Characterization of crystallographic fracture surfaces in Fe–33Mn–6Si alloy

•Fracture surfaces of Fe–33Mn–6Si alloy are analyzed by microscopy.•Observed features indicate quasi-cleavage-type failure with fatigue crack growth.•Various facet morphologies indicate single- versus multiple-slip deformation.•Tensile fractures on γ-twin boundary and surface of ε-martensite are identified.•Fractographic analysis provides information on γ/ε dual-phase steel characteristics. The fatigue fracture surfaces of Fe-33Mn-6Si alloy were observed for detection of the microscopic features of quasi-cleavage fracture, which are reported for γ/ε dual-phase steels. Facet-like morphologies were found in both the fatigue crack origin and growth regions; however, their microscopic features were different. Smooth features implying single-slip deformation were observed at the origin, while characteristic regular triangular and tetrahedral configurations, reflecting {1 1 1} planes and indicating multiple-slip deformation, were observed at the growth region. Some unique morphological features were found in the tensile fracture region following the fatigue fracture region. These distinct microscopic features might indicate the fracture of annealing twin boundaries. Other features on the surface of the primary ε-martensite variant from collision with the secondary ε-martensite variant were also recognized. The variety of microscopic features on the quasi-cleavage fracture surface detected in this study may guide understanding of the deformation and fracture mechanism of γ/ε dual-phase steels.