Hydride Precipitation Research Articles

Direct observation of nanometer size hydride precipitations in superconducting niobium.

Superconducting niobium serves as a key enabling material for superconducting radio frequency (SRF) technology as well as quantum computing devices. Niobium has a high propensity for the uptake of hydrogen. At room temperature, hydrogen commonly occupies tetragonal sites in the Nb lattice as the metal (M)-gas (H) phase. When the temperature is decreased, however, a solid solution of Nb-H begins to precipitate. In this study, we show the first identified topographical features associated with nanometer-size hydride phase (Nb1-xHx) precipitates on the surface of the metallic superconducting niobium using cryogenic-atomic force microscopy (AFM). Further, high energy grazing incidence X-ray diffraction reveals information regarding the structure and stoichiometry of these precipitates. Finally, through time-of-flight secondary ion mass spectroscopy (ToF-SIMS), we locate atomic hydrogen sources near the top surface. This systematic study clarifies nanometer scale hydrides precipitated on the surface of the SRF Nb cavity that exhibit performance degradation at a high accelerating field regime.

Displacement cascade bombardment of delta-hydrides in alpha-zirconium

One of the factors affecting the in-pile performance of Zr-based alloys is the precipitation of hydrides once H concentrations exceed the terminal solubility limit. H transport and hydride precipitation/dissolution is commonly modeled in codes such as BISON, but most of the experimental data supporting these models has been collected on unirradiated materials. As such, there is considerable uncertainty as to the influence of irradiation effects. In this work, molecular dynamics simulations of displacement cascades were performed on δ-hydrides to elucidate: 1) the extent of H dissolution following cascade impacts and 2) any alterations to defect production characteristics when compared to cascades in bulk Zr. The immediate amount of H dissolved in a high-energy cascade impact is notable, but a considerable fraction of the dissolved H atoms are rapidly re-absorbed into the hydride at reactor-relevant temperatures. The amount of dissolved H also decreases with increasing hydride size. When considering the expected volume fractions of hydrides, it is not expected that the irradiation-induced H dissolution rate will significantly affect the availability of H in the Zr lattice. In terms of defect production, cascades which overlap δ-hydrides produced an order of magnitude more stable defects than equivalent-energy cascades in bulk Zr. Vacancy defects are predominantly contained within the hydride structure while interstitials clusters are found adjacent to the hydride surface. Interstitials are strongly repelled by the hydride structure which may drive the expulsion of cascade-generated interstitials to the hydride surface and impede athermal recombination. Thus, the interatomic potential used in this work predicted a significant alteration to the defect survival efficiency and a stark production bias in the availability of mobile defects in bulk Zr following hydride-overlapped displacement cascades.

Hydride induced embrittlement and fracture of non-hardening metals under hydrogen chemical equilibrium

The stress field in the hydride precipitation zone is examined, under conditions of hydrogen chemical equilibrium and constant temperature, in the case of non-hardening metals, by applying slip-line theory. It is proven that the hydride precipitation zone, in any geometry, is a constant stress area. In this area, the principal stresses are equal to the respective principal stresses, before hydride precipitation, minus the difference of hydrostatic stress before and after hydride precipitation. The general relations are applied to the case of a stationary sharp mode-I plane-strain crack and the deviations from Prandtl-field are derived, in the [-π/4, +π/4] sector ahead of the tip, where hydrides precipitate. In this case, the hydride precipitation sector is characterized by a constant hydride volume fraction. In addition, hydride precipitation is associated with the development of elastic sectors along the crack faces and the reduction of the centered fan sectors; the relation between hydride precipitation zone stress trace and the extent of the centered fan sector is presented. The mode-I plane-strain blunted crack is also considered and the deviations from the logarithmic spiral slip-lines is discussed together with the reduction of hydride volume fraction as the blunted crack-tip is approached. A general fracture criterion, based on the strength of hydride platelets, is derived, which indicates that fracture occurs, when a critical hydride precipitation zone stress trace dominates. The criterion is applied, under the condition of a dominant K-field annulus, surrounding the plastic zone, and the estimated threshold stress intensity factor of delayed hydride cracking correlates favorably with experimental measurements.

Effects of intergranular hydride precipitation on the mechanical behavior of bicrystalline zirconium: A molecular dynamics-based study

This article investigated the impact of intergranular hydride precipitation on the mechanical behavior of zirconium (Zr) bi-crystals using molecular dynamics (MD)-based simulations. Uniaxial tensile tests were conducted to explore the effects of hydride precipitation on symmetric and asymmetrical tilt grain boundaries (GBs) of Zr with [0 1‾ 10] as the tilt axis. The stress-strain curves and deformation governing mechanism of bi-crystalline Zr were analyzed using the MD-based simulations in conjunction with the COMB force field. Misfit stress and tensile stress degradation induced by hydride precipitation were evaluated to understand their correlation with GB misorientation angles. Hydride precipitation induced the residual stresses in the vicinity of GB plane and mitigates the overall mechanical properties. It was predicted from the simulations that GBs with low misfit stress develop a lower mismatch between the lattice of Zr and hydride precipitate. Consequently, the effect of hydride on the tensile strength gets nullify. In contrast, the effects of hydrides are more pronounced on the tensile strength of Zr containing low-energy GBs in conjunction with high-misfit stress. The study further reveals that intergranular hydride precipitation causes the hydride embrittlement effect in Zr bicrystals, which intensifies with large size hydride precipitate. These findings contribute to an atomistic-level understanding of intergranular hydride-induced embrittlement and its correlation with Zr grain boundary orientation.

