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

712

5

(111)γ spot suggests that the microtwins nucleated at the RA boundary and did not travel across the entire length but terminated in the interior. Figure 5 (a) and (d) representing the bright field images for NSB_350 Paris law regime specimen display the deformation twins clearly. Unlike NSB_250, microtwins are not observed in the NSB_350 specimen. Figure 6 reveals the presence of α/α’ phase in NSB_350 FCGR tested specimen. Figure 7 reveals the high resolution (HR) TEM image obtained in the case of NSB_350 FCGR tested specimen which shows the atomic arrangement along (111) plane of austenite corresponding to the inter-planar spacing ‘d’ of 0.205 nm. Fast Fourier transformation (FFT) electron diffraction pattern (EDP) has been shown corresponding to the region marked by blue dotted boundary. NSB_350 specimen displayed higher ΔK Th and longer stable crack propagation regime as compared to NSB_250 specimen, Kumar and Singh (2019). This was attributed to the presence of higher RA content in NSB_350 specimen since ductile RA phase absorbs the input mechanical energy upon interaction with the crack tip to transform into martensite. This in turn, lowers the crack growth rate. The values are reported in Table 2, Kumar and Singh (2019).

Table 2. ΔK Th and Paris Law coefficients for NSB_250 and NSB_350 specimen (Kumar and Singh (2019)) Specimen ΔK Th (MPa m -1/2 ) C m NSB_250 8.75 9.83×10 −10 4.08 NSB_350 10 1.95×10 −9 3.64

Fig 4 (a) Bright field image for NSB_250 captured using TEM in the region corresponding to stage II of FCGR test; (b) SAED of the corresponding bright field image; (c) Dark field image corresponding to the spot marked by white arrow; (d) Bright field image for NSB_250 captured using TEM in another region showing the twin formation in austenite; (e) corresponding SAED pattern of the twinned austenite; (f) Dark field image corresponding to spot marked using white arrow The results from this study show that in addition to the transformation of RA into martensite upon deformation, other factors like twinning of RA also absorb the input strain energy, thereby resisting the crack growth and the same has been observed in both NSB_250 and NSB_350. Deformation twins along with the martensitic transformation, resist the crack tip and stabilise the crack growth. Finer RA present in NSB_250 specimen does not undergo extensive twinning and displays the presence of micro-twins which terminate without traversing the entire width of RA. A reduced carbon content decreases the stacking fault energy (SFE) values, Abbasi et al. (2011); Blinov et al. (2022); Lee et al. (2012); Seol et al. (2013) and can change the dominant deformation mode from dislocation plasticity to twinning to martensitic transformation. Lower carbon concentration in γ phase, as deduced from XRD data, has been observed in the case of NSB_350 specimen as compared to NSB_250. This may be the reason behind extensive martensitic transformation and twinning in NSB_350 as compared to NSB_250 specimen, which displayed dislocation pile ups and microtwins as the deformation progressed. This is also manifested in increased hardness close to the crack surface as observed by Kumar and Singh (2020)b.