Transformation and dissolution of second phases during solution treatment of an Al–Zn–Mg–Cu alloy containing high zinc

The transformation and dissolution of Mg(Zn, Cu, Al) 2 phase during solution treatment of an Al–Zn–Mg–Cu alloy containing high zinc were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). The results show that solution temperature is the main factor influencing phase dissolution. With solution temperature increasing, the content of residual phases decreases. Phase transformation from Mg(Zn, Cu, Al) 2 to S(Al 2 CuMg) occurs under solution temperature of 450, 460 and 465 °C. Mg(Zn, Cu, Al) 2 phase is directly dissolved into the matrix under solution temperature of 470 and 475 °C, and no S(Al 2 CuMg) phase transformed from Mg(Zn, Cu, Al) 2 phase is observed. The formation of S(Al 2 CuMg) phase is mainly controlled by Zn elemental diffusion. The mechanism of transformation and dissolution of second phases was investigated. At low temperature, the dissolution of Zn is faster than that of Mg and Cu, resulting in an appropriate condition to form S(Al 2 CuMg) phase. At high temperature, the dissolution of main alloying elements has no significant barrier among them to form S(Al 2 CuMg) phase.