COARSENING IN THE PRESENCE OF VACANCIES

Using Monte Carlo simulations, we explore the effect of vacancy-mediat ed diffusion on the kinetics of coarsening of a two-phase (matrix and precipitate) alloy system. When a vacancy diffuses into the vicinity o f a minority-phase (precipitate) cluster, the further time development of its trajectory is dictated by the interplay of the cluster-matrix interface energy and the thermal fluctuations. For low temperatures, t he vacancy takes a position at the interface; while with increasing te mperatures, thermal fluctuations favor a random location of the vacanc y. Thus, at low temperatures, the probability is relatively higher for minority-phase atoms that evaporate from a cluster, aided by a vacanc y, to recondense onto the same cluster, deforming the shapes of the cl usters. The consequent shift in the center of mass causes random motio n of clusters as a whole. This gives rise to coarsening via cluster-cl uster aggregation (CCA) as the clusters encounter one another. As ther mal fluctuations increase, the tendency of an atom to diffuse far from the original cluster, aided by a vacancy, increases. Thus, with incre asing temperature, the coarsening mechanism gradually changes to Ostwa ld ripening (OR). At the early stages of coarsening, for sufficiently low temperature and sufficiently high minority-phase concentration, th ere is a third process, which is a variant of CCA. At sufficiently low minority-phase cluster spacing, the vacancy interfacial diffusion, an d the consequent particle reshaping, leads to CCA without the need for any further diffusive motion of the minority-phase clusters. As we de scribe in detail, this leads to a more complicated time evolution than in the lower concentration and/or higher temperature regime. We inclu de this discussion as part of our overall detailed discussion of the t ime evolution of the length-scale in coarsening. (C) 1998 Elsevier Sci ence S.A. All rights reserved.