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 eta- and T-phase does not influence hardening significantly. The hardening effect above similar to 300 degrees C reflects the Sc,Zr-addition.
Heat treatment at 300 degrees C for 60 min and 460 degrees C for 45 min is insufficient for homogenization of the alloys. The apparent activation energy values were calculated: dissolution of the GP zones (similar to 106 kJ mol(-1)), formation of the metastable eta'-phase (similar to 111 kJ mol(-1)), formation of the stable eta-phase (similar to 126 kJ mol(-1)) and formation of the T-phase (similar to 144 kJ mol(-1)).