Synergistic insights into the tungsten–tantalum-vacancy system: A DFT-cluster expansion study

Abstract

Vacancies play a fundamental role in determining the mechanical properties of alloys of tungsten and tantalum, which are of interest as plasma-facing materials in nuclear-fusion applications. To understand their behaviour, they have been investigated by means of density functional theory, cluster expansion calculations, as well as Monte Carlo simulations. It is found that vacancy formation energies increase with tungsten concentration, and it is easier to remove a tantalum atom than tungsten. Vacancies tend to cluster and form voids with faceted surfaces. A strong tendency towards tantalum segregation around vacancies and voids is observed at all temperatures considered, up to 3000 K. We show how this cloud of tantalum atoms surrounding the vacancies should slow down vacancy migration, thereby influencing their dynamics. This mechanism could in turn delay void formation, which is responsible for mechanical degradation of the alloy upon neutron irradiation. In conclusion, these findings suggest that vacancies induce partial segregation in alloys, which in turn slows down vacancy diffusion, thereby potentially affecting void formation and materials degradation.