Zn-22Al-2Cu alloy phase transformations at different homogenizing temperatures

The effect of the homogenizing temperature on phase transformations and hardening behavior was studied in a Zn-22Al-2Cu (wt%) alloy homogenized for 7 days at 315, 350, and 390 °C. The homogenized alloy was subsequently quenched and aged naturally at room temperature or artificially at 200 °C for different time periods. The β phase was unstable at room temperature and it decomposed through the β → α + η reaction during natural aging. An increase in the homogenizing temperature caused a decrease in the kinetics of the decomposition of the β phase in the alloy. Natural aging also caused an increase in the hardness due to the formation of nanometric grains with α and η phases and the presence of an intermetallic ɛ phase. This result agrees with the time exponents (n) between 1.5 and 2.6, which were obtained using the Johnson-Mehl-Avrami-Kolmogorov equation for the decomposition of the β phase. The transformed fraction curves exhibited the fastest and slowest growth kinetics of the â phase decomposition at homogenizing temperatures of 350 and 390 °C, respectively. The artificial aging induced the formation of equilibrium phases and began a decrease in hardness due to the coarsening process of the equilibrium α, η, and τ′ phases.