Age hardening is a type of heat treatment used in metallurgy to strengthen metal alloys. It is also called precipitation hardening, as it strengthens metal by creating solid impurities, or precipitates, in the alloy that prevents dislocations in the alloy's crystalline structure. Its name comes from the point in the hardening process in which the metal is aged, either by heating it for an extended of time or keeping it stored at a lower temperature for an extended period before use so that these precipitates can form. This treatment is used on malleable alloys, such as those made from nickel, magnesium, and titanium, as well as some types of steel.
Metal undergoing age hardening is heated to a high temperature, which varies according to the materials being used and the desired properties of the final result. For example, maraging steel is heat treated at about around 1510°F (about 820°C). Alloying materials are added and allowed to diffuse through the metal until the heated metal is supersaturated with them, meaning that the amount of these materials dissolved in the metal is higher than would be possible for a solid solution at room temperature.
Next the metal is aged. In some alloys, this is done by keeping the metal heated for several hours at a temperature lower than that of the initial phase but still much hotter than room temperature. Other alloys are stored for days or weeks at room temperature. At lower temperatures, it is no longer possible for all of the alloying materials to remain dissolved in the supersaturated metal, and so some of it undergoes precipitation and separates from the solid solution, becoming impurities spread throughout the metal. The temperature at which the aging process occurs affects how this precipitation occurs, and so influences the mechanical properties of the resulting alloy.
These impurities created by the hardening process strengthen the metal by interfering with the movement of crystallographic defects called dislocations, which result from misalignments in the atoms that form the metal's crystalline structure. Dislocations make metal more vulnerable to being irreversibly bent by outside forces. Their resistance to dislocation gives age-hardened alloys high yield strength and the ability to resist permanent deformation when under heavy strain.
Alloys created by the age hardening process have many uses, especially in applications where high strength and good performance at high temperatures are needed. Maraging steel is used in engine parts and in the construction of missiles and rockets. Age-hardened aluminum alloys made with metals such as nickel, copper, and zinc have frequently been used in the construction of aircraft. The alloy Rene 41, made from nickel alloyed with molybdenum, titanium, chromium, and cobalt, is used in applications involving extreme strain and temperature, such as jet engines.