Crack nucleation in magnesium oxide bi-crystals under compression

Magnesium oxide bi-crystals having a wide range of misorientation were tested under three point bending. The interaction of slip bands coming from the centre loading beam with the grain boundary were studied. When a single slip band impinged upon the boundary one of three things happened depending upon the misorientation. (I) Small misorientations (i.e. a simple tilt boundary). The shear stress concentration ahead of the pile-up nucleated dislocation loops and thereby slip bands in the adjacent grain. Cracks did not form. (II) Medium misorientations (i.e. a twist plus tilt boundary). The tensile stress concentration beneath an edge slip band nucleated a transgranular crack in the adjacent grain. Under compression, this cleavage crack decelerated and generated dislocation loops which developed slip bands in the adjacent grain. (III) Large misorientation (i.e. a large twist plus large tilt). The tensile stress concentration beneath an edge slip band nucleated an intergranular crack. In particular this work has shown that a crack can nucleate at a grain boundary in MgO at room temperature by the pile-up of a single band of edge dislocations according to the classical Zener model.