The effects of trace yttrium on the microstructure and mechanical properties of as-cast Mg-Al-Sn-Zn alloys were investigated by copper mold casting under the vacuum-Ar atmosphere protection. The addition of yttrium can generate the new phase of Al

2

Y and refine the α-Mg and eutectic Mg

17

Al

12

phase. With the increase of yttrium

the size and fraction of the new phase are increased. The results show that the proper amount of Y can improve the mechanical properties of the based alloy at room temperature. The alloy containing 0.5% yttrium has the satisfied mechanical properties with the tensile strength

yield strength and elongation of 247.7 MPa

112.3 MPa

and 11.52%

respectively

which are 14.5%

9.6%

and 17.1% higher than those of the based alloy

which are attributed to the fine grain strengthening and second phase strengthening.