X-Ray Radioscopic Visualization of the Solutal Convection during Solidification of a Ga-30 Wt Pct In Alloy
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occurrence of melt convection alters the microstructure of solidifying metal alloys.[1–11] For instance, a relative motion of alloy species has been identified to be a reason for macrosegregation, especially arising during solidification of solute rich alloys. If the equilibrium concentration of solute in the primary crystals is lower than in the melt, the growing solid phase rejects solute, which forms a diffusion-controlled boundary layer adjacent to the solidification front. A sufficiently strong melt flow at the solid-liquid interface reduces the thickness of the boundary layer and conveys the solute toward the bulk melt. On one hand, the depletion of solute due to the convective transport promotes the further growth of primary crystals, while on the other hand, it can lead to the formation of segregation zones. For instance, thermosolutal convection occurring in the mushy zone can produce channel segregates called as ‘‘freckles’’ or ‘‘chimneys.’’[6–8] Other studies attempted to find relationships between melt convection and length scales of the solidified microstructure.[9–11] A sufficient understanding of the interactive relationships between the melt flow and the structure formation during solidification requires authentic knowledge of the velocity field, especially in the vicinity of the solidification front. Velocity measurements in liquid metals are complicated by the specific material properties; especially the powerful optical methods as used for measurements in transparent liquids are obviously not available because of the opaqueness of the considered melts. S. BODEN, Postgraduate Student, S. ECKERT, Group Leader, B. WILLERS, Senior Research Scientist, and G. GERBETH, Department Head, are with the MHD Department, Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany. Contact e-mail: [email protected] Manuscript submitted June 28, 2007. Article published online February 1, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
Direct observation of the solidification process and the impact of convection thereon have been performed using transparent organic alloys (TOA) as model liquids.[12,13] During solidification such TOAs show similar interfacial characteristics like metals, however, significant differences in fundamental physicochemical properties restrict a full comparability. The ultrasound doppler method has been applied to measure the melt convection in a solidifying Sn-15 wt pct Pb alloy.[14] It was demonstrated that this technique provides instantaneous profiles of the bulk flow, however, the spatial resolution of the method is not sufficient to reveal the flow structure at smaller scales adjacent to the solidification front. Thermal convection in liquid tin and liquid lead in a thin rectangular mold was studied using radioactive tracer techniques.[15] A rapid quenching locks the tracer distribution, which represents the flow pattern that occurred just at that moment. This method does not allow for a continuous observation of the process. Quantitative data with respect to the velocity field can hardly be derived. Recently,
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