Microscopic Aspects of Cleavage Fracture

River patterns are caused by the merging of several cleavage steps in the vicinity of grain boundaries

• The cleavage steps give the appearance of a river pattern from which the crack direction may be seen

Tongues form when the cleavage crack meets a site of deformation twinning

• The cleavage crack continues to form on the interface of the twin boundary, eventually sticking out like a tongue