Microstructural and mechanical behavior of Mg/Mg2Si composite fabricated by a directional solidification system

In the present work, the effect of cooling rates on the morphology and distribution of precipitated Mg2Si reinforcement particles from a Mg-5 wt.%Si matrix fabricated through directional solidification was studied. Using directional solidification system, different cooling rates were applied to one single cast sample. In higher cooling rates, the Mg2Si precipitated in polygonal while in lower ones, it was solidified in morphology dendritically. It was shown that by decreasing cooling rate, the primary Mg2Si particles nodule counts decreased. This was attributed to chilling effect and high solidification rates imposed by the directional solidification system as well as gravity macrosegregation of particles. In terms of mechanical properties, the structures containing polygonal morphologies and finer Mg2Si particles improved tensile properties.

► In higher cooling rates, the precipitated Mg2Si was formed in polygonal morphology while in lower ones, it precipitated dendritically.
► In the first half of directionally solidified ingot, high cooling rates was found to be responsible for hardness and in the second upper half Mg2Si hard particles segregated in the molten metal make up hardness.
► Mechanical properties were affected by cooling rate due to microstructure changes during the solidification, primary Mg2Si concentrations and porosities.