13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- the mechanical properties of the materials are shown in Table 1 and Table 2, respectively. The phase of Mg alloy, i.e., Mg17Al12, is more sensitive to hydrogen embrittlement than the matrix. Hence it is expected that the SCC behavior is dependent on the content of phase. To investigate the effect of phase on the SCC behavior, the T5 heat treated AZ61 (AZ61-T5) in which phases were precipitated along the grain boundaries was also used. The T5 heat treatment was set to hold the material at the temperature of 170 ºC for 42 hours followed by cooling in the air. Fig. 1 shows the optical micrographs showing the microstructures of (a) AZ31, (b) AZ61 and (c) AZ61-T5. Fig. 2 shows the micrograph by a scanning electron microscope (SEM) showing the microstructure of AZ61-T5 in the high resolution. The phase precipitates were observed along the grain boundaries. Table 1. Chemical compositions of materials (wt%) Material Al Zn Mn Si Cu Ni Fe Mg AZ31 2.7 0.79 0.44 0.004 0.0011 0.0009 0.0012 Bal. AZ61 5.8 0.65 0.29 0.01 0.002 0.002 0.002 Bal. Table 2. Mechanical properties of materials Material 0.2% proof stress, 0.2 (MPa) Tensile strength, B (MPa) Elongation, (%) Vickers hardness, HV Elastic modulus, E (GPa) AZ31 170 248 17 54 46 AZ61 192 301 19 62 44 (a) AZ31 (b) AZ61 (c)AZ61-T5 Figure 1. Microstructures of materials Figure 2. SEM micrograph showing phase precipitates along grain boundaries in AZ61-T5 For the SCC tests, compact tension (CT) specimens with the configuration of a width of 50.8 mm and a thickness of 6 mm were fabricated by machining from the materials. Fig. 3 shows the configuration of a CT specimen. Prior to the SCC tests, a pre-crack with a length of 2 mm was introduced from the artificial notch root of CT specimen. Pre-cracks were introduced by a fatigue test at the frequency of 10Hz, the stress ratio of 0.1 and the initial stress intensity factor range ΔKI of 3 MPa m1/2. 20m 100m 100m 100m
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