The influence of artificial aging on the microstructure and hardness of an Al–Zn–Mg–Zr alloy processed by equal-channel angular pressing

The effect of artificial aging on the microstructure and hardness of an ultrafine-grained (UFG) Al–4.8%Zn–1.2%Mg–0.14%Zr (wt%) alloy was studied. The UFG microstructure with an average grain size of about 260 nm was obtained by severe plastic deformation applying four passes of equal-channel angular pressing (ECAP) at room temperature. Then, artificial aging was performed on the ECAP-processed samples at 120 °C and 170 °C for 2 h. In the ECAP-processed sample Guinier–Preston (GP) zones, MgZn 2 precipitates and a high dislocation density were observed. After aging at 120 °C, coarse MgZn 2 precipitates were formed in the grain boundaries, leading to softening, while the dislocation density did not decrease. Annealing at 170 °C yielded a growth of the matrix grains to ~ 530 nm with a significant decrease in the dislocation density. In addition, GP zones disappeared and MgZn 2 precipitates were formed in both the grain interiors and the boundaries. This overaging of the precipitate structure and the decrease in the dislocation density resulted in a lower hardness than after annealing at 120 °C. It was found that the hardness reduction due to the change of the precipitate structure at 170 °C was higher than that caused by the decrease in the dislocation density.