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

M Christensen and W Wolf and C Freeman and E Wimmer and RB Adamson and L Hallstadius and PE Cantonwine and EV Mader, JOURNAL OF NUCLEAR MATERIALS, 460, 82-96 (2015).

DOI: 10.1016/j.jnucmat.2015.02.013

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,D-basal = 8.6 x 10(-6) e(-Q/(RT)) (m(2)/s) and D-vac,D-axial = 9.9 x 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 x 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. (C) 2015 Elsevier B.V. All rights reserved.

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