Evolution of mechanical properties, localized corrosion resistance and microstructure of a high purity Al-Zn-Mg-Cu alloy during non-isothermal aging

The evolution of mechanical properties, localized corrosion resistance of a high purity Al-Zn-Mg-Cu alloy during non-isothermal aging (NIA) was investigated by hardness test, electrical conductivity test, tensile test, intergranular corrosion test, exfoliation corrosion test, slow strain rate tensile test and electrochemical test, and the mechanism has been discussed based on microstructure examination by optical microscopy, electron back scattered diffraction, scanning electron microscopy and scanning transmission electron microscopy. The NIA treatment includes a heating stage from 40 °C to 180 °C with a rate of 20 °C/h and a cooling stage from 180 °C to 40 °C with a rate of 10 °C/h. The results show that the hardness and strength increase rapidly during the heating stage of NIA since the increasing temperature favors the nucleation and the growth of strengthening precipitates and promotes the transformation of Guinier-Preston (GPI) zones to η′ phase. During the cooling stage, the sizes of η′ phase increase with a little change in the number density, leading to a further slight increase of the hardness and strength. As NIA proceeds, the corroded morphology in the alloy changes from a layering feature to a wavy feature, the maximum corrosion depth decreases, and the reason has been analyzed based on the microstructural and microchemical feature of precipitates at grain boundaries and subgrain boundaries.