Hydrogen Embrittlement Of Nickel Research Articles

On the role of vacancy-hydrogen complexes on dislocation nucleation and propagation in metals

New insights are provided into the role of vacancy-hydrogen (VaH) complexes, compared to the hydrogen atoms alone, on hydrogen embrittlement of nickel. The effect of the concentration of hydrogen atoms and VaH complexes is investigated in different crystal orientations on dislocation emission and propagation in single crystal of nickel using atomistic simulations. At first, embrittlement is studied on the basis of unstable and stable stacking fault energies as well as fracture energy to quantify the embrittlement ratio (unstable stacking fault energy/fracture energy). It is found that VaH complexes lead to high embrittlement compared to H atoms alone. Next, dislocation emission and propagation at pre-cracked single crystal crack-tip are investigated under Mode-I loading. Depending upon the elastic interaction energy and misfit volume, high local concentrations at the crack front lead to the formation of nickel-hydride and nickel-hydride with vacancies phases. These phases are shown to cause softening due to earlier and increased dislocation emission from the interface region. On the other hand, dislocation propagation under the random distribution of hydrogen atoms and VaH complexes at the crack front or along the slip plane shows that VaH complexes lead to hardening that corroborates well with the increased shear stresses observed along the slip plane. Further, VaH complexes lead to the disintegration of partial dislocation and a decrease in dislocation travel distance with respect to time. The softening during emission and hardening during propagation and disintegration of partial dislocation loops due to VaH complexes fit the experimental observations of various dislocation structures on fractured surfaces in the presence of hydrogen, as reported in literature.

Cohesive zone simulation of grain size and misorientation effects on hydrogen embrittlement in nickel

Abstract The size and misorientation effects on hydrogen embrittlement of a four grain nickel aggregate are studied with the help of hydrogen informed cohesive zone model. The grain misorientation angle is parameterized by fixing the lower grains while rotating the upper grains about the out-of-plane axis. Brittle failure of the grain aggregate is observed and nominal strength obtained. In the crack-free situation, the grain misorientation exerts an obvious weakening effect on the nominal strength, which is most pronounced at misorientation angles around 20°. Such trend applies to the pre-cracked situation but is much less pronounced. Both misorientation and pre-crack lead to size effect. The nominal strength shows a decreasing trend with the grain size, indicating that grain refinement tends to improve the load bearing capacity, which coincides with the observation in practice. Further, it is shown that the size effect diagram without hydrogen can be divided into three regimes. The conclusions apply to the case with hydrogen, except that the trend of the size effect curve can be affected by large grain sizes due to the longer absolute distance of hydrogen diffusion. These results provide guidelines for grain boundary engineering and for nanomechanical tests aiming at calibrating the intergranular decohesion parameters.

Continuum level simulation of the grain size and misorientation effects on hydrogen embrittlement in nickel

This paper addresses the size and misorientation effects on hydrogen embrittlement of a four grain nickel aggregate. The grain interior is modelled with orthotropic elasticity and the grain boundary with cohesive zone technique. The grain misorientation angle is parameterized by fixing the lower grains and rotating the upper grains about the out-of-plane axis. The hydrogen effect is accounted for via the three-step hydrogen informed cohesive zone simulation. The grain misorientation exerts an obvious weakening effect on the ultimate strength of the nickel aggregate which reaches its peak at misorientation angles around 20◦, but such effect becomes less pronounced in the case with a pre-crack. The misorientation could induce size effect in the otherwise size independent case without a pre-crack. The contribution of misorientation to the size effect is negligible compare to that caused by the existence of a pre-crack. These findings indicate that the misorientation effect in cases with a deep pre-crack is weaker than expected in shallow-pre-crack situations. Most of these conclusions hold for the hydrogen charging situation except that the ultimate strength is lowered in all the sub-cases due to hydrogen embrittlement. Interestingly, it is observed that the size effect becomes less pronounced with hydrogen taken into account, which is caused by the fact that hydrogen takes more time to reach the failure initiation site in larger grains.

