Formation and migration of point defects in tungsten carbide: Unveiling the sluggish bulk self-diffusivity of WC

The formation and migration of point defects in alpha-WC was investigated using density functional theory atomic simulations. The material exhibits extremely limited deviation from stoichiometry and only at high temperatures and for carbon deficient conditions (WC1-x). Bulk self-diffusivity was found to be remarkably slow (similar to 10(14) cm(2)/s at 2400 degrees C) and strongly anisotropic, with basal diffusivity 3-4 orders of magnitude faster than c-axis diffusivity. Our findings help re-interpret previous experimental observations, which were deemed anomalous due to the surprisingly low diffusivity. At equilibrium, the self-diffusivity is dominated by C vacancies, with C interstitials and W defects providing negligible contribution to the total diffusivity. However, under irradiation conditions, interstitial diffusion is predicted to dominate owing to the higher mobility of interstitials once formed (E-m(C-i) = 1.19 eV vs E-m(V-C) = 3.48 eV). Such diffusion also exhibits strong anisotropy. This may be exploited by means of texture engineering to control the swelling of WC components exposed to radiation fields.