Microstructure evolution during homogenization of a τ-type Mg-Zn-Al alloy

Microstructure evolution during homogenization of a low-alloying τ-type Mg-Zn-Al magnesium alloy with a nominal composition of Mg-7% Zn-3% Al was investigated in this paper, using optical microscopy, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry analysis and quantitative image analysis. A anneal treatment following rapid cooling to ambient temperature from homogenization isothermal temperature was adopted to assess the homogenization degree in respect to microchemical distribution. The results show that shorter holding time results in so-called in situ precipitation phenomena, which is attributed to the insufficient diffusion of the element Zn and Al from solute-rich grain boundary region to inner side of grain during homogenization. A homogenization treatment at 325 °C for at least 50 h is just enough allowing complete homogenization for such alloy, in point of both morphology and microchemistry of the microstructure, which is deemed to be a key precondition for the optimum combined properties through proper thermal-mechanical treatment.