Synergistic hydrogen embrittlement in high-strength steels

Significance

Hydrogen embrittlement (HE) poses a significant challenge to the development of safe and reliable infrastructure for the hydrogen economy. Despite extensive research, the fundamental mechanisms underlying hydrogen-induced degradation remain incompletely understood. Here, we combine in situ hydrogen charging experiments, first-principles calculations, and theoretical modeling to investigate HE mechanisms in high-strength steels. Our findings reveal a synergistic effect: Hydrogen and carbon solutes collectively suppress dislocation cross-slip at the microscale, leading to a significantly reduced work-hardening capacity at the macroscale. This integrated experimental-modeling framework not only advances the understanding of HE but also provides a tool to evaluate hydrogen’s interactions with other alloying elements. These insights facilitate the design of hydrogen-tolerant structural materials, enabling robust and durable components for hydrogen-powered systems.

Abstract

Hydrogen embrittlement (HE) remains a critical scientific challenge in building reliable infrastructure for a carbon-free hydrogen economy. Predictive models for hydrogen-induced material failure are still lacking, largely due to an incomplete understanding of hydrogen’s effects on deformation behavior, especially in multiphase alloys with complex compositions and microstructures. Here, we demonstrate a synergistic hydrogen embrittlement (SHE) phenomenon in high-strength martensitic steels, where hydrogen interacts with carbon in solution to activate hydrogen-enhanced localized plasticity (HELP). Microcantilever bending tests revealed greater hydrogen susceptibility with higher carbon content, evidenced by a significant reduction in work-hardening capacity, promoting slip localization and reduced ductility. First-principles calculations and theoretical modeling revealed that carbon intensifies hydrogen–dislocation interactions and amplifies hydrogen redistribution around screw dislocations, inhibiting cross-slip. This work integrates experimental and modeling approaches to elucidate the synergistic interactions between hydrogen and solute elements, providing critical insights for designing high-strength, hydrogen-tolerant structural materials.

The recent strong momentum toward a carbon-free green hydrogen economy (1) to combat climate change poses an urgent challenge for materials science and engineering: developing cost-effective, high-strength structural materials suitable for use in a hydrogen-rich environment. However, it is well known that structural materials, especially high-strength steels, experience severe degradation in mechanical properties due to hydrogen embrittlement (HE), a phenomenon discovered over a century ago, even though the underlying mechanisms are still not fully understood (2). A puzzling feature of HE is that, despite its macroscopic embrittlement effects, hydrogen induces extensive plasticity at the microscale (3–10). Hydrogen-enhanced localized plasticity (HELP) (11, 12) is regarded as an early stage of HE, which helps create conditions for other embrittlement mechanisms to operate (13). Experimental observations (5–10) suggest that hydrogen can significantly modify dislocation behavior, including mobility, pile-up distance, and slip band formation. Simultaneously, numerical studies (14–21) reveal that hydrogen can affect basic dislocation properties, including Peierls stress and dislocation nucleation energy. Several mechanisms, including hydrogen-enhanced dislocation mobility (5, 10) and hydrogen-hindered cross-slip (13, 22), have been proposed to rationalize the observed localized plastic behavior. However, no conclusive agreement has been reached on the fundamental mechanisms by which hydrogen modifies dislocation structures and facilitates HELP. This gap in understanding arises from the inherent complexity of the multiscale HE phenomenon and the limited availability of integrated experimental and numerical studies at the atomic scale.

Beyond the direct hydrogen–dislocation interaction, an open question remains: Do synergistic interactions between hydrogen and the complex compositions and microstructures of structural materials exist, and can they modify the underlying mechanisms of HE? For instance, carbon and other alloying elements in commercial steels significantly enhance mechanical properties through appropriate heat treatment (23, 24). It is well established that higher-strength steels are generally more susceptible to HE. However, the interaction between hydrogen, carbon, and other alloying elements in the context of HE remains poorly understood. Studies on advanced high-strength steels suggest a strong correlation between hydrogen susceptibility and carbon content, though conflicting results persist regarding carbon’s exact role in these processes (25–27). While both hydrogen and carbon independently affect the plastic behavior of materials (20, 21, 28–32), their interplay and combined influence on HE mechanisms remain largely unexplored, posing a significant gap in the current understanding of HE.

