Stefan Guth

This study investigates the Ni‐based superalloy Inconel 718 produced by laser beam powder bed fusion (PBF‐LB) under thermomechanical fatigue (TMF) loading. Strain‐controlled TMF tests under in‐phase and out‐of‐phase conditions were conducted in the temperature range from 300°C to 650°C. To increase creep–fatigue interaction, tensile dwell times of 10 min at 650°C were introduced in some in‐phase TMF tests. After hot isostatic pressing, solution annealing and age‐hardening, the PBF‐LB material recrystallized and exhibited finely dispersed γ ″ precipitates, typical of Inconel 718. In this state, the material showed a cyclic deformation behavior comparable to that of conventionally‐rolled Inconel 718. In tests without dwell times, the TMF lifetimes of PBF‐LB Inconel 718 reached almost those of conventionally‐rolled Inconel 718. Introducing dwell times significantly reduced the TMF life of PBF‐LB Inconel 718. The results suggest that this effect can be attributed to a lack of grain boundary precipitates, which may facilitate grain boundary sliding and promote intergranular crack propagation.

Hydrogen embrittlement (HE) can significantly degrade the mechanical properties of steels. This phenomenon is particularly relevant for high-strength steels where large elastic stresses lead to detrimental localized concentrations of hydrogen at defects. In this study, unnotched rotating bending specimens of the bearing steel SAE 52100 (100Cr6) quenched and tempered at 180 °C and 400 °C were electrochemically charged with hydrogen. Charged and non-charged specimens then underwent rotating bending fatigue testing, either immediately after charging or after aging at room temperature up to 72 h. The hydrogen-charged specimens annealed at 180 °C showed a sizeable drop in fatigue limit and fatigue lifetime compared to the non-charged specimens with cracks mainly originating from near-surface non-metallic inclusions. In comparison, the specimens annealed at 400 °C exhibited a moderate drop in fatigue limit and lifetime due to hydrogen charging with cracks originating mostly from the surface. Aging had only insignificant effects on the fatigue lifetime. Notably, annealing of charged samples for 2 h at 180 °C restored their lifetime to that of non-charged specimens.

Electron beam melting of Ni‐base superalloy Inconel 718 allows producing a columnar‐grained microstructure with a pronounced texture, which offers exceptional resistance against high‐temperature loading with severe creep–fatigue interaction arising in components of aircraft jet engines. This study considers the deformation, damage, and lifetime behavior of electron‐beam‐melted Inconel 718 under in‐phase thermomechanical fatigue loading with varying amounts of creep–fatigue interaction. Strain‐controlled thermomechanical fatigue tests with equal‐ramp cycles, slow–fast cycles, and dwell time cycles are conducted in the temperature range from 300 to 650 °C. Results show that both dwell time and slow–fast cycles promote intergranular cracking, gradual tensile stress relaxation, as well as precipitate dissolution and coarsening giving rise to cyclic softening. The interplay of these mechanisms leads to increased lifetimes in both dwell time and slow–fast tests compared to equal ramp tests at higher strain amplitudes. Conversely, at lower mechanical strain amplitudes, the opposite is observed. A comparison with results of conventional Inconel 718 indicates that the electron‐beam‐melted material exhibits superior resistance against strain‐controlled loading at elevated temperatures such as thermomechanical fatigue.

Previous studies have shown that a thermomechanical treatment (TMT) consisting of cyclic plastic deformation in the temperature range of dynamic strain aging can increase the fatigue limit of quenched and tempered steels by strengthening the microstructure around non-metallic inclusions. This study considers the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. For this, high cycle fatigue tests on specimens with and without TMT were performed at room temperature at a constant stress amplitude. The TMT increased the average lifetime by about 40%, while there was no effect of the TMT on the form or size of critical inclusions. Surprisingly, no correlation between inclusion size and lifetime could be found for both specimen types. There is also no correlation between inclusion depth and lifetime, which means that the crack propagation stage covers only a small portion of the overall lifetime. The average depth of critical inclusions is considerably higher for TMT specimens indicating that the strengthening effect of the TMT is more pronounced for near-surface inclusions. Fisheye fracture surfaces around the critical inclusions could be found on all tested specimens. With increasing fisheye size, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types.

A thermo-mechanical treatment (TMT) at the temperature of maximum dynamic strain aging has been optimized and performed on quenched and tempered steel SAE4140H (German designation: 42CrMo4) in order to improve the fatigue limit in the high cycle fatigue (HCF) and and very high cycle fatigue (VHCF) regimes. Fatigue tests, with ultimate cycle numbers of 10⁷ and 10⁹, have shown that the TMT can increase both the fatigue lifetime and the fatigue limit in the HCF and VHCF regimes. The increased stress intensity factors of the critical inclusions after the TMT indicate that the effect can be attributed to a stabilized microstructure around critical crack-initiating inclusions through the locking of edge dislocations by carbon atoms during the TMT.

One possibility to improve the fatigue life and strength of metallic materials is shot peening. However, at elevated temperatures, the induced residual stresses may relax. To investigate the influence of shot peening on high-temperature fatigue behavior, isothermal fatigue tests were conducted on shot-peened and untreated samples of gamma TiAl 48-2-2 at 750 °C in air. The shot-peened material was characterized using EBSD, microhardness, and residual stress analyses. Shot peening leads to a significant increase in surface hardness and high compressive residual stresses near the surface. Both effects may have a positive influence on lifetime. However, it also leads to surface notches and tensile residual stresses in the bulk material with a negative impact on cyclic lifetime. During fully reversed uniaxial tension-compression fatigue tests (R = −1) at a stress amplitude of 260 MPa, the positive effects dominate, and the fatigue lifetime increases. At a lower stress amplitude of 230 MPa, the negative effect of internal tensile residual stresses dominates, and the lifetime decreases. Shot peening leads to a transition from surface to volume crack initiation if the surface is not damaged by the shots.