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 recuperat ion of GPII zones, formation of eta'-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 ov er the interval from -15 to +55 degrees C has no apparent effect on th e subsequent behaviour of the alloy; (b) the hardening effect of natur al ageing is positively correlated with the density of small and unifo rmly distributed GPII zones existing in the alloy at the end of the tr eatment; (c) the interruption in the reversion stage of artificial age ing at 150 degrees C is followed, at room temperature and at 70 degree s C, by a recovery that takes place with the same characteristic time constant t(c) as governs GPII formation after the homogenization treat ment; (d) the dependence of t(c) on the temperature agrees with activa tion energies for the migration of the solute that depend on the conce ntrations of Mg and of the quaternary additions (Cu, Mn).