Hydrogen embrittlement in nickel, visited by first principles modeling, cohesive zone simulation and nanomechanical testing

Hydrogen (H) can have dramatic consequences on material properties, especially by reducing the fracture toughness. Degradation by H initiates through mechanisms at the nano-scale, and is normally not detectable prior to the final leakage or component fracture. Computational techniques are therefore being developed in order to provide both a wider understanding of the phenomenon and engineering tools for prediction of materials susceptibility toward hydrogen embrittlement. This work presents a preliminary study for the development of a novel computational approach in which density functional theory (DFT), nanoscale experiments and finite element (FE) modeling are combined and interrelated in order to improve the understanding of hydrogen induced intergranular cracking. Two low angle and low coincidence grain boundaries types have been considered: Σ3 and Σ5. Density Functional Theory has been applied to investigate the influence of an increasing number of H atoms on the cohesive strength of these grain boundaries in pure Nickel. This provided relations between the number of H atoms (coverage of the grain boundary) and the cohesive strength, which is further applied in cohesive zone FE modeling of a triangular nanometer sized fracture mechanics cantilever beam. For verification of the model such specimens will be tested experimentally both with and without in-situ electrochemical charging.The simulation results show that the model is suitable for describing the combined effect of grain boundary misorientation and the reduced cohesive energy due to hydrogen on the grain boundary propensity to cleave.

The effect of Li, He and Ca on grain boundary cohesive strength in Ni

Boron is added to nickel-base superalloys such as Alloy X-750 in order to enhance high temperature strength and ductility so that the alloy may be more easily hot worked[1]. Boron additions also have been shown to ameliorate intergranular hydrogen embrittlement in nickel[2], and to improve the high temperature resistance of Alloy X-750 to aqueous stress corrosion cracking (SCC) in the absence of irradiation[3]. Recent quantum mechanical calculations demonstrate that boron strengthens grain boundaries in pure nickel[4], and may contribute to the observed benefits of boron on workability and fracture resistance of nickel alloys. Alloy X-750 exhibits greater susceptibility to intergranular stress corrosion cracking (IGSCC) when irradiated[5], and it has been proposed that the presence of grain boundary helium and/or lithium is responsible. Arguments have been advanced that helium embrittlement of the grain boundaries is primarily responsible for the greater observed susceptibility to IGSCC in irradiated X-750[1]. Alternatively, it has been proposed that lithium promotes IGSCC either by entering the water at the crack tip and lowering the local pH, or by inducing a restructuring of the grain boundary itself[1]. Direct embrittlement of grain boundaries by lithium also has been investigated by ion bombardment in Nimonic PE16, illustrating that under certain conditions lithium can produce degrees of embrittlement in nickel comparable to that produced by helium[6]. It is important to understand the relative roles of these species in grain boundary embrittlement in nickel alloys so that better predictive abilities and mitigation strategies can be developed. Toward that end, quantum mechanical calculations have been performed to investigate the influence of isolated lithium and helium atoms on the cohesive strength of an ideal grain boundary in pure nickel.

The effects of the addition of B and Fe and a prior deformation on the hydrogen embrittlement of nickel

The resistance to hydrogen embrittlement of pure nickel was improved by the addition of B and Fe. This improvement is thought to be the effect of Fe on reducing the tendency for nickel hydride formation. Moreover, B segregated to the grain boundary improved the ductility of this alloy. The hydrogen embrittlement was reduced by the cold rolling. The cross rolling was better in reducing the embrittlement than the single rolling because the cross rolling developed well the stable dislocation cellular structures acting as a hydrogen trapping site.

The effect of stress state on the hydrogen embrittlement of nickel

The ductility of nickel sheet subjected to in situ cathodic hydrogen charging has been investigated over a range of multiaxial stress states including uniaxial, plane-strain, and equibiaxial tension. The data show that the extent of ductility loss due to the presence of hydrogen increases as the stress state tends from uniaxial to equibiaxial tension. In all instances, the hydrogen embrittlement is characterized by intergranular fracture with failure occurring due to microcrack formation, microcrack link-up, and macrocrack growth. The increased susceptibility of nickel to intergranular hydrogen embrittlement with increasing biaxiality of stress state is shown to be a consequence of an enhanced rate of the link-up of strain-induced intergranular microcracks.

