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INCONEL 718,** a niobium containing nickel**INCONEL 718 and 706 is a trademark of Special Metals Corporation, New Hartford, NY.

based superalloy, is widely used in high-temperature applications, including aircraft- and gas-turbine engines, due to its high-temperature tensile strength, good oxidation resistance, and good creep resistance.[1–4] The strengthening of INCONEL 718 is related to the precipitation of c¢ (Ni3Al, L12-ordered, usually with spherical morphology) and c¢¢ (Ni3Nb, D022-ordered, usually with disc morphology) phases within a c (fcc) matrix. Strengthening in this alloy is related to the formation of antiphase boundary within these precipitates, as well as coherency strains[5–7] in the c matrix. Extensive research has been published related to alloy

YUAN TIAN, Graduate Student, formerly with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43221, is now with the McGill University, Montreal, Canada. DONALD McALLISTER, Graduate Student, HENDRIK COLIJN, Research Specialist, MICHAEL MILLS and DAVE FARSON, Professors, are with the Department of Materials Science and Engineering, The Ohio State University. MARK NORDIN, Engineer, is with Rolls Royce Corporation, Indianapolis, IN 46225. SUDARSANAM BABU, Professor, is with the Department of Mechanical, Aerospace and Biomedical Engineering, The University of Tennessee, Knoxville, TN 37996. Contact e-mail: [email protected] Manuscript submitted September 1, 2013. Article published online June 25, 2014 4470—VOLUME 45A, SEPTEMBER 2014

718 with reference to heat treatments and ensuing microstructures. Interestingly, coprecipitation of c¢ and c¢¢ were observed in the form of a compact sandwich structure.[8–12] The detailed mechanisms for such coprecipitation are still under debate. For example, Burger et al.[13] concluded that c¢ is not the precursor to c¢¢, and either c¢¢ or both c¢ and c¢¢ nucleate initially during precipitation. Alam et al. found clustering of Al/Ti and Nb during early stages of nucleation.[14] Kindrachuk et al.[15] evaluated the energetics of c¢ and c¢¢ using computational models and concluded that the merger of these precipitates is inevitable in INCONEL 706 alloy during high-temperature service. The preceding work showed that aging and strengthening responses of these alloys are very sensitive to local composition and precipitate structure. Our motivation was to evaluate the precipitation behavior of alloy 718 during laser additive manufacturing processes,[16] with processing conditions typical to blade-tip repair conditions. These tip repair conditions involve many weld beads per layer and many layers per build. Although these conditions may appear to be similar to multipass welding,[17] the number of thermal cycles (hundreds) experienced during laser additive manufacturing can be many orders of magnitude greater than welding (tens). Therefore, our focus was to explore the spatial microstructure heterogeneity due to such complex thermal cycles. In addition, the microstructure data will form