Effect of ageing time on strength and microstructures of an Al–Cu–Li–Zn–Mg–Mn–Zr alloy

Yield strength of an Al–3.7Cu–1.5Li–0.50Zn–0.37Mg–0.30Mn–0.14Zr alloy aged at 165 °C has been examined as a function of time. Phases existing in this alloy were identified by selected area electron diffraction analysis. Microstructure evolution was examined by transmission electron microscopy (TEM). The peak-ageing time determined is 50 h with yield strength of 453 MPa. Beside commonly distributed precipitates T1, δ′ and θ′, σ phase has been observed too. This is the first experimental evidence for the existence of σ in Al–Cu–Li alloys. The strength of the alloy increases rapidly during the first 24 h of ageing due to the combinative hardening effect of T1, θ′, θ″, δ′ and σ. Additional strengthening was obtained for further ageing up to 50 h due to evolution of T1 and θ′ plates. Finally, coarsening led to strength decrease at prolonged ageing process.