Hydrogen-dislocation Interactions Research Articles

Evaluation of hydrogen interactions with defects induced by deformation in the duplex stainless steel microstructure: An internal friction study

Despite their good corrosion resistance and optimal mechanical properties, duplex stainless steels are affected by hydrogen embrittlement. Therefore, understanding the hydrogen-defect interactions in these steels is crucial. This study uses internal friction to evaluate the interactions of hydrogen with microstructural defects. Analysis of the internal friction spectra of the steels subjected to straining and hydrogen charging, together with thorough microstructural characterization, gives new insights in hydrogen interactions with defects present in the different phases, i.e. ferrite (body centered cubic) and austenite (face centered cubic).While no significant effect of tensile deformation can be observed by thermal desorption spectroscopy, the internal friction spectra show a clear influence of the presence of defects. Detailed analysis of these spectra reveals the interactions in the austenite as dominant, while no indications for hydrogen-dislocation interactions in ferrite are observed. This can be related to limited trapping in ferrite due to the austenite sink action or to limited dislocation formation in ferrite. Indications for hydrogen interactions with dislocations in the austenite are found, possibly suggesting enhanced dislocation mobility when surrounded by hydrogen. Moreover, a pronounced influence of hydrogen charging on the vacancy cluster related peaks is observed, indicating strong interactions between hydrogen and vacancy clusters in austenite. This can be put in contrast to behavior of pure ferritic steels, where dislocations provided the strongest hydrogen interactions. As these specific defects are of primary interest in the hydrogen embrittlement mechanisms, internal friction is concluded to provide important unique insights in hydrogen-defect interactions, even for complicated multiphase microstructures.

Critical assessment of hydrogen pipe diffusion at dislocations in metals

Hydrogen-dislocation interactions have always been a crucial problem in understanding and predicting the long-standing hydrogen embrittlement (HE) phenomenon in structural metals. Particularly with respect to hydrogen diffusion, dislocations have often been assumed with the ability to facilitate hydrogen transport via dislocation pipe diffusion (DPD). Yet the experimental and theoretical studies supporting DPD remain elusive and even controversial. In this work, hydrogen kinetics at dislocations in two representative metal systems, fcc Ni and bcc Fe, were systematically investigated combining comprehensive atomistic and kinetic activation relaxation technique simulations to reveal atomic details while enabling long-time (i.e., microsecond to second) hydrogen diffusion analysis. The dislocations were found to favor hydrogen trapping and provide regions of low migration barriers for hydrogen. However, these regions of low barriers only lead to localized fast short-circuit H diffusion, but do not translate into DPD over long time, which is attributed to fast hydrogen diffusion pathways along the dislocation line direction being periodically bound by high hydrogen migration barriers. Using the polyhedral structural units (PSUs) as an effective tool, we further quantitatively analyzed the correlation between hydrogen diffusion behaviors and local dislocation structures, illustrating the structural origin leading to local short-circuit diffusion and inhibition of DPD at dislocations. The new mechanistic insights gained not only are critical for understanding hydrogen-dislocation interaction, but also advances the general knowledge on hydrogen-microstructure interaction in structural metals.

Hydrogen-enhanced homogeneous dislocation nucleation of nanoindentation in nickel

Hydrogen-dislocation interactions are the essential problems to comprehend the multi-scale/multi-physics hydrogen embrittlement phenomena in metallic materials. For nano-scale materials, the initiation of dislocation nucleation marks the incipient plastic deformation, making the study of hydrogen’s role on nucleation processes pivotal. Nanoindentation experiments provide evidences of the hydrogen-reduced critical load, i.e., pop-in load, for homogeneous dislocation nucleation (HDN). However, there is no consensus to be reached on the underlying mechanism. Here, by directly calculating the activation energies of HDN through the nudged elastic method (NEB), we reveal that the hydrogen atoms would decrease the energy barrier of HDN and then reduce the corresponding pop-in load. Molecular dynamics simulations also capture the hydrogen-reduced critical resolved shear stress under varying hydrogen background fractions and temperatures, qualitatively verifying the theoretical prediction of NEB calculations. These findings at the atomistic scale will inform the understanding of hydrogen embrittlement, especially on the hydrogen effect on the dislocation multiplication, tangled dislocation, and dislocation wall.

Role of microstructure heterogeneity on hydrogen-assisted fracture toughness degradation of X80 weld metal in H2S-saturated solution

The hydrogen-assisted fracture toughness degradation of X80 weld metal with different microstructure heterogeneity was investigated through a crack tip opening displacement (CTOD) test in air and an H2S-saturated solution combined with a microstructure-based simulation. Differences in microstructure heterogeneity do not lead to variations in corrosion resistance; thus, the degradation of fracture toughness associated with microstructure heterogeneity is mainly controlled by hydrogen embrittlement (HE). The combined effects of increased microstructure heterogeneity and hydrogen promote damage associated with enhanced plastic strain localization. Hydrogen-dislocation interactions make it easier for dislocations to follow the {110} slip plane, resulting in void formation and crack initiation.

