Mg-xZn-3 Y-0.7 Zr alloy was prepared by low-pressure casting.Evolution of the microstructure of Mg-xZn-3 Y-0.7 Zr alloy was observed by optical microscope

scanning electron microscope and differential thermal analysis.The heat treatment process was optimized to improve the mechanical properties through controlling secondary phase distribution in the Mg alloy.The results indicate that the as-cast microstructure of Mg-5 Zn-3 Y-0.7 Zr alloy is composed ofα-Mg and W-Mg3 Zn3 Y2 phase.The W phase with net-like structure is distributed along the grain boundary.With the addition of 8% Zn

dendrites in as-cast alloy are obviously increased

and I-Mg3 Zn6 Y phase appears.After T6 heat treatment

the IMg3 Zn6 Y phase disappears

and the plate-like eutectic structure of W phase is broken

however

fishbone lamellar eutectic structure is reserved at triangle grain boundary.The tensile test shows that the ultimate tensile strength

yielding strength and elongation of as-cast Mg-5 Zn-3 Y-0.7 Zr alloy reach 223.8 MPa

124.9 MPa and 7.3%

respectively.Effects of T6 heat treatment on mechanical properties of the Mg-5 Zn-3 Y-0.7 Zr alloy can be ignored.After T6 heat treatment

the mechanical properties of Mg-8 Zn-3 Y-0.7 Zr alloy are improved greatly.The ultimate tensile strength

yielding strength and elongation reach 263.3 MPa

207.9 MPa and 2.2%

respectively.The fracture mechanism of Mg-5 Zn-3 Y-0.7 Zr is characterized by quasi cleavage crack with tearing ridge

while the Mg-5 Zn-3 Y-0.7 Zr alloy is characterized by cleavage fracture with absence of dimple.