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

Abstract

Two distinct and different unimodal γʹ size distributions have been produced in a disc alloy RR1000 to understand the combined effects of γʹ 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 γʹ variants at 650 °C under trapezoidal waveform loading with dwell times of 1 s and 90 s at maximum load. The fine γʹ 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 γʹ variant exhibits faster and more time dependent FCG behaviour, with more evident intergranular fracture modes and rougher fracture surface than the fine γʹ 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 γʹ 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.