Influence of Mg content on ageing precipitation behavior of Al-Cu-Li-x alloys

The addition of minor Mg (similar to 0.4 wt%) to Al-Cu-Li alloy could not only modify the precipitation path but also promote the precipitation of T-1 (Al2CuLi) phase, however, the influence of higher Mg addition on the microstructures during ageing is still not clear. In this work, the influence of high Mg addition ( > 0.4 wt%) on the precipitation sequence, precipitation kinetics and the corresponding strengthening of an Al-Cu-Li alloy has been investigated by atom probe tomography (APT), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and tensile testing. For an Al-Cu-Li-0.4 wt% Mg alloy in the peak-aged condition, the microstructure contains a high density of fine T-1 precipitates along with some non-negligible theta' phase (Al2Cu). When adding larger amount of Mg (0.8 wt%) to the alloy, however, the precipitation of S' phase (Al2CuMg) is strongly accelerated at the expense of T-1 and theta' phases. Much more Mg clusters are observed by APT at the early ageing stages of the high Mg-containing alloy, leading to more rapid hardening response. The precipitation kinetics for T-1 phase become somewhat sluggish due to decreased nucleation sites and shortage of available solutes caused by formation of more S' precipitates. The precipitation of coarse T-1 and S' phases is significantly detrimental to the ductility and toughness in the peak-aged condition for the high Mg-containing alloy.