13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 2 μm" Load direction" Figure. 7. Backscattered electron images of the 〈001〉 specimen subjected to stress relaxation test at 950°C in tension, N-type of rafting of the γ/γʹ′-microstructure. 4. Discussion The MD2 alloy exhibits an anisotropic creep behaviour as well as a tension/compression asymmetry during stress relaxation, see Fig. 2. It can also be seen that the 〈111〉 direction has the worst creep properties at 750°C. This direction relaxes to a significant lower stress state compared to both 〈001〉 and 〈011〉 direction during the stress relaxation tests. At 750°C, the 〈001〉 and 〈111〉 specimens are stronger in tension than compression. However, for the 〈011〉 specimens the opposite behaviour is observed; compressive loading leads to a higher stress state compared to tensile loading. At 950°C, an opposite tension/compression asymmetry is observed: the 〈001〉 direction is stronger in compression than tension, and the 〈011〉 direction is stronger in tension than compression. It seems that there is a change in tension/compression asymmetry when going from 750°C to 950°C. That the tension/compression asymmetry changes with temperature has been showed by Ezz et al. [6] who studied the tension/compression asymmetry for the critical resolved shear stress (CRSS) for a single-crystal superalloy. They showed that the CRSS is higher in tension than compression at 750°C for orientations close to [001] in the stereographic triangle. On the other hand, for materials close to [011] and [111] in the stereographic triangle, there was a smaller asymmetry in CRSS at this temperature. At higher temperatures the asymmetry decreased. Present study shows that there is a change in tension/compression asymmetry also in stress relaxation behaviour, however the reason to this is still to be determined. The 〈011〉 oriented specimens showed a serrated yielding behaviour during loading into the first TMF cycle in the tests at 750°C. This behaviour is attributed to two things; the fact that only one slip system is active during deformation in this direction and the effect of dynamic strain ageing (DSA). Serrated yielding behaviour of the 〈011〉 direction has previously been reported in the literature [17-20]. One explanation for this has been that only one slip system is active during deformation of the 〈011〉 direction while multiple slip systems are active during plastic deformation in the 〈001〉 direction [17]. The reason to single slip for the 〈011〉 direction is attributed to the lower symmetry, i.e. less equivalent slip systems, for this direction compared to the 〈001〉 and 〈111〉 directions. When several slip systems are active at the same time, there is always one slip system where the dislocations can propagate and a more stable yielding is observed. On the other hand, if
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