The Influence of Defect Concentrations on Migration Energies in AgZn Alloys
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IRATICNS ON MIGRATION ENERGIES THE INFIDENCE OF DEFECT C IN AgZn ALLOYS T.D. Andreadis, M. Rosen, J.M. Eridon, D. J. Rosen Naval Research Laboratory, Washington, DC 20375-5000. ABSTRACT Migration energies in Ag of vacancies, interstitials, Zn impurity atoms, interstitial-iipurity cumplexes, and vacancy-impurity complexes were calculated using Embedded Atom. Method (EAM) potentials in Molecular Statics calculations. A new Zn EAM potential was determined and used in these calculations. The dependence of the migration energies on local defect concentrations was determined in a linear approximation. Binding and formation energies of defects are also presented. A new model for the migration energy appropriate for defect reactions is introduced.
Diffusion in an irradiated solid is influenced by the production and interaction of defects in the target. Interstitials and vacancies diffuse and react with each other and with impurities to form complexes. Diffusion and rate coefficients have the general form: R = R0 exp[-H1/kT], where
R0 Hm k T
= a pre-exponential factor, = migration energy, = Boltzmann's constant, = absolute temperature.
The value of these coefficients can depend sensitively on the migration energy, since it occurs in the exponent. Few measurements of migration energies are available and still fewer in which concentration dependence is measured. In this study we calculate migration energies for AgZn alloys as a function of the local defect environment. Credible results require credible potentials. We used Embedded Atom Method (EAM) potentials in Molecular Statics calculations to calculate the migration energies of vacancies, interstitial, Zn impurity atoms, interstitial impurity complexes, and vacancy impurity complexes. Binding and formation energies of defects are also presented. THE EAM PaIENTIALS The Ag EAM potential was obtained from the work of Daw and Baskes [1], however a Zn potential was newly developed for this study. Zinc is difficult since it has a non-ideal hcp structure (c/a ratio of 1.856). The Zn potential that was developed, however, reasonably reproduced a selected set of bulk properties of Zn and also the heat of solution of Zn in Ag. This potential is adequate for dilute solutions of Zn in Ag. An EAM potential for silver already exists [1]. It is difficult to create a set of embedding functions for zinc, however. Although other zinc potentials have been reported recently [2,3], we believed that it was important to use a potential derived in a manner similar to that of silver. To this end, the electron density was computed using Roothan-Hartree-Fock
Mat. Res. Soc. Symp. Proc. Vol. 128. e1989 Materials Research Society
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calculations[4], and the pair potential was ccmputed using the same parametrization found in the reference [1]. It was not possible to match all five elastic constants using this parametrization, but as we were examining a dilute solution of zinc in silver this was not deemed important. We were able to provide a reasonable match to two of the elastic constants, C11
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