4.2 Influence of local proportion of α-Mg grain on crack initiation behavior
As a dual-phase Mg-Li alloy, there may also exist hard and soft phases in the LZ91 alloy, just similar to the duplex stainless steels that have been studied before. The plastic deformation will preferentially accumulate in the soft phase during the subsequent micro-crack initiation and early propagation [35, 36]. Therefore, we calculated the proportion of α-Mg and β-Li phases in the region near the crack initiation sites. In this way, we tried to explore whether the difference in local microstructure will affect the fatigue performance of the material. As presented in Figure 10 , we calculated the area of α-Mg grains in the region near the crack initiation site in each fracture topography. Note that the area of each calculated region is the same, and the area of the calculated region is determined by the largest area of Zone I (specimen No. 5). In this way, the proportion of α-Mg grains near the crack initiation site can be quantified more intuitively. The conversion of the area of α-Mg and β-Li grains was done by the mathematical software. The statistical results of the area of α and β phases in the calculated region are listed in Table 5 , and the α-Mg ratio-Fatigue life diagram is presented in Fig 10b , respectively. It is obvious that there is a simple linear relationship between the fatigue life of the material and the proportion of α-Mg phase in the region near the crack initiation site. Namely, the lower the proportion of α-Mg phase in the region is, the longer the fatigue life will be. Besides, it is worth noting that the proportion of α-Mg phase in most specimens is below the average value of the material. Based on the discussion above, it can be preliminarily inferred that the α phase is a strengthening phase in the material as compared with the β phase. In other words, fatigue crack is more likely to nucleate and grow in a local region, where the proportion of α-Mg grains is relatively low. As demonstrated in Section 3.2, however, the hardness and the modulus of α-Mg and β-Li phase vary barely. As pointed above, the crystallographic structure of the two phases is different, in which the α-Mg phase is the HCP, and the β-Li phase is the BCC structure, respectively. According to previous studies, the texture of magnesium alloy will significantly affect the fatigue behavior of the material. Therefore, in the present work, the most likely cause for this may be the enhancing effect of the texture and the grain orientation of α-Mg grains.