Coupled segregation mechanisms of Sc, Zr and Mn at [formula omitted] interfaces enhances the strength and thermal stability of Al-Cu alloys

Abstract

The refinement and thermal stability of intermediate theta-prime (θ′) 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 θ′ precipitates. The formation of Al3(Sc, Zr) core/shell dispersoids significantly refine the θ′ 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 θ′ precipitates, and greatly slows the growth and coarsening of the θ′ precipitates at elevated temperatures. Differential scanning calorimetry (DSC) shows that this can be attributed to an enhanced nucleation and improved coarsening resistance of the θ′ 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 θ′ 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 θ′ precipitates, Mn not only refines the θ′ precipitates, but also greatly enhances their coarsening resistance and corresponding alloy's thermal stability.