Microstructures, corrosion and mechanical properties of as-cast Mg-Zn-Y-(Gd) alloys

Long period stacking ordered (LPSO) structure phases were prepared by conventional solidification method in Mg₉₄Zn₃Y_xGd_3-x (x=3, 2, 1.5, 1, mole fraction) alloys, the microstructures, corrosion and compressive mechanical properties of which were investigated, separately. The results reveal that the microstructures of the as-cast Mg₉₄Zn₃Y_xGd_3-x alloys, with n(Zn)/n(Y+Gd)=1:1, consist of α(Mg) phase, Mg₃Zn₃RE₂ (W) phase, Mg₁₂ZnRE (14H-LPSO) phase and a few bright cube-shaped Mg-Y-Gd phases. The formation and the distribution of LPSO-phase in the alloys can be influenced by the content of Gd. The volume fraction of 14H-LPSO phase increases first and then decreases with the increase of the Gd content. For the electrochemical impedance spectroscopy (EIS) measurement, a R(Q(R(QR))) model was used to fit the test results in 3.5% (mass fraction) NaCl solution at room temperature. The corrosion current densities of all samples are about 10^-5 A/cm². When x(Gd)≤1%, Mg-Zn-Y-(Gd) alloy shows good corrosion resistance, which is better than that of the commercial AZ91D magnesium alloy. The corrosion rate increases when the Gd content is higher than 1.5%. At room temperature, the compressive properties of Mg-Zn-Y-(Gd) alloys increase remarkably with the increase of the volume fraction of LPSO phase. In addition, the pinning effect of W-phase and dispersive cube-shaped Mg-Y-Gd phase is beneficial to improving the mechanical properties of as-cast Mg₉₄Zn₃Y_xGd_3-x alloy in deformation process.