Effects of Sc Additions on Annealing Behavior of Heavily Cold-Rolled Al-Cu-Mn Alloys

Effects of minor additions of Sc on the microstructure evolution during long-term heat treatments at 200-300 degrees C have been studied for heavily rolled Al-Cu-Mn alloys, where Sc content varies in the range 0-0.2 wt%. It is found that cold rolling to 90% thickness reduction results in deformation structures with boundary spacing of 58-68 nm measured along the normal direction. This cold-rolled sample is characterized by high hardness (145-147 HV). Precipitation of theta ' particles along deformation-induced boundaries and coarsening of deformation structures via triple junction motion and boundary migration take place during annealing for 100 h at 200 degrees C or 250 degrees C. The extent of structural coarsening is greater in the Sc-free alloy than in the Sc-containing alloys, where the presence of Sc enhances precipitation and stability of theta ' particles. The coarsening of deformation structures leads to reduced hardness in each alloy, with greater reductions seen for the Sc-free alloy. Interestingly, an increase in Sc content from 0.1 wt% to 0.2 wt% does not lead to greater improvement in thermal stability of deformation structures. It is shown that the addition of 0.2 wt% Sc stimulates precipitation of Al3Sc particles during homogenization. As these particles consume Sc, the amount of Sc available for enhanced precipitation and stability of theta ' particles does not increase despite the higher concentration of this element in the chemical composition. Therefore, the number density of theta ' particles in samples heat treated at 200-250 degrees C also does not increase with increasing Sc content from 0.1 wt% to 0.2 wt%. Annealing at 300 degrees C for 100 h leads to fully recrystallized microstructures in each alloy. During recrystallization, finer grains develop in the Sc-containing alloys compared with those in the recrystallized Sc-free alloy. The recrystallized Sc-containing alloys are harder (51-55 HV) than the Sc-free alloy (47 HV).