Microstructure evolution and creep behaviors of a directionally solidified nickel-base alloy under long-life service condition

By means of creep properties measurement, microstructure observation and lattice parameters measurement, the microstructure evolution and creep behaviors of a directionally solidified nickel-based alloy under long-life service condition are investigated. The results show that the creep life of the alloy at 980 °C/90 MPa is measured to be 9714 h. During creep, the various morphologies of γ′ and γ phases display in the different regions of sample. Wherein, the γ′ phase in the region near fracture is firstly transformed into the rafted structure, while the γ′ phase in the stress-free region exhibits the bunch-like structure. The size of the rafted γ′ phase in thickness increases from 0.4 µm to 1.8 µm as the time of creep prolongs to 9714 h, the coarsening regularity of the rafted γ′ phase in thickness obeys the parabolic law. Moreover, the parameters and misfits of γ′/γ phases in the alloy increase with the creep time. When the creeping time is less than 3000 h, the deformation mechanisms of alloy are dislocations slipping in the matrix channels and climbing over the rafted γ′ phase. In the later stage of creep, the deformation mechanisms of alloy are dislocations slipping in the matrix channels and shearing into the rafted γ′ phase, the alternate activation of the main/secondary slipping dislocations promotes the coarsening and twisting of the rafted γ′/γ′ phases. Wherein, the coarsening of the rafted γ′ phase is thought to be a main reason of the alloy having a better creep resistance and longer life.