Phase evolution of a novel Al–Zn–Mg–Cu–Zr–Sm alloy during homogenization annealing treatment

The effects of pretreatment and homogenization on the microstructure of a new Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr-0.3Sm alloy were investigated. The results show that severe dendritic segregation is visible at the grain boundaries of the as-cast microstructure, which consists of the η (Mg (Zn, Cu, Al) _2 ), Al _10 Cu _7 Sm _2 and Fe-rich phases. Besides, some small η and needle-shaped θ (Al _2 Cu) phases are distributed inside of the grains. After pretreatment at 400 °C for 10 h, the η phase and θ phase in the grains are dissolved, and Al _3 Zr particles are precipitated inside of the grains by homogeneous nucleation. However, dendritic segregation still exists at the grain boundaries. After homogenization annealing, the η phase present at the grain boundaries completely dissolves, and only a small number of Al _10 Cu _7 Sm _2 and Fe-rich phases remain. The alloy microstructure becomes more uniform and the volume fraction of residual eutectic decreases to 0.70% after homogenization annealing. The optimal homogenization processing is determined as 400 °C/10 h + 470 °C/24 h, which is in perfect accordance with the results obtained via homogenizing kinetic analysis.