Microstructural evolution and creep mechanism of a directionally solidified superalloy DZ125 under thermal cycling creep

In this study, thermal cycling creep tests under (950 °C/15 min+1100 °C/1 min)/100 MPa were performed to simulate the overheating service condition of a directionally solidified superalloy DZ125. The microstructural evolution and creep mechanism at each creep stage were revealed by systematic microstructure characterization after the interrupted creep tests. The results indicated that DZ125 superalloy exhibited three creep stages: the decelerating stage, the slow accelerating stage and the rapid accelerating stage. During creep, the un-recovered γ′ volume fraction at the lower temperature stages and the deterioration of grain boundary caused by the overheating stages were harmful to the creep resistance. During the decelerating stage, a few dislocations moved on the γ matrix. The rafted γ′ microstructure and dislocation networks formed near the minimum creep rate. Simultaneously, some dislocations sheared the γ′ phase as well. During the slow accelerating stage, the rafted γ′ microstructure and dislocation networks could prevent dislocations from moving. However, the dislocations shearing the γ′ phase, facilitated by the overheating and thermal cycling process, was conducive to creep deformation. During the rapid accelerating stage, more dislocations shearing the γ′ phase, severe γ′ phase degradation and increasing PFZs (precipitate free zones) along the grain boundary accelerated the creep deformation. The coupled effect of overheating and thermal cycling damage accelerated the deformation during the thermal cycling creep.