Mechanical properties, corrosion behavior and microstructures of a non-isothermal ageing treated Al-Zn-Mg-Cu alloy

The mechanical properties, corrosion behavior, and microstructures of Al-Zn-Mg-Cu alloy during non-isothermal ageing (NIA) process were investigated. The results suggest that hardness and tensile strength generally keep improving with increasing ageing degree until the level of 182 HV and 578 MPa (cooled at 120 ℃, or C120), apart from a slight decline at the region where the temperature is beyond 200 ℃. Synchronously, the electrical conductivity increases and corrosion susceptibility decreases as the ageing proceeds. The optimal performance of the alloy is obtained at the C120 condition, whose electrical conductivity, stress corrosion susceptibility rtf and maximum corrosion depth are 40.3 %IACS, 98.2% and 24.7 μm, respectively. GPI zones and η' phases are the primary ageing precipitates at the heating stage (100-210℃, 40℃/h), then η' phases are the main ones at the cooling stage (210-100℃, 20℃/h). Reasonably, the combination of high strength and favorable corrosion resistance of the alloy through NIA treatment are associated with the growth and coarsening of intergranular precipitates during heating and newly formed fine intragranular ones during cooling.