Formation of θ-Al13Fe4 and the multi-step phase transformations to α-Al8Fe2Si, β-Al5FeSi and δ-Al4FeSi2 in Al–20Si-0.7Fe alloy

Formation of Al13Fe4 and the phase transformations on these particles between numbers of different types of Fe-containing intermetallic compounds (FIMCs) were observed in Al–20Si-0.7Fe alloy when solidified at a slow cooling rate (0.01 K/s). The experimental results show that the Al13Fe4 phase (monoclinic) solidified initially and transformed into an intermediate α-Al8Fe2Si (hexagonal) and nano-scaled α-Al grains via a quasi-peritectic reaction. Subsequently, the intermediate α-Al8Fe2Si transformed into β-Al5FeSi (monoclinic). The transformed β-Al5FeSi phase contained many defects, and with heat treatment (530 °C for 30 min) transformed into δ-Al4FeSi2 (tetragonal). The variation in the composition and the lattice parameters of these FIMCs was examined with TEM which shows that the phase transformation among FIMCs starts from FIMCs with lower Si content and process to FIMCs with higher Si content. The interfaces between each of these phases were examined to determine the orientation relationships between various FIMCs. The phase transformation mechanism and the FIMCs transformation sequence have been proposed based on the experimental investigation.