On the fracture behaviour of magnesium oxide bi-crystals

Metallographic studies on compressed bi-crystals of magnesium oxide have revealed cracks formed at the grain boundary by piled-UP groups of edge dislocations. However, these were produced only when two slip bands, one from each grain, met at the boundary; cracks associated with a single pile-up were not observed. Crack propagation was transgranular, and by crystallographic cleavage. A mechanism for crack formation at low strains is proposed in which a source, situated on the opposite side of the grain boundary and some two to eleven microns away from a piled-up group of edge dislocations. can be activated by the stress field of this pile-up and produce dislocations which, under certain conditions, may cooperate with the activating group to form a crack.