Identification of novel diffusion cycles in B2 ordered phases by Monte Carlo simulation

Atomic migration in ordered binary alloys with B2 structure is studied by atomistic Monte Carlo simulations where atom migration results from exchanges with a single vacancy on a rigid lattice. Highly correlated vacancy sequences are observed and studied using improved residence time algorithms. It is shown that, for partially ordered structures, the classical six-jump cycles contribute only partially to the diffusion process, and that a wide range of other correlated sequences are observed, including the recently proposed antisite bridge mechanism. Among the other sequences, we have identified six-jump cycles that are assisted by antisites. Furthermore, when atomic interaction energies present a high degree of asymmetry, two effects have been observed: the ratio of tracer diffusion coefficients increases as a result of additional loops involved in the six-jump cycles; diffusion coefficients exhibit an upward curvature below the order-disorder transition temperature. These two effects have been observed in some alloys such as Co-Ga and therefore can be qualitatively reproduced without invoking triple defects.