The first direct molecular dynamics study of diffusion in B2-NiAl, using one of the most reliable embedded-atom method potentials for this phase, was reported. The simulation was performed for a near-stoichiometric composition at a temperature just below the melting point. An equilibrium point-defect concentration was generated and maintained by using a film sample with periodic boundary conditions only in two directions and free surfaces in the third direction. Two types of point defect (Ni vacancies, Ni antisites) were found in the bulk of the model. It was demonstrated that isolated Ni vacancies strongly predominated in concentration over all of their bound complexes with Ni antisites. Although it was predicted that some attractive interactions should occur between point defects, to form bound Ni vacancy-Ni antisite pairs and bound Ni antisite-Ni vacancy-Ni antisite complexes, only some 2% of Ni vacancies and 1% of Ni antisites statistically randomly associated to form bound Ni vacancy-Ni antisite-Ni vacancy complexes (so-called bound triple-defect complexes). It was thus deduced that the triple-defect diffusion mechanism was unlikely to be the predominant diffusion mechanism in the bulk of the model because this mechanism effectively required that the Ni vacancies and Ni antisites form bound triple-defect complexes. It was found that Ni atoms diffused in the bulk of the model about 2.5 times faster than Al atoms on average. Therefore, it was suggested that isolated Ni vacancies were likely to play a key role in the atomic diffusion of both Ni and Al near to the stoichiometric composition of B2-NiAl and thus the most plausible and widely accepted candidate for dominant diffusion mechanism in B2-NiAl was six-jump cycles of a Ni vacancy. It was supposed that additional next-nearest neighbor jumps of a Ni vacancy might cause Ni atoms to diffuse faster than Al atoms.

Molecular Dynamics Simulation of Diffusion in a (110) B2-NiAl Film. Evteev, A.V., Levchenko, E.V., Belova, I.V., Murch, G.E.:  Intermetallics, 2011, 19[7], 848-54