Hydrogen-dislocation Interaction Research Articles

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-dislocation interaction in relation to hydrogen embrittlement and enhanced plasticity of metals

Interaction between hydrogen atoms and dislocations is studied in the iron-, nickel-, and titanium-based alloys using ab initio calculations and experimental studies of the electron structure, as well as mechanical spectroscopy. It is obtained that hydrogen increases the density of electron states, DOS, at the Fermi level, which is responsible for the increased concentration of free electrons decreasing thereby the elasticity moduli, and results in the decreased line tension of dislocations causing their increased mobility. By means of mechanical spectroscopy, the hydrogen diffusion enthalpy, ΔHd, and that of binding between hydrogen atoms and dislocations, ΔHb, have been measured. It is shown that these values control temperature Tc for condensation of hydrogen atoms at dislocations. Consequently, in a definite range of temperatures and strain rates, dislocations move accompanied by hydrogen atmospheres, which is a reason for hydrogen embrittlement. Hydrogen embrittlement in the iron-, nickel- and titaniun-based alloys is analyzed in terms of this concept. Based on the obtained results, the use of hydrogen as a temporary alloying element increasing techological plasticity of β titanium alloys is interpreted.

Hydrogen-induced intergranular cracking of pure nickel under various strain rates and temperatures in gaseous hydrogen environment

Industrial-grade pure nickel, NI-201, was tensile tested in high-pressure hydrogen gas at various testing temperatures and strain rates. The degree of hydrogen embrittlement (HE) was strongly affected by the testing temperature and strain rate, and elevating the strain rate and lowering the temperature result in suppressed HE. While hydrogen trapped at GBs via local equilibrium dominated the HE degree in hydrogen-charged cases, hydrogen supply to GBs during the tests was required in gaseous hydrogen cases. Although hydrogen–dislocation interaction was one of candidates for the hydrogen supply, the lattice hydrogen, which can interact with dislocations, hardly existed and related behaviors such as increase in flow stress and serrated yielding were not observed in the gaseous hydrogen cases; therefore, the contribution of the hydrogen–dislocation interaction was speculated to be minor. Alternatively, we proposed an HE model considering hydrogen diffusion along grain boundaries (GBs) from the crack tip, which successfully demonstrated the experimental results.

Studying the effects of hydrogen on dislocation mobility and multiplication in nickel by phase-field method

The effects of hydrogen on dislocation mobility and operation of the Frank-Read source are evaluated using our recently developed phase-field model. This is achieved by accounting for long-range elastic hydrogen-dislocation interaction (i.e., hydrogen shielding effect) and short-range inelastic hydrogen-dislocation core interaction through atomically informed generalized stacking fault energy (GSFE, or γ-surface). The results show that, due to the short-range interaction, hydrogen impedes the dislocation motion, which is contrary to the long-range hydrogen shielding effect. The bow-out configurations of the Frank-Read source obtained by the phase-field model can agree well with that from the atomic simulations. Hydrogen can decrease the critical nucleation stress of the Frank-Read source and thus enhance the hydrogen-induced plasticity. Moreover, the hydrogen-induced dislocation core energy reduction prevails over the hydrogen-induced elastic shielding, especially when the hydrogen concentration is low. Our results suggest that the modification of the dislocation core structure due to the short-range interaction plays an important role in understanding the hydrogen-enhanced localized plasticity mechanism. Finally, quantitative influences of hydrogen on dislocation mobility and critical nucleation stress of the Frank-Read source are obtained, which can further serve upscaled dislocation dynamics simulation and dislocation-density based crystal plasticity modelling.

