Diffusion of point defects, nucleation of dislocation loops, and effect of hydrogen in hcp-Zr: Ab initio and classical simulations

Abstract Diffusion of point defects, nucleation of dislocation loops, and the associated dimensional changes of pure and H-loaded hcp-Zr have been investigated by a combination of ab initio calculations and classical simulations. Vacancy diffusion is computed to be anisotropic with D vac,basal  = 8.6 × 10 −6  e − Q /( RT ) (m 2 /s) and D vac,axial  = 9.9 × 10 −6  e − Q /( RT ) (m 2 /s), Q  = 69 and 72 kJ/mol for basal and axial diffusion, respectively. At 550 K vacancy diffusion is about twice as fast in the basal plane as in a direction parallel to the c -axis. Diffusion of self-interstitials is found to be considerably faster and anisotropic involving collective atomic motions. At 550 K diffusion occurs predominantly in the a -directions, but this anisotropy diminishes with increasing temperature. Furthermore, the diffusion anisotropy is very dependent on the local strain ( c/a ratio). Interstitial H atoms are found to diffuse isotropically with D H  = 1.1 × 10 −7  e −42/( RT ) (m 2 /s). These results are consistent with experimental data and other theoretical studies. Molecular dynamics simulations at 550 K with periodic injection of vacancies and self-interstitial atoms reveal the formation of small nanoclusters, which are sufficient to cause a net expansion of the lattice in the a -directions driven by clusters of self-interstitials and a smaller contraction in the c -direction involving nanoclusters of vacancies. This is consistent with and can explain experimental data of irradiation growth. Energy minimizations show that vacancy c -loops can collapse into stacking-fault pyramids and, somewhat unexpectedly, this is associated with a contraction in the a -directions. This collapse can be impeded by hydrogen atoms. Interstitial hydrogen atoms have no marked influence on self-interstitial diffusion and aggregation. These simulations use a new Zr–H embedded atom potential, which is based on ab initio energies.