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

Abstract

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.