Based on the Bragg-Williams approximation, a statistical-thermodynamic approach was presented for non-stoichiometric binary B2 phases with a pure anti-structure defect mechanism. Whereas in the original Bragg-Williams model only the state of long-range order was considered and no correlations were allowed in the arrangement of the point defects, short-range order was here introduced as an equilibrium property of the system. This new approach was applicable not only both to highly ordered intermetallic compounds and to completely disordered alloys but also in the region of the order-disorder transition. The obtained phase boundary which described the order-disorder transition was in good agreement with the results predicted by Monte Carlo calculations.

Correlations in the Arrangement of Antistructure Point Defects in Intermetallic Phases with B2 (CsCl) Structure. R.Krachler, H.Ipser: Philosophical Magazine A, 2002, 82[16], 3013-25