Synergy of γ′ phase, MC carbide and grain boundary phase on creep behavior for nickel-based superalloy K439B

The synergy of γ′ phase, MC carbide and grain boundary phase on 815 °C/379 MPa creep behavior for nickel-based superalloy K439B was investigated by adjusting the time and temperature of the first-stage aging treatment. Increasing aging time from 4 h to 6 h, the total creep life is reduced by 35.8% due to the increased bimodal γ′ phase size. The bimodal γ′ phase transforms into small unimodal γ′ phase when increasing aging temperature from 1000 °C to 1080 °C. In addition, the content of M23C6 carbide increases, and the total creep life is increased by about 1.3 times. Small-size γ′ phase with bimodal distribution redissolves into γ matrix during creep deformation. Dislocations pile up at the γ/γ′ interface, and partial dislocations enter into large-size γ′ phase, mainly forming isolated parallel stacking faults. By comparison, the crossed stacking faults within small unimodal γʹ phase form Lomer-Cottrell locks, APB coupled dislocation pairs shear into γʹ phase, and more granular M23C6 carbide precipitates at the grain boundary, which further decreases the minimum steady creep rate and improves the creep life. At the beginning of creep deformation, MC carbide prevents the movement of dislocations. MC carbide formed at grain boundaries can delay crack propagation to a certain extent. MC carbide debonds from γ matrix subsequently due to the dislocation accumulation and higher interface energy, which provides a pathway for crack initiation and propagation. Moreover, the fragmentation and degeneration inside MC carbide promote crack initiation. •Different microstructures are obtained by adjusting the time and temperature of first-stage aging treatment.•Synergy of γ′ phase, MC carbide and grain boundary phase on creep behavior is clarified.•The crossed stacking faults form Lomer-Cottrell locks in γʹ phase to strengthen the alloy.•Small γʹ phase and high content of M23C6 carbide cause longer steady creep and accelerated creep stages.•MC carbide can prevent dislocation movement and delay crack propagation at grain boundaries.