Positron lifetime spectroscopy and decomposition processes in commercial Al-Zn-Mg-based alloys

Positron lifetime spectroscopy and Vickers microhardness measurements were used for studying the decomposition sequence of Al-Zn-Mg systems. Various microstructural changes (formation, dissolution and recuperation of GPII zones, formation of -particles) were induced by using isochronal annealing and multiple-step ageing thermal treatments. The experimental results give the following information: (a) changing the quenching temperature used for the initial homogenization treatment over the interval from -15 to has no apparent effect on the subsequent behaviour of the alloy; (b) the hardening effect of natural ageing is positively correlated with the density of small and uniformly distributed GPII zones existing in the alloy at the end of the treatment; (c) the interruption in the reversion stage of artificial ageing at is followed, at room temperature and at , by a recovery that takes place with the same characteristic time constant as governs GPII formation after the homogenization treatment; (d) the dependence of on the temperature agrees with activation energies for the migration of the solute that depend on the concentrations of Mg and of the quaternary additions (Cu, Mn).