PSI - Issue 68

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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. 2. Creep curves at 575 °C/310 MPa of the Nb-free alloy and the Nb-rich material.

Large dimples suggest a mostly ductile transgranular fracture for the Nb-free alloy. On the other hand, the Nb-rich alloy exhibit faceted features, whose sizes are similar to that of the grains. This observation would suggest a predominantly ductile intergranular fracture for this specimen. In order to study the creep damage mechanisms further, longitudinal sections are cut out from the damaged part of the broken specimens, mounted in conductive resin and then polished until colloidal silica polishing. The damaged microstructures are observed using the Secondary Electrons (SE) and the BackScattered Electrons (BSE) methods of the ZEISS Sigma 300 SEM. Observations presented in Fig. 4 illustrate that the Nb-free alloy contains shorter and more opened cracks. This implies that damage nucleates on grain boundaries for the rolled plate but does subsequently not readily propagate. This allows substantial void growth (crack opening) from the damaged grain boundaries during plastic deformation. Electron BackScattered Diffraction (EBSD) analyses are performed to characterize the crack initiation in each specimen (Fig. 5).

a

b

Fig. 3. Fracture surfaces at different resolutions of the (a) Nb-free specimen; (b) Nb-rich alloy tested at 575 °C/310 MPa.