Effect of cerium-rich mischmetal content on the growth morphology and microstructure evolution of undercooled Al-70 wt%Si alloys

The Al-70 wt%Si alloys with various cerium-rich rare earth contents undercooled by electromagnetic levitation were investigated, and the nucleation and growth, growth mode, and morphology evolution of primary Si were observed and recorded using a high-speed camera and scanning electron microscope. When the undercooling is lower than ΔT1, primary Si is coarse long plates with specific edges and faces, showing a characteristic of the lateral growth mode. When undercooling approaches ΔT2, the recalescence interface becomes dendritic and massive, and dendritic primary Si covers the surface of the alloys. The growth mode of primary Si shows a mixed growth mode of lateral and non-lateral. When undercooling exceeds ΔT2, the recalescence interface takes on a parallel shape and the morphology of primary Si transforms to homogeneous and fine spherical shapes with smooth surfaces, indicating a characteristic of the non-lateral growth mode. With an increasing RE content, critical undercooling, ΔT1 and ΔT2, gradually decreases, with the lowest values of 78 and 180 K for an RE content of 1.5%.