Hydrogen Segregation Research Articles

Hydrogen embrittlement in Nickel oligocrystals: Experimentation and crystal plasticity-phase field fracture modeling

This work investigates the hydrogen embrittlement (HE) behavior of Nickel-201 alloy using experimental and numerical approaches. In experimentation, the effect of electrochemical hydrogen charging on the deformation behavior of tensile oligocrystals is investigated at different strain rates. Irrespective of the strain rate levels, all the hydrogen-charged tensile oligocrystals exhibited intergranular (IG) fracture exclusively along the random high-angle grain boundaries (RHAGBs). At the same time, no crack was observed along the coincident site lattice (CSL) Σ3 type grain boundaries. The absence of an interconnected grain boundary network, lower stress levels, and insensitivity of H-induced IG fracture to the strain rate levels of investigated FCC oligocrystals (with low lattice hydrogen diffusion coefficient) established the insignificance of dynamic H-redistribution toward HE. In numerical simulations, a crystal plasticity-phase field fracture finite element model simulates the macroscopic tensile response and corresponding microscopic fracture evolution behavior for hydrogen-free and hydrogen-charged tensile oligocrystals. The experimental results corroborated by numerical simulations, validate that the reduction in fracture energy of RHAGBs, induced by hydrogen segregation through thermodynamic equilibrium during hydrogen charging prior to deformation, is the dominating factor governing HE.

Effect of alloying solutes on hydrogen segregation at pure iron Σ3(111) grain boundary: First-principles calculation

Hydrogen segregation behaviors at BCC-Fe Σ3 (111) grain boundary (GB) as well as the effects of alloying solutes were studied by the first-principles method. The segregation energy of alloying solutes at different periods presents a concave-down parabolic-like relationship with the atomic number, and the 4 d transition alloying solutes show a higher averaged segregation tendency. At the favorable trapping site, hydrogen segregation energy decreased by increasing the number of hydrogen atoms up to 0.85/Å2 in the plane vertical to the GB. Mo, Tc, Ru, Ta, W, Re, Os, and Ir strengthen GB and inhibit hydrogen segregation. Significantly, the interaction between alloying solutes and hydrogen segregation was elucidated by emphasizing the separation of the chemical and the mechanical contributions, and appropriate descriptors on hydrogen segregation energy influenced by alloying solutes were screened. This work offers theoretical backing to comprehend hydrogen segregation behaviors and the effects of alloying solutes to design advanced high-strength steels resistant to hydrogen embrittlement.

Predicting hydrogen segregation energy distributions in strained regions of tungsten using artificial neural network

Employing the smooth overlap of atomic position (SOAP) descriptors, we established an artificial neural network (ANN) model with the ability to effectively and accurately predict the segregation energy Eseg distributions of hydrogen (H) atoms in various strained regions of tungsten (W). The model is verified to have comparable accuracy with the molecular statics (MS) relaxations, as quantitively evidenced by the root mean square error (RMSE) values of 0.018 eV for the homogeneously strained regions, averagely 0.025 eV for regions near different dislocations, and 0.032 eV for regions surrounding an interstitial H cluster. Besides, the predictions yield no systematic bias at both high and low Eseg extremes, which is a common issue in the elastic theory (ET) calculations. Moreover, Eseg values are precisely predicted with the RMSE value of 0.041 eV surrounding an interstitial H cluster that is not encountered during the learning phase of the model, demonstrating the model’s robust generalization capability. Considering its markedly enhanced computation speed, this accurate ANN model holds promising prospects in circumstances that require massive Eseg calculations.

Molecular dynamics examination of hydrogen effects on relative fracture properties of interfaces between ferrite, martensite, and cementite in Fe–C steels

Understanding susceptibility to hydrogen embrittlement is a key metric for structural materials for hydrogen applications. While various microstructures from combinations of ferrite, martensite, and cementite phases have been utilized to yield desired mechanical properties for low-cost iron carbon (Fe–C) steels, interface fracture between these phases can be exacerbated in hydrogen-containing environments. We have applied molecular dynamics simulations to study fracture properties of a wide range of possible interfaces in Fe–C steels with and without hydrogen. We also perform a detailed analysis of hydrogen segregation at these interfaces. While there are exceptions to these trends, it was found that promoting block-packet martensite-martensite or ferrite-ferrite interfaces and reducing interfaces with cementite may improve toughness of materials. In general, hydrogen was found to reduce the decohesion energies of interfaces.

