Atomistic simulation of point defects and diffusion in B2 NiAl

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Abstract
Point defect energetics in compounds is important both for diffusion simulations and for the calculation of formation energies of extended defects. We apply the strict thermodynamic definitions to relate the formation energy of an arbitrary crystalline defect in an ordered compound to its ‘raw’ energy (calculated using a perfectly ordered crystal as a reference state) and the chemical potentials of the components in a uniform alloy at zero temperature. In turn, the chemical potentials are expressed in terms of the ‘raw’ formation energies of constitutional defects in off-stoichiometric alloys. We derive expressions for the chemical potentials and true energies of vacancies and antisites in a triple-defect compound. Specific calculations are performed for a B2 compound NiAl using ‘molecular statics’ and the embedded atom method (EAM). We find that the existing EAM potentials for NiAl are inapproriate for this purpose because the point defect energies obtained are inconsistent with the triple-defect model. Since this model has been firmly verified experimentally for NiAl, we had to modify the existing EAM potentials in order to reconcile the simulation results with experimental observations. The modified EAM potentials are empirically fitted to self-diffusion data in pure Ni and Al and render NiAl a triple-defect compound. Using the modified EAM potentials we calculate the chemical potentials of Ni and Al, the true formation energies of point defects, and the binding energies of their complexes in NiAl. The data we obtain can be used for the calculation of the excess energy of extended defects (for example, grain boundaries) in NiAl. These data will also be used in part II of this work for the analysis of diffusion mechanisms in NiAl.