First-principles study of helium in austenitic Fe 6.3 at% Cr alloys: Structural, stability, energetics, and clustering with vacancies

Abstract

The structural, stability and energetics of helium-vacancy-type (HenVm) clusters in Fe 6.3 at% Cr alloys are studied by means of DFT calculations, considering n (m) from 0 (0) to 6 (4). The self-trapping and accumulation of interstitial helium atoms in the vacancy-free lattice caused by strong attraction are more likely to form clusters in Fe 6.3 at% Cr alloys. The trapping capacity of vacancies strengthens with the increase of m, but attenuates gradually with the increase of n, and the addition of Cr prevents the trapping of helium atoms by multi-vacancies, effectively reducing the risk of helium atoms forming clusters. The binding energies and electronic properties confirmed from the macroscopic and microscopic levels respectively that the stability of Hen-clusters gradually decreases with the increase of n. Whether the HenVm complexes are easily formed are determined by the number of n and m. The higher density of the helium bubble nucleate precursor in Fe 6.3 at% Cr alloys, i.e. the small HenVm clusters, supply more annihilation sites for recombination of radiation-induced Frenkel pairs, resulting in better radiation swelling resistance than the bulk γ-Fe. We predict spontaneous emission of vacancies or helium atoms at smaller or larger n/m ratios. In addition, the trends of the binding energy of interstitial helium and monovacancy with helium-vacancy-type complexes are discussed in detail, which provide useful information for the explanation of complex experimental phenomena.