Stage-selective impact embrittlement in a bainitic steel: Direct fracture–microstructure correlation and boundary-network interpretation

Segregation-induced banding is known to degrade the impact toughness of steels, yet the fracture stage most affected by such heterogeneity and the underlying microstructural origin remain unclear. In this work, normal and segregated specimens of a high-strength bainitic rail steel were compared to clarify how segregation-induced martensitic bands control impact fracture. Instrumented Charpy testing was used to resolve the impact energy into different fracture stages, while pre- and post-impact microstructural characterization was used to compare the microstructural features of the martensitic band and the bainitic matrix and to identify microstructural evolution upon impact. The results show that segregation-induced toughness loss is stage-selective, with the most pronounced degradation occurring during stable crack propagation. The decisive microstructural feature responsible for this stage-selective loss is the boundary network: the martensitic band contains higher densities of most hierarchical boundary categories, which would conventionally imply improved crack resistance, but a markedly lower density of high-{110}-specific misorientation angle boundaries than the bainitic matrix. This unfavorable boundary network is proposed to increase the effective spacing between barriers to dislocation slip transfer, promote localized dislocation pile-up and stress concentration, and thereby reduce resistance to stable crack propagation.