Experimental study on the hydrogen diffusion across an individual grain boundary in pure nickel by scanning Kelvin probe force microscopy

There are few existing experimental studies on the controversial diffusion behavior of hydrogen atoms across an individual grain boundary (GB). In this study, by combining Electron Back Scattered Diffraction (EBSD) and scanning Kelvin probe force microscopy (SKPFM) techniques, the transverse diffusion behavior of hydrogen atoms across a random GB with a misorientation of 45° within the directionally solidified high purity nickel was directly observed. The results suggest that the hydrogen atoms diffused perpendicular to the studied GB will be trapped by this GB, resulting in hydrogen segregation at the GB. In addition, the studied GB acts as a two-dimensional barrier that can block the transverse diffusion of hydrogen atoms. This present study reveals the real role played by a specific random GB on the hydrogen transverse diffusion and enriches the existing experimental research for intergranular diffusion.

Effect of hydrogen charging intensities and times on hydrogen embrittlement of Q&P980 steel

Q&P steel has good development prospects because of its excellent mechanical properties, but with the improvement in strength grade, hydrogen-induced delayed fracture (HIDF) is almost inevitable. In this paper, slow strain rate tensile tests and deep-drawn cup tests of Q&P980 steel under different hydrogen charging strengths and times were carried out, and the microstructure and fracture morphology were analysed by SEM. The results show that the plastic loss of Q&P980 steel was more obvious with increasing hydrogen charging intensity and hydrogen charging time, and a good elongation of 6.63% is still retained under the hydrogen content of 2.134 ppm. The deep-drawn cup samples were placed in acidic distilled water and alkaline and acidic solutions, and only a deep-drawn ratio of 1.9 showed HIDF in the three solutions. Specifically, 12 cracks were observed after soaking in HCl solution for two days. The main reason is that the martensite, austenite island and ferrite phase interface of Q&P980 steel increase stress during deformation and with the transformation-induced plasticity (TRIP) effect, resulting in hydrogen segregation at the phase interface and crack initiation leading to HIDF.

Analysis of Hydrogen Distribution and Diffusion in Pre-Strained SUS316L through Scanning Kelvin Probe Force Microscopy and Thermal Desorption Spectroscopy

Austenitic stainless steels (γ-SS) play an important role in the storage of high-pressure hydrogen. However, hydrogen embrittlement (HE) can significantly degrade the mechanical properties of γ-SS. Measuring the distribution of hydrogen in γ-SS is a vital way to learn about HE. In this paper, scanning Kelvin probe force microscopy (SKPFM) and thermal desorption spectroscopy (TDS) have been utilized to analyze the distribution and diffusion of hydrogen in pre-strained SUS316L. Additionally, the McNabb–Foster model is employed to calculate hydrogen in the lattice and phase boundaries along the sample’s thickness direction. The results demonstrate that the combination of SKPFM and TDS is an effective approach for studying hydrogen distribution and diffusion in metals. It was observed that hydrogen segregation occurs at the boundary between the martensitic (α′) and austenite (γ) phases. The inhibitory effect of the oxide film on hydrogen diffusion is more significant at lower temperatures. However, it should be noted that the McNabb–Foster model exhibits relatively high accuracy in predicting hydrogen desorption at higher temperatures while disregarding the influence of the native oxide film.

