Crystal Dislocations

Line defects that make metals soft — click to apply stress and watch dislocations glide.

Dislocations are line defects in crystalline materials — places where the regular atomic arrangement is disrupted along a one-dimensional curve.

An edge dislocation is an extra half-plane of atoms inserted into the lattice. The core is a region of intense stress. Under applied shear stress, the dislocation can glide one lattice spacing at a time, moving through the crystal much more easily than sliding entire planes simultaneously — which is why metals deform plastically at stresses far below the theoretical ideal shear strength.

A screw dislocation has its Burgers vector parallel to the dislocation line, transforming the lattice planes into a helical ramp. Both types are characterised by their Burgers vector — the closure failure of a loop drawn around the defect.

When dislocations pile up against grain boundaries or tangle with each other, the material becomes harder to deform — work hardening. Annealing at elevated temperatures allows dislocations to climb and annihilate, restoring softness.