Experimental measurements of the stress–strain curve from uniaxial tension tests in hydrogen yielded both macroscopic hardening and softening results despite the fact that hydrogen increases the mobility of dislocations according to the hydrogen-enhanced localized plasticity (HELP) mechanism for embrittlement. In his seminal paper, Howard Birnbaum (Scripta Metallurgica et Materialia, 31(2), pp. 149–153, 1994) using dislocation plasticity explained the contradictory outcomes of the uniaxial tension test on the basis of the two fundamental principles of HELP, hydrogen-enhanced dislocation mobility, and hydrogen-induced plastic flow localization; principles whose effects have been experimentally observed in almost all metals and alloys. By way of example, Birnbaum showed that the flow stress increases in hydrogen if the increased dislocation mobility cannot balance the shear localization hardening effect. In this work, we revisit the dexterity of Birnbaum's theory from a continuum plasticity standpoint and develop a framework to calculate macroscopic flow stress versus plastic strain curves in the presence of plastic flow localization in hydrogen. We incorporate the hydrogen effect on the constitutive response of the material through its effect on the activation enthalpy and volume characterizing the motion of dislocations—parameters that can be measured experimentally. Then, on the basis of Birnbaum's theory, we demonstrate that the competing and combined hydrogen effects on enhanced localization and enhanced dislocation mobility can result in either macroscopic hardening or softening response depending on their individual intensity and magnitude.

Understanding and predicting mechanical properties such as proof stress (yield strength), ultimate tensile strength, elongation to failure, and reduction in area are essential for screening the application of austenitic stainless steels in adverse chemomechanical environments. However, experimental determination of these properties is time-consuming, labor-intensive, and costly—especially under extreme conditions, which requires advanced experimental capabilities. In this study, we leverage a large-scale, curated dataset comprising 2180 experimental entries of austenitic alloys to develop machine learning (ML) models capable of predicting these key mechanical properties as functions of composition, solution treatment condition, and testing temperature. We systematically evaluate a range of ML algorithms, namely linear regression, kernel ridge regression, extreme gradient boosting, and artificial neural network. Among these, the extreme gradient boosting achieves the highest predictive accuracy, with R2 scores of 0.946 and 0.985 for proof stress and ultimate tensile strength, respectively. To further enhance model performance, we explore ensemble learning and transfer learning strategies. The transfer learning approach that leverages interdependencies between mechanical properties reduces the mean percentage error by 21.0% and increases R2 score by 3.1% in predicting reduction in area, compared to the original artificial neural network model. Our results show that ML models trained on well-structured experimental data can serve as an efficient and reliable tool for exploring the effect of composition on screening metrics. This work highlights the potential of data-driven approaches to accelerate the design and optimization of high-performance austenitic alloys. Furthermore, obtaining feature importance and insights from extreme gradient boosting using Shapley Additive Explanation tool provides valuable understanding of how features contribute to the prediction of mechanical properties.

We propose a quantitative description of creep acceleration in gaseous hydrogen environment. The central hypothesis is that hydrogen enhances high temperature creep by reducing the vacancy formation energy, thereby facilitating faster dislocation climb. By employing the defactant theory, we derive the ratio of steady-state creep strain rate in gaseous hydrogen ε˙H to creep rate in the absence of hydrogen ε˙ as ε˙H/ε˙=(1−Γ/Γmax)−Γmax, where Γ is the number of hydrogen atoms trapped at a vacancy and Γmax is the maximum number of hydrogen atoms that can be trapped by a vacancy. We demonstrate the predictive capability of the proposed model by comparing with experimental measurements of creep strain rates across a range of hydrogen pressures and temperatures. Notably, this defactant-theory based formulation of creep law introduces no additional fitting parameters and is defined entirely by physically well-established quantities.

