*Article* **Experimental Investigation and Thermodynamic Verification for the Phase Relation around the** ε**-Mg<sup>23</sup> (Al, Zn)<sup>30</sup> Intermetallic Compound in the Mg-Zn-Al System**

**Yan Zheng <sup>1</sup> , Jiaxing Sun 1,2, Kaiming Cheng 1,\*, Jin Wang <sup>1</sup> , Chengwei Zhan <sup>1</sup> , Jingrui Zhao <sup>3</sup> , Xitao Wang <sup>1</sup> , Shouqiu Tang <sup>1</sup> , Jixue Zhou 1,\* , Lijun Zhang <sup>4</sup> and Yong Du 1,4**


**Abstract:** The appearance of the ε phase during the welding process can severely weaken the welding strength of dissimilar metals of Mg-Zn-Al alloy systems. An understanding of the accurate phase diagram, especially the equilibrium phase relation around the ε phase, is thus of particular importance. However, the phase interrelation near the *ε*-Mg23(Al, Zn)<sup>30</sup> phase has not yet been fully studied. In this work, the local phase diagrams of the ε phase and its surrounding phases in the Mg-Zn-Al system are systematically determined by experimental investigation and thermodynamic verification. Five Mg-Zn-Al alloys and one diffusion couple were fabricated and analyzed to get accurate phase constituents and relationships adjacent to ε phase. The current experimental data obtained from Scanning Electron Microscope (SEM), X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC), and Electron Probe Micro Analysis (EPMA) were further compared with the thermodynamically computed phase relations around ε phase for verification, showing good agreements. Several important conclusions are drawn based on current experimental work, which can provide supporting information for the follow-up studies on ε phase in the Mg-Zn-Al alloy systems.

**Keywords:** Mg-Zn-Al alloy; ε phase; phase equilibrium; calculation of phase diagram (CAL-PHAD); diffusion
