Abstract
This study investigates the crack propagation behavior and associated deformation microstructure evolution during hydrogen-related fracture in high-strength martensitic steel, utilizing three-dimensional focused ion beam–scanning electron microscopy serial sectioning, advanced transmission electron microscopy, and finite element simulations. It was found that even sub-micrometer-scale segments of low-angle prior austenite grain boundary (PAGB) segments could act as effective obstacles to intergranular crack propagation. In several instances, crack arrest at such boundaries was accompanied by intense localized plastic deformation sufficient to induce ultrafine-grained structural refinement. For quasi-cleavage fracture, abrupt changes in crystal orientation were observed within approximately 1 μm of the crack tip, alongside elevated dislocation densities and the formation of characteristic deformation substructures, such as low-energy dislocation structures. In contrast, the microstructure beyond ~1.6 μm from the crack tip remained nearly undeformed, resembling the non-deformed specimen. Finite element simulations with a cohesive zone model suggest that the morphology of crack propagation as well as the arrestability of low-angle PAGB segments significantly influence the macroscopic mechanical response to hydrogen-related fractures. These findings demonstrate that the highly localized plasticity near both intergranular and quasi-cleavage crack tips plays a critical role in governing crack growth resistance.