Hydrogen-enhanced Decohesion Mechanism Research Articles

Phase-field simulation of dual-crack system hydrogen embrittlement in metallic materials

Hydrogen embrittlement is a severe form of failure in metallic materials, where hydrogen ions significantly reduce the load-bearing capacity of the material. The presence of hydrogen and multiple cracks complicates the fracture process, rendering fracture prediction considerably more challenging. In this work, we employ the standard phase-field damage model to investigate the propagation of dual cracks based on the hydrogen-enhanced decohesion mechanism (HEDE). Furthermore, we have enhanced the iteration approach of the phase-field method by introducing Anderson acceleration and over-relaxation as complementary strategies on top of the staggered solution scheme. This modification significantly reduces the number of iterations, thereby achieving efficient solutions. Additionally, this paper comprehensively considers the influence of various crack factors, under different crack arrangement configurations, on both the material's load-bearing capacity and the interaction among cracks. These arrangements include horizontal, inclined, collinear, parallel, internal, and boundary positions. The influencing factors encompass crack length, crack tilt angle, the lateral spacing between cracks, the vertical spacing between cracks, and the distance between cracks and boundaries. Through numerical simulations, universal patterns of the influence of various crack factors on the fracture behavior of a dual-crack system in a hydrogen environment were identified. It was demonstrated that when the influence regions of two cracks overlap, it significantly affects their propagation paths and reduces the model's load-carrying capacity.

A Phase-Field Regularized Cohesion Model for Hydrogen-Assisted Cracking

Hydrogen-assisted cracking (HAC) usually causes premature mechanical failure of the material and results in structural damage in hydrogen environments. A phase-field regularized cohesion model (PF-CZM) was proposed to address hydrogen-assisted cracking. It incorporated the hydrogen-enhanced decohesion mechanism to decrease the critical energy release rate to address damage initiation and progression in a chemo-mechanical coupled environment. This model is based on coupled mechanical and hydrogen diffusion responses, driven by chemical potential gradients, and the introduction of hydrogen-related fracture energy degradation laws. The coupling problem is solved by an implicit time integral, in which hydrogen concentration, displacement and phase-field order parameters are the main variables. Three commonly used loading regimes (tension, shear, and three-point bending) were provided for comparing crack growth. Specifically, (i) hydrogen-dependent fracture energy degradation, (ii) mechanical–chemical coupling, and (iii) the diffusion coefficient D is influenced by both the phase field and the chemical field. By considering these factors, the PF-CZM model provided a variational framework by coupling mechanical loading with concentration diffusion for studying the complex interplay between a chemo-mechanical coupled environment and material damage, thereby enhancing our understanding of hydrogen-assisted cracking phenomena.

Hydrogen embrittlement of a precipitation-strengthened high-entropy alloy

The present study explores the hydrogen embrittlement characteristics, cracking mechanism, and the impact of hydrogen on the deformation microstructure of (FeCoNi)86Al7Ti7 high-entropy alloy (HEA). The cracking induced by hydrogen primarily manifests as intergranular cracking, with the interaction between the L21 phase and dislocation slip at grain boundaries promoting the hydrogen-enhanced decohesion mechanism. The distinct work hardening stage is observed during the tensile testing of hydrogen-charged samples. This additional work hardening is attributed to the early formation of microband structures, which correlates with stacking fault energies and directional dislocation substructure. However, the propagation of crack significantly diminishes the ductility.

Design of high-strength martensitic steels by novel mixed-metal nanoprecipitates for high toughness and suppressed hydrogen embrittlement