Here, we reveal a synergistic hydrogen embrittlement (SHE) phenomenon in martensitic advanced high-strength steels (MS-AHSS), driven by a carbon-assisted hydrogen-enhanced localized plasticity (C-HELP) mechanism. This mechanism arises from the redistribution of hydrogen clouds around screw dislocations, inhibiting their cross-slip. Using in situ hydrogen charging microcantilever bending tests, we evaluated the hydrogen susceptibility of MS-AHSS with varying carbon content. The results demonstrate that hydrogen reduces the work-hardening capacity of MS-AHSS, with the severity of this reduction increasing with carbon content. To investigate the underlying mechanism, we employed first-principles density functional theory (DFT) calculations and continuum models, which revealed that carbon, as a solute in body-centered cubic (BCC) iron, enhances hydrogen susceptibility by increasing the attraction between hydrogen and screw dislocations. This interaction amplifies the redistribution of hydrogen Cottrell clouds (33) around screw dislocations during cross-slip, further explaining the observed C-HELP behavior.

Results

In Situ Hydrogen Charging Microcantilever Bending Tests.

Three grades of martensitic steels—M900, M1300, and M1700—containing 0.08 wt%, 0.19 wt%, and 0.29 wt% carbon, respectively (SI Appendix, Supplementary Note 1), were tested to assess their hydrogen susceptibility. Baseline mechanical responses were measured in air, and in situ hydrogen charging tests were conducted to evaluate the effects of hydrogen. The lath martensite in these steels exhibits a hierarchical microstructure (Fig. 1 A–D), comprising prior austenite grains (10 to 100 μm), packets (5 to 10 μm), blocks (1 to 3 μm), and laths (~100 nm). Microcantilevers were fabricated from steel sheets using focused ion beam (FIB) milling, with nominal dimensions of 10 μm (length) × 3.5 μm (width) × 2.5 μm (thickness) (Fig. 1E). These cantilevers feature blocks and packets delineated by parallel laths misoriented along the cantilever’s length. Bending tests were performed using a conical-tip nanoindenter with a ~960 nm radius in displacement control mode, unloading at a maximum cantilever tip displacement of 2,000 nm. For hydrogen-charged tests, microcantilevers were precharged via cathodic charging at 1,000 mV for 500 s in an electrochemical microcell mounted on a Hysitron TI 950 TriboIndenter, with continuous hydrogen charging maintained during the bending tests.

Fig. 1.

Microstructures of the martensitic steels and hydrogen embrittlement (HE) observed in in situ microcantilever bending tests. (A) Schematic of the hierarchical lath martensite microstructure. Micrographs of (B) prior austenite grains, (C) packets/blocks, and (D) laths with decreasing length scale in M900 steel. (E) Scanning electron microscope (SEM) image of the microcantilever with approximate dimensions of 10 μm (length) × 3.5 μm (width) × 2.5 μm (thickness), where blocks and packets are separated by sets of parallel misoriented laths along the length direction. Bending responses of microcantilevers in air and with in situ hydrogen charging for (F) M900, (G) M1300, and (H) M1700 martensitic steels.

