Creep-Fatigue Interactions in Single-Crystal Ni-base Superalloys

Wednesday, September 15, 2021: 4:00 PM
223 (America's Center)
Ms. Sanam Gorgannejad , Georgia Institute of Technology, Atlanta, GA
Dr. Ernesto A. Estrada Rodas , Georgia Institute of Technology, Atlanta, GA
Dr. Richard W. Neu , Georgia Institute of Technology, Atlanta, GA
Materials for extreme service conditions, for example, those used in the hot section of gas turbine systems, are often Ni-base superalloys. The motivation of materials development is to increase the efficiency by pushing these materials to fully utilize their temperature capabilities while maintaining excellent fracture toughness and durability, and at a reasonable cost. Peaker power plants used in the changing energy technology mix are required to withstand on-off cycles more frequently resulting in the need to better understand and quantify creep-fatigue interactions of materials used in the hot section for improving the predictive models for component design and maintenance scheduling.

In this study, the creep‐fatigue behavior of a lower cost, reduced rhenium Ni‐base superalloy, CMSX‐8, a variant of CMSX‐4, cast in a single crystal was experimentally evaluated over a broad range of conditions, from room temperature to 1100°C, and for two loading orientations: <001> and <111>. The fatigue lives and creep responses highly depend on the orientation, cycle type, and temperature. The relative importance of these parameters on influencing the life is identified and discussed. From this understanding, a simple creep-fatigue crack initiation life model is constructed to capture the influence of these broad test conditions.

The creep-fatigue interactions are further explored by decoupling the creep and fatigue steps using sequential experiments and comparing to conventional creep-fatigue interaction experiments. These experiments include long-term sustained loading, followed by fatigue, and several cycles of fatigue, followed by creep. These decoupled creep-fatigue interaction experiments targeted specific temperatures, in particular, 20°C, 750°C, and 1100°C, where the interaction between cyclic deformation and microstructure is known to be quite distinct. The results suggest that temperature is one of the most critical parameters in interpreting creep-fatigue interactions, and that the deformation mechanisms, crack growth behavior, and fracture behavior are all highly sensitive to temperature.