We develop a network simulation platform to assess the feasibility of hydrogen-fueled trucking in Illinois. Given the state's central role in U.S. freight transportation, adopting fuel cell trucking offers significant economic and environmental benefits, including reduced greenhouse gas emissions and improved freight efficiency. Using a discrete event simulator, we model truck movements across major Illinois cities to estimate key metrics, including daily hydrogen demand at refueling stations, total travel times, and infrastructure utilization. Our approach integrates geographic data such as road network structure and average freight speed with truck specifications such as mileage, payload capacity, and refueling constraints. We apply this framework to compare the productivity of hydrogen-fueled trucks with that of battery-electric trucks. Additionally, we use zero-order optimization techniques to determine near-optimal placement of hydrogen refueling stations across the state, with the goal of maximizing freight ton-miles moved per unit of time.

High temperature hydrogen attack (HTHA) is a form of degradation of carbon steels exposed to high temperature and high-pressure hydrogen whereby internal hydrogen reacting with carbides forms methane gas bubbles with an associated loss in strength and toughness due to decarburization. Grain boundary gas bubbles can grow and coalesce leading to microcrack formation and frequently to premature fracture. Current models mainly rely on the Grabke and Martin transient methane generation kinetics which is based on carburization/decarburization experiments on iron surface at temperatures between 600 degrees C and 800 degrees C, though those temperatures are much higher than those at which HTHA is observed in industrial processes. This work presents a coupled chemical kinetics and micromechanics model that addresses methane and hydrogen gas formation along with simultaneous decarburization and bubble growth over a wide temperature range. The energetics of the chemical reactions taking place at the ferrite-matrix/bubble interface are established through DFT calculations. Model calculations unveil the relationship between the rates of hydrogen migration to the bubble interface, carbon and hydrogen atom reactions for methane formation, and attendant volumetric bubble growth. The model predicts equilibrium methane bubble pressures that agree with those predicted by existing models at high temperatures. Significantly, the model predicts equilibrium methane pressures that are remarkably lower than the extrapolated predictions of the existing models at lower temperatures, e.g., 250 degrees C. In summary, the model establishes a methodology to understand and quantify methane pressure development and decarburization across the length and time scales that are relevant to hydrogen attack.

Hydrogen embrittlement (HE) in austenitic stainless steels is advanced by hydrogen enhanced localized plasticity (HELP), typically accompanied by a transition from homogeneous to localized slip. Short-range order (SRO) in face-centered cubic (FCC) alloys is known to promote slip planarity, and recent studies suggest that H may amplify this localization behavior linked to inherent SRO. However, the manner in which the introduction of H affects SRO properties and, conversely, the manner that pre-existing SRO may affect H behavior, are not fully understood. In this work, a spin cluster expansion model combined with Monte Carlo simulation is employed to study the interplay between H and SRO in Fe-Ni-Cr alloys. Chemical order is quantified using Warren-Cowley SRO parameters, and the model predictions are validated against experimental data. We find that the presence of H only slightly alters the intrinsic ordering preference of the Fe-Ni-Cr alloys. As temperature decreases and the alloy evolves from disordered to ordered thermodynamic states, distinct H-metal correlations emerge. In particular, H-Ni and H-Cr pairs exhibit stronger ordering tendencies than H-Fe pairs, suggesting a selective affinity of H for certain atomic environments. On the other hand, we also find that compared to random alloys, when pre-existing SRO is present, it significantly affects the resulting H distribution by promoting local H enrichment in SRO domains. Such SRO-driven local H accumulation may facilitate slip localization and contribute to the early onset of embrittlement. These findings provide thermodynamic and structural insights into the interaction between H and SRO in austenitic stainless steels, highlighting possible implications on how the interaction between HELP and SRO brings about hydrogen embrittlement in austenitic stainless steels.