Investigation on hydrogen embrittlement susceptibility in martensitic steels with 1000 MPa yield strength

Two quenched and tempered cost-saving marine steels with 1000 MPa yield strength were charged with hydrogen by the electrochemical technique to evaluate the hydrogen embrittlement (HE) behavior. The HE susceptibility index (HEI) obtained by slow strain rate tensile tests (SSRT) indicated that the HE resistance of steel A was weaker than that of steel B. Aided by fracture surface analyses, it could be determined that HE was a combination of hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) model. The microstructural characteristics were examined via transmission electron microscopy (TEM) and scanning electron microscope with electron backscattered diffraction analysis (SEM-EBSD). Thermal desorption spectroscopy (TDS) was applied to explore the hydrogen trapping behavior. It was shown that compared to steel A with fine microstructure and low Σ3 fraction, steel B had excellent HE resistance, which could result from the presence of more nano-sized NbC particles. The precipitation of NbC could not only act as an irreversible hydrogen trap to inhibit the hydrogen diffusion, suppressing the HEDE mechanism, but also can restrict hydrogen–dislocation interaction and suppress the HELP mechanism. The role of the interface between carbide and matrix on hydrogen trapping was investigated by density functional theory (DFT) and showed that the hydrogen capture force of NbC was stronger than (Nb,V)C. In addition, with the coarsening of microstructure during tempering at 580 °C, the high-angle grain boundaries (HAGBs) was obviously reduced, led to the HE susceptibility increased, especially steel A.

Interpretation of complex, tensile-fracture phenomena in precipitation-hardened, martensitic stainless steels, 17-4PH, in presence of hydrogen

Hydrogen-induced ductility loss in two precipitation-hardened, martensitic stainless steels was investigated via tensile testing with internal hydrogen, revealing it to be more significant with a higher hydrogen content or tensile strength. A lower hydrogen content and tensile strength resulted in quasi-cleavage fracture and considerable plastic strain, suggesting that hydrogen–dislocation interaction contributed to the loss in ductility. When either the hydrogen content or the tensile strength increased, the intergranular and cleavage surfaces were incorporated into the quasi-cleavage surface, a phenomenon attributed to the hydrogen-induced reduction of cohesive strength. Such a reduction was confirmed using impact tests at cryogenic temperatures, implying that some mechanisms were responsible for the loss in ductility.

Hydrogen, as an alloying element, enables a greater strength-ductility balance in an Fe-Cr-Ni-based, stable austenitic stainless steel

After pre-charging at the gaseous phase with a concentration of ~7,000 at. ppm, solute hydrogen was discovered to have an abnormal effect on both the strength and ductility enhancement of a commercially-available, Fe-24Cr-19Ni-based, stable austenitic stainless steel that had been subjected to tensile testing at various strain-rates. Specifically, the impact of hydrogen on material strength was accompanied by amplified yield and flow stresses, as well as tensile strength, while the improvement in ductility featured extended uniform elongation and strain-to-fracture, both of which became more pronounced as hydrogen concentration intensified. The product between tensile strength and uniform elongation served as indicators of the strength-ductility balance, at which hydrogen maximally optimized the indicator at the particular intermediate strain-rate. The yield/flow stress augmentations were interpreted in terms of solid-solution strengthening, whereas the enhanced ductility was primarily ascribed to the facilitation of mechanical twinning, whereby dynamic hydrogen-dislocation interaction exerted a critical influence as was indirectly revealed by supplemental stress-relaxation experiments.

