallic compound Ni3Si have been determined at 900°C, and their substitution mode has been deduced from the direction of solubility lobe of the compound. It is shown that the alloying behavior in Ni3Si is determined by both size and electronic factors, i.e, the substitution mode is governed by electronic configuration and solubility by the both. The solubilities of alloying elements can be explained on the two dimensional map proposed by Alonso and Simozar.

Four isotherms of Ni-Si-X (X = Fe,Cr, Co,Cu) have been obtained. They are very similar in structure.

Liquid-solid and solid state phase equilibria in the ternary system Ti-Al-V have been studied using a combination of several experimental techniques. A likely surface of primary seperation (i.e., the liquidus surface) is proposed in the form of the usual projection on the triangular base and the directions of the monovariant lines are defined. Four ternary invariant reactions have been identified in this system. Solid state equilibria have been determined at 900°C and are presented in the form of an isothermal section through the phase diagram. These are very similar to the relationships reported at 800°C by Hashimoto et al. [1].

The main features of the phase transformations occurring in alloys based on Ti-48 at% Al are described in order to assess their roles in microstructural development during thermomechanical processing and subsequent heat treatment. It is shown that the presence of Al-rich interdendritic γ formed during solidification plays a critical role during these processes.

Forging in the α2+γ-phase field results in grain refined zones by dynamic recrystallization, but still coarse γ-grains and a residual amount of α2/γ-lamellae are observed. During subsequent heat treatment in the α+γ-phase field the recrystallization leads to inhomogeneous distributions of α2-Ti3Al and grain sizes of γ-TiAl. Due to these inhomogeneities a significant increase of the room temperature ductility is not found.

An investigation of the phase equilibria and phase transformation behavior in the Al-Ti-Ta system was conducted with an emphasis on developing alloys with stable microstructures and properties desirable for advanced aerospace applications. A series of ternary alloys containing 5 to 50 atomic percent Ta and varying amounts of Ti and Al were prepared by arc melting and characterized in the as-cast and heat treated (1450°C) conditions. Three stable two-phase regions (β-σ, β-α2. and Al2 Ta-η) were identified and characterized at 1450°C. A stable ternary B2 phase was identified near the composition 25Al-5OTi-25Ta1 and several binary phases were found to exhibit extensive solid solubilities. The results were used to construct a partial isothermal section at 1450°C for this system and to assess the complex phase transformations that occur upon cooling.

The phase equilibria developed upon substitution of Ti in MoSi2 along the pseudobinary MoSi2-TiSi2 section has been investigated using x-ray diffraction, and electron microscopy. The pseudobinary exhibits orthorhombic (C54), hexagonal (C40), and tetragonal (Cllb) crystalline structures. A hexagonal (C40) ternary silicide (Ti1-xMox)Si2 was observed to exhibit a relatively wide range of solubility with x in the range of 0.50 to 0.75, in contrast to MoSi2 which exhibits limited solubility for Ti. An analysis of the microstructural evolution following solidification reveals that the hexagonal β-(Ti,Mo)Si2 phase forms via a peritectic reaction between liquid and MoSi2 (Cllb). The primary dendritic structure during solidification in (Til-xMo2)Si2, where x z 0.80, has been determined to be tetragonal (Cllb) MoSi2. The results of this study confirm that substitutional alloying on the transition metal sublattice (Mo) is an effective approach to control of the phase stability and crystal structure in intersilicide reactions.

Amorphous structures or metastable crystalline phases are produced in sputter deposited Be-Nb alloys (5–15 at.% Nb) depending on the substrate temperature. The metastable phases transform to the stable Be12Nb, Be17Nb2 or Be3Nb phases on annealing at temperatures >800°C. No Be5Nb phase was found and the Be17Nb2 phase is stable to low temperature. The Be12Nb phase appeared to have a stoichiometric range of about 5.5 to 7.7 at.% Nb. The formation of the metastable phases is consistent with current models and theories.

The results of computer simulations on the effect of elastic energy on APB anisotropy are presented. The model used treats composition and order parameter as local variables defined at crystal planes parallel to the APB. The set that minimizes the free energy, including elastic energy terms, defines the equilibrium APB structure. It was found that, despite their high elastic anisotropy, elastic effects would not result in any substantial APB anisotropy in Fe-Al alloys. However, a model with different parameters did show a high degree of anisotropy.