First-principles calculation of bonding and hydrogen trapping mechanism of Fe3C/α-Fe interface

The Fe3C/α-Fe interface in pearlite is the main hydrogen trapping site. The first-principles calculation method based on density functional theory was employed to investigate the bonding and hydrogen trapping mechanism of the Fe3C/α-Fe interface with three different orientation relationships, specifically the Bagaryatsky orientation, Isaichev orientation, and Pitsch-Petch orientation. The results showed that by calculating the interface energy and separation work, the theoretical bonding strength and stability of the three interfaces presented the following order: Isaichev orientation > Pitsch-Petch orientation > Bagaryatsky orientation. Hydrogen segregation energy revealed that H atoms were more likely to be trapped at the Fe3C/α-Fe interface region and the hydrogen trap ability of three interfaces was in the following order: Isaichev orientation > Pitsch-Petch orientation > Bagaryatsky orientation. The maximum absolute value of segregation energy was found at the interface with Isaichev orientation, which was 0.38 eV. The differential charge density, density of states, and Mulliken bond population calculations revealed that the formation of the interface was related to the covalent bond between C atoms in cementite and Fe atoms in ferrite, and affected by the spatial distribution of C atoms. The C atoms also repelled the H atoms, so the strength of H–Fe bond was identified as the dominant factor of hydrogen trapping behavior. Overall, these insights can potentially aid in improving the design and performance of materials with pearlite structures.

Hydrogen trapping and diffusion in polycrystalline nickel: The spectrum of grain boundary segregation

Hydrogen as an interstitial solute at grain boundaries (GBs) can have a catastrophic impact on the mechanical properties of many metals. Despite the global research effort, the underlying hydrogen-GB interactions in polycrystals remain inadequately understood. In this study, using Voronoi tessellations and atomistic simulations, we elucidate the hydrogen segregation energy spectrum at the GBs of polycrystalline nickel by exploring all the topologically favorable segregation sites. Three distinct peaks in the energy spectrum are identified, corresponding to different structural fingerprints. The first peak (−0.205 eV) represents the most favorable segregation sites at GB core, while the second and third peaks account for the sites at GB surface. By incorporating a thermodynamic model, the spectrum enables the determination of the equilibrium hydrogen concentrations at GBs, unveiling a remarkable two to three orders of magnitude increase compared to the bulk hydrogen concentration reported in experimental studies. The identified structures from the GB spectrum exhibit vastly different behaviors in hydrogen segregation and diffusion, with the low-barrier channels inside GB core contributing to short-circuit diffusion, while the high energy gaps between GB and neighboring lattice serving as on-plane diffusion barriers. Mean square displacement analysis further confirms the findings, and shows that the calculated GB diffusion coefficient is three orders of magnitude greater than that of lattice. The present study has a significant implication for practical applications since it offers a tool to bridge the gap between atomic-scale interactions and macroscopic behaviors in engineering materials.

Hydrogen diffusion in zirconium cladding alloys with an inner liner as quantified by neutron radiography and nanoindentation

Embrittlement caused by hydride precipitates in zirconium claddings constitutes a significant risk factor for the integrity of spent nuclear fuel rods, especially during the handling and transport operations necessary before and after long-term intermediate dry storage. Zircaloy-2 is the most commonly used alloy for cladding tubes in boiling water reactors. It is typically manufactured with an inner liner to better cope with issues related to pellet-cladding interaction. Previous work showed that in clad samples possessing a liner, a significant portion of hydrogen tends to migrate towards the liner region during cool-down. In this work, the hydrogen precipitation and distribution after thermal exposures comparable to service was investigated in different commercial claddings for boiling water reactors. The resulting hydrogen distribution was quantified by means of high-resolution neutron imaging at the SINQ spallation neutron source at Paul Scherrer Institut in Switzerland, and the presence of hydrides was subsequently confirmed by metallography and nanoindentation mapping of selected samples. The results show that when a cooling rate of 30 °C/h is applied, the vast majority of the hydrogen tends to accumulate at the liner/substrate interface when a liner is present. At lower cooling rates, the hydrogen tends to distribute more homogenously into the liner material, and the cladding bulk material is left completely hydrogen-free. Neutron radiography of samples after water quenching from homogenization temperatures between 370 °C and 460 °C revealed a significant amount of hydrogen trapped in the hydrides form at the liner/substrate interface at temperatures up to 400 °C. Undissolved hydrides may act as nucleation seeds for hydrides upon cooling, contributing to the observed hydrogen segregation. A model that takes into account the interfacial properties of the region between the liner and the bulk material is proposed.

