Microstructural evolution of Al–Zn–Mg–Cu alloy during homogenization

In this study, the microstructural evolution of an as-cast Al–Zn–Mg–Cu alloy (AA7085) during various homogenization schemes is investigated. It is found that in a single-stage homogenization scheme, some of the primary eutectic gets transformed into the Al₂CuMg phase at 400 °C, and the primary eutectic and Al₂Cu phase gradually dissolve into the alloy matrix at 450 °C. The Al₃Zr particles are mainly precipitated at the center of the grain because Zr is peritectic. However, the homogeneous distribution of the Al₃Zr particles improves and the fraction of Al₃Zr particles increases in two-stage homogenization scheme. At the first low-temperature (e.g., 400 °C) stage, the Al₃Zr particles are homogeneously precipitated at the center of the grain by homogeneous nucleation and may be heterogeneously nucleated on the residual second-phase particles at the grain boundary regions. At the second elevated-temperature (e.g., 470 °C) stage, the Al₃Zr nuclei become larger. A suitable two-stage homogenization scheme for the present 7085-type Al alloy is 400 °C/12 h + 470 °C/12 h.