Improving the strength contribution of T1 precipitates in Al-4Cu-1.2Li-0.1Sc alloys by dissolving Cu-enriched second phases

To elucidate the effect of solution temperature on microstructures and mechanical properties of the Al-Cu-Li-Sc alloy, the evolution of the second phase and precipitates were analyzed to reveal the strength contribution by dissolving Cu-enriched second phases. It is demonstrated that the diffusion rate of the Cu atom in Cu-enriched second phases, mainly Sc-containing θ (Al2Cu) phases, increased with solution treatment temperature, which induced more T1 (Al2CuLi) precipitates formed during the aging process. More 10 at. % Cu atoms in second phases could dissolve into the Al matrix with the solution temperature increased from 490 °C to 520 °C, and these Cu-enriched second phases transformed into near equilibrium Sc-containing θ phases. Increasing the Cu atomic concentration in Al solid solution reduced GPB zones and inhibited δ′ (Al3Li) phases, thus improving the precipitation kinetics of the T1 phase. Less δ′ phase was observed in the sample at peak aging with a solution treatment at 520 °C, and the T1 phase was dominant. A clear improvement in the peak-aging hardness by 15.5% and yield strength by 19.1% of the sample after solution-treated at 520 °C compared with 490 °C.