Molecular dynamics simulation of diffusion in a (110) B2-NiAl film

We report on the first direct molecular dynamics study of diffusion in B2-NiAl using one of the most reliable embedded-atom method potentials for this phase. The simulation is performed for near the stoichiometric composition at a temperature just below the melting temperature of the model. In the molecular dynamics simulation, an equilibrium point-defect concentration is 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 defects – Ni vacancies and Ni antisites – are found in the bulk of the model. It is demonstrated that isolated Ni vacancies strongly dominate in concentration over all of their bound complexes with Ni antisites. Although we predict 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 about 2% of Ni vacancies and 1% of Ni antisites statistically randomly associate to form bound Ni vacancy-Ni antisite-Ni vacancy complexes (the so-called bound triple-defect complex). As a result, it is deduced that the triple-defect diffusion mechanism is not likely to be the dominant diffusion mechanism in the bulk of the model because this diffusion mechanism effectively requires that the Ni vacancies and Ni antisites must form bound triple-defect complexes. Furthermore, it is found that Ni atoms diffuse in the bulk of the model on average about 2.5 times faster than Al atoms. Therefore, we suggest that isolated Ni vacancies are likely to play a key role in atomic diffusion of both Ni and Al near the stoichiometric composition of B2-NiAl and, consequently, the most plausible and widely accepted candidate for dominant diffusion mechanism in B2-NiAl can be considered to be six-jump cycles of a Ni vacancy. Furthermore, we can suppose that additional next-nearest-neighbor jumps of a Ni vacancy may cause that Ni atoms diffuse faster than Al atoms