This paper is intended to be a general introduction to this conference and is therefore not a review of the state of our current knowledge. Instead, it will address questions like the following: Why are intermetallic compounds interesting? What alloys are being studied, and which are being ignored? Since most research work is now being performed on L12 alloys, with by far the greatest emphasis on Ni3Al, the balanceof the paper will concentrate on strengthening mechanisms and mechanisms of ductility control in Ni3Al, pointing out the interesting questions and controversies which arose during this conference.

The conclusions to be drawn from this paper are that ordered intermetallic alloys are very valuable materials for high temperature use, but engineers probably must become more sophisticated in the use of materials with limited ductilities at low temperatures before intermetallics will gain widespread usage. Furthermore, additional research needs to be performed on more complex intermetallic compounds than L12 since L12 compounds, as a group, do not have particularly high melting temperatures. However,since alloys with complex structures, e.g. Laves phases, are well known for their brittleness at low temperatures, it is all the more important that the properties of such alloys be studied and methods found to improve their ductilities.

We review some of the advances in the calculation of the electronic structure and energetics of ordered and disordered alloys that hold out the possibility of obtaining, in thenot-too-distant future, an ab initio theory of ordering and phase stability in alloys. In particular, we focus on the calculation of the ground state properties of Ni3Al and discuss the competition between the Li2 and D022 ordered structures. We review the ab initio concentration functional theory of ordering developed by Gyorffy and Stocks and its application to the short-range-ordered solid-solution state in CucPd1-c alloys. Finally, we review the generalized perturbation method approach to calculation of multisite interchange potentials in Ni3Al, Pd3V, and Al3Ti and again discuss L12/D022 competition as well as antiphase boundary energies in Ni3Al.

In elements and intermetallic compounds alike, tensile ductility is favoured by a small spacing of obstacles to dislocation movement, whether these obstacles be grain boundaries or other structural features. The evidence for this generalisation is reviewed.

Antiphase domains (APD) form only when Tt is below the freezing temperature, Tf: this isa rule which has not hitherto been recognized. In Ni-Al, Tt < Tf when the Al content is less than ≈23.5at.%. APD's can serve as heterogeneous nuclei where films of disordered (γ) phase deposit. Domains, especially if decorated by γ films, are associated with enhanced ductility, and there is circumstantial evidence that the ductility is indeed caused by these microstructural features.

Effective-pair-interactions (EPI) are computed for alloys of Al with transition metals, Li, and Zn, using a method in which concentration-independent cluster interactions are resummed to obtain the concentration-dependent EPI. The method includes alloy fluctuations in the interatomic charge transfer, enables one to transcend the muffin-tin approximation and thus treat surfaces and layered structures, and allows the inclusion of lattice strain effects. The calculated EPI have a large magnitude when d-bonding effects are important. For transition metals the EPI are strongly concentration-dependent. In Ni-Al, results for bcc and fcc lattices are similar and exhibit a quick decay of the EPI with interatomic separations. The concentration dependence of the transition metalEPI exhibits rapid oscillations with the number of valence electrons. The concentration-averaged EPI varies less dramatically. The oscillations in the concentration dependence of the EPI are interpreted in terms of the position of the Fermi level relative to peaksand valleys in the one-electron density of states.

We report a systematic atomistic simulation study of [001] symmetric tilt grain boundaries (GB) in Ni3Al, Ni, and Al. We found that the grain boundary energies and cohesive energies of Ni3Al and pure fcc Ni are approximately thesame. Grain boundary energies aid cohesive energies in Ni3Al depends stronglyon the grain boundary composition. The Al-rich boundaries have highest grain boundary energies and lowest cohesive energies. This offers an explanation for the stoichiometric effect on the boron ductilization

The structure and energetics of point defects, surfaces and grain boundaries in Ni3A1 are investigated using the Embedded Atom Method. The approach is shown to reproduce the experimental phase diagram of the Ni-Al system and the elastic properties of Ni3AL. The vacancy and anti-site defect energies are calculated and used to predict the vacancy concentration as a function of bulk composition. The preferred geometries and energies of the low index surfaces are also computed. The equilibrium structure of certain ideal grain boundaries are computed by Monte Carlo computer simulations as a function of bulk composition. It is found that the boundaries act as a sink for anti-sitedefects and the degree of ordering at the boundaries is strongly affected by the bulk composition. The cohesive energy of grain boundaries in Ni3A1 is computed and is found to be comparable to that for pure Ni.

