3D characteristics and growth behavior of Fe-rich phases in Al−Si−Fe−Mn alloys during melt holding

The morphological evolution and growth behavior of Fe-rich phases in the Al−7.0Si−1.0Fe−1.2Mn alloy during melt holding were studied using scanning/transmission electron microscopy, thermodynamic calculations and synchrotron X-ray imaging. The results show that with a decrease in holding temperature, the Fe-rich phase underwent a morphological transformation from coarse star-like and polygonal structures to fine Chinese-script and coupled structures consisting of Chinese-script and polygonal shapes. The 3D structure of the star-like phase was composed of multibranched primary phases and irregular secondary phases; the coupled structure was composed of polyhedrons and dendrites; the Chinese-script phase was composed of dendrites. When the holding temperature was decreased, the number and volume fraction of Fe-rich phases were significantly reduced. A similar trend was also obtained for the largest volume particles and their connectivity. In situ synchrotron X-ray radiography results indicated that two types of primary Fe-rich phases were formed during the solidification process, and the smaller phase that remained in the melt could act as nucleation sites for the formation of the coupled eutectic structures.