Analysis of the anisotropy of point defect diffusion in hcp Zr

A combination of density functional theory (DFT), kinetic Monte Carlo and mean-field rate theory is applied to analyze point defect migration and its effect on the observed growth of hexagonal close-packed (hcp) Zr under 1 MeV electron irradiation. DFT is used to study stability of various configurations of vacancies and self-interstitial atoms (SIAs) and migration barriers. The data are used in kinetic Monte Carlo modeling of defect diffusion at different temperatures. It is found that both defects exhibit anisotropic diffusion, predominantly parallel to the basal planes. The ratio of diffusion coefficients parallel and perpendicular to the basal planes is found to be higher for vacancies as compared to SIAs at temperatures below ∼600 K. This raises doubts that the observed radiation growth in Zr irradiated with 1 MeV electrons, namely positive strains in prismatic and negative strains in basal directions, and void alignment along basal planes, can be accounted for by the anisotropy of point defect diffusion, which predicts opposite strain signs. It is speculated that formation of small SIA clusters with higher diffusion anisotropy may be responsible for the experimental observations.