Hydrogen-enhanced Localized Plasticity Research Articles

Stress-Corrosion-Cracking Sensitivity of the Sub-Zones in X80 Steel Welded Joints at Different Potentials.

X80 steel plays a pivotal role in the development of oil and gas pipelines; however, its welded joints, particularly the heat-affected zone (HAZ), are susceptible to stress corrosion cracking (SCC) due to their complex microstructures. This study investigates the SCC initiation mechanisms of X80 steel welded joints under practical pipeline conditions with varying levels of cathodic protection. The SCC behaviors were analyzed through electrochemical measurements, hydrogen permeation tests, and interrupted slow strain rate tensile tests (SSRTs) conducted in a near-neutral pH environment under different potential conditions (OCP, -1.1 VSCE, -1.2 VSCE). These behaviors were influenced by microstructure type, grain size, martensite/austenite (M/A) constituents, and dislocation density. The sub-zones of the weld exhibited differing SCC resistance, with the fine-grain (FG) HAZ, base metal (zone), welded metal (WM) zone, and coarse-grain (CG) HAZ in descending order. In particular, the presence of coarse grains, low dislocation density, and extensive M/A islands collectively increased corrosion susceptibility and SCC sensitivity in the CGHAZ compared to other sub-zones. The SCC initiation mechanisms of the sub-zones within the X80-steel welded joint were primarily anodic dissolution (AD) under open-circuit potential (OCP) condition, shifting to either hydrogen-enhanced local plasticity (HELP) or hydrogen embrittlement (HE) mechanisms at -1.1 VSCE or -1.2 VSCE, respectively.

In-depth understanding in the effect of hydrogen on microstructural evolution, mechanical properties and fracture micro-mechanisms of advanced high-strength steels welded joints

The hydrogen embrittlement (HE) mechanism of QP980 steels laser welded joints with complex microstructures is unclear. The joint with hydrogen pre-charging fractured at the inter-critical heat affected zone (ICHAZ) in slow strain rate tensile test, and the relative loss of elongation amounted to 84.8 %. Hydrogen-induced microcracks mainly initiated inside martensite (∼ 60 %). Hydrogen-induced crack propagation paths of the joint were transgranular propagation of martensite and intergranular propagation of ferrite/martensite interface. The HE mechanism of the joint was the combination of the effects in hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE).

Molecular dynamics study on the effect of hydrogen on the plastic behavior of α-Fe crack tip

In this paper, the unilateral tension of α-Fe in the local region of the crack tip are simulated by molecular dynamics method, the plastic deformation behaviors and hydrogen embrittlement mechanism of the crack tip under different hydrogen concentration and hydrogen positions are analyzed. The results show that for the hydrogen-containing models, there exists a critical hydrogen concentration c0 that can make the plasticity of the α-Fe material reduce greatly. For the crack-free model, the main hydrogen embrittlement mechanism is adsorption-induced dislocation emission (AIDE), and the critical hydrogen concentration c0 is 0.3%. When the hydrogen concentration cH<c0, the plastic deformation mechanism is mainly dominated by phase transformation; when cH≥c0, the plastic deformation mechanism is phase transformation at low strains and dislocation motion at high strains. For the cracked model, the main mechanism of hydrogen embrittlement is hydrogen-enhanced local plasticity at the crack tip, with a critical c0 of 1.7%. When cH<c0, the plastic deformation mechanism of the model is a combination of phase transformation and dislocation; when cH≥c0, the plastic deformation mechanism is controlled by phase transformation. When the hydrogen is far away from the crack tip, the local mechanical behavior of the crack tip does not change significantly, while when hydrogen gathered at the crack tip, the addition of a small amount of hydrogen atoms will make the fracture strain decrease rapidly, which shows a mechanism of hydrogen-enhanced strain-induced vacancies (HESIV). A saturation value of hydrogen concentration at the crack tip is confirmed in this paper, when the local hydrogen concentration exceeds this saturation value, the plastic behaviour tends to be stable. The influence of hydrogen atoms distribution on the crack initiation behavior was determined by atomic configuration analysis.

