Influence of Cu and Ti microalloying on the multiscale microstructure evolution and mechanical properties of 7xxx alloys

Introducing trace rare earth elements (REEs) into L1₂ dispersoids (Al₃(Sc,Zr)) can markedly enhance the mechanical properties of aluminum alloys, However, excessive amounts may cause adverse impacts. This study explores Ti and Cu as transition metal candidates for Al-Zn-Mg-X alloys, aiming to enhance mechanical properties, elucidate microstructure evolution, and identify optimization mechanisms. The addition of Ti to the Al-6.8Zn-2.2Mg-0.2Sc-0.1Zr (AS) alloy results in a notable refinement of the grain size, reducing it from 170 μm to 47 μm. This refinement of Ti can be attributed to its role as a nucleating agent during solidification, its promotion of dynamic recrystallization during hot-rolling, and its inhibition of static recrystallization during solid solution treatment stage. The formation of a new Ti-containing layer, which substitutes Al sites adjacent to Al₃Zr dispersoids, leads to an increase in the phase size from 16 nm to 25 nm. In addition, Cu significantly decreases the aging activation energy of the GP zone and η’ precipitate, thereby facilitating their nucleation and growth, which enhances the mechanical properties of the alloy. Ti markedly improves the hardness and strength of the alloy through grain refinement strengthening, Orowan strengthening and solid solution strengthening, while Cu predominantly enhances solid solution strengthening. Our findings suggest that Ti and Cu microalloying profoundly influences the properties and the microstructures of Al-Zn-Mg-X alloys across various scales offering a promising approach for the advancement of high-performance aluminum alloys.