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

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Fig. 2. Observation of typical fracture surfaces with interior failure at 650 °C: (a) CDA with pore under SEM; (b) Facets and secondary cracks; (c) CDA without defect under SEM; (d) Facets, secondary cracks and slip traces; (e) CDA without defect under SEM; (f) Facets and secondary cracks. 3. Discussion Based on the analysis above, the interior cracking mechanism of SLM Ni-based superalloy in long life regime at elevated temperature is schematically shown in Fig. 3. The main processes contain: (1) Initially, some surface finishing flaws and microstructural defects including dislocations, pores and inclusions are present. As cycling continuous, a growing number of slip lines or bands occur within the grains especially oriented in the highest shear stress plane, and the oxide layer is gradually thickened, as shown in Fig. 3(a). Due to the applied lower stress, the protective layer is not easily broken down by local plasticity. Conversely, it will weaken the detrimental effect of surface flaws (Sujata et al. (2010), Stinville et al. (2018), Petrova et al. (2019)); (2) These intensified slip lines or bands can be the location for microcracks to develop, as shown in Fig. 3(b). However, if the slip bands are just concentrated near the twin boundary with stain concentration effect, and the microcracks are initiated preferentially along twin boundary (Murakami (2021)). Furthermore, the presence of small microstructural defects produced during the SLM process certainly accelerate the cracking process. However, in most circumstances the sizes of defects such as inclusions are too small, the larger strain concentration mainly occurs at twin boundary in the long life regime; (3) The initiated microcracks will discontinuously propagate along the maximum shear stress plane, and the facets are formed especially at positive stress ratios, as shown in Fig. 3(c). Moreover, due to the inhibition effect of high-angle grain boundary, the crack deflection phenomenon will take place. It should be noted that the threshold value for microcrack growth is related to the crack size; (4) When the microcracks spread to an approximately circular region under axial loading, the CDA is formed, as shown in Fig. 3(d). The stress intensity factor range of CDA is equivalent to the threshold value for long crack growth, which also means the subsequent crack growth belongs to the ordinary crack growth behaviour controlled by mechanics.