Line defects

Dislocation

A Dislocation is a line discontinuity in the regular crystal structure.

There are two basic types: Edge dislocations, and Screw dislocations.

• An Edge dislocation in a Metal may be regarded as the insertion (or removal) of an extra half plane of atoms in the crystal structure.

The regions surrounding the dislocation line are made of essentially perfect crystal.

The only severe disruption to the crystal structure occurs along the dislocation line (perpendicular to the page).

Note that perpendicular to the page, the line may step up or down. These steps are known as jogs.

• A Screw Dislocation changes the character of the atom planes.The atom planes no longer exist separately from each other.

They form a single surface, like a screw thread, which "spirals" from one end of the crystal to the other.

(It is actually a helical structure because it winds up in 3D, not like a spiral that is flat.)

In the average crystal structure, there are ~1012 m of dislocation lines per m3 of crystal.

Combinations of edge and screw dislocations are often formed as edge dislocations can be formed by branching off a screw dislocation.