Trapping of hydrogen and helium at grain boundaries in nickel: An atomistic study

Atomistic computer simulations of the trapping of hydrogen and helium at defect free grain boundaries in nickel are presented. Three symmetrical tilt boundaries that encompass a number of compact polyhedra of atoms are considered as regions of potential trapping sites. By employing the structural unit model, these boundaries are shown to be representative of a wide range of grain boundary structures. A general correspondence of trap locations in regions of expansion for both hydrogen and helium has been found; however, the binding energy for helium trapping is much greater than that for hydrogen. Consequently, clean grain boundaries in nickel appear to be important trapping sites for helium, but not significant sites for hydrogen binding. These results are consistent with experimental autoradiography, thermal desorption, and transmission electron microscopy observations. They imply that grain boundary trapping plays an important role in mechanisms of helium embrittlement, but not in hydrogen embrittlement of nickel.

The influence of plastic deformation on the hydrogen embrittlement of nickel

The effects of cold deformation and surface treatment by shot peening on the susceptibility to hydrogen embrittlement of high purity polycrystalline nickel have been examined. Tensile, high cycle fatigue, low cycle fatigue and crack growth experiments have been performed on precharged samples as well as on specimens exposed to hydrogen gas. High vacuum and argon were utilized as reference environments. Shot peening suppressed hydrogen embrittlement in high cycle fatigue tests but did not improve low cycle fatigue properties. Fracture paths tended to be intergranular in all tests on nickel exposed to hydrogen, except in cold-worked samples. Orientation effects (relative to rolling direction) on hydrogen-induced crack growth were found to be significant. The results are discussed in terms of hydrogen distribution, dislocation densities and residual stress states resulting from shot peening or general cold deformation.

Effects of the additions of B, P, dSn, and Sb on oxygen assisted hydrogen embrittlement of nickel

Effects of additions of B, P, Sn, and Sb on hydrogen embrittlement of nickel were examined on specimens cathodically charged with hydrogen. Tensile specimens were annealed either in vacuum or in a dry hydrogen gas. Undoped, Sn-doped, and Sb-doped materials annealed in vacuum revealed high susceptibility to intergranular hydrogen embrittlement, while the susceptibility was greatly reduced when the materials were annealed in hydrogen. The deleterious effect of vacuum annealing was proposed to result from grain boundary penetration of oxygen from the annealing environment. Additions of a small amount of B and P almost completely suppressed the IGHE of specimens annealed in vacuum. It is suggested that the effect of these elements on grain boundary penetration of oxygen is an important factor for their beneficial effects on IGHE. Addition of Sn somewhat increased the ductility of hydrogen charged specimens annealed both in vacuum and in hydrogen, indicating that this element is not deleterious. Addition of Sb increased the embrittlement susceptibility of specimens annealed in hydrogen. However, the deleterious effect of Sb was not serious in spite of the high concentration of Sb that was examined.

Influence of Grain Boundary Penetration of Oxygen on Hydrogen Embrittlement of Nickel

In order to make clear the influence of vacuum annealing of specimens on intergranular hydrogen embrittlement of nickel, tensile properties and surface cracking along grain boundaries of specimens cathodically charged with hydrogen were examined. Specimens were annealed either in vacuum or in dry hydrogen gas before hydrogen charging. High susceptibility to intergranular hydrogen embrittlement and cracking of the surface was observed in specimens annealed in vacuum. Specimens annealed in hydrogen exhibited a significantly low susceptibility; they fractured in a predominantly transgranular mode, accompanied by a marked local contraction. The deleterious effect of vacuum annealing on the embrittlement was presumed to be due to the penetration of oxygen along grain boundaries during vacuum annealing. The penetration depth of oxygen was evaluated to exceed 2 mm in 10.8×10−3 s anneal at 873 K. These observations would have an important implication regarding hydrogen embrittlement of nickel and its alloys, since the effect of vacuum annealing has been disregarded in previous researches.

