Transmission electron microscopy study of the evolution of precipitates in aged Al-Li-Cu alloys: The θ′ and T₁ phases

We have investigated the structures of the θ′ and T₁ precipitates in Al-1.6wt%Li-3.2wt%Cu and Al-2.4wt%Li-3.2wt%Cu alloys aged at 220 °C. The θ′ precipitates in the 1.6 wt%-Li alloy are those known for the Al-Cu binary system (a=0.40 and c=0.58 nm); whereas those in the 2.4 wt%-Li alloy exhibited two atypical structures. One, named a type I T_B′ plate in this study, is isostructural to the known θ phase with a large c value of about 0.64 nm, having a habit plane parallel to the matrix {1 0 0}_α; the other, type II T_B′, is characterized by a=0.41 and c=0.61 nm, having a habit plane inclined at about 20° with {1 0 0}_α, while maintaining a coherent interface. Also images of {1 1 1} precipitates in the 1.6 wt%-Li alloy revealed a continuous change from the T₁ phase (c=0.935 nm), to a structure with c=0.87-0.90 nm. The image and small lattice parameter suggest that this {1 1 1} precipitate is likely to be the Ω phase.