Solidification and Microstructure Control of Mg-rich Alloys in the Mg-Zn-Y Ternary Systemya
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Solidification and Microstructure Control of Mg-rich Alloys in the Mg-Zn-Y Ternary System E.S. Park, S. Yi, J.B. Ok, D.H. Bae, W.T. Kim1 and D.H. Kim Center for Non-crystalline Materials, Department of Metallurgical Engineering, Yonsei University, 134 Shinchondong Seodaemungu, Seoul, 120-749, Korea 1 Center for Non-crystalline Materials, Department of Physics, Chongju University, 36 Naedokdong, Chongju, 360-764, Korea ABSTRACT Solidification microstructures of Mg rich Mg-Zn-Y alloys were studied by using optical microscopy, scanning electron microscopy and transmission electron microscopy. Pseudo eutectic reaction (Liquid→α-Mg + icosahedral phase) takes place during solidification. Alloys containing high Mg content solidifies by primary crystallization of α-Mg followed by the eutectic reaction at interdendritic region. Mg68Zn28Y4 alloy solidifies by primary crystallization of I-phase followed by the eutectic reaction into a mixture of α-Mg and I-phase. Occasionally Dphase and Mg4Zn7 phases were observed to form with orientation relationships with previously formed I-phase and D-phase, respectively. The strength of the alloys increased with increasing the volume fraction of I-phase. INTRODUCTION Icosahedral phases (I-phases) that have quasiperiodic atomic structure have been reported in many alloy systems. I-phases have attractive mechanical/physical properties attributed to their unique atomic structure. For instances, high strength at elevated temperatures and low friction coefficients of the I-phase have been reported in the alloy system Al-Fe-Cu [1-2]. However, monolithic I-phase materials have poor ductility inhibiting structural applications. One reasonable solution for the structural applications is fabricating composite materials consisting of I-phase distributed in a ductile matrix [3]. Especially for in-situ composite material design, thermally stable I-phase equilibrating with a metallic solid solution is needed. Stable I-phases have been found in several alloy systems [4-6]. Even though Al-Cu-Fe icosahedral phase is thermally stable it coexists with brittle intermetallic compounds [7]. In the Mg-Zn-Y system, thermodynamically stable I-phase exists having the stoichiometry of about Mg3Zn6Y1 [6]. Recently isothermal sections of Zn-Mg-Y equilibrium phase diagram have been reported by Tsai et al [8]. I-phase forms by peritectic reaction between hexagonal H- and liquid phases at about 870 K. Two phase mixture region of I + α-Mg exists along the compositional range between I-phase and α-Mg. Considering the small difference between the peritectic temperature and the melting temperature of Mg (923 K), pseudo eutectic reaction between L, I and α-Mg phases is expected to be present. Indeed a eutectic microstructure consisting of I-phase and α– Mg phase has been reported to form in as-cast Mg72Zn25Y3 alloy [3]. The existence of the (Iphase+α-Mg) two phase field in the alloy system Mg-Zn-Y provides an opportunity to develop structural composite materials. The microstructures including the I-phase and a ductile phase,
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