On the Time-Temperature-Transformation Behavior of a New Dual-Superlattice Nickel-Based Superalloy
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On the Time-Temperature-Transformation Behavior of a New Dual-Superlattice Nickel-Based Superalloy P.M. MIGNANELLI, N.G. JONES, M.C. HARDY, and H.J. STONE Recent research has identified compositions of nickel-based superalloys with microstructures containing appreciable and comparable volume fractions of c¢ and c¢¢ precipitates. In this work, an alloy capable of forming such a dual-superlattice microstructure was subjected to a range of thermal exposures between 873 K and 1173 K (600 C and 900 C) for durations of 1 to 1000 hours. The microstructures and nature of the precipitating phases were characterized using synchrotron X-ray diffraction and electron microscopy. These data have enabled the construction of a T-T-T diagram for the precipitating phases. Hardness measurements following each thermal exposure have identified the age-hardening behavior of this alloy and allowed preliminary mechanical properties to be assessed. DOI: 10.1007/s11661-017-4355-8 The Author(s) 2017. This article is an open access publication
I.
INTRODUCTION
RECENT trends in nickel-based superalloy design have primarily focused upon the development of new materials for the highest temperature applications. However, the continued use of Inconel 718 (IN718) in intermediate to high temperature regimes, on account of its excellent mechanical strength and processability, suggests that opportunities still exist for new highstrength nickel-based superalloys with temperature capabilities beyond that of IN718.[1–4] IN718 derives the majority of its strength from gamma double prime (c¢¢) precipitates, which possess the D022 structure (Strukturbericht notation).[5] Conversely, the A1 gamma (c) matrices of higher temperature nickel-based superalloys are strengthened by gamma prime (c¢) precipitates, which possess the L12 structure. The L12 and D022 structures are superlattice structures of the A1 matrix[6] and are able to form coherent interfaces with the matrix. This compatibility of the lattices confers considerable strengthening through order hardening, especially at high temperature.[7] Importantly, the c¢¢ produces mechanical property benefits that are in excess of the c¢ phase on a per volume fraction basis, due to the large coherency strains associated with the c¢¢ precipitates.[8] However, the benefits of c¢¢ reinforcing precipitates cannot be
P.M. MIGNANELLI, N.G. JONES, and H.J. STONE are with the Department of Materials Science and Metallurgy, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK. Contact e-mail: [email protected] M.C. HARDY is with the Rolls-Royce plc, PO BOX 31, Derby, DE24 8BJ, UK. Manuscript submitted June 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
exploited in alloys operating at the highest temperatures, as the c¢¢ is known to be a metastable phase.[9,10] Consequently, the c¢¢ transforms to the thermodynamically stable d phase during prolonged exposure at elevated temperatures.[11] The inability to stabilize the c¢¢ phase through alloying or microstructural modification has resulted in the almost exclusive use o
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