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 Al₃Sc precipitates. At small applied stress, a threshold stress exists, increasing from 9% to 70% of the Orowan stress with increasing Al₃Sc precipitate radius from 2 to 25 nm. An existing model based on a climb-controlled bypass mechanism is in semi-quantitative agreement with the precipitate radius dependence of the threshold stress. The model is, however, only valid for coherent precipitates, and the Al₃Sc 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.