Hot compression test of the Al-Zn-Mg-Cu alloy with Zr addition was carried out on the Gleeble-1500 thermal simulation machine at 300

4

50℃and the strain rates of 0.000 5

1

.000s-1.The microstructure of the alloy under different deformation conditions was observed using optical microscopy（OM）

electron back scattered diffraction（EBSD）and transmission electron microscopy（TEM）.The results show that the dislocation density is decreased and the subgrain size is increased with increasing in deformation temperatures or decreasing in strain rates

leading to the decrease of peak stress.Under the given thermal deformation conditions

the microstructure is mainly controlled by low angle boundaries

and the main softening mechanism is characterized by dynamic recovery.Amounts of Al3 Zr particles with the size of 10

2

5nm are generated after adding minor Zr in Al-Zn-Mg-Cu alloy

which can pin dislocations and subgrain boundaries to inhibit recrystallization during deformation

which are responsible for the alloy keeping dynamic recovery microstructure under the given hot deformation conditions.