A novel GP-Li precursor and the correlated precipitation behaviors in Al-Cu-Li alloys with different Cu/Li ratio

Aluminum-lithium (Al–Li) alloys have become an essential material for aircraft skin in the aerospace industry owing to their high strength and low density. An optimized coherent Cu/Li ratio and well controlled precipitation sequence are critical to upgrade the desired properties. In this study, we systematically evaluate the effect of the Cu/Li ratio (ranging from 0.5 to 1.4) on the aging hardening and precipitation behavior of Al–Cu–Li alloys with specific rare earth elements (Sc, Zr, and Ce). Using state-of-the-art aberration-corrected transmission electron microscopy equipped with super-energy-dispersive X-ray spectroscopy (EDS) and atomic scale spatially resolved electron energy loss spectroscopy (EELS) with K3 camera, classical strengthening phases, such as Cu containing precipitates T1 (Al ₂CuLi), θ ′ (Al ₂Cu) and Li-rich phase δ ′ (Al ₃ Li), have been identified during the artificial aging and natural aging treatment. A new Li-rich phase (namely, the GP-Li zone) with a hexagonal shape has been identified in alloys with Cu/Li ratios of 0.5 and 1. High density GP-Li zones are formed after five months natural aging, which are found fully coherent with the Al matrix. The discovery of GP-Li zones shows a new precipitation path for Li atoms in the supersaturated solid solution. Besides, the GP-Li zone stimulates the nucleation of T1 and θ ′ phase at the interfaces indicating that there is an alternative approach for the nucleation mechanism of T1 and θ ′ . The crossover between the suppression of δ ′ phase formation and the acceleration of T1 and θ ′ precipitation occurs when the Cu/Li ratio is increased during the artificial aging process, resulting in a decrease in the aging hardening ability at first, followed by a large recovery in aging hardening. The findings of this study provide a strategy to design high strength Al–Li alloys through the manipulation of Cu/Li ratios and the newly identified GP-Li phase.