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 (<100ppm) 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 degrees C is associated with a high number density of nanoscale L1(2)-Al-3(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, Al-3(Sc0.8U0.2), 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 Al-3(Sc0.98Th0.02) 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 L1(2)-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 degrees C by forming core/shell structure.