13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- After the tests, all specimens were examined by stereomicroscopy before they were cut parallel to the loading direction for further microstructural investigation by scanning electron microscopy (SEM). The SEM samples were prepared by grinding and mechanical polishing, but no samples were etched. The microstructure investigation was performed in a Hitachi SU70 SEM, using voltages from 10 to 20 kV. 3. Results 3.1. Stress relaxation testing Results from the tests with a Tmax of 750°C show that the 〈011〉 oriented specimens demonstrates a serrated yielding behaviour during the loading phase. This was visible in both tension (IP) and compression (OP). However, the yielding for both the 〈001〉 and 〈111〉 directions during the loading phase was more stable, i.e. no serrated yielding was observed. In addition to the serrated yielding for the 〈011〉 direction, those specimens also demonstrated a clear noise during the loading phase. On the other hand, both the 〈001〉 and 〈111〉 directions were more quite. For the specimens subjected to stress relaxation at Tmax of 950°C no serrated yielding and no noise was observed during the loading phase. 0! 100! 200! 300! 400! 500! 600! 700! 800! 900! 1000! Stress [MPa]! Time [h] 100! 200! 〈001〉 IP! 〈001〉 OP! 〈011〉 IP! 〈011〉 OP! 〈111〉 IP! 〈111〉 OP! 750°C! 950°C! Figure. 2. Stress relaxation over 200 h for all crystal orientations tested.
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