Effect of Mg addition on the creep and yield behavior of an Al–Sc alloy

The relationships between microstructure and strength were studied at room temperature and 300 °C in an Al–2 wt% Mg–0.2 wt% Sc alloy, containing Mg in solid-solution and Al3Sc (L12 structure) as nanosize precipitates. At room temperature, the yield strength is controlled by the superposition of solid-solution and precipitation strengthening. At 300 °C and at large applied stresses, the creep strength, which is characterized by a stress exponent of ~5, is significantly improved compared to binary Al–Sc alloys, and is independent of the size of the Al 3Sc precipitates. At small applied stress, a threshold stress exists, increasing from 9% to 70% of the Orowan stress with increasing Al 3Sc precipitate radius from 2 to 25 nm. An existing model based on a climb-controlled bypass mechanism is in semiquantitative agreement with the precipitate radius dependence of the threshold stress. The model is, however, only valid for coherent precipitates, and the Al 3Sc precipitates lose coherency for radii larger than 11 nm. For semi-coherent precipitates with radii greater than 15 nm, the threshold stress remains high, most likely because of the presence of interfacial misfit dislocations.