Nucleation competition and phase transformation mechanisms in recycled aluminium alloys: Insights into θ-Al13Fe4, Al6(Fe,Mn) and α-Al15(Fe,Mn)3Si2

Aluminium rich Fe-bearing intermetallic compounds (Fe-IMCs) plays a critical role in determining the mechanical properties of recycled aluminium alloys due to inevitable Fe accumulation during recycling. The Fe-IMCs which have a needle-/plate-like morphology are particularly detrimental, impairing the ductility and overall performance of aluminium alloys. Consequently, optimizing phase selection to favour less harmful Fe-IMCs is a critical strategy for improving alloy design and enhancing material properties. The nucleation of Fe-IMCs, however, is challenging because it requires precise structural and compositional templating, involving multiple alloying elements at specific atomic positions, and thus necessitates substantial undercooling. This study examines a complicated primary phase selection among θ-Al 13 Fe 4 and Al 6 (Fe,Mn) and α-Al 15 (Fe,Mn) 3 Si 2 in an Al-5Mg-2Si-0.6Mn-1.3Fe alloy. Experimental results show θ-Al 13 Fe 4 and Al 6 (Fe,Mn) solidify as non-equilibrium primary phases ahead of the equilibrium α-Al 15 (Fe,Mn) 3 Si 2 , with subsequent transformation to α-Al 15 (Fe,Mn) 3 Si 2 during later stages. Phase competition and transformation mechanisms were characterized using scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), with experimental results supported by first-principles modelling. Particular focus was given to the transition from the silicon-unfavourable Al 6 (Fe,Mn) to the silicon-rich α-Al 15 (Fe,Mn) 3 Si 2 . The findings provide a novel framework for designing recycled aluminium alloys with enhanced mechanical properties by optimizing Fe-IMC phase selection and transformation pathways. Phase Evolution of Fe-Rich Intermetallic in Al Alloys: From Al 6 (Fe, Mn) to α-Al 15 (Fe, Mn) 3 Si 2 + α-Al in Al-5Mg-2Si-0.6Mn-1.3Fe alloy during solidification. • First comprehensive study on nucleation competition and phase transformations among Fe-IMCs (θ-Al 13 Fe 4 , Al 6 (Fe,Mn) and α-Al 15 (Fe,Mn) 3 Si 2 ) in recycled Al alloys. • Non-equilibrium θ-Al 13 Fe 4 and Al 6 (Fe,Mn) nucleate preferentially due to lower nucleation barriers, preceding equilibrium α-Al 15 (Fe,Mn) 3 Si 2 . • Spatial phase distribution depends on cooling rate: θ-Al 13 Fe 4 dominates in slower-cooled regions, while Al 6 (Fe,Mn) prevails in faster-cooled areas. • Transformation from Al 6 (Fe,Mn) to α-Al 15 (Fe,Mn) 3 Si 2 follows a lamellar structure formation with no detectable orientation relationship.