By applying the Bragg-Williams approximation, a pair-interaction model was developed for describing the compositional and temperature dependences of the thermodynamic properties of ternary intermetallic B2 phases. By taking the activities and integral enthalpies of formation as input, values were obtained for the enthalpy and entropy of bonding between atoms. An analytical expression was derived for the dependence of the concentration of point defects (vacancies and antistructure atoms) upon composition and temperature. The model was successfully applied to available experimental data, and it was shown that values calculated according to the effective enthalpy and entropy of formation of vacancies agreed well with those deduced from experimental observations and ab initio theoretical calculations.

Thermodynamics of B2 Intermetallic Compounds with Triple Defects - a Bragg-Williams Model for (Ni,Co)Al. J.Breuer, F.Sommer, E.J.Mittemeijer: Metallurgical and Materials Transactions A, 2001, 32[9], 2157-66