Characterization of precipitation and phase transformations in Al–Zn–Mg alloy by the differential dilatometry

The phenomena of phase transformations are very complex particularly in the case of the Al–Zn–Mg alloys because of the various parameters which influence them (heat treatments, chemical composition, types of elements of addition, relationship between different chemical constituents…). In order to contribute to the comprehension of these phenomena we have made a study of phase transformations by using differential dilatometry with some microhardness measurements. The sequence of precipitation for this system of alloys is as follows: solid solution → Guinier–Preston zones (ZGP) → intermediate η′ phase → equilibrium η phase. The analysis of the results suggests principally the formation of GPII zones on one hand and, on the other hand, at the beginning of its formation the equilibrium η phase consists of several types of precipitates and for sufficiently long time there remains one type.