β-Al4.5FeSi: Hierarchical crystal and defect structure: Reconciling experimental and theoretical evidence including the influence of Al vs. Si ordering on the crystal structure

Abstract

The β-Al4.5FeSi phase occurs as plate-shaped particles in secondary, i. e. recycled, Fe-containing hypoeutectic Al-Si alloys and as a component of the intermetallic reaction layer between Al-Si alloys and steels joints. Unsolved issues of the structure interpretation with consequences at all microstructural scales are addressed based on experimental results and new theoretical evidence from DFT calculations. On top of the previously described polytypism (AB and ABCD polytypes, where the letters refer to the positions of a certain type of double layers perpendicular to the stacking direction), ordering of Al vs. Si atoms adds a further aspect of structural variability. Thus, three different ordered crystal structures of β-Al4.5FeSi with different ordering of Al vs. Si atoms and different stacking sequences (AB or ABCD) could be encountered in the same alloy. The two AB structures can be described in space groups A12/a1 with (DFT-base) lattice parameters a = 6.1667 Å, b = 6.1683 Å, c = 20.779 Å and β = 91.51° and A21/e11 with lattice parameters a = 6.1739 Å, b = 6.1602 Å, c = 20.7840 Å and α = 88.63°. The third structure has ABCD stacking. Unit cell distortions away from a pseudotetragonal orthorhombic metric comply with the symmetry are shown to be induced by the ordering of Al vs. Si atoms. Frequently observed δ-like stacking sequences are discussed as consequence of excess Si in β-Al4.5FeSi. Furthermore, recommendation is given for optimal experimental conditions for the characterization of the type of order and polytype in β-Al4.5FeSi.