13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 0 –1 –2 –3 –4 0 10 20 30 Cathodic potential E (V) Threshold stress intensity facto for SCC propagation AZ31 AZ61–T5 AZ61 Magnesium alloy 3% NaCl solution K ISCC (MPam 1/2) Figure 8. Relationship between threshold stress intensity factor for SCC propagation, KISCC and cathodic potential, E 3.2. Fracture surface observation Fig. 9 shows the SEM micrographs showing the typical examples of fracture surfaces. As shown in Fig. 9 (a), the fracture surface of AZ31 under the cathodic potential of -1.4 V at KI of 8.6 MPa m1/2 was covered by corrosion products. However corrosion products became less on the fracture surface at KI of 14 MPa m1/2 as shown in Fig. 9 (b). Under the cathodic potentials of -2.5 V and -3.0 V, no corrosion products were observed as in Fig. 9 (c) and (d). In AZ61 and AZ61-T5, almost all of the fracture surfaces were covered by corrosion products and only a part of fracture surfaces were not covered as in Fig. 9 (e). As the cathodic potential became lower, corrosion products became less. Corrosion products on the fracture surfaces of AZ61 were less than those of AZ61-T5 comparatively. Some of the fracture surfaces of AZ61 remained their own morphologies under the cathodic potential of -4.0 V as in Fig. 9 (f). Except for the corrosion products, the fracture surface morphology is similar in all materials, i.e., fracture surfaces seem to be brittle covered with flat packets with fine steps without dimples as are observed in the fracture surfaces by fatigue tests [3]. 4. Discussion The crack growth rates of AZ31 specimens were much higher than those of AZ61 and AZ61-T5 specimens, which indicates the corrosion resistance of AZ31 is lower than those of AZ61 and AZ61-T5. Aluminum (Al) is added to AZ31 and AZ61 to enhance the strength and also to improve the corrosion resistance. So AZ61, which contains more Al than AZ31, exhibited better corrosion resistance. Comparing AZ61 and AZ61-T5, the crack growth rates of AZ61-T5 specimens were faster. It is attributed to the precipitated phases which are more sensitive to corrosion than the matrix. But the phases were precipitated along the limited region of grain boundaries so the corrosion resistance was not decreased as AZ31. KISCC of AZ61 under the cathodic potential of -1.4 V is the same as that under the cathodic potential of 0 V, which indicates that the anodic dissolution occurred under the cathodic potential of -1.4 V as same as no cathodic potential is applied and the cathodic potential of -1.4 V did not act to decrease
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