Microstructure evolution of Al8.79Zn2.16Mg2.11Cu0.12Zr alloy during homogenization

The microstructure evolution in Al8.79Zn2.16Mg2.11Cu0.12Zr alloy ingot during the one-step low-temperature homogenization and three-steps high-temperature homogenization is investigated by the scanning electron microscopy （SEM）， differential scanning calorimetry （DSC）， X-ray diffraction （XRD）， and the transmission electron microscopy （TEM）. The results indicate that the microstructure of the cast Al8.79Zn2.16Mg2.11Cu0.12Zr alloy is similar to that of traditional commercial Al-Zn-Mg-Cu alloys， consisting of η′（Mg（Zn， Cu， Al）2）， η（MgZn2）， and Fe-rich （Al7Cu2Fe） phases. The θ（Al2Cu）， T（Al2Mg3Zn3），and S（Al2CuMg） phases are not present. The redissolution effect of the second phases is not enough after one-step homogenization heat treatment at low temperature（380 ℃/2，8，16，24 h） and the change of redissolution effect is not obvious even if the homogenization heat treatment time is prolonged. A large number of the eutectic organization of the second phases still remains in the original mesh structure. However， the redissolution effect is improved by increasing the homogenization heat treatment temperature to 475 ℃（430 ℃/12 h+470 ℃/4 h+475 ℃/15 h）， except for a small amount of Fe-rich （Al7Cu2Fe） phases that are difficult to redissolution， the other phases are almost completely redissolution， no η′ phase transformation of S phase is detected during the one-step low-temperature homogenization and three-steps high-temperature homogenization heat treatment processes. The appearance of quench cracks occurs during the three-steps homogenization heat treatment at 475 ℃， which severely deteriorated the mechanical properties of the region near the crack in the alloy.