Phase precipitation and phase stability in nimonic 263
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. INTRODUCTION
NIMONIC 263[1] is a wrought Ni-based superalloy which has found application in gas turbines because of its attractive creep strength and good oxidation resistance. Its nominal composition is 19 to 21Co, 19 to 21Cr, 5.6 to 6.1Mo, 1.9 to 2.4Ti, ⱕ0.6Al, ⱕ0.6Fe, 0.04 to 0.08C, and the balance Ni. (All compositions are in weight percent throughout this article, unless otherwise stated.) This alloy is precipitation hardened; however, only limited information is available in the literature on its precipitation behavior.[2,3] Several phases have been reported: gamma-prime (␥ ⬘),[2,3] eta (),[2,3] M23C6,[2,3] and M6C.[2] The crystal structures and lattice parameters of the phases are listed in Table I.[4,5] The ␥ ⬘Ni3(Ti,Al) phase has an L12 crystal structure (cP4), and the -Ni3Ti phase has a complex hexagonal structure (hP16). The M23C6 phase has an fcc structure (cF116), whereas the M6C phase has a diamond cubic crystal structure (cF112), but their lattice parameters are very close. Hicks and Heap[2] studied two versions of the alloy using optical microscopy and replica electron microscopy along with hardness evaluations. They derived a C-curve for the -phase formation based on microstructure data and observed that the phase did not form at temperatures ⱖ925 ⬚C. In addition, Hicks and Heap observed dissolution of the ␥ ⬘ phase at ⬃900 to 925 ⬚C, which is consistent with the results of Ratna and Sarma,[3] who, based on transmission electron microscopy (TEM) examination of thermal-fatigue samples, reported a dissolution temperature of ␥ ⬘ in the vicinity of 900 ⬚C. Singh and Singh[6] studied the work-hardening behavior of Nimonic 263, but did not examine the microstructures in detail. The aim of the present investigation on Nimonic 263 is two-fold: (1) to provide more data on the precipitation J.-C. ZHAO, Materials Scientist, is with Corporate Research & Development, General Electric (GE) Company, Schenectady, NY 12301. V. RAVIKUMAR, Materials Scientist, is with the GE John F. Welch Jr. Technology Center, Bangalore, Karantaka 560-066, India. A.M. BELTRAN, formerly Manager, Materials and Processes Engineering, GE Power Systems, Schenectady, NY 12345, is retired. Manuscript submitted June 16, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
behavior in order to build a time-temperature-transformation (TTT) diagram; and (2) to study the phase stability by integrating phase-precipitation information with thermodynamic calculation results.
II. EXPERIMENTAL AND CALCULATION PROCEDURES Test samples of about 7 ⫻ 20 ⫻ 20 mm in size were cut from a piece of wrought commercial Nimonic 263. The composition of the alloy as analyzed by a microprobe was, in wt pct, 20.1Co, 19.6Cr, 5.7Mo, 2.1Ti, 0.4Al, 0.5Fe, and 0.06C. (The carbon content was not analyzed, and this value was based on the manufacturer’s nominal composition.) The piece had been rolled and solution annealed. To ensure that the starting piece was in the solution-annealed condition, an extra solution annealing was performed at 1150 ⬚C for 2 hours in air and
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