The role of hydrogen in the edge dislocation mobility and grain boundary-dislocation interaction in α-Fe

The atomistic mechanisms of dislocation mobility depending on the presence of hydrogen were investigated for two edge dislocation systems that are active in the plasticity of α-Fe, specifically ½<111>{110} and ½<111>{112}. In particular, the glide of the dislocation pile-ups through a single crystal, as well as transmission of the pile-ups across the grain boundary were evaluated in bcc iron crystals that contain hydrogen concentrations in different amounts. Additionally, the uniaxial tensile response under a constant strain rate was analyzed for the aforementioned structures. The results reveal that the presence of hydrogen decreases the velocity of the dislocations – in contrast to the commonly invoked HELP (Hydrogen-enhanced localized plasticity) mechanism -, although some localization was observed near the grain boundary where dislocations were pinned by elastic stress fields. In the presence of pre-exisiting dislocations, hydrogen-induced hardening was observed as a consequence of the restriction of the dislocation mobility under uniaxial tension. Furthermore, it was observed that hydrogen accumulation in the grain boundary suppresses the formation of new grains that leads to a hardening response in the stress-strain behaviour which can initiate brittle fracture points.     

The effects of hydrogen and vacancy on the tensile deformation behavior of Σ3 symmetric tilt grain boundaries in pure fe

The interplay of hydrogen, vacancy and grain boundary plays an important role in hydrogen induced premature fracture in the metallic materials. In this work, two models with the representative high angle grain boundaries with the low and high grain boundary energy have been built according to the coincidence site lattice theory. The effects of hydrogen and hydrogen-vacancy combination on the deformation behaviors of the two models were investigated by means of molecular dynamics simulation with the straining direction vertical to the grain boundary. It is found that in both cases hydrogen tends to segregate and maintain a high local stress state around the grain boundary, and promote the premature fracture compared with the hydrogen-free model. Vacancy enhanced the effects of hydrogen in the model with grain boundary of the lower energy. However, vacancy promoted the dislocation evolution behavior in the model with grain boundary of the higher energy. The simulation results were further explained by considering the effects of hydrogen on the generalized stacking fault energy and the work of separation of the grain boundary.     

Hydrogen effect on the intergranular failure in polycrystal ɑ-iron with different crystal sizes

We studied the fracture strain of polycrystal ɑ-iron at three different hydrogen concentrations and for three crystal sizes with molecular dynamics simulations (MDs). As the hydrogen concentration increases, a fine crystal model's fracture resistance is prone to below a comparatively coarse grain model. This finding elucidates that the most vulnerable area can alter from coarse grain zone to fine-grain zone in the welding heat-affected zone (HAZ) with the effect of hydrogen. A simplified model is thus built to predict the strain energy increment in different crystal size systems caused by the introduction of hydrogen atoms. This strain energy increment is not equal to the fracture energy reduction induced by the same amount of hydrogen insertion, demonstrating that elastic volume expansion of grain boundary (GB) caused by hydrogen is not the determining mechanism of intergranular failure. The density of triple or multi-junctions of GBs, which is typically dependent on the volume fraction of GBs, is the crucial factor for intergranular failure caused by hydrogen embrittlement.     

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.     

Atomistic simulations of hydrogen embrittlement

It is well known that hydrogen weakens strengths of metals, and this phenomenon is called hydrogen embrittlement. Despite the extensive investigation concerning hydrogen related fractures, the mechanism has not been enough clarified yet. In this study, we applied the molecular dynamics method to the mode I crack growth in α-Fe single crystals with and without hydrogen, and analyzed the hydrogen effects from atomistic viewpoints. We estimated the hydrogen trap energy in the vicinity of an edge dislocation in order to clarify the distribution of hydrogen atoms, using the molecular statics method. We also evaluated the energy barrier for dislocation motion under a low hydrogen concentration. Based on these results, we propose a mechanism for hydrogen embrittlement of α-Fe under monotonic loading.     

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.     

Effect of temperature on hydrogen embrittlement susceptibility of alloy 718 in Light Water Reactor environment

A 718 superalloy, presenting a standard microstructure, was mechanically tested under uniaxial tensile loading at 80 °C and 300 °C in Light Water Reactor environment after an exposure at 300 °C for 200 h. Hydrogen embrittlement mechanism was clearly observed. In order to identify the most influent metallurgical parameters on hydrogen embrittlement, three “model” microstructures were synthesized to test the efficiency of carbides, δ, γ′ and γ” precipitates to trap hydrogen at different temperatures. Results showed that γ′ and γ” played the major role on the hydrogen embrittlement susceptibility of the alloy even though carbides and δ precipitates could also act as hydrogen traps and influence the final rupture mechanism. Results also characterized the influence of temperature on the fracture modes.     

