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 fro m exchanges with a single vacancy on a rigid lattice. Highly correlate d vacancy sequences are observed and studied using improved residence time algorithms. It is shown that, for partially ordered structures, t he classical six-jump cycles contribute only partially to the diffusio n process, and that a wide range of other correlated sequences are obs erved, including the recently proposed antisite bridge mechanism. Amon g the other sequences, we have identified six-jump cycles that are ass isted by antisites. Furthermore, when atomic interaction energies pres ent a high degree of asymmetry, two effects have been observed: the ra tio of tracer diffusion coefficients increases as a result of addition al loops involved in the six-jump cycles; diffusion coefficients exhib it an upward curvature below the order-disorder transition temperature . These two effects have been observed in some alloys such as Co-Ga an d therefore can be qualitatively reproduced without invoking triple de fects.