Effects of ultrasonic nanocrystal surface modification treatment and subsequent annealing on the microstructure and mechanical properties of rolled AZ31 alloy

This study investigates the variations in the microstructure and mechanical properties of a rolled Mg-3.0Al-0.8Zn-0.3Mn (AZ31, wt.%) alloy caused by ultrasonic nanocrystal surface modification (UNSM) treatment and subsequent annealing. The UNSM-treated material has a gradient microstructure characterized by a nanoscale grain region overlying a twinned region. The nanoscale grain region close to the surface has extremely high strain energy, while the twinned region contains abundant {10–12} twins. Severe plastic deformation near the surface due to UNSM treatment leads to the formation of ultrafine grains and an increase in dislocation density, thereby remarkably improving the hardness and strength of the material. However, the excessive dislocation density reduces the tensile ductility. During subsequent annealing, static recrystallization occurs rapidly in the nanoscale grain region, and partial grain-boundary migration and dislocation recovery occur in the twinned region. Consequently, a gradient microstructure is formed along the thickness direction in the material subjected to UNSM treatment and subsequent annealing, thereby resulting in a tensile yield strength of 172 MPa, an ultimate tensile strength of 259 MPa, and an elongation of 15.4%. Notably, these values are higher than those of the as-rolled material, which are 132 MPa, 246 MPa, and 13.4%, respectively. This demonstrates that the sequential UNSM and annealing process results in a simultaneous improvement in the strength and ductility of the material. The strengthening mechanisms and fracture behavior are comprehensively discussed in this study. • UNSM and subsequent annealing are subjected to a rolled AZ31 sheet. • UNSM forms a gradient microstructure comprising nanoscale grain region and twinned region. • Subsequent annealing induces softening mechanisms such as RX, SIBM, and recovery. • The sequential UNSM and annealing process improves both strength and ductility. • This is mainly due to fine recrystallized grains with low internal strain in near-surface region.