Solution Heat Treatment Optimization of Fourth-Generation Single-Crystal Nickel-Base Superalloys
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TRODUCTION
CURRENT economic and environmental pressures have increased the need for higher aeroengine efficiency to reduce the operating cost and environmental impact from increasing volumes of air travel. To achieve these targets, the engine operating temperature, specifically the turbine entry temperature (TET), has to be increased to improve the thermodynamic efficiency of the aeroengine. However, the current limitation on TET is the high-temperature capability of the materials used, as under such severe operating conditions, the materials in the hot section of the aeroengine, in particular the turbine blades and discs, must possess a good balance of high resistance to creep, fatigue, and environmental HON TONG PANG, Research Associate, HOWARD J. STONE, Assistant Director of Research, and CATHERINE M.F. RAE, Reader, are with the Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. Contact e-mail: [email protected] LIJUAN ZHANG, formerly Research Associate, with the Department of Materials Science and Metallurgy, University of Cambridge, is now Senior Lecturer, with the School of Technology, University of Wolverhampton, Telford Campus, Telford TF2 9NT, U.K. ROBBIE A. HOBBS, formerly Project Manager, University Research (Materials) with Rolls-Royce plc., Derby DE24 8BJ, U.K., is now MBA Candidate, with the MIT Sloan School of Management, Cambridge, MA 02142. Manuscript submitted October 18, 2010. Article published online April 11, 2012 3264—VOLUME 43A, SEPTEMBER 2012
deterioration under service conditions. Nickel-base superalloys have been the material of choice for hot sections of the aeroengine as this class of material offers a good balance of the required properties for operation in the severe high-temperature conditions. Nickel-base superalloys consist of a face-centered cubic (fcc) c matrix phase and a coherent fcc c/ phase in the cuboidal precipitate morphology. Turbine blades are cast as single crystals to confer higher creep resistance via the absence of grain boundaries. To achieve increasingly demanding creep strength requirements of turbine blades, increasing amounts of dense refractory elements are added to newer generations of single-crystal nickel-base superalloys for turbine blade applications.[1,2] Depending on the partitioning to either the c or c/ phase, these dense refractory elements that include Mo, Re, Ta, W, and Ru may give solidsolution strengthening to the c matrix phase, provide strengthening to the c/ precipitates phase, and/or increase the c/ precipitate volume fraction. In addition, these dense refractory elements have sluggish solute diffusivity; therefore, an improvement to creep properties may be expected. The primary solidification of single-crystal nickel-base superalloy commences with the formation of c dendrites. Solute partitioning at the solid-liquid interface occurs where the concentrations of elements that partition to c (e.g., Re and W) are progressively reduced while the METALLURGICAL AND MATERIALS TRANSACTIONS A
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