In-Situ Determination of Precipitation Kinetics During Heat Treatment of Superalloy 718
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RODUCTION
FOR many years, superalloy 718 has been the workhorse material for moderate-temperature applications in the aero-engine and power-generation industries. Combining an attractive blend of low cost, ease of manufacture with respect to both mill products and finished components, and balance of properties, 718 has accounted for ~ 50 pct of the total superalloy market since it was first introduced in the early 1960s.[1–3] The use of the alloy has declined somewhat in recent years due to the need for higher-temperature capability as well as the development of new, moderate-cost, cast-andwrought c–c¢ superalloys with higher strength.[4]
J. CORMIER, P. GADAUD, and M. CZAPLICKI are with the Institut Pprime, UPR CNRS 3346, ISAE-ENSMA, 1 avenue Cle´ment Ader, BP 40109, 86961 Futuroscope-Chasseneuil Cedex, France. Contact e-mail: [email protected] R.Y. ZHANG is with the Open University, Milton Keynes MK7 6AA, UK. H.B. DONG is with the University of Leicester, Leicester LE1 7RH, UK. T.M. SMITH is with the NASA Glenn Research Center, Cleveland, OH 44135. F. ZHANG is with the CompuTherm LLC, Middleton, WI 53562. J.S. TILEY and S.L. SEMIATIN are with the Materials and Manufacturing Directorate, Air Force Research Laboratory, WrightPatterson Air Force Base, OH 45433. Manuscript submitted August 5, 2020; accepted October 19, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Nevertheless, the widespread use of 718 and the absence of proprietary restrictions have promoted extensive research to understand its physical metallurgy, let alone have its formulation and processing methods serve as the basis for emerging, modern alloys.[5] One of the key attributes of 718 includes its ability to be heat treated uniformly for thick-section hardware and its good weldability. In part, these attributes are associated with the moderate aging temperatures and the somewhat sluggish precipitation kinetics of its two ordered strengthening phases, both of which have a nominal composition of Ni3X. These phases are the metastable DO22 bct c¢¢ (for which X is primarily Nb with lesser amounts of Ti and Al) and stable L12 fcc-like c¢ (for which X consists of comparable amounts of Al, Ti, and Nb). To optimize strength, 718 components are usually solution treated just below or above the solvus temperature of the stable DOa orthorhombic d phase (which provides grain-size control), oil quenched, and then given a two-step aging treatment at 993 K and then 893 K. By this means, a uniform dispersion of ellipsoidal c¢¢ and spheroidal c¢ precipitates with diameters of ~ 20 to 30 or ~ 10 to 20 nm, respectively, and a total volume fraction of ~ 15 pct is produced. Aging immediately following hot forging without an intermediate solution treatment, or so-called direct-aging, has also been
applied to further refine the precipitate structure and obtain higher levels of strength.[6] Because of the industrial importance of 718, a number of attempts have been made to quantify its aging response via the determination of both isothermal and continuous-cooling
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