Hydrogen-induced degradation behavior of nickel alloy studied using acoustic emission technique

Hydrogen embrittlement susceptibility of nickel alloy (Inconel 625) is examined by a conventional strain rate tensile test (10−3 s−1) equipped with an acoustic emission (AE) setup. From the spectral analysis of AE signals coupled with the clustering procedure followed by the electron microscopic characterization, we have demonstrated the capability of the AE technique in identifying the hydrogen-dislocation interactions, crack initiation and propagation in various hydrogen-charged samples. The enhanced hardening behavior of hydrogen-charged samples is investigated from the viewpoint on the elementary dislocation theory, using a phenomenological relationship between the acoustic emission power and the experimentally measurable strain hardening parameters derived from the monotonic tensile test. In this way, we have shown that the mobile dislocation density increases with hydrogen content. In addition, the average velocity of dislocations is assessed from the AE data, and it is found to be reduced with the hydrogen concentration. The statistical crack analysis reveals the crack transition from the transgranular to intergranular mode in the samples failed under the hydrogen environment. The competition between these two fracture modes is explained by relating the hydrogen concentration profile with loading conditions.

Observation of the Hydrogen-Dislocation Interactions in a High-Manganese Steel after Hydrogen Adsorption and Desorption

Observation of the Hydrogen-Dislocation Interactions in a High-Manganese Steel after Hydrogen Adsorption and Desorption

Hydrogen-accelerated white etching area formation in bearings under rolling contact fatigue

The presence of hydrogen can dramatically facilitate microstructural alterations in components subjected to rolling contact fatigue (RCF) potentially leading to premature failure. A dislocation-assisted carbon migration model is developed to describe the formation of hydrogen-influenced microstructural alterations such as white etching areas; the model incorporates rolling parameters such as maximum contact stress, number of cycles, rotational speed, and temperature. Kinetic Monte Carlo is adopted to describe hydrogen-dislocation interactions which alters dislocation mobility, accelerating white etching area formation whilst reducing fatigue life. The results are experimentally validated by microstructural characterisation and RCF testing.

Assessment of hydrogen embrittlement on advanced high strength steels revealed by in-situ electrochemical micro-cantilever bending test

In the current study, the hydrogen-induced embrittlement on advanced high strength steels (AHSSs) is evaluated by in-situ electrochemical microcantilever bending (IECB) tests. Microcantilevers of 1200 M and 1400 M steels were bent while hydrogen charged inside a miniaturized electrochemical cell and then compared to bent-cantilevers in the air. The results of bending experiments and post-mortem evaluation of the bent-cantilevers showed that the plastic deformation occurred for the bent-cantilevers in the air. At the same time, the reduction of yield stress and the formation of hydrogen-enhanced cracking happened for the hydrogen-charged cantilevers. The results indicated that the microcracks are initiated and propagated adjacent to the clamped boundaries of the cantilevers, where the stress intensity is topmost. This finding is demonstrated by created step-wise cracks in 1400 M representative bent cantilever. The results show that the hydrogen-enhanced dislocation nucleation and hydrogen-reduced dislocation mobility are responsible for plastic deformation and hydrogen-enhanced cracking behavior. • The effect of hydrogen on advanced high strength steels was evaluated by micro-cantilever bending tests. • Plastic deformation occurred for the bent-cantilevers in the air. • Hydrogen assisted cracking happened due to hydrogen dislocation interactions. • Cracks were initiated and propagated beside the clamped boundaries of the cantilevers.

The interaction of hydrogen with microstructural defects studied by internal friction

In order to evaluate the hydrogen-defect interactions, an internal friction study is performed on hydrogen-charged cold rolled ultra-low carbon steel. For this purpose, the steel is subjected to various thermo-mechanical treatments and information on the hydrogen-dislocation interactions are obtained via thorough analysis of the so-called hydrogen cold work peak obtained by internal friction. Using existing theoretical models, the peak could be decomposed into two contributions from hydrogen segregation at screw and non-screw dislocations.

An In-Situ Electrochemical Nanoindentation (ECNI) Study on the Effect of Hydrogen on the Mechanical Properties of 316L Austenitic Stainless Steel.

In-situ electrochemical nanoindentation (ECNI) has been used to study the effect of hydrogen on the mechanical properties of austenitic stainless steel AISI 316L. Changing the electrode potential (via electrochemical charging) revealed the interconnected nature of the hydrogen effect on the nanomechanical properties of the stainless steel. At more positive cathodic potentials, a softening effect of hydrogen can be noticed, while significant hardening can be observed at more negative cathodic potentials. The hydrogen effects on the nanomechanical properties were analyzed in terms of the homogeneous dislocation nucleation (HDN) and the hydrogen-dislocation interactions from the energy point of view. The effects can be explained with the framework of the defactant theory and the hydrogen-enhanced localized plasticity (HELP) mechanism.

