Low cycle fatigue behavior and microstructural evolution of nickel-based superalloy M951G at elevated temperatures

Low cycle fatigue (LCF) tests of the newly developed nickel-based superalloy M951G have been conducted at 900 and 1000 degrees C under different total strain amplitudes. Results show that the fatigue properties, fracture mechanisms as well as coarsening of gamma' precipitates are dependent on testing temperatures and strain amplitudes. Fatigue life and cyclic stress response under the same total strain amplitude at 1000 degrees C are lower than that at 900 degrees C, which is due to the degradation of microstructures, shearing of gamma' precipitates by dislocations and serious oxidation. Fracture modes change from intergranular cracking to the mixed mode cracking as the strain amplitude increases. At low strain amplitudes, M951G alloy fails in the form of intergranular cracking owing to the oxidation of surface carbides and the relatively low deformation rate. At higher strain amplitudes, the strain localization in grain interior, the distribution of broken carbides and eutectics as well as the relatively higher strain rate are the main reasons for the formation of transgranular microcracks. Ultimately, the effects of fatigue conditions on coarsening of cubic gamma' precipitates are also analyzed from the aspect of gamma' volume fraction, fatigue life and flow stress difference between the gamma/gamma' interfaces.