Plastic deformation sequence and strain gradient characteristics of hydrogen-induced delayed crack propagation in single-crystalline Fe–Si alloy

doi:10.1016/j.scriptamat.2019.11.012

Scripta Materialia

Volume 178, 15 March 2020, Pages 99-103

https://doi.org/10.1016/j.scriptamat.2019.11.012 Get rights and content

Abstract

Microscopic features of hydrogen-induced delayed crack propagation in a thin sheet of single-crystalline Fe-3wt%Si alloy were investigated. The crack tip plastic deformation associated with the effect of hydrogen during the crack propagation leaves three adjacent regions where different plastic strain gradients and dislocation densities are observed. These regions revealed the effects of plastic deformation and hydrogen-dislocations interaction around the crack tip on the rate-limiting process of hydrogen-induced delayed crack propagation in thin specimens.

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Acknowledgments

This work was financially supported by JSPS KAKENHI (JP16H06365) and T.T.H. was funded by the Can Tho University Improvement Project VN14-P6, supported by a Japanese ODA loan.

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Relationship between mechanical response and microscopic crack propagation behavior of hydrogen-related intergranular fracture in as-quenched martensitic steel
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Citation Excerpt :
Takahashi et al. [24] studied hydrogen-related fatigue crack propagation in a thin sheet of a single-crystalline Fe–Si alloy and reported that the crack opening displacement (200 μm behind the crack tip) was independent of the crack length. Further, based on the results reported by Takahashi et al. [24], Huynh et al. [25] proposed that the rate-limiting stage was the hydrogen accumulation around the crack tip. These studies showed that crack opening displacement was an important mechanical parameter for evaluating the relationship between the mechanical response and crack propagation behavior in hydrogen embrittlement.
The present study investigated the relationship between the mechanical response and microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steel. In contrast to cracks in the uncharged specimen, the cracks in the hydrogen-charged specimen propagated under a small crack opening displacement. Crack tip bluntings were frequently observed in the uncharged specimen, whereas no obvious blunting of the tip was observed in the hydrogen-related intergranular cracks. In addition, a high strain was localized around the hydrogen-related intergranular crack tip. The results indicate that strain localization can induce the formation of new quasi-cleavage cracks inside prior austenite grains ahead of the existing crack tip. The hydrogen-enhanced decohesion at prior austenite grain boundaries and the quasi-cleavage crack formation/propagation ahead of the arrested intergranular crack result in hydrogen-related crack propagation with a small crack opening displacement.
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Subsequently, the plastic strain distribution was analyzed by electron backscattering diffraction (EBSD) at 15 kV with a beam step size of 100 nm, and the dislocation structures were investigated by electron channeling contrast imaging (ECCI) at an accelerating voltage of 30 kV. To compare the material behaviors between air and hydrogen environments, a specimen broken in hydrogen pressure (580 kPa) under the sustained load (reported previously [19,20]) was further analyzed in greater detail. Fig. 4 shows the plot of half crack length (a) with time.
Effects of hydrogen on macroscopic and microscopic features of crack growth in thin specimens were investigated using a thin sheet of single-crystal Fe-3wt%Si alloy. Center-cracked specimens were tested under a sustained load in a hydrogen environment, and under continuous stretching in an air environment. The fracture features were compared to elucidate the role of hydrogen in hydrogen-induced delayed crack growth. In both air and hydrogen environments, the crack growth mode of the thin specimens was the same as that of the thick specimens, despite the significantly reduced thickness. Surprisingly, the crack grew discontinuously, and left striations on the fracture surface, in which shorter striation spacing was observed in hydrogen. In addition, there was a similarity in the deformation microstructures beneath the fracture surface, that is, both microstructures were composed of three distinct layers characterized by different plastic strain gradients and dislocation densities. In the hydrogen environment, the hydrogen-enhanced localized plasticity (HELP) mechanism is believed to be relevant to the crack growth process. Also, HELP was supposed to cause different characteristics (the magnitude of plastic strain, the plastic strain gradient, and dislocation structure) of the three layers in the hydrogen compared to those in the air. Reverse plastic deformation occurred in the regions behind the crack front during crack growth, which is speculated to contribute not only to enlarge the crack tip opening angle (CTOA) but also to blunt the crack tip.
Microscopic examination of striation spacing during ductile crack growth in Fe-3wt%Si single-crystalline thin plates in air and hydrogen
2021, Materials Science and Engineering: A
Citation Excerpt :
Moreover, based on the concept of fracture mechanics, the crack tip stresses and strains increase during the crack growth, the amount of crack growth should be increased as the crack lengthens. In contrast, Huynh et al. showed that the unit distance of crack extension (striation spacing) is independent of the crack length [14,15]. To reconcile this discrepancy, it is useful to identify aspects related to discontinuous crack growth and the independence of striation spacing from crack length.
Microscopic features of crack growth in thin plates of single-crystalline Fe-3wt%Si alloy in both air and hydrogen environments were investigated by electron channeling contrast imaging (ECCI), electron backscattering diffraction (EBSD), as well as scanning electron microscopy fractography. The goal was to elucidate the discontinuous crack growth as well as the constant unit distance of crack extension (striation spacing). Center-cracked specimens were tested under a sustained load in a hydrogen environment, while they were under continuous stretching in the air environment. The following results were obtained. (1) Striation is formed by extensive slips emitted from the crack tip, mainly contributed from specific ( $1 \overline{1} 2$ )[ $\overline{1} 11$ ] and ( $\overline{1} 12$ )[ $\overline{1} 1 \overline{1}$ ] slip systems. The discontinuous crack growth is mainly caused by interaction of the crack and ( $1 \overline{1} 2$ )[ $\overline{1} 11$ ] and ( $\overline{1} 12$ )[ $\overline{1} 1 \overline{1}$ ] slip bands/cell walls formed ahead of the crack tip. These slip bands/cells show that the spacing between slip bands/cells is constant and independent of the crack length. Hence, the striation spacing is the same as that of the slip bands/cells ahead of the crack tip. (2) Hydrogen may affect the slip behavior by reducing the spacing between slip bands/cells ahead of the crack tip compared to that in the air environment.
Crack growth behavior in air and hydrogen of iron-3% silicon single-crystal thin sheet
2023, AIP Conference Proceedings

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Plastic deformation sequence and strain gradient characteristics of hydrogen-induced delayed crack propagation in single-crystalline Fe–Si alloy

Abstract

Graphical abstract

Section snippets

Acknowledgments

Acta Mater.

Acta Metall. Mater.

Eng. Frac. Mech.

Scri. Mater.

Mater. Charac.

J. Mech. Phys. Sol.

Acta Metall.

Mater. Charac.

Nucl. Eng. Des.

Acta Mater.

J. Mech. Phys. Sol.

Eng. Frac. Mech.