Atomistic simulation study of the grain-size effect on hydrogen embrittlement of nanograined Fe

Although hydrogen-induced fracture at grain boundaries has been widely studied and several mechanisms have been proposed, few studies of nanograined materials have been conducted, especially for grain sizes below the critical size for the inverse Hall-Petch relation. In this research work, molecular dynamics (MD) simulations are performed to investigate the hydrogen segregation and hydrogen embrittlement mechanism in polycrystalline Fe models. When the same concentration of H atoms is added, the H segregation ratio in the model with the smallest grain size is the highest observed herein, showing the high hydrogen trapping ability of small-grain Fe, while the H concentration at the grain boundaries (GBs) is, on the contrary, the lowest. Uniaxial tensile test simulations demonstrate that as the grain size decreases, the models show an increased resistance to hydrogen embrittlement, and for small-grain models (d < 10 nm), the GB-related deformation modes dominate the plastic deformation, where the segregated H mainly influences the toughness by inhibiting GB-related processes.     

Hydrogen effects on the mechanical behaviour and deformation mechanisms of inclined twin boundaries

It has been observed that coherent twin boundaries (CTBs) are resistant to hydrogen embrittlement (HE). However, little is known about the role of inclined twin boundaries in the H-related deformation and failure. Here we comprehensively investigate H segregation and its influence on the mechanical behaviour and deformation mechanisms of inclined Σ3 twin boundaries at inclination 0°≤Φ ≤ 90° using molecular dynamics simulations. Our results demonstrate that for Φ = 0° CTB and Φ = 90° symmetric incoherent twin boundary (SITB), the presence of H reduces the yield stress required for dislocation nucleation under uniaxial tension, while for inclined twin boundaries (0°<Φ < 90°), the yield stress increases with increasing H concentration. Under shear deformation, solute H increases the critical shear stress for the SITB and inclined twin boundaries (0°<Φ < 90°). The underlying deformation mechanisms are directly associated with H-modified atomic structure and GB motion. These findings deepen our understanding of the HE mechanisms of inclined twin boundaries, and provide a pathway for designing materials with high HE resistance.     

Strain and defect engineering of graphene for hydrogen storage via atomistic modelling

The adsorption of hydrogen molecules on monolayer graphene is investigated using molecular dynamics simulations (MDS). Interatomic interactions of the graphene layer are described using the well-known AIREBO potential, while the interactions between graphene and hydrogen molecule are described using Lennard-Jones potential. In particular, the effect of strain and different point defects on the hydrogen storage capability of graphene is studied. The strained graphene layer is found to be more active for hydrogen and show 6.28 wt% of H₂ storage at 0.1 strain at 77 K temperature and 10 bar pressure. We also studied the effect of temperature and pressure on the adsorption energy and gravimetric density of H₂ on graphene. We considered different point defects in the graphene layer like monovacancy (MV), Stone Wales (SW), 5-8-5 double vacancy (DV), 555–777 DV, and 5555-6-7777 DV which usually occur during the synthesis of graphene. At 100 bar pressure, graphene with 1% concentration of MV defects leads to 9.3 wt% and 2.208 wt% of H₂ storage at 77 K and 300 K, respectively, which is about 42% higher than the adsorption capacity of pristine graphene. Impact of defects on the critical stress and strain of defected graphene layers is also studied.     

Temperature dependent nanomechanics of Si-C-N nanocomposites with an account of particle clustering and grain boundaries

