Gamma Prime Precipitate Evolution During Aging of a Model Nickel-Based Superalloy
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Gamma Prime Precipitate Evolution During Aging of a Model Nickel-Based Superalloy A.J. GOODFELLOW, E.I. GALINDO-NAVA, K.A. CHRISTOFIDOU, N.G. JONES, T. MARTIN, P.A.J. BAGOT, C.D. BOYER, M.C. HARDY, and H.J. STONE The microstructural stability of nickel-based superalloys is critical for maintaining alloy performance during service in gas turbine engines. In this study, the precipitate evolution in a model polycrystalline Ni-based superalloy during aging to 1000 hours has been studied via transmission electron microscopy, atom probe tomography, and neutron diffraction. Variations in phase composition and precipitate morphology, size, and volume fraction were observed during aging, while the constrained lattice misfit remained constant at approximately zero. The experimental composition of the γ matrix phase was consistent with thermodynamic equilibrium predictions, while significant differences were identified between the experimental and predicted results from the γ′ phase. These results have implications for the evolution of mechanical properties in service and their prediction using modeling methods. DOI: 10.1007/s11661-017-4336-y © The Author(s) 2017. This article is an open access publication
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INTRODUCTION
POLYCRYSTALLINE Ni-based superalloys are the material of choice for many high-temperature structural applications in gas turbine engines. Their remarkable mechanical performance is derived from the presence of an ordered L12 (strukturbericht notation) γ′ precipitate phase within the disordered A1 γ matrix. The principal mechanisms by which these alloys are strengthened include order and coherency strengthening from the γ′ precipitates, as well as solid solution strengthening of the γ matrix phase and grain boundary hardening.[1] As the extent of precipitation strengthening is dependent on the γ′ morphology and particle size distribution, these are carefully controlled through sagacious selection of heat treatments. However, care is taken to ensure that the resultant microstructure delivers an appropriate balance of properties. The precipitate size at which peak strength is obtained corresponds to the transition from weak to strong pair dislocation coupling and, as such, varies with alloy composition. For example, this occurs for 55 to 85 nm γ′ A.J. GOODFELLOW, E.I. GALINDO-NAVA, K.A. CHRISTOFIDOU, N.G. JONES, and H.J. STONE are with the Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Contact e-mail: [email protected] T. MARTIN and P.A.J. BAGOT are with the Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. C.D. BOYER is with the Canadian Neutron Beam Centre, Chalk River, ON K0J 1J0, Canada. M.C. HARDY is with the Rolls-Royce plc, PO Box 31, Derby DE24 8BJ, UK. Manuscript submitted July 4, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitates in Nimonic 105, but just 26 to 30 nm for PE16.[2] However, a unimodal distribution of such fine precipitates is difficult to achieve in practice and has been associa
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