DISLOCATION INDUCED MECHANISMS OF HYDROGENE EMBRITTLEMENT OF METALS AND ALLOYES

The paper discusses some models of hydrogen-stress cracking of metals and alloys. These models are based on hydrogen-dislocation interaction. It is shown that the critical role of dislocation emissions in AIDE mechanism is, in its turn, similar to HELP except for a higher localization of deformations compared with microvoids coalescence that is related with HELP, because that stresses needed for the dislocation propagation are high enough to boost general dislocation activity in deformation zones in front of cracks. This results in the formation of small voids on intersecting deformation bands. It has been observed that a crack is essentially growing due to the emission of dislocations. However the emission of dislocation towards the tip of a crack and the formation of voids in front of a crack contribute a lot to the process. Furthermore, the formation of voids in front of a crack makes for a short radius of the crack tip and low angles of the crack tip opening displacement The paper considers crack growing in inert media in plastic materials. Crack plastic growth takes place mainly due to dislocations that originate from the sources in the deformation zone in front of the crack tip and are propagating backwards along the crack tip plane with a small or zero emission of the dislocations that start from the crack tip. Small number of the dislocations that originate in the sources lying closest to the crack tip will intersect the tip of the crack precisely thus promoting the crack development while the majority of the dislocation will have either blunting effect or contribute to the deformation in front of the crack. Thus to cause a crack growth due to microvoid coalescence and deep cavities with shallow depressions therein on fracture surfaces there must be a large deformation in front of the crack. It is demonstrated that the cracking mechanism resulting from the AIDE mechanism will be either intergranular or transcrystalline depending on the location where the propagation of dislocations and formation of voids run mostly easily. In case of transcrystalline cracking alternative sliding motion along the planes on either side of the crack will tend to minimize the reverse stress caused by previously emitted dislocations. Then the macroscopic transcrystalline cracking plane will divide the angle between the slide planes and the crack front will be located on the intersection line of the crack planes and the slide planes. However, if there is a difference in the number of slides that occur on either crack side because of big differences in shear stresses on different slide planes, there will be deviations from the planes and directions with low refraction index. If the plane index is not low, there still can be deviations in the failure planes depending on the location of nucleus voids in front of the crack. A detailed description of the relationship between hydrogen effect on the behavior of dislocations and voids, sliding motion localization and hydrogen embrittlement is still lacking, moreover, it presents a serious problem that can be solved by describing the kinetics of hydrogen embrittlement process. Thanks to their sophisticated nature HELP and AIDE mechanisms can be embrittlement contributors both in cracking and in the formation of cavities due to ductile fracture.

Hydrogen migration and hydrogen-dislocation interaction in austenitic steels and titanium alloy in relation to hydrogen embrittlement

CrNi austenitic steels and titanium alloy Ti-10V-2Fe-3Al are studied aiming to clarify a reason for difference between two classes of engineering materials in their sensitivity to hydrogen brittleness. Using ab initio calculations, it is found that hydrogen increases density of electron states at the Fermi level in both materials except for its decrease in the titanium alloy at extremely high hydrogen contents. Migration of hydrogen atoms and their interaction with dislocations are studied using mechanical spectroscopy. The enthalpies of hydrogen atoms migration and their binding to dislocations, as well as temperature for condensation of hydrogen clouds around dislocations, are shown to be significantly larger in austenitic steels in comparison with the β titanium alloy. This is a reason for lower temperature range of hydrogen embrittlement in the titanium alloys. The different hydrogen effect in the studied materials and usage of hydrogen as temporary alloying element increasing plasticity of titanium alloys in the course of their processing are interpreted within the frame of HELP theory.

Hydrogen diffusivities as a measure of relative dislocation densities in palladium and increase of the density by plastic deformation in the presence of dissolved hydrogen

The effect of dissolved hydrogen on the dislocation density in cold-rolled palladium was investigated in order to provide evidence of a line energy reduction caused by hydrogen–dislocation interaction as proposed by the defactant concept. For this issue, palladium samples were electrochemically charged with hydrogen and subsequently cold rolled. Using conventional methods (X-ray diffraction, transmission electron microscopy) and a newly developed diffusion method, it was shown that the dislocation density after deformation increases with increasing hydrogen concentration.

Strain field of interstitial hydrogen atom in body-centered cubic iron and its effect on hydrogen–dislocation interaction

The effect of hydrogen in body-centered cubic iron is explored by using the density function theory. Hydrogen atoms increase the concentration of free electrons in the simulation cell and undergo bonding interaction with Fe atoms. Elastic interaction between hydrogen and screw dislocations was found, caused by the anisotropic strain components of the hydrogen atoms in the tetrahedral sites. The dependence of the hydrogen–screw dislocation interaction on hydrogen concentration is confirmed by repeated stress relaxation tests.

