The Role Of Various Zr Additions In Static Softening Behavior Of Al-Zn-Mg-Cu Alloys During Double-stage Hot Deformation

Al-Zn-Mg-Cu alloys are very attractive materials for their applications in the aerospace industries due to their excellent combination of properties such as high strength-to-density ratio,high fracture toughness and resistance to stress corrosion cracking.In Al-Zn-Mg-Cu alloys,the addition of Zr can play vital roles in retarding recrystallization by the formation of fine Al₃Zr dispersoids during hot deformation and improving the strength,stress corrosion resistance and fracture toughness.Hot working processes of Al-Zn-Mg-Cu alloys generally consist of several successive deformation passes,it is important to understand the effect of Zr additions on the static softening behavior and mechanisms of Al-Zn-Mg-Cu alloys during interval holding of multi-stage thermomechanical processing.In the present study,Double-stage interrupted uniaxial hot compression tests were conducted on Gleeble-3500 to investigate the static softening behavior in Al-Zn-Mg-Cu alloys with various Zr additions at temperature of 300? and 400? and strain rates of 0.01 s^-1 and 0.1 s^-1.The microstructural evolution was also characterized by utilizing electron backscatter diffraction and transmission electron microscopy.1.The static softening fraction of the studied alloys increased with increasing Zr additions.Al₃Zr particles pinned dislocation and inhibited the dynamic recovery and/or recrystallization remarkably during first pass hot deformation,which led to higher stored deformation energy in the deformed alloys with higher Zr additions.Consequently,more driving force for static recovery and recrystallization during interval holding,as well as enhanced static softening were presented with increasing Zr additions.2.Static softening fraction is also proportionate to increase deformation temperature and strain rate.Static softening curves exhibited shorter plateaus period during post-deformation holding at 300?,which could be attributed to static recovery,static precipitation and its coarsening based on EBSD and TEM observations.Clear precipitate coarsening was observed when holding for 1000 s.3.Longer plateaus period was observed at 400? and 0.1 s^-1 owing to that completed static recovery and recrystallization occurred within very short time.Accordingly,the variation of softening fraction was small with further increasing holding time.