Co-precipitation on the basal and prismatic planes in Mg–Gd–Ag–Zr alloy subjected to over-ageing

Precipitation hardening is one of the dominant strengthening mechanisms for Mg alloys, especially for Mg alloys containing rare earth elements. However, precipitation hardening of conventional Mg alloys (such as AZ91D, ZK60 or even WE54/43) is relatively weaker than in high performance Al alloys [such as Al–Cu–Mg based alloys (2024) and Al–Zn–Mg–Cu based alloys (7075, 7050)]. Further avenues for improving precipitation hardening of Mg alloy are therefore being actively explored. In this paper, co-precipitation on the basal and prismatic planes of the α-Mg matrix in a Mg–2.4Gd–0.4Ag–0.1Zr alloy after over-ageing at 200 °C for 2048 h was investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS). Ag was observed within precipitates on the basal plane, while Gd was observed within precipitates on both the basal and prismatic planes. Furthermore, Ag, Gd-rich clusters were also observed in the vicinity of these precipitates, which were proposed to form during solution treatment and to coarsen during subsequent ageing treatment.