Mg-Zn-Y alloys with a Zn/Y=6:1(atomic ratio) were prepared by steel mould casting method. The effect of alloy composition on phase constituents as well as the morphology, volume fraction of Mg3Zn6Y quasi-crystalline phase (quasi-crystalline I-phase) in Mg-Zn-Y alloys was investigated by XRD, SEM, EDS, TEM and DSC. Results show that the phase constituents, the morphology, volume fraction and formation mechanism of Mg3Zn6Y quasi-crystalline I-phase in the Mg-Zn-Y alloys are closely related to alloys' compositions. The formation reaction of quasi-crystalline I-phase is transferred from fully peritectic reaction to the combination of peritectic and eutectic reactions, and to completely eutectic reaction with the decrease of Zn and Y content in the alloys. When the Y content is and more than 7%(atom fraction), the alloys are composed of (Mg,Zn)5Y, quasi-crystalline I-phase, Mg2Zn3 and Mg7Zn3 phases and they are distributed in a layered form in the alloy, and the quasi-crystalline I-phase takes the polygonal form by peritectic reaction during solidification. When the Y content is less than 7%, Mg phase precipitates in the alloys and their microstructure consists of (Mg,Zn)5Y, quasi-crystalline I-phase, Mg7Zn3 and Mg phases. When the Y content is between 5% and 7%, the quasi-crystalline I-phase is precipitated by peritectic and eutectic reaction, and eutectic reaction, which dominates the precipitation of quasi-crystalline I-phase. When the Y content is and less than 4%, the quasi-crystalline I-phase forms totally a (Mg+I-phase) lamellar eutectic microstructure by eutectic reaction. The volume fraction of quasi-crystalline I-phase is all greater than 27%,and in Mg30Zn60Y10 alloy is the maximum (about 77%) among the studied alloys.
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