EFFECT OF HEAT TREATMENT ON MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES OF Al-Zn-Mg-Cu(-Sc-Zr) ALLOYS

The as-cast commercial Al-6 wt% Zn-2.5 wt% Mg-1.3 wt% Cu-0.8 wt% Fe, Si, Cr (AA7075) and AA7075-ScZr (0.19 wt% Sc-0.06 wt% Zr) alloys were investigated during isochronal annealing from room temperature up to 450 degrees C. Precipitation reactions were studied by microhardness, conductivity measurements and differential scanning calorimetry. Microstructure observation by scanning electron microscopy and transmission electron microscopy proved the Zn, Mg, Cu-containing eutectic phase at grain boundaries in the as- cast state of both alloys. The melting of this eutectic phase was observed for both alloys at similar to 480 degrees C in the DSC curves at heating rate of 20 K/min. The distinct changes in microhardness and conductivity curves as well as in heat flow of the alloys studied are mainly caused by dissolution of GP zones and by formation of the metastable phase particles of the Al-Zn-Mg-Cu system. The hardening effect after isochronal annealing at temperatures above similar to 270 degrees C reflects the Sc, Zr-addition in the AA7075-ScZr alloy - the secondary Al-3( Sc,Zr) particles precipitation causes a pronounced hardening in the temperature range of 270-450 degrees C. The eutectic Zn,Mg,Cu-containing phase partly disappeared after the isochronal annealing at 450 degrees C as it was observed by scanning electron microscopy.