The thermodynamic properties of conservative (111) antiphase boundaries in L12 ordered structures are modeled using the tetrahedron approximation of the cluster variation method. The concentration and long-range order parameter profiles are determined as a function of temperature and composition of the bulk alloy. Characteristic wetting transitions, with a macroscopic disordered layer growing from the antiphase boundary as the transition temperature is approached, are found for all cases investigated. The effectof antiphase boundaries on the disordering of ordered alloys and on the gliding of superdislocations are discussed.

The order-disorder behavior of a Σ = 5 grain boundary was investigated using a two dimensional latice gas model and the cluster variation method. It is found that a disordered layer forms in the grain boundary region at temperatures significantly below theorder—disorder temperature for the bulk. Under certain assumptions for the pair interaction energies the model predicts grain boundary compositions different from the bulk.

APB tubes have been observed in both a B2 and an L12 ordered alloy by means of weak beamtransmission electron microscopy. The tubes are attached to near—edge dislocations, either to single superdislocations or to superdipoles. The majority appearto have been formed by cross slip of screw dislocations. A computer model of the cross slip process in B2 alloys is described. The tubes formed by cross slip drag on edge dislocations and are capable in principle of explaining the extra work hardening shown by ordered over disordered crystals. The temperature and orientation dependence of the work hardening are similar to those of the proof stress which is also thought to be controlled by cross slip. This new mechanism of work hardening is shown to give order of magnitudeagreement with experiment.

Field—ion microscopy and atom probe microanalysis have been used to determine thestructure and chemistry of grain boundaries in ductile, recrystallized Ni3Al containing 0.23 at % B (500 wt.ppm). The results indicate that the boron concentration fluctuates along the boundary plane and that the boron enriched zone is wider than expected from equilibrium—type adsorption. It was also found that boron lowers the aluminum concentration of some of the boundary regions.

The characteristics of perfect grain boundary dislocations in ordered alloys are discussed, and consideration is given to the possible formation of perfect and imperfect grain boundary dislocations by the impingement of crystal lattice dislocations. It is shown that many dislocation reactions at grain boundaries in ordered alloys are made unfavorableif chemical co—ordination must be maintained in the structure of the grain boundary. This leads to a suggestion that the effect of boron in the grain boundaries of Ni3AI is to reduce the importance of chemical ordering, and thus to promote deformation by allowing greater freedom for dislocation reactions to occur.

Atom probe analyses of rapidly solidified, boron-doped Ni-24 at. % Al subjected to various heat treatments have shown boron segregation to a wide range of linear and planar defects. These include dislocations, superlattice intrinsic stacking faults, antiphase boundaries, twin boundaries, low-angle boundaries (dislocation cell walls), and high-angle grain boundaries. Boron coverage of these features was found to vary along a particular linear or planar defect and from boundary to boundary.

Hypostoichiometric Ni3Al alloys of composition 76.2 Ni:23.8 Al containing impurity levels of boron and hafnium (supplied by Oak Ridge National Laboratory) were either cold rolled or pressed. Rolled and pressed samples were deformed by 20% and 10% thickness reductions, respectively. Samples were annealed isochronally at approximately 50ºC intervals up to 1050ºC. Two major annealing stages were apparent in all three alloys studied. These could be attributed to vacancy migration to sinks and annealingof dislocations and(or) recrystallization. The onset of vacancy migration occurred at approximately 200ºC in all three alloys. Annealing of dislocations started at 650ºC to 700ºC and was complete at 1000ºC for alloys which contained boron and or hafnium impurities. In the pure alloy the onset of dislocation annealing occurred at 800ºC and was incomplete at the highest (1050ºC) annealing temperatures reached.

The site occupation of three substitutional elements, hafnium, iron and cobalt, in substoichiometric Ni3Al was determined from atom probe field-ion microscopy. The hafnium was found to have a strong preference for the aluminum sites, the cobalt had a strong preference for the nickel sites, and the iron had a weak preference for the aluminum sites. The atom probe results were in agreement with zone axis electron channeling microanalysis of the same alloys and predictions from the position of the solubility lobes in the ternary phase diagrams.

