Crystallography of tough microcracks and relaxation of stresses initiated in packet martensite by tips of cracks

A study is made of the crystal geometry of microcracks formed in thin specimens 0.5-1 hr after preparation for electron microscopic investigations. It is shown in engineering steels that fracture is crystallographic and takes place in the martensite in planes 〈100〉 and 〈110〉. The crack passes through each lath and packet with deviations, but the overall direction does not change. The principal direction of a crack within a packet is governed by how close planes with low indices are for different orientations of the martensite and in the initial austenite. It is found that relaxation of stresses initiated by the tips of microcracks is by slip and deformation twinning.