Microstructure evolution of Al-Zn-Mg-Cu-Zr alloy during hot deformation

Compression tests of the Al-Zn-Mg-Cu-Zr(7055) alloy were performed at various strains and temperatures from 300 to 450 C under a constant strain rate between 10(-2) s(-1) and 1 s(-1). Microstructures during hot deformation were studied by transmission electron microscopy (TEM). Dislocation density, dislocation cells and subgrains of the deformed samples were investigated in detail and compared to make a better understanding of the microstructure evolution. The results showed that stress-strain curves under the experimental conditions belonged to the type of dynamic recovery. When the alloy deformed at various strains and 300 degrees C, the microstructure underwent a process of disordered dislocations to cell structure, subgrain structure and subgrain coarsening. With the temperature increasing, subgrains grew and dislocation density in the interior decreased at a strain rate of 1 s(-1). At the temperature of 350 degrees C, the average diameter of subgrains decreased, sub-boundaries broadened and dislocation density in the interior decreased when the strain rate was increased. The deformed samples of 7055 alloy had smaller subgrains than that of 7005 alloy at the same compression condition because of high alloy content.