Abstract
Metallographic and fractographic analysis of ferritic specimens subjected to mechanical tests in the transition region proved that deformation twinning is an integral part of plastic deformation in the moving crack tip region at transition and sub-transition temperatures. Cleavage microcracks, acting as inherent nuclei of cleavage, initiate on the intersections of active slip bands with unfavorably oriented twins ahead of the approaching crack tip. These pre-cleavage microcracks (PCMC) grow towards the approaching main crack. Thus, the actual fracture mode of ferritic steel is governed by the extent of twinning in the plastically strained volume, i.e. by the size and spacing of the twins.

Key words
ductile-to-brittle transition, cleavage fracture, deformation twins, ferritic steel, pre-cleavage microcrack

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