Hydrogen modulated dislocation reaction and defect accumulation in bcc metals

The interaction between dislocations is fundamental to plastic deformation, work hardening, and defect accumulation. While much attention has been focused on effects of solutes on individual dislocations, their influence on dislocation-dislocation reactions remains largely unexplored. In this work, using atomistic simulations of iron as a model bcc system, we uncover a novel mechanism by which hydrogen (H) fundamentally alters the reaction dynamics between 〈111〉/2 screw dislocations, promoting the formation of 〈001〉 edge dislocation junctions, a process that would normally be unfavorable in H-free conditions. This phenomenon arises from the dislocation-character-dependent segregation behavior of H, which reduces the line energy of 〈001〉 edge dislocation and stabilizes the junction. Once formed, these junctions serve as strong pinning sites, impeding the motion of 〈111〉/2 dislocations and facilitating the formation of 〈001〉 vacancy-type dislocation loops. Under continued deformation, these H-decorated loops accumulate locally, providing nucleation sites for structural damages such as cracking and blistering. This mechanism is generic to bcc metals and highlights the critical role of H in dislocation reactions, defect accumulation, and failure initiation. Our findings bridge atomistic mechanisms with recent experimental observations, reshaping our understanding of dislocation behavior in H-rich environments.