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

Abstract The 2xxx series aluminium alloys, which contains Cu as the main addition element, are widely used in the aeronautic industry for their high specific strength. The main strengthening in these alloys comes from the formation of θ’ precipitates. In this work, we study the impact of alloying a model Al-Cu alloy with Sc and Zr. Significant strengthening contribution comes from the formation of Al3(Sc,Zr) dispersoids. In the present work, we aim at understanding the impact of dispersoids, aging temperature and stretching on the θ’ precipitation. In order to separate these different contributions, specific thermo-mechanical routes were conducted on an Al-3.5wt.%Cu and an Al-3.5wt.%Cu-0.1wt.Sc-0.14wt.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-3.5wt.%Cu alloy, it has a reduced impact in the Al-3.5wt.%Cu-0.1wt.%Sc-0.14wt.%Zr alloy. This is due to a strong ability of θ’ to nucleate heterogeneously on the finely dispersed Al3(Sc,Zr) phase. This nucleation mechanism was found to dominate when aging at 160°C. When increasing the aging temperature to 190°C, θ’ increasingly nucleates homogeneously and less on the dispersoids. The nucleation mechanisms of θ’ are thought to be temperature dependant. Aging at higher temperatures 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.