Dislocation: Encyclopedia II - Dislocation - Dislocation geometry

Dislocation - Dislocation geometry

Any dislocation can be described by the Burgers vector and the dislocation line. However, an introduction to these and other terms used to describe dislocations can be difficult and it is easer to begin with a simple description of an edge dislocation.

Dislocation - Edge dislocations

Edge dislocations can be visualised as being formed by adding an extra half-plane of atoms to a perfect crystal, so that a defect is created in the regular crystal structure along the line where the extra half-plane ends (Figure 1). Such visualisations can be difficult to interpret. Initially, it can be helpful to follow the process of simplification involved in arriving at such representations.One approach is to begin by considering a 3-d representation of a perfect crystal lattice, with the atoms represented by spheres (Figure A). The viewer may then start to simplify the representation by visualising planes of atoms instead of the atoms themselves (Figures B and C).

Finally a simple schematic diagram of such atomic planes can be used to illustrate lattice defects such as dislocations. (Figure D represents the "extra half-plane" concept of an edge type dislocation).

Dislocation - Burgers vector

Once a picture of an edge dislocation has been formed it is possible to begin to explain the important characteristics used to describe it.

The orientation and magnitude of a dislocation is characterised by its Burgers vector (marked in black in Figure D), which is perpendicular to the dislocation line (marked in blue in Figure D) in the case of the edge, and parallel to it in the case of the screw. In metallic materials, b is alligned with close-packed crystallographic directions and its magnitude is equivalent to one interatomic spacing.

Dislocation - Screw and mixed dislocations

Screw dislocations are more difficult to visualise, but can be considered as being formed by the insertion of a "parking garage ramp" that extends to the "edges of the garage" into an otherwise perfectly layered structure. Basically it comprises a structure in which a helical path is traced around the linear defect (dislocation line) by the atomic planes in the crystal lattice (Figure E).

In fact, the dislocations present in real crystalline solids are rarely of a pure edge nature or pure screw, rather they exhibit aspects of both types, and are therefore termed "mixed" dislocations.

Adapted from the Wikipedia article "Dislocation geometry", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki