Thermo-mechanical fatigue deformation and fracture mechanisms of nickel-based powder metallurgy superalloy

Thermo-mechanical fatigue (TMF) failure is one of the major concerns for aero-engine turbine discs under complex service loading conditions. By conducting both in-phase (IP) and out-of-phase (OP) TMF tests, the stress-strain hysteresis loops and the cyclic stress responses were analyzed for the third-generation nickel-based powder metallurgy (P/M) superalloy FGH4098. To reveal the failure mechanism, advanced microscopy analysis was carried out to characterize the fracture surface and microstructure of the alloy after failure. The phase angle between alternating mechanical and thermal loads plays an important role for the magnitude and evolution of mean stress. According to SEM fractography, the crack growth exhibits intergranular fracture under IP TMF and transgranular fracture under OP TMF. From the electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) characterization results, the geometrically necessary dislocation (GND) density is higher for IP TMF, indicating more severe plastic deformation. The material shows cyclic softening in the high-temperature half-cycle, but cyclic hardening in the low-temperature half-cycle. The cyclic deformation is mainly related to dislocation activities under IP loading conditions, for which fatigue damage dominates the failure process. Under OP loading conditions, the deformation behavior is mainly associated with the generation of stacking faults (SF), and its failure mechanism includes both fatigue and oxidation damage.