To obtain a fundamental understanding of mechanisms of hydrogen embrittlement (HE) and its prevention in advanced high-strength steels containing novel nanoscale mixed-metal precipitates, it is necessary to study local microstructure, H trapping, and crack path with new multiscale experimental and simulation approach. Spatially resolved hydrogen mapping via SKPFM is used together with investigation of the crack path using high-resolution EBSD and HMPT, and global trapping behavior of the alloys by TDS. These results are combined with newly introduced method to elucidate real-time distribution of hydrogen in the alloy using high-energy synchrotron X-ray diffraction (HES-XRD). Mixed-metal precipitates improves HE resistance of the alloy, due to nature of the trapping sites, e.g. irreversible H-trapping by carbon vacancies inside novel nanoprecipitates and high total length of PAGBs. This is because of lower possibility of build-up of critical local hydrogen content at PAGBs for intergranular hydrogen-assisted cracking due to hydrogen-enhanced decohesion mechanism. Less weakly trapped hydrogen also reduces frequency of dislocation activation and enhanced dislocation slip in {011} slip plane due to hydrogen-enhanced localized plasticity in regions with affinity for transgranular hydrogen-assisted cracking at lower local hydrogen content. Direct evidence of carbon vacancies in novel nanoprecipitates is observed for the first time via HAADF-STEM.

Hydrogen-enhanced decohesion mechanism of the Ni-Ni3X interfaces in precipitation-hardened Ni-based alloys

Ni and its alloys are susceptible to hydrogen embrittlement. In this study, we investigate the phenomenon of hydrogen-enhanced decohesion at inter-phase interfaces in precipitation-hardened Ni-based alloys using a systematic first-principles approach. We demonstrate that hydrogen atoms primarily prefer to localize at the Ni3Al phase in the Ni/Ni3Al interface, while they tend to be trapped by Ni in the Ni/Ni3Nb interface. Our findings reveal that hydrogen induces inter-phase embrittlement in both the Ni/Ni3Al and Ni/Ni3Nb interfaces. Moreover, we show that the hydrogen-enhanced decohesion at these interfaces is influenced by various factors such as hydrogen pressure, hydrogen content, temperature, and strain. Finally, we discuss in detail the hydrogen-enhanced decohesion mechanisms at the Ni/Ni3Al and Ni/Ni3Nb interfaces, including their electronic structures, energy landscape of hydrogen at trapping sites, and schematics of crack propagation.

Hydrogen Embrittlement Failure Behavior of Fatigue-Damaged Welded TC4 Alloy Joints

The interaction between fatigue damage and hydrogen in TC4 welded joints was analyzed considering specimens with different pre-cycles. The fracture failure caused by hydrogen-induced fatigue damage was investigated using tensile testing and microstructural observations. The results indicate that increasing the number of pre-cycles increases the specimens’ initial dislocation density, enhancing the severity of the hydrogen-induced embrittlement phenomenon. The hydrogen-enhanced local plasticity mechanism, manifested by fracture along the grain boundary, dominates the tensile process in fatigue-damaged hydrogen-filled welded joints after 10,000 and 20,000 pre-cycles. After 30,000 pre-cycles, the hydrogen-enhanced decohesion mechanism dominates the tensile process, manifesting as a transgranular cleavage decoupling fracture.

Hydrogen-prompted heterogeneous development of dislocation structure in Ni

First documented in 1875, the deterioration of mechanical properties of hydrogen-containing metals is a longstanding yet unsolved problem in materials science. In this work, the evolution of dislocation structures in differently orientated grains (i.e., near [100], [110], and [111]) of the uncharged and hydrogen-charged (400 and 1200 ppm) polycrystalline Ni were systematically investigated by combining electron backscatter diffraction, focused ion beam and scanning transmission electron microscopy. By using site-specific characterization methods, for the first time, we discover that hydrogen-enhanced localized plasticity (HELP) is orientation-dependent, with the following sequence: [100] > [111] > [110]. Massive incompatibility between differently orientated grains, induced by the orientation dependence of HELP, contributes to the premature intergranular fracture of Ni, especially for the 400 ppm H-charged Ni. Our results suggest that optimizing orientation distribution is a potential approach for enhancing metals' resistance to hydrogen damage. The relative contribution of HELP and hydrogen-enhanced decohesion (HEDE) mechanisms in hydrogen embrittlement of Ni is also analyzed quantitatively for 400 and 1200 ppm H-charged samples. In the 400 ppm H-charged Ni, a strong synergistic interaction exists between HELP and HEDE mechanisms, and the HELP mechanism plays a critical role in premature fracture. By contrast, in the 1200 ppm H-charged Ni, the HELP effect on final failure is much less significant and HEDE is the dominant embrittlement mechanism.

