Effects of Zn and Gd on the as-cast microstructure and mechanical properties of Mg-Zn-Gd alloys were investigated by menas of optical microscope（OM）

XRD（X-ray diffraction）

SEM（scanning electron microscope） and tensile testing at room temperature.The results reveal that Gd content plays an important role in the phase constitution of the alloys.When given a fixed Zn content

with the increase of Gd content

the second phase transformation was observed as follows:Mg7Zn3 phase +Iconsahedral quasicrystalline phase（I phase）→I phase →I phase +W phase.Meanwhile

the secondary dendrite arm is evidently decreased and morphology of the interdendrite is evolved from particle into needle-like structure and then into closed net-like structure.When the atomic ratio of Zn to Gd keeps constant of 5.8

with increasing in total content of Zn and Gd contents

the second phase constituent sustains stable

however

the more fine dendrite phase is distributed into the microstructure with the increase of second phase volume fraction.Tensile testing indicates that when the Zn is given a fixed value

with increasing in Gd content

both the tensile strength and elongation of the alloy are improved

however

the yield strength is firstly increased and then decreased.In addition

when the atomic ratio of Zn to Gd keeps a constant value of 5.8

with increase of the total content Zn and Gd

the alloy exhibits higher strength and lower elongation.