A dislocation is a line defect in a crystal. Its presence allows one part of the crystal to slip relative to another through the progressive rearrangement of bonds near the dislocation line, rather than by shearing an entire atomic plane at once.

Two idealized geometries are useful. An edge dislocation can be pictured as an extra half-plane of atoms terminating inside the crystal. A screw dislocation produces a helical mismatch of atomic planes. Real dislocations commonly contain both characters.

The magnitude and direction of lattice mismatch are described by the Burgers vector $\mathbf b$.

Slip systems

Dislocation motion is easiest on particular crystallographic planes and directions. A combination of slip plane and slip direction is a slip system. For example, the common close-packed slip systems of FCC metals are written ${111}\langle110\rangle$: slip occurs on a member of the ${111}$ plane family along a compatible $\langle110\rangle$ direction.

Crystal structure therefore affects how many easy deformation modes are available.

A resolved shear stress drives slip. In a single crystal loaded in tension, the shear stress acting on a chosen slip system depends on its orientation relative to the applied load; this is why differently oriented grains begin plastic deformation at different macroscopic stresses.

Why dislocations control strength

A perfect crystal would require an enormous shear stress to move one half of the lattice over the other simultaneously. A mobile dislocation lowers that barrier dramatically. Conversely, obstacles that impede dislocation motion—other dislocations, solute atoms, precipitates and grain boundaries—raise the stress required for plastic flow.

Dislocations therefore connect atomic-scale crystal defects to macroscopic yield strength, ductility and work hardening.