Temperature-dependent hydrogen-induced crack propagation behaviour and mechanism in polycrystalline α-iron: Insights from molecular dynamics simulations

Understanding the interactions between hydrogen and material integrity in polycrystalline α-Fe is essential for advancing the reliability of critical infrastructure and energy systems. In this study, molecular dynamics simulations were implemented to pinpoint the crack propagation behaviour and mechanism in polycrystalline α-Fe under various hydrogen concentrations and temperatures. The results show that a phase transition from body-centred cubic to face-centred cubic structure first occurs at the crack tip, followed by grain boundary-mediated plasticity activities at room temperature devoid of hydrogen. A limited amount of hydrogen atoms (H/Fe atomic ratio<1%) induces twinning emission from the tip, and increasing temperature further enhances dislocation plasticity as a consequence of decreased unstable stacking fault energy, thereby leading to the blunting of the crack tip. At high hydrogen concentrations (H/Fe atomic ratio>1%), the formed hydrides ahead of the crack tip suppress the phase transition, and concurrently temperature-enhanced dislocation plasticity disappears. As a consequence, the crack propagation proceeds via grain boundary cavity nucleation and growth, and ultimately evolves into intergranular fracture. These findings provide an atomistic-level explanation for temperature-dependent hydrogen-crack interaction mechanisms, and reveal a transition in the fracture mode from ductile transgranular to intergranular failure associated with locally high hydrogen concentrations found in the experiments.