Precipitation Kinetics and Morphology of Grain Boundary Carbides in Ni-Base Superalloy Haynes 282
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ewly developed Ni-based superalloy Haynes 282 is specifically aimed for high-temperature applications in components for aero- and land-based turbine engines because of its excellent high-temperature creep resistance and strength, fabricability and superior weldability.[1–4] As in other superalloys, the mechanical properties are primarily governed by the morphology and distribution of precipitated phases in the microstructure, where gamma prime (c¢) imparts strength while the carbides at the grain boundaries contribute to the good creep resistance. The conventional heat treatment of Haynes 282 is a two-step aging at 1010 C/2 hours/AC and 788 C/8 hours/AC, where AC denotes air cooling, which is carried out directly on mill-annealed material. The first aging step stabilizes the desired grain boundary carbides, and the second aging
C. JOSEPH is with the Department of Industrial and Materials Sciences, Chalmers University of Technology, 41296, Go¨teborg, Sweden and also with the GKN Aerospace Engine Systems AB, 461 38, Trollha¨ttan, Sweden. C. PERSSON is with the Department of Industrial and Materials Sciences, Chalmers University of Technology. M. HO¨RNQVIST COLLIANDER is with the Department of Physics, Chalmers University of Technology, 41296 Go¨teborg, Sweden. Contact e-mail: [email protected] Manuscript submitted July 3, 2020 and accepted September 4,2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
step precipitates c¢. A recent study showed that the morphology of grain boundary carbides can significantly affect the tensile ductility of Haynes 282.[5] When carbides were present in an interconnected ‘‘brick wall’’ structure, the ductility decreased by almost 50 pct compared to material with discrete carbides in the grain boundaries. Fahrmann and Pike[6] studied the kinetics of secondary carbide formation in Haynes 282 and showed that the precipitation of M6C was significantly slower than expected. Polkowska et al.[7] showed that the temperature stability of M6C was higher than that of M23C6, as the latter did not appear after aging at ‡ 1010 C. This is contradictory to the results presented in Reference 6, where both M23C6 and M6C were observed up to 1100 C. However, a systematic study of the grain boundary carbide morphology has not yet been reported, which appears to be warranted given the previously observed effect on tensile ductility[5] and known reduction in creep resistance for superalloys.[8–10] It is also important for the development of alternative heat treatments, such as the single-stage aging proposed in Reference 11, where the stabilization treatment was omitted and carbides were instead precipitated together with c¢ at 800 C. The objective of this work is therefore to characterize the carbide morphology development in Haynes 282 with temperature and time, as there are insufficient literature data on the phase stability and precipitation in this newly developed alloy. To this end, we subjected a millannealed Haynes 282 sheet to aging heat treatments in the temperature range 650 C to 1120 C a
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