Coupled segregation mechanisms of Sc, Zr and Mn at θ ′ interfaces enhances the strength and thermal stability of Al-Cu alloys

The refinement and thermal stability of intermediate theta-prime (${\theta }^{\prime }$) precipitates are critical in the development of new high strength 2xxx series aluminium-copper (Al-Cu) alloys for high temperature applications. In this work, we use trace additions of Sc, Zr and Mn in an Al-6.5 wt.% Cu alloy to refine and stabilise the ${\theta }^{\prime }$ precipitates. The formation of Al₃(Sc, Zr) core/shell dispersoids significantly refine the ${\theta }^{\prime }$ precipitates by acting as preferential nucleation sites during artificial ageing. Adding Mn results in a significant increase of hardness during ageing at 190 °C. Hardness is maintained during thermal exposure at 280 °C for up to 24 h. Transmission electron microscopy (TEM) reveals that the addition of Mn leads to a finer and denser distribution of ${\theta }^{\prime }$ precipitates, and greatly slows the growth and coarsening of the ${\theta }^{\prime }$ precipitates at elevated temperatures. Differential scanning calorimetry (DSC) shows that this can be attributed to an enhanced nucleation and improved coarsening resistance of the ${\theta }^{\prime }$ precipitates in the presence of Mn. Atom probe tomography (APT) reveals that the enhanced age-hardening kinetics and thermal stability arise from the independent segregation mechanisms of Mn, Sc and Zr at the semi-coherent and coherent interfaces of the ${\theta }^{\prime }$ precipitates. The segregation is quantified by calculating the Gibbsian interfacial excess and corresponding reduction in interfacial energy. These calculations reveal that while Sc and Zr play a significant role in the refinement of the ${\theta }^{\prime }$ precipitates, Mn not only refines the ${\theta }^{\prime }$ precipitates, but also greatly enhances their coarsening resistance and corresponding alloy's thermal stability.