Towards understanding the trapping, migration and clustering of He atoms in W–Ta alloy

We investigated the properties of He defects, the migration of a single He atom and the formation of He clusters in W–Ta (atomic ratio 1:1) alloy via first-principles density functional theory calculations. Results showed that He most preferred to occupy the substitutional site first and then the tetrahedral and octahedral interstitial sites. The formation energies of interstitial He in W–Ta alloy are between in bulk W and Ta. While the formation energies of substitutional He in W–Ta alloy were higher than those in bulk W and Ta. Structural relaxation and energy calculations revealed that the formation energies of He clusters linearly increased with cluster size. The binding energies of even-numbered He clusters were lower than those of their neighbouring odd-numbered He clusters. The migration calculation showed that the energy of SubTa was lower, and it had stronger attraction to He atom. Density of state and electronic structure were analysed to reveal the underlying mechanisms governing cluster formation. Results demonstrated that the hybridizations of p-projected DOS of He and its neighbouring metal atoms were responsible for the site-preference order of He in DOS. Moreover, the differences in charge density between He and W(Ta) were negative under all conditions, suggesting the repulsive interaction of He and W(Ta) in W–Ta alloy. This study provides a good reference for evaluating the influence of He on the performance of W–Ta alloy for plasma-facing materials.