dislocations

Dislocations are line defects in crystalline solids that represent irregularities in the arrangement of atoms along a line. They permit plastic deformation at stresses much lower than would be needed to shear perfect crystals. A dislocation is characterized by its Burgers vector, which measures the lattice distortion around the defect, and by the direction of the dislocation line.

Idealized dislocations come in three basic types: edge, screw, and mixed. An edge dislocation has a Burgers vector perpendicular to its line and represents the termination of an extra half-plane of atoms. A screw dislocation has a Burgers vector parallel to the dislocation line, producing a helical distortion of the lattice. Mixed dislocations have components of both. The core region where the lattice is highly distorted is surrounded by an elastic field that extends into the crystal.

Dislocations move by glide on crystallographic slip planes. Screw dislocations can glide on any plane containing their line, while edge dislocations glide on specific slip planes associated with their Burgers vector. At higher temperatures, climb can occur, allowing dislocations to move out of their glide planes via diffusion of vacancies or interstitials. The energy of a dislocation per unit length (line energy) and the Peierls stress, a lattice resistance to motion, influence how easily dislocations move.

Dislocations multiply and interact, giving rise to work hardening and complex microstructures. Sources such as Frank–Read generators produce dislocation loops that expand under load. Interactions can lead to annihilation, pinning, jog formation, or cross-slip, all shaping the strength and ductility of the material. Dislocation density increases with plastic deformation, and materials science seeks to control dislocations through alloying, heat treatment, and processing to tailor mechanical properties.

Observation and analysis use techniques like transmission electron microscopy and diffraction. Understanding dislocations is central to designing metals, ceramics, and semiconductor materials with desired performance.