A rationale for modelling hydrogen-induced softening in fcc single crystals

Several atomistic-scale mechanisms have been proposed to explain the phenomenon of hydrogen-enhanced localized plasticity (HELP). However, the relative contributions of each of these atomistic mechanisms in determining the constitutive behaviour of fcc single crystals undergoing HELP are yet to be fully understood. In this study, we address this issue by presenting a constitutive model of H-induced softening for fcc single crystals. Here, we first compute H concentrations in the trapped state close to the dislocation cores and in the normal interstitial lattice sites (NILS), which are then used to determine the efficacy of different HELP mechanisms contingent on them. The H in NILS forms atmospheres around dislocations and shields the long-range elastic forces acting on them, while the H in deep traps around dislocations reduces the activation barrier associated with short-range obstacles and enhances the ease of dislocation nucleation. These mechanisms are incorporated in a coarse-grained form into a dislocation density-based constitutive model of fcc single crystals, and the constitutive response is simulated for each one of the underlying mechanisms. Our simulations reveal that the mechanism of enhancement of dislocation nucleation in the presence of H is the most potent to cause softening of the constitutive response. In comparison, the flow softening caused by the mechanisms of reduction of the activation barrier of short-range obstacles and the shielding effect of H appears to be insignificant. The presented constitutive model can be extended to simulate H-induced softening in polycrystalline fcc materials as well.

Hydrogen in metallic alloys ─ embrittlement and enhanced plasticity: a review

Abstract The evolution of ideas concerning the nature of hydrogen embrittlement of engineering metallic materials is described based on a number of the proposed hypotheses and corresponding experiments. The main attention is paid to two of them, namely hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). Recent attempts to interconnect the both models as HELP + HEDE and HELP-mediated HEDE ones are also estimated. A conclusion is made that HELP model is preferential for understanding the entire array of experimental data with a caveat that it is necessary to consider the chemical nature of hydrogen atoms and view them not only as point defects. Based on the studies of hydrogen effect on the atomic interactions in iron, nickel, titanium, and its alloys, it is shown that the electron approach to HELP phenomenon adequately describes two competitive features of hydrogen behavior in metals: increased brittleness and enhanced plasticity. Due to the increase in the concentration of free electrons, hydrogen decreases the elasticity moduli, which causes the crystal lattice to soften. For this reason, the formation of hydrogen atmospheres around the dislocations decreases the start stress of dislocation sources, as well as line tension of emitted dislocations, enhancing thereby their mobility, and weakens repulsion between dislocations in their pile-ups. The range of temperatures and strain rates in which hydrogen embrittlement occurs is controlled by the enthalpies of hydrogen atoms’ diffusion and their binding to dislocations. The resulting consequences for mechanical properties depend on the short-range atomic order, SRO, which inherently occurs in the metallic solid solutions and localizes plastic deformation both in the cases of short-range atomic ordering and of short-range atomic decomposition. Hydrogen enhances slip localization because of its different solubility in the submicrovolumes of short-range decomposed solid solutions. If SRO is absent or not remarkably formed, the hydrogen-increased concentration of free electrons results in enhanced plasticity. Available positive hydrogen effects on the plasticity of titanium β-alloys and austenitic steels are presented and interpreted.

Hydrogen-assisted cracking and damage tolerance of high-strength lath-martensite steel bars for structural applications

The paper addresses the hydrogen-assisted cracking and damage tolerance of high-strength lath martensite steel bars assessed from slow strain rate tensile tests. These were carried out on fatigue precracked specimens simultaneously charged with hydrogen in a standardized 20 % aqueous solution of ammonium thiocyanate at 50 °C and in air, at room temperature, for comparison. The fractographic analysis of the tested specimens showed the steel sensitivity to hydrogen uptake, which gives rise to the sub-critical extension of the pre-existing crack in two phases, initiation and growth, with both hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) being synergically involved as damage mechanisms. Despite this, the damage tolerance behavior of the steel is highly satisfactory because the crack size causing the steel failure in tension slightly exceeds the theoretical limit of plastic collapse under combined tension and bending loading, regardless of the environment aggressiveness. Although H-assisted failure loads fit the theoretical limit loads, the fast propagation of the crack under constant load beyond the small-scale yielding regime could represent a real threat for the structural use of these steel bars in aggressive media unless the service loads are limited in accordance with such behavior.

Hydrogen embrittlement of FeCrAl alloys: The significant role of Laves phase and AlN particles

The addition of Nb to FeCrAl alloy enhanced its mechanical properties owing to the precipitated-phase strengthening effect. However, the additive amount and its effect on hydrogen embrittlement (HE) susceptibility have to be evaluated before implementation, as the hydrogen produced by fission and corrosion in the reactor may affect its mechanical properties. Therefore, hydrogen pre-charged tensile bar specimens were tested, and the HE mechanism was investigated. The results demonstrate that though FeCrAl alloys show low HE susceptibility, the Nb-doped FeCrAl alloys still exhibit a slightly higher HE susceptibility than Nb-free FeCrAl alloys. Their reduced ductility was mainly caused by decohesion and cracking of Fe2(Nb, Mo) precipitates and quasi-cleavage cracking induced by AlN inclusions. The underlying HE mechanisms for the Nb-doped FeCrAl alloys are hydrogen-enhanced localized plasticity (HELP) and HELP-mediated hydrogen-enhanced decohesion (HEDE).

