Plastic deformation in crystalline metals occurs largely through the motion of dislocations. Many strengthening methods work by making that motion more difficult.

Grain-size strengthening

Grain boundaries obstruct dislocation motion. Finer grains therefore generally increase yield strength over a useful range.

Solid-solution strengthening

Alloying atoms that differ in size or bonding from the host lattice distort the crystal and interact with dislocations, increasing the stress required for slip.

Work hardening

Plastic deformation creates and multiplies dislocations. As dislocation density rises, the defects obstruct one another, so continued plastic deformation requires larger stress.

Precipitation strengthening

Heat treatment can produce fine second-phase particles that impede dislocation motion. Their size, spacing and coherence with the surrounding lattice strongly affect the resulting strength.

Strength and ductility

Strengthening mechanisms often reduce ductility because they restrict the same dislocation motion that permits plastic deformation. Material design therefore involves balancing properties rather than maximizing one quantity independently.