Dislocations

Dislocations

Definition: A linear or 1D defect around which some of the atoms are irregular or misaligned. This occurs during plastic deformation.

Dislocation density is given as total dislocation length per unit volume or the number of dislocations intersecting a given unit area.

Units - mm^-2

Common values for metals displayed in table:

Edge dislocation – This is an extra plane - half plane of atoms in which extra half plane terminates in the

crystal. This linear defect centres on the dislocation line (direction going into page). Surrounding the

dislocation line there is localised lattice distortion.

The Burgers vector, b is used to express the magnitude and direction of the lattice distortion. It is perpendicular to the dislocation line (displayed in the diagram).

Motion is caused by shear stress acting perpendicularly to the dislocation line.

Initially, all the atoms in plane A are part of the half plane. Then the dislocation moves to the right by a single atom and so the upper part of A bonds to the lower part of B. Hence the half plane shifts to B and the motion continues to C, then D...

Screw dislocation

A screw dislocation is a topological defect of a crystal lattice. It is formed when a shear stress is exerted on the crystal. Consequently, the upper front region of the crystal is shifted one atomic distance. If one moves around the dislocation, the lattice plane shifts by one layer (or more layers), similar to a spiral staircase. The Burgers vector of a screw dislocation is parallel to the dislocation line.

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