PSI - Issue 68

Yuki Tampa et al. / Procedia Structural Integrity 68 (2025) 681–686 Y. Tampa et al. / Structural Integrity Procedia 00 (2025) 000–000

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specimen (Figs. 6d and 6e). The fracture surface of FCV-O was entirely covered with cleavage features (Fig. 6f). These findings suggest that the V segregation at grain boundaries (Fig. 1) prevents the intergranular cracking and plastic deformation accommodate the local stress concentration. By contrast, the FCV-D specimen exhibited typical ductile fracture features (Figs. 6g–6i). 3.3. Effect of plastic constraint by neighbouring grains on the brittle fracture For the ordered-phase Permendur alloys, the single-crystalline specimen exhibited bending due to shear banding under uniaxial tensile loading, whereas in the poly-crystalline specimen, brittle fractures such as intergranular and cleavage cracking occurred. To clarify the effect of plastic constraint by neighbouring grains on the brittle fracture, we prepared two types of multi-grains specimens on the micro-scale. Figure 7 shows the SEM images overlapped with EBSD maps of micro multi-grains specimens in the initial state and corresponding nominal stress–nominal strain curves. The FC-O specimen with gauge section dimensions of 150 μm × 60 μm × 30 μm had a so-called bamboo structure, as shown in Fig. 7a. The FCV-O specimen with gauge section dimensions of 150 μm × 60 μm × 60 μm had more than two grains in the transverse direction of specimen (Fig. 7b). The FC-O specimen exhibited moderate strain hardening after the onset of yielding, whereas in the FCV-O specimen, fracture occurred after rapid strain hardening. Figure 8 shows the deformation and fracture morphologies of micro multi-grains specimens. Similar to the single-crystalline specimens, the FC-O specimen with the bamboo structure was deformed by shear banding, which resulted in specimen bending (Figs. 8a and 8b). By contrast, in the FCV-O specimen, having plastic constraint by the neighbouring grains, cleavage fracture occurred at the shoulder of specimen without bending (Figs. 8c–8d). These findings suggest that the plastic constraint by the neighbouring grains inhibits the deformation with multiple slip systems, which increases the local stress to the cleavage fracture stress.

Fig. 7 SEM images overlapped with EBSD maps of micro multi-grains specimens in the initial state (a–b) and nominal stress– nominal strain curves obtained for micro multi-grains specimens (c).

Fig. 8 Optical micrographs captured in the middle of deformation processes (a and c) and fractography SEM micrographs (c and d).