Microstructures and mechanical properties of Mg-Zn-Y and Mg-Zn-Al-Y alloys have been investigated to find the optimum Zn/Y ratio for the formation of two-phase microstructures consisting of α-Mg and icosahedral phase, as well as to enhance the aging characteristics of Mg-Zn-Y alloys by the addition of aluminum. The optimum range of Zn/Y ratio for the two-phase microstructure of α-Mg and I-phase is 3-5. With a decreasing amount of icosahedral phase in the Mg-Zn-Y alloys, the compressive yield stress decreased. The strengthening effect of the icosahedral phase is explained to originate from the low energy interface between the icosahedral phase and α-Mg matrix. Hardness measurement shows that the aging response is improved when Al is added to the Mg-Zn-Y alloy. Although the Al containing alloy shows higher strength level in the peak-aged condition than Mg-Zn-Y alloy, the ductility increase is marginal possibly due to the decreased fraction of icosahedral phase particles and formation of intermetallic compound during heat treatment.
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