Atomic scale understanding of the role of hydrogen and oxygen segregation in the embrittlement of grain boundaries in a twinning induced plasticity steel

High strength twinning induced plasticity (TWIP) steels have potential for applications in the automotive industry. However, they are prone to hydrogen embrittlement (HE) and galvanic corrosion. Here, we report on the susceptibility towards HE and oxidation of a model Fe 27Mn 0.3C (wt%) TWIP steel by atom probe tomography. We measured the segregation of hydrogen and oxygen at grain boundaries, which appears correlated with manganese depletion. Our study suggests a correlation between HE and oxidation mechanisms in TWIP steels, which can combine to favor the previously reported hydrogen enhanced decohesion of grain boundaries.

Analysis of grain-boundary segregation of hydrogen in bcc-Fe polycrystals via a nano-polycrystalline grain-boundary model

Hydrogen embrittlement caused by hydrogen segregation at grain boundaries (GBs) is the most serious issue in the development of high-strength steels, but the mechanisms behind this process are still not well understood. The GB segregation behavior of hydrogen in body-centered cubic (bcc)-Fe polycrystals was comprehensively analyzed based on the interatomic potentials derived from first-principles calculations. Considering that the atomic structure of GBs is almost independent of the grain size, the GBs in polycrystals were modeled as nano-polycrystalline GBs with random orientations. The segregation energies of hydrogen for ∼17 million interstitial sites in this GB model were calculated. From these segregation energies, the effective segregation energy for the polycrystalline GB at thermal equilibrium under various temperature and hydrogen content conditions were determined, and the validity of the calculation method was verified by comparing the results with experimental data. The relationship between the segregation energy of hydrogen at each segregation site and the surrounding local atomic environment was used to identify the major hydrogen segregation sites at the atomic level, and the changes in the crystal structure near the GB that dominated segregation were clarified. The effective segregation energies at the polycrystalline GBs were in the range of −0.48 to −0.42 eV, which are in good agreement with the experimentally reported binding energy of hydrogen at GBs of bcc-Fe polycrystals (−0.52 eV). The major hydrogen segregation sites were octahedral sites with Voronoi volumes larger than 7.0 Å3, and the segregation energy was mainly due to the uniaxially distorted crystal structure in the short-axis direction of octahedral sites. Our findings and the developed calculation method contribute to the understanding of the hydrogen segregation behavior and hydrogen embrittlement mechanism in polycrystalline metallic materials.

Hydrogen segregation by local chemical ordering structure in CrCoNi medium-entropy alloys: A first principle study

Hydrogen segregation by local chemical ordering structure in CrCoNi medium-entropy alloys: A first principle study

The high strength and hydrogen embrittlement resistance of selective laser melted 304 austenitic stainless steel

3D printing technology can be used to produce a variety of complex parts for hydrogen service. In this study, 304 austenitic stainless steel (ASS) sample was manufactured by selective laser melting (SLM), one of the 3D printing technologies, then its high strength and hydrogen embrittlement resistance were discovered in comparison to traditional solution-treated 304 ASS. The unique high-density dislocation cellular structure of SLM sample remarkably restrained the nucleation of α'-martensite and slowed down the diffusion of hydrogen. Moreover, the possibility of hydrogen induced intergranular fracture was significantly reduced because of the alleviated hydrogen segregation at the grain boundary.

