Investigation of microstructural evolution and mechanical properties during two-step ageing treatment at 115 and 160 °C in an Al-Zn-Mg-Cu alloy pre-stretched thick plate

The microstructure evolution and age-strengthening characteristics of an Al-Zn-Mg-Cu alloy thick plate during a two-step ageing treatment at 115 °C and 160 °C were studied. The results show that the first step ageing treatment contributed to a major increase of tensile strength, the UTS value achieves 569 MPa after ageing for 7 h at 115 °C, both the GPI and GPII zones are dominant precipitates under this ageing condition. During the second step ageing at 160 °C, the peak strength value is achieved after a short ageing time, and then the strength decreases obviously, the UTS value reaches 493 MPa after ageing for 36 h. The microstructure in the matrix of the alloy at this ageing stage contains mostly metastable η′ phase and stable η phase. The grain boundary precipitates in two-step aged alloy were coarse and sparsely distributed, and clear precipitation free zone was observed.