This paper briefly summarizes recent advances in intermetallic research and development. Ordered intermetallics based on aluminides and silicides possess attractive properties for structural applications at elevated temperatures in hostile environments; however, brittle fracture and poor fracture resistance limit their use as engineering materials in many cases. In recent years, considerable efforts have been devoted to the study of the brittle fracture behavior of intermetallic alloys; as a result, both intrinsic and extrinsic factors governing brittle fracture have been identified. Recent advances in first-principles calculations and atomistic simulations further help us in understanding atomic bonding, dislocation configuration, and alloying effects in intermetallics. The basic understanding has led to the development of nickel, iron, and titanium aluminide alloys with improved mechanical and metallurgical properties for structural use. Industrial interest in ductile intermetallic alloys is high, and several examples of industrial involvement are mentioned.

First-principles quantum mechanical calculations based on local-density-functional theory have been used to investigate the fundamental factors that govern the deformation and fracture behavior of ordered intermetallic alloys. Unlike in Ni3Al, the calculated elastic constants and shear fault energies indicate that anomalous yield strength behavior is not likely to occur in Ni3Si. From the calculated Griffith strength and a phenomenological theory relating fracture toughness to ideal cleavage strength, Ni3Si is predicted to be ductile with respect to cleavage fracture. For TiAl, we find the absence of structural vacancies due to the strong Ti-Al bonding and similar atomic radii for Ti and Al. For NiAl, the defect structure is found to be dominated by two types of defects - monovacancies on the Ni sites and substitutional antisite defects on the Al sites. For FeAl, on the other hand, we find a more complex defect structure, which is closely related to the importance of electronic structure effect in FeAl. More importantly, we predict the strong tendency for vacancy clustering in FeAl due to the large binding energy found for divacancies. Effects of thermomechanical history on microhardness are discussed in terms of the calculated results.

The implications of the basic assumptions of the local pinning theories for the yield stress anomaly in Ll2 alloys are discussed. An alternative theory is presented in which the superpartials on (111) cross-slip on to (010) to form long locks lying partially on (111) and (010) or completely on (010) planes (Kear-Wilsdorf locks). The ends of the locks are joined by glissile superkinks. The yield stress is controlled by superkinks bypassing the screw dislocation locks, and the increase of the yield stress with increasing temperature is due to the decrease of the lengths of superkinks. The theory accounts satisfactorily for the mechanical properties including the small strain-rate dependence of the yield stress and is consistent with electron microscope observations.

The high temperature deformation properties of single crystals of stoichiometric NiAl have been studied in tension creep and in constant strain rate compression at temperatures between 850 and 1200°C. Samples were tested in a “soft”, [223], orientation and the “hard”, [001], orientation. The samples exhibit a strong orientation dependence of the strength and show other revealing deformation characteristics. The activation energy for steady state flow in both hard and soft orientations is near that for lattice self diffusion. Soft oriented crystals reach steady state rapidly and develop little dislocation substructure. Deformation of these soft oriented crystals occurs by the glide of b=<001> dislocations.

The creep curves of hard oriented crystals show pronounced sigmoidal creep, suggesting that the dislocations move in a sluggish manner, multiply in the early stages of creep and, ultimately, lead to strain hardening. Hard oriented crystals also develop extensive dislocation substructure during creep. This dislocation substructure is composed of b=<100> dislocations, which have no resolved shear stress for glide. Evidence for {101}<101> glide in hard oriented crystals is presented and a model is developed by which the decomposition of gliding b=<101> dislocations can produce the observed b=<100> dislocation networks. The increased creep resistance of hard oriented crystals compared to soft oriented crystals is described in terms of the differences in dislocation mobility and substructure formation for deformation in these directions.

Recent studies of fatigue behavior of intermetallic compounds are reviewed. Emphasis is upon fatigue damage leading to crack initiation, effects of environment on crack propagation and behavior of intermetallic matrix composites. Future research directions are suggested.

