Microstructure Evolution of a High Zinc Containing Al-Zn-Mg-Cu Alloy during Homogenization

Abstract The microstructural evolution of a high Zn-containing Al-Zn-Mg-Cu alloy during homogenization was investigated by optical microscopy, differential scanning calorimetry, scanning electron microscope and X-ray diffraction. A homogenization kinetic equation derived from a diffusion kinetic model was established to confirm the optimum homogenization parameter. Results indicate that severe segregation exists in the as-cast alloy. The non-equilibrium eutectics consist of α (Al), Mg(Zn, Cu, Al) 2 , S(Al 2 CuMg), θ (Al 2 Cu) and Fe-enriched phases. In the present work, no transformation from Mg(Zn, Cu, Al) 2 to S(Al 2 CuMg) phases occurs during homogenization and Mg(Zn, Cu, Al) 2 phases directly dissolve into the matrix. θ (Al 2 Cu) phases dissolve into the matrix while Fe-enriched phases still exist after homogenization. In addition, the contents of Zn and Mg elements in Fe-enriched phases are reduced or even disappear by prolonging the holding time of homogenization. The proper homogenization parameter is 440 °C/12 h + 468 °C/24 h, which is in consistent with the results of homogenization kinetic analysis.