Intermetallic compounds have been extensively studied because of their superior strength, low creep rate, and high melting point [1,2]. However, room temperature ductility for the L12 and B2 phases are a continuing problem. Both L12 Ni3Al [3,4] and B2 NiAl [5,6] exhibit an intergranular fracture mode. Understanding grain boundaries in these materials is of particular importance since intergranular fracture limits the applicability of these otherwise promising materials. In an effort to understand the fracture mechanism, we have used embedded atom potentials [7] to study the properties of Ni 3Al [8,9,10] and NiAl [11]. We also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni3Al and NiAl. Many of the results presented here appear in literature elsewhere [8,9,10,11].

Grain boundary regions in B-doped as well as B-free Ni3AI were studied by field-ion microscopy and atom probe microanalysis. In the ductile, recrystallized, Ni-rich alloys the segregation of boron was often accompanied by an enrichment of nickel. Such an enrichment was not observed at boundaries in B-free alloys. Boron was also observed to segregate to the boundaries in a 25.2A1 - IB alloy which was reported to contain boron clusters. Such clusters were not observed, instead a high concentration of boron pairs were found.

Faceted crystals with well-defined {100}, {110}, and {1111 surfaces were observed in the interdiffusion zone during chemical interdiffusion of Ni/Ni 3 Al diffusion couple in the temperature range of 800°−1200°C. Composition of the diffusion-induced crystals was determined to be 87.6Ni-12.4A1 (wt%) by electron microprobe.

The shape of Ni3 Al crystals showed a 26-sided polyhedron (rhombicuboctahedron) with a m3m point group symmetry at low temperature range (800° to 1000°C). At high temperatures, for example at 1212°C, the crystal shape changes to a 14-sided polyhedral form (tetrakaidecahedron) with the same point group symmetry.

Based on the crystal geometry, equations describing the surface energy anisotropy, γ{111}/γ{100}, and γ{100}/γ{100}, were obtained. The surface-energy anisotropy was then calculated from the crystal dimension. For the equilibrium crystals with tetrakaidecahedral form, the inequality γ{110} > γ{110} > γ{111} holds. On the other hand, for crystals of rhombicuboctahedral form, the inequality becomes γ{110} > γ{111} > γ{100}. The experimental values were consistent with theoretical calculations based on the broken-bond model.

Microstructural development in helium gas atomized Ti-50A1-2Nb (at.%) powder has been characterized using x-ray diffractometry and transmission electron microscopy. The structure of the as-atomized powder was a mixture of two phases: an ordered fct (L10)γ TiAl phase and an hcp α Ti phase containing fine (≈ 10–20 nm) D019 α2 domains. Large powder (≈ 250 μm) contained ≈ 14 volume % hcp phase, while fine powder (≈37 μm) contained ≈ 72 volume % hcp phase. However, this α+α2 constituent was metastable and transformed to the γ phase during annealing.

It is only within the last five years that, with the increasing interest of several specialized groups, the amount of accurate TEM information on dislocations in ordered alloys, and in particular in theL12 structure, has become significant. Our knowledge on superdislocation cores related todeformation under various temperature and deformation modes, is now sufficiently documented in order to be critically reviewed and this is the object of the present contribution.

Mechanistic understanding of deformation twinning in ordered superlattice structures is reviewed, and the inter-relationships between twinning and generalized plastic flow or fracture toughness are discussed. While general discussions refer to all the fcc-based and bcc-based cubic and noncubic ordered intermetallic alloys, specific calculations of the energetic and kinetic aspects of deformation twinning are made for TiAl. The importance of the twin-slip conjugate relationship on high temperature mechanical properties is emphasized. Discussion is given of possible effects of macro- and micro-alloying on twinning propensity.

Dislocations in single crystals of Ni3 (Al, Ti) deformed at temperatures above the peak in flow stress have been studied by the TEM “weak-beam” technique. <110> dislocations on the primary cube plane are mostly of edge character, and they have been observed to transform into “super” Lomer- Cottrell locks. <100> dislocations, by contrast, are principally of 45° character. They are believed also to become immobilized by dissociation on {111} planes. Properties of both dislocations on cube planes are discussed and are related to the deformation behaviour of L12 ordered intermetallic alloys.

