Microstructure and creep of the Al-2Mg-0.2Sc alloy

Microstructure and creep properties at 300°C were studied in the Al-2wt.%Mg-0.2wt.% OscoSc alloy containing Mg in solid solution and Al₃Sc as nano-size precipitates. At large applied stresses, the creep strength is characterized by a stress exponent is ∼5, is significantly improved as compared to binary Al-Sc alloys, and is independent of the size of the Al₃Sc precipitates. At small applied stress, a threshold stress was measured, increasing from 9 to 63% of the Orowan stress with increasing Al₃Sc precipitates radius form 2 to 20 nm. An existing model for climb-controlled bypass mechanisms is in qualitative 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 ∼12 nm. For semi-coherent precipitates with radii above 12 nm, the threshold stress remains high, probably because of interface dislocations.