Effect of the scandium and zirconium additions on the ageing behaviour of Al-Cu alloys

2xxx-series Al-Cu alloys are heavily used in the transport sector due to their high strength to weight ratio. The main strengthening contribution in these alloys comes from the formation of the θ’ precipitates. Recent work has shown that adding Sc and Zr to 2xxx-series can further increase the strength by refining the θ’ precipitates which nucleate on the Al3(Sc,Zr) core-shell dispersoids. However, the effect of Sc and Zr on the full precipitation sequence of the Cu containing phases is still not fully understood.

In the present work, we explore the effect of Sc and Zr and pre-stretching on the natural and artificial ageing behaviour of Al-Cu alloys. A combination of hardness and differential scanning calorimetry is used to quantify the kinetics of GP-zones formation in Al-Cu alloys with and without Sc and Zr. We find that the addition of Sc and Zr significantly slows down the natural ageing response in these alloys. Transmission electron microscopy and atom probe tomography are used to further quantify this effect. The GP-zones are found in high number density in the binary Al-Cu alloy but are absent from the alloy containing Sc and Zr. This is explained by the ability of scandium to bind vacancies which reduces the mobility of solute atoms at room temperature. However, the θ’ formation kinetics during artificial ageing is significantly accelerated. The segregation of scandium at the Al/θ' semi-coherent interfaces is quantified with atom probe tomography. This segregation lowers the interfacial energy of the θ' precipitates, thus lowering their nucleation energy barrier. Pre-stretching is commonly used to generate dislocations that act as nucleation site for θ’. However due to the anisotropy of dislocation distributions this can end up in anisotropic distribution of θ’ between the different habit planes. We here show that Al3(Sc,Zr) dispersoids can reduce this anisotropy by nucleating θ’ in all its habit planes. This new understanding of the precipitation sequence in these alloys gives clues for the design of optimal heat treatments for Sc containing 2xxx-series alloys.