Effect of Mo addition on hydrogen segregation at α-Fe grain boundaries: A first-principles investigation of the mechanism by which Mo addition improves hydrogen embrittlement resistance in high-strength steels

The development of high-strength steels is essential for realizing a decarbonized society; however, to achieve this, the problem of hydrogen embrittlement must be solved. The effectiveness of adding Mo has been demonstrated to address this issue, but its underlying mechanism has not yet been clarified. To investigate this mechanism, we developed a calculation method to evaluate the effect of adding alloying elements on the grain boundary segregation of H using parameters calculated via first-principles calculations. Accordingly, the segregation energies of H with and without Mo segregation near the grain boundary were calculated. Based on the obtained segregation energies, the concentration of H at the grain boundary was calculated for a given amount of Mo under the heat treatment conditions normally assumed. The results showed that a small amount of Mo segregates to α-Fe grain boundaries and significantly reduces the grain boundary concentration of H owing to repulsive interactions between Mo and H. This indicates that Mo addition improves the resistance to hydrogen embrittlement by reducing the grain boundary H concentration. The physical origin of these repulsive interactions was further investigated by decomposing them into mechanical and chemical contributions; we found that these repulsive interactions are mainly caused by the chemical contribution. Furthermore, the crystal orbital Hamiltonian population indicated that the chemical contribution can be attributed to the large core repulsion between Mo–H at α-Fe grain boundaries (in comparison to that between Fe–H) owing to the larger atomic orbital of Mo. The proposed calculation method as well as the information obtained from our evaluations can help in the development of high-strength steels with excellent resistance to hydrogen embrittlement and in the improvement the resistance to hydrogen embrittlement of various metallic materials.

Defeating hydrogen-induced grain-boundary embrittlement via triggering unusual interfacial segregation in FeCrCoNi-type high-entropy alloys

Metallic materials are mostly susceptible to hydrogen embrittlement (HE), which severely deteriorates their mechanical properties and causes catastrophic failures with poor ductility. In this study, we found that such a long-standing HE problem can be effectively eliminated in the Fex(CrCoNi)1-x face-centered-cubic (fcc) high-entropy alloys (HEAs) by triggering the localized segregation of Cr at grain boundaries (GBs). It was revealed that increasing the Fe concentration from 2.5 to 25 at. % leads to substantially improved HE resistance, i.e., the ductility loss decreases from 70% to 6%. Meanwhile, the fracture mode transformed from the intergranular to the transgranular mode. Multiscale microstructural analyses demonstrated that the Fe2.5Cr32.5Co32.5Ni32.5 and Fe25Cr25Co25Ni25 alloys show negligible differences in the phase structure, grain size, and grain-boundary (GB) character. However, interestingly, the near atomic-resolution elemental mapping revealed that an increased Fe concentration promotes the nanoscale Cr segregation at the GBs, which is primarily motivated by the strong repulsive force between Cr and Fe and the low self-binding energy of Cr. Such unusual interfacial segregation of Cr, which has not been reported before in the Fe25Cr25Co25Ni25 alloy, helps enhance the GBs’ cohesive strength and suppresses the local hydrogen segregation at GBs due to the deceased GB energy, leading to the outstanding HE resistance. These findings decipher the origins of the vastly-improved HE resistance in current FeCrCoNi-type HEAs, and meanwhile, provide new insight into the future development of novel high-performance structural alloys with extraordinary immunity to hydrogen-induced damages.

Hydrogen-Induced Dislocation Nucleation and Plastic Deformation of 〈001〉 and 〈11¯0〉 Grain Boundaries in Nickel.

The grain boundary (GB) plays a crucial role in dominating hydrogen-induced plastic deformation and intergranular failure in polycrystal metals. In the present study, molecular dynamics simulations were employed to study the effects of hydrogen segregation on dislocation plasticity of a series of symmetrical tilt grain boundaries (STGBs) with various hydrogen concentrations. Our study shows that hydrogen both enhances and reduces dislocation nucleation events from STGBs, depending on different GB structures. Specifically, for STGBs, hydrogen does not affect the mode of heterogeneous dislocation nucleation (HDN), but facilitates nucleation events as a consequence of hydrogen disordering the GB structure. Conversely, hydrogen retards dislocation nucleation due to the fact that hydrogen segregation disrupts the transformation of boundary structure such as Σ9 () STGB. These results are helpful for deepening our understanding of GB-mediated hydrogen embrittlement (HE) mechanisms.

