SPINODAL DECOMPOSITION OF A 2-DIMENSIONAL MODEL ALLOY WITH MOBILE VACANCIES

Monte Carlo simulations are performed for the initial stages of phase separation in a model binary alloy (AB), where unmixing is caused by a repulsive energy between atoms of different kind (epsilon-(AA) = epsilon-(BB) = 0, epsilon-(AB) = epsilon), and a small fraction C(v) of mobile vacancies is present (typically c(v) = 0.04.) Unlike previous work, where interdiffusion was modelled in an unrealistic way by direct interchange of A and B atoms for c(v) = 0, we use here the vacancy mechanism of diffusion: A-atoms may jump to vacant sites with a rate GAMMA-(A), and B-atoms may jump to vacant sites with a rate GAMMA-(B), no direct A-B interchange being permitted. It is shown that the overall time-scale on which phase separation proceeds typically is controlled by the slower of these rates, while otherwise there is little dependence of the structure factor S(k, t) on the parameter GAMMA = GAMMA-(B)/GAMMA-(A); in fact, S(k, t) is qualitatively very similar to the direct exchange model, thus justifying the use of the latter. We also observe a slight anisotropy of S(k, t) for different directions of the scattering vector k. No significant enrichment of the vacancies in the A-B-interfacial regions is observed. Finally, the decrease of the critical temperature with increasing vacancy content is also studied.