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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log 2N f = 1 log σ a − 1 log σ f ´ ,

(2) where the fatigue strength coefficient σ f ´ and the fatigue strength exponent b were evaluated using non-linear regression analysis in the Slidewrite Plus 7.0 software, and their values are shown in Table 2. The fatigue life curves in the Coffin-Manson representation of the EEQ- 111 superalloy tested at 800 °C and 900 °C are plotted in bilogarithmic coordinate system in Fig. 4b. The data pairs of ε ap and N f were approximated by: log 2N f = 1 log ε ap − 1 log ε f ´ , (3) where fatigue ductility coefficient ε f ´ and the fatigue ductility exponent c were evaluated in the same way as parameters of Basquin representation, and the values are listed in Table 2. As can be seen, the fatigue life curve for the temperatu re of 900 °C is shifted to a higher number of cycles to failure. This is due to higher plasticity at 900 °C, which is strongly reflected in this representation. However, the slope is slightly different, and a more pronounced shift occurs in the low strain amplitude domain.

Fig. 4. Fatigue lifetime curves of the EEQ- 111 superalloy obtained at 800 °C and 900 °C (a) Basquin representation (b) Coffin -Manson representation. Table 2. The LCF parameters of the EEQ- 111 superalloy obtained at 800 °C and 900 °C. ´ (MPa) b (-) ´ (-) c (-) 800 °C 2190 ± 120 - 0.16 ± 0.01 0.92 ± 0.66 - 1.06 ± 0.09 900 °C 1530 ± 80 - 0.15 ± 0.01 0.33 ± 0.19 - 0.82 ± 0.07 A comparison of the fatigue life of EEQ-111 with other first (IN 713LC) and second (MAR-M247) generation cast superalloys is shown in Fig. 5 for 800 °C and 900 °C separately. The diagrams are plotted as a function of total strain amplitude vs. the number of cycles to failure. As can be seen, for both temperatures, the superalloy EEQ-111 shows a superior fatigue performance compared to the other superalloys, which probably has the origin in the excellent microstructural stability of the γ´ precipitates (Sajjadi et al., 2006) and the high oxidation resistance of the surface (see Fig. 6 and Fig. 7). However, for low values of total strain amplitudes, the fatigue life curves approach each other, and the differences in a lifetime are nearly erased. The fatigue data of IN 713 LC and MAR-M247 used in Fig. 5 with details of microstructural degradation and damage mechanisms can be found elsewhere (Šulák et al., 2020, 2018, 2016; Šulák and Obrtlík, 2020) .