Microstructural evolution of the as-cast and the peak-aged Mg-xYb-0.5Zn-0.4Zr (x = 0.5, 1, 2, and 3 wt.%) alloy

Detailed microstructures of the as-cast and the peak-aged metal mould casting Mg-xYb-0.5Zn-0.4Zr (x = 0.5, 1, 2, and 3 wt.%) alloys were thoroughly investigated. The results indicate that Yb has better grain refinement effect on the Mg-Zn-Zr based alloys compared with the conventional rare earth elements such as Nd, Sm, and Gd, and the dominant intermetallic phase in the as-cast samples is Mg2Yb, although with a few Mg41Sm5 particles. In addition, the studied alloys also exhibit good aging hardening response when aged at 200 °C. During aging treatment, obvious precipitation of T phase formed on grain boundaries can be observed in the alloys with Yb is less than 2 wt.%, while Mg2Yb precipitate chains in the grain center and discrete Mg2Yb particles homogeneously distributed near grain boundaries in the samples with Yb addition over 2 wt.%. Finally, ultra-fine needle-like precipitates were detected in all peak-aged samples, although with distinct difference on sizes and distribution densities. Based on energy dispersive spectrometer mappings of Mg, Yb, Zn and Zr and the SAED patterns, the needle-like precipitates were identified as γ′ phase (MgZnRE containing, hexagonal structure, a = 0.55 nm and c = 0.52 nm) and followed an orientation relationship as (11¯.0)γ′||(11.0)Mg and [00.1]γ′||[00.1]Mg.