Effects of γ′ size and carbide distribution on fatigue crack growth mechanisms at 650 °C in an advanced Ni-based superalloy

Two distinct and different unimodal gamma ' size distributions have been produced in a disc alloy RR1000 to understand the combined effects of gamma ' size (linked to slip character), grain boundary (GB) precipitates/carbides and dwell time on fatigue crack growth (FCG) mechanisms at elevated temperature. The FCG behaviour has been investigated on single edge notched bend samples of both gamma ' variants at 650 degrees C under trapezoidal waveform loading with dwell times of 1 s and 90 s at maximum load. The fine gamma ' variant has better FCG resistance for both dwell times, and the difference in FCG rate becomes even more marked with longer dwell time. The coarse gamma ' variant exhibits faster and more time dependent FCG behaviour, with more evident intergranular fracture modes and rougher fracture surface than the fine gamma ' variant. Intergranular failure modes become dominant with the 90 s dwell time in both variants. A more continuous distribution of carbides is seen on the GB in the coarse gamma ' variant, which is likely to influence oxidation behaviour on GB and intergranular fracture behaviour. The effects of dwell time and consequent oxidation damage at the crack tip on fatigue behaviour are further investigated by transitioning between cyclic and time dependent regimes to assess the damage zones formed. The results show that an oxidation damage zone contributes to enhanced FCG after switching frequency from cyclic to time dependent regimes while a tortuous crack path is related to crack wake (shielding due to prior crack path tortuosity), resulting in improved FCG resistance.

Automated summary

Two distinct unimodal gamma' size distributions were produced in a disc alloy to study the effects on fatigue crack growth mechanisms at elevated temperature. The fine gamma' variant showed better resistance to fatigue crack growth for both dwell times, while the coarse gamma' variant exhibited faster and more time dependent behavior. Additionally, the presence of a more continuous distribution of carbides on the grain boundary in the coarse gamma' variant may influence oxidation behavior and intergranular fracture behavior.