ON THE 2-STEP AGING OF A COMMERCIAL AL-ZN-MG ALLOY - A STUDY BY POSITRON LIFETIME SPECTROSCOPY

Formation of Guinier-Preston zones and precipitation, induced by two-s tep ageing thermal treatments in an age-hardenable commercial Al-Zn-Mg -based alloy, have been studied by positron lifetime spectroscopy and Vickers microhardness measurements. In particular, an initial softenin g of the specimens, occurring in the early stages of the second ageing step (artificial ageing at 150 degrees C), appears to be correlated w ith a strong decrease in the positron lifetime, which indicates a part ial dissolution of the GP zones. If the treatment at 150 degrees C is interrupted at this stage, and the ageing continues at a lower tempera ture (either room temperature or 70 degrees C), the positron lifetime and the microhardness recover to the values reached at the end of the first ageing step (natural ageing at room temperature for five days). The kinetics of this process is discussed in terms of reconstruction o f GP zones; the activation energy value of this process is also obtain ed.