Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys

Differential scanning calorimetry (DSC) has been combined with transmission electron microscopy (TEM) to investigate the low-temperature decomposition processes taking place in an Al-5 wt pet Zn-1 wt pct Mg alloy. It was conf irmed that two types of GP zones, i.e., GP(I) (solute-rich clusters) and GP (II) (vacancy-rich clusters), formed independently during decomposition of the supersaturated solid solution. The GP(I) zones form at a relatively low aging temperature and dissolve when the aging temperature is increased. Th e GP(II) zones are stable over a wider range of temperatures. To investigat e the nature of the zones in the Al-Zn-Mg alloy, differential scanning calo rimetry and transmission electron microscopy have also been carried out on binary Al-Zn alloys containing 5 wt pet and 10 wt pct Zn. In these Al-Zn al loys, GP zones formed rapidly during quenching, and they nave rise to chara cteristic electron diffraction patterns identical to those from GP(II) in t he Al-Zn-Mg alloy system, implying that GP(II) zones in Al-Zn-Mg alloys are very similar to the zones formed in binary Al-Zn alloys. Thus, it is likel y that GP(II) zones in Al-Zn-Mg alloys are zinc-rich clusters. In the Al-5 wt pet Zn-l wt pet Mg alloy, both GP(I) and GP(II) were found to transform to eta' and/or eta particles during heating in the differential scanning ca lorimeter. The eta' was also observed to form after prolonged isothermal ag ing of the Al-Zn-Mg alloy at 75 degrees C or after short aging times at 125 degrees C.