Unveiling the role of hydrogen on the creep behaviors of nanograined α-Fe via molecular dynamics simulations

Hydrogen embrittlement (HE) substantially deteriorates the mechanical properties of metals. The HE behavior of nanograined (NG) materials with a high fraction of grain boundaries (GBs) may significantly differ from those of their coarse-grained counterparts. Herein, molecular dynamics (MD) simulations were performed to investigate the HE behavior and mechanism of NG α-Fe under creep loading. The effects of temperature, sustained stress, and grain size on the creep mechanism was examined based on the Mukherjee-Bird-Dorn (MBD) equation. The deformation mechanisms were found to be highly dependent on temperature, applied stress, and grain size. Hydrogen charging was found to have an inhibitory effect on the GB-related deformation mechanism. As the grain size increased, the HE mechanism transitioned from H-induced inhibition of GB-related deformation to H-enhanced GB decohesion. The current results might provide theoretical guidance for designing NG structural materials with low HE sensitivity and better mechanical performance.     

Hydrogen-assisted fracture resistance of pipeline welds in gaseous hydrogen

Fracture resistance of pipeline welds from a range of strength grades and welding techniques was measured in air and 21 MPa hydrogen gas, including electric resistance weld of X52, friction stir weld of X100 and gas metal arc welds (GMAW) of X52, X65 and X100. Welds exhibited a decrease in fracture resistance in hydrogen compared to complementary tests in air. A general trend was observed that fracture resistance in 21 MPa hydrogen gas decreased with increasing yield strength. To accommodate material constraints, two different fracture coupon geometries were used in this study, which were shown to yield similar fracture resistance values in air and 21 MPa hydrogen gas; values using different coupons resulted in less than 15% difference. In addition, fracture coupons were removed from controlled locations in select welds to examine the potential influence of orientation and residual stress. The two orientations examined in the X100 GMAW exhibited negligible differences in fracture resistance in air and, similarly, negligible differences in hydrogen. Residual stress exhibited a modest influence on fracture resistance; however, a consistent trend was not observed between tests in air and hydrogen, suggesting further studies are necessary to better understand the influence of residual stress. A comparison of welds and base metals tested in hydrogen gas showed similar susceptibility to hydrogen-assisted fracture. The overall dominant factor in determining the susceptibility to fracture resistance in hydrogen is the yield strength.     

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.     

Mechanisms of metastability in hydrogenated amorphous silicon

We survey theoretical approaches to understanding the diverse metastable behavior in hydrogenated amorphous silicon. We discuss a recently developed network-rebonding model involving bonding rearrangements of silicon and hydrogen atoms. Using tight-binding molecular dynamics we find non-radiative recombination can break weak silicon bonds with low activation energies, producing dangling bond–floating bond pairs. The transient floating bonds annihilate generating local hydrogen motion and leaving behind isolated dangling bonds. Charged defects are also observed. Major experimental features of metastability including electron-spin resonance, t^1/3 kinetics, dangling-bond H anti-correlation, and hysteretic annealing are explained. In the second part we focus on large metastable structural changes observed in a-Si:H. We find H atoms have a local metastability involving the flipping of the H to the backside of the Si–H bond that results in a local increase of strain and increase of dipole moments. This naturally explains the larger infrared absorption found after light soaking, and may be related to other large structural changes in the network. Directions for future research are surveyed.     

Detection of voids in hydrogen embrittled iron using transmission X-ray microscopy

Hydrogen embrittlement remains a barrier to widespread adoption of hydrogen as a carbon-neutral energy source. Here, hydrogen embrittlement mechanisms are investigated across length scales in iron using transmission X-ray microscopy (TXM), digital image correlation (DIC), and notched tensile testing during in-situ electrochemical hydrogen charging. TXM reveals void size and spatial distribution ahead of a propagating crack. We find hydrogen charging leads to voids within ～10 μm of the crack tip and suppression of voids beyond this distance. Near the crack tip, voids are elongated in the direction of the crack and are smaller than voids in an uncharged sample. In the presence of hydrogen, these voids lead to quasi-cleavage fracture and a sharper crack tip. DIC shows localization and reduction of plastic strain with hydrogen charging, and tensile testing reveals a reduction in fracture energy and elongation at failure. These results are discussed in the context of hydrogen embrittlement mechanisms.