recrystallization

Keep heating a cold-worked metal past the recovery stage and something dramatic happens: brand-new, defect-free crystals sprout inside the beaten-up structure and grow until they have eaten it entirely. This is recrystallization. The old grains, packed with dislocations from cold work, are replaced by fresh soft grains with almost none, so the metal is reborn soft and ductile, as if the cold work never happened.

The driving force is the strain energy stored in all those tangled dislocations; the metal lowers its energy by nucleating clean new grains that consume the deformed ones. Because it depends on that stored energy, recrystallization needs prior cold work to occur, since an already-soft metal has nothing to recrystallize. It happens fastest above the recrystallization temperature, a rough benchmark of about one-third to one-half of the absolute melting temperature (for many metals around 0.4 times T_melt in kelvin). More cold work, finer starting grains, and higher purity all lower that temperature. This is exactly the annealing step that resets a work-hardened metal so it can be shaped further.

Recrystallization is the dividing line between hot and cold working. Deform below it and dislocations accumulate (the metal work-hardens); deform above it and new grains form as fast as you damage the structure (the metal stays soft, which is hot working). One design lever falls out of this: the more you cold-work a metal before annealing, the more nucleation sites, so the finer the recrystallized grains, a way to refine grain size and thus strengthen via Hall-Petch. Over-anneal, though, and the fresh grains keep growing coarse (grain growth), giving strength back.