Multiscale evaluation of hydrogen-assisted mechanical degradation in grade 2 titanium

The mechanism of hydrogen embrittlement in grade 2 titanium was studied by a combined micro and nanoscale approach. Insights into the hydrogen related deformation and degradation mechanisms were acquired via a unique characterisation procedure, correlating the topological observations on the fracture surface after tensile testing to the dislocation arrangement underneath. Focused ion beam lift-out was employed to extract a subsurface slice perpendicular to the fracture surface. High resolution characterisation provided information about the dislocation configuration and active hydrogen-assisted degradation mechanism. A high hydrogen content forms a brittle titanium hydride phase, failing via hydride cleavage. At a moderate hydrogen quantity, hydrogen in solid solution is responsible for a ductility decrease via the hydrogen-enhanced localised plasticity mechanism.

A continuum scale chemo-mechanical model for multi-trap hydrogen transport in deformed polycrystalline metals

This study presents a coupled multi-trap hydrogen diffusion and crystal plasticity model within a thermodynamically consistent framework. The developed modeling scheme incorporates the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced strain-induced plasticity (HESIV) based on thermodynamic considerations. The theoretical formulations are implemented in the finite element method, which considers the concentration- and chemical potential-based balance equations. The validity of the coupled model is established through preliminary single element simulations under isothermal conditions, showcasing the predictability of evolution of hydrogen concentration and occupancies in the deformed microstructure of polycrystalline metal. Then, the developed computational approach is applied to two materials exhibiting hydrogen-induced hardening and softening behaviors in accordance with the HELP mechanism and vacancy-hydrogen complex (VaH) formation under isothermal condition. The model successfully reproduces the change of mechanical behavior induced by hydrogen in both materials. The effects of hydrogen diffusivity and the binding energy of defects on hydrogen transport are also investigated. Furthermore, the sensitivity of hydrogen diffusion to different trapping mechanisms, local equilibrium trapping and kinetics trapping, is examined. The proposed modeling framework can be applied to new material design strategies for developing metals resistant to hydrogen embrittlement.

An atomistic study on the HELP mechanism of hydrogen embrittlement in pure metal Fe

The Hydrogen Enhanced Localized Plasticity (HELP) mechanism is one of the most important theories explaining Hydrogen Embrittlement in metallic materials. While much research has focused on hydrogen's impact on dislocation core structure and dislocation mobility, its effect on local dislocation density and plasticity remains less explored. This study examines both aspects using two distinct atomistic simulations: one for a single edge dislocation under shear and another for a bulk model under cyclic loading, both across varying hydrogen concentrations. We find that hydrogen stabilizes the edge dislocation and exhibits a dual impact on dislocation mobility. Specifically, mobility increases below a shear load of 900 MPa but progressively decreases above this threshold. Furthermore, dislocation accumulation is notably suppressed at around 1 % hydrogen concentration. These findings offer key insights for future research on Hydrogen Embrittlement, particularly in fatigue scenarios.

Hydrogen enhanced localized plasticity in zirconium as observed by digital image correlation

Hydrogen embrittlement in zirconium alloys is a highly relevant research topic, important for ensuring the integrity of nuclear fuel clads. While the presence of hydrides in zirconium-based clad is a well-known risk factor due to their brittleness, the effect of the presence of hydrogen in solid solution is unclear.According to the Hydrogen Enhanced Localized Plasticity (HELP) model, hydrogen in solid solution may affect the mechanical performance of a metal by increasing dislocation mobility due to the shielding effect that hydrogen has on the interaction of dislocations with other types of defects. In the presented work, the presence of HELP has been assessed in Zircaloy-4 samples, under conditions relevant to the storage of spent nuclear fuel.Elevated temperature tensile tests were performed on samples containing localized areas of higher hydrogen concentration, as quantified by neutron radiography. Digital Image Correlation (DIC) analysis was performed on the specimen surfaces for quantitative characterization of strain heterogeneity during deformation to identify the regions where the plastic strain localization occurs.These regions could be correlated to elevated amounts of hydrogen in solid solution, that is, in the location immediately adjacent to dissolving hydrides, or in the areas of the samples with local hydrogen concentration close to the expected terminal solid solubility limit (TSS) in the test conditions.The results confirm the presence of localized softening caused by solid solution hydrogen, indicating that the HELP mechanism needs to be taken into account when modeling the mechanical response of zirconium-based nuclear fuel clads at elevated temperatures.

