Revealing the formation mechanisms of twinned substructure in low-carbon martensitic steel through multiscale characterization

The formation mechanisms of twinned substructure in low-carbon martensitic steel were systematically investigated through a synergistic analysis of spatial distribution, atomic arrangement, localized strain, and elemental segregation. The findings show that heterogeneous carbon distribution creates favorable conditions for twinning deformation. Twinned substructure preferentially nucleates in carbon-enriched depressed areas, demonstrating substantial spatial heterogeneity in both individual dimension and population distribution. In this substructure, irregular area disrupts the regular crystallographic arrangement between the matrix and twin. The width of this area varies from atomic scale (at twin boundary) to nanometer scale (within matrix/twin interior), depending on the matrix/twin dimensions. The irregular area has a hexagonal configuration and maintain a coherent relationship with the adjacent matrix/twin phase. This structural characteristic enhances carbon diffusivity, accommodates localized strain, and reduces transformation resistance, which together promote the formation of twinned substructure. Carbon concentration disparities critically regulate twin morphology and microscopic configuration. As the carbon content increases, the width of the irregular area decreases, and its preferential location shifts from matrix/twin interior to twin boundary. The identified mechanisms exhibit strong generalizability across carbon concentrations and are expected to establish a unified mechanistic framework for the formation of twinned substructure across the full carbon content spectrum.