Abstract
• The effects of Er micro-alloying on the mechanical properties and microstructural evolution of Al-Sc-Zr at different isothermal ageing temperatures were investigated. • The Er additions significantly improve the mechanical properties of Al-Sc-Zr alloy during elevated temperature ageing, especially at 400 °C. • The Er additions can increase the nucleation rate of precipitates and consequently enhance the age-hardening response of the Al-Sc-Zr alloy. • DFT calculations were conducted to rationalize the observed precipitate structure and the underlying formation mechanism in the Al-Sc-Zr(-Er) alloys. Correlation between precipitates evolution and mechanical properties of Al-Sc-Zr alloy with Er additions during isothermal ageing were investigated by microhardness measurements, transmission electron microscopy, atom probe tomography and density functional theory-based simulations. The results demonstrate that the Er additions significantly improve the hardness during elevated temperature ageing, especially at 400 °C. This is mainly because Er additions increase the nucleation rate of the Al 3 (Er,Sc,Zr) precipitates, resulting in a higher density of fine and uniform dispersion of L1 2 structured nanoparticles. First-principles calculations demonstrate that the second nearest neighboring solute-solute interactions for the species Sc, Zr and Er are energetically favored – a key feature to rationalize the observed precipitate structure and the underlying formation mechanism. The sequential formation of the core/shell precipitates in the Er-free alloy and core/double-shell precipitates in the Er-containing alloy arises due to the different solute-solute and solute-vacancy interaction energies, and the relative diffusivities of the Er, Sc and Zr species in Al. These results shed light on the beneficial effects of Er additions on the age-hardening behavior of Al-Sc-Zr alloy and provide guidance for designing the ageing treatments for the Al-Sc-Zr(-Er) alloys.
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