age hardening

Some alloys get stronger just by sitting: you dissolve a bit of alloying element at high temperature, freeze it in place by quenching, then let fine particles slowly precipitate out, and each particle becomes an obstacle that snags dislocations. Age hardening (precipitation hardening) is this three-step trick, and it is how aluminium aircraft alloys reach steel-like strength at a third of the weight.

The three steps are: (1) solution treat, heating so all the solute (for example copper in aluminium) dissolves into a single solid solution; (2) quench, cooling fast to trap a supersaturated solid solution, holding more solute than equilibrium allows; (3) age, holding at a moderate temperature (or even room temperature) so the excess solute precipitates as a fine dispersion of tiny particles. Those particles impede dislocation motion, and strength climbs. For example, Al with 4 percent Cu aged at around 190 degrees C peaks in a few hours.

Strength rises to a peak then falls again if you age too long or too hot: the particles coarsen and become fewer and farther apart, so dislocations slip between them. This over-aging is why aging is timed. And it only works for alloy systems whose solubility drops steeply on cooling (like Al-Cu, and some Ni and Mg alloys); it is a different mechanism from steel's martensite hardening, with no phase change of the matrix, just precipitates.