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The oxidation behavior of grain boundaries (GBs) was investigated on Alloy 690 after exposure to simulated pressurized water reactor primary water. The tendency to form passive film above GB is positively related to Cr diffusivity along GB which is determined by GB structure. Highly-ordered GB like twin boundary is subject to penetrative oxidation as Cr diffusivity is very low. GBs with misorientation angle θ ≤17.4° can't support fast diffusion of Cr either and also show penetrative oxidation. When θ ranges between 17.4° and 28.7°, Cr diffusivity is enhanced a little and GB shows unstable passivation. Meanwhile, the Cr diffusivity along GB is also related to the atomic packing density of GB planes. GB with higher atom packing density exhibits lower Cr diffusivity and deeper intergranular oxide depth. As θ increases above 28.7°, Cr diffusion is fast enough to support the formation of protective film above GB. [Display omitted] [ABSTRACT FROM AUTHOR]

A numerical calculation method based on the angle in a triple junction composed of a random grain boundary is proposed to predict the connectivity and stability of a grain boundary in a B10 copper-nickel alloy. The grain-boundary connectivity and its effect on corrosion resistance are studied combining computer-aided analysis with electrochemical impedance testing. The results show that the prediction of corrosion resistance using a grain-boundary connectivity numerical method is consistent with immersion experimental results. The B10 alloy exhibited the best corrosion resistance after cold rolling with a 9% reduction rate. The relationship between the grain-boundary characteristics and corrosion resistance is well established using the numerical method to quantify the grain-boundary connectivity. A higher connective frequency and a low proportion of grain-boundary angles between 60° and 180° in the triple junction is detrimental to corrosion resistance of the B10 alloy. [ABSTRACT FROM AUTHOR]

We investigated the connectivity of high-energy random grain boundaries through fractal analyses of specimens with different grain boundary (GB) microstructures in BFe10-1-1 copper–nickel alloy. It was found that the profile of maximum random boundary network possesses a fractal nature and more than one fractal dimension can exist. The fraction of special boundaries and grain size homogeneity can play an important role on GB character distribution. Here, GB microstructures are combined with quantitative materials structure–property relationship models to predict intergranular corrosion properties. The experimental results are accurately consistent with the theoretical predictions. [ABSTRACT FROM AUTHOR]

The intergranular degradation of seven different types of high-angle grain boundaries (HAGBs) were investigated on Alloy 600 after exposure to simulated pressurized water reactor primary water. All boundaries are susceptible to preferential intergranular oxidation (PIO) except for ideal coherent twin boundary. Diffusion induced grain boundary migration (DIGM) normally occurs and its depth is positively correlated with the PIO extent. Interestingly, the PIO susceptibility is independent on the grain boundary misorientation angle or Σ value, but related to the grain boundary atom packing density (GBAPD). Grain boundaries with higher GBAPD values show higher PIO resistance as the element diffusion is slower. • The full grain boundary (GB) structure parameters were characterized. • All boundaries are susceptible to oxidation except for ideal coherent twin boundary. • Diffusion induced GB migration depth is positively correlated with oxidation depth. • Intergranular oxidation susceptibility is related to the GB atom packing density. [ABSTRACT FROM AUTHOR]

The grain boundary (GB) corrosion behavior of Ni15Cr alloy in 650℃/1bar CO 2 gas was investigated. The oxidation behavior of coherent twin boundary is similar to that of grain matrix due to the immobility of Cr. However, the incoherent twin boundary is prone to preferential intergranular oxidation due to the limited Cr diffusivity and the ingress of oxygen. Likewise, low angle GB is also susceptible to intergranular oxidation because Cr diffusion is not fast enough although it can induce GB migration and result in a protective scale at oxide tip. In contrast, random high angle GB is immune to intergranular oxidation because Cr can diffuse quickly to form a protective scale above GB with the formation of a large GB migration zone. Therefore, the oxidation behavior of GB is closely related to the diffusivity of Cr along GB which is determined by the GB structure. [Display omitted] [ABSTRACT FROM AUTHOR]

