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:
- 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.
- 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.
- 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.
- 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.