Experimentally obtained Silicon Carbide (SiC)-Silicon Nitride (Si₃N₄) nanocomposites have SiC particles with nearly circular cross-section placed in Si₃N₄ matrix either along grain boundaries (GBs) or in inter-granular locations. In the present investigation, 3-D molecular dynamics (MD) analyses of SiC-Si₃N₄ nanocomposite deformation are performed at 300 K, 900 K, and 1500 K to understand the effect of SiC particle position with respect to the Si₃N₄ GBs on the nanocomposite mechanical strength. A range of SiC-Si₃N₄ nanocomposite phase morphologies having cylindrical SiC particles with diameter of 4 nm distributed in different manners in three different types of Si₃N₄ phase matrices (single crystalline, bicrystalline, and nanocrystalline) are generated. Analyses reveal that the second phase particles act as significant stress raisers in the case of single crystalline Si₃N₄ phase matrix. However, the particle’s presence does not have a significant effect on the mechanical strength of bicrystalline or nanocrystalline Si₃N₄ phase matrices. In order to understand the effect of grain boundary (GB) contamination on mechanical strength of the nanocomposites, structures having GBs with two different configurations: (1) sharp and (2) diffused; are analyzed. The strength of structures with diffused GBs decreased with increase in temperature with one exception for structures where due to the particle clustering the strength improved with increase in temperature. The findings indicate that the SiC-Si₃N₄ interface plays an important role in strengthening the nanocomposite microstructure with increase in temperature through stronger Si-C-N bonding with increasing temperature. GBs on the other hand soften the structures by facilitating GB sliding based deformation mechanism. Overall, analyses confirm that the temperature dependent strengthening of the nanocomposite owing to SiC second phase particles is a strong function of particle placement along GBs, particle clustering, relative volume fractions of atoms in the interfaces and GBs, and GB thickness.     

Role of grain boundaries in hydrogen embrittlement of alloy 725: single and bi-crystal microcantilever bending study

In situ electrochemical microcantilever bending tests were conducted in this study to investigate the role of grain boundaries (GBs) in hydrogen embrittlement (HE) of Alloy 725. Specimens were prepared under three different heat treatment conditions and denoted as solution-annealed (SA), aged (AG) and over-aged (OA) samples. For single-crystal beams in an H-containing environment, all three heat-treated samples exhibited crack formation and propagation; however, crack propagation was more severe in the OA sample. The anodic extraction of H presented similar results as those under the H-free condition, indicating the reversibility of the H effect under the tested conditions. Bi-crystal micro-cantilevers bent under H-free and H-charged conditions revealed the significant role of the GB in the HE of the beams. The results indicated that the GB in the SA sample facilitated dislocation dissipation, whereas for the OA sample, it caused the retardation of crack propagation. For the AG sample, testing in an H-containing environment led to the formation of a sharp, severe crack along the GB path.     

Application of atomic simulation for studying hydrogen embrittlement phenomena and mechanism in iron-based alloys

High-strength iron-based alloys serving in hydrogen-containing environments often faces a critical problem of hydrogen embrittlement, which involves intricate mechanisms across multiple lengths and time scales resulting in catastrophic consequences. It is challenging to track the evolution or/and nanoscale distribution of hydrogen atoms via experiments directly, whereas atomic simulation displays its great advantages in revealing the hydrogen-related behaviors and interaction mechanism. Most studies on hydrogen embrittlement mechanisms via atomic simulations focused on iron, as it is the matrix composition of steel. Herein, we summarize recent advances about applying atomic simulations, including density functional theory and molecular dynamics, in understanding the interaction between hydrogen atoms and various defects in iron-based alloys. Finally, some scientific issues and challenges in this field are discussed to provide insight for future researches.     

Mechanical and microstructural analysis on hydrogen-related fracture in a martensitic steel

The present study quantitatively evaluated mechanical response of hydrogen-related fracture in the as-quenched martensitic steel and correlated it to crack propagation behavior analyzed by microstructure observations. The crack-growth resistance curves revealed that the hydrogen-related intergranular cracks propagated in a stable manner even when the diffusible hydrogen content was large. Fracture initiation toughness was decreased significantly by small amounts of diffusible hydrogen. With further increasing diffusible hydrogen content, however, the fracture initiation toughness did not change and remained almost constant. On the other hand, tearing modulus, corresponding to crack-growth resistance, decreased rather gradually with increasing diffusible hydrogen content. The microstructure observations confirmed that the hydrogen-related crack propagated discontinuously in a stepwise manner on a microscopic scale. Accordingly, it was proposed that the microscopic discontinuous crack propagation could be the possible reason for the stable crack propagation.     

