Effect of Ruthenium on Precipitation Behavior of the Topologically Close-Packed Phase in a Single-Crystal Ni-Based Super
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INTRODUCTION
AS the primary candidate materials for the gas turbine blades and vanes, single-crystal Ni-based superalloys have achieved great progress in alloy design and processing during the past decades.[1] So far, alloy design has gone through five generations of development in total.[2] In an attempt to obtain a superior creep resistance at increasing temperatures, large amounts of refractory elements are added into the single-crystal superalloys. In practice, however, there is a limit to the concentrations of the refractory elements that can be added.[1] Excessive amounts of Cr, Mo, W, and Re promote the precipitation of the topologically closepacked (TCP) phases r, l, P, and R, which are frequently observed in Ni-based superalloys.[3,4] It is known that many adverse effects may be caused by the precipitation of TCP phase, i.e., the depletion of important strengthening elements from the matrix, the interruption of microstructural continuity, the cracks initiation induced by stress concentration, and the occurrence of micropores during high-temperature creep, which apparently would degrade the mechanical properties.[3,5,6] It has been known that the microstructural XIPENG TAN, PhD Student, JINLAI LIU, Associate Professor, and TAO JIN and ZHUANGQI HU, Professors, are with the Superalloy Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] HYUN UK HONG, formerly Senior Researcher, with the High Temperature Materials Research Group, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea, is now Associate Professor, with the Department of Materials Science and Engineering, Changwon National University, Changwon 641-773, Republic of Korea. BAIG GYU CHOI, IN SOO KIM, YOUNG SOO YOO, and CHANG YONG JO, Principal Researchers, are with the High Temperature Materials Research Group, Korea Institute of Materials Science. Manuscript submitted September 13, 2011. Article published online April 7, 2012 3608—VOLUME 43A, OCTOBER 2012
instability caused by the TCP precipitation limits the development of single-crystal superalloys for higher requirements. In recent years, the biggest concern is the role played by Ru as a symbol element in the fourth- and fifth-generation, single-crystal Ni-based superalloys.[7] It is generally accepted that Ru improves the microstructural stability, i.e., suppresses the precipitation of TCP phases. It has been reported that the so-called reverse partitioning may be the primary reason for Ru’s beneficial effect on the suppression of TCP phases.[8–10] Sato et al.[11] showed that Ru increased the solubility limits of Re and W in the c phase, which eliminated the precipitation of the TCP phases. Hobbs et al.[12] pointed out that the suppression of the TCP phase by Ru seems to be the net effect of its limited solubility in the TCP phase, a lower density of structural growth ledges for atomic attachment at the TCP/matrix interface, and destabilization of the c¢ phase at elevated temperatures. Chen et al.[13] rece
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