The load-vertical deflection curves (Fig. 1 F–H) from the bending tests exhibit an initial linear region, indicating elastic behavior, followed by a nonlinear segment with a decreasing slope, signifying the onset of plasticity and work hardening. In hydrogen-charged samples, minor changes were observed in the onset of the nonlinear region, reflecting slight variations in yield load for nominally similar cross-sections. However, the work-hardening capacity was noticeably reduced, particularly in the M1700 steel with the highest carbon content. This steel exhibited the most significant reduction in work-hardening capacity under hydrogen charging, leading to slip localization and a macroscale loss of ductility. The microcantilever bending tests clearly demonstrate that hydrogen susceptibility increases with higher carbon content in these MS-AHSS. This finding is consistent with the result of the bulk uniaxial tensile test, which shows a more severe reduction in ductility as carbon content increases (SI Appendix, Fig. S2). More importantly, the microlength scale of the microcantilever bending tests provides an amplified demonstration of the effects of hydrogen on measurable mechanical response, which may not be as evident in macroscopic tests. Notably, no cracks or significant microstructural changes were detected in the microcantilevers posttesting. These results suggest that the synergistic effect of carbon and hydrogen in reducing work-hardening capacity likely originates from hydrogen-modified dislocation mobility. This observation aligns with previous experimental findings of the HELP mechanism, further supporting the hypothesis that hydrogen and carbon collectively influence dislocation behavior (11).

Carbon and Hydrogen Segregation to Screw Dislocations.

Martensitic steels possess a body-centered tetragonal (BCT) crystal structure, resulting from carbon-induced lattice distortion of the body-centered cubic (BCC) structure of iron. DFT calculations have shown that this carbon-induced tetragonality in the BCC Fe supercell is relatively minor across a wide range of carbon contents (34). Therefore, in our atomistic study of hydrogen–dislocation interactions, low-carbon martensitic steels were approximated as BCC Fe. This simplification allows us to investigate local hydrogen–carbon–dislocation interactions while disregarding the deformation anisotropy inherent in the BCT structure, which has fewer equivalent slip systems. Plasticity in BCC materials is primarily governed by the mobility of screw dislocations, which have a significantly higher Peierls stress compared to edge dislocations (23). Recent experiments (35, 36) indicate that both hydrogen and carbon can segregate to dislocations, forming solute atom atmospheres. Our microcantilever bending tests further suggest that hydrogen alters screw dislocation behavior, and carbon atoms in martensitic steels may enhance this hydrogen–dislocation interaction. To investigate the deformation mechanisms underlying the synergistic effect of carbon and hydrogen on dislocation behavior, we employed first-principles DFT calculations to study the interactions between carbon, hydrogen, and screw dislocations. These calculations aim to elucidate the atomic-scale processes driving the observed changes in mechanical properties.

The <111> screw dislocation in BCC Fe, modeled with a periodic dislocation dipole cell of length equal to two Burgers vectors (2b) (SI Appendix, Fig. S3A), exhibits a compact easy-core structure with three-fold symmetry (SI Appendix, Fig. S3B), consistent with previous studies (37). The effect of hydrogen on the dislocation core structure was analyzed by placing a hydrogen atom at the most favorable site in the core. The relaxed core structure (Fig. 2A) shows only slight distortion from the original compact easy-core structure. The hydrogen–dislocation core interaction energy is defined as the energy difference between two configurations: one with hydrogen near the dislocation core (Fig. 2A) and the other with hydrogen in a tetrahedral lattice site far from the dislocation (Fig. 2C). The calculated interaction energy of −0.16 eV indicates a strong attraction between hydrogen and the dislocation core.

Fig. 2.

Solute-modified screw dislocation core structure and solute–dislocation interaction. Differential displacement plots of (A) an easy-core screw dislocation with hydrogen and (B) a carbon-decorated hard-core screw dislocation with hydrogen. The red dots represent the center of the dislocation, which is also the preferable site for carbon solute in (B) and the blue dots represent the locations of hydrogen atoms. (C) Interstitial hydrogen at a tetrahedral site in a BCC perfect crystal. (D) Changes in hydrogen–dislocation interaction energy and binding site Voronoi volume with increasing carbon concentration along the screw dislocation line. In the 3D representations of the atomic structure around the dislocation core in (A, B, and D), white, red, and blue atoms represent iron, carbon, and hydrogen atoms, respectively.

