Effect of U and Th trace additions on the precipitation strengthening of Al–0.09Sc (at.%) alloy

The age hardening response of Al–0.09Sc (at.%), to which trace amounts (< 100 ppm) of actinides (An = U or Th) were added, is studied by microhardness, conductivity, transmission electron microscopy, and atom probe tomography (APT). Peak-age hardening at 300 °C is associated with a high number density of nanoscale L1₂-Al₃(Sc_{1 − x}An_x) precipitates with core/shell structure. The first alloy Al–0.09Sc–0.006U (at.%) has a peak microhardness similar to that of binary Al–0.09Sc (at.%), but a shorter incubation period for hardening which is consistent with U diffusing faster than Sc in Al and acting as nucleant for Al₃Sc. This is confirmed by APT measurements of precipitate composition, Al₃(Sc_0.8U_0.2), showing that U has high solubility in Al₃Sc precipitates and segregates at their core. The second alloy, Al–0.09Sc–0.008Th (at.%), exhibits Th-poor Al₃(Sc_0.98Th_0.02) precipitates with Th segregation in their shells and it has microhardness evolution undistinguishable from binary Al–0.09Sc; this is indicative of low solubility of Th in L1₂-Al₃Sc and/or low diffusivity of Th in Al. These two primordial actinides -U and Th- show different abilities to coprecipitate with Al₃Sc precipitate in aluminum, they, however, both improve coarsening resistance after 143 days at 300 °C by forming core/shell structure.