Effect of dislocations, Al-3(Sc,Zr) distribution and ageing temperature on theta ' precipitation in Al-Cu-(Sc)-(Zr) alloys

The 2xxx series aluminium alloys, which contain Cu as the main addition element, are widely used in the aerospace industry for their high specific strength. The main strengthening contribution in these alloys comes from the formation of theta' precipitates. Recent work, has revealed that minor additions of Sc and Zr result in significant strength increase through the formation of fine Al-3(Sc,Zr) dispersoids that help nucleate and refine the theta' precipitates. The aim of the present work is to understand the impact of dispersoids distribution, ageing temperature and stretching on theta' precipitation. In order to separate these different contributions, specific thermo-mechanical routes were conducted on an Al-3.5 wt.%Cu and an Al-3.5 wt.%Cu-0.1 wt.Sc-0.14 wt.Zr alloy. Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) characterisations were conducted to study the nano-precipitation. While stretching has a significant impact on the theta' distribution in the Al-3.5 wt.%Cu alloy, it has a reduced impact in the Al-3.5 wt.%Cu-0.1 wt.%Sc-0.14 wt.%Zr alloy. In the latter alloy, the theta' precipitates are already refined, even without stretching, due to preferential heterogeneous nucleation on the finely dispersed Al-3(Sc,Zr) phase. This nucleation mechanism was found to dominate when ageing at 160 degrees C. When increasing the ageing temperature to 190 degrees C, theta' increasingly nucleates homogeneously and less on the dispersoids. Ageing at a higher temperature allows for a redistribution of Sc at the theta' interface hence increasing its thermal stability. This competition in nucleation mechanisms is revealed for the first time and provides different strategies to design high strength 2xxx-series alloys with purpose-specific properties.