When a carbon atom is placed near the dislocation core, it causes a core structure change from easy-core to hard-core (Fig. 2B), attributed to the larger Voronoi volume of the interstitial site at the center of the reconstructed core. This observation aligns with previous studies (29, 38). The carbon-induced core structure change also modifies the preferred hydrogen binding sites. The hydrogen–dislocation interaction energy for the most energetically favorable site near the carbon-occupied hard-core (Fig. 2B) is calculated to be −0.22 eV, representing nearly a 40% increase in magnitude compared to the easy-core configuration without carbon.

The effect of carbon on the hydrogen–dislocation interaction energy was further investigated by varying the carbon concentration along the dislocation line. As shown in Fig. 2D, the absolute value of hydrogen interaction energy and the Voronoi volume of hydrogen sites increase with carbon content until a saturation point is reached. This behavior reflects the competition between the attraction of carbon and hydrogen to the dislocation core and the internal repulsion between carbon and hydrogen atoms. The unique geometry of the dislocation core and the carbon-induced core structure change allow for the cosegregation of hydrogen and carbon. HE is highly sensitive to hydrogen concentration within the material, requiring sufficient hydrogen at critical microstructural features, such as dislocation cores and grain boundaries, for embrittlement mechanisms to operate (13). This suggests that carbon-assisted hydrogen segregation to screw dislocation cores can make steels with higher carbon content more susceptible to HE than lower-carbon steels. This is because higher carbon content results in a greater effective hydrogen concentration at dislocations. The segregation of hydrogen around screw dislocations is an essential step in how hydrogen mediates dislocation mobility, with carbon playing a facilitating role in this process.

Screw Dislocation Cross-Slip and Slip Planarity.

The above DFT calculations confirm the viability of cosegregation of carbon and hydrogen to a screw dislocation in the BCC structure. To explore the implications of this solute segregation, external strains were applied to the simulation cell to investigate the combined effect of carbon and hydrogen on screw dislocation mobility. At the macroscopic scale, work hardening arises from the collective behavior of dislocation glide, interaction, and multiplication in crystalline materials (39). Microscale processes such as dislocation pinning, cross-slip, and junction formation influence strain-hardening behavior, though their quantitative relationships remain poorly understood. Experimental observations (5, 7) indicate that steels susceptible to HELP exhibit coarse slip bands with greater height, suggesting reduced dislocation cross-slip activity. In this study, we focus on the combined effects of carbon and hydrogen on dislocation cross-slip. Using DFT calculations, the dislocation glide direction was controlled by applying various strains (SI Appendix, Supplementary Note 3) to investigate how these solutes modify cross-slip behavior and contribute to the observed strain-hardening characteristics.

Fig. 3A shows the <111> screw dislocation core structure in pure Fe under external strain ${ϵ}_{\mathit{yz}}$, just before gliding in the $\left[-112\right]$ direction on the ($1\overline{1}0$) plane. Unlike the stress-free dislocation core (SI Appendix, Fig. S3B), the loaded core loses its three-fold symmetry and contracts to the slip plane. With an additional applied strain ${ϵ}_{\mathit{xz}}$, the dislocation changes its gliding direction, as shown in Fig. 3B, which captures the shifted core structure before gliding in the new direction. The ability of the contracted dislocation core to shift between gliding planes is a key aspect of the cross-slip process. For the carbon-decorated hard core (Fig. 3C), the shear strain required to distort the core is three times higher than that of the easy-core dislocation in pure Fe, owing to the pinning effect of carbon. Under these conditions, the dislocation core exhibits a broader, more distorted region due to the higher stress levels, which redistributes in response to the external loading during cross-slip (Fig. 3D).

Fig. 3.

