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NTRODUCTION

Ni-BASED superalloys show remarkable mechanical and chemical properties at high temperatures.[1,2] This makes them suitable as blade material for gas turbines and aero engines. Ni-based superalloys consist of the typical c/c¢-microstructure where the c¢-phase works as strengthener in a matrix of c. The c¢-phase has an L12ordered structure and is rich of Al, Ta, and Ti, while the c-matrix typically contains higher amount of W and Mo. By using a material in single-crystal form instead of poly-crystal form both creep and fatigue properties are enhanced.[3] Gas turbine blades in single-crystal form are most commonly casted with the h001i crystallographic direction upwards. This direction has the lowest stiffness of all directions and therefore the best fatigue properties, which is of great importance for gas turbine blade components. Single-crystal superalloys exhibit an anisotropic behavior as well as a tension/compression asymmetry.[4–7] In the literature, stress relaxation of single-crystal superalloys is often associated with shorter dwell times at high temperatures during thermomechanical fatigue (TMF) testing, for example dwell times of 5 minutes during each TMF cycle.[8,9] However, sometimes longer dwell times are applied, for instance Ro et al.[10] studied MIKAEL SEGERSA¨LL, Ph.D. Student, DANIEL LEIDERMARK, Associate Professor, and KJELL SIMONSSON, Professor, are with the Department of Management and Engineering, Linko¨ping University, 58183 Linko¨ping, Sweden. Contact e-mail: [email protected] JOHAN J. MOVERARE, Associate Professor, is with the Department of Management and Engineering, Linko¨ping University, and also with the Materials Technology, Siemens Industrial Turbomachinery AB, 61283 Finspa˚ng, Sweden. Manuscript submitted April 25, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

the effect of dwell times up to 10 hours during TMF and found that the fatigue life decreased with an increase in dwell time. Further, Zhang et al.[11] applied a dwell time of 1 hour at 1173 K (900 C) in compression at each cycle during TMF testing. In that study, three different stages of stress relaxation during the TMF tests were found and each stage was connected to a specific microstructural behavior. In another study,[12] dwell times of 30 to 60 minutes were applied during tensile loadings at temperatures from 973 K to 1273 K (700 C to 1000 C). Results showed that after the dwell time the stress had relaxed to an asymptotic stress value. During stress relaxation in Ni-based superalloys it seems like dislocation rearrangement and a decreased dislocation density are microstructural factors that influence the behavior.[13] When superalloys are subjected to mechanical loads at high temperatures ~1273 K (1000 C) rafting of the microstructure is observed. Rafting is a directional coarsening of the c¢-particles and is a diffusion-controlled process.[14] Several studies have shown that rafting plays a vital role during high temperature creep.[15,16] The orientation of the rafting is dependent on whether the all