On the interactions between alloying element niobium and interstitial/ vacancy in Zr-Nb alloy: A first-principles study

Zirconium-niobium alloy is widely used in pressurized-water-reactors (PWRs) due to its excellent performance. Herein, the effects of Nb on the formation, migration, and clustering of vacancies/interstitials in Zr-Nb solid solution are studied using first-principles calculations. Nb lowers the formation energy of Zr interstitials and significantly enhances the anisotropy of Zr interstitial migration in the vicinity. Nb shows thermodynamic attraction to vacancies while causes a significant decrease in the vacancy migration barrier, which enhances the mobility of vacancies around Nb. In addition, we find that the vacancy-mediated Nb migration is anisotropic, and these Nb-vacancy interactions play an important role in revealing the mechanism of the precipitation of needle- like Nb phases under high irradiation fluences. Finally, we verified the anisotropic attraction of Nb to vacancies with CI-NEB method. These findings will contribute to a more in-depth comprehension on the mechanism how Nb influences the evolution process of irradiation defects in Zr-Nb alloys.