The anomalous increase in the yield strength of single crystals of Ni3AI(1%Ta) as a function of temperature has been correlated with the post-deformation substructure using weak beam TEM techniques. At low temperatures (77K), there is evidence for abundant cross-slip between (111) and (111). With increasing temperature, this conventional cross-slip process is gradually replaced by the formation of KW locks--straight screw segments which have crossslipped and completely dissociated on (010). This change in the mode of cross slip corresponds with the onset of the yield strength anomaly. At still higher temperatures (544K and 715K), the KW locks become mobile and bow out on (010). The density of dislocations on the cube plane also increases sharply at higher temperatures, even though deformation occurs principally by glide on the primary (111) based on slip trace analysis. These observations are inconsistent with the widely accepted cross slip pinning model and suggest that deformation occurs primarily by the motion of the non-screw components. A dislocation model is introduced which attempts to account for the observed dislocation configurations while remaining consistent with the yield strength and work hardening behavior of these alloys.

Prestrain of bcc metals at temperatures T>0.2Tm decreases the flow stress at lower temperatures (T<O.15Tm) where the yield strength has a large negative temperature dependence. This investigation has examined the influence of prestrain on the flow stress of Ni3A1, for which the yield strength has a large positive temperature dependence above 25°C. Nickel-rich Ni3Al single crystals with axial orientations near <001> or <123> were prestrained in compression up to 20% shear strain at -196°C and subsequently compression tested at 550°C. Specimens near the <123> axial orientation were also prestrained at 550°C and then tested at -196°C. The initial flow stress of samples prestrained at - 196°C and tested 550°C was reduced up to 50% compared to samples solely compression tested at 550°C. The magnitude of the reduced flow stress and its extent as a function of plastic strain were dependent upon the amount of prestrain and orientation. Prestraining at 550°C and subsequent testing at -196°C increased the flow stress by as much as 60% over samples solely tested at -196°C. Dislocation substructures obtained from selected samples coupled with arguments based on dislocation dynamics and obstacle strengthening are used to explain the results.

The deformation behavior of Ni3Al alloys with various chemical compositions and subjected to different heat treatments were in-situ observed in SEN. Moreover, in-situ observations of slip trace are supplemented by the direct observation of dislocation arrangements by TEM. In boron-doped Ni3Al alloys it is shown that close to the grain boundary there exists a thin slip transition region, within which slip lines are reoriented or other slip systems are operated to produce a local strain accommodation and to relax the stress concentration at grain boundaries. However, boron-enhenced ductility is seriously affected by alloy stoichiometry, the addition of a tertiary alloy element and heat treatment, etc.

An effort to modify the slip vector in the B2 compound NiAl was undertaken to overcome the brittle failure associated with the <100> slip vector which typically operates in this compound. Alloying additions were made to reduce the ordering energy of NiAl, and hence to promote <111> slip. Preliminary indications showed that Cr and Mn were effective in producing <111> slip in polycrystalline NiAl tested at room temperature. Two-surface slip trace analyses and TEM g·b analyses performed on aligned crystals of NiAl showed that <111> dislocations operate on either { 110} or {112} slip planes. However, the slip vector at 660°C was seen to revert to <100>, which operates on both {100} and {110} planes.

The compression deformation behavior of single crystalline TiAl was examined by transmission electron microscopy (T.E.M.). The relatively pure Ti–56 a/o Al crystal was containerless processed in ultrapure hydrogen. The crystal growth direction is 18° off (011) and 60° off [111] in the [011]– [111]-[010] unit triangle. At low stresses, a/2 [110] type dislocations were observed. a/2 [110] dislocations appeared at slightly higher stresses. Additional plastic deformation initiates twinning. Twinning plays an important role at higher stresses. Diffraction results indicate that most of the twins have the (111) mirror plane.

A small amount of (111) twins were also observed. Superdislocations of the a<011> type were not observed to contribute to the plastic deformation in this crystal. The results indicate that plastic deformation by twinning follows the low density of ordinary dislocations.

