Multimodal Precipitation in the Superalloy IN738LC
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IN738LC is a polycrystalline, nickel-base superalloy, which is used in aggressive environments at high temperatures. The required strength is provided by precipitate strengthening. Both unimodal and multimodal precipitate distributions are observed in IN738LC. After reaching a critical size, a unimodal precipitate microstructure transforms to a bimodal one. This transformation is controlled by the precipitate-matrix interface, which is under compression in IN738LC. As the unimodal precipitates grow, the strained interface, due to differential lattice parameter of the matrix and the precipitate phase, stops solute diffusion into the growing precipitates. Hence, the solute atoms, entrapped in the matrix, saturate the matrix and form new, fine precipitates. Dissolution of some large precipitates also supplies solute to supersaturate the matrix. On the other hand, a multimodal precipitate distribution tends to become unimodal at low aging temperatures and bimodal at high aging temperatures. Interestingly, the activation energy is calculated for the coarsening of large precipitates in multimodal distribution and is found to vary with aging time.
KeywordsSolution Treatment Large Precipitate Growth Exponent Bimodal Size Distribution Bimodal Microstructure
The authors appreciate the financial support provided by Bogazici University Scientific Research Projects (BAP) through Grant No. 05HA601. EB also acknowledges with gratitude the help in aging treatments by undergraduate senior student Mr. Mustafa Sengor and in metallography by MSc student Mr. Daghan Arpaci.
- 1.P.W. Schilke: GER 3569G, GE Energy Report, “Advanced Gas Turbine Materials and Coatings”, New York, NY, 2004.Google Scholar
- 2.C.G. Bieber and J.J. Galka: U.S. Patent No. 3,459,545, Aug. 5, 1969.Google Scholar
- 6.R. Roshental and D.R.F. West: Mater. Sci. Technol., 1994, vol. 15, pp. 1387–94.Google Scholar
- 8.C.G. Bieber and J.R. Mihalisin: 2nd Int. Conf. on the Strength of Metals and Alloys, Pacific Grove, CA, 30 Aug–4 Sept 1970, The American Society for Metals, Metals Park, Ohio, 1970.Google Scholar
- 10.C. Wagner: Z. Elektrochemie, 1961, vol. 65 (7–8), pp. 581–91.Google Scholar
- 18.E. Balikci, R.A. Mirshams, and A. Raman: Z. Metallkd., 1999, vol. 90 (2), pp. 132–40.Google Scholar
- 24.W.S. Rasband: Image J, U.S. National Institutes of Health, Bethesda, MD, http://rsb.info.nih.gov/ij/, 1997–2007.
- 25.D.A. Porter and K.E. Easterling: Phase Transformations in Metals and Alloys, 2nd ed., Chapman and Hall, New York, NY, 1992, pp. 112, 265, 315, and 316.Google Scholar
- 26.M. Durand-Charre: The Microstructure of Superalloys, Gordon and Breach Science Publishers, Amsterdam, 1997, pp. 68–69.Google Scholar
- 28.G.I. Rosen, S.F. Dirnfeld, M. Bamberg, and B. Prinz: Z. Metallkd., 1994, vol. 85, pp. 127–30.Google Scholar
- 30.E. Balikci, R.E. Ferrell, Jr., and A. Raman: Z. Metallkd., 1999, vol. 90, pp. 141–46.Google Scholar
- 31.R.S. Moshtaghin and S. Asgari: Mater. Des., 2003, vol. 24, pp. 325–30.Google Scholar