Characterization of phase development in commercial Al-Zn-Mg(-Mn, Fe) alloy with and without Sc, Zr-addition

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).