Co Effect on As-cast and Heat-Treated Microstructures in Ru-Containing Single-Crystal Superalloys
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Ni-BASED single-crystal superalloys with superior properties at high temperature are widely used as turbine blades in aircraft engines. In the past 30 years, various alloying approaches were employed to develop multiple classes or generations of Ni-based superalloys with improved performance.[1] To enhance the degree of solid solution strengthening in the c matrix and control the coarsening kinetics of the c¢ precipitates, the level of Re content was continuously increased, from 0 wt pct in the first-generation single-crystal superalloys to 5 to 6 wt pct in the third and fourth generation superalloys.[2,3] Since Re additions exhibit limited solubility in the c¢ phase, it partitions preferentially to the c phase. The overall composition of single-crystal Ni-based superalloys needs to be carefully controlled as supersaturation of the c phase with Re, W, Mo, and Cr will lead to the microstructural instabilities and the formation of detrimental topologiQIANYING SHI, Ph.D. Candidate, and JIANPING HE, Senior Engineer, are with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, P.R. China. XIANFEI DING and MEILING WANG, Assistant Research Scientists, and YUNRONG ZHENG, Research Scientist, are with National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing, P.R. China. SAMMY TIN, Associate Professor, is with Department of Mechanical, Materials, and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL 60616. QIANG FENG, Professor, is with National Center for Materials Service Safety, University of Science and Technology Beijing, and also with State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing. Contact e-mail: [email protected] Manuscript submitted January 7, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
cally close-packed (TCP) phases during exposures at elevated temperature. Ru additions have recently been identified as an effective suppressor of TCP phases and are characteristic additions in the fourth-generation singlecrystal superalloys.[4–7] Since alloys containing high levels of Re and Ru additions tend to have relatively high densities and cost, the overall levels of these alloying additions need to be reduced to meet the practical demands of aircraft engines. Co is present in the large majority of commercially available multi-component Ni-based superalloys. As an alloying addition, Co increases the strength at elevated temperature by reducing the stacking fault energy of c matrix.[8] It has been shown that Co influences the formation of c¢ phase, carbides, and TCP phases in Nibased superalloys.[9–13] Previous studies had shown that increasing the level of Co in wrought Ni-based superalloys lowered the solvus temperatures of c¢ phase and carbides.[9] The volume fraction, mean size, and composition of c¢ phase in the cast polycrystalline superalloy MAR-M247 was also influenced by Co additions. Moreover, increasing the level of Co add
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