Changes of microstructure in Al-Mg₂Si alloys containing several Mg₂Si contents during heating

Differential scanning calorimetry (DSC) measurements and high resolution transmission electron microscope (HRTEM) observations were performed in order to confirm the precipitation sequence of Al-Mg₂Si alloys containing of 0.6, 1.0 and 1.6 mass%Mg₂Si during heating. Peaks in the DSC curves were clearly observed to form when the Mg₂Si content was increased from 0.6 to 1.6 mass%. Exothermic peak A, endothermic peak B, and three exothermic peaks C, D and E appeared in the Al-1.0 mass %Mg₂Si alloy upon heating from room temperature. Peak C was the highest exothermic peak among them. It was difficult to detect clear peaks in the DSC curve of the Al-0.6 mass%Mg₂Si alloy. Needle-shaped precipitates were observed at 557 K, corresponding to peak C in the Al-1.0 mass%Mg₂Si alloy, and coarsened with increasing temperature. There was far less precipitate in the Al-0.6 mass%Mg₂Si alloy, and the precipitates were coarser. The highest density of random-type precipitate occurred in the temperature range of 493 K to 557 K in the Al-1.0 mass%Mg₂Si alloy, as determined by HRTEM. Many parallelogram-type precipitates occur at 557 K, and the β′ phase appears at temperatures higher than 580 K. This tendency is similar to that of the Al-1.6 mass%Mg₂Si alloy. The precipitation sequence in the Al-0.6 mass%Mg₂Si alloy is similar to that in the Al-1.0 and 1.6 mass%Mg₂Si alloys, however there is temperature range in the Al-0.6 mass%Mg₂Si alloy where the abundance of parallelogram-type precipitate equals that of the β′ phase. No β″ phase was observed in any of the alloys. The hardness of a s was greatest at temperatures corresponding to the respective peaks in the DSC curves in Al-1.0 and 1.6 mass%Mg₂Si alloys, which is in agreement with the temperature at which the density of random-type precipitate is highest.