Microstructure and phase components of as-cast and homogenized advanced Al-Zn-Mg-Cu alloys with high strength and toughness

The microstructure and phase components of as-cast and homogenized Al-Zn-Mg-Cu alloys with high strength and toughness were studied by combining thermodynamic calculation with experiments. It is found the secondary phases presented in as-cast alloy A are Mg(Zn,Al,Cu)₂ and a small amount of Al₂Cu; however, only Mg(Zn,Al,Cu)₂ is found in as-cast alloy B. Thermodynamic calculation results show that the real solidification paths of alloys A and B fall between simulations by the Lever rule and the Scheil model, and the differences in content of phases in the two as-cast alloys are consistent with the regularity of the calculated mole fraction of phases formed after solidification in the Scheil model. Mg(Zn,Al,Cu)₂ or Al₂Cu phase in as-cast alloys A and B can be dissolved completely after conventional homogenization treatment (holding at 460℃ for 24 h), single α(Al) phase can be obtained, and this is consistent with the calculated isothermal section of the Al-Zn-Mg-Cu phase diagram with 7.5% Zn at 460℃.