Strengthening Al-Cu-Li alloys using nano-sandwiched δ′/θ'/δ′ precipitates induced by Sc-clusters' diffusion drags and precipitation nuclei

The new generation of cast Al-Cu-Li alloys offers a benefit in both high strength and stiffness, while the ductility of these alloys is generally poor, restricting their application in aerospace industries. In this work, the cast Al-Cu-Li alloy micro-alloyed by Ti, Zr, and Sc was prepared by argon-shielded metal mold casting. The as-cast alloy possesses the complete equiaxed grains which exhibit good resistance to coarsening during solution annealing treatment. The microstructure including secondary phase, grain size, and precipitates were characterized by SEM, EBSD, and TEM, respectively. We find that the using of rare earth element Sc combined with its dragged effect on precipitation kinetics at 120 °C can optimize the strength-ductility balance of the cast Al-Cu-Li alloy. The alloy yields evident heterogeneous nucleation of “ δ′+θ' ” precipitates with δ′ phase wetting outside θ′ layer around Sc-clusters when aged at 120 °C for 100 h. Those fine precipitates result in an improved work-hardening capability compared to the coarse " T₁+δ′+S' " precipitates forming at 160 °C aging process. Applying those principles to the new cast Al-Cu-Li alloy, a combination of mechanical properties with tensile strength at 482 MPa and elongation at 6.0 % has been achieved.