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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regime, the crack nucleation mode of AM Ni-based superalloy is quite different from that of traditional fatigue (Sakai et al. (2016)). The subsurface or interior crack nucleation will become a remarkable failure behavior, greatly closed to the microstructure (Zhu et al. (2018), Grbović et al. (2019), Balokhonov et al. (2020), Cooke et al. (2020)). Furthermore, combined with the service environment of material, the elevated temperature will inevitably have an impact. Unfortunately, since the subsurface or interior cracking process cannot be directly observed, it is still a huge challenge for evaluating local deformation and crack nucleation under service environment. In the present study, combined with the multiple observation technologies containing scanning electron microscopy (SEM) and electron backscattered diffraction (ESBD), the axial loading tests with stress ratios R = -1 and 0.1 were performed to clarify the long-life fatigue property of an AM superalloy at high temperature.

Nomenclature SLM

selective laser melting

CDA circular rough area AM high cycle fatigue VHCF very high cycle fatigue SEM scanning electron microscopy ESBD electron backscattered diffraction EDS energy dispersive spectrometer CGR crack growth region FGA fine-granular-area additive manufacturing LCF HCF low cycle fatigue

2. Results 2.1. Microstructures and S-N curves

Combined with the analysis of energy dispersive spectrometer (EDS), some tiny circular inclusions of Al2O3 and chain-like or punctate Laves phrases can be found in the microstructure, as shown in Fig. 1(a). According to the analysis of grain orientation distribution in pole figures, the polar density in the {100} plane is the highest, about 13.17, as shown in Fig. 1(b). This indicates that there is a preferred orientation along <100>. Furthermore, based on the inverse pole figure shown in Fig. 1(c), the majority of grains are located in the (001) plane. Based on these, one can conclude that the grains have a strong cubic texture of {100} <001> in the building direction. With the hourglass shaped specimen machined from the as-deposited round bar, the axial loading fatigue tests with stress ratios R of 0.1 and -1 were conducted by a testing machine with 100 Hz at elevated temperature of 650 °C. After experiment, the S– N (stress-number of cycle) diagram indicated by stress amplitude σa vs. fatigue life Nf in the life region around 106 109 cycles is shown in Fig. 1(d). Apparently, the fatigue strength is lower at R = 0.1 due to the effect of mean stress, but there is no traditional fatigue limit (Mlikota et al. (2018)). Thus, two monotonically descending curves are plotted to approximately represent S–N property of SLM Ni-based superalloy under R = 0.1 and -1 at 650 °C.