Hydrogen-enhanced entropy (HEENT): A concept for hydrogen embrittlement prediction

The concept of hydrogen-enhanced entropy (HEENT) for hydrogen embrittlement prediction was firstly introduced and discussed through experimental validation methods, multiscale characterization, and trapping studies. The accumulated generated entropy reaches a constant value (fatigue fracture entropy (FFE)) regardless of hydrogen content. Hydrogen-enhanced localized plasticity (HELP) is the dominant mechanism for hydrogen-assisted cracking in the studied material, due to quasi-cleavage fracture pattern with serrated marking and H-enhanced planar slip. Fatigue life is reduced due to increasing hydrogen uptake with increasing current density of H charge, and the accumulated generated entropy reaches a constant FFE value. As H content increases, a fraction of weakly trapped hydrogen at interstitials and dislocations in the ferrite increases, and the possibility of reaching hydrogen to crack tip and slip planes increases. Direct evidence of HELP mechanisms with a well-developed dislocation substructure is reported for the first time in pearlitic steels with nanoscale observations near the H-assisted crack path. The contribution of microplasticity due to the HELP mechanism to the total entropy compensates for the total entropy reduction generated due to reduced fatigue life. This introduces the hydrogen-enhanced entropy (HEENT) concept for hydrogen embrittlement prediction.

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.

Simultaneous enhancement of strength and hydrogen embrittlement resistance of laser-powder bed fusion maraging steel via long-term low-temperature aging

This study reported an effective strategy for simultaneously enhancing the strength and hydrogen embrittlement resistance of laser-powder bed fusion(L-PBF) 18Ni300 maraging steel through a long-term low-temperature aging treatment (LTA). The beneficial roles of LTA are: (i) LTA produced a multi-scale and multi-dimensional hydrogen trap network composed of dispersed ω nanoprecipitates and dense cellular structures, which strongly hindered hydrogen enrichment at grain boundaries, thereby inhibiting hydrogen-enhanced decohesion. (ii) High-density ω precipitates not only enhanced the strength, but also affected H-dislocation interactions, thereby hindering hydrogen-enhanced localized plasticity. (iii) LTA eliminated the formation of austenite aggregation zone, reducing the additional risk of cracking.

Effect of hydrogen on the dislocation evolution in an aluminum alloy under cyclic loading by two-dimensional discrete dislocation dynamics

The aim of this research is to reveal the effect of hydrogen on fatigue crack initiation in an aluminum alloy. First, the effect of hydrogen on the dislocation movement, that is, the hydrogen-enhanced localized plasticity (HELP), was modeled. Two mechanisms of HELP were modeled: reduction of the dislocation interaction stress and an increase of the dislocation velocity by hydrogen. Next, simulations of the early stage of crack initiation were performed by discrete dislocation dynamics with the HELP model. The simulation results indicated that HELP accelerates fatigue crack initiation and the increase of the dislocation velocity by hydrogen is the main mechanism of HELP.

Hydrogen embrittlement prompt fracture in Ni-based single crystal superalloy

Hydrogen-fueled and hydrogen-hybridized aircraft engines are a new trend in the aviation industry for environmental reasons. Single crystalline Ni-based superalloys are the most commonly used engine materials and their hydrogen embrittlement properties need urgent investigation. In this study, the hydrogen embrittlement behavior and underlying fracture mechanism of a second-generation Ni-based single crystal superalloy with electrochemical hydrogen pre-charge were investigated. The superalloy showed tremendous susceptibility to hydrogen embrittlement with reduced strength and ductility. A large number of micropores and cracks on the fracture surface are found in hydrogen-charged specimens, leading to embrittlement and ultimate cracking. More dislocations, stacking faults and DSBs are observed in specimens with hydrogen uptake. Hydrogen-induced micropores first form at the γ/γ′ interface and then propagate into the γ′ phase, leading to cracking, which was analyzed using in situ environmental studies with a transmission electron microscope. Hydrogen reduces the cohesive strength between the γ- and γ′-phase and accelerates crack propagation along the voids. Hydrogen embrittlement fracture in Ni-based single crystal superalloys is due to synergistic hydrogen-enhanced local plasticity, strain-induced vacancies and decohesion in the hydrogen-induced cracking process.