The transformation from short-range↔long-range order in Haynes® Alloy 242, a nominal Ni-25Mo-8Cr (in wt.%) alloy, during both continuous heating and cooling and isothermal holding at temperatures between 550 and 750°C is reported using dilatometry and microhardness measurements, and optical and transmission electron microscopy The long-range to short-range order transformation temperature was observed to increase with an increase in the degree of prior order. Aging below the transformation temperature led to considerable hardening; this hardening was associated with the formation of a very high volume fraction of Ni2Mo domains/precipitates. On aging at 700°C and below, the hardness initially increases and then levels off, with no evidence of overaging for times to 2000 hours. The role of both short- and long-range order on the transformation temperature, thermal expansion and hardening behavior and the mechanisms of transformation from short-range to long-range order were established and will be discussed.

It is now generally accepted that in many L12 compounds the yielding behavior is controlled by the special features of the cores of screw dislocations. In this paper we first summarize results of the atomistic studies of the core structures of the <110> screw dislocations in these compounds. At this point we show that, depending on the atomic bonding, two distinct classes of L12 alloys exist. In the first, represented by alloys like Ni3Al, a glissile configuration of the core exists on {111} planes although a sessile configuration is energetically more favored. In the second class, represented by alloys like Pt3A1 and A13Ti modified by alloying into L12 structure, the cores of screw dislocations are always sessile. Using the results of the atomistic studies we present physical models explaining the temperature dependences of the yield stress in both classes of L12 alloys. At this point we also present a further development of the model for the anomalous temperature dependence of the yield stress in alloys like Ni3A1, originally put forward by Paidar et al. [15]. In this development strain rate effects are included and it is shown that the model explains not only the orientation dependences of the yield stress in the anomalous regime but also the very low strain rate sensitivity observed in this regime.

The strain, strain rate and temperature dependencies of the yield stresses of the model L12 materials Ni3Al, Ni3Ga and Co3Ti are described, as well as two associated properties, the initial work-hardening rate and the inverted creep. These mechanical properties, the electron microscope observations of <110> {111} APB coupled slip and the violations of Schmid's laws point to glissile-sessile-glissile transitions by cross slip of screw dislocations as the explanation for the yield stress anomaly. Similar explanations are probable for h.c.p. Be, B2 CoTi and tetragonal TiAl and MoSi2. A different kind of model, based on the behavior of edge dislocations is required for h.c.p. Mg, B2 CuZn and DO19 Ti3Al.

Dislocation core structures have been calculated using atomistic computer simulations in NiAl and other B2 compounds. In the present work the calculated dislocation core structure are correlated with the known deforniatiorn behavior of B2 alloys. It is found that for the high ordering energy compounds <111> dislocations do not split in the simulations, in agreement with the experimental observations. It is also found that core structures for certain <111> and 1/2 <111> dislocations are spread in { 112} planes, which is consistent with the slip plane often reported for these dislocations. For the < 100> dislocations several orientations of the dislocation line produce sessile core configurations, whereas other orientations produce relatively more glissile cores. However, a structural transition of each of these dislocation cores may be required before < 100> dislocations become mobile, and this is consistent with the limited tensile ductility observed in NiAl “soft” single crystals below 200°C. Core structure simulations for < 110> dislocations are also reported and are discussed with respect to the importance of these dislocations in the deformation of NiAl.

We investigate the structures of dislocation cores in a model DO22 type intermetallic alloy in the absence of external stresses. The tetragonal distortion produces sessile configurations of superpartial dislocations even when the energies of planar faults are reasonably low. The influence of the core configurations on the mechanical behavior at low temperatures is discussed.

Current models for predicting the unusual deformation characteristics of Ni3Al alloys are based upon the idea of cross-slip pinning of screw dislocation segments. These models assume an Arrhenius type framework which employs a stress-modified activation barrier for forward cross slip. Use of such models has been successful in describing the orientation, asymmetry and temperature characteristics of yielding. A limitation of current models is their inability to predict the apparent lack of a strain rate effect in Ni3Al alloys. The current model approaches the problem using both kinetic and thermodynamic arguments to describe the physical process. It is then demonstrated that the driving force for cross-slip controls the process. The model describes all aspects of deformation including apparent strain rate insensitivity. An additional feature of the model is its ability to provide analytical and physical justification for the use of Arrhenius type equations for representing experimental data.

