Inconel 939 Performance Under Isothermal and Thermomechanical Fatigue Loading

In this work, cylindrical bars with a total height of 150 mm and a diameter of 13 mm were additively manufactured from EOS IN939 superalloy powder. The vertically built cylinders underwent solid solution annealing, followed by two‐step precipitation hardening and subsequent machining. The specimens exhibit elongated grains with a strong <001> texture along the building direction. Thermomechanical fatigue (TMF) experiments in strain‐controlled mode were performed under in‐phase (TMF‐IP) and out‐of‐phase (TMF‐OP) loading conditions in the temperature range from 400°C to 800°C. The heating and cooling rate was set to 10°C s −1 . Isothermal low‐cycle fatigue (ILCF) experiments at a peak temperature of 800°C complement the TMF investigations. ILCF tests showed continuous cyclic softening, whereas TMF tests exhibited early softening, then stabilization with phase‐angle‐dependent mean‐stress evolution. Life curves showed the highest fatigue life for ILCF loading and revealed the characteristic TMF‐IP/TMF‐OP crossover. TMF‐OP resulted in longer lifetimes at high strain amplitudes, while IP lifetimes exceeded OP lifetimes at low strain amplitudes. Microstructural scrutiny by means of electron backscatter diffraction/scanning electron microscope/transmission electron microscope linked the fatigue behavior with deformation and damage. Cracks initiated predominantly at the surface and propagated by mixed transgranular/intergranular paths governed by elongated grains, porosity, and carbides. TMF‐IP favored creep‐assisted intergranular facets, whereas TMF‐OP promoted transgranular crack propagation.