The difficulties to obtain valid fracture toughness values in brittle intermetallics are discussed. Different intermetallic alloys with the same specimen size were tested in four point bending to allow the direct comparison of the bnttleness of different alloys. The influence ofgrain size, phase distribution, and temperature on the fracture toughness was measured. The fracture toughness of many intermetallic alloys was found to be extremely rate sensitive. This is discussed in view of dynamic models of the brittle/ductile transition developed recently. NiAl single crystals with <100>-specimen axis snowed the largest toughness at room temperature and compared to other orientations the highest transition temperature. The apparent activation energy for the brittle%ductile transition depends on orientation. Evidence was given that in multiphase alloys the yielding of a second phase initiates interfacial fracture. For polycrystals it was shown that different processes can cause brittle/ductile transitions.

A number of techniques are available for making metals, non-metals, and intermetallic materials into high-purity single crystals. The most common of these for producing large crystals involve solidification from the melt. The high melting temperatures of most intermetallics of interest for structural applications result in the expected problems of achieving the required high temperatures and temperature gradients while containing the molten material in a chemically, thermally and mechanically stable environment. Processes which have produced intermetallic single crystals, and the materials which have been crystallized, are reviewed. The largest known single crystals of a high temperature intermetallic have been produced in alloys based on NiAl using a modified Bridgman-type directional solidification process, an evolution of the process commonly used to create large jet engine turbine airfoils in Ni-base superalloys. Issues related to processing are described, and the resultant solidification structures are compared with those typical of superalloys. Finally, the prospects for the various processes, and the advances required to push them toward more practical applications, are addressed.

Alloys based on the ordered intermetallic Nb3Al were fabricated by powder metallurgy processing, infiltration processing and clad-chip extrusion processing. By controlling processing variables, single phase Nb3Al and two phase Nb3Al-Nb solid solution alloys were obtained with various microstructures. The single phase Nb3Al deforms by {001 }<100> slip above 1400K and stress-strain curves exhibit deformation softening after showing a high peak stress. Transmission electron microscopic observations revealed that dislocations are dissociated into two partial dislocations which bound a stacking fault and the dissociation width between the partial dislocations depends on aluminum content in Nb3Al. The deformation softening was found to be caused by dynamic recrystallization. The two phase alloy shows similar stress-strain curves to the single phase alloys. However, sub-boundaries and dislocation networks were observed without recrystallized grains.

A method is presented to predict the oxidative life of intermetallics. The method is demonstrated by predicting the lifetimes of several aluminides undergoing cyclic oxidation at 1200°C. For NiAl and NiAI-Zr alloys, the lifetimes were predicted at several other temperatures as well. Using a critical surface recession failure criterion, it is shown that several aluminides (e.g., NiAl and FeAl) have long-term oxidation resistance (∼ 10,000 hours) to approximately 1150°C. Aluminides containing reactive elements (e.g., NiAI-Zr alloys) have long-term oxidation resistance to approximately 1 200°C. The method is also applied to predict the oxidative lifetime of MoSi2 at temperatures of 1200°-1400°C which shows that the oxidation resistance of MoSi2 is significantly better than that for the aluminides. For the same failure criterion, it is shown that MoSi2 can exhibit long-term oxidation resistance (∼ 10,000 hours) at temperatures in excess of 1400°C. Use of the method to predict the maximum use temperature is also demonstrated for NiAl and NiAI-Zr alloys.

Recent results on the development of Ti3AI base materials, including orthorhombic alloys, are reviewed. Included in this discussion are new data and ideas concerning alloy development, phase equilibria, processing for improved toughness and ductility, environmental degradation, oxidation and corrosion resistance, and coating development. Several new families of alloys have been developed in recent years and the microstruc-ture%processing%property relationships in these alloys are reviewed. It is shown that errors in processing this class of alloy can lead to microstructural instabilities under service conditions. Finally, some immediate needs and some directions for further development are suggested.