Recent advances in the modelling of dislocation nucleation from a crack tip are used here to compare the critical energy release rate associated with emission in Ni3Al and Ni. The method for analyzing nucleation makes use of a Peierls-type stress versus displacement relation ahead of the crack tip. It has been shown recently by Rice [1] that the energy release rate for emission scales with γus, the “unstable stacking” energy associated with the sliding of atomic planes past one another. The advantage of this approach is that it allows for an extended dislocation core during nucleation, thus eliminating the need for a core cutoff radius. Preliminary calculations which take into account only the shear stress on a slip plane show that it is more difficult to emit a dislocation in Ni3Al than in Ni. Working within the framework of the competition between atomic decohesion and blunting by dislocation emission, the implications for explaining the brittleness of Ni3Al are also discussed.

In molecular dynamics studies of 10,000 atom arrays of stoichiometric B2 NiAl containing a crack under external stress in Mode I loading, it has been observed that a martensitic transformation generally occurs (starting in the vicinity of the crack tip) prior to the generation of dislocations and/or the propagation of the crack. The martensitic phase appears to be 2H, in agreement with experimental observations of SIM at higher Ni compositions (62 at % Ni). The interatomic interactions used in the simulations were the Embedded Atom Method (EAM) potentials developed by Voter and Chen [1] for the Ni-Al alloy system.

A detailed study of deformation of NiAl single crystals in two soft orientations, <110> and <111>, has been conducted. The Schmid factor favors {100} slip in the former and {110} slip in the latter. Detailed dislocation analysis, critical resolved shear stress measurements, and slip trace analysis have been performed to determine the nature of dislocation motion and interactions in this material. Particular attention is given to prismatic loops formed during deformation, since the shapes of these loops reveal the active slip planes. Similar loop morphologies observed in elevated temperature [001] oriented tensile specimens are also discussed.

The present work was undertaken to understand the transition in slip direction In [001] oriented NiAl as a function of temperature. Single crystals of [001] NiAl were deformed at room temperature to produce <111> dislocations and were subsequently annealed at various temperatures in order to see the effect of temperature on the stability of the <111> dislocation. These results were compared with corresponding dislocation substructures of specimens deformed directly at high temperatures. During annealing <111> dislocations are dissociated into <001>+<110> dislocations at 773–923 K. It is also observed that <111> dislocations can be dissociated into <001>+<110> dislocations during deformation in the vicinity of the transition temperature of around 773 K. The quantitative details of these changes depend on deformation or annealing temperatures.

Polycrystalline NiAl exhibits <001> slip at temperatures below about 450°C. In Fe-Al alloys there is a transition in the Burgers vector from a/2<111> at low temperatures to a<001> at intermediate temperatures. The transition temperature is dependent upon composition.

A transition in the Burgers vector from a/2<111> to a<001> has been found to exist as a function of composition and temperature in the B2 pseudo-binary (Fe,Ni)-40Al (at%) system. At room temperature, this transition occurs at a composition between Fe-40Al-10Ni, which exhibits <111> slip, and Fe-40Al-3ONi, which exhibits <001> slip. At 400°C this transition shifts further toward the iron-rich compositions. Fe-40Al-10Ni exhibits both <111> and <001> slip, whereas Fe-40Al-20Ni exhibits <001> slip.

B2-type CoTi single crystals which exhibit the yield strength anomaly were deformed at various temperatures and on various compression axis in order to investigate the deformation microstructures. The morphological feature and Burgers vectors of the activated dislocations were investigated by means of transmission electron microscopy. At a low temperature of 300 K, relatively straight dislocations with the <001>-type Burgers vector were observed. They consisted of the mixed components of edge and screw, and strongly tended to form the dipoles. At a temperature of 773 K where the yield stress increases with increasing temperature, the screw components of dislocations with a <001>-type Burgers vector were dominant and showed peculiar morphology revealing the pinning or cross slip. However, the examination using the weak-beam method could not show the evidence of any dissociation. At a high temperature of 973 K above the peak temperature, the Burgers vectors of activated dislocations were determined to be a <001>- type for compressive orientation axes close to [111] and [011] whereas a <110>-type for orientation axis close to [001]. These dislocation microstructures were discussed in correlation with the yield strength anomaly observed in these intermetallics.