13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Fig. 2 displays how the stress is relaxing during the 200 h hold time for all specimens. The figure shows that the stresses relax to lower stress states at 950°C compared to 750°C. The other significant differences in the stress relaxation behaviour are summarized below. All crystal orientations show a tension/compression asymmetry. However, the asymmetry is most pronounced the 〈001〉 direction at 750°C. Here, tensile loading leads to a higher stress state compared to compressive loading, and the difference in stress is approximately 80 MPa after 200 h. The 〈011〉 oriented specimens show an opposite behaviour at this temperature; tensile loading leads to a lower stress state compared to compressive loading. Another difference at this temperature, is that the unloading after 100 h leads to an increase in stress state for the 〈011〉 direction when the material is loaded into the second 100 h TMF cycle. However, after some time the material relaxes and adapts into the pre-unloading behaviour. For the 〈001〉 direction the unloading after 100 h does not have an influence on the stress state. Concerning the 〈111〉 oriented specimens, they relax to a significant lower stress state compared to both 〈001〉 and 〈011〉 directions. After the first 100 h TMF cycle, the stress is approximately 360 MPa for the 〈111〉 direction, while both 〈001〉 and 〈011〉 directions are well above 500 MPa. It should be noted that the stress relaxation in the 〈111〉 direction is very unstable compared to the other directions. The instability should be attributed to TMF machine instabilities, rather than any microstructural difference between the three directions. One reason can be that the high stiffness in the 〈111〉 direction makes it more difficult for the TMF machine to take care of instabilities. At 950°C, only the 〈001〉 and 〈011〉 directions were tested. At this temperature, an opposite tension/compression asymmetry is observed compared to 750°C. The 〈001〉 direction is stronger in compression than tension, while the 〈011〉 direction is stronger in tension than compression. Here the unloading after 100 h also seems to influence the specimens to a greater extent compared to 750°C. A significant higher stress state is observed in the beginning of the second TMF cycle. This is the case for both 〈001〉 and 〈011〉 directions at 950°C, but only for the 〈011〉 direction at 750°C. 3.2. Microscopy Crystallographic deformation bands across the specimens are found on the 〈011〉 oriented materials subjected to hold times at 750°C, both in tension (IP) and compression (OP). See Fig. 3a for the 〈011〉 oriented specimen subjected to 750°C in compression. Neither the 〈001〉 nor 〈111〉 oriented specimens show similar deformation bands at this temperature. However at 950°C, both 〈001〉 and 〈001〉 oriented specimens show crystallographic deformation bands (the 〈111〉 direction was not tested at this temperature). Another difference between the two temperatures is that the deformation bands become more distinct at 950°C compared to 750°C. All slip traces are consistent with slip along the {111}〈110〉 slip systems. In Fig. 3b, the more distinct deformation bands are visible. This figure shows the deformation bands on the 〈011〉 oriented specimen subjected to tensile stresses at 950°C. Here, slip traces on different {111} planes are found. In Fig. 3c, the deformation bands on the 〈001〉 oriented specimen subjected to tensile stresses are shown. In this case, in addition to the crystallographic deformation bands, also wavy deformation bands are found. This indicates that several slip systems are active during deformation in this direction at 950°C.
RkJQdWJsaXNoZXIy MjM0NDE=