PNNL

Abstract

The nucleation and growth of helium (He) bubbles in the bulk and at S3 {112} and ?73b {661} grain boundaries (GBs) in bcc iron have been investigated using molecular dynamics simulations. The results show that a 1/2 {111} dislocation loop is formed with the sequential collection of interstitial crowdions at the periphery of the He cluster and is eventually emitted from the He cluster. Insertion of 45 He atoms into a He cluster leads to the formation of a 1/2 dislocation loop in S3 GB. It is of interest to notice that the transition of a dislocation segment through the GB leads to the formation of a step at the GB plane following the loop formation, accounting for the formation of a residual GB defect. A 1/2 loop, with a {110} habit plane, is emitted with further increase of the He bubble size in the S3 GB. In contrast, the sequential insertion of He atoms in ?73b GB continuously emits self-interstitial atoms (SIAs), but these SIAs rearrange at the core of the inherent GB dislocation, instead of forming a dislocation loop, which leads the GB dislocation to propagate along the [-1 -1 12] direction. In the bulk and S3 GB, the He bubble exhibits three-dimensionally spherical shape, but it forms longitudinal shape along the dislocation line in the ?73 GB, a shape commonly observed at GBs in experiments.