Towards understanding the mechanism of titanium suppressing vacancy clustering/void nucleation in vanadium alloy

The energetics and dynamics of vacancy/vacancyn (Vacn) clusters in presence of Ti in vanadium alloy were systemically investigated by first-principles calculations to elucidate the mechanism of Ti suppressing vacancy clustering/void nucleation. Energetically, Ti can reduce vacancy formation energy and stabilize vacancy/Vacn cluster, and the TimVacn clusters are more stable than alone Vacn over 30% with m, n=5, corresponding to the smaller saturated size. Kinetically, mono-/di-vacancy diffusion and vacancy clustering near Ti are predicted by the CI-NEB methods and empirical formula. Vacancy diffusing towards Ti shows a lower barrier of 0.04 eV than 0.39 eV in vanadium and the energy barriers of vacancy migrating towards Ti-Vac complexes decrease gradually and are even close to zero during vacancy clustering. The migration barriers of vacancy dissociating from Ti-Vac complexes are relatively higher, and Ti changes the migration mechanism of di-vacancy from one-step to two-step. Ti significantly reduces vacancy effective diffusivity, and the effective diffusivity decreases with Ti concentration. The present results show that Ti additions provide more nucleation sites of vacancies, forming large numbers of stable small-size complexes and hindering vacancy diffusion, and eventually suppress the formation and growth of large vacancy clusters/voids.