An Investigation of High-Temperature Precipitation in Powder-Metallurgy, Gamma/Gamma-Prime Nickel-Base Superalloys
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COMMERCIAL nickel-base (gamma/gamma-prime) superalloys comprise a class of material for which the understanding and control of precipitation behavior is extremely important. The size and volume fraction of gamma-prime precipitates play a key role in controlling strength, fatigue, and other properties.[1–3] Typically, the final heat treatment of components made from gamma/gamma-prime superalloys includes a solution treatment in the single-phase-gamma field or high in the two-phase, gamma-plus-gamma-prime field, followed by cooling at a rate which is determined by the cooling/quenching medium, the size of the part, and the location within in the part. The cooling rates that are imparted within the bulk (i.e., away from the free surface which is removed by machining) are usually of the order of 10 to 200 K/min (10 to 200 C/min). Following cool-down, an isothermal aging treatment at S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, and J.S. TILEY, Senior Materials Engineer, 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] S-L. KIM, Research Scientist, is with UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432. F. ZHANG, President, is with CompuTherm LLC, Madison, WI 53719. Manuscript submitted September 22, 2014. Article published online 29 January 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
a temperature approximately 50 K to 200 K (50 C to 200 C) above the service temperature is common. For ingot-metallurgy alloys having relatively-low amounts of gamma prime in the fully-hardened state, the formation of precipitates can be avoided during cooling from the solution-treatment temperature. Precipitation hardening then relies on a final aging treatment. By contrast, more-highly-alloyed, powder-metallurgy (PM) superalloys often decompose during cooling. This precipitation usually occurs in several ‘‘bursts’’. At relatively small undercooling relative to the gamma prime solvus, secondary gamma prime (so-called to differentiate it from the coarse, primary gamma-prime dispersion developed during subsolvus extrusion or isothermal forging) nucleates over a relatively narrow temperature interval. During continued cooling, the secondary gamma prime grows via diffusion to a size typically in the range of 100 to 500 nm. At temperatures several hundred kelvins below the solvus, diffusional growth becomes sluggish, matrix supersaturation increases again, and additional bursts of gamma-prime precipitates, referred to as tertiary (with a size of approximately 10 to 50 nm) and quatary (~3 to 10 nm in diameter) are formed. Because of its importance with regard to the strengthening of metallic materials such as nickel-base superalloys, the modeling of precipitation reactions has received considerable attention in the literature. The phenomenon is frequently described in terms of nucleation, growth, and coarsening processes.[4,5] VOLUME 46A, APRIL 2015—1715
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