1.Introduction
In recent decades, magnesium alloy is one of the lightest structural
metallic materials, the advantages of this alloy are its outstanding
specific strength, sound damping capabilities [1-3]. However, due to
its hexagonal closed-packed (HCP) lattice structure, magnesium alloy
possesses limited slip systems, low ductility, and unfavorable
formability at room temperature [4-6]. Recent studies have shown
that the formability and ductility of magnesium alloys would be highly
improved by adding an appropriate amount of Lithium (BCC, body-centered
cubic structure) [7, 8]. It is reported that Lithium addition would
significantly decrease the CRSS value of non-basal slip systems by
introducing more slip systems and reducing the c/a ratio [9].
Besides, with the addition of lithium, the lightest metal (about 0.534
g/cm3), the Mg-Li dual-phase alloy could significantly
reduce its density without losing too much mechanical strength [10,
11]. Therefore, Mg-Li alloy is widely considered as an ideal
structural material in the aerospace, automotive, and electronic
industry. Usually, the structural material is subjected to repeated
fatigue loadings during its service life, which may exceed more than
107 cycles and reach the VHCF (very-high cycle
fatigue) regime [12-14]. However, knowledge of the VHCF properties
of Mg-Li is fairly limited, which prevents its wide application. Thus,
it is essential to fully investigate its VHCF behavior.
As we all know, small crack initiation behavior accounts for the
majority (about 90%-95%) of the total fatigue life [15-17] of the
material and it is closely related to the microstructure of the
material. For conventional magnesium alloy with a single phase of HCP
structure, it is widely reported that crack initiation is closely
related to the deformation texture. Crack is mainly induced by twinning
deformation in Mg alloys with strong deformation texture [18], while
basal slip-induced crack is the predominant crack initiation mode in Mg
alloys with weak deformation texture [19-21]. In the case of
dual-phase Mg-Li alloy, the plastic deformation behavior of the material
becomes more complicated with the introduction of
the β-Li phase. Currently, plastic
deformation and fracture behavior have been examined by some researchers
[22, 23]. For instance, Guo et al. [23] investigated the slip
behavior of dual-phase Mg-Li alloy by tracing the microstructure and
texture evolution during the rolling procedure. It proposed that
{001}<110> was the dominant texture component
in the β-Li phase and basal slip, pyramidal I and II slip worked
together to coordinate deformation strain in the α-Mg phase. On the
other hand, Dong et al. [22] found that deformation and fracture
occurred mainly in the β-Li phase, and α-Mg phase participated in
fracture at high tensile speed. However, what remains unclear is how the
α-Mg and β-Li phase coordinate the deformation strain as well as the
fracture mechanism of dual-phase Mg- Li alloys in the regimes of
high-cycle and very high-cycle fatigue.
Therefore, the purpose of this research has been to study the behavior
of dual-phase Mg-Li alloy LZ91 in the VHCF regime. The relation between
fatigue crack initiation and propagation and the microstructure of the
material was carefully investigated through microstructural
characterization and quantitative statistics. This research may increase
understanding of the formation mechanism of the fatigue crack of LZ91
alloy and potentially provide a way to improve its fatigue properties.