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 degreesC in an Al-2 wt% Mg-0.2 wt% Sc alloy, containing Mg in solid-solution and Al3Sc (L1(2) structure) as nanosize precipitates. At room temperature, the yield strength is controlled by the superposition of solid-solution and precipitation strengthening. At 300 degreesC and at large applied stresses, the creep strength, which is characterized by a stress exponent of similar to5, 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. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.