Binary effect of He and H on the intra- and inter-granular embrittlement in Fe

Both helium and hydrogen are known to degrade the mechanical performance of reactor materials, but their binary effect has not been well understood. Using ab initio density functional theory calculations, we have investigated the He–H interactions both in the bulk and at the grain boundary in bcc Fe. We find an attraction of 0.56 eV between one substitutional He and one interstitial H in the bulk and a repulsion of 0.20 eV at the Σ3 (1 1 1) [ 1 1 ¯ 0 ] grain boundary. The attraction of He and H inside a grain means a reduction of combined embrittling effect in intra-granular fracture and the repulsion at the grain boundary means an enhancement of combined embrittling effect in inter-granular fracture. The interaction of He and H are interpreted with electronic structure analyses.