3.3 Very high-cycle fatigue
The S-N data associated with stress amplitudes σa and fatigue life Nf of LZ91 alloy under asymmetric axial loading with a stress ratio of R = -1 are shown in Figure 5 . The S-N data of some other Mg-Li alloys and magnesium alloys were also included for comparison [19, 31, 32]. “Run-out specimens” represents that the fatigue life of the specimen reached a limit of 1×107 (for LAZ832-0.5Y alloy) and 1×107 (for the rest three alloys). The nonlinear fitting curves of these fatigue data were obtained by Basquin’s equation, which were plotted by dash lines.
It can be seen that the S-N curve of LZ91 alloy exhibits a continuous decreasing trend, even in the range of 107-109 cycles. By comparing the S-N curve of LZ91 alloy and LAZ832-0.5Y alloy, it can be found that the curve of LZ91 alloy decreases much more slowly. Meanwhile, the fatigue performance of the LZ91 alloy is much better than that of the LAZ832-0.5Y alloy when the fatigue life exceeds 105cycles. As compared to conventional magnesium alloys [19, 32], whose fatigue tests were also conducted at R=-1 and a loading frequency of ~20 kHz, the fatigue resistance of the LZ91 alloy is obviously lower, about 20-35 MPa. However, as the fatigue life approaching 109 cycles, the fatigue strength gap between them is narrowing, less than 20 MPa at 109cycles. Due to the significant difference in mechanical properties between LZ91 and these two alloys, the lag of LZ91 in fatigue strength seems inevitable. After that, in order to compare the fatigue performance of these alloys, we calculated their fatigue ratios (the ratio of fatigue strength to the ultimate tensile strength), and the results are displayed in Table 4 . Clearly, the fatigue ratio of the LZ91 alloy, about 0.46, is significantly higher than that of the other three alloys (0.29, 0.27, 0.30 for LAZ832-0.5Y alloy, ZK60, AZ31, respectively), which indicates that the LZ91 alloy exhibits relatively good fatigue performance.