Notched zinc, broken by impact at room temperature

This is a notched zinc specimen broken by impact at room temperature. It has failed by a cleavage mechanism. One can observe the cleavage planes clearly separated by regions of local shear, where the fracture path crosses to adjacent planes. The cleavage planes are atomically flat and are not associated with much plastic deformation. At low temperature metals having b.c.c. and h.c.p structures have insufficient independent slip systems operating and are brittle, fracturing by cleavage.