PSI - Issue 52

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

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automatically adjusts the applied force. Thus a constant stress is applied upon the specimen. The displacement is measured by a half-bridge induction coil sensor connected to a measuring amplifier. The induction sensor provides a measuring range of ± 10 mm. The grips are placed in a furnace with a Eurotherm 2416 electronic regulation with a controlling Ni-Cr or Pt-Rh thermocouple situated close to the specimen gauge length. The creep tests were carried out in tension in a protective argon atmosphere in the temperature range of 600- 880 °C and in the stress range of 30 -150 MPa. 3. Results and discussion 3.1. Microstructure The microstructure of bulk and LPBF Alloy 400 is presented in Fig. 2. At first glance, the differences in the microstructure brought about by different production methods are apparent. In the case of bulk Alloy 400, the equiaxed grains with the presence of twins (Fig. 2a) can be seen. No preferential orientation was identified (EBSD map in Fig. 2b). An average grain size of (63.3 ± 4 9 .1) µm was established using an equivalent circle diameter from EBSD data. The grain size distribution histogram can be seen in Fig. 2c. The microstructure of LPBF Alloy 400 is typical with comparably finer grains with an average grain size of (14.7 ± 10.1) µm. The grains are preferentially orientated along the 〈011〉 direction (Fig. 2f) in the building direction (Chlupová et al., 2023) . The microstructure exhibits a so-called honeycomb structure with melt-pools going through several grains (Fig. 2e). The dislocation arrangement as revealed by TEM (Fig. 2d and Fig. 2h) is also different for both material batches. In the bulk material, we can observe relatively low dislocation density with dislocations arranged in bands along the {111} planes (Fig. 2d). However, in the case of LPBF Alloy 400, a cell-like structure with dislocations densely allocated to the cell walls and relatively clean centre is formed (Fig. 2h), which is a typical manifestation of 3D printed materials (Liu et al., 2022), where multiple reflows create significant strain fields that in some ways resemble thermomechanical loading (Babinský et al., 2023) .

Fig. 2. The microstructure and grain size distribution of Alloy 400 a)-d) bulk; e)-h) LPBF.

3.2. Fatigue The S-N curves for both bulk and LPBF Alloy 400 are displayed in Fig. 3 in the representation of stress amplitude versus the number of cycles to failure. Due to the limited amount of material (especially the LPBF variant), only a few fatigue tests were performed at each temperature (each point corresponds to one experiment). However, even with this limitation, the results are representative. Two regions with different behaviour can be observed. As can be seen, from the S-N curves, the fatigue life of LPBF Alloy 400 is comparable with bulk Alloy 400 at 400 °C and 550 °C. However, a significant drop in the fatigue life of LPBF Alloy 400 compared to bulk Alloy 400 occurs when fatigue loading is applied at 650 °C and 750 °C. The explanation for this behaviour can be seen in both the grain size and the