PSI - Issue 80

Tomáš Vražina et al. / Procedia Structural Integrity 80 (2026) 244 – 255 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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images of a TEM foil prepared from Sanicro 25. The PSBs appear as thin and sharply defined structures, interacting directly with GBs and adjacent carbides.

Fig. 7 (a-c) STEM micrographs of FeAlOY (0.35%) with detail on PSBs, (d-e) STEM micrographs of PSBs in different grains obtained from Sanicro 25 (0.45%) sample.

4. Discussion The Sanicro 25 and FeAlOY, both Fe-based alloys but differing in crystallography, chemical composition (Table 1) and phase composition (Fig. 2a-b) exhibited similitudes in fatigue crack growth mechanisms within the same strain amplitude range. The stage II of cyclically strained Sanicro 25 fracture surface was compromised by a mixture of intercrystalline and transcrystalline cracking. The PSMs identified on intercrystalline facets were typically thin and faint as shown in Fig. 6(e-f). Detailed examination of the underlying dislocation structures further confirmed the presence of thin PSBs (Fig. 7(e-f)) observable in STEM micrographs and potentially with HR-EBSD KAM maps Fig. 8(a-b). Fig. 8(c), together with the EBSD KAM map indicated that the cyclic plasticity mechanism extends throughout the cyclically strained volume, not just at the surface. These results further verify the mechanism proposed by Mazánová (Mazánová et al., 2022) arguing that PSBs contribute to the formation of extrusions that exert pressure on neighboring grains. However at higher strain amplitudes, surface features on intergranular facets changed and resemble more the striation-like feature Fig. 6d. This observation is consistent with work of Chlupová (Chlupová et al., 2022) performed on polycrystalline copper specimens, where dense PSMs initially identified on the specimen surface were later observed to extend onto the fracture surface in polycrystalline copper. Polák (Polák et al., 2024a, 2025.) further documented these dense structures on fracture surfaces, interpreting them as PSMs. Building upon these findings and accounting for distinct morphology of striation and PSMs the dense features observed in this study, in the intergranularly cracked areas are more accurately characterized as PSMs. In tested specimens of Sanicro 25, STEM micrographs revealed PSB-GB interactions, particularly near carbides, emphasizing these regions as potential initiation sites for intergranular cracking, a mechanism previously highlighted by Polák (Polák et al., 2024c) and Mu (Mu and Aubin, 2010) in their studies on cyclically strained FCC alloys. The results further support the notion that PSB-GB induced cyclic slip can locally concentrate plastic strain, particularly at sites where PSBs intersect GBs or secondary phases. And thus promote intercrystalline fatigue damage. Contrary to Sanicro 25 the FeAlOY fracture surface stage II cracking mode was mainly intercrystalline. The intercrystalline nature of cracking was verified based on comparison of irregular facets morphology like that in Fig.