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Work hardening (cold working)

Work hardening is that plastic deformation at low temperature increases strength, because more dislocations form and start blocking each other's movement. The effect can be removed by annealing, where heating causes recrystallization and gives back a softer, more ductile material.

plastic deformation→more dislocations→higher strength\text{plastic deformation} \to \text{more dislocations} \to \text{higher strength}the mechanism behind work hardening

Example

A copper wire bent back and forth several times becomes stiffer and eventually more brittle — it is work hardened.

Annealing "resets" a work-hardened material, but costs energy and an extra production step.
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Part of Materials Technology: Mechanical properties.