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 delta T-1, primary Si is coarse long plates with specific edges and faces, showing a characteristic of the lateral growth mode. When undercooling approaches delta T-2, 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 delta T-2, 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, delta T-1 and delta T-2, gradually decreases, with the lowest values of 78 and 180 K for an RE content of 1.5%.