Aging, or precipitation hardening, is a heat treatment used in metallurgy to strengthen metal alloys by creating solid impurities that prevent dislocations in the crystalline structure. The metal is heated to a high temperature, then aged at a lower temperature to allow impurities to form. This process is used on malleable alloys and results in high yield strength and resistance to permanent deformation. Alloys created by aging have many uses, including in engine parts, rocket construction, and aircraft construction.
Aging is a type of heat treatment used in metallurgy to strengthen metal alloys. It is also called precipitation hardening, as it strengthens the metal by creating solid impurities, or precipitates, in the alloy that prevent dislocations in the crystalline structure of the alloy. Its name comes from the point in the hardening process where the metal is aged, either by heating it for an extended period of time or by 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 based on nickel, magnesium and titanium, as well as on some types of steel.
The age-hardened metal is heated to a high temperature, which varies depending on the materials used and the desired properties of the end result. For example, maraging steel is heat treated to about 1510°F (about 820°C). The 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 greater than possible for a solid solution at room temperature.
Subsequently the metal is aged. In some alloys this occurs by keeping the metal heated for several hours at a temperature lower than that of the initial phase but still much hotter than ambient temperature. Other alloys are stored for days or weeks at room temperature. At lower temperatures, it is no longer possible for all the alloying materials to remain dissolved in the supersaturated metal, and therefore some of it undergoes precipitation and separates from the solid solution, becoming impurities scattered throughout the metal. The temperature at which the aging process occurs affects how this precipitation occurs and therefore affects 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 make up the metal’s crystal structure. Dislocations make the metal more vulnerable to being irreversibly bent by external forces. Their resistance to dislocation gives age-hardened alloys a high yield strength and the ability to resist permanent deformation when subjected to severe stresses.
Alloys created by the age-hardening process have many uses, especially in applications where high strength and good high temperature performance are needed. Maraging steel is used in engine parts and in rocket and rocket construction. Age-hardened aluminum alloys made from metals such as nickel, copper, and zinc were often used in aircraft construction. Rene 41 alloy, composed of nickel alloyed with molybdenum, titanium, chromium and cobalt, is used in applications that require extreme stresses and temperatures, such as jet engines.
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