Structure of the grain boundary region in an Al–15 at.% Zn and Al–2.0 at.% Zn–1.3 at.% Mg alloy aged at elevated temperatures

Abstract

AbstractThe results of TEM investigations are presented of the effects of various solutions and ageing treatments, cooling and quenching rates as well as of tensile stress at elevated temperatures on the microstructure of the grain boundary (GB) region of the examined AlZn and AlZnMg alloys.A pronounced influence was found of partial coherency of GB stable β(Zn) precipitates with the α matrix in the AlZn alloy on the asymmetry of Zn depleted GB region and on the shape of GB precipitates. The role of GB's as sources and sinks of vacancies and the effect of stress gradients in GB regions were emphasized in the establishment of differences in the nucleation and decomposition characteristics within the GB region as compared to those within the grain interior. The existence of a negative stress gradient towards the GB has been suggested to lead to the observed gradual retardation of transformation kinetics within the GB region in AlZn alloys. A rapid quench from annealing to ageing temperature introduced large local stresses close to GB giving rise to “band of precipitates” on subsequent ageing within the former precipitate free region (PFZ) in AlZnMg alloy. Similar enhancement of local stresses accompanied by acceleration of decomposition kinetics was also observed in the aged AlZn sample subjected to a prior tensile creep exposure within the one‐phase region.