Multiscale precipitates in a cross-alloyed Al-Cu-Zn-Mg alloy to enhance intergranular corrosion resistance via microalloying of Sc and Hf

The effects of microalloying of Sc and Hf on the microstructure and intergranular corrosion (IGC) behavior of a cross-alloyed Al-Cu-Zn-Mg alloy were investigated. Results indicate that the addition of Sc and Hf significantly refines grain size in both as-cast and hot-rolled alloys, altering grain structure and multiscale precipitates characteristics. It was revealed that the corrosion behavior of the alloy transformed from severe intragranular corrosion into IGC after the addition of Sc. In contrast, the Hf-microalloyed alloy exhibited predominantly pitting with excellent IGC resistance. This was attributed to the segregation of Hf atoms at grain boundaries (GBs) suppressed the continuous precipitation of η and S phases, narrowing the precipitation-free zone (PFZ). The discontinuous distribution of grain boundary precipitates (GBPs) and the narrowed PFZ inhibited the anodic dissolution, thereby reducing IGC sensitivity. Interestingly, the Hf-containing alloy exhibit smaller grain sizes, a higher proportion of low-angle grain boundaries (LAGBs) and coincident site lattices (CSL), which synergistically contribute to the enhanced IGC resistance.