PSI - Issue 19

Yasuhiro Yamazaki et al. / Procedia Structural Integrity 19 (2019) 538–547 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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oxidation layer can be observed at the crack tip. The typical result of EDS analysis around the crack tip is shown in Fig. 10. It is found in Fig. 10 that the oxide layer around the crack tip consists of alumina and chromia mainly. On the other hand, little oxide product was observed around the crack under the lower temperature condition as shown in Fig. 9(b). It can be considered from Fig. 9(b) that the effects of oxidation at the crack tip can be negligible on the crack propagation under the lower temperature conditions. In other words, Fig. 9(b) suggests that the crack propagation controlled only by the mechanical stress field at the crack tip under the lower temperature conditions. In addition, it is also found from Fig. 9 (b) that the short fatigue crack propagates in the γ matrix under the lower temperature condition. The above experimental results suggest that not only the mechanical factor based on the fracture mechanics but also the other factor, such as an environmental effect, should pay the important role on the crack propagation behavior.

Fig. 9. Microstructures around crack tip, (a) at 500-900°C, (b) at 200-600°C.

Fig. 8. Temperature dependence in the relationships between the d a /d N and the ∆ K eff / E .

Fig. 10. Element map by EDS analysis of the specimen tested at 500-900°C.

3.2. Oxidation behavior

The thickness of the oxide layer on the (100) surface was measured from the cross-sectional SEM images of the oxidized specimen shown in Fig. 11. It was revealed from the EDS analysis that the oxide layer on the specimen surface consisted of alumina and chromia mainly. The growth behaviors of the oxide layer without the loading are shown in Fig. 12. From Fig. 12, the growth behavior of the oxide layer on the (100) surface can be expressed as follows.