Vacancy diffusion in non-stoichiometric ε-WB2-x

The diffusion of boron (V_B) and tungsten vacancies (V_W) in the hypo-stoichiometric ε-phase of tungsten boride was studied using an Atomic (Lattice) Kinetic Monte Carlo (AKMC) approach, informed by Density Functional Theory (DFT) simulations. To account for the hypo-stoichiometric nature of the ε-phase, two limiting compositions, B-poor and B-rich, were considered. Results showed that both V_B and V_W exhibit strong anisotropic behaviour, with basal migration requiring significantly less energy than c-axis migration. Consequently, diffusion coefficients for both vacancies are orders of magnitude smaller in the c-direction regardless of B stoichiometry, indicating a predominance of 2D diffusion. This behaviour impacts the evolution of radiation-induced defects, potentially leading to anisotropic swelling. While the basal diffusivity of V_B remains largely unaffected by the stoichiometry, its c-axis diffusivity is found to be highly sensitive to boron content, which enhances vacancy migration through additional pathways. Although diffusion is found to be faster in the basal planes, increased boron occupancy slightly reduces the level of anisotropy, a trend that also diminishes at higher temperatures. Nonetheless, the V_B diffusion remains significantly anisotropic, exceeding a factor of 100 even at extreme temperatures. These findings underscore the critical role of stoichiometry in regulating vacancy behaviour and promoting densification, which is essential for optimizing tungsten boride materials in compact fusion reactor applications.