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 (Al2CuLi), θ′ (Al2Cu) and Li-rich phase δ′ (Al3Li), 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.