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For the Nickel-based superalloy Inconel 718, electron-beam melting (EBM) enables the production of microstructures with columnar grains and a pronounced [001]-texture. A previous study demonstrated that EBM Inconel 718, after hot-isostatic pressing (HIP), solution annealing, and two-step ageing, exhibits longer thermomechanical fatigue (TMF) life compared to conventionally rolled Inconel 718 [1]. This improvement was primarily attributed to the [001] texture aligned with the loading direction, which results in a lower elastic modulus and reduced stresses during strain-controlled TMF loading.
In the present study, comparative TMF tests are conducted on as-built EBM Inconel 718 to analyse the influence of HIP and age-hardening treatment and to assess the potential of the as-built…
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For the Nickel-based superalloy Inconel 718, electron-beam melting (EBM) enables the production of microstructures with columnar grains and a pronounced [001]-texture. A previous study demonstrated that EBM Inconel 718, after hot-isostatic pressing (HIP), solution annealing, and two-step ageing, exhibits longer thermomechanical fatigue (TMF) life compared to conventionally rolled Inconel 718 [1]. This improvement was primarily attributed to the [001] texture aligned with the loading direction, which results in a lower elastic modulus and reduced stresses during strain-controlled TMF loading.
In the present study, comparative TMF tests are conducted on as-built EBM Inconel 718 to analyse the influence of HIP and age-hardening treatment and to assess the potential of the as-built microstructure. The TMF tests were conducted under strain control with a temperature range of 300 – 650 °C. The phase angle between temperature and mechanical load was 0° (in-phase) and 180° (out-of-phase). In some in-phase tests, dwell times of 600 s were introduced at the maximum temperature to provoke severe creep-fatigue interaction. Transmission electron microscopy and scanning electron microscopy were used to characterise the microstructure and damage evolution during TMF testing.
Since neither HIP nor heat treatment caused recrystallisation, the as-built and the heat-treated specimens exhibit comparable grain structures. Although not heat-treated, the as-built material shows finely dispersed γ’’ precipitates, which are similar to those of the heat-treated material. This results in similar cyclic stress-strain behaviour during TMF-testing. While the as-built material reaches nearly the TMF lifetime of the heat-treated material in tests without dwell times, it shows longer lifetimes in tests with dwell time. The TMF damage depends on the phase angle and is similar for both as-built and heat-treated specimens. The reason for the better resistance to dwell times of the as-built specimens appears to be a lower elastic modulus in loading direction, resulting in lower stresses during the dwells. Pores and printing defects in the as-built specimens have only insignificant influence on the TMF lifetime. In summary, the results demonstrate that as-built EBM Inconel 718 is a viable option for TMF-loaded high-temperature components, potentially eliminating the need for HIP and heat treatment.