Using the perturbed angular correlations technique we have investigated the intermetallic alloy Ni76Al24Hf1 in order to determine the lattice location of the Hf in the ordered Ni3Al structure. From the fact that the Hf atoms sense an electric field gradient it is concluded that most of the Hf resides on Ni-sublattice sites.

Experiments have made use of Rutherford backscattering (RBS), ion channeling techniques and analyses for nuclear reaction products to study lattice location of B and Hf dopantsin an ordered nickel aluminide. Studies were of an alloy single crystal of composition Ni76A123Hf1 with about 0.1 at. % of B added. Analysis for B was accomplished by detection of alpha particles resulting from the reaction 11B(p,αi)8 Be, in ion channeling experiments in which RBS from the Ni constituent was used to control channeling in the host lattice. Yield of the reaction product from proton interaction with B decreased relative to random for channeling in a <100> direction, but increased relative to random for channeling in a <110= direction. It was concluded that B occupies primarily octahedral interstitial sites. RBS/channeling half-angles for Hf in a <100> direction are somewhat smaller than those for Ni, but nevertheless considerably larger than half-angles expected for Al. It is concluded that the majority of Hf atoms are on the Ni sublattice.

The effects of cooling rate and composition on the nucleation and growth kinetics of NiAl, Ni2Al3, and NiAl3 intermetallics were studied for alloys containing 25 and 31.5 ht.% Ni. Fgr the former composition, the peritectic regction Ni Al + Liquid → NiAl3 was studied over cooling rates from 20 to 105 K/s. For the latter composhtion, the reaction NiAl + Liquid → Ni2Al3 was studied at cooling rates ranging form 10 to 600 K/sec. The amounis of constituent phases are shown to depend on the cooling rate, and for the peritectic phases, on the surface area of the primary phase. The nucleation of Ni2Al3 and NiAl3, and the ordering of the aluminum-rich NiAl were also examines uding trans ission electron microscopy. Cooling the NiAl + liquid below the peritectic temperature results in a metastable extension of NiAl with a high dislocation and stacking fault density, followed by the epitaxial nucleation and almost dislocation-free growth of Ni2Al3 In contrast, the nucleation of the NiAl3 on Ni2Al3 occurs directly witgoud the formationof an intermediate region.

The microstructural development in a Ti3Al alloy modified with an addition ofil w/o Mo has been studied. Under equilibrium conditions (arcmelted) a two phase ordered hexagonal and ordered bcc structure is present. Evidence of omega phase was alsoobserved. By rapid solidification the hexagonal structure is suppressed, which contrastswith the behavior of a Ti3Al alloy modified with a similar amount of Nb. The mechanism by which the phases present form is discussed in terms of the atomic movements required.

Rapid solidification has an effect on lattice parameters and long&range order parameter in L1o TiAl compound alloys. In these compounds rapid quenching from the liquid state significantly decreases the long—range order parameter while the tetragonal distortion c/a decreases as a result of rapid quenching. The lattice parameter change due to rapid quenching becomes pronounced with increasing aluminum concentration beyond the stoichiometric alloy composition. The recovery from the disordered state and the relaxation from the tetragonal distortion were found to be two different kinetic processes.

The long—range ordering requires low temperature and long annealing time in comparison with high temperature and short annealing time for the recovery of the tetragonal distortion.

The as—extruded microstructures of two alloys in the two phase field consisting of Ni3Al and NiAl in the Ni—Al phase diagram exhibit fibrous morphologyand consist of Ll2 Ni3Al and B2 NiAl. These as—extruded microstructures can be modified dramatically by suitable heat treatments. Martensite plus NiAl or martensite plus Ni3Al microstructures are obtained upon quenching from1523 K. Aging of martensite at 873 K results in the recently identified phase Ni5Al3 whereas aging at 1123 K reverts the microstructures to Ni Al plui NAl. The microstructures with predominantly martensite or Ni5Al3 phases are brittle in tension at room temperature. The latter microstructure does not deformplastically even in compression at room temperature. However, some promise of room temperature tensile ductility is indicated by the Ni3Al plus NiAl phase mixtures.