Investigations on the Fatigue Behavior of High-Temperature Alloys for High-Temperature Gas-Cooled Reactor Components

For the development of a high-temperature gas-cooled reactor that is to be operated at temperatures up to 950°C, the low-cycle fatigue (LCF) as well as the high-cycle fatigue (HCF) behavior of several high-temperature alloys have been evaluated. The tests, performed between room temperature and 950°C, include the influence of the environment, hold times, and strain rate in the case of LCF behavior and of mean stresses in the case of HCF behavior. At high strain ranges, alloys with a high ductility like Incoloy-800H appear to be superior, whereas at low strain ranges and under HCF conditions, high-strength alloys like Inconel-617 and Nimonic-86 show a better fatigue resistance. Hold times decrease LCF resistance, especially at low strain ranges, which can be explained by the large stress relaxation. The better LCF resistance in impure helium compared to tests in air was correlated to differences in the deformation and crack initiation mechanisms. At high temperatures, strain rate plays an important role for the stress response under LCF loading. The HCF behavior was found to be very sensitive to superimposed mean stresses because of the considerable creep strain induced.