Insights into GPB Zones Evolution and S’ Phase Formation in Al-Cu-Mg Alloy

The precipitation behavior of an Al-Cu-Mg alloy during high-temperature aging was systematically examined, with particular emphasis on the evolution of Guinier–Preston–Bagaryatsky (GPB) zones and their role in the formation of the S’ phase. Aging at 250 °C led to a progressive evolution of the precipitate state, as revealed by high-resolution transmission electron microscopy. At a short-term aging of 0.5 h, only GPB zones were observed as fine rod-like features preferentially formed on the {012} Al planes. With increased aging time, these zones gradually diminished, while S’ precipitates became predominant after approximately 2 h. Crystallographic analysis indicates that the growth direction of GPB zones is parallel to the g-(012) Al vector, corresponding to the habit plane of S’ phase. This crystallographic continuity suggests that GPB zones act as an effective precursor for S’ precipitation, contributing to not only nucleation but also to subsequent precipitate thickening. The present results provide new insight into the phase transformation pathway and precipitation mechanism governing high-temperature aging in Al-Cu-Mg alloys.