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

Abstract

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-Al13Fe4 and Al6(Fe,Mn) and α-Al15(Fe,Mn)3Si2 in an Al-5Mg-2Si-0.6Mn-1.3Fe alloy. Experimental results show θ-Al13Fe4 and Al6(Fe,Mn) solidify as non-equilibrium primary phases ahead of the equilibrium α-Al15(Fe,Mn)3Si2, with subsequent transformation to α-Al15(Fe,Mn)3Si2 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 Al6(Fe,Mn) to the silicon-rich α-Al15(Fe,Mn)3Si2. The findings provide a novel framework for designing recycled aluminium alloys with enhanced mechanical properties by optimizing Fe-IMC phase selection and transformation pathways.