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

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Abstract
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.