Highly interdependent dual precipitation and its effect on mechanical properties of Al–Cu-Sc alloys

Dual precipitation in a microalloyed Al-2.4 wt% Cu-0.1 wt% Sc alloy was investigated in comparison with single precipitation in binary Al-2.4 wt% Cu and Al-0.1 wt% Sc alloys, respectively. Although Al₃Sc nanoparticles are more readily produced at higher aging temperatures in the binary Al–Sc alloys, the Al₃Sc precipitation in the Cu-added ternary alloy was completely suppressed at 723 K while unexpectedly promoted at 573 K-aging. Meanwhile, a significant Sc-dependence of θ^′ precipitation was also evidenced in the 573 K-aged Al–Cu-Sc alloy that the thickness of θ^′ precipitates kept almost unchanged even to prolonged time, resulting in a high aspect ratio of >200 for the θ^′ precipitates. The complex precipitation behaviors are rationalized in terms of strong interactions between the Sc and Cu solutes, which could be tailored to optimize the Al₃Sc + θ^′ dual precipitation and hence to improve mechanical properties. Moreover, the Sc atoms fully stabilized in Al solid solution by Cu solute at 723 K was discussed, and the high strength in Al–Cu-Sc alloy with dual precipitation was evaluated by quantifying the strengthen contribution from different strengthening mechanism.