Synergistic effects of Ag and Sc addition on superior thermal stability in Al-Mg-Si-Cu alloy

Al-Mg-Si-Cu-Ag-Sc alloy with superior thermal stability has been designed, and a new mechanism for enhancing thermal stability was suggested in this study. The complex addition of Ag and Sc and the increased Cu content significantly enhanced the thermal stability of the Al-Mg-Si-Cu alloy by preventing the growth of over-aged precipitates. Particularly, Ag addition only delayed hardness degradation at the over-aging stage, whereas Sc addition lowered both the precipitation kinetics during the early and over-aging stages. The different roles of Ag and Sc create a synergistic effect so that excellent thermal stability can be obtained by the complex addition of Ag and Sc. The role of Ag in enhancing thermal stability generally depends on the conventional mechanisms of disordering and solute segregation. However, since the amount of Sc detecting was too low to have a sufficient influence on the thermal stability of the precipitate, the presence of Sc solute in the matrix was predicted. Solute analysis of the matrix revealed that Sc enhanced the thermal stability by disturbing Cu diffusion to Cu-containing precipitates. The strong Sc-Cu interaction enabled the temporary catching of Cu atoms by forming an Sc-Cu pair. Therefore, Sc not only enhances thermal stability but also delays age-hardening kinetics because the formation of Cu-containing precipitates is disturbed by the delaying mechanism depending on Sc-Cu pair formation. Additionally, the observation that the complex addition of Ag and Sc accelerated the formation of Mg-rich clusters during early stage aging provides minor evidence for the enhanced thermal stability of the Ag-Sc added Al-Mg-Si-Cu alloy. This is because Mg-rich clusters may be related to precursors of Cu-containing precipitate due to their high Cu contents and high Mg composition of Q′ and L phases.