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

111

3

2. Experimental material and methods For the experimental procedures, the Inconel alloy 718 was used. The SPECTROMAXx chemical elements analysis of the alloy is in Table 1. The experimental material, wrought superalloy IN 718, was supplied by BIBUS METALS s.r.o., Brno, Czech Republic in the form of bar semi-finished products with different diameters and the form of a cut strip with dimensions of 10x11x56 mm.

Table 1. The chemical composition of IN718 alloy (in wt. %), the Ni content is a balance. Alloy C Co Nb Ti Cr Al Mo Fe Mn IN718 0.026 0.14 5.3 0.96 19.31 0.57 2.99 11.15 0.07

Rounded cross-section samples were used for fatigue tests with cycle asymmetry parameter R = -1 (push-pull load), and square samples for fatigue tests with cycle asymmetry parameter R  1 (three-point bending load). Tests with alternating push-pull loading were performed at RT (high-frequency, high-cycle HFHC) with a loading frequency of f  20 000 kHz on the ŽU KAUP device with a half-wave Al attachment. The number of cycles to fracture Nf  1.10 8 was considered the so-called "run-out", the stress amplitude corresponding to this number of cycles was considered as the fatigue limit  c . Fatigue tests with alternating push-pull loading at a temperature of 700 °C (low-frequency, high-cycle LFHC) were performed on a ZWICK/ROELL AMSLER 150 HFP 5100 device, controlled by TestExpert software. The loading frequency ranged from f = 51 – 64 Hz, the number of cycles to fracture Nf = 2.10 7 was considered the so-called "run-out", and the stress amplitude corresponding to this number of cycles was considered as the fatigue limit  c . Three-point bending tests were performed on samples at RT, but one set was in the initial state, and the other set after applied heat treatment, annealing 800 °C/72 hours. The purpose of the heat treatment used was to simulate a change in the microstructure corresponding to fatigue tests at a temperature of 700 °C. As in the previous case, the test equipment used was ZWICK/ROELL AMSLER 150 HFP 5100, controlled by TestExpert software. The loading frequency was in the range f = 147 – 151 Hz, the number of cycles to fracture Nf = 2.10 7 was considered the so-called "run-out", and the stress amplitude corresponding to this number of cycles was considered as the fatigue limit  c . All sample sets for fractography analysis were prepared by classic metallographic procedures optimized for nickel superalloys. Fractography analysis of samples after fatigue tests was carried out using a TESCAN VEGA II scanning electron microscope. 3. Results Fractography analysis of fracture surfaces of samples after fatigue tests are detailed in Fig. 1 – 2. It is generally known that the fatigue process of structural materials consists of several stages. The first stage is a change in mechanical properties, which in this case is caused by cyclic loading. The density and configuration of lattice defects change thus the physical and mechanical properties. These changes are most pronounced at the beginning of cyclic loading, their intensity decreases as the number of cycles increases, and after a certain number of cycles they, no longer change. Cyclic loading and deformation of superalloys are related to dislocation motion. An effective obstacle to dislocation movement in superalloys is a coherent precipitate  -phase (Ni 3 Al, Ti) or  -phase (Ni 3 Nb) (Donachie et al. 2002). The second stage is fatigue crack initiation. Even in the case of nickel superalloys, the fact that the fatigue process is surface and structurally sensitive and the initiation of fatigue cracks takes place on the surface through the so-called PSB  s (Persistent Slip Bands) (Huang et al. 2016) or just below the surface on inclusions, carbides or carbo nitrides (Man et al. 2009). Experiments have shown that PSB  s are likely to form in the range of plastic deformation amplitude 1. 10 -4 ≤ ε pa ≤ 1. 10 -3 in appropriately oriented grains along the slip belt (Buque 2001). Fatigue crack initiation in the IN718 superalloy is the most important stage of the fatigue life itself. The method and speed of crack initiation depend primarily on the load, temperature and environment. According to Yoshiba (1991), the crack initiation stage in IN718 superalloy represents 75-80% of the total life, with this percentage increasing with the number of cycles required to fracture. Based on the performed tests and fractography analysis of the samples, it was found that at higher stress amplitudes multiple crack initiations occur, Fig. 1a, e, and 2a on the contrary, at small stress amplitudes the crack was initiated only at one place (Belan, 2015, Zhang, 2013), Fig. 1c and 2c, e and f.