Thermodynamics of B2 intermetallic compounds with triple defects: a Bragg-Williams model for (Ni,Co)Al

Applying the Bragg-Williams approximation, a pair interaction model has been developed for describing the composition and temperature dependence of thermodynamic properties of ternary intermetallic B2 phases. Taking the activities and the integral enthalpies of formation as input data, values for the enthalpy and the entropy of bonds between atoms have been obtained. An analytical expression has been derived for the dependence of the concentration of point defects (vacancies and antistructure atoms) on composition and temperature. The model has been applied successfully to the available experimental data for the Ni-Al, Co-Al, and Ni-Co-Al systems. It has been shown that the values calculated accordingly for the effective enthalpy (and entropy) of formation of vacancies agree well with those from experimental observations and theoretical ab initio calculations.