A superalloy is a metal alloy that performs well at very high temperatures. The base metal is usually iron, nickel, or cobalt. Normal metals, by contrast, fail in various ways as the temperature increases.
As a normal metal's temperature approaches its melting point, the metal loses strength and exhibits creep under load. Layers within the crystalline structure take advantage of crystalline defects to slide with respect to each other. Adding to this behavior, a normal metal is a collection of small crystal "domains" such that the domain boundaries are additional weak points. Metals also become more chemically reactive at high temperature. So, a hot metal surface exposed to air (oxygen) may corrode significantly. This can be disastrous.
The strongest superalloys are formed as a single metal crystal, of nickel for example, with alloying atoms selected to lock the crystal lattice against creep. Corrosion resistance is provided by fairly traditional means. Atoms of aluminum or chromium near the surface increase the resistance. The metal surface may also receive a thin coating of a more inert metal. Aluminum chloride is a common choice, possibly supplemented with traces of platinum.
In practice, superalloys are used in a variety of harsh situations (see below) and meet a variety of needs even in the same machine. A component might need to be less rigid and more tough, or it might be easy to replace and less durable. Metallurgists have developed a wide variety of superalloys with various costs and properties.
Superalloys are a natural progression of metallurgy that was underway before and during World War Two. Military jets started to appear, and their performance is limited by the temperatures and pressures that can be achieved in the engines. Traditional iron and steel alloys were too constraining.
In the 1940s and 1950s, improved casting techniques were developed for cobalt alloys and then nickel alloys. Precisely-shaped components became possible, and both metals can tolerate higher temperatures than the iron that was used historically.
Parallel research throughout the last half of the 20th century improved the precision with which superalloy items could be fabricated evenly and with various alloy mixes. The ability to grow single-crystal alloys is one of the later developments.
Superalloys are found in the turbines of jet engines and electrical generators (right). Turbine blades are the point where fuel combustion creates great heat, and the blade spin demands durability and mechanical strength.
Other parts of a gas turbine use somewhat different superalloys. For example, the turbine shaft might be designed for less heat and the exhaust duct for less load.
Diesel engines are another environment with fuel combustion at high temperature. The valves are made of a superalloy, primarily for the corrosion resistance.
Other extreme environments that use superalloys are the chemical processing vessels used in industry and nuclear power reactors such as on submarines.