Fracture by hydrogen accelerated fatigue crack growth is a severe type of environmental failure. Although fatigue crack growth has been the subject of intense investigation over several decades, a complete mechanistic and predictive model is still lacking. Such a crack growth model is even more rare in the case of hydrogen in view of the lack also of constitutive models for material deformation under cycling loading that account for the hydrogen effect. In this study, we present a model for fatigue crack propagation induced by alternating crack tip plastic blunting and re-sharpening, which in the presence of hydrogen can be accelerated by hydrogen enhanced dislocation motion and generation. The Chaboche constitutive model, which is a nonlinear kinematic hardening model capable of capturing many features of material behavior under cyclic loading, is used for the calculation of the stress and strain fields at the propagating crack tip. The Chaboche model is calibrated using a sequence of experimental data from uniaxial strain-controlled cyclic loading tests and uniaxial stress-controlled ratcheting tests with a low carbon steel, JIS SM490YB, in the absence and presence of hydrogen. The numerical simulation results indicate that the proposed crack propagation model can predict Paris law behavior and can successfully demonstrate acceleration of fatigue crack growth in the presence of hydrogen. Significantly, the profiles of the steady-state opening stress and strain ahead of the fatigue crack tip in a compact tension (C(T)) specimen were found to have sections over which they vary as ln(1/r) with distance r from the crack tip, consistent with the crack-tip strain field for a non-stationary crack.

High temperature hydrogen attack (HTHA) is a severe degradation of mechanical properties of steel components operating in hydrogen at high pressure and temperature. Hydrogen solute atoms react with carbides forming methane bubbles, typically at grain boundaries. These pressurized bubbles grow and coalesce to form microcracks which can compromise the structural integrity of the steel component. From an engineering design perspective, the interaction of HTHA damage with pre-existing cracks is most detrimental. This work investigates the growth and coalescence of a cluster of methane bubbles in the neighborhood of an axial crack on the inner diameter surface of a pressure vessel. For a given crack depth and hydrogen pressure and operating temperature, simulations were carried out to predict bubble growth and coalescence leading to microcrack formation by combined creep and grain boundary diffusion. Important effects of stress triaxiality ahead of the crack tip on bubble growth are included. By considering the time for microcrack formation as time to crack propagation and failure—a conservative measure of failure time for a component—Nelson-type curves were constructed for the case of 2.25Cr–1Mo steel indicating time to failure on a hydrogen pressure vs temperature diagram for different crack sizes. The proposed flaw-sensitive design curves offer a promising initial step toward improving the empirical API Nelson-curve diagram while maintaining its simplicity.

High temperature hydrogen attack (HTHA) is degradation of steels exposed to hydrogen gas at high temperatures and pressures. Hydrogen in steels reacts with carbon from carbides to produce methane gas bubbles typically on grain boundaries which grow and coalesce, leading to loss of strength and fracture toughness. Current design practice against HTHA is based on the Nelson curves which define the conditions for safe operation in a temperature/hydrogen-partial-pressure diagram. Nelson curves are phenomenological in nature and do not account for the underlying failure mechanism(s), material microstructure, carbide stability, and applied stresses. In light of experimental evidence of predominant cavitation ahead of cracks reported by Martin et al. (Acta Mater 140:300–304, 2017), it is expected that void growth is accelerated by the triaxial stresses associated with microstructural flaws. To this end, we propose a three-dimensional, axisymmetric, constraint-based void-growth model extending the “one-dimensional” model of Dadfarnia et al. (Int J Fract 219:1–17, 2019). The present model is shown to yield satisfactory agreement with the available experimental data from hydrogen attack of 2¼Cr–1Mo steel at temperatures ranging from 500 to 600 °C. In addition, the model is used to construct Nelson type curves in the temperature/hydrogen-partial-pressure diagram. These curves represent failure times for given applied stresses and triaxiality. The proposed methodology can be viewed as providing a step toward improving the current design practice against HTHA while maintaining the simplicity of the original Nelson curve approach.

It is expected that high-temperature hydrogen technologies, such as the solid oxide fuel cell and high-temperature water electrolysis, will play important roles in the hydrogen society. In this study, creep tests of JIS SUS304, SUS304L and SUS310S austenitic stainless steels and SUY-1 pure iron were carried out in hydrogen and argon at 873 K. For all the materials, the creep rate in the secondary creep region in hydrogen environment was increased. It resulted in reduction of the creep life. In argon, the fracture surface of the SUS304 changed from dimple to intergranular fracture with the increase in the creep life. In hydrogen, this change was delayed. Regarding the mechanism, it was confirmed that the effect of carbide formation, HELP and HTHA were not activated in this study. Instead, enhanced dislocation climb mediated by an increased vacancy density is a plausible mechanism, although further investigation is needed.

