Solidification Behavior and Microstructural Evolution of Near-Eutectic Zn-Al Alloys under Intensive Shear
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f shear-induced convection on the solidification behavior and microstructural evolution of eutectic alloys has been a promising subject of many studies in the last several decades. In the 1960s, on the basis of Jackson and Hunt’s model,[1] Verhoeven and Homer[2] reported that the convective flow produced little effect on eutectic growth in the eutectic solidification. However, from the mid 1970s, the shear-induced convection was found to result in modifications of eutectic microstructure during solidification. Larson[3] reported that the increased convection causes a significant reduction of interlamellar spacing k for Mn-Bi fibrous eutectics. Conversely, Quenisset et al.[4,5] found that the vigorous convection can cause the eutectic spacing to increase in directional solidification of Pb-Sn and Cd-Zn alloys. The experimental results carried by Jin et al.[6] from the solidification of Al-Cu eutectics with electromagnetic stirring of the melt indicated that the eutectic interlamellar spacing k increases if the Peclet number Pe is >1, whereas k changes little for Pe < 1. Li et al.[7] investigated the effect of convection on upward directional solidification of Pb-Sn alloy. They found that S. JI, SCM Manager, is with Arcadia Products plc, Surrey RH1 5DY, United Kingdom. Contact e-mail: [email protected] Z. FAN, Professor, is with the Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge, UB8 3PH, United Kingdom. Manuscript submitted February 28, 2008. Article published online November 18, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
the average of interlamellar spacing becomes smaller with the increase of solid fraction during solidification. On the other hand, Apaydin[8] observed that the mechanical stirring during solidification of Al-Si eutectics brings uncommon eutectic microstructures, in which eutectic phases separate from each other and grow in an uncoupled manner. Chen and Davis[9,10] also found that the fluid motions during the eutectic solidification perturb the solute distribution, and therefore modify the crystal growth direction. Apparently, the aforementioned theoretical understandings and experimental observations are not quite consistent and some conclusions are conflicting. Therefore, further work is necessary to consolidate the experimental observation and to extend the understanding of the solidification under the forced convection. The shear-induced convection can be produced at different approaches, including by linear flow over a chill, rotation of the crucible and internal disk or cylinder at various angles to the heat flow vector, and by electromagnetic forces.[11] However, the shear produced by existing techniques seems relatively weak, although there was no special value available in literature. The publications about the solidification behavior, the effect of intensive shear on the phase formation, and morphology of eutectic phases are still absent for alloys at compositions close to the eutectic point (denoted as near-eutectic alloys). Therefore, this article aims to study
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