Negative effect of nano-scale Al3(Sc, Zr) on strength of Sc-Zr micro-alloyed Al-Cu-Mg-Ag alloys

Abstract

The Al-Cu-Mg-Ag alloy is widely used in the aerospace industry due to its high strength and heat resistance and microalloying is the main strategy to improve its mechanical properties. Adding excessive Sc to Al-Cu-Mg-Ag alloys was reported to decrease strength, and this was attributed to the formation of micro-scale AlCuSc constituents. It is widely expected to avoid the generation of AlCuSc constituents, generate Sc-containing nano precipitates and improve mechanical properties, by adding a suitable amount of Sc. Unfortunately, according to the multi-scale microstructures and mechanical properties of a Sc-Zr micro-alloyed Al-Cu-Mg-Ag alloy studied in this work, it is shown that even if the formation of large AlCuSc constituents was prohibited and Al₃(Sc, Zr) nano precipitates were promoted through a multi-step homogenization, the alloy strength was still decreased due to Sc-Zr microalloying. This is mainly attributed to the heterogeneous nucleation of θ' precipitates alongside Al₃(Sc, Zr) particles, which consumes the available Cu solutes for Ω precipitation in the alloy, thereby reducing the volume fraction of the main strengthening phase Ω and the overall strengthening effect in both aged and thermally exposed states. Our findings provide insights into the effective control of the microstructure of Al-Cu-Mg-Ag alloys with Sc or Sc-Zr addition.