Vacancy-assisted phase separation with asymmetric atomic mobility: Coarsening rates, precipitate composition, and morphology - art. no. 184114

We have investigated the coarsening kinetics and the morphology of precipit ates after quenching dilute binary alloy into its miscibility gap. Three-di mensional kinetic Monte Carlo simulations with a vacancy-diffusion mechanis m are performed. The atomic diffusion model accounts for the asymmetry of t he two terminal phases with respect to the vacancy concentration and diffus ivity. II is shown that, at a fixed low temperature of about 0.25T(c), this asymmetry has a profound effect on the mechanism responsible for precipita te coarsening and on precipitate morphology. For positive asymmetry (i.e., when the vacancy is mostly diffusing inside the precipitates), precipitate diffusion and coagulation are favored. Nearly pure solute precipitates with atomically sharp interfaces are formed in a persistently supersaturated ma trix. For negative asymmetry (i.e., when,the vacancy is mostly diffusing in the matrix), the evaporation condensation of solute atoms becomes dominant even at early stages. The lack of interfacial mobility produces disordered , diffuse interfaces, which then result in highly supersaturated precipitat es. These last results offer an explanation to recent atomic observations o f the precipitate morphology in the Cu-Co system. A mean-field model is int roduced to rationalize how the asymmetry parameter controls the distributio n of vacancies in a two-phase alloy. This model predicts that cluster mobil ity increases when the asymmetry parameter is increased, resulting in an in crease of the coagulation exponent in agreement with the simulations. Furth ermore, this model offers a rationalization for persistent supersaturations of the matrix or the precipitates, and this provides some insight into the formation of diffuse interfaces.