Niの粒界水素割れにおよぼす酸素の影響

Niの粒界水素割れにおよぼす酸素の影響

The role of interfaces in the hydrogen embrittlement of nickel

The role of interfaces in the hydrogen embrittlement of nickel

Niの水素脆化におよばすBの影響

Niの水素脆化におよばすBの影響

純NiおよびNi-Fe合金の水素脆化について

In order to clarify the mechanism of hydrogen embrittlement in nickel and nickel-iron alloys, tensile tests at various temperatures and strain rates were carried out using thin specimens occluded with hydrogen in the form of solid solution and hydride.Embrittlement occurred over the whole investigated temperature range of −196 to +100°C, revealing two maxima near −50°C and ambient temperatures. In lower testing temperatures where the hydride is stable, supersaturated hydrogen in the form of solid solution is segregated to grain boundaries or high triaxial stress fields, precipitating hydride during the tensile test. Embrittlement near −50°C decreases with the increase in iron content and strain rate, since they make hydride precipitation difficult. On the other hand, in the higher testing temperature range where the hydride is unstable, embrittlement is accelerated not only by the generation of structural damage like surface cracks caused by the decomposition of the unstable hydride but also by the segregation of the hydrogen to regions of triaxial stresses around the damage. Embrittlement of this type occurs more pronouncedly during aging at 20°C; the ductility of hydrogenated metals decreases with hydride decomposition, showing a minimum when the hydride has been decomposed completely.Hydrogen embrittlement is considered to take place by two distinct causes; one is precipitation of brittle hydride, and the other is the clustering of hydrogen around the metallographic damage introduced by hydride decomposition.

Further observations on the effect of grain boundary segregation in the hydrogen embrittlement of nickel

Abstract : Earlier work has shown an association between the intergranular embrittlement of nickel by hydrogen and the presence of hydrogen recombination poisons at the grain boundaries. The present effort demonstrates that the susceptibility of nickel to embrittlement may be controlled by heat treatment sequences which affect the partitioning of impurities. It is also suggested that the association between impurities and microchemistry may apply in part as well as to the temper embrittlement of steels. (Author)

The effects of hydrogen on the deformation and fracture of nickel-iron alloys

Slow strain-rate tensile tests have been performed on hydrogen-charged f.c.c. nickel-iron alloys. The hydrogen embrittlement observed in pure nickel is suppressed by alloying with iron so that the deformation and fracture behaviour of alloys containing 50 and 60 % of iron is unaffected by the presence of hydrogen at levels in the order of 3 cm 3/100 g. These and other results indicate that the hydrogen embrittlement of nickel and nickel-iron alloys is associated with the grain boundary precipitation of a hydride which decomposes during or after fracture. As nickel is alloyed with iron the tendency for hydride formation decreases and the hydrogen embrittlement is correspondingly reduced.

ニッケルの水素脆化について

The brittleness caused by heating in hydrogen gas was examined on several kinds of nickel specimens containing oxygen from 0.004% to 0.024%. It was clarified that this hydrogen embrittlement depends on the oxygen content, and by eliminating oxygen by preannealing at high temperature, it decreases considerably. Such a finding is similar to the hydrogen brittleness in copper. Also the brittleness attributed perhaps to the effect of residual hydrogen in nickel was observed. Namely, the recovery of brittleness and the so-called delayed failure phenomenon which occur in steels during atmospheric-temperature aging were observed. And, as a special characteristic, the embrittlement was obvious only in the case of rapid cooling and especially when tested by reverse bending. From these results it was concluded that the hydrogen embrittlement in nickel is due to the formation and growth of micro-cracks along grain boundaries by the reduction of oxide and the effect of residual hydrogen accumulated along them.