Current knowledge of the properties of vacancies and the slip behavior and mechanisms of CoSi2 is reviewed based on the results of our recent studies on CoSi2. The concentration of thermal vacancies in CoSi2 is much higher than that in ordinary metals and alloys with melting points comparable with that of CoSi2. Vacancy defects are easily retained in CoSi2 even after air cooling from high temperatures. An annealing stage observed at around 310 K after electron irradiation is concluded to occur by the migration of vacancies to form secondary defects. Peculiar phenomena recently reported on CoSi2 such as an anisotropy of electrical resistivity and the climbing of dislocations at room temperature can be understood on the basis of the current knowledge of defect properties. Slip in CoSi2 occurs along <100> on {001} at low temperatures. The selection of {001}<100> as the primary slip system in CoSi2 can be interpreted in terms of high covalency of Co-Si bonding. a<100> dislocations have a strong tendency to align along their edge orientation and are dissociated into two a / 2<100> partial dislocations separated by a stacking fault on {001}. {001}<100> slip is augmented by {111}<110> and {110}<110> slip at high temperatures. Thermal activation analysis of deformation indicates that while deformation at low temperatures is controlled by the Peierls mechanism, the greatly increased concentration of thermal vacancies influence the mobility of dislocations at high temperatures.

Extensive research activity, over the last 15 years, has been conducted on structure/property relationships and processing of intermetallic compounds for high temperature use. Progress has been made in improving a number of properties of these compounds; however, the demanding balance of properties required (high strength, good strength retention at temperatures exceeding 1000°C, low density, damage tolerance at ambient temperatures, good creep and stress rupture characteristics and environmental stability at high temperatures) are not likely to be achieved in a monolithic (single phase) compound.

Initial work on intermetallic matrix composites has proven to be quite promising. It has already been shown that compounds, brittle at room temperature, may be toughened by the inclusion of appropriate reinforcements, either strong or tough and ductile. Both artificial and natural or in-situcomposite fabrication techniques have been used to manufacture these composite systems. The properties of two specific intermetallic matrix composite systems, NiAl/Al2O3 and Cr2Nb/Nb are summarized to elucidate their strengths, weaknesses and potential. Candidate composite systems are also discussed along with the rationale behind their selection.

In this paper we present results of a first-principles study of phase stability and structural and thermodynamic properties of fcc- and hcp-based Ti-Al alloys. In particular, the full-potential linear muffin tin orbital method has been used to determine heats of formation and other zero-temperature properties of 9 fcc and 7 hcp ordered superstructures as well as fcc and hep Ti and Al. From these results a set of effective cluster interactions are determined which are used in a cluster variation method calculation of the solid-state portion of the composition-temperature phase diagram for fcc- and hcp-based alloys.

The phase stability of MoSi2 with Cr additions has been investigated in order to explore the issues of ternary solubility and structural stability of MoSi2- The solidification microstructure of MoSi2-rich alloys, along the MoSi2-CrSi2 ternary section, displays a two phase mixture of primary MoSi2 (C11b) and intercellular ternary CrSi2 (C40). The development of the phase equilbria between the C11b and C40 disilicides, as observed in this system, is characteristic of a broad class of intersilicide reactions involving MoSi2. The issues of chemical reactivity and structural stability of MoSi2 composite designs underscores the importance of phase equilibria investigations. The solubility of Cr in annealed MoSi2 was observed to be on the order of 3 atomic percent. Past studies demonstrated that Ti and Ta have limited solution in MoSi2; the minor solubility of Cr in MoSi2 corroborates the trend of limited solubility of transition metals in the MoSi2 (C11b) structure. The relatively small changes in the lattice parameters of MoSi2 with Cr additions point to an inability of the C11b disilicide structure to accommodate the lattice perturbation resulting from solute atoms. The observations of this investigation suggest that the phase stability of MoSi2 is primarily controlled by geometrical factors.

Solid solution hardening is sought and observed in two B2(cP2) CsCl and two C15(cF24) Cu2Mg type structures. New data are presented for the high-temperature compounds AlCo, AlRu, and Cr2Zr; prior data of Livingston on Cu2Mg are analyzed. From lattice parameters and specific gravities, cell occupancy numbers have been measured and used to infer likely defect types and concentrations. The effects of constitutional defects in binary compounds correlate with traditional substitutional solution hardening from elastic interactions due to size differences and modulus differences. The hardening from ternary solutes is similar in magnitude, but the defect structures are complicated and not yet adequately understood.

