PSI - Issue 43

Ivo Šulák et al. / Procedia Structural Integrity 43 (2023) 209–214 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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The microstructural observations of fatigued specimens was performed via TESCAN Lyra3 XMU scanning electron microscope (SEM) and JEOL JEM-2100F transmission electron microscope (TEM).

Fig. 1. (a) macrostructure of B1914 superalloy with typical coarse dendritic grains (light microscopy); (b) SEM micrograph of B1914 microstructure consisting of γ matrix, γ´ precipitates, γ/γ´ eutectic , MC carbide and Mo 3 B 2 borides; (c) EDS maps of local chemical composition. 3. Results The stress amplitude versus the number of elapsed cycles N acquired for the B1914 superalloy at 800 °C and 900 °C for various total strain amplitudes is shown in Fig. 2. The stress response seems to be independent of both the temperature and the applied total strain amplitude. If transients are neglected in the first few cycles when changes in the stress response are caused by the inability of the testing machine to achieve accurate values of the specified strain amplitude, the material response is stable and plateaus or very mild softening can be observed across the selected amplitudes. This indicates a fairly balanced interplay of strengthening and softening mechanisms, with the increase in dislocation density being compensated for by the apparent formation of low-energy dislocation structures, annihilation of dislocations, and gradual degradation of γ´ precipitates (Šulák et al., 2020; Šulák and Obrtlík, 2023, 2020) . The Basquin and Coffin-Manson fatigue life curves of the B1914 superalloy tested at 800 °C and 900 °C are displayed in Fig. 3. The stress ampl itude σ a at half-life vs. the number of cycles to failure N f is plotted in the bilogarithmic representation in Fig. 3a. Experimental data were fitted by: (2 ) = ( 1 ) − ( 1 ) ´ . (1) The fatigue strength coefficient ´ and fatigue strength exponent b are listed in Table 1. In this representation, as the temperature is increasing, the LCF life of B1914 decreases significantly, however, the slopes of the fatigue life curves are nearly the same. Also, the values of the fatigue hardening exponent b are very close to each other (see Table 1).