Effect of Si Content on the Size of Fe-rich Intermetallic Particles in Al-xSi-0.8Fe Alloys

Al-Si-Fe plates with Si contents of 4.5, 9 and 11 mass %, unmodified and Sr-modified, were quasi-directionally solidified in sand moulds with chills at one end. The size and nature of the Fe-rich intermetallics were determined along the plates. Two forms of the intermetallic were observed, alpha-Al8FeSi and beta-Al5FeSi, in proportions and scale dependent on the cooling rate and the Si concentration. The size of the beta-phase increased with the concentration of Si at low cooling rates. At high cooling rates the tendency to form alpha-Al8FeSi phase increased with increasing Si content reducing the size of the beta-plates. Modification generally increased the size of the pre-eutectically formed plates while reducing the post eutectically formed ones.