work hardening
Bend a paperclip once and it moves easily; bend it back and forth at the same spot and it gets stiffer and harder each time, until it snaps. That growing resistance is work hardening (also called strain hardening): the more you plastically deform a metal, the stronger and harder it becomes. It is deformation making the metal fight back.
The mechanism is dislocations getting in each other's way. Plastic deformation does not just move dislocations, it creates new ones, so their number multiplies as you work the metal. Soon the crystal is crowded with them, and because each dislocation carries its own strain field, they repel and tangle with one another like traffic in gridlock. A dislocation trying to glide now has to push through a thicket of other dislocations, which takes ever more stress. So the metal's yield strength climbs as you deform it, captured by the strain-hardening relation sigma = K times epsilon^n, where the exponent n (typically 0.1 to 0.5) measures how fast it hardens.
Work hardening is both a tool and a nuisance. It is how cold-rolled sheet, drawn wire, and hammered blades are strengthened without any alloying or heat treatment. But it also uses up ductility: a heavily worked metal is strong yet close to cracking, so it must eventually be softened by annealing before it can be shaped further. It vanishes on heating; warm the metal enough and the tangles clear (recovery and recrystallization), returning it to soft.
Cold-rolling annealed copper to 50 percent reduction can raise its yield strength several-fold while its elongation-to-fracture drops from tens of percent to a few percent; stronger but far less able to stretch.
Every pass of the rolling mill multiplies dislocations that tangle and jam, hardening the metal but spending its ductility.
Work hardening only sticks below the recrystallization temperature; deform a metal while it is hot enough and it re-softens as fast as you harden it, which is exactly the difference between cold and hot working.