PSI - Issue 52

9

Ivo Šulák et al. / Procedia Structural Integrity 52 (2024) 143–153 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 11. The comparison of creep curves for bulk and LPBF Alloy 400, both crept at 100 MPa and 650 °C.

Fig. 12. Representative fracture surface of bulk Alloy 400 crept at a) 600°C ; b) 650 °C ; c) 700 °C .

Fig. 13: Representative fracture surface of LPBF Alloy 400 crept at a) 600°C ; b) 650 °C ; c) 700 °C .

The creep results of the bulk and LPBF Alloy 400 differ significantly. Particularly, the steady strain rates for the bulk material are lower and the true strain achieved is higher than for the LPBF Alloy 400 (see Fig. 11). This can be partially explained by the grain size difference. The bulk material consists of coarse grains with a mean grain size of about 63 μm, whereas the LPBF Alloy 400 has an approximately four times smaller value of mean grain size (≈14 μm) . Although the smaller grain size might improve the strength and creep properties of the material (Čadek, 1988) , the LPBF Alloy 400 contains a significant amount of manufacturing defects (as can be seen from fracture surfaces) (Chlupová et al., 2023) . These defects have a detrimental effect on the creep properties, as the voids, gas porosity and keyholes facilitate the crack initiation. The SEM micrographs in Fig. 12 and Fig. 13 indicate that the creep tests of both bulk and LPBF Alloy 400 terminate in intergranular fracture. However, it is also possible to observe several defects on the fracture surfaces of LPBF Alloy 400, which accelerated the fracture. The inferior creep properties of the LPBF Alloy 400 are also indicated by the stress exponent. Whi le the bulk material crept at 650 °C has n = 5.7, the