Precipitation phenomena in an ultrafine-grained Al alloy

To provide insight into the influence of length scales on precipitation phenomena, a bulk Al 7075 alloy with an ultrafine grain (UFG) structure was fabricated via cryomilling, hot isostatic pressing and extrusion, followed by solution treatment and artificial aging. The precipitation behavior in the UFG material was studied by transmission electron microscopy (TEM) and compared to its coarse grain (CG) counterpart. Results indicate that there are important differences in the size, chemistry and spatial distribution of precipitates between these two bulk materials. Differences in grain size were noted to influence the dislocation substructure, which in turn governed precipitation kinetics. Specifically, a high volume of grain boundaries (GBs) leads to a low concentration of vacancies in the UFG material, which hinders homogeneous nucleation of precipitates during aging. The nucleation of Guinier–Preston (GP) zones in UFG after extrusion is influenced by the presence of dislocations, which provide heterogeneous nucleation sites. The density of precipitates in the UFG was comparatively lower than that in the CG counterpart, because of the lower dislocation density in the grain interior. Solution treatment promoted recovery of dislocations in the UFG material. Then GP zones and nanoscale platelet η′ phase precipitates nucleated homogeneously during artificial aging. In comparison, the network of dislocations that remained in the CG material after solution treatment facilitated the formation of plate-like η′ phase precipitates around dislocations during artificial aging. Moreover, copious GP zones were observed in the CG interior after artificial aging with a comparatively higher density than those in the UFG.