Next-generation reactors are expected to play a crucial role in power production in the foreseeable future. Due to the extreme anticipated operating temperatures of next-generation plants, a major concern for candidate materials is failure by creep cavitation. Indeed, many commonly used component lifetime estimates are based on how quickly intergranular voids grow. Void growth is known to be caused by three processes: diffusion along the void surface, diffusion along the grain boundary, and creep of the surrounding grains. However, until now, previous creep cavitation models have neglected to account explicitly for both the surface diffusion process and primary creep effects. More precisely, previous models assume that surface diffusion occurs rapidly enough to sustain quasi-equilibrium void growth, and that the creep response of the grains is accurately modeled by power-law secondary creep. To illustrate the potential ramifications of these assumptions, we present here novel finite element simulations of intergranular void growth under the combined effects of surface diffusion, grain boundary diffusion, and bulk primary/secondary creep. Our results indicate that crack-like void growth may be more prevalent at high temperatures than previously assumed, and that void growth of any kind is substantially accelerated during the primary creep regime. This could have serious implications for previous creep rupture models, which may underestimate the rate of void growth by almost two orders of magnitude or more. Based on our results, we establish quantitative criteria for quasi-equilibrium and crack-like void growth, and we suggest quantitative improvements to the existing models.

Fossil fuels continue to exacerbate climate change due to large carbon emissions resulting from their use across a number of sectors. An energy transition away from fossil fuels seems inevitable, and energy sources such as renewables and hydrogen may provide a low carbon alternative for the future energy system, particularly in large emitting nations such as the United States. This research quantifies and maps potential hydrogen fuel distribution pathways for the continental US, reflecting technological changes, barriers to deployment, and end-use-cases from 2020 to 2100, clarifying the potential role of hydrogen in the US energy transition. The methodology consists of two parts, a linear optimization of the global energy system constrained by carbon reduction targets and system cost, followed by a projection of hydrogen infrastructure development. Key findings include the emergence of trade pattern diversification, with a greater variety of end-uses associated with imported fuels and greater annual hydrogen consumption over time. Further, sensitivity analysis identified the influence of complementary technologies including nuclear power and carbon capture and storage technologies. We conclude that hydrogen penetration into the US energy system is economically viable and can contribute toward achieving Paris Agreement and more aggressive carbon reduction targets in the future.

Taking advantage of in-situ fracture testing method inside a transmission electron microscope (TEM), crack evolution in a low alloy steel under low triaxiality conditions is studied and the interaction between cementite particles and the crack is revealed. It is found that the ferrite matrix is the major void initiation site due to the low stress triaxiality in the thin TEM sample (plane stress condition), which contrasts the behavior under plane strain conditions in bulk specimens, where voids are typically found to initiate by decohesion at the particle/matrix interface. This work reveals that fracture behavior proceeds differently under low triaxiality conditions, such as the shear lip region of fractured bulk sample, and demonstrates the possibility to avoid interface decohesion and thereby to enhance ductility in steels.

This paper presents a review of the current state of scientific understanding of the corrosion phenomenon known as Hydrogen-Induced Cracking (HIC). HIC is defined as cracking in low-to medium-strength steels where cracking is driven by the precipitation of gaseous hydrogen molecules within the crack, which typically occurs in sour (H2S containing) environments. It is a complicated phenomenon, encompassing a surface reaction for hydrogen uptake, hydrogen diffusion to vulnerable microstructural sites, hydrogen gas precipitation creating an incipient crack, and crack growth driven by hydrogen gas pressure within the crack. While HIC has been studied for decades, understanding of the critical factors controlling each step of the phenomenon has been elusive. The maturation of many characterization techniques gives hope that a full mechanistic understanding may occur in the near future.

