Refinement of Eutectic Si Phase in Al-5Si Alloys with Yb Additions
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L-SI-BASED alloys, i.e., A356, are dominant for the foundry application which constitutes the majority of all castings. The size and shape of eutectic Si in hypoeutectic Al-Si alloys and primary Si in hypereutectic Al-Si alloys have a key influence on the final mechanical properties of the manufactured parts. The modification of the Si morphology from flake-like to fibrous form greatly improves the mechanical properties. Therefore, the modification of eutectic Si in Al-Si alloys has been widely investigated in the field of solidification since the first modification phenomenon was discovered by Pacz,[1] where an Al-15 wt pct Si alloy was stirred in a sodium fluoride flux and a remarkable increase in the mechanical properties was achieved. The modification of the eutectic Si in hypoeutectic Al-Si alloys is normally achieved in two different ways: by addition of certain modified elements (chemical modification)[2–13] or by rapid solidification (quench modification),[13] although ultrasonic vibration[14] and electromagnetic field[15] were also reported to refine the eutectic Si. With respect to chemical modification, several common modifying elements, i.e., Sr and Na, have been widely investigated over the years and are J.H. LI, University Assistant at Chair of Casting Research, is with the Department of Metallurgy, University of Leoben, 8700 Leoben, Austria. Contact e-mail: [email protected] S. SUETSUGU, Master Student, and Y. TSUNEKAWA, Professor, are with Toyota Technological Institute, Hisakata 2-12-1, Tempaku, Nagoya, 4688511, Japan. P. SCHUMACHER, Professor at Chair of Casting Research, is with the Department of Metallurgy, University of Leoben, and also Managing Director, with Austrian Foundry Research Institute (O¨GI), Parkstrasse 21, Leoben, Styria 8700, Austria. Manuscript submitted January 2, 2012. Article published online September 22, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
widely used in the casting industry to modify the Si morphology from flake-like to fine fibrous. However, the exact mechanism involved behind this process is still under debate. Even for the earliest discovered element Na, several mechanisms still coexist. For example, the addition of Na into the Al-Si alloys was proposed to cause the obstruction of Si crystal growth by surface adsorption,[16] although Si crystal growth was also postulated to be hindered via the presence of sodium-rich NaAlSi1.25 or NaAlSi1.33 compounds.[17] In contrast, the addition of Na was also reported to retard the nucleation of Si by adsorption on the nuclei interface, during eutectic solidification.[18] Another modification mechanism, proposed by Guillet[19,20] suggests that the addition of Na poisons the Al2O3 and SiO2 impurity particles, which could act as heterogeneous nucleation sites. It was demonstrated by Flood and Hunt[21] using quench experiments that the addition of Na not only changed the growth morphology but also prevented nucleation ahead of the eutectic growth front, so that a higher undercooling and finer eutectic lammelar spacing can be achieved. It should be
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