Evolution of microstructural characteristics during creep behavior of Inconel 718 alloy

In this work, the microstructural evolution and creep behavior of Inconel 718 alloy under different temperatures and stresses were studied comprehensively. The results show that the increase of temperature and stress significantly accelerates the creep process, steady creep rate increases by two orders of magnitude and the creep life decreased to only 11 h (690 degrees C/690 MPa). During creep, gamma '' phase coarsened and transformed to delta phase, which was dominated by the diffusion of Nb (based on the activation energy calculation). Long holding time at high temperature/stress induced the concentration of dislocations around the delta phase, resulting in the creation of voids. With the progress of creep, the delta phase was separated from the matrix gradually and the voids coalesced to form cracks. Additionally, the crystal symmetry of twin within grains also provides energetically favorable condition for precipitation of delta phase. The increased delta phase provides more cracks, resulting in a tendency of transgranular fracture. Twinning coordinates grain deformation and obstructs dislocation motion, competing with the crack formation and propagation induced by intragranular delta phase. Based on the detailed analysis of creep fracture mechanism, a model for creep fracture was developed.