Cleavage Fracture of Metal Materials

The fracture form of metal materials separated along a specific crystal plane under the action of tensile stress is called cleavage fracture. The plane where this fracture occurs is called the cleavage plane.

Cleavage fracture is a brittle fracture. Cleavage fracture does not often occur in all metals, but is common in metals based on body-centered cubic and close-packed hexagonal lattice metals.

Cleavage Fracture Process

The initial crack that occurs in cleavage is related to plastic deformation, that is, it is related to the slip of dislocations.

Taking body-centered cubic metals as an example, since the slip of dislocations along the close-packed plane has the least resistance, the slip planes are (011), (0-11) and other crystal planes.

These two planes are at a certain angle, and when dislocations slip, stress concentration will occur at the intersection. If the strength of the material is high, the stress cannot be released through plastic deformation at the stress concentration point, and the initial crack will be generated on the (001) crystal plane. See Figure 1.

The (001) crystal plane spacing is relatively large, and crack propagation is relatively easy. This (001) plane is the cleavage plane. The cleavage plane of close-packed hexagonal metal is the {0001} plane.

The microscopic morphology of the ferrite cleavage section is shown in Figure 2.

The initial crack of the cleavage crack is formed in the crystal, because the restraint stress in the crystal is large and it is difficult to deform.

The cleavage crack often ends at the grain boundary, and sometimes dimple strips are generated at the grain boundary. Because the atomic arrangement of the grain boundary is relatively messy, there are many defects such as vacancies, and the restraint is small, the degree of freedom of these parts will increase relative to the crystal.

The microscopic morphology of cleavage fracture is often in the form of a river pattern, and sometimes in the form of a tongue pattern, as shown in Figure 3.

The essence of the river pattern is the steps between the cleavage planes that are parallel to each other but have different heights. In actual metal materials, the arrangement of the lattice is not ideal, but there are subgrain boundaries and dislocations, which leads to some offsets when many crystal planes are arranged.

This causes a series of steps to form when the cleavage crack expands. These steps merge with each other to form the so-called river pattern, as shown in Figure 4.

In failure analysis, the crack direction is often determined based on the direction of the river pattern, so that the origin of the fracture can be inferred.