13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- only one slip system is active, the dislocations are sometimes stopped if the stress is not large enough and a more serrated yielding is obtained. The fact that deformation bands are found only on the 〈011〉 specimens at 750°C can also be attributed to the fact that only one slip system is active in this direction and temperature. The deformation bands on the surfaces are topographic differences, which are more probable to obtain when only one slip system is active compared to the case when several systems are active. This since slip on several systems will lead to a more homogeneous surface. The serrated yielding behaviour of the 〈011〉 direction at 750°C can also be attributed to dynamic strain aging (DSA). Localized deformation bands are found on these specimens, and those deformation bands are likely glide bands with a high dislocation density. DSA is the interaction between solute atoms and moving dislocations, and is favoured in areas with a high dislocation density, i.e. within those localized deformation bands. A clear noise was heard from the 〈011〉 oriented specimens when loading into the first TMF cycle for tests at 750°C. This behaviour of the 〈011〉 direction has been reported before when acoustic emission (AE) was used to measure noise during plastic deformation [21]. In that study, loading in the 〈011〉 direction led to higher AE signals and higher noise compared to 〈001〉. It is thus likely, that during loading of the 〈011〉 direction at this temperature, in which serrated yielding occurs, a high noise is created for each serration. 4. Conclusions From this study it can be concluded that the Ni-based single-crystal superalloy MD2 shows an anisotropic creep behaviour as well as a tension/compression asymmetry during stress relaxation. At 750°C, the tensile creep properties is best for the 〈001〉 direction, followed by the 〈011〉 direction while the 〈111〉 direction has the worst creep properties. At the same temperature in compression, the 〈001〉 and 〈011〉 directions show similar creep properties, and also here the 〈111〉 direction shows the worst creep properties. During stress relaxation at 950°C, the creep properties are slightly better in the 〈011〉 direction compared to the 〈001〉 direction for both compressive and tensile loadings. Acknowledgements The work has been financially supported by Siemens Industrial Turbomachinery AB in Finspång, Sweden and the Swedish Energy Agency, via the Research Consortium of Materials Technology for Thermal Energy Processes, Grant No. KME-502. References [1] R.C. Reed, T. Tao, N. Warnken, Alloys-By-Design: Application to nickel-based single crystal superalloys. Acta Materialia, 57 (2009) 5898-5913. [2] T.M. Pollock, S. Tin, Nickel-based superalloys for advanced turbine engines: Chemistry, microstructure, and properties. J. Propul, Power. 22 (2006) 361-374. [3] R.C. Reed, The Superalloys - Fundamentals and Applications, Cambridge University Press, Cambridge, 2006. [4] N. Tsuno, S. Shimabayashi, K. Kakehi, C.M.F. Rae, R.C. Reed, Tension/compression asymmetry in yield and creep strengths of Ni-based superalloys. Proceedings of the
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