Heterogeneous Nucleation of α-Al Grain on Primary α-AlFeMnSi Intermetallic Investigated Using 3D SEM Ultramicrotomy and HRTEM

Microstructural examination of the Al-5.3Mg-2.4Si-0.6Mn-1.0Fe alloy in the die-cast condition revealed that a significant number of the primary α-AlFeMnSi intermetallic particles were found inside both the coarse α-Al dendrite fragments formed in the shot sleeve and the fine α-Al grains formed in the die cavity. The heterogeneous nucleation of α-Al phase on primary α-AlFeMnSi intermetallic particle was further investigated experimentally. 3-Dimension (3D) scanning electron microscopy ultramicrotomy revealed that the probability of finding at least one primary α-AlFeMnSi intermetallic particle inside each α-Al grain was almost 90 pct. The detailed microstructural analysis identified the primary α-AlFeMnSi intermetallic particle as the α-Al₁₂(Fe,Mn)₃Si composition with a body-centered cubic structure and a lattice parameter of a = 1.265 nm. It was found that the primary α-Al₁₂(Fe,Mn)₃Si intermetallic particle had a faceted morphology with {110} planes exposed as its natural surfaces. High resolution transmission electron microscopy further confirmed that the crystallographic orientation relationship between α-Al₁₂(Fe,Mn)₃Si intermetallic particle and α-Al phase was: [111]_α-AlFeMnSi//[110]_Al and (1\( \overline{1} \)0)_α-AlFeMnSi~6 deg from (1\( \overline{1} \)1)_α-Al, and the corresponding interface between two phases could be confirmed as a semi-coherent interface with a lattice misfit of 2.67 pct at 933 K (660 °C), which was considerably smaller than the theoretical limit (5.7 pct) for epitaxial nucleation. Finally, based on these experimental evidences and the epitaxial nucleation model, we concluded that the primary α-Al₁₂(Fe,Mn)₃Si intermetallic particles were both potent and effective nucleating substrates for the α-Al phase.