A Monte-Carlo study of B2 ordering and precipitation via vacancy mechanism in b.c.c. lattices

Atomistic Monte-Carlo simulations of precipitation and B2 ordering in a binary alloy on a b.c.c. lattice are performed using a vacancy mechanism. They reproduce the following experimental observations: for a composition close to the A2 solubility limit, precipitation and ordering proceed simultaneously and lead to subsequent coarsening of B2 precipitates. At higher solute concentrations, a rapid short range ordering is observed before A2 phase appears via thickening of the antiphase boundaries. Closer to the B2 solubility limit, one also observes nucleation of A2 phase inside B2 domains. Moreover, it is observed that the very early stages of ordering are strongly dependent on atomic mobility parameters. Depending on the relative exchange frequency of vacancies with A and B atoms, and depending on the composition, two extreme regimes of ordering are identified, leading either to localized or delocalized ordering.