The effect of charging conditions on hydrogen embrittlement behavior of ultra-high-strength steel 22MnB5

The effect of charging conditions on hydrogen embrittlement (HE) of the hot-stamping boron steel was investigated with its hydrogen distributions and reversibility properties. With low hydrogen concentration, no noticeable hydrogen-induced cracking was detected, coupled with the uniform distributed diffusible hydrogen, realizing reversible HE. The quasi-cleavage fracture modes suggested the hydrogen-enhanced localized plasticity (HELP) mechanism. At high hydrogen concentration, the diffusible hydrogen distributed at the martensitic block and prior austenite grain boundaries which facilitated hydrogen-induced cracking initiation and propagated along the {110} M plane, leading to irreversible HE. The intergranular fracture modes suggested the transition of HELP to hydrogen-enhanced decohesion mechanism (HEDE). • The HE mechanism transits from HELP to HEDE as hydrogen concentration increases. • The distribution of hydrogen atoms is visualized by hydrogen microprint technology. • The reversibility of HE depends on the formation of hydrogen-induced cracking.

Hydrogen embrittlement behavior in interstitial Mn–N austenitic stainless steel

The susceptibility to hydrogen embrittlement behavior was investigated in an interstitial Mn–N austenitic steel HR183 and stainless steel 316L. Hydrogen was introduced by cathodic hydrogen charging at 363 K. HR183 has stronger austenite stability than 316L despite its lower nickel content, the addition of manganese and nitrogen inhibited martensitic transformation during the slow strain rate tensile deformation. Due to the diffusion of hydrogen being delayed by the interstitial solution of nitrogen atoms and the uniform dislocation slips, hydrogen permeates more slowly in HR183 than 316L, contributing to an 84.79 μm thinner brittle fracture layer in HR183 steel. Hydrogen charging caused elongation losses in both 316L and HR183 steels associated with the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanism. However, the hydrogen embrittlement susceptibility of HR183 is 3.4 times lower than that of 316L according to the difference in elongation loss between the two steel after hydrogen charging. Deformation twins trapped a lot amount of hydrogen leading to brittle intergranular fracture in 316L. The multiple directions of slip in HR183 steel suppressed the strain localization inside grains and delayed the adverse effects conducted by HELP and HEDE mechanism, eventually inhibiting server hydrogen embrittlement in the HR183 steel. This study is assisting in the development of low-cost stainless steel with excellent hydrogen embrittlement resistance that can be used in harsh hydrogen-containing environments.

The effect of an Al-induced ferritic microfilm on the hydrogen embrittlement mechanism in martensitic steels

The addition of 2 wt% aluminium to a martensitic Fe-0.4C steel leads to a ferritic microfilm along the prior austenitic grain boundaries, which was demonstrated to significantly increase the ductility without affecting the bulk hardness. This study investigates its effect on the hydrogen embrittlement behavior. For Al-free martensitic steel without grain boundary ferrite, the presence of hydrogen causes a brittle cleavage type of fracture due to the embrittlement of the martensitic packets/blocks according to the hydrogen enhanced decohesion mechanism. Al-containing steels, with a ferritic grain boundary microfilm, on the other hand, experience a delay in hydrogen-induced fracture, and the crack propagation is redirected along the ferrite/martensite interface resulting in an intergranular fracture surface. This can be linked to an interplay between the hydrogen enhanced decohesion and hydrogen enhanced localized plasticity mechanism. However, by applying slow deformation rates in a hydrogen rich environment, the ferritic microfilm itself embrittles and loses its capability to influence the crack propagation, thus resulting in a similar fracture surface and ductility as the samples without the ferritic microfilm.

