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Dislocations are line defects in crystalline solids in which the regular arrangement of atoms is disrupted along a line. They are central to plastic deformation because they allow atoms to move incrementally, enabling much lower applied stresses than would be needed for a perfect lattice. The two primary types are edge dislocations, where an extra half-plane of atoms ends within the crystal, and screw dislocations, where the lattice is twisted around a helical line; many dislocations are mixed with both characters. The Burgers vector, defined by tracing a closed circuit around the defect, quantifies the magnitude and direction of the lattice misfit caused by the dislocation.

Dislocations arise during crystal growth, phase transformations, severe plastic deformation, and thermal processing. Under applied shear stress, dislocations glide along specific crystallographic planes; at higher temperatures they can climb via diffusion of atoms to the dislocation core. Their motion is impeded by lattice friction, interactions with other dislocations, solute atoms, and obstacles such as precipitates and grain boundaries.

The density and interactions of dislocations govern a material’s yield strength and ductility. As dislocations multiply and entangle, materials harden through work hardening. Strengthening strategies exploit dislocations: refining grain size, solid-solution strengthening, precipitation hardening, and dispersion of obstacles reduce dislocation mobility. Dislocations also influence diffusion and phase transformations, particularly in metals and ceramics.

Dislocations are routinely studied by transmission electron microscopy and diffraction techniques. In nanostructured and two-dimensional materials, dislocations can dominate mechanical response and affect electronic properties. While most common in metals, dislocations also occur in ceramics, semiconductors, and some polymers, where their behavior can differ notably from metallic systems.