What Does Cleavage Mean?
Cleavage may refer to:
- Fracture of a crystal by crack propagation across a crystallographic plane
- The tendency to cleave or split along definite crystallographic planes
- Breakage of covalent bonds
Cleavage is a physical property traditionally used in mineral identification, and crystal cleavage is of technical importance in the electronics industry.
Brittle transgranular fracture of a crystalline material takes place by cleavage between two adjacent crystallographic planes within the individual grains. The crack grows through the grains (transgranular cleavage), which ultimately leads to corrosion or material failure.
Corrosionpedia Explains Cleavage
Cleavage is a low-energy fracture that propagates along well-defined low-index crystallographic planes known as cleavage planes. Cleavage is a brittle process that occurs on the plane of maximum normal stress. It occurs in body-centered cube (BCC) or hexagonal close-packed (HCP) metals, particularly in irons and steels, below the ductile-to-brittle transition temperature.
In metals that possess these microstructures, the movement of dislocations is affected by the thermal agitation of atoms. The cleavage process in BCC and HCP metals occurs by separation of normal to crystallographic planes of high atomic density. Microscopic examination of a fracture surface from cleavage typically reveals distinctive "river lines" indicative of propagation by fracture along nearly parallel sets of cleavage planes.
Since engineering alloys are polycrystalline and contain grain and subgrain boundaries, inclusions, dislocations and other imperfections that affect a propagating cleavage fracture, truely featureless cleavage is seldom observed. These imperfections and changes in crystal lattice orientation, such as possible mismatch of the low-index planes across grain or subgrain boundaries, produce distinct cleavage fracture surface features, such as:
- Cleavage steps
- River patterns
- Feather markings
- Herringbone patterns
In brittle crystalline materials, fracture can occur by cleavage as the result of tensile stress acting normal to crystallographic planes with low bonding. In amorphous solids, by contrast, the lack of a crystalline structure results in a conchoidal fracture, with cracks proceeding normal to the applied tension.