First-principles study of diffusion of intrinsic point defects in tungsten borides

Abstract

The evolution of point defects is crucial in determining the radiation resistance of materials in a fusion environment. A comprehensive investigation of diffusion mechanisms for intrinsic point defects in six stoichiometric tungsten borides (WxBy), including W2B, WB, WB2, W2B5, WB3 and WB4, has been performed using first-principles density functional theory (DFT) calculations based on the W boride with a minimum formation enthalpy for each involved stoichiometry. The calculations indicate that boron point defects are more mobile than W point defects in each W boride. The activation energy of the W vacancy (VW) diffusion in W borides increases with B content. In general, with increasing B content, the activation energy for one-dimensional (1D) diffusion of B vacancy (VB) slightly decreases, while the activation energy for 3D diffusion of VB does not show a clear trend with B content. VW in B-rich WxBy (y/x > 1) enhances the mobility of adjacent B lattice atoms. In W2B, WB and WB2 compounds, complex configurations of W interstitials (IW), which involve B displacement, are more energetically stable than single W atoms at interstitial sites; while no or less stable complexes of W interstitials are observed in W2B5, WB3 or WB4. The activation energy for 1D/2D diffusion of IW in B-rich compounds increases with B content, and the 3D diffusion of IW in WB3 has the largest activation energy. The activation energy for 1D/2D diffusion of B interstitials (IB) generally increases with B content; however, the activation energy for 3D diffusion of IB is dependent on specific W boride composites. This study is helpful in building a database related to the diffusion behavior of defects in stoichiometric W borides. It is important to note pre-existing, as well as irradiation-induced, defects in non-stoichiometric WxBy will impact the diffusion kinetics of all intrinsic species, which needs further investigation.