Phase diagrams of two-dimensional binary alloy with vacancies (ABV) is
studied with pairwise interaction epsilon(AA), epsilon(BB), and epsil
on(AB) between the nearest neighbors. Attention is focussed on the cas
e where the vacancies are annealed and epsilon(BA) = epsilon, epsilon(
AA) = epsilon(BB) = 0. For vacancy concentrations c(upsilon) < 0.2, th
e critical temperature decreases linearly with c(upsilon). The vacanci
es remain randomly distributed within the A and B phase separated regi
ons. For c(upsilon) > 0.2 the critical temperature no longer shows thi
s linear trend and falls to zero very sharply at c(upsilon) = 0.39. Th
is critical vacancy concentration is slightly lower than the one expec
ted for quenched vacancies, where the percolation threshold of atomic
species is 0.59 (c(upsilon) = 0.41). The mobility of vacancies and the
nature of atomic interactions makes it energetically favorable for th
e vacancies to occupy the boundary layers between A and B phase separa
ted regions, thus making the process of phase separation difficult. Th
is lowers the critical temperature below that of the case where the va
cancies are quenched. Salient features of the phase diagram are discus
sed by comparison with previously studied similar systems with differe
nt interaction energies.