Developing 300 °C heat-resistant Al-Cu-Sc alloys via microstructural reconstruction during a novel three-step aging process

Achieving superior thermal stability at 300 °C in Al-Cu alloys remains a challenge owing to the rapid coarsening of θ′ precipitates. This study proposes a synergistic strategy that combines Sc microalloying with a novel three-step aging protocol (275 °C/6 h → 400 °C/40 min → 225 °C/4 h). The high-temperature retrogression stage (400 °C) is identified as the critical window for microstructural reconstruction, which induces the partial dissolution of unstable θ′ phases, concomitant with the activation of long-range Sc diffusion to precipitate thermally stable L 12 -Al 3 Sc nanoprecipitates. During subsequent re-aging, these Al 3 Sc particles serve as heterogeneous nucleation sites, triggering the re-precipitation of densely distributed, nanoscale metastable θ′ platelets. Consequently, a hierarchical microstructure featuring the coexistence of coarsened residual θ′ phases and refined new θ′ precipitates was established. After 100 h of thermal exposure at 300 °C, the Al-4.5Cu-0.15Sc alloy maintains a hardness retention rate of 83.3% and a UTS retention rate of 86%, significantly outperforming the Sc-free base alloy (55% retention). HRTEM analysis revealed that the re-aging treatment effectively reduced the lattice misfit (the in-plane misfit decreased from 2.73% to 1.43%), thereby lowering the interfacial energy and kinetically inhibiting Ostwald ripening. This study provides a robust microstructural control strategy for developing next-generation heat-resistant aluminum alloys for use in extreme service environments.