Reconstruction of the dislocation core field during cross-slip. Differential displacement plots of screw dislocations with (A) easy core under shear strain ${ϵ}_{\mathit{yz}}=0.0102$, (B) easy core under shear strain ${ϵ}_{\mathit{yz}}=0.0102 \mathrm{a}\mathrm{n}\mathrm{d} {ϵ}_{\mathit{xz}}=-0.0068$, (C) carbon-decorated hard core under shear strain ${ϵ}_{\mathit{yz}}=0.034$, (D) carbon-decorated hard core under shear strain ${ϵ}_{\mathit{yz}}=0.034$ and ${ϵ}_{\mathit{xz}}=-0.034$, (E) carbon-decorated hard core with hydrogen under shear strain ${ϵ}_{\mathit{yz}}=0.034$, and (F) carbon-decorated hard core with hydrogen under shear strain ${ϵ}_{\mathit{yz}}=0.034$ and ${ϵ}_{\mathit{xz}}=-0.0374$. The color contour shows the screw component ${\alpha }_{33}$ of the Nye tensor. Carbon increases the load level required for cross-slip, resulting in a wider dislocation core field distortion and reorientation. The red dots represent the location of the center of the dislocation, which is also the preferable site for carbon solute in (C−F), and the blue dots represent the location of hydrogen atoms. In the 3D representations of the atomic structure around the dislocation core, white, red, and blue atoms represent iron, carbon, and hydrogen atoms, respectively.

As shown in Fig. 3 E and F, hydrogen at the dislocation core interacts with the distorted core field, increasing the external load required for cross-slip. The ease of cross-slip is quantified by the ratio $\left|{ϵ}_{\mathit{xz}}^{\mathit{cross}}\right|/\left|{ϵ}_{\mathit{yz}}^{\mathit{glide}}\right|$, which compares the critical applied strain in the cross-slip direction (${ϵ}_{\mathit{xz}}^{\mathit{cross}}$) to that in the original gliding direction (${ϵ}_{\mathit{yz}}^{\mathit{glide}}$). For a carbon-decorated screw dislocation, the insertion of one hydrogen atom raises this ratio from 0.85 to 0.92, reflecting a higher energy barrier for cross-slip. This increase is attributed to changes in the hydrogen–dislocation core field interaction. Following cross-slip, the hydrogen–dislocation interaction energy in Fig. 3F decreases by 60 meV compared to that in Fig. 3E before cross-slip. This reduction requires additional external loading to compensate for the diminished interaction energy. This added load arises from the hindered cross-slip process, illustrating the combined effect of hydrogen and carbon in modifying dislocation mobility and contributing to the mechanisms of HE.

Therefore, our simulations reveal that cosegregated carbon and hydrogen at the dislocation core suppress cross-slip and promote slip planarity. These DFT calculations provide direct evidence of the microscale mechanism underlying hydrogen-induced slip planarity in BCC iron, a phenomenon that remains experimentally inaccessible. The effect of hydrogen on cross-slip is linked to changes in the hydrogen–dislocation interaction energy, which stem from the altered distribution of the dislocation core field. This distribution is further influenced by the presence of carbon atoms, highlighting the synergistic role of carbon and hydrogen in modifying dislocation behavior.

Hydrogen Redistribution Around the Screw Dislocation.

Our DFT calculations demonstrate that cross-slip of a screw dislocation is hindered when both a single carbon and hydrogen atom are present at the dislocation core. Experimental observations (36) indicate that solute atmospheres can extend several nanometers from the dislocation core. It has long been hypothesized that the redistribution of the hydrogen atmosphere around a dislocation during cross-slip contributes significantly to slip planarity (13), as additional driving force is required to facilitate this process. Building on our study of hydrogen–dislocation interactions using DFT calculations, we directly calculate hydrogen redistribution around a screw dislocation through a cross-scale framework (Fig. 4A). Continuum representations (40) of hydrogen and the screw dislocation, which account for the distorted core field, are derived from the atomic-scale DFT results. These representations are used to investigate hydrogen redistribution under various loading conditions, as detailed in SI Appendix, Supplementary Notes 4 and 5.

Fig. 4.

