Three-dimensional crystallographic and local variant configuration analysis of lath martensite in medium-carbon steel

Akinobu Shibata; Goro Miyamoto; Shigekazu Morito; Akiko Nakamura; Taku Moronaga; Houichi Kitano; Ivan Gutierrez-Urrutia; Toru Hara

doi:10.1016/j.actamat.2026.121981

ジャーナル論文 Three-dimensional crystallographic and local variant configuration analysis of lath martensite in medium-carbon steel

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Akinobu Shibata, Goro Miyamoto, Shigekazu Morito, Akiko Nakamura, Taku Moronaga, Houichi Kitano, Ivan Gutierrez-Urrutia, Toru Hara. Three-dimensional crystallographic and local variant configuration analysis of lath martensite in medium-carbon steel. Acta Materialia. 2026, 307 (), 121981. https://doi.org/10.1016/j.actamat.2026.121981

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The microstructures and crystallographic features of lath martensite in a medium-carbon steel were investigated using focused ion beam-scanning electron microscopy serial sectioning and automated crystallographic orientation mapping in transmission electron microscopy techniques. The analysis revealed that, despite the complex and interwoven overall morphology, individual laths possess well-defined, plate-like shapes. Habit plane / lath boundary plane orientations determined by two-surface trace analysis spanned a range from (2 5 2)A to (2 9 5)A in the austenite coordinate system and were centered around (3 8 1)M in the martensite coordinate system. These results are consistent with predictions from the phenomenological theory of martensitic crystallography incorporating double lattice-invariant deformations and a high degree of tetragonality. Crystallographic variant analysis showed pairing tendencies among variants sharing parallel close-packed planes, along with the presence of nanoscale twin-related domains distinct from typical (1 1 2)M transformation twins. These domains were interpreted as twin-orientation-related intralath structures that formed ahead of the longitudinal growth front of a martensite plate and were subsequently embedded by its lateral growth. Micromechanics calculations further suggested that stress-induced variant selection, enhanced by high tetragonality, plays a critical role in the preferential formation of these twin-orientation-related intralath structures. Although the lath has traditionally been regarded as the smallest structural unit, detailed characterization of a twin-orientation-related intralath structure reveals an additional level of hierarchy, distinct from conventional defect substructures, fundamentally redefining the established microstructural concept of lath martensite.

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