Achieving superior strength and ductility synergy in bulk ultrafine grained Al-Mg-Sc-Zr alloy via powder pre-aging

Introducing high density of nano-precipitates to recrystallized ultrafine grains is helpful to realize strength-ductility synergy but is a challenging task, because recrystallization and precipitate growth/coarsening usually concur. Here we develop a pre-aging powder metallurgy processing route to achieve such microstructure in Al-Mg-Sc-Zr alloy. During the pre-aging stage, atomic clusters including short-range order are formed within the grains, which provide new sites for the nucleation and enable the formation of fine Al₃(Sc, Zr) precipitates. Subsequent high-temperature sintering and hot extrusion lead to grain recrystallization. The nano-precipitates not only further strengthen the ultrafine-grained alloy by Orowan mechanism, but also greatly enhance the strain-hardening rate by dislocation-precipitate interaction, resulting in excellent strength-ductility synergy. The utilization of digital image correlation (DIC) analysis allows for the observation of dynamic strain aging during the tensile process, whereby the strain demonstrates a distinctive step-like transition coinciding with the passage of the Portevin-Le Chatelier (PLC) band. This work provides a new path for improving the mechanical properties of the same type of metallic materials.