Microstructural stability during low temperature exposure of alloys within the Al–Li–Cu–Mg system

Al–1.7%Li, Al–1.7%Li–Cu, and Al–1.7%Li–Cu–Mg alloys have been given a damage-tolerant age of 24 h at 150 °C and then exposed for 1000 h at 70 °C. As a result of exposure, all alloys underwent an increase in strength and a decrease in toughness largely as a result of continued precipitation of δ′. The magnitude of this change in mechanical properties increased in the sequence Al–Li < Al–Li–Cu < Al–Li–Cu–Mg. The reasons for the different behaviour during exposure of the various alloys have been sought by TEM, DSC and electrical resistivity measurements. From these results it is proposed that the mechanisms of δ′ formation during exposure are widely different in the three alloy systems.