Investigation on Morphology of Primary Mg2Si in Al-20wt.%Mg2Si Composite with Experiment and First-Principle Calculations

The morphology of refined primary Mg2Si always exhibits the faceted crystal growing pattern in the Al-Mg2Si composites. The 2D and 3D morphologies of primary Mg2Si particles were examined by scanning electron microscopy (SEM) and field emission scanning electron microscope (FE-SEM) respectively. The morphology evolution mechanism of the faceted primary Mg2Si in Al-Mg2Si composite was investigated. The results show that there were only (100) planes and (111) planes emerging in Mg2Si particles. The morphology evolution of faceted Mg2Si has only two growing paths including Cube to Truncated Cube and Octahedron to Truncated Octahedron. The morphology evolution, critical nucleus and growth, was determined by r value which was deduced from the ratio of the surface free energy of (100) planes to the surface free energy of (111) planes. The surface free energies were identified by First-principle simulation. The validity of the above two growing path modes were verified. The configurations of the surface elements in faceted Mg2Si particles are predicted according to the simulating results.