Lattice gas decomposition model for vacancy formation correlated with B2 atomic ordering in intermetallics

Thermal vacancy formation correlated with atomic ordering was modelled in B2-ordering A–B binary intermetallics. Ising Hamiltonian was implemented with a specific Bragg–Williams-type thermodynamic formalism for thermal vacancy formation based on the phase equilibria in a lattice gas composed of atoms and vacancies. It has been demonstrated that for pair-interaction energetics favouring vacancy formation on A-atom sublattice, equilibrium concentrations of vacancies and antisite defects result mutually proportional in well defined temperature ranges. The effect observed in both stoichiometric and non-stoichiometric binary alloys was interpreted as a tendency for triple defect formation. In B-rich non-stoichiometric binary alloys vacancy concentration did not extrapolate to zero at temperature approaching zero, which indicated the formation of constitutional vacancies. Energetic conditions for the occurrence of the effects were analysed in detail.