Quasi chemical and defect correlation models for intermetallic compounds with B2-structure: new applications

A generalized, quantitative, and predictive statistical-thermodynamic model is proposed employing a quasi-chemical argument for description of thermodynamic behavior and ordering phenomena in B2 ordered binary intermetallic materials. A modified procedure is given for the application of the Bethe-Guggenheim QC method which takes into account the presence of all possible defects in the structure, both vacancies and anti-structure atoms. Interactions between nearest-neighbor atoms result in correlations between their arrangements in the crystal lattice. The QC method which can be regarded as the simplest form of the cluster variation method takes these correlations into consideration by describing the formation of point defect pairs and assuming random distribution of these point defect pairs. The results are compared with those obtained by the defect correlation model, a simple Ising-type model which is, however, able to describe defect clusters formed by two, three or even more atoms or vacancies in non-frustrated crystal lattices like the B2 structure. Both modeling approaches were applied for description of thermodynamic activities and vacancy concentrations over composition range and temperature in non-stoichiometric B2 ordered intermetallic compounds with triple-defect mechanism (PdIn, NiGa, and CoGa), as well as with substitutional (anti-structure) defect mechanism (FeCo, NiZn, AgMg). The (hypothetical) critical temperatures of the order–disorder transformations were derived from the model calculations. Comparison with a wealth of experimental data provided in the literature confirms the viability of these simple models.