ON FORMATION AND MIGRATION ENTROPIES OF VACANCIES IN METALS

An approach has been developed for calculating defect entropies from an entirely discrete vibrational point of view. Calculations have been performed within a discrete finite spherical array of atoms which includes all contributions to the defect entropy predicted by linear isotropic continuum elasticity. The method used, however, does not demand an explicit knowledge of the strength of the defect and thus avoids certain conceptual difficulties that arise in both the elastic and the combined discrete-elastic approaches. The vibrational spectra for the various defect configurations have been obtained using a quasiharmonic Einstein model in conjunction with a modified Morse function as force law. Values of the vacancy formation and migration entropies were computed for the cases of copper, nickel and aluminum. Where experimental values were available for comparison, fair agreement was found with the calculated entropies. In addition, Arrhenius D0's calculated on the assumption that diffusion is controlled by the migration of single vacancies were found to be lower than experimental D0's, thus lending support to previous suggestions in the literature that divacancies make a significant contribution to the diffusion flux in f.c.c. metals at high temperatures. © 1969.