Current greenhouse gas emissions suggest that keeping global temperature increase below 1.5 degrees, as espoused in the Paris Agreements will be challenging, and to do so, the achievement of carbon neutrality is of utmost importance. It is also clear that no single solution can meet the carbon neutral challenge, so it is essential for scientific research to cover a broad range of technologies and initiatives which will enable the realization of a carbon free energy system. This study details the broad, yet targeted research themes being pioneered within the International Institute for Carbon-Neutral Energy Research (I2CNER). These approaches include hydrogen materials, bio-mimetic catalysts, electrochemistry, thermal energy and absorption, carbon capture, storage and management and refrigerants. Here we outline the state of the art for this suite of technologies and detail how their deployment, alongside prudent energy policy implementation can engender a carbon neutral Japan by 2050. Recognizing that just as no single technological solution will engender carbon neutrality, no single nation can expect to achieve this goal alone. This study represents a recognition of conducive international policy agendas and is representative of interdisciplinary, international collaboration.

The objective of this study is to accumulate creep data in hydrogen of various materials in order to consider the mechanisms that hydrogen affects creep properties. Creep testing was performed in argon and hydrogen gases at 873K. The materials were JIS SUS304, SUS304L and SUS310S austenitic stainless steels and JIS SUY-1 commercial pure iron. For all materials, the creep life was reduced in hydrogen compared to that in argon to a greater or lesser extent. The creep ductility in hydrogen was higher than that in argon except for the pure iron. The mechanism that hydrogen reduced the creep life of the SUS304 we considered was the accelerated dislocation climb mediated by hydrogen increased vacancy concentration. According to the literatures, decarburization, carbide formation and hydrogen enhanced localized plasticity (HELP) were investigated. It was confirmed that these mechanisms were not activated in our creep test for the SUS304.

The effect of internal hydrogen (up to 104.2 mass ppm) on the tensile and fatigue properties of SUS316L austenitic stainless steel was investigated. Internal hydrogen had minimal impact on the tensile properties but reduced the fatigue lifetime, but not monotonically with hydrogen concentration. The evolved microstructural state beneath the fatigue fracture surface showed commonalities and differences with crack length and the presence of hydrogen. Hydrogen influenced the evolved microstructural state, resulting in the formation of smaller dislocation cells with thicker cell walls, and modified the distribution of deformation twins. The non-linear dependence of the response on fatigue lifetime with increasing hydrogen concentration was attributed to hydrogen-induced changes in the macroscopic mechanical properties at the highest concentration. As the emphasis of this paper is on relating the hydrogen-induced changes in the deformed microstructural state to those in the mechanical properties, the results are discussed in terms of the hydrogen-enhanced localized plasticity mechanism.

Hydrogen-accelerated fatigue crack growth is a most severe manifestation of hydrogen embrittlement. A mechanistic and predictive model is still lacking partly due to the lack of a descriptive constitutive model of the hydrogen/material interaction at the macroscale under cyclic loading. Such a model could be used to assess the nature of the stress and strain fields in the neighborhood of a crack, a development that could potentially lead to the association of these fields with proper macroscopic parameters. Toward this goal, a constitutive model for cyclic response should be capable of capturing hardening or softening under cyclic straining or ratcheting under stress-controlled testing. In this work, we attempt a constitutive description by using data from uniaxial strain-controlled cyclic loading and stress-controlled ratcheting tests with a low carbon steel, Japanese Industrial Standard (JIS) SM490YB, conducted in air and 1 MPa H-2 gas environment at room temperature. We explore the Chaboche constitutive model which is a nonlinear kinematic hardening model that was developed as an extension to the Frederick and Armstrong model, and propose an approach to calibrate the parameters involved. From the combined experimental data and the calibrated Chaboche model, we may conclude that hydrogen decreases the yield stress and the amount of cyclic hardening. On the other hand, hydrogen increases ratcheting, the rate of cyclic hardening, and promotes stronger recovery.