© 2013 Siemens Energy, Inc. All rights reserved. Figure 7. Crack growth behaviors of D type specimens at T0 and T0+250 0C 3.3. Fractography All FCG tests were run until the specimen was separated into two parts. The fracture surfaces of all post-test specimens were observed through SEM and OM to understand the underlying mechanisms responsible for different damage modes. In regard to crack propagation behavior of single crystal superalloys, it is well known that cracks tend to propagate on <111> slip planes at low temperature, while cracks usually propagate normal to loading direction independent of crystal orientation at high temperature [8-11]. The former cracking behavior is called stage-I cracking, and the latter is called stage-II cracking. In current project, both stage-I and stage-II cracking modes can also be found, as can be seen by the side faces of fracture surfaces. Table 2 lists the side faces of specimens tested at both temperatures, in which cracks propagated from left to right, and a black arrow in each figure corresponds to a machined notch tip. For TL type specimens (i.e., cracks propagate along the longitudinal direction), crack follow an angle of 0 14 α≈ to the loading direction at T0, which seems as a stage-I cracking mode, while both at T0+250 0C show stage-II type of cracking modes. For LT type specimens (i.e., cracks travel along the transverse direction), trends are totally reversed, i.e., cracks at T0 follow an angle of 08 α≈ to the loading direction, while those at T0+250 0C all travel along a constant angle of 0 25 α≈ ; For diagonal specimens, stage-I cracking mode can also be seen at lower temperature ( 0 27 α≈ ), while at higher temperature, cracking surfaces show stage-II type cracking mode ( 08 α≈ ). To sum up, cracks in different orientations show different cracking modes, namely, cracks travel along longitudinal and diagonal directions show stage-I cracking at lower temperature and stage-II cracking at higher temperature, which is in accordance with literature. However, cracks travel in
RkJQdWJsaXNoZXIy MjM0NDE=