PSI - Issue 52

Ivo Šulák et al. / Procedia Structural Integrity 52 (2024) 154–164 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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based superalloys is the coherent γ´ precipitates, which possess an unusual property that the yield strength increases with increasing temperature over a significant range of temperatures. Generally, in service, components made of nickel-based superalloys experience severe elastic-plastic deformations resulting in low cycle fatigue (LCF) damage that is characterized by fatigue crack initiation and subsequent fatigue crack propagation (Gallardo et al., 2002; Polák, 1991; Puspitasari et al., 2021) . In the case of nickel-based superalloys, which are produced via investment casting, either surface (oxidized carbides, grain boundaries, and intrusions/extrusions) or casting defects (gas porosity, shrinkage defects, etc.) are preferential fatigue crack initiation sites. To minimize the harmful effect of casting defects, components are often isostatically pressed at high temperatures, which leads to a significant increase in fatigue properties (Šulák et al., 2016; Yu et al., 2022) . Once a fatigue crack is initiated, its propagation can proceed crystallographically along active octahedral slip systems or non crystallographically across grains (transgranular crack) or along grain boundaries (intergranular crack) (Š míd et al., 2020; Šulák and Obrtlík, 2020) . As the EEQ-111 superalloy is intended to operate for the majority of its lifetime at high temperatures, it is necessary to investigate its mechanical properties at high temperatures. Several studies reported on the tensile and creep properties of GTD-111 (Agh and Amini, 2018; Sajjadi and Zebarjad, 2006; Sajjadi et al., 2004; Sajjadi and Nategh, 2001). Sajjadi and Zebarjad (Sajjadi and Zebarjad, 2006) correlated the fracture behaviour and tensile properties. They distinguish three different regions with different operating deformation mechanisms. The peak of the yield strength was reached at 750 °C. Sajjadi and Nategh (Sajjadi and Nategh, 2001) studied creep behaviour in the temperature range of 750 °C to 950 °C to measure the stress exponent and at five different stress levels from 250 MPa to 450 MPa to elucidate the activation energy of GTD-111. Agh and Amini (Agh and Amini, 2018) recently published a study on the effects of vacuum induction melting and vacuum arc re-melting processes on the creep properties of the GTD-111 superalloy at 1000 °C. Thermomechanical fatigue of GTD-111 up to 800 °C was investigated in the last decade (Kim et al., 2013; Patel et al., 2015). However, fatigue properties of the EEQ-111 superalloy at high temperatures have been scarcely investigated (Yang et al., 2011). This work aims to report the LCF behaviour of the EEQ-111 superalloy at temperatures of 800 °C and 900 °C. The fatigue hardening/softening curves and fatigue life curves were assessed. A comparison with other industrially successful superalloys is provided. Microstructural degradation is documented by means of advanced microscopy techniques.

Nomenclature b

fatigue strength exponent ( – )

c fatigue ductility exponent ( – ) EBSD electron backscatter diffraction EDS

X-ray energy dispersive spectroscopy scanning electron microscope number of cycles to failure (cycles)

SEM

N f

γ

gamma matrix (fcc)

γ´ ε a

gamma prime precipitates (L1 2 crystal structure)

strain amplitude (%) ε ap plastic strain amplitude ( – ) ´ σ a stress amplitude (MPa) ´ σ m mean stress (MPa)

fatigue ductility coefficient ( – )

fatigue strength coefficient (MPa)