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Thermal analysis with cooling curve has been used for investigation of microstructure evolution with composition variation during solidification process in Mg-Zn-Y-Zr alloys (denoted as ZW alloys). The obtained results reveal that two pseudobinary eutectic reactions appear one after another during solidification due to the different Zn/Y ratio (ranged front 0.35 to 3.33). In ZW62 alloy, a peritectic reaction happens before I-phase Formation through pseudobinary eutectic reaction distinguishing it from other alloys. The changes of characteristic temperature points for phase transformation are studied in detail. Microstructure observation shows dominating secondary phases in the alloys differ from one another according to various Zn/Y ratios. Divorced massive eutectic Z-phase is formed during solidification in a low Zn/Y ratio: W-phase comes out when Zn/Y ratio grows to 0.85; if the Zn/Y ratio increases further to a certain level, I-phase forms by a peritectic reaction through W-phase and the liquid. (C) 2008 Elsevier B.V. All rights reserved.