Diffusion in Irradiated AgZn Alloys using Defect-Concentration Dependent Migration Energies
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DIFFUSION IN IRRADIATED AgZn ALLOYS USING DETECII ATIýt DEPENDENT MIGRATION ENERGIES T.D. Andreadis, M. Rosen, J.M. Eridon Naval Research Laboratory, Washington, DC 20375-5000.
ABSTRACT Concentration dependent migration and reaction migration energies, calculated with the molecular dynamics code DYNAMD using Embedded Atom Method Potentials, were inserted into the appropriate diffusion and rate coefficients. The time and space evolution of the concentration of interstitials, vacancies, Zn impurity, interstitial-inpurity, aid vacancyimpurity complexes was calculated for an irradiated AgZn alloy in the framework of the approach presented in Johnson and Lam. A significant effect on defect segregation and profiles was found.
INTRODUCTION Johnson and Iam[l] (JL) have presented a segregation model for defect diffusion in an irradiated AgZn alloy. They investigated both spatial and temporal profiles of interstitials, vacancies, and dilute Zn impurities, interstitial-impurity complexes and vacancy-inpurity complexes. Hcoever the activation energies for migration, or migration energies, which occur in the diffusion and rate coefficients in the diffusion equation, depend on local
concentrations of defects. We have determined, within the framework of the JL model, how significant an affect this defect concentration dependence has on the calculated defect profiles using the NRL nonlinear diffusion code, DELOS. We used the Molecular Dynamics code DYNAM0[2] in its static mode and Embedded Atom Method (EAM) potentials to calculate defect migration energies and their concentration dependence for dilute solutions of Zn in Ag (Table 1). The calculation process will be briefly described here. First, migration energies were calculated for each defect under dilute conditions (a defect in pure Ag). The concentration dependence as a function of the concentration of the other defects was obtained by recalculation of the migration energy with defects present. A linear concentration dependence was assumed and the slopes for the curves of migration energy vs. concentration were obtained. Multiplying the slopes, which are given in Table 1, by the concentration of the corresponding specie gives the change in migration energy from the dilute case. The affects of different species on a migration energy are assumed to be additive. Details of the Molecular Statics calculations are presented elsewhere in these proceedings[3].
REACTION MIGRATION ENERGIES
Diffusion and rate coefficients have the general form:
R= Roexp[-I6kT], Mat. Res. Soc. Symp. Proc. Vol. 128.
1989 Materials Research Society
202
where
R0 Hm k T
= = = =
a pre-exponential factor, migration energy, Boltzmann's constant, and Absolute temperature .
Rate coefficients for the formation and breakup of complexes of defect species are generally calculated in the literature[l] using a migration energy which is the same as that for diffusion of the individual species. However, even when the concentration of two reactants is extremely dilute, the migration energies of reacting pair
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