Stability and stoichiometry of L12 Al3(Sc,Zr) dispersoids in Al-(Si)-Sc-Zr alloys

Abstract

This work studies the impact of adding 0.5 wt.% Si on the formation of Al3(Sc,Zr) L12 precipitates in a lean Al-Sc-Zr alloy. Precipitation kinetics are significantly accelerated when ageing at 300 and 375 °C in the presence of Si. At 375 °C, peak hardness occurs in a few minutes and is maintained up to 200 h at 375 °C . High resolution TEM reveals the presence of very fine core-shell L12 dispersoids ~5 nm in size that remain stable for up to 200 h at 375 °C. Using high-resolution energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT), the presence of Si and Fe is detected in the core of the dispersoids, where their content decreases as a function of ageing time. Similar Si and Fe compositions were observed inside the dispersoids for both alloys regardless of bulk Si content. Both Si and Fe are found to have preference for the Al site in the L12 structure, which is supported by density functional theory (DFT) calculations. DFT also explains the decrease of Si and Fe over time, showing that there is no thermodynamic driving force for either Si or Fe to be present in the L12. Instead Si and Fe are found to play a crucial role at the early stages of clustering, but are expelled from the particles when they stabilise into L12 structures.