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 alpha-AlFeMnSi intermetallic particles were found inside both the coarse alpha-Al dendrite fragments formed in the shot sleeve and the fine alpha-Al grains formed in the die cavity. The heterogeneous nucleation of alpha-Al phase on primary alpha-AlFeMnSi intermetallic particle was further investigated experimentally. 3-Dimension (3D) scanning electron microscopy ultramicrotomy revealed that the probability of finding at least one primary alpha-AlFeMnSi intermetallic particle inside each alpha-Al grain was almost 90 pct. The detailed microstructural analysis identified the primary alpha-AlFeMnSi intermetallic particle as the alpha-Al-12(Fe,Mn)(3)Si composition with a body-centered cubic structure and a lattice parameter of a = 1.265 nm. It was found that the primary alpha-Al-12(Fe,Mn)(3)Si 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 alpha-Al-12(Fe,Mn)(3)Si intermetallic particle and alpha-Al phase was: [111](alpha-AlFeMnSi)//[110](Al) and (10)(alpha-AlFeMnSi)similar to 6 deg from (11)(alpha-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 A degrees 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 alpha-Al-12(Fe,Mn)(3)Si intermetallic particles were both potent and effective nucleating substrates for the alpha-Al phase.