Kinetic Analysis of Precipitation Process in the Two-Step Age Hardening of an Al-6%Zn-0.75%Mg Alloy

Yamamoto’s rate equation was applied to two-step age hardening of an Al-6%Zn-0.75%Mg alloy. All aging processes at 160–200°C were described as the sum of three precipitation reactions, that is, spherical, plate-like, and rod-like precipitation independent of the holding time at room temperature (RT). These results agreed with the observation of TEM microstructures. Based on the activation energy of diffusion calculated from the rate equation, it was suggested that the diffusion of zinc controlled the spherical precipitation reaction, and the diffusion of magnesium did the rod-like one. The plate-like precipitation reaction which depended on the holding time at RT is controlled by the diffusion of zinc for a short time and the one of magnesium for a long time. These precipitation reactions were related to the meta-stable and stable phase particles. The comparison between electrical resistivity (ER) and Vickers hardness (HV) changes at 160°C aging showed that the HV change was faster than ER one in the long holding time at RT. On the other hand, the ER change became faster than HV change in the short one at RT. This difference is related to the formation of clusters and GP zones that are formed by holding at room temperature.