We have investigated the coarsening kinetics and the morphology of precipitates after quenching dilute binary alloy into its miscibility gap. Three-dimensional kinetic Monte Carlo simulations with a vacancy-diffusion mechanism are performed. The atomic diffusion model accounts for the asymmetry of the two terminal phases with respect to the vacancy concentration and diffusivity. 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 precipitate 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 matrix. 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 precipitates. These last results offer an explanation to recent atomic observations of the precipitate morphology in the Cu-Co system. A mean-field model is introduced to rationalize how the asymmetry parameter controls the distribution of vacancies in a two-phase alloy. This model predicts that cluster mobility increases when the asymmetry parameter is increased, resulting in an increase of the coagulation exponent in agreement with the simulations. Furthermore, 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.
