Semi-grand canonical Monte Carlo simulation of ternary bcc lattice-gas decomposition: Vacancy formation correlated with B2 atomic ordering in A–B intermetallics

Lattice-gas decomposition model for equilibrium vacancy concentration in B2-ordered triple defect A–B binary intermetallic systems previously formulated and solved within the Bragg–Williams approximation was now simulated by means of Semi-Grand Canonical Monte Carlo (SGCMC) technique. Simulations of a ternary Ising lattice gas A–B–V with pair-interaction energies promoting the generation of A-antisites and A-vacancies revealed a miscibility gap and a coexistence of vacancy-poor and vacancy-rich phases within a wide range of temperature. The B2-ordered vacancy-poor phases were identified with intermetallic compounds containing equilibrium number of vacancies, which, at sufficiently low temperature, resulted proportional to the number of antisite defects. The latter effect additionally accompanied by the presence of constitutional vacancies in B-rich systems was interpreted as a signature of triple defect formation. The reliability of the model was checked by its implementation with Ni–Al EAM potential and by comparing the calculated vacancy concentrations in NiAl with the available experimental data. The modelled temperature dependence of the equilibrium vacancy concentration and configuration in triple defect B2-ordering binary intermetallics will be applied in Kinetic Monte Carlo (KMC) simulations of “order–order” kinetics in these systems.