3.5 Morphology of specimen surfaces
Figure 7 presents an oblique view of the specimen surface near the crack initiation site, the α-Mg grains were indicated by white arrows. Note that the crack exhibit different characteristics when passing through the α-Mg phase and β-Li phase. The fatigue crack passes directly through the β-Li phase in the process of crack growth. On the contrary, the crack presents a ‘zigzag’ pattern when passing through the α-Mg grains, as enclosed by yellow dash circles. An enlarged optical image of a second crack near the crack initiation site is presented inFigure 8 . Clearly, there are numerous turning points in the crack path, especially when the crack passes through α-Mg grains, making the crack appear pretty tortuous. This is quite analogous to the fatigue behavior of duplex stainless steel [38]. It was reported that the fatigue crack growth was affected by the slip directions of the γ-austenite grains, which is a relatively hard phase in the material. The crack path appears quite tortuous results from the influence of the local microstructure. Therefore, it can be inferred that the α-Mg phase may also act as a hard phase (i.e. strengthening phase) in the material. The microstructure (grain orientation or grain boundary) of the α-Mg phase would hinder the initiation and early propagation of fatigue crack to a certain extent. In the following discussion section, we attempt to quantitatively analyze and figure out how the local microstructure has a macro effect on the crack behavior of the material.