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

The synergy of gamma ' phase, MC carbide and grain boundary phase on 815 degrees 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 gamma ' phase size. The bimodal gamma ' phase transforms into small unimodal gamma ' phase when increasing aging temperature from 1000 degrees C to 1080 degrees C. In addition, the content of M23C6 carbide increases, and the total creep life is increased by about 1.3 times. Small-size gamma ' phase with bimodal distribution redissolves into gamma matrix during creep deformation. Dislocations pile up at the gamma/gamma ' interface, and partial dislocations enter into large-size gamma ' phase, mainly forming isolated parallel stacking faults. By comparison, the crossed stacking faults within small unimodal gamma' phase form Lomer-Cottrell locks, APB coupled dislocation pairs shear into gamma' 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 gamma 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.