On the Deformation of Dendrites During Directional Solidification of a Nickel-Based Superalloy
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THE microstructural characteristics of a casting are governed by the thermophysical processes that take place in the mushy zone.[1,2] A detailed understanding of the coupled effects of fluid flow and solute partitioning with the stress, pressure, and temperature fields is therefore required if solidification microstructures are to be controlled. Much of the early work on this topic has emerged from the semi-solid metal (SSM) processing community in an effort to decrease grain size and improve homogeneity by stirring. It has been proposed that crystal multiplication through stirring occurs through dendrite fragmentation via mechanisms involving deformation, recovery, and finally grain boundary wetting.[3,4] Problematically, calculations using
J.W. AVESON, C.J.L. GODDARD, J.R. DAVENPORT, 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] G. REINHART, H. NGUYEN-THI, and N. MANGELINCK-NOE¨L are with the Aix Marseille Universite´ & CNRS, IM2NP UMR 7334, Campus SaintJe´rome, Case 142, 13397 Marseille Cedex 20, France. A. TANDJAOUI is with the Aix Marseille Universite´ & CNRS, IM2NP UMR 7334 and also with the Laboratoire de Me´canique de Lille (UMR CNRS 8107), Ecole Centrale de Lille, CS-20048, 59651 Villeneuve d’Ascq Cedex, France. N. WARNKEN is with the Department of Metallurgy and Materials, University of Birmingham, Edgbaston B15 2TT, UK. F. DI GIOACCHINO is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, 1500 Illinois St, Golden, CO. T.A. LAFFORD is with the European Synchrotron Radiation Facility, 71 avenue des Martyrs, CS 40220, 38043 Grenoble Cedex 9, France. N. D’SOUZA is with the Rolls-Royce plc, Derby, DE24 8BJ, UK. Manuscript submitted November 24, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
tabulated high-temperature material property data have suggested that the fluid flow velocities experienced in all but rapid solidification conditions would be insufficient to damage dendrites mechanically.[5–8] Despite this, there remains a need to incorporate mechanical behavior into the understanding of dendrite deformation and fragmentation, owing to observations of bent dendrites in conventional castings and stirred castings[9–11] and notable efforts are now being made to systematically elucidate the origins of such behavior.[12,13] There are many possible ways in which dendrites may be mechanically loaded during solidification in the mushy zone during investment casting, for example, differential thermal contraction between mold and metal, and buoyancy forces.[14] In single crystal investment castings, used most notably for aerofoil blades for turbine engines, permanent dendrite deformation during solidification is intolerable since grain boundaries reduce creep performance and cannot be removed by subsequent thermomechanical processing. Understanding the mechanistic origins of high- and low-angle boundaries has therefore been an important resea
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