Atomistic simulations of hydrogen distribution in Fe–C steels

To engineer low-cost Fe–C based steels for application in hydrogen energy technologies, an understanding of the hydrogen distribution inside the material and how it is affected by the microstructure is vital. Molecular statics and molecular dynamics simulations are used to study hydrogen distribution and transport kinetics in the ferritic and martensitic phases of Fe–C steels, with and without dislocations present. We find that hydrogen preferentially resides in martensite especially in high dislocation density regions near the martensite/ferrite boundaries, in agreement with experiments. Furthermore, the rate of hydrogen transport through ferrite is up to an order of magnitude greater than that in martensite. The fundamental mechanisms behind this phenomenon are analyzed.     

Investigation for enhancing the resistance of H-induced delayed cracking for martensitic steel by quenching tempering and quenching partitioning: Influence of microstructure on hydrogen embrittlement and cracking behavior

The objective of the present study is to enhance the hydrogen embrittlement (HE) of the commercial martensitic steel (QT220). For this purpose, the heat treatments of quenching tempering and quenching partitioning are conducted, labeled as QT400 and Q&QP400, respectively. Compared to QT220, the mechanical properties of the both heat-treated specimens are reduced, nevertheless, the HE resistance is extremely promoted, resulting from the lesser dislocations, the more Mo_yC_x, and the existence of the strained interface of cementite. Besides the above favorable factors, the presence of the ferrite is another important factor which contributes to the lowest HE susceptibility in Q&QP400, resulting from the propagation's inhibition of hydrogen induced cracks (HICs) by ferrite. The HICs behavior of QT220, QT400 and Q&QP400 are mainly influenced by the dislocation glide, the cementite at the high angle boundaries and ferrite, respectively, mainly resulting in the fractographs of quasi-cleavage, intergranular and finely fragmented quasi-cleavage, respectively. In addition, HICs always deflect when propagating to the RD//<112～114> orientations, providing a valuable direction for research to enhance the HE resistance in the future.     

Hydrogen embrittlement of low carbon structural steel at macro-, micro- and nano-levels

This paper aims to investigate the effect of hydrogen-induced mechanical degradation of low carbon steel at macro-, micro- and nano-levels in the hydrogen-rich acidic environments. From the test results of specimens, a relationship in hydrogen concentration and corrosion propagation was observed that led to the significant reductions of bulk elastic modulus after 28 days of exposure to the hydrogen-rich acidic environments. Through microstructural analysis, the deformation of larger grains, cracks, and blisters caused by hydrogen penetration was found as the possible cause for this reduction. Moreover, by performing nanoindentation on the areas of interest of various specimens at planned time periods, the influence of hydrogen on the nano-elastic and nano-hardness properties of grains was determined. The 3D surface profiles of the nano-elastic modulus and nano-hardness of various specimens are presented in this paper.     

Important role of chemical interaction on flame extinction in downstream interaction between stretched premixed H2-air and CO-air flames

Important role of chemical interaction in flame extinction is numerically investigated in downstream interaction among lean (rich) and lean (rich) premixed as well as partially premixed H₂- and CO-air flames. The strain rate varies from 30 to 5917 s⁻¹ until interacting flames cannot be sustained anymore. Flame stability diagrams mapping lower and upper limit fuel concentrations for flame extinction as a function of strain rate are presented. Highly stretched interacting flames are survived only within two islands in the flame stability map where partially premixed mixture consists of rich H₂-air flame, extremely lean CO-air flame, and a diffusion flame. Further increase in strain rate finally converges to two points. It is found that hydrogen penetrated from H₂-air flame (even at lean flame condition) participates in CO oxidation vigorously due to the high diffusivity such that it modifies the slow main reaction route CO + O₂ → CO₂ + O into the fast cyclic reaction route involving CO + OH → CO₂ + H. These chemical interactions force even rich extinction boundaries with deficient reactant Lewis numbers larger than unity to be slanted at high strain rate. Appreciable amount of hydrogen in the side of lean H₂-air flame also oxidizes the CO penetrated from CO-air flame, and this reduces flame speed of the H₂-air flame, leading to flame extinction. At extremely high strain rates, interacting flames are survived only by a partially premixed flame such that it consists of a very rich H₂-air flame, an extremely lean CO-air flame, and a diffusion flame. In such a situation, both the weaker H₂- and CO-air flames are parasite on the stronger diffusion flame such that it can lead to flame extinction in the situation of weakening the stronger diffusion flame. Important role of chemical interaction in flame extinction is discussed in detail.     

