Unraveling the plastic deformation, recrystallization, and oxidation behavior of Waspaloy during thermal fatigue crack propagation

Thermal fatigue is one typical failure mode for engine components involving cyclic thermal stresses/strains. However, the microstructure evolution and its microscopic interaction with thermal fatigue cracking in Ni-based superalloys featuring intragranular γ′ and intergranular carbides have yet to be clarified. In this study, cyclic temperature variations ranging from 25 °C to 700 °C were conducted on Waspaloy to unravel the synergistic damage mechanisms including stress distribution, plastic deformation, recrystallization, oxidation, and crack propagation. The thermal fatigue cracks are found to predominantly propagate along grain boundaries with a gradually descending rate from 3.5 μm/cycle to 1.5 μm/cycle. Compared with the macroscopic thermal stress up to ∼800 MPa, the local thermal stress induced by different sizes of M ₂₃C ₆ carbides marginally affects the crack propagation. Intensive slip bands and sparse deformation twins in the deformed matrix are observed, indicating the plastic deformation is mainly achieved by dislocation slipping and supplemented by twinning. Trans-granular cracks parallel to {111} planes form occasionally when the propagation directions of intergranular cracks are almost parallel to the {111} slip bands. Besides, dislocations promote the oxidation damage of the crack surface through pipe diffusion of solutes, and the γ′-precipitate free zones (PFZs) and recrystallized areas are produced accordingly. The quantitation analysis of the PFZ thickness provides a reference for predicting crack initiation time during thermal fatigue.