Molecular statics methods, with embedded atom potentials, were used to study the energetics of point defect formation and migration. The basic assumption made was that the mechanism which required the lowest activation energy would dominate the diffusion process. The calculations showed that Ni diffusion was dominated by next-nearest neighbor vacancy jumps; except in Ni-rich compositions (above about 52at%Ni). Here, the ASB mechanism also made a significant contribution and resulted in an apparent reduction in the activation energy. This was quantitatively consistent with experimental data. The Al diffusion was dominated by the 6JC(VNi) mechanism in stoichiometric and Al-rich alloys; with a possible contribution of the ASB mechanism at CNi < 0.48, and the mechanism of next-nearest neighbor vacancy jumps in Ni-rich alloys. The activation volumes for the various mechanisms were essentially different. That for self-diffusion in pure metals was usually close to the atomic volume, but the activation volume in NiAl varied from unusually high (6JC) to unusually low (ASB); depending upon the mechanism. This made the various mechanisms either extremely sensitive (large atomic volume) or anomalously insensitive (small atomic volume) to the mechanical stresses that might build up in intermetallic alloys. It was suggested that diffusion measurements performed under high pressures could be used to identify atomic diffusion mechanisms in NiAl.

Atomistic Simulation of Point Defects and Diffusion in B2 NiAl Y.Mishin, D.Farkas: Scripta Materialia, 1998, 39[4-5], 625-30