Characterization of the morphology of primary silicon particles using synchrotron X-ray tomography

The 3D morphology of primary silicon particles in a hypereutectic Al-20 wt.% Si alloy was characterized using synchrotron X-ray tomography. According to the statistical analysis of the primary silicon particles, it was found that both octahedral shape and twinned spinel shape were identified as the typical morphologies. For the octahedral shape, the measured growth speed ratio between < 100 > and < 111 > directions was close to 3, i.e. approaching the theoretical value for a regular octahedron. The measured growth speed ratio between < 111 > and <112¯> for most twinned particles was about 0.93, indicating a perfect twinned spinel shape. However, influenced by surrounding solidification conditions, such perfect twinned growth of the primary silicon particles was hard to maintain, i.e. with medium or low cooling rate, the particle morphology tended to be regular while with high cooling rate, a plate-like morphology would become favorable.