PSI - Issue 68

Marwa Ben Bettaieb et al. / Procedia Structural Integrity 68 (2025) 297–302 M. Ben Bettaieb et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 4. SEM-BSE observations of longitudinal specimens of the damaged parts of the (a) Nb-free material; (b) Nb-rich alloy tested at 575 °C/310 MPa.

These analyses were performed with an acceleration voltage at 20 kV and a step size of 2 µm for both materials. Post-processing is done on the OIM Software. It is found that the grains of the Nb-free material are more deformed than those of the Nb-rich alloy, which is consistent with the respective macroscopic creep ductility. The crack initiation in the Nb-free specimen is intergranular followed by a transgranular crack propagation. On the other hand, the crack initiation and propagation are both intergranular for the Nb-rich case, as revealed by the cracks following the grain boundaries specifically that perpendicular to the loading direction. 4. Microstructural evolution Magnified SEM micrographs of the damaged parts of the two specimens are presented in Fig. 6. Intergranular precipitates are highlighted in the Nb-free and the Nb-rich materials. For both materials, cavities nucleate from these intergranular precipitates. Intragranular precipitates are also observed for the Nb-rich alloy. According to (PADILHA et al., 2007), the intergranular precipitates in the Nb-free alloy should be M 23 C 6 carbides based on the creep conditions. This phase has an fcc structure and is the first phase that forms in austenitic stainless steels during aging at 550-600 °C because of its easy nucleation (PADILHA et al., 2007; SOURMAIL, 2001). Precipitates observed after creep test in the Nb-rich material are not characterized here but could be the niobium carbonitrides already observed at the as received state or a new phase. FEG-SEM observations on the heads of specimens, considered as non-strained parts yet thermal-aged ones, are also performed.

Fig. 5. SEM-EBSD observations of longitudinal specimens of the damaged parts of the (a) Nb-free material; (b) Nb-rich alloy tested at 575 °C/310 MPa.