Influence of Tertiary Gamma Prime (γ') Size Evolution on Dwell Fatigue Crack Growth Behavior in CG RR1000

This work investigates coarsening behavior of strengthening precipitates (gamma') in coarse-grained RR1000 after isothermal exposure for various times (50 to 500 hours) at 700 degrees C and 750 degrees C. The impact of these isothermal treatments is then studied by carrying out dwell (1 hour) fatigue crack growth tests at 700 degrees C in air. Such dwell periods can increase crack growth rates by nearly two orders of magnitude compared with baseline (0.25 Hz trapezoidal waveform) fatigue crack growth tests. Predominantly, scanning electron microscopy was used for gamma' analysis. Transmission electron microscopy was also utilized when necessary. Overaging as a thermodynamically driven and diffusion-controlled process strongly affects tertiary gamma' precipitates for the temperatures and treatment times investigated here. No influence of overaging on baseline fatigue behavior is observed. However, after overaging at 750 degrees C for 500 hours (which increases the mean tertiary gamma' size by a factor of two from 17 to 37 nm), dwell crack growth rates at 700 degrees C are reduced by one order of magnitude. Increased dwell fatigue crack growth resistance is also measured after overaging for 100 hours at 700 degrees C. The potential influence of gamma' distributions on dwell fatigue crack growth resistance is discussed in terms of stress relaxation behavior during dwell periods.