An Fe–Ni–Cr–H interatomic potential and predictions of hydrogen-affected stacking fault energies in austenitic stainless steels

While Fe–Ni–Cr austenitic stainless steels exhibit relatively good resistance to hydrogen embrittlement, they still suffer from significant degradation of ductility, fatigue and fracture properties in gaseous hydrogen environments. Experimental studies in the literature suggest that hydrogen reduces stacking fault energy in austenitic stainless steels. This phenomenon causes a large separation of partial dislocations and lower propensity for cross-slip. Whereas lower stacking fault energy does not correlate well with loss of ductility in the absence of hydrogen, lower stacking fault energy trends toward greater loss of ductility when hydrogen is present. Calculations of stacking fault energy are challenging for austenitic stainless steels. One main issue is that in alloys, stacking fault energy is not a single value but rather varies depending on local composition. Herein, we first report an Fe–Ni–Cr–H quaternary interatomic potential and then use this potential to perform time-averaged molecular dynamics simulations to calculate stacking fault energies for tens of thousands of realizations of local compositions for selected stainless steels alloys with and without internal hydrogen. From statistical analyses, our results suggest that hydrogen reduces stacking fault energy, which likely impacts deformation mechanisms of Fe–Ni–Cr austenitic stainless steels when exposed to hydrogen environments. We then perform validation MD simulation tests to show that hydrogen indeed statistically increases the stacking fault widths due to statistically reduced stacking fault energies.     

Numerical simulation of the interaction between shock train and combustion in three-dimensional M12-02 scramjet model

The characteristics of combustion flow fields and performance for hypersonic M12-02 scramjet were numerically simulated and analyzed. The compressible two-equation k-w SST turbulence model was employed for the turbulence model and the 9-species, 27-reaction-step hydrogen-air reaction mechanism was used as the reaction kinetics model. The numerical method was verified and a good agreement was obtained between the results of the numerical simulations and the experimental data. The results showed that shock waves from the upper and lower walls respectively crossed with each other near the central axis, forming a ‘diamond’ shape in the high-temperature combustion region. Compared to the conventional scramjet engine, most of the fuel reaction was in pure supersonic combustion mode for this hypersonic scramjet engine. Changes in the distribution of fuel on the upper and lower walls could have an appreciable impact on the combustion flow field. Average fuel distribution between upper and lower walls is benefit for combustion enhancement while the heat transfer in the corner of the side wall is severe and should be avoided during operation. The flame investigation showed that it cannot automatically predict the flame surface temperature in advance only based on the equivalence ratio Φ according to diffusion combustion theory. Compared to Φ = 1.0 condition, the flame surface temperature for Φ = 0.8 condition is higher as the complicated interaction between shock waves and combustion, which makes the local air temperature and mixing extent in flame surface is more appropriate. However, in terms of the overall engine performance, the Φ = 1.0 condition has the better combustion efficiency along the whole flow path.     

Electrical activity of grain boundaries in silicon bicrystals and its modification by hydrogen plasma treatment

Electrical activity of grain boundaries (GBs) and its transformation under the influence of low-energy hydrogen plasma treatment in p-type silicon bicrystalline samples cut from EFG silicon polycrystals were investigated. Comprehensive studies have enabled one to investigate the electrical activity of GBs relative to the minority (MIC) and majority (MAC) carriers and to demonstrate the possibility of controlling this activity by different processing methods. These studies allowed for establishing the correlation between the type, structure and individual electrical activity of GBs and also thermal pre-history of samples. Among the tested modes, hydrogenation was found to be the most radical method of electrical activity modification for all types of GBs. In the process, results on hydrogenation of GBs in EFG silicon crystals depend essentially on three factors: type of GBs, state of ribbons (as-grown or annealed) and concurrence of grain boundary dangling bonds and boron passivation effects.     

