Atomic-Scale Insights into Hydrogen-Induced Ductile-to-Brittle Transition in Polycrystalline α-Fe with Varying Hydrogen Concentrations and Grain Sizes

With the development of hydrogen pipelines, hydrogen embrittlement has become a critical issue affecting the safe service of pipeline steels. However, the critical hydrogen concentration for the hydrogen-induced ductile-to-brittle transition and its dependence on grain size remain unclear. Molecular dynamics simulations were performed to elucidate the effects of hydrogen concentration and grain size on the hydrogen-induced ductile-to-brittle transition by investigating hydrogen diffusion, dislocation emission, and fracture evolution in polycrystalline α-Fe. The results demonstrate that, with increasing hydrogen concentration, hydrogen atoms preferentially diffuse along grain boundaries and gradually reach segregation saturation, resulting in continuous degradation of mechanical properties. The initial fracture strain of both fine-grained and coarse-grained models decreases by more than 40%, while fracture energy decreases by more than 50%. Crack surface areas increase by factors of 2.48 and 1.86, respectively, indicating greater hydrogen sensitivity of the fine-grained model. At grain-boundary hydrogen concentrations of approximately 7–9%, suppressed grain-boundary dislocation emission triggers the transition from plastic deformation to brittle intergranular fracture, marking the onset of the hydrogen-induced ductile-to-brittle transition. A quantitative relationship between grain size and grain-boundary atom fraction was established to predict the critical hydrogen concentration for this transition. This study reveals the role of hydrogen concentration in regulating dislocation emission and intergranular fracture, providing insights for hydrogen embrittlement assessment and pipeline material design.