First-principles study of helium solution and diffusion in tungsten borides

The behavior of helium (He) in plasma facing materials (PFMs) is crucial for the performance of PFMs under neutron irradiation in a fusion environment. First-principles density functional theory calculations have been performed to model the solution and diffusion of He in six stoichiometric ground-state tungsten borides (WxBy), including W2B, WB, WB2, W2B5, WB3 and WB4. The solution energy of He interstitial in W borides linearly decreases with increasing B content, except for the deviation in WB and WB3. The mechanical and electronic contributions to He solubility in WxBy have been discussed. The calculated diffusion energetics indicates that He transport in W2B5, WB3 and WB4 is much more difficult than in W2B, WB and WB2. Subsequently, we investigate the influence of vacancies on the He behavior. Except for WB3, He is more strongly bound in W vacancies rather than B vacancies and the He binding energy to a W vacancy generally decreases with increasing B content in WxBy. These findings can be used to predict the potential influence of He on the properties of different W borides in the fusion environment.