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.