4.1. Effect of Er on the microstruc

4.1. Effect of Er on the microstructure evolution in LZ105 alloyAccording to the Mg-Li binary phase diagram, the β-Li phase coexists with α-Mg phase when Li content is between 5.7 wt% and10.3 wt% and the matrix is totally β-Li phase if the Li content is over 10.3 wt% [22]. The actual Li content of the studied alloys is ~10.2 wt %, which is near the region of single β-Li phase which indicate the most matrix of the studied alloys is β-Li phase. It has also been reported that addition of Zn and Y into the Mg-6 wt%Li alloy increases the volume fraction of β-Li phase, which is probably ascribed to the barrier effect of the formed eutectic pockets on the homogenous diffusion of Li during the solidification [16]. What's more, since part of Mg atoms in LZ105- xEr alloy are consumed for the formation of abundant Mg-Li-Zn or MgZn-Er particles, the decreased Mg content can hardly provide the driving force for the growth of α-Mg precipitates. Thus the Zn and Er addition also increase the content of β-Li phase in Mg-10Li alloy which results in high volume fraction of β-Li phase that can be detected in optical images. And low volume fraction of α-Mg phase is found in TEM characterization.The former references have been reported that the content variation of Er and Zn elements in the Mg-Zn-Er alloys can influence the phase formation greatly. In general, they are mainly two kinds of ternary equilibrium phases: I-phase (icosahedral quasicrystalline structure) and W-phase (FCC structure) in cast Mg-Zn-Er alloys [8]. Through controlling the Zn/Er ratio, I-phase or W-phase can be generated selectively. Li et al. designed a series of Mg-xZn-yEr alloys and the results shows that Zn/Er weight ratio from 10 to 6 results in the formation of Iphase separately as main secondary phase [8]. When the ratio is less than 0.8, the second phase is mainly W-phase. I-phase coexisting with W-phase is inclined to be formed in the range from 1 to 4. However, the situation totally changes with ~10.2 wt% Li addition. In this work, the actual Zn/Er ratio is varied from 11.6 to 1.4 and only W-phase is confirmed in these LZ105-xEr alloys. The similar result has also been reported in Mg-Li-Zn-Y-Zr alloy. Chen et al. discovered that Mg-5.21Li-3.44Zn-0.32Y-0.01Zr (wt%) alloy consists α-Mg phase, β-Li phase, Wphase and Mg0.97Zn0.03 compound and phase-forming mechanism in Mg-Li-Zn-Y-Zr alloy is not consistent with that in Mg-Zn-Y alloys [23]. It has been reported that the W-phase firstly forms, and then reacts with the liquid phase to form I-phase in Mg-Zn-Er alloy [24]. While, high Li content up to ~10 wt% may make the thermodynamic conditions and element segregation for forming I-phase not meet that accounts for this. The formation of I phase might be controlled by several concurrent factors including the atomic polarizations and ratios of Mg, Zn and Er elements, the conditional cooling rate during solidification and so on, which will be the subject in our later research. Previous work reported that formation of I-phase can stimulate the formation of more β-Li phase in duplex Mg-Li based alloy [16]. However, I-phase isn’t found in LZ105-xEr alloys both by XRD and TEM in LZ105-xEr alloys that indicates W-phase has similar effect