Effects of Er and Gd compound addition on microstructure and mechanical properties of hypereutectic Mg-3.2Si ally were investigated by optical microscopy

scanning electron microscopy and EMPA.And modification mechanism of the Er and Gd was also described.With 0.6%（mass fraction

the same below）Er addition

size of the primary Mg

2

Si phase in Mg-3.2Si alloy is refined from 150μm to 40μm with structure transformed from bulky dendritic into irregular polyhedron shape.And then adding0.6% Gd

polyhedron primary Mg

2

Si phase with size in 20

3

0μm can be observed

exhibiting the desirable modification effects.However

when addition of rare earth is excessive

the primary Mg

2

Si phase becomes coarse.Er and Gd can enrich on the primary Mg

2

Si phase to control its growth process and change growth mode.Moreover

Er and Gd can generate the continuous solid solution during solidification because of the same lattice structure.With compound adding 0.6% Er and 0.6% Gd

tensile strength and elongation of the alloy reach 112 MPa and 3.2%

respectively.