Correlative evolution of microstructure, particle dissolution, hardness and strength of ultrafine grained AA6063 alloy during annealing

In this research, evolution of hardness and strength in ultrafine grained AA6063 alloy during annealing is investigated and correlated to the evolution of microstructure and particle dissolution. After four passes ECAP, formation of ultrafine grained (UFG) structure with average grain size less than 1 μm is demonstrated. Evolution of hardness and strength in addition to the microstructural stability of the samples are investigated during annealing at 300-500 °C. At 300 °C, volume fraction recrystallized gradually increases by time and after 1 h, the sample is fully recrystallized. An in-line trend is observed for the evolution of hardness. In fact, hardness decreases gradually with annealing time as the volume fraction of recrystallization increases. However, at 350 and 420 °C, the samples are fully recrystallized after 30 s and consequently, a severe drop in hardness is observed. At 500 °C, the microstructure is quite unstable. By 10 and 30 s annealing, the samples exhibit fully recrystallized structures which turns to abnormal grain growth when the annealing time increases to 120 s. The average grain size is in general larger than the samples annealed at 350 and 420 °C. Therefore, a lower hardness is expected. However, an inverse trend is observed. In fact, hardness of the sample annealed at 500 °C is higher than those in the samples annealed at 350 and 420 °C. In addition, after the initial significant drop in hardness due to full recrystallization after 30 s at 500 °C, a gradual increase is observed. Higher hardness at 500 °C and its increase by annealing time are correlated to dissolution of second phase particles which contribute in more significant solid solution hardening.