Continuum representation of the hydrogen cloud and hydrogen redistribution around a screw dislocation. (A) Schematic of the adopted cross-scale modeling framework, where the distorted dislocation core field is characterized from DFT calculations and used to obtain the equilibrium distribution of the hydrogen cloud around the screw dislocation under various external loadings. Equilibrium hydrogen concentration around the easy core (B) and the hard core (D) screw dislocation under shear strain ${ϵ}_{\mathit{yz}}$. Percentage hydrogen concentration change around the easy core (C) and the hard core (E) screw dislocation under additional shear strain ${ϵ}_{\mathit{xz}}$. Hydrogen redistribution around the hard core is significantly larger than around the easy core. In (B−E), regions within 3b of the dislocation core center are excluded from the hydrogen distribution map.

Interstitial hydrogen is represented by a point expansion (16), while the displacement field of the screw dislocation is extended from a previous elastic dipole model (41) and expressed as

$$u=Re[\sum \frac{1}{z\pi i}\left(D_{\alpha }, ,\text{ln},(,z\right)+C_{\alpha }^1\frac{1}{z}+C_{\alpha }^2\frac{1}{z^2})],$$ [1]

where $C_{\alpha }^1$ and $C_{\alpha }^2$ are coefficients calibrated from the DFT displacement field. In the stress-free state, the screw dislocation core field in BCC Fe induces an equibiaxial in-plane dilatation (37), characterized by Eq. 1 without the second-order term. The equilibrium hydrogen distribution profile $c_h$ around the screw dislocation can be determined from thermodynamic principles as (16):

$$c_h=\frac{c_0}{c_0+\left(1-c_0\right)\exp (W_h/k_{b}T)},$$ [2]

where $c_0$ denotes the hydrogen concentration in the matrix, and $W_h$ is the elastic interaction energy between hydrogen and the screw dislocation.

The continuum representations of the distorted core field (with coefficients reported in SI Appendix, Table S1) under various loading conditions are obtained by minimizing the L₂ error between the analytical solution in Eq. 1 and the DFT results (Fig. 3). These representations are then used to determine the equilibrium hydrogen profile around the screw dislocation at different stages of the cross-slip process through Eq. 2. Under external loading ${ϵ}_{\mathit{yz}}$, the dislocation core field deviates from the symmetric distribution in the stress-free state, as shown in Fig. 4 B and D. The carbon-decorated screw dislocation core exhibits more significant distortion under loading, leading to a larger deviation in the hydrogen distribution compared to the symmetric stress-free case. This is evident in Fig. 4D, where the hydrogen profile around the carbon-decorated screw dislocation shows greater asymmetry compared to the baseline screw dislocation shown in Fig. 4B.

When subjected to additional external loading ${ϵ}_{\mathit{xz}}$, the hydrogen distribution evolves further, aligning with the changes in the dislocation core field, which contracts along the gliding direction, as depicted in Fig. 3 B, D, and F. The carbon-decorated screw dislocation (Fig. 4E) exhibits significantly greater hydrogen redistribution, measured by the percentage change in local equilibrium hydrogen concentration, compared to the baseline screw dislocation (Fig. 4C).

Our cross-scale study reveals that carbon induces a dislocation core structure change and increases the stress required for dislocation gliding, resulting in greater core field distortion and enhanced hydrogen redistribution around the dislocation. This amplified hydrogen redistribution significantly hinders cross-slip. The synergistic effect of carbon and hydrogen in promoting slip planarity reduces the likelihood of dislocations in different slip planes interacting to form dislocation junctions. Consequently, this leads to a decrease in work-hardening capacity, consistent with the experimental observations shown in Fig. 1 F−H. These findings provide a mechanistic explanation for the reduction in ductility and work-hardening observed in high-carbon steels under hydrogen-charging conditions.

