Effect of In Addition on the Microstructure and Mechanical Properties of an Al–Cu–Mg–Ag Alloy

In this work, the effects of small In addition (0.11 wt pct) on the microstructure and mechanical performance of an Al–Cu–Mg–Ag alloy in natural aging (NA) state and peak aging (PA) state are systematically investigated by using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), texture, differential scanning calorimetry (DSC), and density functional theory (DFT). The results reveal that the number density of Mg–Ag co-clusters decreases and that of Cu–Mg co-clusters and Ag–In co-clusters increases in Al–Cu–Mg–Ag alloys with small In addition, while the differences in grain size and recrystallization degree are relatively small. The In atoms have a strong vacancy binding energy, which is capable of trapping the vacancies in the as-quenched alloys, leading to a decrease in the strength of 0.11In–NA alloy. 0.11In–PA alloy exhibits a smaller Taylor factor, lower aspect ratio, and volume fraction of Ω phase, which are the major reasons for the decrease in strength of the alloy. Experiments show that In atoms are enriched around the Ω phase, and it is calculated and verified that the In atoms can form a more stable structure by replacing some atoms in the Ω phase by DFT.