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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1. Introduction Fatigue crack initiation in metals is primarily associated with the localization of cyclic plastic strain, typically manifested by the formation of persistent slip markings (PSM) on the specimen surface (Polák, 2023). These surface relief features arise from the underlying dislocation structures known as persistent slip bands (PSBs) and are widely recognized as one of the primary origins of fatigue cracks in pure metals and single crystals (Essmann et al., 1981; Mughrabi, 1983; Polák, 1991). In addition to PSMs, fatigue cracks can also nucleate at inclusions (Tanaka and Mura, 1982), grain boundary (GB) (Sangid et al., 2011; Tanaka and Iizuka, 1985), and, in some cases, at twin boundaries (Blochwitz and Tirschler, 2005; Li et al., 2023). Inclusions in commercial alloys frequently serve as fatigue crack initiation sites, not only due to their tendency to be separated from the matrix but also because they create localized stress concentrators or surface notches (Pineau and Antolovich, 2016; Tanaka and Mura, 1982). GB cracking, on the other hand, is associated with high-temperature fatigue, where wedge-type cracks and intergranular cavities form through creep mechanisms, as proposed by Tanaka (Tanaka and Iizuka, 1985). Fatigue crack nucleation is not always governed by a single dominant mechanism. Instead, it typically results from the competitive interplay of multiple contributing factors. Early theoretical work by Tanaka (Tanaka and Mura, 1981) and subsequent research performed by Mughrabi (Mughrabi, 1983) emphasized the importance of PSB interactions with grain boundaries, where dislocation pile-ups at PSB – GB interfaces generate localized stress concentrations that facilitate intergranular crack initiation. Christ (Christ, 1989) later expanded this concept by developing a semiquantitative model describing crack initiation at PSB – GB intersections in fatigued FCC metals. The model proposes that edge dislocations accumulate at the PSB – matrix interface and are obstructed by neighboring GBs, leading to crack formation. Theoretical analysis demonstrated that specific PSB – GB orientations are more susceptible to cracking. Grain size was later shown to play a crucial role in the development of intergranular cracking mechanisms (Liang and Laird, 1989a). This finding aligned well with the conclusions of Thompson (Thompson, 1972), who observed that larger grain sizes and lower stacking fault energies lead to more pronounced dislocation pile-ups and increased secondary slip, both of which promote stress concentration and facilitate crack initiation. Liang (Liang and Laird, 1989b) also proposed that repeated PSB impingement can accumulate local plastic deformation in a stepwise manner, gradually producing an intergranular crack. However, Mazánová (Mazánová et al., 2022) introduced an alternative view centered on extrusion/intrusion-driven GB cracking. In Sanicro 25 steel, extrusions were observed to grow in orientations normal to GBs, gradually distorting the surrounding microstructure and pushing neighboring grains away, while intrusions gave rise to void-like defects. The combined action of PSMs and with local discontinuities results in the formation of GB cracks. Additional observations by Chlupová (Chlupová et al., 2022) and Polák (Polák et al., 2024a, 2024b, 2024c) confirmed that these interactions are not confined to specimen surface features alone. PSBs were also found to penetrate onto fracture surface. Polák (Polák et al., 2025) identified PSB-GB interactions on intergranular facets as a distinguishing feature of stage II fatigue fracture, contrasting with striations typically associated with transgranular propagation. Following the abovementioned work, the facets resulting from intercrystalline cracking of Sanicro 25 are revealed and analyzed. Although the significance of PSMs in intergranular fatigue cracking has been documented in several face centered cubic (FCC) systems (Larrouy et al., 2015; Li et al., 2023; Polák et al., 2024b), much less is known about their role in body-centered cubic (BCC) materials, particularly in oxide dispersion strengthened (ODS) alloys. ODS alloys, such as Fe-10Al-4Cr- 4Y₂O₃ (FeAlOY), are designed for exceptional high-temperature strength and oxidation resistance through the incorporation of highly stable oxide nanoparticles. The mechanical performance of ODS alloys, particularly their fatigue behavior is closely tied to GB stability and dislocation activity in the vicinity of these boundaries (Gamanov et al., 2023; Šulák et al., 2024; Svoboda et al., 2023) . Yet, despite their technological relevance, the interactions between cyclic slip bands and large GBs in these complex, high-strength materials have received limited attention. In this article. we aim to investigate existing research gap and provide a comprehensive understanding of intergranular fatigue crack nucleation mechanisms in FCC and BCC metals

Nomenclature BCC

Body centered cubic