First-principles investigations of intrinsic point defects and helium impurities in vanadium monocarbide

Abstract

Helium (He) trapping at carbide precipitates is significant for understanding the macroscopic effects of He in the normal reduced activation ferritic/martensitic (RAFM) steels and castable nanostructured alloys (CNAs) under irradiation. In this study, the behavior of intrinsic point defects and He impurities in VC is investigated by first-principles calculations. The formation of carbon (C) vacancy is quite easy and vacancy makes covalent interaction of neighboring C and V atoms more obvious. Besides, the diffusion of vacancy in VC is significantly slower than that in Fe matrix. It's worth noting that the interaction between carbon vacancies is repulsive which means that the formation of large voids maybe difficult in VC. Apart from vacancy, He impurities are also investigated. Interstitial He is less stable and diffuses more easily inside VC precipitate than inside TaC and Fe matrix. Besides, vacancy can trap much more He in VC than in Fe matrix. The result means that vacancy-abundant VC particle may have stronger ability to trap He than Fe matrix. Due to the repulsion of vacancies, we suggest that the formation of large He bubbles in VC is difficult. That is to say, little He bubbles have a dispersive distribution in VC. These theoretical results can help to understand the behavior of vacancy and He impurities in VC precipitate and further the bubble formation at precipitate in normal RAFM steels and CNAs.