Deformation twins — probable inherent nuclei of cleavage fracture in ferritic steels

The vicinity of cleavage fracture of ferritic structural steels was analyzed. A strong tendency to deformation twinning was observed in the vicinity of the main cleavage crack. Numerous cleavage nuclei were revealed on the twins in the close vicinity of cleavage facets. It is concluded that deformation twinning represents an integral part of the deformation processes in the region of the propagating cleavage crack tip and that the deformation twins, which nucleate in front of the crack tip act as nucleation sites for cleavage fracture nuclei — pre-cleavage microcracks (PCMC). The size of PCMCs corresponds to that of the assumed inherent cleavage crack nuclei predicted by brittle/cleavage fracture theory. However, such nuclei have not hitherto been observed in ferritic materials.