First-principles study of interaction between vacancies and nitrogen atoms in fcc iron

Abstract

The interaction between vacancies and nitrogen atoms in fcc iron has been systematically studied by first-principles calculations. The interstitial N atoms bind strongly with vacancies into VnNm clusters, and each vacancy prefers only two N atoms that locate at the first nearest neighboring octahedral interstitial sites to the vacancy and align along the 〈1 0 0〉 directions. The number of N atoms is determined by the local lattice distortion evaluated by the expansion of atomic coordination polyhedrons with respect to the vacancy at the center. The most stable V1N2 cluster can be considered as a structural unit for the cluster development. Moreover, in larger VnNm clusters with relatively high stability, the vacancies aggregate at the center and the N atoms are around the vacancies. Then, the effect of N solute on radiation swelling depending on the N concentration is discussed from a perspective of the cluster formation. The radiation swelling increases with the N concentration because the existence of N atoms can promote the vacancy aggregation and, hence, the nucleation and growth of voids. However, as the concentration further increases, N atoms will saturate the surfaces of vacancy clusters to form barrier layers, which can hinder the combination of the VnNm clusters and the void formation.