Visualization of dislocations through electron channeling contrast imaging at fatigue crack tip, interacting with pre-existing dislocations

Habib, Kishan; Koyama, Motomichi; Tsuchiyama, Toshihiro; Nishikawa, Hiroshi

doi:10.1080/21663831.2017.1392370

Cited by 19 publications

(5 citation statements)

References 22 publications

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“…In addition, the target area shows a black contrast, which indicates low backscatter electron intensity. That is, the channeling condition was nearly satisfied in the grain interior [ 1 , 3 , 27 ]. In Figure 3(c ), locally different contrast along the (111) plane was observed, which was perhaps due to the residual stress resulting from the pre-strain-induced dislocation motion on the (111) plane and subsequent unloading.…”

Section: Resultsmentioning

confidence: 99%

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Stacking fault aggregation during cooling composing FCC–HCP martensitic transformation revealed by in-situ electron channeling contrast imaging in an Fe-high Mn alloy

Koyama

Seo

Nakafuji

et al. 2021

Science and Technology of Advanced Materials

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To understand the mechanism of FCC–HCP martensitic transformation, we applied electron channeling contrast imaging under cooling to −51°C and subsequent heating to 150°C. The stacking faults were randomly extended and aggregated during cooling. The stacking fault aggregates were indexed as HCP. Furthermore, the shrink of stacking faults due to reverse motion of Shockley partials was observed during heating, but some SFs remained even after heating to the finishing temperature for reverse transformation (A f : 104°C). This fact implies that the chemical driving force of the FCC/HCP phases does not contribute to the motion of a single SF but works for group motion of stacking faults.

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Section: Resultsmentioning

confidence: 99%

“…Electron channeling contrast imaging (ECCI) enables observations of dislocations and stacking faults [1][2][3]. Therefore, the ECCI technique has been used for the characterization of dislocation-driven phenomena such as the plastic deformation of metals [4,5].…”

Section: Introductionmentioning

confidence: 99%

Stacking fault aggregation during cooling composing FCC–HCP martensitic transformation revealed by in-situ electron channeling contrast imaging in an Fe-high Mn alloy

Koyama

Seo

Nakafuji

et al. 2021

Science and Technology of Advanced Materials

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“…The resulting images look similar as the micrographs from dark-field imaging in transmission electron microscopy (TEM). This technique has shown its capability in studying HE related deformation information in a variety of materials with satisfying results [25,[45][46][47][48][49]. The theoretical and practical details of the ECCI technique can be found in Ref [50].…”

Section: Characterizationmentioning

confidence: 98%

Hydrogen-enhanced fatigue crack growth behaviors in a ferritic Fe-3wt%Si steel studied by fractography and dislocation structure analysis

Wan

Alvaro

Olden

et al. 2019

International Journal of Hydrogen Energy

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The effect of hydrogen (H) on the fatigue behavior is of significant importance for metallic structures. In this study, the hydrogen-enhanced fatigue crack growth rate (FCGR) tests on in-situ electrochemically H-charged ferritic Fe-3wt%Si steel with coarse grain size were conducted. Results showed strong difference between the H-charged and the non-charged conditions (reference test in laboratory air) and were in good agreement with the results from literature. With H-charging, the fracture morphology changed from transgranular (TG) type to "quasi-cleavage" ("QC"), with a different fraction depending on the loading frequency. With the help of electron channeling contrast imaging (ECCI) inside a scanning electron microscope (SEM), a relatively large area in the failed bulk specimen could be easily observed with high-resolution down to dislocation level. In this work, the dislocation sub-structure immediately under the fracture surfaces were investigated by ECCI to depict the difference in the plasticity evolution during fatigue crack growth (FCG). Based on the analysis, the H-enhanced FCG mechanisms were discussed.

