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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coherent γ´ precipitates with ordered L1 2 crystal structure. The SEM micrograph showed in Fig. 1d presents a detailed view of the bimodal morphology of γ´ precipitates involving cubic-shaped precipitates with an average edge size of (790 ± 120) nm and spherical nano-precipitates with a radius of (60 ± 15) nm. The presence of η and σ phases typical for the GTD-111 superalloy has not been observed. These phases emerging from the reaction between MC carbides and the γ´ phase are usually formed during heat treatment and adversely affect mechanical properties (Agh and Amini, 2018; Choi et al., 2004; Sajjadi et al., 2006).

Fig. 1. Initial microstructure of the EEQ-111 superalloy (a) Coarse dendritic grains as depicted by light microscopy; (b) EBSD map with IPF colouring showing no preferential grain orientation; (c) SEM micrograph in backscattered electrons; (d) detail of cubic γ´ precipitates and spherical γ´ n ano-precipitates. The representative stress-strain hysteresis loops depicted as a function of stress vs. strain for both testing temperatures are shown in Fig. 2. The selected hysteresis loops represent the 10th, N f /2, and N f loading cycles for a total strain amplitude of 0.70%. At both temperatures, the hysteresis loops are symmetrical in tension and compression until the principal fatigue crack starts to propagate across the specimen body. The mean stress is for the majority of the fatigue life near zero. Towards the end of the fatigue life, the hysteretic loop begins to bend significantly compressive, indicating the propagation of the principal crack across the fatigue specimen, and the mean stress becomes. The width of the hysteresis loops remains almost constant throughout life. As expected, the width of the hysteresis loop is broader at 900 °C .