PSI - Issue 46

W. Li et al. / Procedia Structural Integrity 46 (2023) 119–124 Wei Li et al. / Structural Integrity Procedia 00 (2021) 000–000

121

3

Fig. 1. Microstructure, microtexture and S-N results: (a) Inclusions; (b) Pole figure; (c) Inverse pole figure; (d) Descending S-N curves at two stress ratios.

2.2. Observation of fracture surfaces No matter under R = -1 or R = 0.1, fatigue failures are all induced from the interior, and fracture surfaces are divided into three distinct regions consisting of circular dark area (CDA), crack growth region (CGR) and final fracture area. However, the fisheye morphology outside the CDA or the fine-granular-area (FGA), corresponding to the early microcrack growth behavior, cannot be observed (Božić et al. (2014), Božić et al. (2018), Li et al. (2021)). Obviously, it is related to the elevated temperature effect. Moreover, compared with the surface morphology in the CGR, the auxiliary effect of oxidation on cracking within the CDA should be lower, which is attributed to the interior vacuum environment. Next, based on the observation at high magnification, a larger pore acted as stress concentrator is observed as shown in Fig. 2(a). Some relatively smaller facets, as well as secondary cracks, are found around the pore, as shown in Fig. 2(b). However, in most cases, especially in the VHCF regime beyond 107 cycles, only a number of facets can be found in the crack initiation area at 650 °C no matter under R = -1 or R = 0.1, as shown in Figs. 2(c) and 2(d). Moreover, some slip traces or patterns on the surface of facet, as well as secondary cracks, are observed as shown in Figs. 2(e) and 2(f). Again, it confirms that the fracture of facets is greatly associated with the slip under shear stress. Combined with the cracking mode at room temperature (Li et al. (2021)), it can be concluded that the facets induced failure is the predominant failure behavior of SLM Ni-based superalloy in the VHCF regime no matter at room temperature or at elevated temperature, especially under the assistance of defects such as pore or inclusion.