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A superalloy, or high-performance alloy, is an alloy that exhibits several key characteristics: excellent mechanical strength, resistance to thermal creep deformation, good surface stability and resistance to corrosion or oxidation. The crystal structure is typically face-centered cubic austenitic. Examples of such alloys are Hastelloy, Inconel, Waspaloy, Rene alloys, Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys. Superalloy development has relied heavily on both chemical and process innovations. Superalloys develop high temperature strength through solid solution strengthening. An important strengthening mechanism is precipitation strengthening which forms secondary phase precipitates such as gamma prime and carbides. Oxidation or corrosion resistance is provided by elements such as aluminium and chromium. The primary application for such alloys is in turbine engines, both aerospace and marine. ==Chemical development== Because these alloys are intended to be used for high temperature applications, in addition to these materials being able to withstand loading at temperatures near their melting point, their creep and oxidation resistance are of primary importance. Ni based superalloys have emerged as the material of choice for these applications.〔Reed, Roger C. The Superalloys: Fundamentals and Applications. Cambridge, UK: Cambridge UP, 2006.〕 The properties of these Ni based superalloys can be tailored to a certain extent through the addition of many other elements, both common and exotic, including not only metals, but also metalloids and nonmetals; chromium, iron, cobalt, molybdenum, tungsten, tantalum, aluminium, titanium, zirconium, niobium, rhenium, yttrium, vanadium, carbon, boron or hafnium are some examples of the alloying additions used. Each of these additions has been chosen to serve a particular purpose in optimizing the properties for high temperature application. Creep resistance is dependent on slowing the speed of dislocation motion within a crystal structure. In modern Ni based superalloys the γ’-Ni3(Al,Ti) phase present acts as a barrier to dislocation motion. For this reason, this γ’ intermetallic phase, when present in high volume fractions, drastically increases the strength of these alloys due to its ordered nature and high coherency with the γ matrix. The chemical additions of aluminum and titanium promote the creation of the γ’ phase. The γ’ phase size can be precisely controlled by careful precipitation strengthening heat treatments. Many superalloys are produced using a two-phase heat treatment that creates a dispersion of cuboidal γ’ particles known as the primary phase, with a fine dispersion between these known as secondary γ’. In order to improve the oxidation resistance of these alloys, Al, Cr, B, and Y are added. The Al and Cr form oxide layers that passivate the surface and protect the superalloy from further oxidation while B and Y are used to improve the adhesion of this oxide scale to the substrate.〔Klein, L., Y. Shen, M. S. Killian, and S. Virtanen. "Effect of B and Cr on the High Temperature Oxidation Behavior of Novel γ/γ′Strengthened Co-base Superalloys." Corrosion Science 53 (2011): 2713-720.〕 Cr, Fe, Co, Mo and Re all preferentially partition to the γ matrix while Al, Ti, Nb, Ta, and V preferentially partition to the γ’ precipitates and solid solution strengthen the matrix and precipitates respectively. In addition to solid solution strengthening, if grain boundaries are present, certain elements are chosen for grain boundary strengthening. B and Zr tend to segregate to the grain boundaries which reduces the grain boundary energy and results in better grain boundary cohesion and ductility.〔Shinagawa, K., Toshihiro Omori, Katsunari Oikawa, Ryosuke Kainuma, and Kiyohito Ishida. "Ductility Enhancement by Boron Addition in Co–Al–W High-temperature Alloys." Scripta Materialia 61.6 (2009): 612-15.〕 Another form of grain boundary strengthening is achieved through the addition of C and a carbide former, such as Cr, Mo, W, Nb, Ta, Ti, or Hf, which drives precipitation of carbides at grain boundaries and thereby reduces grain boundary sliding. While Ni based superalloys are excellent high temperature materials and have proven very useful, Co based superalloys potentially possess superior hot corrosion, oxidation, and wear resistance as compared to Ni-based superalloys. For this reason, efforts have also been put into developing Co based superalloys over the past several years. Despite that, traditional Co based superalloys have not found widespread usage because they have a lower strength at high temperature than Ni based superalloys.〔Sato, J. "Cobalt-Base High-Temperature Alloys." Science 312.5770 (2006): 90-91.〕 The main reason for this is that they appear to lack the γ’ precipitation strengthening that is so important in the high temperature strength of Ni-based superalloys. But report on metastable γ’-Co3(Al,W) intermetallic compound with the L12 structure in 2006 has given a path to think Co based alloys as alternative to traditional Ni based superalloys. However these class of alloys have been discovered first by C S Lee in 1971 and report exists in the form of his PhD thesis at Arizona State University (USA) in 1971.〔(【引用サイトリンク】first1=CS )〕 The two-phase microstructure consists of cuboidal γ’ precipitates embedded in a continuous γ matrix and is therefore morphologically identical to the microstructure observed in Ni based superalloys. Like in the Ni-based system, there is a high degree of coherency between the two phases which is one of the main factors resulting in the superior strength at high temperatures. This provides a pathway for the development of a new class of load-bearing Co based superalloys for application in severe environments.〔Suzuki, A., Garret C. DeNolf, and Tresa M. Pollock. "Flow Stress Anomalies in γ/γ′ Two-phase Co–Al–W-base Alloys." Scripta Materialia 56.5 (2007): 385-88.〕 In these alloys 'W' is the crucial addition for getting γ’ intermetallic compound that makes them much denser (>9.6 gm/cm3) compared to Ni-based superalloys. Recently a new class of γ - γ’ cobalt based superalloys have been developed that are "W" free and have much lower density comparable to nickel based superalloys. In addition to the fact that many of the properties of these new Co based superalloys could be better than those of the more traditional Ni based ones, Co also has a higher melting temperature than Ni. Therefore, if the high temperature strength could be improved, the development of novel Co based superalloys could allow for an increase in jet engine operation temperature resulting in an increased efficiency. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Superalloy」の詳細全文を読む スポンサード リンク
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