Establishing the chemical dependence of thermally activated processes which govern plasticity in intermetallic alloys requires that the dislocation dissociation reactions be determined as a function of composition. A major parameter governing such reactions is the relative fault stability as a function of composition. Here the results of first principles electronic structure calculations, using the layer Korringa-Kohn-Rostoker method, are reported for planar faults in γ TiAl at various compositions. The influence of dilute substitutional impurities on the fault energies is treated using the coherent potential approximation. The variation of fault energies as a function of binary composition (TixAl1−x where 52≤x≤49) and the addition of transition metals (Cr, Mn and Nb at 2% concentration) are presented. The influence of this chemical dependence on the stability of <101] super-dislocations is discussed, along with expected trends in the flow stress behavior.

Cubic trialuminides deform by the movement of <110> dislocations which are clearly dissociated as APB superdislocations at high temperatures, with debate about whether these are dissociated as APB or SISF superdislocations at low temperatures. These materials are characterized by the following mechanical behaviour: (i) strength variable with ternary element addition or titanium content; (ii) mild strength anomaly at high temperature, sometimes; (iii) serrations in stress-strain curve at intermediate temperatures; (iv) low tensile ductility (≈0) at room temperature, increasing at higher temperatures, with an intermediate temperature minimum.

These properties are explained by the fine microstructure and its variation locally and with temperature: (a) a tetragonal component of order in ternary alloys in addition to the basic L12 order, varying with ternary element and content; (b) precipitation (sometimes on dislocations) during high temperature testing - accounting for the strength anomaly; (c) extra-ordinarily rapid solute collection at dislocations and APB's - accounting for strain aging and minimum ductility phenomena; (d) strain relaxation at crack tips restricted to single slip planes by the structural modification produced by shear - making a major contribution to brittleness.

All these processes are particularly acute in the titanium trialuminides because of the structural instabilities involved, and the fast kinetics of atom rearrangement; thus low temperatures during testing or during cooling after prior heat treatment play a major role. Similar effects may be of importance in other intermetallics such as FeAl, TiAl.

Equilibrium grain boundary segregation at symmetrical <110> tilt grain boundaries of Ni3AI bicrystals containing 0.17 at% B and traces of S has been studied by Auger electron spectroscopy and compared directly to surface segregation by applying a special specimen geometry. From the experimentally determined interfacial concentrations after annealing at different temperatures, segregation free energies for B and S at both types of interfaces have been determined using the Langmuir-McLean equation. The effect of B and S segregation on the grain boundary cohesive energy has been calculated.

The free energy simulation method and EAM type potentials are employed to study segregation to the Σ5 (310)/[001] tilt grain boundary and its free energy in ordered Ni3−xAl1+x alloys. For 300K≤T≤900K, there is weak Al and Ni segregation to the grain boundary in the stoichiometric and Ni-rich (76.6 at. %) alloys, respectively. In the Al-rich (73.5 at. %) alloy, however, there is strong Al segregation. Al segregation induces the formation of a thin, orderedx AlNi layer at the boundary. The width of this layer grows as the temperature is decreased to approximately 6 Å at 300K. The grain boundary segregation decreases the grain boundary free energy by less than 10% in the stoichiometric and Ni-rich alloys, and by 30% in the Al-rich alloys, respectively. These results suggest a new model to explain the observed brittleness of grain boundaries in polycrystalline Ni3Al. In this model, the brittleness of Ni3Al alloys is attributed to the formation of an intrinsically brittle, thin AlNi layer at grain boundaries. This brittle layer can be removed by increasing the grain boundary Ni content - either by changing the alloy composition or due to cosegregation with B.

Monte Carlo computer simulations with embedded atom method potentials are used to study the structure and energy of symmetric tilt grain boundaries in Ni3Al at high temperatures and compared with those at absolute zero, which is conducted with static relaxation method. The effect of stoichiometry and boron addition on the structure and energy of grain boundary are also investigated. The simulation results show that there exists more compositional disorder at grain boundaries than in the bulk in non-stoichiometric Ni3Al with and without boron. At absolute zero the grain boundary consists of periodically distributed structural units which are distorted with the increasing temperature. The effect of temperature on the structure and energy of grain boundaries in Ni3Al with and without boron was discussed.