Discussion

The cross-slip process (42) is critical for influencing work hardening in metals, as it mediates dislocation interactions such as junction formation, multiplication, and annihilation, thereby distributing slip. Discrete dislocation dynamics simulations (43) have shown that dislocation density, the number of dislocation junctions, and the work-hardening rate are significantly reduced when cross-slip is suppressed. For a critical dislocation segment with a length of 20b in iron, the estimated increase in the cross-slip energy barrier due to the synergistic effects of carbon and hydrogen is highly sensitive to hydrogen concentration. It rises from just a few percent at a nominal background hydrogen concentration of 10 appm to 50% under high hydrogen levels (SI Appendix, Supplementary Note 6). This suggests that local hydrogen segregation—driven by stress concentrations from chemical heterogeneities, microcracks, internal boundaries, and dislocation-assisted hydrogen transport (44–47)—plays a critical role in activating or modifying localized plasticity in bulk materials. Such effects may not be apparent in macroscopic stress–strain curves, which might show only marginal reductions in work-hardening capacity, yet still result in embrittlement. Compared to the easy-core structure in pure Fe, the carbon-decorated hard-core structure shows an order-of-magnitude increase in the energy barrier at the same hydrogen concentration, suggesting that the presence of carbon solute makes HE much more likely.

With its low diffusivity at room temperature, carbon atoms can be considered immobile solutes distributed in steels and swept by dislocation lines. Continuous carbon–dislocation interaction is expected in M1700 steel with 0.29 wt.% carbon for typical dislocation line gliding over the distance of the dislocation mean free path before storage (SI Appendix, Supplementary Note 7), ensuring that the SHE mechanism operates throughout the entire deformation process. When the nominal carbon concentration is too low, as in M900 steel, the nonuniform distribution of carbon atoms and their segregation into other microscopic features, such as lath boundaries, can result in insufficient carbon–dislocation interactions during deformation. This leads to an ineffective C-HELP mechanism and only marginal changes in work hardening behavior (Fig. 1F).

Our proposed C-HELP mechanism directly demonstrates how hydrogen interacts synergistically with carbon to influence screw dislocation behavior and modify plasticity in BCC materials. While this study focuses on the interactions between hydrogen, carbon, and dislocations, the C-HELP mechanism leads to reduced work-hardening capacity and plastic strain localization, contributing to premature failure. Additionally, hydrogen-modified dislocation motion alters the local stress/strain distribution and hydrogen concentration within the material, creating favorable conditions for other decohesion mechanisms (48–56), such as grain boundary embrittlement and the ductile-to-brittle transition at crack tips, to occur.

The findings in this work provide a fundamental understanding of the SHE mechanism and lay the foundation for developing physics-based deformation laws for meso/macroscale modeling (57–60). Such models can incorporate multiple embrittlement mechanisms to create predictive frameworks for evaluating the complex multiscale HE process, which depends on material microstructure, composition, and loading conditions. In particular, the strain rate plays a critical role in the hydrogen transport process that affects the local hydrogen concentration and hydrogen redistribution behavior, especially at high strain rates where the time scale of hydrogen diffusion becomes comparable to that of dislocation motion. As a result, the hydrogen distribution around the dislocation core becomes rate-dependent, as does the energy barrier for cross-slip in our proposed C-HELP mechanism. These factors must be carefully incorporated into meso- and macroscale models to accurately capture localized plastic behavior and the associated HE process in structural materials.

This study uses carbon as an example to show how solutes can significantly modify hydrogen–dislocation interactions and influence hydrogen susceptibility. Other common solutes in Fe, such as B and O, can similarly induce changes in dislocation core structures (61). Therefore, the proposed SHE and C-HELP mechanisms may operate in a range of solute-strengthened materials containing various interstitial and substitutional solutes (62), including sulfur, manganese, nickel, and silicon. However, there remains no systematic understanding of the cooperative effects of different alloying elements and hydrogen on plastic deformation, hindering the evaluation and selection of optimal strengthening solutes for structural materials in hydrogen-rich environments.

