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The effects of Ba-Nd composite modification on the microstructure and mechanical properties of Mg-3Si-4Zn cast alloy were investigated. The microstructure was characterized by OM, SEM, EDS and XRD. The mechanical properties were tested by hardness test. The best modification effect was achieved when a single denaturant Ba of 1.2wt% was added to the Mg-3Si-4Zn alloy. Results show that the formed phase BaMg₂Si₂ can act as a heterogeneous nucleation core for the primary Mg₂Si, refining the primary Mg₂Si. Ba-Nd composite modification is achieved by adding the modifier Nd to the Mg-3Si-4Zn-1.2Ba alloy. By the calculation of the Gibbs free energy using the Miedema model and a linear fit, it is found that the growth rate of primary Mg₂Si is suppressed and the primary Mg₂Si phase in the microstructure becomes smaller because more stable compounds like NdSi, NdSi₂, Ba₂Si, and BaSi₂ can be formed by Nd and/or Ba atoms with Si atoms, preventing Si atoms from binding to Mg atoms at the initial stage of solidification. The best Ba-Nd composite modification effect is achieved when the Nd content is 2.0wt%, i.e. the primary Mg₂Si changes from a dendritic shape with an average area of about 600 μm² to a nearly square-shape with an average side length of about 5 μm and the eutectic Mg₂Si changes from complex and coarse Chinese-script shape with an average area of about 444 μm² to simpler shape with a mean area of about 89 μm². The hardness of the alloy is increased from 575.75 MPa to 612.11 MPa, increased by 6.31%.