Abstract
Precipitation reactions of the Al-Zn-Mg(-Mn,Fe)-based alloys with/without Sc,Zr-addition were studied by microhardness and resistivity measurements, and differential scanning calorimetry. Microstructure observation proved the Zn,Mg-containing eutectic phase at grain boundaries. Positron spectroscopy confirmed the presence of Guinier-Preston (GP) zones in the initial state. The changes in resistivity and microhardness curves as well as in heat flow are mainly caused by the formation and/or dissolution of the Guinier-Preston zones and precipitation of the particles from the Al-Zn-Mg system. Formation of the Mn,Fe-containing particles as well as of the η- and T-phase does not influence hardening significantly. The hardening effect above ∼ 300 °C reflects the Sc,Zr-addition. Heat treatment at 300 °C for 60 min and 460 °C for 45 min is insufficient for homogenization of the alloys. The apparent activation energy values were calculated: dissolution of the GP zones (∼ 106 kJ mol^–1), formation of the metastable η❜-phase (∼ 111 kJ mol^–1), formation of the stable η-phase (∼ 126 kJ mol^–1) and formation of the T-phase (∼ 144 kJ mol^–1).

Key words
AA7xxx alloys, early precipitation stages, electrical resistivity, GP zones, Al₃ (Sc,Zr) particles, activation energy

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