Journal of Statistical Physics, Vol. 62, Nos. 1/2, 1991, p. 161-175.
As a model for a binary alloy undergoing an unmixing phase transition, we consider a square lattice where each site can be either taken by an A atom, a B atom, or a vacancy (V), and there exists a repulsive interaction between AB nearest neighbor pairs. Starting from a random initial configuration, unmixing proceeds via random jumps of A atoms or B atoms to nearest neighbor vacant sites. In the absence of any interaction, these jumps occur at jump rates Г_A and Г_B, respectively. For a small concentration of vacancies (C_v=0.04) the dynamics of the structure factor S(k, t) and its first two moments k_l(t), (k_2)^2(t) is studied during the early stages of phase separation, for several choices of concentration c_B of B atoms. For c_B = 0.18 also the time evolution of the cluster size distribution is studied. Apart from very early times, the mean cluster size l(t) as well as the moments of the structure function depend on time t and the ratio Г of the jump rates (Г= Г_B/Г_A) only via a scaled time t/z(Г). Qualitatively, the behavior is very similar to the direct exchange model containing no vacancies. Consequences for phase separation of real alloys are briefly discussed.