A generalized defect correlation model for B2 compounds

A generalized statistical-thermodynamic model for non-stoichiometric binary B2 phases was developed on the basis of a Bethe–Bragg–Williams approximation, with pair interaction energies as input parameters. The model considers both vacancies and anti-structure atoms as possible point defects on the two sublattices; it allows therefore a description of all types of B2 compounds, independently of their particular defect mechanism. At the same time, the model describes both long-range ordering and short-range ordering in the crystal lattice, i.e. it includes the possible correlation of nearest neighbour point defect combinations. It was tested with ab initio values of pair interaction energies for NiAl from the literature, and good agreement was obtained between calculated values and experimental data for both, aluminum activities and vacancy concentrations. Full sets of pair interaction energies were suggested for three other B2 phases, i.e. FeAl, CoAl, and PdAl, based on experimental data for thermodynamic activities as well as, in the case of FeAl, also for vacancy and Fe anti-structure atom concentrations. The (hypothetical) critical temperatures of the order–disorder transformations were derived from the model calculations.

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