Improving microstructure and deformation surface quality of novel Al-Mg-Zn-Zr alloy by varying homogenization regimes

Al-Mg-Zn-based alloys are newly developed high-performance age-hardenable 5xxx series alloys. However, they suffer from poor surface quality due to the Lüders bands and the Portevin-Le-Chatelier (PLC) effect during plastic deformation, and the elongation (EL) is impaired by the large constituent particles. In this work, various homogenization regimes combined with minor addition of Zr were employed to manipulate the microstructural evolution of Al-7Mg-2Zn-0.15Zr alloy and thus the deformation characteristics. The results demonstrate that a two-step homogenization treatment of 400°C/12 h + 465°C/12 h can sufficiently dissolve the constituent phase and eliminate the micro-segregation, as well as achieve a fine and uniform distribution of Al3Zr dispersoids, which results in the enhanced yield strength (YS) of 201 MPa and ultimate strength (UTS) of 376 MPa after rolling and annealing. Most importantly, it has been revealed that the dispersed Al3Zr formed via two-step homogenization is favorable to impede the movement of dislocations, which extends the waiting time, increases the stress amplitude, and thus diminishes the number of PLC bands. Moreover, the dislocation network induced by dispersed Al3Zr can further inhibit the movement of active dislocations, which inhibits the formation of Lüders bands. This study provides a simple and effective homogenization regime to optimize the mechanical response and surface quality of alloys with high-Mg contents.