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

Abstract

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), Al10Cu7Sm2 and Fe-rich phases. Besides, some small η and needle-shaped θ (Al2Cu) 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 Al3Zr 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 Al10Cu7Sm2 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.