Effect of external stress on the microstructure and mechanical properties of creep-aged Al-Cu-Li-Ag alloy.

The effects of external stress on the precipitation of T precipitates and mechanical properties of creep-aged Al-Cu-Li-Ag alloys are investigated. Promotion mechanisms of external stress to the precipitation of T precipitates are discussed. It is found that external stress significantly promotes the precipitation and improves the distribution of the T precipitates in the creep-aged alloys. There is a threshold stress, close to the yield stress, that has only a limited promotion effect on the precipitation of T precipitates. The external stress below and above the threshold stress promotes the precipitation of T precipitates by two different mechanisms. One is the promotion mechanism of lattice distortion produced by the elastic stress. Another is the promotion mechanism of multiplication of dislocations produced by the plastic stress. Both elastic and plastic external stress can synergistically improve the strength and ductility. Especially, the plastic external stress resulted in the best improvement to ductility of creep-aged alloys. Hence, the creep ageing with plastic external stress is an alternative method to synergistically improve the strength and ductility of Al-Cu-Li-Ag alloys. However, it is necessary to avoid using excessive plastic stress for the creep ageing because it may cause creep damage and degrade its mechanical properties.