PSI - Issue 52

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

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Fig. 2. Hysteresis loops for selected number of cycles (10 th , N

f /2 and N f ) of the EEQ- 111 superalloy cyclically strained with ε a = 0.7 %. (a) 800 °C; (b) 900 °C.

Fatigue hardening/softening curves in the representation of the stress amplitude σ a versus the number of elapsed cycles N are shown in Fig. 3 for both testing temperatures. Fig. 3a shows fatigue hardening/softening curves obtained at 800 °C . Cyclic straining with amplitudes higher than 0.5% resulted in initial short (up to 5 cycles) cyclic hardening followed by moderate cyclic softening till the end of fatigue life. The initial hardening can be attributed to a significant increase in dislocat ion density and interactions of dislocations with each other and with the γ´ precipitates (Antolovich, 2015; Choe and Lee, 1995). In contrast, cyclic softening indicates that shear, dissolution, directional coarsening of γ´ precipitates, and formation of persistent slip bands is happening (Šulák et al., 2022; Šulák and Obrtlík, 2023) . Fig. 3b shows fatigue hardening/softening curves at 900 °C . Cyclic softening can be seen for strain amplitudes equal and higher than 0.27%. Below this strain amplitude, the character of cyclic hardening/softening curves shows a saturated stress response indicating the balance between hardening and softening mechanisms.

Fig. 3. Cyclic hardening/softening curves of the EEQ-111 superalloy (a) 800 °C (b) 900 °C.

Fatigue life curves were determined as the dependence of stress amplitude and plastic strain amplitude (both set from hysteresis loops at half-life) vs. the number of cycles to failure. The fatigue life curves for both testing temperatures of the EEQ-111 superalloy in the Basquin representation are shown in Fig. 4a. It is clear that with the increase in temperature, the strength of the material decreases. The shift is approximately 100 MPa for the lowest strain amplitudes, and it is accentuated with increasing strain amplitudes up to circa 230 MPa for the highest strain amplitudes. Experimental data were fitted by the Basquin equation expressed in the form (Obrtlík et al., 2009) :