Effects of Re and Ru on the Solidification Characteristics of Nickel-Base Single-Crystal Superalloys
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NICKEL-BASE single-crystal superalloys have been widely used in manufacturing the critical components of aero-engines, such as turbine blades and vanes. To improve the high-temperature performance and fuel efficiency of aero-engines, modern Ni-base single-crystal superalloys have been added with large amounts of refractory elements, such as Ta, W, and Re.[1] In particular, as an efficient strengthening element, Re retards the diffusion-controlled c¢ coarsening and significantly improves the high-temperature creep properties.[2–4] However, the Re additions also lead to a series of problems. (1) Re strongly segregates to the dendrite core during directional solidification. Due to the low diffusion ability, the severe segregation of Re is difficult to eliminate even by the complicated homogeneous heat treatments.[5] (2) Re addition promotes the precipitation of detrimental topologically close-packed (TCP) phase during long time exposure at high temperature.[6,7] (3) Re addition increases the tendency of grain defect formation.[8,9] During directional solidification, the low density element Al segregates preferentially to the solute in the mushy zone, whereas the high density elements Re and W segregate to the dendrite core. The strong segregation of alloying elements creates a density inversion between the bulk liquid and the segregated solute in the mushy zone. When the buoyancy force of GANG LIU and BINGMING GE, Postdoctoral Candidates, LIN LIU, JUN ZHANG, and HENGZHI FU, Professors, and XINBAO ZHAO, Research Assistant, are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. Contact e-mail: linliu@ nwpu.edu.cn Manuscript submitted October 20, 2010. Article published online March 31, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
the segregate solute exceeds the surrounding viscous force, the resulting thermosolutal convection in the mushy zone increases the tendency of grain defect formation. To increase the phase stability of Re-containing superalloys, the newly developed fourth- and fifthgeneration Ni-base single-crystal alloys were added with approximately 3 to 6 wt pct Ru.[10–12] The Ru addition can suppress the brittle TCP phase formation and improve the high-temperature creep strength. There has been much research focused on the solidification characteristics of Re- and Ru-containing superalloys.[13–16] However, because Ni-base superalloys are multicomponent and multiphase alloys, it is difficult to independently investigate the effects of individual elements. A better understanding of solidification characteristics of Re- and Ru-containing single-crystal superalloys is significantly important for enhancing the high-temperature capability of Ni-base superalloys. In this experiment, four Ni-base single-crystal superalloys containing varied contents of Re and Ru were directionally solidified under high thermal gradient. The influences of Re and Ru on the solidification characteristics were systematically investigated.
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MATERIALS AND EXPE
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