Abstract: The stability of small vacancy clusters including divacancy, trivacancy and tetravacancy has been studied in body-centered cubic high-entropy alloy Nb_0.75ZrTiV_0.5 in structures of random solid solution and short-range order by first-principles calculations and molecular dynamics simulations. Different from conventional body-centered cubic metals, the tightly bound configurations have a lower structural stability and are not preferred energetically in the studied high-entropy alloy. Instability of vacancy configurations leads to vacancy-atom exchanges that favor less compact configurations. The formation energy of small vacancy clusters is much smaller than its constituent elements of Nb and V due to the large structural adjustment induced by severe local lattice distortion. The difference in local lattice distortion and elemental arrangement in the vacancy neighborhood leads to significant site-to-site variation in vacancy cluster energy and configuration. The formation energy has a strong correlation with the local energy state of the vacancy configuration and the extent of structural relaxation. Compared to random solid solution, the structure of short-range order has a higher stability for the most compact cluster configurations and tends to have higher vacancy cluster formation energy. According to classical molecular dynamics simulations of cluster diffusion at high temperature, the studied high-entropy alloy has a higher probability of cluster dissociation compared to Nb and V. The unconventional energetics of small vacancy clusters is expected to have a profound impact on their generation, diffusion, dissociation, coalescence, as well as the defect microstructure evolution during irradiation.

Key words: Refractory high-entropy alloys, Small vacancy clusters, First-principles calculations, Defect energetics, Radiation defect evolution