Eutectic Morphology of Al-7Si-0.3Mg Alloys with Scandium Additions
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THE combination of fine-equiaxed a-Al grains and fine-fibrous eutectic Si morphology can significantly improve the mechanical properties of hypoeutectic Al-Si cast components.[1] This combination is usually achieved by applying separate chemical treatments such as Al-5Ti1B and Al-10Sr master alloys. There has been recent interest in scandium additions to Al-Si casting alloys because recent studies indicate that the addition of 0.20.4 wt pct Sc to Al-6Si-0.25Mg can achieve both a-Al grain refinement[2] and modification of the eutectic Si from a coarse plate-like to a fibrous structure[3,4] at cooling rates of 3 K/s. However, the mechanisms of Al-Si eutectic refinement due to scandium additions are not well understood. It is well known that Al-Si eutectic modification can be achieved by rapid cooling or chemical modification[5] with elements such as Na and Sr. Some authors define Al-Si eutectic modification as a dramatic change in morphology from flake/plate to fine fibrous due to the influence of impurity-induced twinning (IIT) on Si PHROMPHONG PANDEE, Ph.D. Student, is with the Department of Production Engineering, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand, and also with the Department of Materials, Imperial College, London SW7 2AZ, U.K. Contact e-mails: [email protected]; p.pandee@ imperial.ac.uk C.M. GOURLAY, Senior Lecturer, and S.A. BELYAKOV, Postdoctoral Research Associate, are with the Department of Materials, Imperial College. RYOTA OZAKI, Masters Student, is with the Department of Adaptive Machine Systems, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. HIDEYUKI YASUDA, Professor, is with the Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. CHAOWALIT LIMMANEEVICHITR, Associate Professor, is with the Department of Production Engineering, King Mongkut’s University of Technology Thonburi. Manuscript submitted April 2, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
growth.[6] Other authors include any significant refinement of the Si phase in their definition of Al-Si eutectic modification.[3,7,8] Several theories have been put forward to explain the mechanisms of chemical modification of eutectic Si in hypoeutectic Al-Si alloys. According to the restricted growth theory, modifier atoms adsorb on the twin-plane re-entrant edge (TPRE) sites of growing Si crystals[9] and prevent or retard the subsequent attachment of Si atoms during eutectic Si growth. Additionally, the twin density is increased via the IIT mechanism,[10] and eutectic solidification occurs at larger undercooling. This causes the eutectic Si phase to grow more isotropically with a fine-fibrous structure. Evidence for this mechanism comes from the increased density of twinning in modified fibrous Si (e.g., Reference[11]), the fact that the atomic radius ratio of some potent modifiers is close to 1.65 as predicted by the IIT theory,[10] and from the measurement of Sr[12] and Eu[13] strongly segregated to and uniformly distributed within the fibrous Si phase. R
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