Effect of vanadium content on hydrogen embrittlement of 1400 MPa grade high strength bolt steels

In this work, the hydrogen embrittlement (HE) characteristics of 1400 MPa bolt steels with three different vanadium contents of 0, 0.17 wt% and 0.34 wt% were evaluated. The characteristics of the microstructure and dislocation density of the experimental steels were analyzed, and their effects on HE were also discussed. The results showed that with increasing V content, the HE resistance of the experimental steels was improved, and the experimental steel with the highest V content possessed the best HE resistance. The V-precipitates of steels with V contents of 0.17 wt% and 0.34 wt% were reversible hydrogen traps, and the inhibitory effect of V-precipitates on hydrogen-dislocation interactions improved HE resistance. In addition, a lower dislocation density and finer martensitic structure were also beneficial for hindering hydrogen-induced cracking (HIC).

Recent studies of tungsten-based plasma-facing materials in the linear plasma device STEP

This contribution summarized the recent studies of tungsten-based plasma-facing materials in the linear plasma device like the simulator for tokamak edge plasma (STEP), focusing on the examination of newly developed tungsten (W)-based materials and plasma-induced defects in pure W. Pure W, W-V, W-Y2O3 and W-ZrC samples were exposed to a high-flux plasma of ~ 1021–1022 m−2 s−1 with a fluence up to 1026 m−2 at a surface temperature below 500 K. The investigation of fundamental evolution of plasma-induced defects in pure W indicated a critical role of hydrogen-dislocation interactions. Suppressed surface blistering was observed in all W-based materials, but deuterium desorption behavior and retention were distinct with respect to different materials. The studies showed that the linear plasma device like the STEP was indispensable in the understanding of plasma-material interactions and the qualification of new materials for future fusion reactors.

Incorporating hydrogen in mesoscale models

Hydrogen embrittlement is a major and longstanding challenge in materials science. Mesoscale mechanical models are currently being developed which are providing new insight into hydrogen-dislocation interactions and the subsequent detrimental effect on ductility and ultimate tensile strength. Here, a review of some recent approaches to incorporating hydrogen within crystal plasticity, discrete dislocation plasticity and cohesive zone models are reviewed. Modelling at this scale bridges the gap between atomistic models and continuum engineering models which will be essential to fully understand hydrogen embrittlement.

The influence of microstructure on the hydrogen embrittlement susceptibility of martensitic advanced high strength steels

This work addresses the influence of microstructure on the hydrogen embrittlement of advanced high-strength steels. With sufficient hydrogen, fracture initiates at the ultimate tensile stress, but initiation is at the specimen surface. Fracture initiation is typically intergranular, transgranular or quasi-cleavage. These micro-fracture modes suggest that planar defects are important, such as prior austenite grain boundaries and lath and block boundaries. Fracture propagation is by shear fracture indicating a major influence of hydrogen-dislocation interactions. These hydrogen assisted fractures occur at a fast rate, probably approaching the speed of sound, that is at fracture velocities greater than 1000 m/s.

Hydrogen embrittlement revealed via novel in situ fracture experiments using notched micro-cantilever specimens

The susceptibility of the FeAl intermetallic alloy to hydrogen-assisted cracking was investigated by in situ fracture experiments using notched micrometre-sized specimens using an Environmental Scanning Electron Microscope (ESEM). The notched beams were loaded under two different environmental conditions: one in high vacuum (5 × 10−4 Pa) to avoid hydrogen effects and one under a certain water vapor pressure (450 Pa) to promote hydrogen uptake. The fracture behaviour on a non-ASTM-standard micro-sized specimen was successfully studied by the experimental approach, and the microstructure of the whole crack area was analysed by Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscopy (TEM) techniques. Three crack growth stages were observed in all the specimens: i) elastic regime, ii) notch blunting and micro-crack formation; and iii) stable crack growth. We observed an accelerated crack propagation rate in specimens under hydrogen exposure. The hydrogen embrittlement phenomenon was found to occur because of the strong hydrogen-dislocation interactions. The combined effect of hydrogen-enhanced dislocation nucleation and hydrogen-restricted dislocation mobility is responsible for the hydrogen-enhanced cracking behaviour.

