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A spin-1/2 Ising model, with nearest-neighbor exchange and next-nearest-neighbor superexchange interactions, is proposed to describe the phase diagram of disordered Fe p -Al q (p + q = 1) alloys in the body centered cubic lattice. The size of the aluminum clusters have been taken into account in order to induce a superexchange interaction among the Fe atoms. Extensive Monte Carlo simulations have been employed by using a hybrid algorithm consisting of Metropolis spin flip and Wolff cluster algorithm, together with single histograms techniques. A good fit to the experimental phase diagram has been achieved, including the anomalous region for Al concentration in the range q ≤ 0.2.