Atomistic simulations of helium clustering and grain boundary reconstruction in alpha-iron

The accumulation and clustering of He atoms at Σ3Σ3 〈1 1 0〉 {1 1 2} and Σ73bΣ73b 〈1 1 0〉 {6 6 1} grain boundaries (GBs) in body-centred cubic Fe, as well as their effects on GB reconstruction, have been investigated using atomic-level computer simulations. The accumulation of He atoms and the evolution of the GB structure all depend on local He concentration, temperature and the original GB structure. At a local He concentration of 1%, small He clusters are formed in the Σ3Σ3 GB, accompanied by the emission of single self-interstitial Fe atoms (SIAs). At a He concentration of 5%, a large number of SIAs are emitted from He clusters in the Σ3Σ3 GB and collect at the periphery of these clusters. The SIAs eventually form 〈1 0 0〉 dislocation loops between two He clusters. It is likely that impurities may promote the formation of 〈1 0 0〉 loops and enhance their stabilities in α-Fe. At a He concentration of 10%, the large number of emitted SIAs are able to rearrange themselves, forming a new GB plane within the Σ3Σ3 GB, which results in self-healing of the GB and leads to GB migration. In contrast to the Σ3Σ3 GB, He clusters are mainly formed along the GB dislocation lines in the Σ73bΣ73b, and the emitted SIAs accumulate at the cores of the GB dislocations, leading to the climb of the dislocations within the GB plane. As compared to bulk Fe, a higher number density of clusters form at GBs, but the average cluster size is smaller. The product of cluster density and average cluster size is roughly constant at a given He level, and is about the same in bulk and GB regions, and varies linearly with the He concentration.