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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Fig. 6. Microstructural degradation of the EEQ- 111 superalloy tested with ε a = 0.24% at 800 °C (a) γ´ precipitates morphology after fatigue; (b) thin oxide scale and typical fatigue crack initiated in the vicinity of carbide (c) EBSD map of transgranular fatigue crack; (d) corresponding KAM map showing local misorientation around fatigue crack. The microstructural degradation due to fatigue loading at 900 °C is shown in Fig. 7. The cubic γ´ precipitates have a tendency to slightly round the edges. Nonetheless, their size remains within the statistical scatter same as of the initial state (compare with Fig. 1d). The width of the oxide scale is more pronounced this time. The formation of a γ´ precipitate-depleted subsurface layer also occurs (Fig. 7b), which is a typical example of the diffusion of Al and Ti from the γ´ precipitates towards the surface (Šulák et al., 2018; Šulák and Obrtlík, 2020) . The thickness of this subsurface γ´ precipitate-depleted zone and, in particular, the kinetics at which the zone forms in the vicinity of the fatigue crack affects the mechanical properties. Because the γ´ precipitates, which are the origin of the strength of nickel-based superalloys, are absent at the crack tip, and only a relatively weak γ matrix remains. The initiation of fatigue cracks was again from the surface (Fig. 7c). A s in the case of 800 °C , the majority of secondary fatigue cracks propagate transgranularly. However, there were also frequent cases where the fatigue crack initiated at the grain boundary and propagated in the early stages along the grain boundary (intergranularly), see the EBSD map in Fig. 7d. The propagation of fatigue cracks along grain boundaries may indicate grain weakening, either due to oxidation or creep. Both of these mechanisms are time-dependent. Hence the proportion of fatigue cracks that initiated and propagated along grain boundaries increased with decreasing total strain amplitude. The change in the fracture mechanisms with the increase in the temperature was also reported for the GTD-111 superalloy (Sajjadi and Zebarjad, 2006; Sajjadi et al., 2004).