13th International Conference on Fracture June 16–21, 2013, Beijing, China IMR, China. Through inductively coupled plasma atomic emission spectrum (ICP-AES) apparatus, the chemical compositions of alloys I-IV were determined, as listed in Table 1. The extrusion ratio was 10:1. Vickers hardness testing was performed with a load of 250g (HV). Table 1 Chemical composition of the as-extruded Mg-Zn-Y-Zr alloys Composition (wt %) Nominal alloy Mg Zn Y Zr Zn/Y HV Alloy I Bulk 5.68 0 0.78 ------ 62 ± 5 Alloy II Bulk 5.53 1.08 0.83 5.12 78 ± 4 Alloy III Bulk 5.64 1.97 0.73 2.86 74 ± 4 Alloy IV Bulk 5.49 3.08 0.82 1.78 68 ± 4 The specimens used for the super-long fatigue study were taken along extrusion direction (ED) of the plates. The dimensions of the specimens for fatigue test are shown in Fig. 1. Fatigue test was conducted on a Shimadazu USF-2000 ultrasonic fatigue testing machine at a resonance frequency of 20,000 Hz, with a resonance interval of 150 ms and a stress ratio of R = -1 in ambient air (temperature of 25-35 °C, relative humidity of 40-60 %). Specimens were cycled at constant amplitude until failure or until at least 109 cycles were reached. After test, fracture surfaces of failed samples were examined using Environmental Scanning Electron Microscope (XL30-FEG-ESEM). Fig. 1. Dimensions of the ultrasonic fatigue specimen. 3. Results and discussion Fig. 2 shows the fatigue strength (S) versus lifetime (N) curves of four Mg-Zn-Y-Zr alloys. It can be seen that for all measured S-N curves, a plateau exists in the regime of 5×106-108 cyc, and then the fatigue strength gradually decreases between 108 and 109 cyc. Therefore, only fatigue strength corresponding to 109 cyc can be determined. Compared with other alloys, the alloy with Y content of 1 wt% has the highest fatigue strength and its value is 105 MPa. To understand and compare the crack initiation mechanism of different alloys that failed in the super-long fatigue regime, four representative samples designated as S1, S2, S3 and S4 are chosen for analysis, as shown in Fig. 2. Fig. 3 shows the overall fracture surfaces of samples. It can be seen that for all samples, the fatigue crack preferentially initiates at the surface or subsurface. Based on the backscattered electron image, the overall fracture surface can be divided into three regions, i.e. crack initiation region (Region 1), steady crack propagation region (Region 2) and tearing region (Region 3), as shown in Fig. 4. It is very interesting to find that basically no phase -2-
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