In summary, we have reported a SHE phenomenon in MS-AHSS, evidenced by the reduced work-hardening capacity in microcantilever bending tests. This behavior can be attributed to carbon-assisted hydrogen-enhanced localized plasticity. The atomistic origin of reduced work hardening and localized plasticity is linked to carbon-enhanced hydrogen segregation to screw dislocations and carbon-amplified hydrogen redistribution during dislocation cross-slip, which promotes slip planarity. Our proposed SHE and C-HELP mechanisms are broadly applicable to BCC materials, where screw dislocation mobility limits plasticity. The C-HELP mechanism serves as a specific demonstration of SHE in structural materials, highlighting the need for further studies to unravel the complex interactions between hydrogen, composition, and microstructure. The combined experimental-modeling framework developed in this work offers an integrated approach to evaluating the effects of alloying elements on hydrogen susceptibility, providing a foundation for designing hydrogen-tolerant, high-strength structural materials.

Materials and Methods

Microcantilever Bending Test.

Martensitic steel specimens were prepared from sheet form and cut into small pieces (7.6 mm × 12.6 mm) using electrodischarge machining (EDM). A 1:2 wedge was created on one of the short sides of each piece to produce a sharp edge. The specimen faces were ground and mechanically polished, followed by electrochemical polishing using a solution of 10 mL of 70% perchloric acid dissolved in 90 mL of acetic acid at 5 V for approximately 3 s. Smooth microcantilevers were fabricated using FIB milling with nominal dimensions of 10 μm (length) × 3.5 μm (width) × 2.5 μm (thickness). Two specimens were cut from the same prior austenite grain each time to ensure consistent microstructure for subsequent tests, both in air and with in situ hydrogen charging. Both air and hydrogen charging tests were conducted using the Hysitron TI 950 TriboIndenter equipped with an electrochemical fluid cell. A conical-tip indenter with an approximate radius of 960 nm was used to bend the cantilever at its free end. Displacement control was applied with a maximum displacement of 2,000 nm, a loading time of 300 s, and an unloading time of 300 s. For hydrogen charging tests, the microcantilevers were immersed in a borate buffer solution (pH = 9.0) and precharged cathodically at 1,000 mV for 500 s. Continuous charging was maintained during bending tests. The experimental setup for in situ hydrogen charging microcantilever bending tests is shown in SI Appendix, Fig. S1. The initial microstructure was characterized using standard optical metallography and transmission electron microscopy (TEM). A microcantilever beam specimen was FIB-milled to electron transparency, enabling TEM observation of the alignment of the lath microstructure along the beam.

First-Principles DFT Calculations.

First-principles DFT calculations were carried out using the Vienna Ab-initio Simulation Package (VASP) with the projector augmented wave (PAW) method (63, 64) and the Perdew–Burke–Ernzerhof (PBE) Generalized Gradient Approximation (GGA) (65) for exchange-correlation. The PAW potential used in this work explicitly includes 8, 4, and 1 valence electrons for Fe, C, and H atoms, respectively. A plane wave cutoff energy of 400 eV was used, and all calculations employed spin polarization. A supercell containing 270 atoms with a screw dislocation dipole, having a length of 2b (where b is the Burgers vector length), oriented along the 1/2[111] direction, as shown in SI Appendix, Fig. S3, was used to calculate the dislocation core structure in BCC Fe. The atomic positions were initially deformed using the displacement field of screw dislocations calculated via anisotropic elasticity (40) and then fully relaxed with the conjugate gradient algorithm until forces converged to less than 1 meV/Å. Brillouin zone integrations were performed with a gamma-centered Monkhorst–Pack k-point mesh of 1 × 2 × 8, with the third direction aligned along the dislocation line. This setup, widely used in previous studies, has proven effective for calculating dislocation core structures in BCC metals (37, 66).

Supplementary Material

Appendix 01 (PDF)

Data, Materials, and Software Availability

All study data are included in the article and/or SI Appendix.