First-principles study of constitutional point defects in B2 NiAl using special quasirandom structures

We developed special quasirandom structures (SQSs) for substitutionally random pseudobinary A_{1 - x}B_xC B2 alloys at compositions x = 0.25 and 0.5. The structures mimic the local pair and multisite correlation functions of the corresponding random simple-cubic alloy. Our SQSs were applied to study non-stoichiometric B2 NiAl alloys containing high concentrations of constitutional point defects. Direct first-principles calculations on the SQSs provide formation enthalpies, equilibrium lattice parameters and elastic constants of non-stoichiometric B2 NiAl alloys in satisfactory agreement with existing experimental data in the literature. Our calculations unambiguously show that Ni vacancies and Ni antisites are the stable constitutional point defects in Al-rich and Ni-rich B2 NiAl, respectively, up to large deviations from stoichiometry. Our SQS calculations also confirmed the experimentally observed structural instability of B2 NiAl at high Ni concentrations. Finally, we demonstrated that our SQSs can even give formation enthalpies of isolated defects in good agreement with 54-atom supercell calculations.