Thermomechanical fatigue performance of additively manufactured Inconel 939

• Building direction affects stress response and the resulting TMF lifetime. • TMF-IP loading is typical of intergranular/subsurface degradation. • Grain boundary cracking parallel to the loading direction for TMF-OP. • Deformation twinning observed for horizontally oriented specimens. Additively manufactured nickel-based superalloy Inconel 939 (IN939) was subjected to in-phase and out-of-phase thermomechanical fatigue loading in the temperature range of 400–800 °C. Horizontally and vertically built cylindrical specimens were subjected to a three-step heat treatment and subsequently tested with mechanical strain amplitudes in the range of 0.3–0.9%. A constant heating and cooling rate of 10 °C/s was utilised, making the cycle period 80 s. Representative hysteresis loops, fatigue hardening/softening curves, cyclic stress–strain curves, and fatigue life curves are reported. The results show that, regardless of the load cycle, the horizontally built IN939 exhibits lower lifetimes than the vertically built alloy. This stems from a distinctive 〈001〉 texture in the building direction, which influences the stress response of the material. Higher stress amplitude values observed for horizontally built material contribute to faster fatigue crack initiation and propagation. The SEM observation revealed that, regardless of the building direction, the damage is mainly intergranular for in-phase loading and mixed for out-of-phase loading. Plastic strain localisation into persistent slip markings and formation of nanotwins was typical for out-of-phase loading. In contrast, dense dislocation networks and stacking fault formation within γ́ precipitates were observed for in-phase loading.