GP-zones in Al-Zn-Mg alloys and their role in artificial aging

The structure of GP-zones in an industrial, 7xxx-series Al-Zn-Mg alloy has been investigated by transmission electron microscope methods: selected area diffraction, conventional and high-resolution imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)zones are formed over a wide temperature range, from room temperature to 140-150 degreesC, independently of quenching temperature, The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450 degreesC, by aging at temperatures above 70 degreesC. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated < 110 > domains, The structural relation to the eta'-precipitate is discussed. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.