Effects of quenching agents, two-step aging and microalloying on tensile properties and stress corrosion cracking of Al-Zn-Mg-Cu alloys

Abstract

In this paper, the tensile properties and stress corrosion resistance of two Al-Zn-Mg-Cu alloys were tested by tensile test and slow stress rate tensile test. The microstructures of the precipitation were observed by transmission electron microscopy, and the proportion and size of the precipitates were measured by ImageJ software. This work revealed that Sc and Zr microalloying will instead reduce the SCC resistance of Al-Zn-Mg-Cu alloys and increase the strength reduction of Al-Zn-Mg-Cu alloys, under certain heat treatment. The area fraction of GBP can be effectively adjusted by the combine of changing quenching agents, step-aging and minor additive of Sc and Zr. With the increase of area fraction of precipitates at the grain boundary, the slow stress corrosion susceptibility decreased first and then increased. This is because when the area fraction exceeds 22.3%, the low stress cracking mechanism will change from hydrogen embrittlement mechanism to anodic dissolution mechanism. The combination of heat treatment and Sc, Zr microalloying enable the area fraction reach approximately 22.3 %, which will make the Al-Zn-Mg-Cu alloys have high strength and low slow stress corrosion susceptibility.