4. ConclusionsThis work explores the effect of Y on the microstructure and mechanical properties of Mg-Sn-Zn alloy in the cast condition. The major conclusions can be drawn as follows: (1) Mg-5Sn-3Zn as-cast alloy mainly consists of a-Mg, Mg2Sn, and MgZn2, thermal stable MgSnY phase emerged by the addition of Y, and these phases gradually grow in amount with the increases of Y. The grains turn from dendrite to equiaxed structure by the Y addition. (2) Mg2Sn would be the nucleating agent for MgZn2 precipitates, because the Mg2Sn could separates out in advance and the mismatch degree between Mg2Sn and MgZn2 is relatively low which means Mg2Sn would stimulate the precipitation of MgZn2 to enhance the mechanical properties. (3) The addition of Y improves the mechanical properties of Mg-5Sn-3Zn alloy effectively especially the elongation, the comprehensive mechanical properties considered to reach to the best in Mg-5Sn-3Zn-0.5Y, which's ultimate tensile strength is 229Mpa and the elongation reaches to 19.1%.