PSI - Issue 52

7

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

160

Fig. 5 . Comparison of ε a vs. N f fatigue lifetime curves of the EEQ-111 superalloy with MAR-M247 and IN713LC superalloys (a) 800 °C (b) 900 °C.

Fig. 6 shows the microstructure and fatigue damage of the EEQ- 111 superalloy after fatigue loading at 800 °C. As can be seen in Fig. 6a, the γ´ precipitates remain cubic. Moreover, there is no dissolution of the spherical nano precipitates even after a very long time at 800 °C (test duration was approximately 60 hours) . A thin oxide scale is formed on the surface (Fig. 6b). As with other types of superalloys (Stinville et al., 2018, 2017; Šulák et al., 2018; Šulák and Obrtlík, 2020) , this layer is composed of oxides rich in Al, Cr, and Ti. Its thickness, even in the longest LCF test at 800 °C , was less than 0.5 µm which is four times lower than for the MAR-M247 superalloy exposed to 800 °C for the comparable time (Šulák and Obrtlík, 2020) . It can be concluded that at 800 °C , the effect of oxidation on the initiation and subsequent propagation of the fatigue crack (see Fig. 6b) is insignificant. These facts have an undeniably positive effect on the fatigue life of the EEQ-111 superalloy. Transgranular fatigue crack propagation perpendicular to the loading axis is typical, as can be seen from the EBSD map in Fig. 6c. The fatigue crack propagates in a straight line. Slight deviations occur only when carbides are nearby (Fig. 6b). Kernel average misorientation (KAM) map, Fig. 6d, shows a local grain misorientation that is bigger in the vicinity of the crack than in the rest of the material, suggesting an accumulation of plastic deformation in the vicinity of the growing crack.