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 degrees 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 T-1, delta' and theta', sigma phase has been observed too. This is the first experimental evidence for the existence of sigma 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 T-1, theta',theta '', delta' and sigma. Additional strengthening was obtained for further ageing up to 50 h due to evolution of T-1 and theta' plates. Finally, coarsening led to strength decrease at prolonged ageing process. (C) 2008 Elsevier B.V. All rights reserved.