Zirconium based materials, which are widely used in the nuclear industry, have a particularly challenging microstructural evolution under irradiation because of their hexagonal close packed structure. Which are the configurations adopted by self-interstitial atoms (SIAs) among the non less than eight possible ones is not clear, and whether point defects exhibit or not diffusion anisotropy is an open question. We have investigated the structural and migration properties of vacancies and SIAs by combining calculations based on an empirical potential (EP) and on first principles (FP) methods. The latter are performed with the Density Functional Theory SIESTA code on cells with up to 385 atoms. The extensive study of the convergence as function of size and shape of the supercell of the SIA formation energies confirms the conclusion of previous DFT works according to which four SIA configurations are nearly degenerate in energy in Zr. We have in addition evidenced a new family of low-energy configurations, both within EP and FP calculations. These new configurations are shown to play a particularly important role: they are stabilized at high temperature by low-frequency vibrational modes and they are intermediate configurations for the migration mechanism between other configurations. We used the ART method (Activation Relaxation Technique) in order to search for other configurations and to determine in a systematic way the various possible migration pathways. The energy landscape which emerges from the FP calculation of the SIA migration barriers is compatible with internal friction experiments. The migration barrier of the vacancy within the basal plane is shown to be lowered by electronic effects, which yield a significant diffusion anisotropy (~0.15 eV) within FP calculations.

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1. Stability and mobility of interstitial clusters in alpha-iron: ab initio and empirical potential calculations