Quantitative analysis of the influences of pre-treatments on the microstructure evolution and mechanical properties during artificial ageing of an Al–Cu–Li–Mg–Ag alloy

The quantitative effects of pre-treatments, namely pre-stretching and natural ageing (NA), on the precipitation sequence, precipitation kinetics and mechanical properties of an Al–Cu–Li–Mg–Ag alloy during artificial ageing were systematically investigated, aiming to elucidate the mechanisms by which these effects proceed. At the early ageing stage, pre-stretching could inhibit the formation of GP zones by restricting the Mg–Cu co-clustering, and could result in a higher nucleation rate of the T1 precipitates due to an increase in the quantity of dislocations covered by solute segregations. The introduction of NA after pre-stretching could further increase the number density of T1 by providing more nucleation sites, which evolved from the much more Mg–Ag co-clusters forming during the NA process. With prolonging ageing time, the precipitation kinetics for T1 were strongly enhanced by pre-stretching due to a more efficient nucleation of T1 and a more rapid growth rate of this phase, which was attributed to the combination of the suppression effect on the GP zone precipitation and the pipe diffusion effect derived from the introduced dislocations. When applying NA after pre-stretching, a further acceleration of the precipitation kinetics was observed, which was related to the formation of much more T1 nuclei. At the peak-aged condition, both the two pre-treatments could not only refine the precipitates but also result in a higher T1 volume fraction. Mechanisms of the strength changes induced by the pre-treatments under various ageing conditions were discussed in detail in view of these experimental results.