ATOMISTIC SIMULATION OF POINT-DEFECTS AND DIFFUSION IN B2 NIAL .1. POINT-DEFECT ENERGETICS

Point defect energetics in compounds is important both for diffusion s imulations and for the calculation of formation energies of extended d efects. We apply the strict thermodynamic definitions to relate the fo rmation energy of an arbitrary crystalline defect in an ordered compou nd to its 'raw' energy (calculated using a perfectly ordered crystal a s 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 constitution al 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 inapproria te for this purpose because the point defect energies obtained are inc onsistent with the triple-defect model. Since this model has been firm ly verified experimentally for NiAl, we had to modify the existing EAM potentials in order to reconcile the simulation results with experime ntal observations. ?The modified EAM potentials are empirically fitted to self-diffusion data in pure Ni and Al and render NiAl a triple-def ect compound. Using the modified EAM potentials we calculate the chemi cal potentials of Ni and Al, the true formation energies of point defe cts, and the binding energies of their complexes in NiAl. The data we obtain can be used for the calculation of the excess energy of extende d defects (for example, grain boundaries) in NiAl. These data will als o be used in part II of this work for the analysis of diffusion mechan isms in NiAl.