Regulating the microstructure and mechanical properties in cast Al–3Li–2Cu-0.15Zr alloy via Sn addition and heat treatment

Abstract

The cast Al–3Li–2Cu-0.15Zr alloy is a promising material due to its high strength-to-weight ratio and stiffness. In this work, the Sn addition and heat treatment were employed to regulate its microstructure and mechanical properties. The results indicate that Sn exists in the matrix as Sn solute atoms and Sn-rich particles including Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5. The number of these particles increases as the Sn content rises. The addition of Sn significantly enhances the grain refinement, reducing the average grain size to 50.2 μm when 0.3 wt% Sn is incorporated. During the aging process, the δ′ and δ'/β′ precipitates gradually grow and coarsen, while the T1 and θ′ precipitates nucleate and grow. Some δ′ precipitates attach to the board of θ′ to form sandwich-like phases, such as δ'/θ'/δ'. The addition of Sn promotes the nucleation of θ′ and T1 precipitates. This is achieved by leveraging the Sn-rich particles as nucleation sites and utilizing Sn solute atoms to decrease the interface energy between these precipitates and α-Al. Moreover, the addition of Sn with a high vacancy binding energy and the reduction of aging temperature can effectively decrease the diffusion rate of Li atoms and vacancies, thereby inhibiting the broadening of the δ′-PFZ. After a suitable heat treatment, the Sn-modified Al–Li alloy exhibits a microstructure characterized by fine and equiaxed grain, fine and uniform δ′ precipitates, and a narrow δ′-PFZ, contributing to excellent mechanical properties of the alloy. Significantly, the underlying mechanisms behind the aforementioned observations were discussed analytically.