Study on the dynamic characteristics of the free piston in the ionic liquid compressor for hydrogen refuelling stations by the fluid-structure interaction modelling

The ionic liquid compressor is promising for hydrogen refuelling stations, where the dynamic characteristics of the free piston are crucial for adjusting the compressor performance. This paper presents an investigation of the dynamic characteristics of the free piston in the ionic liquid compressor through a fluid-structure interaction modelling in three typical conditions. The results show that in the typical condition with no impact, phenomenons of buffering, oil charging, and oil overflow are observed in the oil pressure variation. Three features are found in the motion curve: asymmetric motion with a delay of reversal due to the buffering effect, variable location of the dead centre, and fluctuation in the piston velocity. When the impact occurs at the TDC, an opposite variation trend is observed in the gas and oil pressure curve. In the typical condition with impact at the BDC, the oil pressure drops below the atmospheric pressure.     

Herringbone nanofiber CVD synthesis and high pressure hydrogen adsorption performance analysis by molecular modelling

In order to interpret adsorption results of hydrogen storage by adsorption in graphite nanofiber (GNF) materials at molecular scale and to propose optimized structures of graphitic materials, we have realized both experimental and numerical studies of gas adsorption in GNF. The porous materials have been synthesized by CVD method. The adsorption experiments were performed at 293 K by a volumetric method at high pressure until 40 MPa. We completed the surface reactivity analysis by performing structural characterizations of the samples using different structural techniques and numerical modelling computed in the grand canonical Gibbs ensemble. Within the cell, stacks of plans of graphite are arranged periodically using boundary conditions. The present numerical approach enables to interpret the results based on the solid–gas molecular interactions reactivity analysis.     

Dissociative adsorption of hydrogen and methane molecules at high-angle grain boundaries of pipeline steel studied by density functional theory modeling

Pipelines provide an economic and efficient means for hydrogen transport, contributing to accelerated realization of a full-scale hydrogen economy. Dissociative adsorption of hydrogen molecules (H₂) occurring on pipe steels generates hydrogen atoms (H), potentially resulting in hydrogen embrittlement of the pipelines. This is particularly important for existing pipelines transporting hydrogen in blended form with methane (CH₄). In this work, a density functional theory model was developed to investigate the dissociative adsorption of H₂ and CH₄ at high-angle grain boundaries (HAGB), a typical type of hydrogen traps contained in steels, and the stable adsorption configurations. Results demonstrate that the dissociative adsorption of both H₂ and CH₄ at the HAGB is thermodynamically feasible under pipeline operating conditions. Compared with crystalline lattice sites, the HAGB possesses the most negative free energy change, a lower energy barrier and the lowest H-adsorption energy, making the HAGB, especially the quasi three-fold site, become the most stable site for hydrogen adsorption. The saturation coverage of hydrogen at HAGB is calculated to be 1.33. The iron-H bonds are formed at the HAGB by charge consumption at Fe atoms and electron accumulation at H atoms, following a so-called electron hybridization mechanism. The CH₄ adsorption at HAGB affects the H₂ adsorption. Without pre-adsorption of CH₄, the hydrogen adsorption at the HAGB is more stable. Although an elevated CH₄ partial pressure decreases the thermodynamic tendency for H₂ adsorption, it cannot hinder occurrence of the H₂ dissociative adsorption.     

Genetic variation and genotype by environment interaction in Jatropha curcas L. germplasm evaluated in different environments of Cameroon

Jatropha curcas L. (jatropha) is a perennial plant with a high untapped potential towards sustainable production of food and bioenergy. The transformation of jatropha into a competitive crop requires intensive breeding efforts. The objectives of our study were to (i) assess genetic variation of agronomic and quality traits in different environments, (ii) investigate genotype by environment interactions, and (iii) discuss potential selection strategies. Agronomic and quality traits were assessed on 277 jatropha genotypes that were evaluated over three environments in Cameroon. Genetic variation and heritability of agronomic and quality traits showed excellent prospects to select and breed improved cultivars rapidly. Selection for accumulated seed yield over years seems to be the best choice to increase annual seed and oil yield. Seed yield per month might be incorporated in selection indices to improve the efficiency of fruit harvesting. Selection based on a single testing environment was always inferior to the selection based on multiple environments. The magnitude of genotype by environment interaction (GxE) in jatropha is large. Therefore, testing in multiple environments is a requirement to select improved cultivars with local and broad adaptation.