Analysis of the various processes downstream cathodic hydrogen charging, V : Dissolved hydrogen, dislocations and dislocation drag

The new views issued from corrosion on the mandatory replacement of the atomic hydrogen H by a proton H + are first reminded for metallurgist, as well as the evolution of the physical approach to the metallic state, its consequences on the understanding of ab initio calculations (e.g. incompatibility between nuclear and conventional applications), and the reciprocal hydrogen-dislocation interactions. In an edge dislocation, especially, and for relieving the involvement of lattice elasticity on the expanded and contracted sides of the slip plane, a flat and extended cluster of repulsive protons progressively replaces the missing half-plane, and a symmetrical proton depletion dampens the additional half-plane. But when becoming too broad, flat proton clusters can also turn to incipient decohesions, which explains the systematic morphology of cleavage-like cracking in yet ductile steels. The illusion of the thermodynamic of irreversible processes is then reminded in the prospect of a possible hydrogen diffusion under stress gradient. The dislocation drag of hydrogen is finally shown to be a convective transport, which results in an unexpected exponential enrichment at the arrival, explaining both SSC and SOHIC. The propensity of protons to form flat clusters also explains the characteristic morphologies of cleavage-like or intergranular cracking in HE and corrosion fatigue.

Atomistic simulations of the interactions of hydrogen with dislocations in fcc metals

The interactions of hydrogen with both edge and screw dislocations in face-centered-cubic (fcc) metals are investigated using molecular statics simulations of nickel-hydrogen as a model system. It is shown that the most energetically favorable sites for H occupation are octahedral sites in the perfect fcc lattice, tetrahedral sites in the stacking fault, and both octahedral and tetrahedral sites in the Shockley partial cores of dislocations. Moreover, the diffusion barrier for H is relatively high except for pipe diffusion near the Shockley partial cores. It is also shown that partial dislocation cores have the strongest interactions with hydrogen. The hydrogen-dislocation interactions (attractive or repulsive) and the change in stacking width (increase or decrease) depend on the hydrogen-occupying sites (octahedral or tetrahedral) and the positions of the hydrogen atoms relative to the strongest binding energy sites. In particular, on the dislocation glide plane, only hydrogen atoms in both of the Shockley partial core regions can result in increasing the stacking fault width, while those in the stacking fault region or in the perfect fcc lattice region have no observable effects. On the other hand, uniformly distributed hydrogen in the tension region near the center of a dislocation can result in decreasing the stacking fault width. The stable stacking fault energy also decreases with increasing hydrogen concentration due to the resulting negative binding energy of hydrogen to the stacking fault, while the unstable stacking fault energy increases with increasing hydrogen concentration. This may result in pinning the dislocation due to the nature of short-range interaction between interstitial hydrogen atoms and their neighboring Ni atoms.

Solute-dislocation interactions: modelling and experiments in hydrogenated nickel and nickel base alloys

Abstract The dynamic interaction between diffusing hydrogen and mobile dislocations in nickel and binary nickel–16 wt.% chromium is analysed in terms of the solute drag phenomenon. Static strain ageing experiments are used to measure the saturated dislocation pinning force as a function of the hydrogen concentration. Hydrogen-dislocation interactions cause a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the line tension of curved dislocations, and we assess its consequences on the critical force for the expansion of a dislocation loop and on the dissociation mechanism. These results are used to interpret experimental results on the plastic flow of hydrogen-charged nickel single crystals oriented for easy glide. This study illustrates the mechanisms of H-dislocation interactions and their consequences on the different contributions of hydrogen to the flow stress of nickel.

The use of hydrogenated nickel as a model system for studying solute-dislocation interactions

The effect of hydrogen on tensile tests of nickel and binary nickel – 16 wt. % chromium alloy is analysed in terms of solute drag phenomenon. Static Strain Ageing experiments are used to measure the saturated dislocation pinning force as a function of the H concentration. First order hydrogen-dislocation interactions cause a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the self-energy and line tension of curved dislocations, the critical force for the expansion of a dislocation loop and the dissociation mechanism. These results are used to interpret experimental results on the plastic flow of hydrogen-charged nickel single crystals oriented for easy glide.This study illustrates the mechanisms of H-dislocation interactions and their consequences on the different contributions of hydrogen to the flow stress of nickel.

Numerical simulations of hydrogen–dislocation interactions in fcc stainless steels. : part I: hydrogen–dislocation interactions in bulk crystals

The basic equations for the coupling between stress and hydrogen diffusion are presented. A numerical simulation method is proposed. It is based on the diffusion equation including a hydrostatic stress gradient term, and on a discretisation of the hydrogen concentration field. The simulation scheme is applied to elementary dislocation configurations in order to compute the influence of solute hydrogen on dislocation interactions. A simple general expression is derived for the hydrogen screening effect on the pair interactions between co-planar dislocations. This expression is used to investigate the influence of diffusing hydrogen on the cross-slip probability of screw dislocations. First numerical results are presented. They are discussed in relation with the modelling of transgranular stress corrosion crack propagation.