Pre-strain-dependent natural ageing and its effect on subsequent artificial ageing of an Al-Cu-Li alloy

The effects of pre-strain and natural ageing (NA) on the precipitation hardening behavior in an Al-Cu-Li alloy have been systematically investigated. Applying pre-strain significantly restricts the solute clustering-induced formation of fine GPI zones (single Cu-layer) and thus hardness increase during NA. NA before artificial ageing (AA) has a marginally positive effect on the age hardening in samples without pre-strain, giving rise to a strength improvement of 15-20 MPa without loss of ductility. NA slightly enhances the precipitation of T-1 phase during AA while shows no obvious effect on theta', sigma, delta' and X phases. The X phase hasn't been reported and is isostructural to the theta' phase, but the habit plane is (011)(Al) instead of (001)(Al). With the increase of pre-strain level, the T-1 plates become dominant and finer. However, NA after pre-strain has no appreciable effect on the age hardening and microstructure of the alloy. The analysis by complementary methods suggests that the low stability of NA clusters and their inhibition by pre-strain are responsible for the observed NA effect on AA in Al-Cu-Li alloy. (C) 2019 Elsevier B.V. All rights reserved.