Superalloy 2025632 222474022 2008-06-29T15:08:18Z DocWatson42 38455 Cleaned up formating and other matters. A '''superalloy''', or '''high-performance alloy''', is an [[alloy]] that exhibits excellent mechanical strength and [[Creep (deformation)|creep]] resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have 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]]. Examples of superalloys are [[Hastelloy]], [[Inconel]], Waspaloy, [[Rene 41|Rene alloys]] (e.g. Rene 41, Rene 80, Rene 95), Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys. ==Introduction== '''Superalloys''' are [[metal]]lic materials for service at high temperatures, particularly in the hot zones of [[gas turbine]]s. Such materials allow the turbine to operate more efficiently by withstanding higher temperatures. Turbine Inlet Temperature (TIT), which is a direct indicator of the efficiency of a gas turbine engine, depends on the temperature capability of 1st stage high pressure turbine blade made of Ni base superalloys exclusively. One of the most important superalloy properties is [[creep (deformation)|high temperature creep]] resistance. Other crucial material properties are [[fracture|fatigue life]], phase stability, as well as [[oxidation]] and [[corrosion]] resistance. 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 [[aluminium]] 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]]. ==Chemical development== Creep resistance is dependent on slowing the speed of [[dislocation]]s within the crystal structure. The gamma prime [[Phase diagram#Other phase diagrams|phase]] [Ni<sub>3</sub>(Al,Ti)] present in nickel and nickel-iron superalloys presents a barrier to dislocations. Chemical additions such as aluminum and [[titanium]] promote the creation of the gamma prime phase. The gamma prime phase size can be precisely controlled by careful precipitation hardening heat treatments. Cobalt base superalloys do not have a strengthening secondary phase like gamma prime. Many other elements, both common and exotic, (including not only [[metals]], but also [[metalloid]]s and [[nonmetal]]s) can be present; [[chromium]], [[cobalt]], [[molybdenum]], [[tungsten]], [[tantalum]], [[aluminium]], [[titanium]], [[zirconium]], [[niobium]], [[rhenium]], [[carbon]], [[boron]] or [[hafnium]] are just a few examples. ==Process development== The historical developments in superalloy processing have brought about considerable increases in superalloy operating temperatures. Superalloys were originally iron based and cold wrought prior to the 1940s. In the 1940s [[investment casting]] of cobalt base alloys significantly raised operating temperatures. The development of vacuum melting in the 1950s allowed for very fine control of the chemical composition of superalloys and reduction in contamination and in turn led to a revolution in processing techniques such as [[directional solidification]] of alloys and single crystal superalloys. Single-crystal superalloys (SC superalloys) are formed as a [[single crystal]] using a modified version of the directional solidification technique, so there are no [[crystallite|grain boundaries]] in the material. The mechanical properties of most other alloys depend on the presence of grain boundaries, but at high temperatures, they would participate in [[creep (deformation)|creep]] and must be replaced by other mechanisms. In many such alloys, islands of an ordered [[Intermetallics|intermetallic]] phase sit in a matrix of disordered phase, all with the same crystalline lattice. This approximates the [[dislocation]]-pinning behavior of grain boundaries, without introducing any [[amorphous solid]] into the structure. ==Applications== Superalloys are used where there is a need for high temperature strength and corrosion/oxidation resistance. The largest applications of superalloys are the following: aircraft and industrial [[gas turbine]]s; rocket engines; space vehicles; [[submarines]]; [[nuclear reactors]]; military electric motors, chemical processing vessels, and heat exchanger tubing. Many of the industrial nickel-based superalloys contain alloying [[chemical element|elements]], including [[chromium]], [[aluminium]], and [[titanium]], also [[molybdenum]], [[tungsten]], [[niobium]], [[tantalum]] and [[cobalt]]. ==Metallurgy of superalloys== The superalloys of the first generation were intended for operation up to 700 °C (973 K). The up-to-date superalloys of the fourth generation are used as single or [[Monocrystal]]s and are extra alloyed, especially with [[ruthenium]]. They can operate up to 1100 °C (1373 K). The structure of most [[precipitation strengthening|precipitation strengthened]] nickel-base superalloys consists of the gamma matrix, and of intermetallic γ' precipitates. The γ-phase is a solid solution with a face-centered crystal lattice and randomly distributed different species of atoms. By contrast, the γ'-phase has an ordered crystalline lattice of type L<sub>1</sub>2. In pure Ni<sub>3</sub>Al phase [[atoms]] of aluminium are placed at the vertices of the cubic cell and form the sublattice A. Atoms of nickel are located at centers of the faces and form the sublattice B. The phase is not strictly [[stoichiometric]]. There may exist an excess of vacancies in one of the sublattices, which leads to deviations from