Solidification phases and their evolution during homogenization of a DC cast Al–8.35Zn–2.5Mg–2.25Cu alloy

• Mg(Zn,Cu,Al)2 had the structure similar to MgZn2 containing Al and Cu elements.
• The composition of Mg(Zn,Al,Cu)2 was not affected by the homogenization treatment.
• Very few S phase that transformed from Mg(Zn,Cu,Al)2 phase was found when Zn% > 8.0 wt.%.
• Homogenization time increased with thickness and spacing of Mg(Zn,Cu,Al)2 phase.

In the present work, the microstructure evolution of Al–8.35Zn–2.5Mg–2.25Cu alloy during homogenization treatments was investigated. The main grain boundary phases in Al–Zn–Mg–Cu alloy were Mg(Zn,Cu,Al)2, which had the structure similar to MgZn2 containing Al and Cu elements, and Al7Cu2Fe. The content of Mg(Zn,Cu,Al)2 phase firstly decreased sharply within the initial 1 h, and then the mesh-like Mg(Zn,Cu,Al)2 structure was gradually evolved to be discontinuous particles. The ratio of Al, Zn, Mg and Cu elements in Mg(Zn,Cu,Al)2 phase was slightly affected by the homogenization treatment. Combining with other literature results, it could be concluded that the phase transition from Mg(Zn,Al,Cu)2 to Al2CuMg phases was very difficult when Zn content was higher than 8 wt.%. It is suggested that the significant effect of Zn might be explained by the diffusion behavior of Zn element.

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