PSI - Issue 51

Juraj Belan et al. / Procedia Structural Integrity 51 (2023) 109–114 J. Belan et al. / Structural Integrity Procedia 00 (2022) 000–000

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Based on several studies (Donachie et al. 2002), the fatigue process depends on several parameters. The basic factors influencing the initiation and subsequent propagation of the fatigue crack and thus the overall fatigue life are:  the microstructure of the alloy (size and orientation of the  -matrix grain, method and form of the excluded  - phase and δ-phase and carbides and inclusions),  the method applied load characterized by the cycle asymmetry parameter R (R = -1 alternating push-pull loading or combined steady and cyclic stress, R  1 three-point bending, etc.) (Belan et al., 2021),  load frequency f (low-frequency load with frequency up to f = 100Hz, high-frequency or ultra-high -frequency load with f  20,000kHz) and,  environment, test temperature (room temperature - RT or elevated temperatures) (Ono et al., 2004). Several authors have discussed the influence of the microstructure, specifically, the grain size and the morphology of the excluded precipitate  or  -phase since the introduction of the IN718 superalloy. Pieraggi et al. (1994) in his work studied the influence of grain size and structural components (carbides,  -phase and δ-phase) on fatigue life and creep life. The coarse-grained microstructure is characterized by a single initiation site where the fatigue crack is initiated on the crystallographic facet of the large grain. The fine-grained microstructure is characterized by multiple initiations, where the initiation took place on carbide phases or inclusions just below the surface. The shape, size or distribution of the δ-phase, carbides, nitrides or inclusions do not significantly affect the fatigue life under the given test conditions (Liu et al. 2018 and Ghorbanpour et al. 2021). Increased frequency (and thus strain rate) will lead to higher fatigue life values (or unchanged crack propagation velocity values) based on overall strain hardening (Zhang et al., 2013).

Nomenclature BCT

Body Centered Tetragonal fatigue cycle asymmetry parameter

R

RT Room Temperature SEM Scanning Electron Microscopy

HFHC High Frequency and High Cycle fatigue loading LFHC Low Frequency and High Cycle fatigue loading N f Number of cycles to failure  c Fatigue limits stress PSB  s Persistent Slip Bands  pa Plastic deformation amplitude  a Fatigue stress amplitude

The effect of elevated temperature on the fatigue characteristics of IN 718 superalloy in the high-cycle fatigue region is much more complicated and not as satisfactorily elucidated compared to low-cycle fatigue. High-cycle fatigue in the area of cryogenic temperatures of 4K (-269.15 °C), 77K (-196.15 °C) and 273K (-0.15 °C) was dealt with in the work done by Ono et al. (2004). He found that the fatigue strength, as well as the tensile strength and the agreed yield strength of the IN 718 superalloy paradoxically increases with decreasing temperature. He attributes such behaviour to control the size and distribution of carbides with high Nb content. The paper aims to assess the effect of the applied load and fatigue test conditions on the initiation mechanism and the overall fatigue life of the nickel superalloy IN 718. For this purpose, experimental samples of the superalloy were loaded with alternating symmetrical loading with cycle asymmetry parameter R = -1 and combined steady and cyclic stress with cycle asymmetry parameter R  1 at ambient temperature and a test temperature of 700 °C with a different frequency of loading. The area of fatigue crack initiation and propagation was evaluated using SEM.