Fully reversed strain controlled low cycle fatigue and creep-fatigue testing was completed on Alloy 617 at a total strain range of 0.3% and 0.6% in air and a simulated VHTR helium at 950°C. A triangular waveform and a ramp rate of 10-3/s were used for low cycle fatigue testing. Creep-fatigue testing followed a trapezoidal waveform with a tensile hold imposed at the maximum tensile strain. Tensile holds of 180, 600, and 1800 sec were investigated. The number of cycles to failure, Nf, is defined as the point at which the ratio of the peak tensile stress to the peak compressive stress initially deviated from its level slope and the point at which the ratio was 80% of the value at deviation12. Test completion was prior to actual specimen separation.
For testing in helium, a gas mixture was selected to roughly simulate the controlled VHTR coolant. It contained 50 μbar CO and 350 μbar H2 in addition to approximately 8 μbar H2O at 1.1 bar total helium pressure to be oxidizing with respect to Alloy 617