A MONTE-CARLO STUDY OF B2 ORDERING AND PRECIPITATION VIA VACANCY MECHANISM IN BCC 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 mecha nism. They reproduce the following experimental observations: for a co mposition close to the A2 solubility limit, precipitation and ordering proceed simultaneously and lead to subsequent coarsening of B2 precip itates. At higher solute concentrations, a rapid short range ordering is observed before A2 phase appears via thickening of the antiphase bo undaries. Closer to the B2 solubility limit, one also observes nucleat ion of A2 phase inside BZ domains. Moreover, it is observed that the v ery 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 reg imes of ordering are identified, leading either to localized or deloca lized ordering. Copyright (C) 1996 Acta Metallurgica Inc.