Microstructure and Solidification Sequence of the Interdendritic Region in a Third Generation Single-Crystal Nickel-Base
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xceptional properties of nickel-base superalloys at elevated temperatures have led to their widespread use in high-temperature structural applications. In gas turbine engines, the engine operating temperatures are continuously being pushed ever higher to increase the thermodynamic efficiency of these engines. To achieve these increasingly severe engine operating temperatures, the components in the hot sections of the engines such as the turbine blades require superior hightemperature creep resistance; these requirements have led to the development of single-crystal turbine blades manufactured by directional solidification of nickel-base superalloys. The alloy chemistries of the nickel-base superalloys have also been developed continuously, where the evolution of this class of alloy is generally described as occurring over four generations, each characterized by the addition of key elements.[1] These elements include refractory elements such as Ta, W, and Re, which are added for increased strength and creep resistance via increased solid solution strengthening or an optimized volume fraction of the c¢ precipitates. H.T. PANG, Postdoctoral Research Associate, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. H.B. DONG, Lecturer, is with the Department of Engineering, University of Leicester, Leicester LE1 7RH, United Kingdom. R. BEANLAND, Postdoctoral Research Associate, H.J. STONE, Assistant Research Director, C.M.F. RAE, Lecturer, and P.A. MIDGLEY, Professor, are with the Department of Materials Science and Metallurgy, University of Cambridge. G. BREWSTER, Technologist–Surface Engineering, and N. D’SOUZA, Casting Specialist, are with Rolls-Royce plc, Derby DE24 8BJ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted December 2, 2008. Article published online June 6, 2009 1660—VOLUME 40A, JULY 2009
Under equilibrium solidification conditions, solidification terminates within the single-phase c field, and therefore, the microstructure is expected to comprise of only the c phase prior to the precipitation of the c¢ phase (ordered Ni3(Al,Ta) type) below the c¢ solvus temperature. However, during solidification of these alloys, solute partitioning of many of the alloying elements at the liquid-solid solidification interface as well as the retarded diffusivity of the refractory alloying additions in the solid-state leads to an as-cast single-crystal microstructure comprising of cored c dendrites (primary solidification) and a separately identifiable interdendritic constituent containing both the c and c¢ phases. The extent of coring and the volume fraction of the interdendritic constituent are consequently exacerbated in alloys containing increasing amounts of refractory elements owing to the sluggish diffusivity of these solute elements in the solid state. The microstructure, phase fractions, and constituents of the interdendritic region depend on the solidification path. In nickel-base superalloys, two main types of solidification react
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