Heterogeneous nucleation and phase transformation of Fe-rich intermetallic compounds in Al–Mg–Si alloys

The phase selection among different types of Fe-containing intermetallic compounds (FIMCs) during solidification process cannot be controlled. Thus, resultant microstructures and mechanical property profiles for Al–Fe–Si alloys are not consistent. In this work, the phase competition and phase relationship among different types of Fe-containing intermetallic compounds (FIMCs) were investigated in an Al–5Mg–2Si-0.6Mn-1.3Fe alloy by studying the heterogeneous nucleation of these FIMCs. The experimental results show that primary FIMCs, such as α-Al15(Fe, Mn)3Si2 and θ-Al13Fe4, are co-selected during a given solidification condition (e.g. cast at 720 °C, 3.5K/s). The primary(P) θ-Al13Fe4 was identified to be heterogeneously nucleated on the in-situ MgAl2O4 particles, and then the primary α-Al15(Fe, Mn)3Si2 forms on the primary θ-Al13Fe4 via a peritectic reaction: L (liquid) + θ-Al13Fe4 → α-Al15(Fe, Mn)3Si2 + α-Al. These FIMCs were identified and the phase relationships among those FIMCs were studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The orientation relationships between P-θ-Al13Fe4 and MgAl2O4, P-θ-Al13Fe4 and transformed α-Al15(Fe, Mn)3Si2, were identified with TEM. The nature of the complicated phase selection of FIMCs in Al–Mg–Si alloys demonstrate the competition between two different FIMCs during heterogeneous nucleation.