Development of Sustainable Mixed Recycled Wrought Aluminium Alloys with Modified Al3(Sc,Ti) for Improved Impurity Tolerance

Abstract

A representative cross-recycled wrought aluminium alloy, Al–1.33Si–1.28Mg–0.97Cu–0.88Zn–0.51Fe–0.57Mn–0.14Cr–0.09Ti (wt.%), was designed based on mixed 1xxx–7xxx series compositions with maximised impurity levels to emulate realistic recycling streams. To enhance impurity tolerance and microstructural stability in such complex recycled alloys, the role of scandium (Sc) was systematically investigated. While Sc is widely recognised in aluminium alloys for grain refinement via primary Al₃Sc particles and precipitation strengthening through coherent nanoscale Al₃Sc dispersoids, its behaviour in recycled aluminium systems remains largely unexplored. Here, we examine the formation, morphology, composition, crystallography, and impurity tolerance, particularly with respect to Fe and Si, of Al₃(Sc,Ti) in this cross-recycled alloy. Competitive formation of primary Al₃(Sc,Ti) and eutectic (Al₃(Sc,Ti) + α-Al) is observed. Combined experimental characterisation and density functional theory calculations reveal preferential substitution of Ti for Sc in the L1₂ lattice, reducing the lattice misfit with α-Al to nearly zero and promoting heterogeneous nucleation. Al₃(Sc,Ti) exhibits remarkable tolerance to a broad range of impurities, with Fe, Mn, Cr, Si, Cu, Mg, Ti and Zn accommodated through defect-mediated incorporation and local chemical interactions at elevated temperatures. Atomic-resolution STEM further reveals Fe clusters embedded within as-cast Al₃(Sc,Ti) particles, while heat treatment induces surface segregation of major alloying elements, indicating dynamic solute redistribution. These results establish the intrinsic impurity tolerance of Al₃(Sc,Ti) and provide mechanistic insight into solute partitioning and phase stability in complex recycled aluminium alloys, offering guidance for the design of sustainable, high-performance aluminium systems from mixed recycled feedstocks.