Constitutive equation and hot deformation behavior of homogenized Al–7.68Zn–2.12Mg–1.98Cu–0.12Zr alloy during compression at elevated temperature

Hot compression tests over the temperature range from 300 °C to 450 °C and strain rates range from 0.01 s−1 to 10 s−1 of homogenized Al–7.68Zn–2.12Mg–1.98Cu–0.12Zr alloy were carried out on a Gleeble-3500 thermal simulation machine to characterize its hot deformation behavior. The results showed that the flow stress can be predicted by a two-stage constitutive model based on the dislocation density theory and kinetics of dynamic recrystallization (DRX) with the hot deformation activation energy of 125.4 kJ/mol. The associated microstructure was studied using transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD) technique. With the decrease of ln Z (Z represents the Zener–Hollomon parameter) value from 22.4 to 18.6, the microstructure analysis revealed a decrease of low angle boundaries (misorientation below 15°) from 96.1% to 78.2%, and an increase of the higher angle boundaries with the misorientation angles between 15° and 60° from 3.9% to 21.8%. Combining the results from processing map, it can be concluded that such an evolution is mainly due to the decrease of subgrains with the increase of Z values, and partly due to the partial DRX at low Z values. The softening mechanisms of homogenized Al–7.68Zn–2.12Mg–1.98Cu–0.12Zr alloy is dynamic recovery (DRV), together with a partial DRX at high temperature and low strain rate conditions (low Z value).