Comparison of precipitates between excess Si-type and balanced-type Al-Mg-Si alloys during continuous heating

Abstract

Differential scanning calorimetry (DSC) curves were obtained from Al-1.0 mass pct Mg2Si (balanced) and Al-1.0 mass pct Mg2Si −0.4 mass pct Si (excess Si) alloys, and precipitates corresponding to each peak at the DSC curve were interpreted by means of high-resolution transmission electron microscopy (HRTEM) observation in order to understand the precipitation sequence of metastable phases. Five peaks were obtained on the DSC curves, from which four were exothermic (A, C, D, and E) and one endothermic (B). Upon HRTEM observation, the peaks for the excess Si alloy were explained as follows: peak A–B: Guinier-Preston (GP) zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the β″ phase; peak C: β″ phase and precipitation of type B; peak D: dissolution of the β″ phase; precipitation of type A and β′ phase; and peak E: dissolution of the type B, type A, and β′ precipitation of the (β+Si) phase. This result is quite different from that in the balanced alloy as follows: peak A–B: GP zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the parallelogram-type precipitate; peak C: parallelogram-type precipitate and precipitation of β′ phase; peak D: β′ phase, dissolution of parallelogram-type precipitate; and peak E: the β-(Mg2Si).