4.4 A proposed model of the fatigue behavior of LZ91 dual-phase Mg-Li alloy
As discussed above, small crack initiation and propagation are highly affected by the microstructure of the material. Figure 11 thus presents the schematic illustration of crack initiation and early propagation process. The stripe phase in white is the α-Mg phase, and the matrix in blue is the β-Li phase, respectively. Severe plastic deformation tends to occur in the ‘soft’ phase, i.e., the β-Li phase during cyclic loading, as shown in Figure 11a. As crack grows and has to pass through α-Mg grains, the crack propagation will be inevitably hindered by α-Mg grain due to the hard grain orientations. As a result, there are some turning points (marked by black arrows) in the crack path when it passes through α-Mg grains, which make it appear more tortuous. Eventually, crack continues to propagate, resulting in the final failure of the material.
In the present investigation, the experimental results and microstructural analysis of the crack initiation will fill the gap of our understanding of the VHCF behavior of the dual-phase Mg-Li alloy. More importantly, they would provide more valuable advice to improve the anti-fatigue ability of the Mg-Li alloys.