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.