Interatomic potentials of the embedded atom type were developed for the Ni-AI system by emplncal fitting to the properties of BZ NiAl and NiiAI,. Consideration was dsa given to the properties of L12 NijAl as well as the martensitic Lla phase.The B? phase is predicted a the stable phase for the qui-atomic composition. The potentials also predict the stability of the 3R martensitic structure with respect to the 8 2 phase for 62.5% Ni alloys. The globally stable phase for this composition is the NiSAl3 structure. The predicted lattice panmeters and tetragonality ratios for NiSAlj and 3R martensite are "er)' close to experimental valuesThe strncture and energy of various defecls was calculated using the new potentials and the results compared with those given by other potentials in the literamre.

We performed embedded atom method calculations of surface energies and unstable stacking fault energies for a series of intermetallics for which interatomic potentials of the embedded atom type have recently been developed. These results were analyzed and applied to the prediction of relative ductility of these materials using the various current theories. Series of alloys with the B2 ordered structure were studied, and the results were compared to those in pure body-centered cubic (bcc) Fe. Ordered compounds with L1 2 and L1 0 structures based on the face-centered cubic (fcc) lattice were also studied. It was found that there is a correlation between the values of the antiphase boundary (APB) energies in B2 alloys and their unstable stacking fault energies. Materials with higher APB energies tend to have higher unstable stacking fault energies, leading to an increased tendency to brittle fracture.

The relaxed atomistic grain boundary structures in B2 aluminides were investigated using molecular statics and embedded atom potentials in order to explore general trends for a series of B2 compounds. We studied free surface energies and grain boundary structures in three compounds: FeAl, NiAl and CoAl. These alloys represent a series of materials with increasing antiphase boundary energies. The misorientations chosen for detailed study correspond to the Σ5(310) and Σ5(210) symmetrical tilt grain boundaries. The effects of both boundary stoichiometry and bulk simulation block stoichiometry on grain boundary energetics were investigated in detail. The structures obtained for the three alloys are very similar. Defect energies were calculated for boundaries contained in both stoichiometric and off-stoichiometric bulks. The surface energies for these B2 aluminides were also calculated so that trends concerning the cohesive energy of the boundaries could be studied. The implications of the increasing ordering energy, stoichiometry, and multiplicity of possible structures for grain boundary brittleness are discussed.

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