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“…Electron channeling contrast imaging (ECCI) has recently been used for dislocation-resolved microstructure observation. − Because the ECCI technique is applicable to bulk specimens, microstructures at a specific site, such as the crack tip region, can be easily observed. , Furthermore, because the specimen is a bulk specimen, in situ observation of dislocation motion under specific mechanical loading conditions can be performed. , Although the spatial resolution of ECCI is worse than that of transmission electron microscopy, it is sufficiently high to capture/characterize dislocation motion and associated microstructure evolution. Figure shows the in situ ECCI results obtained near the fatigue crack tip in an austenitic steel.…”

Section: Damage-specific Microstructure Characterizationmentioning

confidence: 99%

Quantification and Characterization of Microdamage Resistance in Metals for Designing High-Strength Ductile Microstructures

Koyama

2021

Acc. Mater. Res.

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Conspectus Fractures in metallic materials such as ductile austenitic, ferritic, and dual-phase steels often occur after significant plastic deformation. The dislocation-driven plasticity enhances the microscopic stress concentration and induces vacancies (e.g., by the jogged screw dislocation motion), resulting in microstructure-scale cracks and voids at specific crystallographic planes or microstructure boundaries. (Hereafter, cracks and voids are referred to as damage.) In addition, plasticity plays important roles in microscopic damage growth in terms of damage tip blunting and coalescence. In fact, plasticity-related damage evolution in various fracture phenomena such as metal fatigue and hydrogen embrittlement still contains many uncertainties, particularly in high-strength materials containing fine and complex microstructures. Therefore, microscopy-based damage quantification and characterization are required for designing damage-resistant microstructures. In this context, because damage evolution is a multiscale phenomenon from atomistic to over millimeter scales, the target size of damage in the measurements is important to realize reliable analyses. The intrinsic origin of damage evolution is an atomistic/nanometer scale phenomenon, which requires transmission electron microscopy for its analysis. However, from mechanical viewpoints, revealing damage nucleation behavior is not necessary, because general mechanical analyses are performed for oversubmicrometer-sized damages that can be observed by optical microscopy and scanning electron microscopy. Hence, submicrometer/micrometer-sized damage is the major target in this damage quantification. As a post-mortem analysis method, the damage area fraction, the number of damages, the damage size, and the damage shape at various plastic strains can be quantified by observing micrometer-sized damages. In the case of monotonic tensile deformation, the number density of damages plotted against strain indicate the damage initiation probability. In addition, when damages stop growing, a strain range where the average damage size remains nearly constant appears. The strain range quantitatively indicates damage arrestability. For instance, dual-phase steel consisting of soft and hard body-centered cubic phases clearly shows three strain-dependent damage evolution regimes: (1) damage incubation regime (no damage appears), (2) damage arrest regime (damage initiates, but stops growing immediately), and (3) damage growth regime. Hydrogen uptake increases damage initiation probability and decreases damage arrestability. Specifically, a comparison of the quantified damage parameters between specimens with and without hydrogen revealed that large amounts of hydrogen in the dual-phase steel degrades the resistance to microscopic damage growth, which critically decreases the macroscopic ductility. In this case, the damage arrestability (damage growth resistance) is more important than crack initiation resistance, which is dependent on the dislocation-driven stress accommodatio...

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Visualization of dislocations through electron channeling contrast imaging at fatigue crack tip, interacting with pre-existing dislocations

Cited by 19 publications

References 22 publications

Stacking fault aggregation during cooling composing FCC–HCP martensitic transformation revealed by in-situ electron channeling contrast imaging in an Fe-high Mn alloy

Stacking fault aggregation during cooling composing FCC–HCP martensitic transformation revealed by in-situ electron channeling contrast imaging in an Fe-high Mn alloy

Hydrogen-enhanced fatigue crack growth behaviors in a ferritic Fe-3wt%Si steel studied by fractography and dislocation structure analysis

Quantification and Characterization of Microdamage Resistance in Metals for Designing High-Strength Ductile Microstructures

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