Energetics of vacancy clusters in TiVTaW low-activation high-entropy alloy investigated by ab initio calculations

High-entropy alloys (HEAs) have attracted attention as potential candidates for nuclear fusion reactor materials due to their superior mechanical properties and irradiation resistance. The TiVTaW low-activation HEA exhibits an especially large variation in the cohesive energies of the pure metals of its alloying elements, leading to the characteristic of energetically heterogeneous atomic environments that differ at each atomic site, a focal point of this study. To evaluate the influence of this site-to-site variation on vacancy cluster formation, in this study, density functional theory calculations were conducted on small vacancy clusters consisting of the nearest neighbor configurations in equimolar TiVTaW. The monovacancy formation energies in TiVTaW were found to span a significantly broader range compared to those in other HEAs. Moreover, the range of vacancy cluster formation energies in TiVTaW was observed to be nearly identical to the range of the vacancy cluster formation energies of its alloying elements in the pure metals. Our calculations also showed that clusters with five or fewer vacancies exhibit a lower mean binding energy compared to the pure metals of the alloying elements, indicating that vacancy clusters in TiVTaW are less stable than those in pure metals. Additionally, the distribution of cluster binding energies for sizes less than five-vacancy clusters was found to range from-1.60 to 3.12 eV, which includes unstable clusters with negative binding energies. These energetic characteristics arising from the heterogeneous atomic environments unique to HEAs can influence void nucleation under irradiation. Furthermore, based on a thermodynamic model, local maxima in the free energy change correlating with cluster size were observed during the nucleation of vacancy clusters, indicating the generation of metastable small vacancy clusters. These findings provide a new perspective on irradiation damage mechanisms in general HEAs.