Precipitate Size in the Superalloy IN738LC During Compression Creep
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INTRODUCTION
SUPERALLOYS are metallic high-temperature materials which are employed in aggressive environments.[1] Nickel-base superalloys are the most widespread and popular superalloys in industry and are heavily used in the production of many turbine engine components like turbine blades and turbine discs.[2] A face centered cubic (fcc) gamma (c) phase is the matrix for nickel-base superalloys, and the primary strengthening comes from the Ni3(Al,Ti) c¢ precipitate phase with an L12 crystal structure.[3] A close match between the lattice parameters of matrix and precipitate phases results in a low interfacial energy and allows the c¢ to precipitate homogeneously throughout the matrix and have long-time stability. The lattice misfit provides an elastic coherency strain between c¢ and c. This strain is the main factor for strengthening.[2,3] One of the many Ni-base superalloys is IN738, which is developed to produce a stable alloy that combines the strength of IN713-C with the oxidation and hot corrosion resistance of Udimet-500 superalloys. There are two types of this superalloy, a high-carbon one (0.17 wt pct C) and a low-carbon one (0.11 wt pct C), which are designated as IN738C and IN738LC, respectively.[4] ARUN ALTINCEKIC, Ph.D. Candidate, and ERCAN BALIKCI, Associate Professor, are with the Department of Mechanical Engineering, Bogazici University, South Campus, Bebek, Istanbul 34340, Turkey. Contact e-mail: [email protected] Manuscript submitted August 6, 2012. Article published online February 13, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
IN738LC has a melting range between 1505 K and 1588 K (1232 C and 1315 C),[5] and it can be used effectively up to 1253 K (980 C).[4,6] Table I shows the chemical composition of IN738LC, which is used extensively in land-based turbine parts such as nozzle guide vanes and rotary blades. Furthermore, deleterious topologically close packed phases are not observed in IN738LC because of its low-electron vacancy number of 2.31, which is under a threshold value of 2.36 for the formation of these phases.[4] During aging, precipitates grow by acquiring solute atoms from the matrix until the matrix is exhausted of the dissolved solute atoms.[7] The growth is followed by the Ostwald Ripening (coarsening) where the number of precipitate particles decreases at a constant volume fraction leading to a reduction in the total interfacial energy of the system, which is proportional to the precipitate/matrix interfacial area. According to the well-known Lifshitz–Slyozov–Wagner (LSW) theory that formulates the coarsening,[8,9] because the areato-volume ratio is high in small precipitates, they are consumed by the larger precipitates through diffusion in the matrix. In fact, due to the Gibbs-Thomson effect, a concentration gradient forms between a fine precipitate and a coarse one. This gradient causes the fine precipitates (high-concentration interface) to eventually disappear and coarse ones (low-concentration interface) to become even larger.[7,10] According to the LSW theory,[8
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