Atomistic understanding of helium behaviors at grain boundaries in vanadium

Using first-principles calculations, we systematically investigated the interaction of He (helium) with high-angle grain boundaries (GBs) in vanadium alloy. Three representative grain boundaries were examined: Sigma 3(1 2 1) GB with a compact structure, Sigma 3(1 1 1) and Sigma 5(0 1 2) GBs with open structures. Our results reveal that Sigma 3(1 2 1) GB has higher resistance to He segregation compared with the other GBs, since the segregation energy of He at Sigma 3(1 2 1) is high and He atoms are hardly trapped. All three GBs can decrease the He diffusivity along GB compared with bulk diffusion in vanadium and provide obstacles for He migration between the GB cavities. For the effect of He on the theoretical tensile strength of GBs, the cohesive strength of V GBs is weaken significantly, especially for Sigma 3(1 1 1) and Sigma 5(0 1 2) GBs, which results from that He can deplete considerably the charge density of GB region and weaken the interlayer bonding strength of GBs. It is worth noting that the low content of He (when one atom segregates at GBs) decrease slightly the cohesive strength of Sigma 3(1 2 1) GB, indicating that Sigma 3(1 2 1) exhibits better resistance to He-induced GB embrittlement compared with the other GBs. The physical mechanisms involved in He behaviors in the three distinct GBs are discussed in detail.