LAUSR

In this paper, the morphology transition and the growth process of the primary Mg 2 Si in the Al–Mg 2 Si in situ composites were three-dimensionally investigated by observing the extracted Mg 2 Si particles. The primary Mg 2 Si transforms from perfect octahedron to truncated octahedron with increasing cooling rate. Combining with the crystal morphologies obtained at different growth stages, the growth mechanism of octahedral Mg 2 Si was discussed. In the early growth stage of the octahedral Mg 2 Si, the secondary branches preferentially grow on the advancing tips of the first branches. Then, the hollows in the {111} faces shrink gradually and the octahedral Mg 2 Si forms finally. With the increase of Mg 2 Si content, dendritic Mg 2 Si phases were observed and the truncated octahedron Mg 2 Si connect mutually to form the complicated morphology at low cooling rate. The high cooling rate transforms the morphology of the Mg 2 Si crystal. The growth rates of the 〈100〉 and 〈111〉 axes can be manipulated by adjusting the cooling rates, which are responsible for the evolution of the Mg 2 Si crystals.