Effect of a Transverse Magnetic Field on Solidification Structures in Unmodified and Sr-Modified Al-7wtpctSi Alloys Duri
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come one of the most widely used and studied alloys in recent decades due to their good strength to weight ratio, and excellent fluidity.[1] Modification of the eutectic silicon phase in hypoeutectic Al-Si alloys is carried out extensively in industry to improve mechanical properties. Many works have been performed to develop various agent modifiers to modify the eutectic silicon.[2–6] For instance, addition of trace levels of Sr or Na to these alloys (termed ‘‘modification treatment’’) results in the transformation of plate-like eutectic silicon to a fibrous morphology, thereby improving the mechanical properties and castability. The magnetic field was wildly applied to material processing for over half a century, for example, damping convections to avoid solute banding during crystal growth,[7,8]
XI LI, Professor, is with the State Key Laboratory of Advanced Special Steels, Shanghai University, Shanghai 200072, P.R. China, and also with the SIMAP-EPM-Madylam/G-INP/CNRS, PHELMA, BP 75, 38402 St Martin d’Heres Cedex, France. Contact e-mail: [email protected] ANNIE GAGNOUD, YVES FAUTRELLE, and RENE MOREAU, Professors, are with the SIMAP-EPM-Madylam/ G-INP/CNRS, PHELMA. DAFAN DU, Ph.D. Student, and ZHONGMING REN and XIONGGANG LU, Professors, are with the State Key Laboratory of Advanced Special Steels, Shanghai University. Manuscript submitted June 17, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
levitation,[9] and controlling crystal morphology.[10–12] However, some unexpected results were observed. Bottinger et al.[13] found that a 0.1T axial magnetic field had no influence on microstructure or longitudinal macrosegregation in directionally Pb-rich Pb-Sn off-eutectic alloys. Tewari et al.[14] studied the effect of a 0.45T transverse magnetic field on macrosegregation in directionally solidified Sn-Pb alloy and found that the cellular array was severely distorted at low growth velocities (£1 lm/s). This suggested that, except damping the melt flows, applying static magnetic field may lead to flows at the vicinity of the solid/liquid interface during solidification. The origin of these flows (we name thermoelectric magnetic convection (TEMC) here) had been noticed by the magnetohydrodynamic scientists when they studied flows of liquid metal in the heat exchanging system of fusion power generation. The TEMC is driven by the forces in the melt that are produced by the interaction between applied magnetic field and the thermoelectric currents.[15] Recently, in situ and real-time observation of shape evolution of a planar solid/liquid interface and the movement of the exquisite grain has been performed in directionally solidified Al-Cu alloy under a transverse magnetic field. The existence of the TEMC in liquid and its impact during solidification has been proven.[16–18] The present work aims to investigate the effect of the magnetic field on the microstructures in unmodified and Sr-modified Al-7wtpctSi alloys during directional solidification. It has been revealed that the magnetic field caused
the formation of the channel and freckle macroseg
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