Plasticity in cryogenic brittle fracture of ferritic steels: Dislocation versus twinning

The sub-surface deformation structure after cryogenic (77 K) brittle fracture in a ferritic steel was characterized by scanning electron microscopy (SEM). Twin-like structures were found in many grains below the fracture surface. Electron backscatter diffraction (EBSD) was used to identify the crystallography of the structures, and the twin relation in body-centered cubic (BCC) systems was indexed. The twins in the sub-surface area were characterized further by electron channeling contrast imaging (ECCI). This study clarifies the low temperature brittle fracture behavior in the ferritic Fe-3 wt%Si steel consists of both cleavage and plastic deformation in the form of dislocation activities and twinning.