Optimizing the mechanical performance of extruded AA5454 aluminum alloys microalloyed with Sc and Zr through post-aging treatments

This study investigates the influence of post-aging treatments applied after extrusion on the microstructure and mechanical performance of extruded AA5454 aluminum alloys microalloyed with Sc and Zr. While the strengthening effects of Sc/Zr during solidification and thermomechanical processing are established, their response to post-extrusion aging remains insufficiently understood. Three alloys, AA5454 (base), AA5454-S (0.1 wt% Sc), and AA5454-SZ (0.1 wt% Sc + 0.1 wt% Zr), were extruded, and then subjected to different one-step (300 °C/12 h and 350 °C/4 h) and two-step (275 °C/12 h + 300 °C/12 h and 275 °C/12 h + 350 °C/4 h) post-aging treatments. The results demonstrate that the two-step aging strategy (275 °C/12 h + 300 °C/12 h) provides the most effective strengthening, leading to a yield strength increase of 82 MPa in AA5454-S and 35 MPa for AA5454-SZ. TEM analysis revealed that AA5454-S develops a high number density of L1 2 -Al 3 Sc nanoparticles during post-aging, while AA5454-SZ exhibits a bimodal distribution of fine newly formed nanoprecipitates and coarse pre-existing Al 3 (Sc,Zr) particles owing to their high thermal stability. The α-Al( Mn , Fe , Cr )Si dispersoids in all three alloys are thermally stable and remain unchanged during post-aging. Quantitative analysis based on the Orowan bypass mechanism shows that precipitation strengthening contributes up to 71 MPa and 101 MPa of the yield strength in AA5454-S and AA5454-SZ, respectively. The results indicate that the optimized two-step post-aging enables controlled precipitation strengthening, providing a simple and scalable strategy to enhance the strength of 5xxx alloys.