A change in the properties of certain metals and alloys that occurs at ambient or moderately elevated temperatures after hot working or a heat treatment (quench ageing in ferrous alloys, natural or artificial ageing in ferrous and nonferrous alloys) or after a cold-working operation (strain aging). The change in properties is often, but not always, due to a phase change (precipitation), but never involves a change in chemical composition of the metal or alloy. See also age hardening, artificial aging, natural aging, averaging, precipitation hardening, precipitation heat treatment, quench aging, and strain aging. .