Analysis of Hydrogen-Assisted Brittle Fracture Using Phase-Field Damage Modelling Considering Hydrogen Enhanced Decohesion Mechanism

This study proposes a hydrogen-assisted fracture analysis methodology considering associated deformation and hydrogen transport inside a phase-field-based formulation. First, the hydrogen transport around a crack tip is calculated, and then the effect of hydrogen enhanced decohesion (HEDE) is modeled by defining the critical energy release rate as a function of hydrogen concentration. The proposed method is based on a coupled hydrogen mechanical damage under phase-field and implemented through a user subroutine in ABAQUS software. The test using compact tension (CT) sample is investigated numerically to study the hydrogen embrittlement on 45CrNiMoVA steel. Experimentally, the microstructural fracture presents a mixed brittle fracture mode, consisting of quasi-cleavage (QC) and intergranular (IG) fracture with hydrogen. This fracture mode is consistent with the suggested HEDE mechanism in the model. The simulation results show that hydrogen accumulates at the crack tip where positive hydrostatic stress is located. Moreover, the model estimates the initial hydrogen concentration through iterations. The simulated load-line displacement curves show good agreement with the experimental plots, demonstrating the predictive capabilities of the presented scheme.

Combined thermal desorption spectroscopy, hydrogen visualization, HRTEM and EBSD investigation of a Ni–Fe–Cr alloy: The role of hydrogen trapping behavior in hydrogen-assisted fracture

The role of hydrogen trapping behavior in hydrogen-assisted cracking in a Ni–Fe–Cr alloy has been investigated using thermal desorption spectroscopy (TDS), hydrogen visualization, and high-resolution microstructural characterization methods. The results indicate that hydrogen resides at interstitial lattice sites, dislocations, and vacancies with the ascending order of the desorption activation energies. This study firstly shows that hydrogen is trapped reversibly at dislocations including misfit dislocations at the austenite/δ phase interface, and deformation-induced dislocations. The hydrogen trapping behavior is proved by TDS and hydrogen visualization technique. The weakly trapped hydrogen at dislocations provides sufficient hydrogen to build up a required critical hydrogen concentration at potential flaws for crack nucleation and to activate hydrogen enhanced decohesion mechanism. Further, it was found that δ phase precipitation exacerbates the hydrogen embrittlement sensitivity. For the first time, evidence for hydrogen transport by dislocations in a Ni-based alloy is observed by the hydrogen visualization technique.

Simulating intergranular hydrogen enhanced decohesion in aluminium using density functional theory

Materials modelling at the atomistic scale provides a useful way of investigating the widely debated fundamental mechanisms of hydrogen embrittlement in materials like aluminium alloys. Density functional theory based tensile tests of grain boundaries (GBs) can be used to understand the hydrogen enhanced decohesion mechanism (HEDE). The cohesive zone model was employed to understand intergranular fracture from energies obtained in electronic structure calculations at small separation increments during ab initio tensile tests of an aluminium Σ11 GB supercell with variable coverages of H. The standard rigid grain shift (RGS) test and a quasistatic sequential test, which aims to be faster and more realistic than the RGS method, were implemented. Both methods demonstrated the effects of H on the cohesive strength of the interface. The sequential method showed discrete structural changes during decohesion, along with significant deformation in general compared to the standard rigid approach. H was found to considerably weaken the GB, where increasing H content led to enhanced embrittlement such that, for the highest coverages of H, GB strength was reduced to approximately 20% of the strength of a pure Al GB—it is proposed that these results simulate HEDE. The possibility of finding H coverages required to induce this effect in real alloy systems is discussed in context by using calculations of the heat of segregation of H.

Influence of tempering time on the fracture toughness of hydrogen pre-charged 42CrMo4 steel

This work aims to find suitable 42CrMo4 steel grades for working in energetic applications dealing with high pressure hydrogen gas. Consequently, the influence of the tempering time on the hydrogen embrittlement (HE) sensitivity of 42CrMo4 steel quenched and tempered at 600 °C is studied. Compact tension (CT) specimens were pre-charged with gaseous hydrogen in a pressurized reactor at 19.5 MPa and 450 °C for 21 h and tested for fracture toughness afterwards in air. The hydrogen concentration introduced in the steels was determined using thermal desorption analysis (TDA). The fracture micromechanisms were subsequently identified by means of scanning electron microscopy (SEM). Electrochemical hydrogen permeation (EHP) tests were also performed to gain better insight into the interaction between hydrogen atoms and the microstructure of these steel grades, and consequently to justify the results. The fracture toughness of hydrogenated 42CrMo4 steel increases considerably with the tempering time, being this behaviour correlated with the density of hydrogen traps in the steel. Brittle fracture micromechanisms, associated with the hydrogen-enhanced decohesion mechanism HEDE (martensitic lath decohesion and intergranular fracture) were present to a lesser or greater degree in all the tempering times.

