Solute clustering behaviour during natural ageing in Al-Cu alloys containing Sc and Zr

This study investigates the effects of scandium (Sc) and zirconium (Zr) microalloying on solute clustering in Al-Cu alloys during natural ageing. Using atom probe tomography (APT), we reveal a full suppression of Cu-rich cluster formation in the AlCuScZr alloy compared to its Sc- and Zr-free counterpart. Instead, a distinct type of cluster, comprising Si, Sc and Zr forms, preferentially at Al 3 (Sc, Zr) dispersoids/Al interfaces, driven by strong Si-Sc-vacancy and Si-Zr-vacancy binding energies. These sites act as vacancy traps, redirecting clustering pathways away from the Cu-rich clusters typically observed in Al-Cu alloys. Small-angle x-ray scattering (SAXS) indicates suggest that the natural ageing kinetics in the AlCuScZr alloys are significantly delayed, likely due to the limited availability of Si, Sc and Zr solutes in the matrix. As a result, the AlCuScZr alloy exhibits slower hardening response during natural ageing. This is the first report of Si-Sc-Zr cluster formation during natural ageing in Al alloys, providing critical insights into how dispersoids, solute interactions, and vacancies govern early-stage clustering and mechanical evolution.