Comparison of electron-beam-melted and conventionally rolled Inconel 718 under thermomechanical creep-fatigue loading

Abstract

To evaluate the potential of additively manufactured superalloys for high-temperature components, strain-controlled thermomechanical fatigue tests were performed on the Ni-based superalloy Inconel 718 in conventional and electron-beam-melted (EBM) form. While EBM specimens feature columnar grains with strong [001]-texture along the building direction, conventional specimens exhibit equiaxed polygonal grains without pronounced texture. All tests ran under in-phase conditions with a temperature range of 300–650 °C. In some tests, 10 min dwell times at 650 °C at the peak tensile strain were added to induce severe creep-fatigue interaction. For a given mechanical strain amplitude, the lifetimes of EBM specimens exceed those of conventional ones. This is mainly caused by the lower elastic modulus of the EBM specimens due to their strong [001]-texture resulting in lower cyclic stress amplitudes. Typical for creep-fatigue loading, the damage is mainly intergranular. The EBM material cracks predominantly at boundaries of fine equiaxed grains, while the conventional material suffers also from twin boundary cracking. Electron microscopy characterisation reveals that the strengthening γ’ and γ’’ precipitates develop differently in conventional and EBM specimens during thermomechanical cycling, which affects their deformation and lifetime behaviour. The findings suggest that EBM-manufactured superalloys can be a beneficial alternative for hot-operating components.