Hydrogen-Dislocation Interaction in Austenitic Stainless Steel Studied with Mechanical Loss Spectroscopy

Mechanical loss of pre-strained and hydrogen-charged specimens of 316L austenitic stainless steel has been measured by the free-decay method using an inverted torsion pendulum at frequencies around 1 Hz. It is found that in addition to known hydrogen Snoek-like relaxation process observed at about 240 K a new peak of mechanical loss appears around 270 K. The peak has a relaxation origin with enthalpy close to 0.78 eV and pre-exponential factor of relaxation time is found to be about 10-12.5s. Effects of the pre-strain and parameters of electrochemical hydrogen charging on the hydrogen peak in deformed austenitic stainless steels are measured. The obtained results are discussed in terms of the Snoek-Köster mechanism and specifics of hydrogen atomic distribution in a multi-component substitutional alloy of austenitic stainless steel. Based on the analysis the binding energy of hydrogen to dislocation in 316L steel is evaluated to be about 0.16 eV.

Anelastic mechanical loss spectrometry of hydrogen in austenitic stainless steels

Atomic distribution of hydrogen, its elemental diffusion jumps and its interaction with dislocations in a number of austenitic stainless steels are studied with anelastic mechanical loss (AML) spectrometry in combination with the hydrogen thermal desorption method. Austenitic stainless steels of different chemical composition, namely, AISI 310, AISI 201, and AISI 301LN, as well as LDX 2101 duplex stainless steel are studied to clarify the role of different alloying elements on the hydrogen behavior. Activation analyses of the hydrogen Snoek-like peaks are performed with their decomposition to sets of Gaussian components. Fine structure of the composite hydrogen peaks is analyzed under the assumption that each component corresponds to diffusion transfer of hydrogen between octahedral positions with certain atomic compositions of the nearest neighbouring lattice sites. An additional component originating from hydrogen-dislocation interaction is considered. Binding energies for hydrogen-dislocation interaction are also estimated for the studied austenitic stainless steels.

Hydrogen embrittlement studies of aged and retrogressed-reaged Al–Zn–Mg alloys

The hydrogen embrittlement (HE) of Al–4.35Zn–1.4Mg–0.059Zr alloys in different artificial aging tempers and after retrogression and reaging (RRA) treatments has been investigated by tensile testing hydrogen precharged specimens. The influence of RRA and hydrogen charging on the dislocation structure was studied by TEM. The under-aged temper was the most susceptible while the over-aged temper was the most resistant to HE. The RRA treatment improved the HE resistance of all the tempers. This has been attributed to the reduction in dislocation density upon retrogression and reaging. Flat fractography features near the surface of the hydrogen charged specimen have been correlated to the depth of hydrogen penetration. The hydrogen dislocation interaction and hydride cracking mechanism of HE have been addressed.

Effect of Hydrogen on Damping Capacity of Ti<SUB>50</SUB>Ni<SUB>25</SUB>Cu<SUB>25</SUB> Alloy

Ternary Ti–Ni–Cu alloy is known to exhibit higher damping capacity than binary Ti–Ni alloy. Hydrogen doping was used to further improve the damping capacity of this ternary alloy. Considering ease of manufacture, the Ti50Ni25Cu25 alloy was produced by a lamination process. As a result of hydrogen doping, a new damping peak appeared at around 260 K. The peak temperature shifted with changes in oscillation frequency. The activation energy (H) and the relaxation time constant (� 0) of the damping peak were calculated according to the Arrhenius plot as H ¼ 57:5 kJ/mol, and � 0 ¼ 2:2 � 10 � 13 s, respectively. The damping capacity due to hydrogen doping increased with increasing hydrogen concentration up to 0.45 at%, showing the maximum peak value of tan � ¼ 0:05, but thereafter decreased. On the other hand, the peak temperature increased monotonically with increasing hydrogen concentration, and the peak shape was broader than the single Debye relaxation peak. From these results, we concluded that this damping peak was not the Snoek peak but the Snoek-Koester peak caused by hydrogendislocation interaction. The decrease of the Snoek-Koester peak with increasing hydrogen concentration is explained by formation of hydrogen clusters or hydrides. Schoeck’s theory for the Snoek-Koester peak may be applicable. On further hydrogen doping, both the Snoek-Koester peak and the transformation peak were hardly visible above 5 at% hydrogen. This was found to be due to destruction of the martensitic phase by excess hydrogen doping.

