4.3 Effect of the grain orientation of α-Mg grains on fatigue behavior
It is demonstrated in the previous section that there is a certain relationship between the local proportion of α-Mg in the region near the crack initiation site and the fatigue life of the material. Since the hardness and elastic modulus of the two phases vary slightly, the microstructure of the material may be the governing affecting factor of the fatigue properties. As displayed in Figure 2 , the grain orientation of most α-Mg grains is around {10-10} and {11-20} cylindrical direction, namely, the basal plane of them is almost parallel to the loading direction during fatigue tests. It is well known that the magnesium possesses only 12 slip systems [48-50], with three basal slip systems {0001} <-12-20>, three prismatic slip systems {10-10} <-12-20>, and six pyramidal slip systems {10-10} <11-23>. It has been reported that the initial CRSS (critical resolved shear stress) value of basal slip systems (about 0.45~0.81 MPa) and {10-12} tensile twinning deformation (about 2.0~2.8 MPa) is far below that of non-basal slip systems (about 39.2 MPa of prismatic slip and 45~91 MPa of pyramidal slip) and compressive twinning deformation (about 76-153 MPa) at room temperature. Thus, the basal slip systems and tensile twinning deformation are most easily activated during the process of deformation. However, due to the fact that the basal plane of the α phase is nearly perpendicular to the loading direction, the activation of basal slip is highly hindered. Meanwhile, the loading-depth curve of the α-Mg grain obtained by the nano-indentation test demonstrates that the α-Mg and β-Li phases exhibit distinct characteristics. Clearly, the loading-depth curve of the α-Mg phase exhibits a “pop-in” effect comparing to the β-Li phase, which is proved to be caused by the twinning deformation of α-Mg grains under compressive loading. Therefore, it can be preliminary deduced that the twinning deformation is the predominating deformation pattern of α-Mg grains when subjected to axial loading.
As been revealed in Section 4.2, there is a linear relationship between the fatigue life and the proportion of α-Mg phase in the region near the crack initiation site. Namely, a higher local proportion of α-Mg phase, which acts as a hard phase, seems to increase the fatigue life of the material to a certain extent. Two main reasons, which make the α-Mg phase the hard phase in the material, can be concluded from the above discussion. Firstly, the slip system of the HCP structure is less than the BCC structure, thus the local coordinate deformability of the β-Li phase is much better than that of the α-Mg phase. Secondly, the orientation of the α-Mg phase is mainly around the {10-10} and {11-20} cylindrical direction, which makes the basal slip of the α-Mg phase difficult to be activated. Namely, the α-Mg phase becomes the hard phase in the material, whose deformability is much lower than the β-Li phase. As a result, the β-Li phase takes the major part of the plastic deformation during the fatigue testing, and the α-Mg grains become a strengthening phase that is difficult to deform. Therefore, the fatigue crack tends to nucleate and initiate from the region where the α-Mg phase is relatively sparse.