A study of high-temperature precipitation in Al–Mg–Si alloys with an excess of silicon

The high-temperature decomposition phenomena were examined for two Al–Mg–Si alloys with an excess of silicon. The kinetics of precipitation were studied by transmission electron microscopy and by measurement of the thermoelectric power in the temperature range from 300 to 525 °C. It was shown that decomposition occurs either at 300–400 °C by the precipitation of intermediate β′ phase (different structures are possible depending on the alloy), which is then gradually replaced by the equilibrium β phase or, between 400 and 500 °C, by direct precipitation of the β phase from the supersaturated solid solution. The precipitation of silicon was also observed. At 300–350 °C, a near equilibrium in composition solid solution can coexist with Mg2Si phases out of equilibrium. Structures of intermediate phases and their orientation relationships with the matrix are given. The diagram of isothermal precipitation is constructed as an S-curve.