Influence of cooling rate on morphology evolution of primary Mg2Si in Al–15Mg2Si-1.0Gd in-situ composite

The effects of cooling rate on morphology of primary Mg2Si in Al–15Mg2Si composite modified with 1.0 wt. % Gd during solidification process were studied. To obtain different cooling rates, the step casting with four different thicknesses was used. The results showed that with increasing the cooling rate in Al-15%Mg2Si-1.0% Gd composite, the absorption of Gd atoms as external factor on {100} facets of primary Mg2Si crystal increases, leads to retarding of the growth rates of primary Mg2Si along the directions; results in the formation of primary Mg2Si with various morphologies. With decreasing the thickness of mold from 50 to 40, 30 and finally to 20mm, the morphology of primary Mg2Si altered from a dendrite to an octahedral and then truncated octahedral and finally to a truncated octahedral with larger {100} faces. This study can be beneficial to offer a simple method to control the morphology of primary Mg2Si crystals in Al-Mg2Si composite to tailor new composites with high mechanical properties