Atomistic simulations were used to predict thermal equilibrium vacancy concentrations in ordered samples, and the results were compared with new positron annihilation spectroscopy data. By using a tight-binding second-moment approximation potential to describe the atomic interactions, single point-defect formation free energies were estimated as a function of temperature. This involved both a quasi-harmonic approximation method and a so-called exact technique which was based upon non-equilibrium free-energy estimations that included all anharmonic effects. The corresponding thermal equilibrium concentrations were then predicted by minimizing the crystal free energy, with respect to the defect concentration, within the non-interacting defect approximation. It was found that agreement between the experimental data, and non-equilibrium free-energy estimates for the effective formation enthalpies and entropies, was good for 3 near-stoichiometric compositions. However, quasi-harmonic approximation results for the same compounds deviated systematically and markedly from the experimental results; thus suggesting that the effect of anharmonicities upon the thermodynamics of formation of vacancies in ordered Ni3Al compounds was appreciable.

Atomistic Prediction of Equilibrium Vacancy Concentrations in Ni3Al. De Koning, M., Miranda, C.R., Antonelli, A.: Physical Review B, 2002, 66[10], 104110