TiAl (γ) intermetallic has been investigated with ternary additions of up to 10 at% Ga. Optical microscopy reveals that gallium can be retained in solution to at least 7 at%. Microhardness data reveals that Ga substituted for Al such that Ti/(Al+Ga)=l causes solution softening. Deformation behavior of the single phase alloy in compression is reported. Perfect dislocations of the a/2[110] type and superdislocations of the a[011] type and a/2[112] type have been observed. TEM results indicate that primary deformation is contributed by a/2[110] type dislocations and twinning on (111) planes with wavy slip optically observed

Prototype Laves Phases MgCu2 and MgZn2 have been deformed in compression at various temperatures and strain rates, and dislocations produced by deformation have been characterized by TEM. Ternary cubic Laves phases Mg(Cu,M)2, where M-Al, Ni, Si, and Zn, have also been deformed, and substantial solution hardening was observed

The ordered Cu2NiZn alloy is virtually unique in showing a changing antiphase domain structure according to annealing temperature. At low annealing temperatures there are isotropic domains and at high annealing temperatures there are cube-oriented domains characteristic of highly anisotropic boundary energies. This property allows a comparison of dislocation behaviour for otherwise identical materials: the roles of cube crossslip and cube glide can then be distinguished from other mechanisms, for example elastic torque effects and dipole formation. Mechanical properties and dislocation structures are examined over a range of test temperatures. The material having cube-oriented domain boundaries shows a much higher work hardening rate over an intermediate temperature range. Strengthening at low temperatures is dominated by dipole and multipole formation, whilst deformation above ambient temperature leads to the appearance of non-octahedral glide, more frequently on planes near {110} than near {001}. For the material with cube-oriented domain boundaries there is some evidence to suggest point blockage by cross-slip onto cube planes, leading to the higher work hardening rate at the intermediate temperature. This study emphasizes the multiplicity of slip systems which may operate and discusses the relative importance of anisotropic antiphase boundary energies and anisotropic elasticity effects.

A modified fatigue test matrix that provided both creep (high mean stress) and fatigue (alternating stress) damage was used to evaluate the creep-fatigue behavior of D.S. Ni3 Al(B,Hf). The three generic regimes of below, at, and above peak yield strength temperature were investigated. It was found that the creep-fatigue resistance of this intermetallic remains high through the three temperature regimes, allowing it to be further considered as a monolith or as the matrix of high-temperature fiber reinforced composite.

Transmission electron microscopy of single-slip oriented Ni3Al single crystals cyclically deformed at room temperature revealed a high density of dislocation dipoles and point defect clusters. The majority of the defect clusters were identified by weak-beam TEM to be of vacancy type. The observations of circular perfect dislocation loops, Frank loops, and spherical voids revealed the evidence of vacancy condensation during fatigue cycling at room temperature. Together with SEM observations of surface topography, it is suggested that fatigue crack initiation in Ni3Al single crystals can be rationalized by formation of microvoids at PSB/matrix interfaces.

The creep behavior of single crystals of γ′ and γ alloys were investigated and compared to the response of two phase superalloys tested previously. High temperature deformation in the γ alloys was characteristic of a climb controlled mechanism, whereas the γ′ based materials exhibited glide controlled creep behavior. The superalloys were much more creep resistant than their constituent phases, which indicates the importance of the γ-γ′ interface as a barrier for dislocation motion during creep.

Ni-Al-Cr single crystals with 4 at% Cr and different levels of aluminum content are grown in a directional solidification facility. The crystals are creep tested at temperatures from 760°C to 860°C. Steady state, as well as transient, creep rates are measured. The results show that Cr modified Ni3 A1, when tested along the <100> direction, exhibit remarkably high values of activation energy for creep. Along with evidence from dislocation structures, it is likely that there is a change in the dislocation core structure upon Cr addition.

This paper reports a study of grain boundary segregation and fracture modes in Ll2 intermetallic compounds. Data obtained on Ni3A1, Ni3Si, Ni3Ga, Ni3Ge, and Pt3Ga will be presented. It will be shown that the amount of boron segregation and its ability to improve cohesion depends on the total composition of the compound. The beneficial effects of boron can be counteracted by the presence of borides on the grain boundaries. Carbon additions also produce some improvement in ductility in Ni3Si.