Solidification sequence and four-phase eutectic in AlSi6Cu4Fe2 alloy

Differential thermal analyses of an AlSi6Cu4Fe2 alloy at various cooling rates from the liquid state were performed which showed: i) possible discrepancies in the first solidification steps and; ii) solidification ending with multiple peaks at low cooling rate. Metallographic observation of the samples gave hint for understanding the discrepancies while the presence of rounded multi-phase pools – or so-called rosettes - entrapped within dendrite arms explained the multiple peaks because they solidified independently of each other. The very fine eutectic microstructure in these rosettes was investigated by scanning electron microscopy which showed the precipitation of a four-phase eutectic. Focused ions beam milling was then used to investigate the topology of the distribution of the four phases. Electron backscattered diffraction analysis was carried out to investigate their local crystallographic orientations and transmission electron microscopy analysis was performed to identify the very fine crystalline structures. This eutectic appears as one of the very few regular four-phase eutectics reported in the literature.