DSC measurement and HRTEM observation of precipitates in an Al-1.6 mass%Mg₂Si alloy

Precipitation sequence in an Al-1.6 mass%Mg₂Si alloy was investigated by DSC measurement and HRTEM observation. Four exothermic peaks (A, C, D and E) and one endothermic peak (B) were obtained when samples were heated at 10 K/min. Precipitates were confirmed when samples aged to the temperature of each peak were observed using HRTEM, and were classified into several types. Products in the region between peaks A and B were classified as GP zones. Precipitates between peaks B and C were classified as the random-type precipitates. Precipitates at peak C were classified as the parallelogram-type precipitates. Precipitates at peak D were classified as metastable phase β′. Precipitates at peak E were classified as equilibrium phase β. Although there have been many reports that the β″ phase is formed at peak C, this was not observed in the present study, rather random-type precipitates were observed. The hardness of samples heated to various temperatures was measured. The temperature that resulted in maximum hardness correspond to the temperaturethat yielded the maximum amount of random-type precipitates.