Microstructural evolution and creep properties of precipitation-strengthened Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys

The aging behavior at 300 °C of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys is studied by local-electrode atom-probe tomography, transmission electron microscopy and microhardness measurements. The ternary alloys exhibit high number densities of coherent L12 precipitates (Nv≅1022m-3) at aging times up to 1536 h (64 days). In the Al–0.06Sc–0.02Gd alloy, the Al3(Sc1−xGdx) precipitates are always Sc-rich, displaying a small Gd concentration (x < 0.12) in the precipitates. In the Al–0.06Sc–0.02Yb alloy, the precipitates are initially Yb-rich, Al3(Yb1−xScx), with Sc diffusing subsequently to the precipitates, resulting in a core/shell structure and an overall Sc-rich composition, Al3(Sc1−xYbx). Gd and Yb, like other lanthanides but unlike the transition metals Zr and Ti, do not retard the coarsening kinetics compared with binary Al–Sc alloys. Additionally, the creep resistance of these alloys is greater than that of Al–Sc alloys. The coarsening kinetics and creep properties of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb alloys are compared with other Al–Sc-based alloys and with coarsening models for ternary alloys.