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.