Direct observation of the ledge-typed twin boundary in a martensitic steel

The structure at the {112} twin boundaries in martensitic steels has been extensively debated recently. On the one hand, it was claimed to be the omega phase, similar to Ti alloys. On the other hand, it was considered artifacts induced by the double diffraction effect from the overlaps of twin-matrix crystals at their boundaries. In this study, we clarify the twin structure in a martensitic steel providing a detailed investigation of the α′ martensite twin boundaries using experiment and simulation. The fundamental for rejecting the existence of the omega phase in steels is based on the atomic resolution imaging from the <111> direction of twin structure, which was so far challenged to be acquired for such nano-sized twinned systems. The obtained result confirms the ledge-typed twin boundary that causes the visually omega-like forms when observed from other zone axes. The ledges occurred on {110} planes, making them secondary twin planes with coherent characteristics. Furthermore, these ledges were found to transit in a gradual manner from the primary (112) twin plane to (011) and then to ( 1 ¯ 10 ) secondary twin planes; a perpendicular transition from (112) to ( 1 ¯ 10 ) was not observed. Such a transitional behavior was explained in view of lowering total twin boundary energy, evaluated by the theoretical prediction of system energy and the geometric phase analysis of the twin planes. Double diffraction and rotational Moiré effects clarified the extra spots in the diffraction pattern obtained at the twin-matrix overlapped regions. This study gives light of understanding twinning in the bcc crystals.