strainhardening

Strain hardening, also known as work hardening, is the increase in a metal's yield strength and hardness that occurs when it undergoes plastic deformation, most commonly during cold working. The process raises resistance to further deformation without significant change in composition.

The primary mechanism is the accumulation of dislocations within the crystal lattice. Each plastic step creates new dislocations and causes existing dislocations to interact and entangle with others, forming dislocation forests and cells. These interactions hinder the motion of dislocations and require higher applied stress to continue deforming the material, increasing the yield strength and hardness while typically reducing ductility. The relation between stress and strain during hardening is often described by Hollomon's equation, σ = σ_y + K ε^n, where n is the strain hardening exponent and K is a strength coefficient.

Strain hardening is temperature dependent. At room temperature and moderate strains, metals show substantial work hardening. Elevated temperatures enable recovery and recrystallization, which reduce dislocation density and diminish hardening. Strain rate can also influence hardening: higher rates generally raise flow stress and can increase apparent hardening, although microstructural processes may vary with temperature and alloy.

Implications include the design of forming processes, such as rolling, drawing, and bending, where work hardening affects formability and final properties. To restore ductility after excessive hardening, annealing or recrystallization heat treatments are used. Some alloys are engineered to balance strain hardening with other strengthening mechanisms, such as precipitation hardening, to achieve a combination of strength and ductility.

While strain hardening is most often discussed for metals, similar effects occur in some polymers and ceramics under certain conditions, where increased entanglements or interactions impede molecular motion and increase stiffness.