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

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.