Anisotropy and grain size dependence of the effects of hydrogen on the shock-induced spallation in iron

Hydrogen (H) embrittlement in metals has long been a source of concern in academia and industry. However, the fundamental mechanisms remain not well understood in dynamic events. Using atomic simulations, shock responses of monocrystalline α-Fe along the [100] and [111] crystal orientations and nanocrystalline Fe with and without H atoms are investigated systematically. For the first time, the anisotropic dependences of the effects of H atoms on the spall strength are revealed. In the case of the [100] crystal orientation, H atoms prevent the phase transition of α-iron under shock compression, further slowing down the growth of voids during spall fracture and enhancing the spall strength at high strain rates. While in the case of [111] crystal orientation, H atoms promote dislocation formations under shock compression which is consistent with the known hydrogen-enhanced local plasticity (HELP) mechanism; H atoms accelerate the growth of voids during the spallation and reduce loads-bearing capacity, which finally lowers the spall strength of α-iron. In nanocrystalline samples, voids favor along GBs. The presence of H atoms would cause a local disturbance at the GB but block the long-distance movement of the surrounding atoms resulting in a slight increase in spall strength. The combined effects of H atoms and GBs influence the void evolution. This work provides deep insights into the hydrogen effect on metals under dynamic loading and should benefit a wide broad of research communities in materials and mechanics.

Effect of hydrogen on very high cycle fatigue properties of 17-4 PH martensite stainless steel

Very high cycle fatigue properties of 17-4 PH stainless steel aged for 1 h and 4 h after hydrogen charging were studied. Results show that hydrogen reduces fatigue strength and changes crack initiation mode to mainly inducing by inclusions. The existence of intergranular facets on fracture surface indicates that hydrogen accelerates crack initiation and early crack growth. Hydrogen-granular bright facet (GBF) mechanism may be interpreted as lath martensite fracture and dislocation cells formation, where hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanisms occur. The degradation of hydrogen on fatigue properties is suppressed by hydrogen trapping effect of ε-copper precipitates.

Effects of Diffusible Hydrogen on Fatigue Life of Spot Welds in High-tensile-strength Steel Sheets

Spot-welded advanced high-strength steel (AHSS) sheets are used to decrease car body weight and enhance crashworthiness simultaneously. Such welds are susceptible to hydrogen embrittlement, but research in this area is limited. We have evaluated the hydrogen embrittlement properties of spot welds fabricated with AHSS sheets and clarified the dependence of diffusible hydrogen content and tensile rate on tensile shear strength. However, the performance of spot welds must be evaluated not only by static testing such as tensile shear tests but also by dynamic fatigue testing. Herein, we investigated the effects of diffusible hydrogen on their fatigue life via tensile–shear fatigue tests under cathodic hydrogen charging. The number of cycles to failure and the endurance limit decreased with diffusible hydrogen content. The number of cycles to failure decreased as the constant amplitude-loading frequency declined. The intergranular cracking is more likely to occurs as the amount of diffusible hydrogen increases and as the frequency decreases. These results are explained by hydrogen accumulation and crack growth behavior caused by mechanism for hydrogen-enhanced local plasticity and hydrogen enhancement of the strain-induced generation of vacancies mechanism. Furthermore, the correlation between static testing and dynamic fatigue testing was investigated, and the relationship between fatigue strength and the amount of diffusible hydrogen was in good agreement with the diffusible hydrogen dependence of tensile shear strength obtained from slow rate tensile shear tests under hydrogen charging.

Hydrogen Embrittlement of CrCoNi Medium-Entropy Alloy with Millimeter-Scale Grain Size: An In Situ Hydrogen Charging Study

In this study, we examined the effect of charging current density on the hydrogen embrittlement (HE) of MEA and the associated HE mechanisms using electron backscattered diffraction (EBSD). Results show that MEA is susceptible to HE, but is stronger than as-rolled and 3D-printed Cantor alloy and stainless steel. The HE susceptibility of MEA decreases with increasing current density. Ductile fracture with transgranular dimples switches to intergranular brittle fracture with clear slip bands in the interior of grains. EBSD results uncovered that hydrogen facilitates localized slips and deformation twins. Hydrogen-enhanced localized plasticity and hydrogen decohesion are the possible HE mechanisms.