A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and resistance to creep (tendency for solids to slowly move or deform under stress) at high temperatures; good surface stability; and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. A superalloy's base alloying element is usually nickel, cobalt, or nickel-iron. Superalloy development has relied heavily on both chemical and process innovations and has been driven primarily by the aerospace and power industries. Typical applications are in the aerospace, industrial gas turbine and marine turbine industry, e.g. for turbine blades for hot sections of jet engines, and bi-metallic engine valves for use in diesel and automotive applications.
Examples of superalloys are Hastelloy, Inconel (e.g. IN100, IN600, IN713), Waspaloy, Rene alloys (e.g. Rene 41, Rene 80, Rene 95, Rene N5), Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX (e.g. CMSX-4) single crystal alloys.
Superalloys are commonly used in parts of gas turbine engines that are subject to high temperatures and require high strength, excellent high temperature creep resistance, fatigue life, phase stability, and oxidation and corrosion resistance.
Superalloys develop high temperature strength through solid solution strengthening. The most important strengthening mechanism is through the formation of secondary phase precipitates such as gamma prime and carbides through precipitation strengthening. Oxidation and corrosion resistance is provided by the formation of a thermal barrier coating (TBC), which forms when the metal is exposed to oxygen and encapsulates the material, and thus protecting the rest of the component. Oxidation or corrosion resistance is provided by elements such as aluminium and chromium. Air cooling (such as the air cooling holes seen in the picture above) can additionally cool the components and allow them to operate under such conditions, protecting the base material from the thermal effects as well as corrosion and oxidation.
In most turbine engines this is in the high-pressure turbine, where air-cooled blades can face temperatures 200 °C above the melting temperature of the superalloy used.
Turbine Inlet Temperature (TIT), which is a direct parameter controlling the efficiency of a gas turbine engine, depends on the temperature capability of 1st stage high-pressure turbine blade. This component is exclusively made of nickel base superalloys.
Turbocharger turbines also use superalloys, typically electron beam welded to a steel shaft. Common superalloys in this application are for instance Inconel 713 and Mar-M 247. The latter is particularly useful for gasoline engines as it reduces the need for fuel enrichment at high loads, which improve engine efficiency.
They are also used where corrosion by media would rule-out other metal materials (e.g.) instead of stainless steel in acidic or saltwater environments.
Superalloys (such as Nimonic 80A) are also used in the poppet valves of piston engines, both for diesel and gasoline engines. This is either in the form of a single solid valve or as a bi-metallic valve. The corrosions resistance is particularly useful when dealing with the high temperatures and pressures found in a diesel engine. The superalloys resist pitting and degradation allowing operating conditions that would not be possible with regular stainless steel.
Additional applications of superalloys include: gas turbines (commercial and military aircraft, power generation, and marine propulsion); space vehicles; submarines; nuclear reactors; military electric motors; racing and high-performance vehicles, chemical processing vessels, bomb casings and heat exchanger tubing.
Read more about Superalloy: Chemical Development, Process Development, Metallurgy of Superalloys, Coating of Superalloys, Research and Development of New Superalloys, See Also