3.2 Nano-indentation results
The mechanical properties of α-Mg and β-Li phase obtained by the nano-indentation test are listed in Table 3 , and the loading-displacement curves are shown in Figure 4 , respectively. By comparing the mechanical properties of these two phases, it can be found that the hardness and elastic modulus values of the α-Mg phase and the β-Li phase are fairly close, about 75 HV and 50.5 GPa, respectively. Notably, however, the loading-depth curve of the α-Mg phase varies greatly from the one of β-Li phase. It can be seen that the depth of the indentation in the β-Li phase increased smoothly with the increasing of the loading stress applied by the probe. As shown inFigure 4a , however, the loading-depth curve of the α-Mg phase is quite discontinuous and presents a ladder-like pattern. Similar results were obtained by some previous nano-indentation experiments of Mg-Li alloy done by V. Kumar et al. and W. Wang et al. [27-29], and this phenomenon was named as “pop-in” effect. Guo et al. [30] carried out nano-indentation teste on magnesium grains of {10-10} and {11-20} crystal planes. It was concluded that the “pop-in” effect in loading-depth curves is ascribed to the initiation of {10-12} extension twinning. This can be indirectly supported by the EBSD results of the material. As stated in section 3.1, the axial grain orientation of α-Mg grains is mostly concentrated in {10-10} and {11-20} cylindrical directions. Therefore, when the α-Mg phase is subjected to an axial loading applied by the probe, the twinning deformation will be activated discontinuously when the shearing stress reaches the critical value, as presented in the schematic diagram in Figure 4a1 . On the other hand, due to the BCC (body-centered cubic) structure, the β-Li phase possesses 48 slip deformation systems, which is far more than that of the HCP structure. In other words, there are more slip systems for β-Li grains to coordinate the plastic deformation when they are subjected to axial loadings. For these reasons, the displacement increases more continuously in the loading-displacement curve of the β-Li phase as compared with that of the α-Mg phase.