Irradiation-assisted stress corrosion cracks (IASCCs) from a proton-irradiated 316 L stainless steel (2.5 dpa) after constant load test in simulated PWR primary water were analyzed. The irradiation damages are consistent with those from neutron irradiations at similar dose. The IASCC cracks show microstructural and microchemical features similar to those reported from neutron-irradiated samples, confirming that proton is effective in emulating neutron for IASCC study. Si segregated at grain boundary gets oxidized preferentially beyond the crack tip and then dissolves into solution, resulting in enhanced porosity within the intergranular oxide. The Si effect is an important factor behind IASCC in reducing environment. • Proton irradiation induces similar IASCC features in SS as neutron irradiation. • Intergranular oxidation is a necessary precursor of IASCC of SS at low doses. • Si tends to get oxidized preferentially and dissolves during intergranular oxidation. • The Si effect is likely a key element of IASCC mechanism for stainless steel. [ABSTRACT FROM AUTHOR]

The preferential intergranular oxidation (PIO) behavior of low-angle grain boundaries (LAGBs) with misorientation angle θ ranging from 5.7 to 14.0° on alloy 600 were investigated after exposure to simulated pressurized water reactor primary water. Interestingly, all LAGBs are susceptible to PIO. When θ <8.8°, diffusion-induced grain boundary migration (DIGM) does not occur as Cr is immobile and the PIO depth varies slightly among those LAGBs. As θ increases above 8.8°, Cr starts to diffuse outwards and DIGM occurs. The depths of PIO and DIGM are positively correlated in this case. Also, the PIO depth is not related to the change of θ , but instead poses an inverse relationship with the atom packing density of GB planes. [Display omitted] [ABSTRACT FROM AUTHOR]

A novel approach using focused ion beam (FIB) and electron backscatter diffraction (EBSD) or transmission Kikuchi diffraction (TKD) was developed to characterize all five parameters of the individual grain boundary. With this approach, the grain orientations and the grain boundary trace angles on two perpendicular surfaces can be measured. Then the grain boundary plane normal in the sample coordinate system can be determined and transformed to plane indices in the two crystal coordinate systems. FIB-TKD is a viable alternative to FIB-EBSD when EBSD measurement can't be conducted on the sample surface which is degraded by oxidation or corrosion. This approach was verified on a coherent twin boundary in alloy 690. The accuracy of the present FIB-EBSD method is better than 3°. This approach provides a convenient and efficient solution for measuring all five parameters of a site-specific grain boundary during FIB sample preparation. [Display omitted] • The grain boundary character can be investigated by focused ion beam (FIB) and electron backscatter diffraction (EBSD) or transmission Kikuchi diffraction (TKD). • The measurement accuracy of the FIB-EBSD method is better than 3º. • The FIB-TKD method provides a feasible way to index grain boundary plane when the surface of sample cannot meet the conventional EBSD test requirement. [ABSTRACT FROM AUTHOR]

• The acceleration mechanism in oxidation kinetics ahead of crack tip was clarified. • Compact chromia forms along identical carbide/matrix interfaces at different rates. • Stress concentration near the crack tip only slightly increases the oxidation rate. • Deformation localization near the crack greatly promotes oxygen diffusivity. The acceleration in oxidation kinetics beyond a stress corrosion crack of alloy 690 in simulated PWR primary water was clarified by comparing the oxide depths at two identical carbide/matrix interfaces along a single grain boundary. Compact chromia layer formed along these two interfaces at vastly different rates. Stress concentration ahead of the crack tip only slightly increased the oxidation rate as the crack is still very shallow. Deformation localization near the crack plays a dominant role in accelerating oxidation kinetics as the associated high vacancy concentration can promote the diffusivity of oxygen by over an order of magnitude. [ABSTRACT FROM AUTHOR]