Superalloys
Superalloys are rightly named due to their extraordinary mechanical properties and oxidation resistance at elevated temperatures. These alloys find application at most demanding service conditions like the hottest section of jet turbine engines, space shuttle engines etc. Some superalloys operate in applications that go as high as 90% of their melting point while maintaining its high temperature properties. Superalloys consist of three groups of alloys namely nickel based, cobalt based and iron-nickel based. Base metals are extensively alloyed to achieve its superior properties.
Applications
Superalloys display extraordinary resistance to mechanical and chemical degradation at temperatures close to their melting points. These are generally used above 1000 deg F/ 540 deg C in a very demanding oxidizing atmosphere where parts are highly stressed. Apart from turbine engines, superalloys are used in marine, cryogenic, chemical processing plants etc. Extensive addition of alloying elements and special processing help superalloys to achieve its superior quality in extreme service conditions.
Metallurgy
Alloying elements are added to achieve the extraordinary combination of high temperature strength, toughness, resistance against thermal degradation due to oxidation and corrosion at elevated temperature and stress. Some of these elements are very expensive. Alloying additions range from being a very small constituent (as in ppm (parts per million) range) to as high as a major constituent.
Superalloy matrix is face centered cubic (FCC). Other phases are various types of precipitates and carbides. Alloying elements and special processing transforms iron and cobalt’s regular room temperature structure from body centered cubic (BCC) and hexagonal close-packed (HCP) respectively to FCC.
Superalloys are strengthened by solid solution and precipitation strengthening. ϒ’ and ϒ” are the primary precipitates responsible for precipitation strengthening. FCC matrix (ϒ) is strengthened by chromium, molybdenum, tungsten etc through solid solution mechanism. Superalloys are also strengthened by carbides that prevent grain boundary sliding and grain growth. There are several types of carbides (M23C6, M6C, M7C3 etc where M stands for metal atom like Cr, Mo etc). Not all carbides are present at the same time. Depending on temperature, some type of carbides dissolve while the other types of carbides form. There are several undesirable phases too like Laves phase, μ phase and σ phase. These phases cause lower rupture strength and lower ductility and hence are not acceptable in more than trace amounts. Superalloys can be strengthened by work hardening too. Superalloys are often coated to improve corrosion resistance at high temperature.
Extraction and Manufacturing
Superalloy ingots are produced in electric arc furnace or in vacuum induction furnace. These ingots are used as electrodes for re-melting. There are two common method of re-melting namely; vacuum arc re-melting (VAR) and electroslag re-melting (ESR). Solidification of superalloys is very complicated due to its high solute content. Superalloys are prone to have solute rich hard continuous defects called “freckles”. VAR and ESR techniques reduce defects like freckles because of their higher cooling rate.
Both cast and wrought (forged, rolled, sheet metal etc.) products are extensively used based on the final property requirements. Cast parts are better for high temperature creep properties due to larger grains. Forged parts provide better strength, low cycle fatigue, fracture toughness properties at moderately high temperature because of its finer grains. Investment casting is a common method of casting. Cast parts can be polycrystalline or single crystal. Directional solidification is a common technique too. Forging uses rolled or billets as forging stock. Another common method of manufacturing is through powder metallurgy (PM). PM parts are hot iso-statically pressed (HIP) and then forged.
Stress relieving, annealing, solutionizing treatment and aging are primary heat treatment types. Solutionizing and aging are done mainly for strengthening purpose. Depending upon the alloy type and requirements, parts are subjected to single or multiple aging cycles. Generally, aging cycles are performed at successively lower temperatures compared to the previous cycles.
New Applications
While superalloys gained their popularity through uses in jet engines and space shuttle engines, they continue to find other uses where high temperature performance is demanded – like in next-gen high performance turbochargers for automotive engines. Additional benefits derived from reduced maintenance continue to justify the cost of these alloys.
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