Crystallographic characterization of the middle shelf in a two-step ductile to brittle transition for a ferrite + pearlite structure steel sheet

The middle shelf (MS) exhibits medium absorbed energy during a two-step ductile to brittle transition and appears clearly in a ferrite + pearlite steel sheet. The analysis involved using the electron backscattered diffraction technique and indicated that the traces of fracture surface and secondary cracks at MS and lower shelf (LS) correspond to low index crystallographic planes in bcc iron. Although the {001} cleavage plane in the ferrite grain is dominant for the fracture propagation path at LS, the traces at MS correspond to {001}, {112}, and {011} planes. The potential of fracture in the {001} plane at MS (i.e., the number of traces normalized by the number of equivalent planes) exceeds that in the {112} plane. However, the difference between the same is significantly lower than that at LS. Furthermore, the{112} and {011} planes are typical slip planes in bcc iron. This suggests that the fracture at MS propagates on the {001} plane with cleavage fracture as well as on the {112} and {011} planes with a slipping-off mechanism.