Some metals are classified as precipitation hardening metals. When a precipitation hardening alloy is quenched, the alloying elements are retained in solution, resulting in a soft metal. Ageing a "dissolved" metal allows the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of solution and act as the reinforcing phase, thus increasing the strength of the alloy. Alloys can be divided into naturally ageable, meaning that precipitates form at room temperature, and artificially ageable, meaning that precipitates form at elevated temperatures. The 2000, 6000 and 7000 series aluminium are some of the materials that can be hardened in this way.