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