Agradecimentos: This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. We thank A. Caro and S. Debiaggi for helpful discussions. M.K. acknowledges partial support from the Brazilian agency FAPESP under Contract No. 97/14290-6. C.R.M. and A.A. gratefully acknowledge support from the Brazilian funding agencies FAPESP, CNPq, and FAEP Ver menos

Abstract: We conduct a series of atomistic simulations to predict thermal equilibrium vacancy concentrations in ordered Ni3Al 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 Ni3Al compounds is significant Ver menos