Precipitates, zones and transitions during aging of Al-Zn-Mg-Zr7000series alloy

Age hardening of an industrial 7000 series alloy in the temperature range 70-150degreesC has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate eta' and the stable phase eta-MgZn2, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450degreesC is emphasized.