Edge and Screw dislocations

Edge Dislocation

Interstitial type dislocation.

An extra half plane of atoms is added to the crystal structure. The lattice distorts around it.

Under sheer stress the plane moves to the edge of the lattice as the bonds break and reform in each plane. This eventually leaves a half plane of atoms at the edge.

Screw Dislocation

In a screw dislocation part of the crystal structure is shifted to one side (as shown on the top image) leaving a spiral path in the atoms (as shown on the bottom image). Screw dislocations are neither vacancy or interstitial dislocations.

Most dislocations are mixed dislocations, having part screw and parts of edge dislocations.

Burgess Vector

Mathematical vector (magnitude and direction) to describe distortion of the structure due to the dislocation. In a screw dislocation the dislocation line and vector are parallel, where as in an edge dislocation they are perpendicular as the atoms are shifted across.