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 beta ' 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-0.6mass%Mg2Si alloy where the abundance of parallelogram-type precipitate equals that of the beta ' phase. No beta " phase was observed in any of the alloys. The hardness of alloys 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.