Phase diagram of a binary alloy with annealed impurities has been stud
ied with Monte Carlo computer simulation technique. A combination of t
he traditional Metropolis method and the recently developed Swendsen-W
ang technique are used to relax the system to the temperature of inter
est. For small vacancy concentrations ''v'' the transition temperature
, KT(c)/J, (k is the Boltzmann constant, and J is the strength of inte
raction between nearest neighbours) decreases linearly with ''v''. Thi
s is consistent with earlier results on quenched binary systems. Below
the coexistence curve the system phase separates into A-rich and B-ri
ch regions with vacancies randomly distributed amongst them. At lower
temperatures the vacancies also phase separate out. At higher vacancy
concentrations the spins representing atoms A and B separate into smal
l islands of up (or down) spins. The tricritical point occurs at a tem
perature kT/J=0.65+/-0.02 and a vacancy concentration of 0.38+/-0.01.
Because of a change in the nature of the phase transition at the tricr
itical point the region around this point is difficult to explore. The
phase boundary corresponding to phase separation of vacancies for v <
0.38 and those of spins for v > 0.38 is located by the abrupt changes
in the vacancy concentration as a function of the field strength.