13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- High temperature stress relaxation of a Ni-based single-crystal superalloy Mikael Segersäll1,*, Johan J. Moverare1,2, Daniel Leidermark1, Kjell Simonsson1 1 Department of Management and Engineering, Linköping University, Linköping SE-58183, Sweden 2 Siemens Industrial Turbomachinery AB, Finspång SE-61283, Sweden * Corresponding author: mikael.segersall@liu.se Abstract Nickel-based superalloys are often selected due to their remarkable mechanical and chemical properties at high temperatures. This makes the material suitable for high temperature applications such as gas turbines and aero engines. By use of single-crystal instead of poly-crystal material, both creep and fatigue properties are enhanced. Superalloys in single-crystal form exhibit an anisotropic behaviour and a tension/compression asymmetry. This makes it interesting to study different crystal orientations in both tension and compression. In this study, high temperature stress relaxation of a Ni-based single-crystal superalloy is investigated. Specimens with three different crystal orientations, 〈001〉, 〈011〉 and 〈111〉, were tested in both tension and compression. Results show an anisotropic stress relaxation behaviour, as well as a tension/compression asymmetry for all directions. During stress relaxation at 750°C, the creep properties decreases in the sequence 〈001〉, 〈011〉 and 〈111〉 in tension, while in compression 〈001〉 = 〈011〉, 〈111〉. However, at 950°C the creep properties are slightly better in the 〈011〉 direction compared to the 〈001〉 direction. Keywords single-crystal superalloy, thermomechanical fatigue, stress relaxation 1. Introduction Ni-based superalloys show remarkable mechanical and chemical properties at high temperatures, which make them a suitable as blade material in gas turbines and aero engines [1, 2]. The superalloys consist of the typical γ/γʹ′-microstructure, where the γʹ′-phase works as strengthener in a matrix of γ. The strengthening γʹ′-phase has an L12-ordered structure and is rich of Al, Ta and Ti. By using a single-crystal material instead of poly-crystal material both creep and fatigue properties are enhanced [3]. Gas turbine blades in single-crystal form are most common casted with the 〈001〉 crystallographic direction upwards, since this direction has the lowest stiffness of all crystal directions. Low stiffness implies good fatigue properties, which is of great importance for gas turbine blade components. Single-crystal superalloys exhibit an anisotropic behaviour and they also show a tension/compression asymmetry [4-7] . Stress relaxation testing of superalloys is often associated with shorter hold times at high temperatures during thermomechanical fatigue (TMF) testing, for example hold times of 5 min during each cycle [8, 9]. However, Zhang et al. [10] applied a hold time of 1h at 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 behaviour. Other research showed that when applying hold times of 30-60 minutes during tensile loadings at temperatures from 700 to 1000°C, the stress relaxed to an asymptotic stress value after the hold time [11]. At high temperatures and loads a directional coarsening of the γ/γʹ′-microstructure is obtained. This phenomenon is called rafting and it is a diffusion controlled directional coarsening of the γʹ′-particles [12]. The orientation of the rafting is dependent on whether the alloy has a positive or negative lattice misfit [13]. Rafting doesn't always have to be negative for the material. For instance, research has shown how pre-rafting can improve creep properties [14]. Also isothermal high-temperature fatigue lives have proven to be enhanced by a pre-rafted
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