stoichiometry. Sublattices A and B of the γ'-phase can solute a considerable proportion of other elements. The alloying elements are dissolved in the γ-phase as well. The γ'-phase hardens the alloy through an unusual mechanism called the [[yield stress]] anomaly. [[Dislocation]]s dissociate in the γ'-phase, leading to the formation of an [[Crystallographic defect|anti-phase boundary]]. It turns out that at elevated temperature, the free energy associated with the anti-phase boundary (APB) is considerably reduced if it lies on a particular plane, which by coincidence is not a permitted slip plane. One set of partial dislocations bounding the APB cross-slips so that the APB lies on the low-energy plane, and, since this low-energy plane is not a permitted slip plane, the dissociated dislocation is now effectively locked. By this mechanism, the yield strength of γ'-phase Ni<sub>3</sub>Al actually ''increases'' with temperature up to about 1000 °C, giving superalloys their currently unrivalled high-temperature strength. In addition, it is often beneficial for a grain boundary containing nickel-base alloy to contain carbides for improvements in creep strength. Where the carbides (e.g. MC where M is a metal and C is a carbon atom) are precipitated at the grain boundaries, they act to pin the grain boundaries and improve the resistance to sliding and migration that would occur during creep diffusion. However if they precipitate within a grain, or if they form as a continuous grain boundary film, the fracture toughness of the alloy is reduced, together with the ductility and rupture strength. ==Coating of superalloys== Products from superalloys, which are subjected to high working temperatures and corosive atmosphere (like first stages of [[turbine blades]] of the [[jet engine]]s) are coated with various kinds of [[coating]]s. Mainly, two kinds of coating processes are applied: pack cementation process and gas phase coating. Both of them are [[Chemical vapor deposition|CVD]] coatings. In most cases, after the coating process, near-surface regions of parts are enriched with [[aluminum]], the matrix of the coating is [[nickel aluminide]]. ===Pack cementation process=== The pack cementation process is carried out at lower temperatures (about 750°C). The parts are loaded into boxes, which contain a mixture of powders: active coating material, containing [[aluminum]], [[activator]] ([[chloride]] or [[fluoride]]) and thermal ballast, like [[aluminum oxide]]). At high temperatures the gaseous aluminum chloride (or fluoride) is transferred to the surface of part and diffuses inside (mostly, inward diffusion). After the end of the process the so-called "green coating" is produced, which is too thin and brittle for direct use. A subsequent diffusion heat treatment (several hours at temperatures about 1080°C) leads to the further inward diffusion and formation of the desired coating. ===Gas phase coating=== This process is carried out at higher temperatures: about 1080°C. The coating material is usually loaded on special trays without physical contact with parts. The coating mixture contains active coating material and activator, but, usually does not contain thermal ballast. Like in the pack cementation process, the gaseous aluminum chloride (or fluoride) is transferred to the surface of the part. However, in this case, the diffusion is outwards. This kind of coating also requires diffusion heat treatment. ==Superalloys in the future== The availability of superalloys during past decades has led to a steady increase in the turbine entry temperatures and the trend is expected to continue. [[Sandia National Laboratories]] is studying a new method for making superalloys, known as [[radiolysis]]. It introduces an entirely new area of research into creating alloys and superalloys through [[nanoparticle]] synthesis. “This process holds promise as a universal method of [[nanoparticle]] formation. By developing our understanding of the basic [[material science]] behind these nanoparticle formations, we’ll then be able to expand our research into other aspects of superalloys, like nickel-based [[alloys]].” Tina Nenoff says. ==References== * {{cite book | author=Levitin, Valim | title=High Temperature Strain of Metals and Alloys: Physical Fundamentals | publisher=WILEY-VCH | year=2006 | editor= | id=ISBN 978-3-527-31338-9 }} * {{cite book | last = Sims | first = Chester T. | authorlink = | coauthors = Stolloff, Norman S., Hagel, William C. | editor = | others = | title = Superalloys II: High Temperature Materials for Aerospace and Industrial Power | origdate = 1987 | origyear = | origmonth = | url = | format = | accessdate = | accessyear = | accessmonth = | edition = | date = | year = | month = | publisher = John Wiley & Sons | location = | language = | id = | pages = | chapter = | chapterurl = | quote = }} ==External links== * [http://www.msm.cam.ac.uk/phase-trans/2003/nickel.html Extensive resource on Superalloys, Cambridge University] * [http://www.sandia.gov/news/resources/releases/2007/nanopart.html Sandia news release on superalloys] [[Category:Metallurgy]] [[Category:Alloys]] [[Category:Materials science]] [[de:Superlegierung]] [[it:Superleghe]] [[ja:超合金]] [[ru:Суперсплавы]] [[sk:Superzliatina]] [[sr:Суперлегуре]] [[fi:Superseos]] [[zh:高温合金]]