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 °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 θ′ 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 °C or 250 °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 θ′ 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 θ′ particles does not increase despite the higher concentration of this element in the chemical composition. Therefore, the number density of θ′ particles in samples heat treated at 200–250 °C also does not increase with increasing Sc content from 0.1 wt% to 0.2 wt%. Annealing at 300 °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).