The hydrogen-induced modification of the properties of the metal surface coated with oil and lubricant

The change of elastic and damping properties of the subsurface layer of iron and steel due to the coating with mineral oil and lubricants has been found in internal friction measurements. The effects were not observed for oil coated ceramics but did occur for iron and steel charged with hydrogen. In the case of coated metals, the formation of new internal friction peaks, increase in yield strain and increase in density of the slip lines of stretched foils were accompanied by the hydrogen uptake, was similar to the case of hydrogen charged metals. The origin of the phenomena is to be the hydrogen evolution from oil and lubricants at their catalytic decomposition, the hydrogen entry into the metal and the hydrogen-dislocation interaction within the subsurface layer of a metal. The hydrogen-induced modification of this layer and the possibility of hydrogen-induced softening or hardening should be taken into account in consideration of the tribological contacts.

Hydrogen embrittlement of an HSLA 80 steel in sea water under cathodic charging conditions

Hydrogen embrittlement of a copper precipitation strengthened and niobium microalloyed HSLA 80 steel on cathodic charging in synthetic sea water has been studied using a slow strain rate technique. The effects of strain rate and potential applied for hydrogen charging have been studied. Hydrogen measurement at different potentials has also been carried out. A loss in ductility in terms of a drop in percentage elongation and percentage reduction in area has been observed, the effect being prominent at potentials beyond —800 mV(SCE). Fractography by SEM shows a dominance of microvoid coalescence with increasing quasicleavage features at higher negative potentials. A hardening effect of hydrogen charging up to —600 mV(SCE) followed by a softening effect has been observed. The results are discussed in the light of the existing models of hydrogen–dislocation interaction and hydrogen induced microvoid coalescence.

低合金高強度鋼中の水素による内部摩擦ピーク

Internal friction was measured as a function of temperature for several types of low-alloy high-strength martensitic steels. In the spring steel of SUP7, tempered at 773 K of the usual standard condition, an internal friction peak was observed at about 200 K after cathodic hydrogen charging. The peak is attributable to the hydrogen cold work peak or the Snoek-Köster relaxation, although it was observed without cold work in this tempered martensite.The above internal friction peak disappeared after tempering at 973 K, but reappeared at the same temperature only in the cold-worked state. This means that it is caused by the Snoek-Köster relaxation based on the hydrogen-dislocation interaction in this high-strength steel, as has been well-established in pure iron. When tempered below 773 K, the relaxation was not seen clearly because of a very high background. A requisite condition for observation of the relaxation is therefore a moderate density and an adequate mobility of dislocations.

Simulations numériques des interactions hydrogène-dislocations en corrosion sous contrainte dans les matériaux cubiques à faces centrées

Many transgranular stress corrosion cracking models take into account the effects of hydrogen on the crack-tip plasticity. Our numerical simulations allow a quantitative approach of these hydrogen-dislocation interaction mechanisms. Hydrogen is shown to segregate on edge dislocations and to decrease their elastic interactions, thus causing a localised softening effect. This emphasises the formation of strong pile-ups which can promote local microfracture. First, it increases the dislocation density in the pile-up and second, it decreases the cross-slip ability.

Hydrogen embrittlement of aged and retrogressed-reaged AlLiCuMg alloys

The hydrogen embrittlement (HE) of Al-2.30 Li-1.24 Cu-0.80 Mg-0.12 Zr and Al-1.90 Li-1.80 Cu-1 Mg-0.09 Zr alloys in different artificial aging tempers and after retrogression and reaging (RRA) treatments has been investigated by tensile testing hydrogen precharged specimens. The influence of RRA and hydrogen charging on the dislocation structure was studied by TEM. The under-aged temper was the most susceptible while the peak-aged temper was the most resistant to HE. The RRA treatment improved the HE resistance of all the tempers. This has been attributed to the reduction in dislocation density upon retrogression and reaging. The alloy with the lower Li content exhibited improved HE resistance. Flat fractographic features near the surface of the hydrogen charged specimen have been correlated to the depth of hydrogen penetration. The formation of LiAlH 4 and LiH in hydrogen charged AlLi alloys has been confirmed by X-ray diffraction studies. The hydrogen-dislocation interaction and hydride cracking mechanisms of HE have been addressed.