A new method of approximation for studying the order-disorder transition in binary alloys is presented. The formulation is in terms of face-centered cubic alloys, although the method can also be applied to other structures. The technique is essentially a generalization of the constant-coupling approximation. We have applied the principles of that method to derive the free energy of the alloy using a tetrahedral cluster of nearest neighbouring sites as the basic unit of the calculation. We only consider the case of nearest neighbour pair interactions, but show how the method can be generalized to include many body and second neighbour interactions. Numerical results are presented and comparison of these results is made with results of cluster variation calculations on the same system and with experimental results on the copper-gold alloy system.