Hydrogen-Induced Martensitic Transformation and Twinning in Fe45Mn35Cr10Co10

Hydrogen embrittlement can occur in steels with metastable phases, due to activation of the hydrogen-enhanced decohesion mechanism upon transformation. Meanwhile, recent investigations suggest that alloys undergoing ε-martensite transformation may exhibit resistance to hydrogen embrittlement. To better understand hydrogen effects in these alloys, we investigate the hydrogen-induced microstructural transformations in a metastable Fe45Mn35Co10Cr10 alloy. To this end, we electrochemically charge unstrained samples, quantify the hydrogen evolution by thermal desorption spectroscopy, and observe microstructural transformations by scanning electron microscopy techniques. Through these analyses, we find that the hydrogen-induced ε-martensite formation is dependent on the crystallographic orientation of the austenite grains, and takes place preferentially along Σ3 boundaries. Further charging of hydrogen induces extension twinning within the martensite. We examine the microstructural factors influencing these transformations to better understand the hydrogen-microstructure interactions.Graphical Abstract

Investigation on hydrogen embrittlement susceptibility in martensitic steels with 1000 MPa yield strength

Two quenched and tempered cost-saving marine steels with 1000 MPa yield strength were charged with hydrogen by the electrochemical technique to evaluate the hydrogen embrittlement (HE) behavior. The HE susceptibility index (HEI) obtained by slow strain rate tensile tests (SSRT) indicated that the HE resistance of steel A was weaker than that of steel B. Aided by fracture surface analyses, it could be determined that HE was a combination of hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) model. The microstructural characteristics were examined via transmission electron microscopy (TEM) and scanning electron microscope with electron backscattered diffraction analysis (SEM-EBSD). Thermal desorption spectroscopy (TDS) was applied to explore the hydrogen trapping behavior. It was shown that compared to steel A with fine microstructure and low Σ3 fraction, steel B had excellent HE resistance, which could result from the presence of more nano-sized NbC particles. The precipitation of NbC could not only act as an irreversible hydrogen trap to inhibit the hydrogen diffusion, suppressing the HEDE mechanism, but also can restrict hydrogen–dislocation interaction and suppress the HELP mechanism. The role of the interface between carbide and matrix on hydrogen trapping was investigated by density functional theory (DFT) and showed that the hydrogen capture force of NbC was stronger than (Nb,V)C. In addition, with the coarsening of microstructure during tempering at 580 °C, the high-angle grain boundaries (HAGBs) was obviously reduced, led to the HE susceptibility increased, especially steel A.

Hydrogen embrittlement fracture mechanism of 430 ferritic stainless steel: The significant role of carbides and dislocations

The effect of electrochemical hydrogen charging on the mechanical properties and the fracture mechanism of AISI430 ferritic stainless steel were studied by tensile test. Electrochemical hydrogenation experiments of 2, 4, 8 and 12 h on the tensile specimen with a current density of 50 mA/cm2. Hydrogen has a significant impact on the mechanical properties of the material: the elongation of the material is reduced from 27.8% to 13.0%, the yield strength is increased from 325 MPa to 380 MPa, and the tensile strength is increased from 500 MPa to 575 MPa. The obvious increase in tensile strength is attributed to two aspects: (1) the hydrogen-induced dislocations pinning; (2) the influence of the second phase carbide particles on the critical shear stress of dislocation glide. As the hydrogen outgassing time increases, the hardness measurement proves the softening effect of hydrogen. Since the reduction of the binding energy of the carbide and ferrite matrix interface is attributed to the HEDE mechanism, the crack nucleation in the vicinity of the carbide leads to the generation of the cleavage (C) region. Under low diffusible hydrogen content, the samples are mainly C mode and F-MVC mode guided by the synergistic effect of hydrogen-enhanced decohesion (HEDE), hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced strain-induced vacancies (HESIV) mechanisms and under high diffusible hydrogen content, the transgranular (TG) mode replaces the F-MVC mode, and the HEDE mechanism is more dominant than the HELP mechanism.

