Role of Elemental Sublimation during Solution Heat Treatment of Ni-Based Superalloys
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NTRODUCTION
SUPERALLOYS are a class of materials that have been specifically developed for high-temperature applications. The evolution from the 1st to 4th generation Ni-based superalloys has been motivated by the stringent demands on improved creep and fatigue resistance at elevated temperatures that is achieved by (1) increased solid solution strengthening and (2) the increased volume fraction of the precipitated c¢ phases in the solid state. In order to achieve these goals, the alloys contain increasing amounts of refractory alloying elements such as Mo, Re, Ta, and W. The as-cast microstructure in the latest generation alloys is therefore associated with increasing levels of microsegregation and is consequently required to be heat treated to dissolve the low-melting interdendritic phases and to homogenize the microstructure.[1–7] During solutioning, the alloy is heated above the c¢-solvus in the c phase-field over a period of about 8 hours to homogenize the c phase. However, recently, we have observed that microstructural instability develops across certain regions (that were scaled—NiOx surface oxide) of the aerofoil surface during solutioning,[8] the morphology of which is characteristic of a discontinuous reaction that shows a c¢ matrix with TCP precipitates and c-lamellae and the existence of a polycrystalline microstructure. This is unusual given that solutioning occurs within the c phase-field. From the evolution of the microstructural instability at the surface of the aerofoil after heat treatment, it was deduced that Ni and Cr are N. D’SOUZA, Materials Technologist, is with the Rolls-Royce Plc, PO Box 31, Derby DE24 8BJ, U.K. S. SIMMONDS, Ph.D. Student, and H.B. DONG, Professor, are with the Department of Engineering, University of Leicester, Leicester LE1 7RH, U.K. Contact e-mail: [email protected] G.D. WEST, Senior Research Fellow, is with the Department of Materials, Loughborough University, Leicestershire, LE10 2RF, U.K. Manuscript submitted July 20, 2012. Article published online June 14, 2013 4764—VOLUME 44A, OCTOBER 2013
most likely lost via evaporation, thereby destabilizing the c phase followed by redistribution of Re. However, certain key questions remain unexplained, specifically: (a) It was observed that such microstructural instability only affected the scaled regions of the aerofoil (regions of the as-cast aerofoil covered with a NiOx scale, arising from transient oxidation due to mold/metal separation during casting,[9]) whereas regions that were unscaled are seen to be unaffected. (b) During solutioning both oxidation and evaporation occur; however, the role of these two processes during solutioning on the microstructural instability is unknown. The aim of the current study is therefore to examine the role of evaporation and oxidation on structural instability during solution heat treatment of Ni-based superalloys. The sequential evolution of the microstructure during solutioning and aging will be analyzed to reveal the role of elemental evaporation.
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EXPERIMENTS
A. Sample Preparation and
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