Hydrogen trapping and dynamic distribution in iron voids: A molecular dynamics study

Abstract

The supersaturated vacancies in the structural materials of nuclear fusion reactor blankets, induced by high-energy neutron irradiation, migrate and aggregate to form voids. Hydrogen (H) isotopes are captured and absorbed by these voids, forming gas bubbles within the voids, leading to H isotope retention and undesirable structural properties. However, most existing studies have primarily focused on small-sized vacancy clusters, with limited attention given to the behavior of H atoms in large-sized nanovoids. This study investigates the dynamic distribution of H in nanovoids of α-iron (Fe). The capture behavior of H atoms by vacancy clusters is calculated using dynamic annealing relaxation and molecular statics methods. Studies indicate that H primarily attaches to the quasi-octahedral interstitial sites at the void boundary in atomic form. Additionally, the number of H atoms absorbed by the vacancy clusters before saturation is linearly correlated with the cluster surface area, while the number of H molecules is linearly proportional to the cluster volume. As the amount of H increases, H molecules are generated in the voids, and the void surface gradually forms saturated H adsorption. After saturation, the H molecules subsequently dissociate into H atoms and diffuse out of the voids. H atoms permeating the Fe lattice displace vacancies and Fe atoms, causing the Fe atoms to collapse inward into the voids. Consequently, voids with a high H-to-vacancy ratio cannot remain stable. This study not only quantifies the capture efficiency and pressure evolution characteristics of nanovoid-H complexes but also provides a theoretical basis for the design of H-resistant alloys.