On the Two-Step Ageing in a Commercial Al-Zn-Mg Alloy. A Study by Positron Lifetime Spectroscopy.

Formation of Guinier-Preston zones and precipitation, induced by two-step 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 softening of the specimens, occurring in the early stages of the second ageing step (artificial ageing at 150°C), appears to be correlated with a strong decrease in the positron lifetime, which indicates a partial dissolution of the GP zones. If the treatment at 150°C is interrupted at this stage, and the ageing continues at a lower temperature (either room temperature or 70°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 of GP zones; the activation energy value of this process is also obtained.