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A superalloy, or high-performance alloy, is a metallic material that exhibits excellent mechanical strength and creep resistance at high temperatures, fatigue life, phase stability, good surface stability, and corrosion and oxidation resistance.
One of the most important superalloy properties is high temperature creep resistance. In superalloys the gamma prime phase [Ni3(Al,Ti)] present acts as a coherent barrier to dislocation motion and is a precipitate strengthener. Superalloys develop high temperature strength through solid solution strengthening. Oxidation and corrosion resistance is provided by the formation of a protective oxide layer which is formed 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 aluminum and chromium. By far the most important strengthening mechanism is through the formation of secondary phase precipitates such as gamma prime and carbides through precipitation strengthening.
Superalloys are based on Group VIIIB elements and usually consist of various combinations of Fe, Ni, Co, and Cr, as well as lesser amounts of W, Mo, Ta, Nb, Ti, and Al. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure.
The three major classes of superalloys are nickel-based, cobalt-based and iron-Nickel based alloys.
Nickel-based alloys can be either solid solution or precipitation strengthened. Solid solution strengthened alloys, such as Hastelloy X, are used in applications requiring only modest strength. They have particularly high-temperature corrosion resistance, excellent fabricability and weldability but lower mechanical strength.
In the most demanding applications, such as hot sections of gas turbine engines, a precipitation strengthened alloy is required. They are constituted in applications requiring high-temperature strength and good corrosion and creep resistance, for instance as turbine blades and vanes in the gas turbine. Nickel-based superalloys can be used for a higher fraction of melting temperature and are therefore more favourable than cobalt-based and iron-nickel-based superalloys at service temperatures close to the melting temperature of the materials.
Most nickel-based alloys contain 10-20% Cr, up to 8% Al and Ti, 5-10% Co, and small amounts of B, Zr, and C. Other common additions are Mo, W, Ta, Hf, and Nb .
Cobalt-based alloys with carbon content in the range of 1 to 3 wt% are widely used as wear-resistant solid materials and weld overlays. Depending on the alloy composition and heat treatment, M23C6, M6C, and MC carbides are formed. Materials with lower carbon content are mostly designed for corrosion resistance and for heat resistance, sometimes combined with wear resistance. The metals W, Mo, and Ta are essentially added for solid solution strengthening. In a few alloys Ti and Al are added. They serve to form a coherent ordered Co3 (Ti, Al) phase which precipitates and leads to strengthening by age hardening. The Cr content is generally rather high to provide oxidation and hot corrosion resistance.
Iron-Nickel based alloys The most important class of iron-nickel-base superalloys includes those strengthened by intermetallic compound precipitation in an fcc matrix. Other iron-nickel-base superalloys consist of modified stainless steels strengthened primarily by solid-solution hardening. Alloys in this last category vary from 19-9DL (18-8 stainless with slight chromium and nickel adjustments, additional solution hardeners, and higher carbon) to Incoloy 800H (21 wt% Cr, high nickel with small additions of titanium and aluminum). Other iron-nickel superalloys include Multimet, Discaloy, and Pyromet types.
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.
Superalloys are commonly used in gas turbine engines in regions that are subject to high temperatures which require high strength, excellent creep resistance, as well as corrosion and oxidation resistance. 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. Air cooling (such as the air cooling holes seen in the picture above) and thermal barrier coatings (TBCs) play an important role in blades, allowing them to operate under such conditions, protecting the base material from the thermal effects as well as corrosion and oxidation. Turbocharger turbines also use superalloys, typically electron beam welded to a steel shaft. 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 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 a 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.
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