The Kinetics of Precipitate Dissolution in a Nickel-Base Superalloy
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ion of a precipitate phase plays an important role in the thermomechanical processing (TMP) of two-phase metallic alloys. Perhaps the most common application is age hardening. In such operations, coarse, second-phase particles are dissolved in a high-temperature single-phase field, the material is cooled quickly to retain a high supersaturation, and then it is aged at a moderate temperature in a two-phase field to develop a uniform distribution of precipitates with a substantially-refined size. Alloy systems that rely on age hardening include conventional aluminum-, iron-, beta-titanium-, and nickel- base alloys.
S.L. SEMIATIN, D.W. MAHAFFEY, and E.J. PAYTON are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RXCM, Wright-Patterson Air Force Base, OH 45433. Contact e-mail: [email protected] N.C. LEVKULICH is with Wright-State University, Dayton, OH 45435. A.E. SAURBER is with the University of Dayton, Dayton, OH 45409. O.N. SENKOV is with UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432. Manuscript submitted June 20, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Fine-grain, powder-metallurgy (PM) nickel-base superalloys strengthened by the c¢ phase comprise an alloy class for which an understanding of dissolution and subsequent precipitation is especially important. This is because c¢ often serves a dual role as both a strengthening and a structure-control phase. With regard to the latter function, coarse, primary c¢ (so-called because of its presence during hot working) pins c boundaries and is thus instrumental in the development of fine, duplex microstructures during subsolvus processing. An excellent combination of strength and resistance to fatigue crack initiation are thereby obtained. By contrast, applications requiring service at which creep or creep-crack growth are life-limiting usually benefit from a coarse c grain size produced by supersolvus solution treatment followed by controlled cooling. This operation sometimes leads to highly-undesirable abnormal grain growth (AGG) that can make a component unsuitable for service.[1] Factors such as the strain rate imposed during subsolvus hot working[2–4] are thought to contribute to AGG during supersolvus heat-treatment of c–c¢ superalloys. In this respect, Soucail, Huron, and their coworkers[2,3] have demonstrated that subsolvus isothermal forging at strain rates near the transition from stage
II (superplastic) to stage III (power-law-creep) deformation are detrimental with regard to AGG during subsequent supersolvus heat treatment. In later work, the strain rate imposed during subsolvus forging was shown specifically to affect the location of c¢ precipitates relative to the c grain boundaries.[4,5] Thus, the rate of heating to and through the solvus, the manner in which intragranular vs grain-boundary c¢ particles dissolve, and the release of the associated pinning forces may each play a significant role in AGG, as has been suggested in various investigations.[6–9] During static, supersolvus heat treatment, the
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