Precipitate evolution in underaged Al–Mg–Si alloy during thermal cycling between 25 °C and 65 °C

The evolution of metastable precipitates and the aging response in underaged Al–Mg–Si alloy during environmental temperature cycling was investigated using transmission electron microscopy (TEM) and hardness tests. After the alloy underwent thermal cycling between 25 °C and 65 °C, the hardness tests revealed that hardness decreased slightly, rather than following a concave downward curve, with the cycle times. Needle-shaped G.P. zones transformed during the environmental thermal cycling. The fraction of the zones declined sharply from almost 100% to only approximately 10% after 90 cycles, accompanied by an increase in the fraction of lath-shaped precipitates and the formation of β″ precipitates in the matrix. The precipitate developed with the 25–65 °C cycling time as follows: needle-shaped G.P. zones ⇒ lath-shaped ppt + β″ ppt + needle-shaped G.P. zones ⇒ lath-shaped ppt + β″ ppt + rod-shaped ppt + needle-shaped G.P. zones. Therefore, three or four precipitates coexisted in the underaged alloy following prolonged cycling. The formation of a limited number of β″ precipitates and the presence of a rod-shaped phase in the alloy during environmental temperature cycling reduced the hardness as the cycle time increases.