Order-Disorder Events Produced by Single Vacancy Migration

Monte Carlo computational experiments, based on the Flinn-McManus method, were used to study the interaction between ordering (disordering) and single vacancy migration in AB alloys. Square planar, simple cubic, and bcc lattices were studied. If the alloy is initially disordered (short-range order near zero), the extent of vacancy migration for N jumps is slightly larger than that for a symmetric random walk. However, when the initial short-range order is either comparable to or larger than the equilibrium value for the critical temperature, the ordering process contracts the extent of vacancy migration relative to that for a symmetric random walk. In two dimensions (surface diffusion) the degree of contraction depends upon the number of jumps and is larger than that for three dimensions (bulk diffusion) which is independent of the number of jumps. The long- and short-range order established in expanding migration regions by 10⁴ to 3×{}10⁴ vacancy jumps are very close to the values predicted by Bethe's model for the equilibrium state. A system of contiguous antiphase domains was formed in an initially disordered alloy, by single vacancy migration, rather than a single ordered nucleus.