Effects of combined addition of (cerium plus ytterbium) on microstructure and mechanical properties of aluminium-siliconmagnesium-copper-chromium casting alloy are investigated. The results show that α-aluminium grains refined and eutectic silicon modified simultaneously, and the morphology of eutectic silicon transforms from plate-like to fine granular. Combined addition of (cerium plus ytterbium) and heat treatment also changes the morphology of β-Al5(Fe, Cr)Si. Cerium- and/or ytterbium-rich intermetallic compounds appear to be responsible for the increasing strength and elongation. With increasing (cerium plus ytterbium) addition, the microhardness, ultimate tensile strength, yield strength and elongation increase firstly, and then decrease gradually. When adding 0.4 wt.% (cerium plus ytterbium), the eutectic silicon and β- Al5(Fe, Cr)Si phases fined and the formation of cerium- and/or ytterbium-rich intermetallic compounds lead to the values of microhardness, ultimate tensile strength, yield strength and elongation reach the maximum values of 130.5 HV 10, 336.8 MPa, 204.6 MPa and 7.5 %, respectively. However, the excessive addition of (cerium plus ytterbium) promotes the formation of thick cerium- and/or ytterbium-rich intermetallic compounds, deteriorating the ultimate tensile strength and elongation of the modified alloy. Finally, combined addition of (cerium plus ytterbium) as well as transforms the fracture
