© 2013 Siemens Energy, Inc. All rights reserved. Figure 10. Crack growth rate corresponding to their typical microscopy To sum up, cracks in all three orientations follow a similar propagation characteristic as KΔ arising. Figure 10 shows the typical microscopy corresponding to different KΔ regimes. It can be seen that as KΔ keeps increasing, the fracture surfaces become rougher and rougher. Meanwhile, more crystal facets appear when KΔ exceeds region II and runs into the fast-propagation region III. 4. Conclusions Fatigue crack growth behaviors of a newly developed DS superalloy have been studied to investigate the effects of temperatures and orientations. Test specimens were subjected to two temperatures (T0 and T0+250 0C) and three crystal orientations (longitudinal, transverse and diagonal). Following conclusions can be drawn: 1) Weak orientation dependence have been found, and all / da dN K −Δ curves fall into a [1/3,3] narrow bond. Generally cracks in longitudinal and transverse directions have comparable crack growth rate, while those in diagonal direction propagate more slowly, especially in low- KΔ regimes; 2) Temperature dependence seems to vary in different orientations, i.e., cracks travel in longitudinal orientation show no temperature dependence, while those in transverse orientations show a little temperature dependence, and temperatures do have large effect on cracks travel in diagonal direction; 3) At lower temperatures, stage-I type cracking mode has been found in longitudinal and diagonal directions, while stage-II type cracking mode has been found in transverse direction; Besides, at higher temperatures, stage-II type cracking is preferred in longitudinal and diagonal directions, while stage-I type cracking is preferred in transverse direction.
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