PSI - Issue 68

Bhawesh Chhajed et al. / Procedia Structural Integrity 68 (2025) 708–714 Bhawesh Chhajed et al. / Structural Integrity Procedia 00 (2025) 000–000

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have been used to mark BF, FA and BA type RA in Figure 1. It can be clearly observed that NSB_250 displays finer microstructure as compared to NSB_350 specimen. Several SEM images have been used to measure the bainitic lath thickness using linear intercept method and multiplying the average bainitic lath thickness by a factor of 2/ , which is the stereological correction, Garcia-Mateo et al. (2016); Kumar and Singh (2019); van Bohemen (2018). NSB_250 specimen has an average bainitic lath thickness of 37 ± 9 nm, while NSB_350 specimen displays an average bainitic lath thickness of 101 ± 44 nm as reported in Kumar and Singh (2019).

Fig 1. SEM image for (a) NSB_250 and (b) NSB_350

Table 1 Values of average bainitic lath thickness (Tα) and RA thickness (Tγ) calculated from SEM images; volume fraction of BF (Vα) and RA (Vγ) determined using XRD [12] Specimen T α (nm) T γ (nm) V α V γ C γ (wt%) NSB_250 37 ± 9 22 ± 7 81.6 18.4 1.87 NSB_350 101 ± 44 177 ± 31 51.1 48.9 1.52 3.2 XRD XRD data has been analyzed and the integrated intensities obtained from corresponding peaks of ferrite and austenite were used to calculate the volume fraction of BF and RA phase. As reported in Table 1, Kumar and Singh (2019), NSB_250 specimen shows a higher BF volume fraction and lower RA content as compared to NSB_350 TEM micrographs of undeformed specimens in bright-field mode along with SAED are shown in Figure 2. TEM micrographs revealed that bainitic ferrite was present in higher proportions in case of NSB_250 as compared to NSB_350 while higher proportions of RA were present in NSB_350 as compared to NSB_250. These results are well in accordance with XRD results discussed in section 3.2. High-angle annular dark field (HAADF) images of NSB_250 were captured in undeformed condition and shown in Figure 3. EDS mapping done for the same region revealed the presence of Fe, Co and Mn in larger proportions in regions appearing brighter in HAADF image (marked by yellow dotted boundary). Some regions marked by red dotted boundary represent slightly less bright features which consist of Fe as the primary constituent. specimen. 3.3 TEM