Microstructural Evolution and Enhanced Thermal Stability of Al–Sc–Zr Alloys Modified with Sm and Si: A Multiscale Analysis

This research investigates novel samarium (0.10 at.%) and/or silicon (0.10 at.%) additions to a cast base Al–0.02Sc–0.08Zr (at.%) alloy, focusing on their impact on L12Al3(Sc, Zr) nanoprecipitates. Precipitation was studied via isochronal (25–575 °C), isothermal (400, 425 °C), and double-aging (400 °C + 425–475 °C) treatments, revealing superior thermal stability in Sm-containing alloys. Scanning electron microscopy (SEM) revealed significant grain refinement from Sm. Transmission electron microscopy (TEM) confirmed spherical L12 nanoprecipitates across all compositions, indicating that Sm does not alter the fundamental precipitate characteristics. Atom probe tomography (APT) revealed a core–shell structure with Sc in the core and Sm/Zr in the shell. Sm partitioned to nanoprecipitates (4.68–13.74 at.%) during aging, confirmed by HAADF-STEM and EDS mapping, without forming separate tri–aluminide intermetallics. The Sm/Si-modified alloy exhibited a cumulative effect between Sm and Si, the highest precipitate density (∼3.87 × 1022 m−3), and enhanced coarsening resistance. Creep tests at 300 °C showed a substantial threshold stress increase, from 9.6 MPa (base alloy) to 15.1 MPa (Sm/Si-modified alloy). This correlated with increased Sc content in L12Al3(Sc, Sm, Zr) nanoprecipitates, enhancing dislocation pinning. While Si slightly reduced coarsening resistance, it increased strength (∼80–150 MPa), contributing to improved creep resistance.