precipitation strengthening

Precipitation strengthening seeds a metal with a fine dust of tiny hard particles, a second phase sprinkled through the crystal, that a dislocation cannot glide past without a fight. It is the trick that made aluminum strong enough to build airplanes: an alloy that is soft when freshly quenched grows harder over hours or days as the particles form, which is why it is also called age hardening.

It works in three steps. First, heat the alloy until the alloying element fully dissolves into a single solid solution (solution heat treatment). Second, quench it fast so that solute stays trapped in a supersaturated solution, more dissolved than equilibrium allows. Third, hold it at a moderate temperature (aging) so the excess solute precipitates out as a vast number of tiny, evenly spread particles. These particles pin dislocations: a dislocation must either cut through a particle or bow out and loop around it (Orowan looping), and both cost extra stress. The strength peaks when the particles are small and closely spaced.

The catch is over-aging. Age too long or too hot and the particles coarsen, a few growing large while the rest dissolve, so they spread far apart and become easy to bypass, and the strength falls back. So there is a sweet spot in time and temperature (peak aging) that alloy designers chase. Precipitation strengthening is the basis of high-strength aluminum (the 2xxx and 7xxx aircraft alloys), nickel superalloys for jet engines, and precipitation-hardening stainless steels. It is powerful but, like the others, trades away some ductility and needs the right alloy chemistry (a solubility that shrinks on cooling) to work at all.