Crystallography of brittle fracture and deformation twinning in ferritic steels

Abstract

The crystallography of brittle fracture and deformation twinning in ferritic steels is difficult to study experimentally, because of its three-dimensional aspects. The present paper reports the development of methodologies to study the phenomenon via customisation of various electron backscatter diffraction and SEM routes. It is shown that both direct (from the fracture surface) and indirect (from an adjacent polished side) measurements yield valuable information on crystallographic aspects of the fracture processes. Specifically, brittle fracture in three ferritic steels is studied: a C–Mn weld metal, a low alloy Mn–Mo–Ni steel similar to grade A533 and an ultralow carbon (0·002 wt-%C, 0·058 wt-%P) steel plate. The main conclusions resulting from development of the experimental techniques are that cleavage fracture occurs only on {001} planes, and that intergranular accommodation is present at the fracture surface. Further observations suggest that a cleavage side crack, initially deflected by a deformation twin, eventually blunts at the intersection of two deformation twins. This provides a mechanism for limiting the mean length of microcracks during brittle fracture.