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The microstructure of alloy Mg-3Si(mass fraction/%, same as below) after successive additions with different elements of Zn, Nd, Gd and Y was observed and the microstructure evolution was investigated by scanning electron microscopy and X-ray diffraction. The results show the primary Mg₂Si particles co-exist with eutectic Mg₂Si particles in binary alloy Mg-Si. With minor addition of Zn element, only primary Mg₂Si can be found in ternary Mg-3Si-3Zn system while eutectic Mg₂Si particles disappear. In quaternary alloy Mg-2.0Nd-3.0Zn-3.0Si, the addition of Nd element can effectively refine the primary Mg₂Si particles and form some Mg₄₁Nd₅ particles. After continuous adding of Gd and Y elements into quaternary system, Gd₅Si₃ and YSi particles increase significantly in the alloy Mg-8.0Gd-4.0Y-2.0Nd-3.0Zn-3.0Si, while volume fraction of primary Mg₂Si decrease significantly. Thermo-Calc calculation predicts that the Gibbs free energy for primary particles Gd₅Si₃, YSi is lower, and therefore Gd, Y atom and Si are more likely to form compounds. In Mg-8Gd-4Y-2Nd-3Zn-3Si alloy, room temperature Gibbs free energy for primary particles Mg₂Si, Gd₅Si₃, YSi is -9.56×10⁴, -8.72×10⁴, -2.83×10⁴J/mol, respectively, and the mass fraction of these particles is 8.07%, 5.27%, 1.40% respectively.