Analysis of Hydrogen Embrittlement on Aluminum Alloys for Vehicle-Mounted Hydrogen Storage Tanks: A Review

High-pressure hydrogen tanks which are composed of an aluminum alloy liner and a carbon fiber wound layer are currently the most popular means to store hydrogen on vehicles. Nevertheless, the aluminum alloy is easily affected by high-pressure hydrogen, which leads to the appearance of hydrogen embrittlement (HE). Serious HE of hydrogen tank represents a huge dangers to the safety of vehicles and passengers. It is critical and timely to outline the mainstream approach and point out potential avenues for further investigation of HE. An analysis, including the mechanism (including hydrogen-enhanced local plasticity model, hydrogen-enhanced decohesion mechanism and hydrogen pressure theory), the detection (including slow strain rate test, linearly increasing stress test and so on) and methods for the prevention of HE on aluminum alloys of hydrogen vehicles (such as coating) are systematically presented in this work. Moreover, the entire experimental detection procedures for HE are expounded. Ultimately, the prevention measures are discussed in detail. It is believed that further prevention measures will rely on the integration of multiple prevention methods. Successfully solving this problem is of great significance to reduce the risk of failure of hydrogen storage tanks and improve the reliability of aluminum alloys for engineering applications in various industries including automotive and aerospace.

Influence of hydrogen-enhanced plasticity and decohesion mechanisms of hydrogen embrittlement on the fracture resistance of steel

Hydrogen gas is a renewable energy source for electrical and transportation fuel for vehicular applications. However, the storage and transportation of hydrogen gas are challenging because of its very nature and impact on pipelines and storage tank/facility materials. This paper investigates the influence of hydrogen on the candidate fracture toughness (KQ) of low carbon steel immersed in acidic hydrogen environments for one year which has limited previous research. Steel specimens were coated from all sides except one surface to accurately quantify the influence of hydrogen diffusing from the environments into the specimens. Specimens were tested for crack tip opening displacement (CTOD) fracture toughness at six- and twelve-month intervals of immersion in acidic environments. Before KQ testing at various intervals, the hydrogen contents of the specimens were determined by an electrochemical approach. Based on test results, models for the degradation of KQ of steel were developed in accordance with the proposed hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) model (HELP + HEDE model) of hydrogen embrittlement. Furthermore, fractography of the specimens was performed to observe the synergistic action of HELP and HEDE mechanisms (HE), and their subsequent effects on the microstructure and fracture resistance of steel. The significance of the research is highlighted by its practical application for assessing the durability of steel structures and infrastructure against hydrogen environmental assisted cracking (HEAC). Furthermore, this paper highlights the synergistic activity of HELP and HEDE mechanisms of HE in steel and the importance of developing structures for storing hydrogen on a large scale.

Comparative study of phase-field damage models for hydrogen assisted cracking

Hydrogen assisted cracking (HAC) usually causes premature failure of metallic materials and results in unexpected collapse of structures under the environmental exposure of hydrogen. Therefore, computational modeling of HAC is of paramount significance to quantify the adverse effects of HAC on the integrity and safety of structures. In this respect phase-field models for fracture are promising since they are able to seamlessly deal with complex crack patterns like nucleation, branching, merging and even fragmentation in a standalone framework. In this work we provide a comparative study of three frequently adopted phase-field models for hydrogen assisted cracking. Both the phase-field models for brittle fracture (e.g., the AT1/2 and WN models) and the phase-field regularized cohesive zone model (PF-CZM) are considered within the unified phase-field theory for damage and fracture, and are extended to incorporate the hydrogen enhanced decohesion mechanism. The numerical implementation of the phase-field models for HAC is also presented, with a simple scheme for calculating the gradient of the hydrostatic stress. Representative numerical examples show that, the PF-CZM with an increasing Irwin’s characteristic (internal) length and the PF-CZM with a constant one are both insensitive to the regularization length scale parameter. This merit make them promising for the computational modeling of HAC.