Response of as-cast Al-Zn-Mg alloy with and without Sc, Zr addition to annealing with constant heating rate
Abstract
The as-cast Al-Zn-Mg-based alloy with and without Sc, Zr-addition was investigated during isochronal annealing from room temperature up to 480 °C. Precipitation reactions were studied by electrical resistometry, microhardness measurements and differential scanning calorimetry.
These measurements were compared to microstructure development that was observed by optical microscopy and transmission and scanning electron microscopy. Microstructure observation proved the Zn, Mg-containing eutectic phase at grain boundaries in the as-cast state of both alloys.
It was also observed that the Sc, Zr-content is not homogeneously distributed but concentrated in randomly localized matrix regions and together with Zn and Mg in the particles at grain boundaries. The distinct changes in resistivity and microhardness curves as well as in heat flow of the alloys studied are mainly caused by dissolution of the Zn, Mg-containing Guinier-Preston (GP) zones and subsequent precipitation of the metastable particles from the Al-Zn-Mg system.
The hardening effect after isochronal annealing at temperatures above ~ 280 °C reflects the Sc, Zr-addition. The eutectic Zn, Mg-containing phase partly disappeared during the isochronal annealing above this temperature.
Precipitation of the Mn,Fecontaning particles was also observed in the alloys. The apparent activation energy values were calculated regardless of Sc, Zr- addition as: dissolution of the GP zones (~ 100 kJ/mol) and formation of the metastable Zn, Mg-containing particles (~ 100 kJ/mol).
Melting of the eutectic phase was observed by differential scanning calorimetry at ~ 475 °C.