Abstract
For polycrystalline materials, hydrogen could influence the dislocation behavior and induce intergranular fracture. However, the nanoscale interaction behaviors and underlying mechanisms remain unclear. By uniaxial straining of bi-crystalline nickel with a Σ5(210)[001] and Σ9(1-10)[22-1] grain boundaries, a transgranular to intergranular fracture transition facilitated by hydrogen is elucidated by atomistic modeling, and a specific hydrogen-controlled plasticity mechanism is revealed. Hydrogen is found to form a local atmosphere in the vicinity of grain boundary, which induces a local stress concentration and inhibits the subsequent stress relaxation at the grain boundary during deformation. It is this local stress concentration that promotes earlier dislocation emission, twinning evolution, and generation of more vacancies that facilitate nanovoiding. The nucleation and growth of nanovoids finally lead to intergranular fracture at the grain boundary, in contrast to the transgranular fracture of hydrogen-free sample.