Effect of dislocations, Al3(Sc,Zr) distribution and ageing temperature on θ′ precipitation in Al-Cu-(Sc)-(Zr) alloys

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Department of Materials Science and Engineering


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 θ′ precipitates. Recent work, has revealed that minor additions of Sc and Zr result in significant strength increase through the formation of fine Al3(Sc,Zr) dispersoids that help nucleate and refine the θ′ precipitates. The aim of the present work is to understand the impact of dispersoids distribution, ageing temperature and stretching on θ′ 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 θ′ 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 θ′ precipitates are already refined, even without stretching, due to preferential heterogeneous nucleation on the finely dispersed Al3(Sc,Zr) phase. This nucleation mechanism was found to dominate when ageing at 160 °C. When increasing the ageing temperature to 190 °C, θ′ increasingly nucleates homogeneously and less on the dispersoids. Ageing at a higher temperature allows for a redistribution of Sc at the θ′ 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.

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