PSI - Issue 52

D. Kujawski et al. / Procedia Structural Integrity 52 (2024) 293–308 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 6 A lack of or minimal dependency of ΔK th threshold with R-ratio in vacuum for several engineering alloys, Vasudevan et al. (2005).

3.2 On oxide-induced crack closure [Vasudevan et al. (2022b)] Recently, we reviewed the chemical reactions that occur at the tip of a fatigue crack in Al and Fe systems exposed to humid air and aqueous solutions under ambient conditions, Vasudevan et al. (2022b). This study carefully reviewed literature on the reactions, intermediate phases, and kinetics for the transition of metal ions into hard phases and considered in recent publications. Here we will briefly summarize the findings of that study and their impact on OICC, Vasudevan et al. (2022b). When a bare metallic alloy is exposed to an environment at ambient pressures (gaseous or aqueous), an adsorbed layer of environmental species forms quickly Roth (1976). In non-inert environments, this adsorbed layer will contain oxidizing species that are readily reduced by accepting electrons from active constituents of the alloy to form chemisorbed and reaction product layers Oudar (1995), Nguyen et al. (2018). Oxides nucleate rapidly and spread laterally to form a continuous thin film. This results in a diffusion couple with cations being generated on the metal side of the insulating oxide and anions on the environment side. For oxide growth to occur, electrons, and at least one of the ion types, must transport through the insulating oxide. Mott (1939), (1940), (1947) examined this situation and proposed that oxide growth would arrest at a thickness determined by the tunneling limit for electrons Cabrera (2010). Cabrera and Mott (1949) presented gas phase measurements supporting this model and showed that the limiting thickness for pure Al was ≈ 1.8 nm at 20 °C. Furthermore, they showed that the oxide thickness remained at this value