Effect of the ferrite morphology on hydrogen embrittlement of MAG welded 304 austenitic stainless steel

Ferrite usually plays an important role in the hydrogen embrittlement (HE) of austenitic stainless steel (γ-SS) weld. However, there are few investigations on the effect of ferrite morphology on HE. In this study, the relationship between the ferrite morphology and HE of metal active gas (MAG) welded 304 austenitic stainless steel is investigated. Hydrogen distribution testing and simulation reveal that the ferrite/austenite interfaces become the hydrogen segregation regions and the degree of hydrogen segregation at the interface is affected by ferrite morphology, which shows the order of lathy ferrite > reticular ferrite > skeletal ferrite. The slow strain rate tensile (SSRT) test indicates that the susceptibility to HE of welds containing lathy ferrite, skeletal ferrite and reticular ferrite reaches 34.6%, 30.7% and 33.2%, respectively. It can be found that the HE resistance shows the order of skeletal ferrite > reticular ferrite > lathy ferrite, which corresponds to the order of increasing hydrogen segregation. Hence, the ferrite morphology influences the resistance to HE of welds by affecting hydrogen segregation at the ferrite/austenite interface. A similar ferrite morphological dependence is observed from the weld fractures.

Key role of plastic strain gradient in hydrogen transport in polycrystalline materials

Hydrogen transport by moving dislocations is one of the key mechanisms for hydrogen embrittlement, which is considered responsible for local hydrogen accumulation at preferential crack initiation sites. Although numerous experiments corroborate this mechanism, there are few theoretical models for it available. In the present work, a novel hydrogen transport model is developed within the coupled framework of crystal plasticity and hydrogen balance. The hydrogen transport flux in the present model is divided into two parts: the first-order hydrogen transport flux driven by plastic strain and the second-order hydrogen transport flux driven by plastic strain gradient. With the calibrated parameters by fitting experimental results, the first-order hydrogen transport flux is negligible even for nickel with a low hydrogen diffusivity. In polycrystals, due to the intrinsic plastic heterogeneity induced by grain orientation mismatch, the second-order hydrogen transport flux results in significant dynamic hydrogen segregation at grain boundaries during deformation, which is expected to drive the hydrogen-induced intergranular cracking as observed in experiments. This dynamic segregation behavior is related to the evolution rate of geometrically necessary dislocations, depending on the grain boundary characters. The roles of grain refinement in hydrogen transport are clarified with the present model. This study shows the vital role of plastic strain gradient induced by grain boundary constraint in hydrogen transport, which is essential in understanding hydrogen migration and accumulation in deformed polycrystalline metals.

On the propensity of lath martensite intervariant boundaries to hydrogen absorption using molecular statics simulation

In the current study, the influence of boundary character (i.e., tilt, twist and mixed) on hydrogen absorption was, for the first time, investigated for lath martensite using molecular statics simulations. The energies of the martensitic intervariant boundaries varied widely from 306 to 1109 mJ/m2 depending on their character. The symmetric tilt and quasi-tilt boundaries revealed the lowest and highest boundary energies, respectively. This was, to some extent, reflected by distinct boundary unit structure/s and their complexity within a given intervariant boundary type. Interestingly, the intervariant boundary energy was not the main criteria to regulate the propensity of the boundary to hydrogen absorption. Rather, absorption depended on a distinct arrangement of unit structure/s within a given boundary plane, providing different interstitial sites for hydrogen. Indeed, the propensity for hydrogen atom/s was largely determined by the number of inequivalent interstitial sites, which resulted from the interaction of wide varieties of boundary unit structures. The intervariant boundary that consisted of regular arrangements of atoms with fewer boundary unit structure variety exhibited less tendency to H/H2 absorption (e.g., 60°/[111¯] symmetrical tilt boundary). On the contrary, the presence of a substantial number of diverse unit structures within the boundary plane significantly enhanced the tendency for hydrogen segregation (e.g., 51.73°/[116¯11] quasi-tilt and 10.53°/[01¯1¯] twist intervariant boundaries). The current findings provide a fundamental basis for the engineering of the intervariant boundary network within the martensitic microstructure to significantly enhance the hydrogen embrittlement resistance. This is critical for steels used in severe environments, such as hydrogen storage and pipelines for hydrogen transportation.