Role of hydrogen in stability and mobility of vacancy clusters in tungsten

We investigated the influence of hydrogen(H) on the stability and mobility of small vacancy clusters in tungsten(W) using the first-principles calculations. It is found that the presence of H not only increases the binding energy of small vacancy clusters but also changes their most favorable configurations, owing to the strong attractive interaction between H and vacancies in W. Specifically, the binding energy of di-vacancy changes from negative to positive when the H atoms are introduced. These results suggest that the H addition can significantly promote the stability of small vacancy clusters in W. More importantly, although the migration energy barriers of H-vacancy(H-V) complexes are slightly higher than that of pure vacancy clusters, the activation energy required for complexes migration is always lower than that for dissociation. Therefore, contrary to the conventional view, the collaborative motion of H and small vacancy clusters is possible, because of the low migration energy barriers. Based on the energetic results obtained by first-principles calculations, the mean lifetime and mean diffusion distance of small H–V complexes at different temperatures are examined by the object kinetic Monte Carlo simulations. The small H–V complexes are found to be stable and mobile at moderate temperature, and thus may affect the co-evolution of H and vacancies. These results shed light on the important role of H on the vacancy behaviors and provide a good reference for understanding the defects evolution in W.