The as-cast Al–Zn–Mg–Cu-based alloy with and without Sc,Zr-addition was studied during isochronal annealing from room temperature up to 480 °C. Precipitation reactions were studied by differential scanning calorimetry and microhardness measurements. These measurements were compared to microstructure development that was observed by scanning electron microscopy. Microstructure observation proved eutectic phase at grain boundaries in the as-cast state of both alloys. The measurements of positron lifetime (LT) spectroscopy confirmed the presence of Guinier-Preston (GP) zones in the initial state of both studied alloys. Neither structure nor volume fraction of eutectic phase shows changes after annealing up to 480 °C. The distinct changes in microhardness curves as well as in heat flow of the alloys studied are mainly caused by dissolution of GP zones and by formation of the particles of the Al–Zn–Mg–Cu system. The apparent activation energy values of the thermal processes were calculated. The hardening effect after isochronal annealing at temperatures above ~ 300 °C reflects the Sc,Zr-addition.Keywords: Differential scanning calorimetry, hardness Vickers, Al–Zn–Mg–Cu system, Al3(Sc,Zr) phase
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