Abstract :
Dilute Al-0.06 at.% Sc alloys with microalloying
additions of 50 at. ppm of ytterbium (Yb) or
gadolinium (Gd) are studied with 3D local-electrode
atom-probe (LEAP) tomography for different aging
times at 300 C. Peak-aged alloys exhibit Al3(Sc1–x
Ybx) orAl3(Sc1–xGdx) precipitates (L12 structure) with
a higher number density (and therefore higher peak
hardness) than a binary Al-0.06 at.% Sc alloy. The
Al–Sc–Gd alloy exhibits a higher number density of
precipitates with a smaller average radius than the
Al–Sc–Yb alloy, leading to a higher hardness. In the
Al–Sc–Gd alloy, only a small amount of the Sc is
replaced by Gd in the Al3(Sc1–xGdx) precipitates,
where x = 0.08. By contrast, the hardness incubation
time is significantly shorter in the Al–Sc–Yb alloy, due
to the formation of Yb-rich Al3(Yb1–xScx) precipitates
to which Sc subsequently diffuses, eventually forming
Sc-rich Al3(Sc1–xYbx) precipitates. For both alloys, the
precipitate radii are found to be almost constant to an
aging time of 24 h, although the concentration and
distribution of the RE elements in the precipitates
continues to evolve temporally. Similar to microhardness
at ambient temperature, the creep resistance at
300 C is significantly improved by RE microalloying
of the binary Al-0.06 at.% Sc alloy
