Evolutions of diffusion activation energy and Zener–Hollomon parameter of ultra-high strength Al-Zn-Mg-Cu-Zr alloy during hot compression.

Abstract The changes of stress level for the ultra-high strength Al-Zn-Mg-Cu-Zr alloy were described by constitutive equation with considering lattice diffusion of aluminum, zinc, magnesium and copper. Zener–Hollomon (Z) parameter expression based on the constitutive equation with considering lattice diffusion was used to reflect the changes of microstructure. The critical stress σ c for the initiation of dynamic recrystallization (DRX) was introduced to calculate the Z parameter. Steady-state dislocation density ρ sat and critical dislocation density ρ c for the initiation of DRX decreased with the increase of deformation temperature. The dependence of diffusion activation energy Q on temperature and strain rate was given and the effects of deformation conditions on Q were discussed in detail. Microstructural evolution revealed that low angle boundaries (2–5°) created in the process of dynamic recovery (DRV) could convert into subgrain boundary, thus the original grains were divided into subgrains, and then subgrains transformed into DRX grains by the way of progressive rotation. When the Z value was high (ln Z > 30.9), DRV was the main softening mechanism. With the decrease of Z value, both of DRV and DRX played an important roles in softening effect, while with the further decrease of Z value (ln Z < 28.6), DRX became the main softening mechanism. Continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) operated together under the condition of lower Z value, but CDRX was confirmed as the dominant DRX mechanism. Highlights • The dislocation density under different deformation conditions was calculated. • The Z parameter based on the model with considering lattice diffusion was constructed. • The effects of deformation conditions on diffusion activation energy were discussed. • The effects of Z parameter on deformation mechanism were discussed in detail. [ABSTRACT FROM AUTHOR]