PSI - Issue 43

Ivo Šulák et al. / Procedia Structural Integrity 43 (2023) 209–214 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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subsequent propagation of fatigue cracks along grain boundaries (Fig. 4b) are more typical, mainly due to a higher incidence of impurities at grain boundaries. However, with increasing fatigue crack length, the intergranular propagation was soon replaced by transgranular propagation perpendicular to the loading axis. It is also worth noting that the fatigue crack initiation at grain boundaries was more frequent at the high amplitude domain. This may explain the change in the slope of the Coffin-Manson life curve (see Fig. 3b). Similar behaviour was also observed for second generation nickel-based superalloy MAR-M247 fatigued under different strain rate conditions (Šulák et al., 2022). Fig. 4c shows the TEM image of rafted γ´ precipitates after fatigue loading at 900 °C with the total strain amplitude of 0.27% obtained using beam vector B parallel to [001] direction. The mechanism of γ´ coarsening is generally controlled by the lattice misfit between the γ matrix and γ´ strengthening precipitates producing the elastic strain energy and interfacial energy in the material, allowing higher mobility of the interface due to the higher density of interfacial dislocations (Ardell, 2014; Porter et al., 1983; Reed, 2008). The anisotropic relaxation of the coherency stresses induced by the motion of dislocations during high-temperature LCF loading can cause the rafting of γ´ precipitates. The γ´ rafting was observed for both temperatures, but as it is a temperature-dependent damage mechanism, it was more pronounced at 900 °C.

Fig. 4. Fatigue damage of B1914 (a) fatigue crack initiation at 800 °C; (b) fatigue crack initiation at 900 °C; ( d) rafting of γ´ precipitates after fatigue test at 900 °C.

4. Conclusions The LCF behavior of boron- doped superalloy B1914 was studied at 800 °C and 900 °C in strain c ontrol mode with the following conclusions: 1) Independent of temperature and applied strain amplitude, the material exhibited a stable stress response over the entire lifetime. 2) Fatigue crack initiation starts at the surface and propagates transgranularly. Grain boundary fatigue crack initiation is typical for tests performed with high strain amplitudes at 900 °C. 3) With an increase in temperature, the fatigue life decreases. The change in the slope of the Coffin-Manson fatigue life curve at 900 °C can be attributed to the grain boundary cracking. 4) The formation of interfacial dislocations during fatigue loading is associated with γ´ rafting that was observed for both temperatures. Acknowledgements Authors would like to acknowledge the financial support from the project “ Advanced precision casting technologies for new types of blade castings and blade segments of gas turbines and turbochargers from modern superalloys with increased service life ” ( FW03010190) from the Technology Agency of the Czech Republic. The support of the Czech Academy of Sciences is acknowledged.