Effects of Thermoelectric Magnetic Convection on the Solidification Structure During Directional Solidification under Lo
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on the formation of the microstructure neglects the influence of the convection (both in the melt and in the mushy zone) and considers diffusional transport of either heat or mass away from the growing crystal only (and not the coupled heat and mass transport, which may occur in alloy solidification). However, the convection and its influence on the microstructure formation and development at the low growth rate will be dominant and should not be neglected.[1,2] Convection can be driven by a number of phenomena: buoyancy, surface tension differences (Marangoni convection), externally induced flows (by electromagnetic fields), and advection (produced by the volume change on the solidification). The convection will further affect solute redistribution and the transport XI LI and ZHONGMING REN, Professors, are with the Department of Material Science and Engineering, Shanghai University, Shanghai 200072, People’s Republic of China. Contact e-mail: [email protected] ANNIE GAGNOUD and OLGA BUDEBKOVA, Researchers, and YVES FAUTRELLE, Professor, are with SIMAP-EPM-Madylam/G-INP/CNRS, PHELMA, BP 75, 38402 St Martin d’Heres Cedex, France. Manuscript submitted January 10, 2011. Article published online May 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
of solid fragments and generate compositional nonuniformity at the macroscopic level. Therefore, controlling the convection may result in significant variation of physical and mechanical properties of the crystal. However, the real effect of the convection on the interface shape and the dendritic network is still far from being completely understood and quantified. Therefore, it is necessary and valuable to investigate the effect of convection on solidification structure. The effect of a permanent magnetic field on convection in directional solidification was discussed extensively. It was determined that the application of a magnetic field during directional solidification of materials can significantly reduce the thermosolutal buoyant flow.[3–5] Nevertheless, the effect of a steady-state magnetic field applied during the solidification of the metallic alloy is not always well understood. During directional solidification in the dendritic regime, some unexpected behaviors were observed.[6–8] For instance, Tewari et al.[6] found that the cellular array was severely distorted and the stripes of freckles on the plane perpendicular to the magnetic field formed when a Pb-Sn alloy was solidified vertically under the 0.45 T transverse magnetic field and at very low growth speed. Alboussie`re et al.[7] and Laskar[8] also found that freckles appeared in the Bi-60 wt pct Sn and VOLUME 42A, NOVEMBER 2011—3459
Cu-45 wt pct Ag alloys solidified vertically with a 0.6 T transverse or 1.5 T axial magnetic fields. They suggested that this new flow was induced by the interaction between the magnetic field and thermoelectric (TE) effects. Subsequently, Lehmann et al.[9] offered some experimental evidence for the thermoelectric magnetic convection (TEMC). Nevertheless, sufficient works are still lacking to prove the TEMC and its
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