Phase transformation during solid-solution and aging treatment of a Mg-Gd-Y-Zn-Zr alloy containing LPSO and W phases

In this study, solid-solution and aging treatments were conducted on a hot-drawn Mg-9.5Gd-4Y-2Zn-0.3Zr alloy wire, which included long period stacking order (LPSO) phases and a Mg3Zn3RE2 (W phase) with a diameter of 4 mm. Subsequently, the phase transformations within the alloy were meticulously analyzed. Following solid solution treatment, the W-Mg3Zn3RE2 phase in the edge region almost completely dissolved, leaving behind blocky RE-rich phases. Conversely, the core region retained more of the original LPSO phase, resulting in significant microstructural differences between the core and the edge. The aging treatment induced a more pronounced hardening response in the core region, achieving a peak hardness of 136.33 HV at 96 h. By contrast, the edge regions exhibited a weaker aging response. Specifically, edge region 1 showed minimal fluctuations in hardness without significant hardening, while edge region 2 reached its peak hardness of 103.13 HV at 96 h. The differential hardening effects induced by aging treatment primarily stemmed from the formation and distribution of precipitate phases. Both the core region and edge regions initially precipitated beta-Mg5Gd at the grain boundaries, with stacking faults (SFs) forming within the grains. However, dense and uniform precipitation of the beta '-Mg7Gd phase also occurred within the core region. Therefore, controlling the precipitation of the W phase was crucial for tuning the mechanical properties of the Mg-9.5Gd-4Y-2Zn-0.3Zr alloy.