Atomistic prediction of equilibrium vacancy concentrations inNi3Al

Abstract

We conduct a series of atomistic simulations to predict thermal equilibrium vacancy concentrations in ordered ${\mathrm{Ni}}_{3}\mathrm{Al}$ and compare the results to recent positron-annihilation spectroscopy experiments. Using a tight-binding second-moment-approximation potential to describe the atomic interactions, we compute single point-defect formation free energies as a function of temperature using both a quasiharmonic approximation (QHA) method and an ``exact'' technique based on nonequilibrium free-energy estimation (NFE), which includes all anharmonic effects. The corresponding thermal equilibrium concentrations are then computed by minimizing the crystal free energy with respect to the defect concentrations within the noninteracting-defect approximation, following the canonical ensemble approach of Hagen and Finnis [Philos. Mag. A 77, 447 (1998)]. It is found that the agreement between the NFE predictions for the effective formation enthalpies and entropies and experimental data is good for three near-stoichiometric compositions. The QHA results for the same compounds, however, deviate systematically and substantially from the experimental results, suggesting that the influence of anharmonicities on the formation thermodynamics of vacancies in ordered ${\mathrm{Ni}}_{3}\mathrm{Al}$ compounds is significant.