5. Conclusions
In the present paper, the VHCF behavior of a dual-phase Mg-Li alloy (LZ91) was studied. The underlying mechanisms of the crack initiation and early propagation have been fully investigated. The following conclusions can be drawn from the discussions:
  1. The S-N curve of the LZ91 alloy decreases continuously without a fatigue limit. The fatigue strength is evaluated to 78 MPa at 109 cycles, and the fatigue ratio of the material is calculated to 0.46, respectively.
  2. The α-Mg grains in the material exhibit strong textures due to the extrusion, in which the basal planes α-Mg grains are almost parallel to the loading direction. The twinning deformation is the predominant pattern of α-Mg grains since the basal slip is highly hindered during fatigue tests, making the α-Mg the strengthening phase in the material.
  3. The fractography of the material presents a noticeable microstructural sensitivity. The distinct morphologies of Zone I, II, and III resulted from the differences in the size of the plastic zone at the crack tip.
  4. The fatigue crack initiation is significantly affected by the local microstructure of the material. The proportion of the α-Mg phase of the region near the crack initiation region is lower than the average level. Cracks tend to nucleate and initiate from the local region with fewer α-Mg grains, and the fatigue life is linearly positively correlated with the proportion of α-Mg phase in the region.