In order to reduce the cost of Al–Sc alloys and maintain their mechanical properties, the microstructure and mechanical properties of Al–0.24wt% Sc–0.07wt% Yb in comparison with Al–0.28wt% Sc alloys were studied. The aging behaviour, precipitate morphologies, precipitate coarsening and precipitation hardening of both alloys were investigated. The average diameter and the size distribution of nanoscale Al 3 Sc and Al 3 (Sc,Yb) precipitates at various aging conditions were measured. Transmission electron microscopy (TEM) and high-resolution TEM were used to deeply understand the precipitate evolution. A maximum hardness around 73 (HV 30 ) was obtained with a precipitate diameter from 4.3 to 5.6nm for both alloys.