The role of symmetric tilt grain boundaries on the precipitation of hydrides in zirconium: A molecular dynamics study

Zirconium (Zr) alloys are widely used as structural materials in the core of nuclear reactors. However, the absorption of hydrogen by these alloys during the reactor operation may lead to the precipitation of hydrides. Such hydrides have considerable effects on the microstructure and mechanical properties of the alloys, thus significantly impacting their overall performance. In this work, we investigate the behavior of hydrogen in the presence of symmetric tilt grain boundaries in Zr using molecular dynamics simulations. To systematically explore the conditions for the formation of hydrides, we consider hydrogen concentration levels of up to 50 at.%. We find that hydrogen concentrations of 30 at.% or lower are below the threshold for the precipitation of hydrides, while fcc coordinated hydrides precipitate at 50 at.%, which is very close to the solubility limit of hydrogen in Zr at 800 K. We find that the grain boundaries (GBs) play a pivotal role in the behavior of hydrogen, as well as the formation of hydrides in Zr systems. In most cases, large amounts of hydrogen are found to accumulate at GBs. However, both homogeneous and heterogeneous nucleation are observed during the formation of hydrides at the grain interiors and GBs, respectively. Nonetheless, in some cases the GB inhibit the formation of hydrides.

The micromechanics of fracture of zirconium hydrides

Zirconium alloys are susceptible to hydrogen embrittlement and hydride precipitation. The precipitation of hydrides is accompanied by a transformation strain, but the contribution of this strain to the fracture of hydrides is not well-understood. In this study, deformation mechanisms and the micromechanics of fracture of hydrides are investigated by conducting in-situ and interrupted ex-situ tensile experiments on hydrided zirconium specimens. Electron backscatter diffraction (EBSD) is used to measure the macro-EBSD maps that contain the grains located in the gauge section of the specimens’ surfaces. Further, high spatial resolution EBSD is used to determine orientation variations induced by the precipitation of hydrides and by the external mechanical load. The measured grain orientations are mapped into a crystal plasticity finite element (CPFE) model to examine the performance of eight different crack initiation criteria. It is shown that a multiscale approach is essential for studying the fracture of hydrides as the details of hydride morphology, orientation, local grain neighborhood, and hydride-hydride interactions are important in such analysis. It is shown that neglecting the effects of hydride-induced transformation strain leads to inaccuracies in predicting both the location and direction of microcracks within hydrides. Among the examined methods, the combination of resolved shear stress and resolved shear strain on slip systems, i.e., the highest shear energy density, consistently predicts the correct locations of hydride microcracks as well as their propagation direction. Further, it is shown that the significant deformation that takes place within hydrides is the main driving force for the fracture of hydrides.

Effect of grain boundary character on intergranular hydrides precipitation in zirconium

Grain boundary characteristics play a crucial role in the formation of intergranular hydrides, which may cause brittle fracture in zirconium alloys. However, the impact of grain boundary character on microscopic hydride precipitation remains unclear. Here, we investigate the nucleation and precipitation of intergranular hydrides in zirconium by using In-situ transmission electron microscopy observations and post-precipitation electron backscattered diffraction analysis. The nucleation of intergranular hydrides is highly dependent on the interacting angle (αGB-BP) between the grain boundary plane and the basal plane. For low-angle grain boundaries, hydrides prefer to grow into the grain interior when the αGB-BP is less than 60°, otherwise, no hydride precipitation occurs. For high-angle grain boundaries, hydrides nucleate along the grain boundary when the αGB-BP is less than 39°. As the αGB-BP increases, hydrides grow into the grain interior. A thermodynamic model is developed to analyze the nucleation of needle-shaped hydrides. The variation of the intergranular hydride as a function of the grain boundary energy and the αGB-BP is predicted. This finding provides new insight into utilizing grain boundary engineering to modulate intergranular hydride precipitation and offers a pathway to control hydrogen embrittlement in zirconium alloys.

Comparison of the hydrogen damage of different rolling surfaces of TC4 Ti alloy sheet

Abstract The microstructure of three rolling surfaces of TC4 sheet is different, and their resistance ability to hydrogen damage lacks systematic research. Thus, the hydrogen damage behavior of TC4 rolling sheet was investigated in this paper. The hydrogen diffusion law along different rolling directions and the precipitation of hydrides on different rolling surfaces were compared. It is found that the shape and distribution of α and β phases are changed under the action of extrusion force during the rolling process, and they are arranged in striped shape on the R-N surface along the R direction, and the diffusion of hydrogen along the R direction is faster due to the existence of continuous β phases as hydrogen diffusion channels, resulting in the more serious hydrogen damage. Besides the hydrides mainly deposited at the α and β phase boundaries, the hydrides precipitated in the interior of α phases on the R-N surface are more than that on the R-T surface due to the different distribution state of β phases.

Effect of hydride orientation on tensile properties and crack formation in zirconium alloy cladding tubes

Hydride precipitation and reorientation are of great importance to evaluate considering their detrimental influence on mechanical properties of nuclear fuel claddings. In this work, a fully recrystallized Zr-Sn-Nb cladding tube was hydrided with 120 and 220 wppm concentration, followed by thermo-mechanical treatment to produce specimens with varying hydrogen contents and hydride orientations. Ring tensile test demonstrates that in contrast to circumferential ones, the radial hydrides lead to significant reductions in plastic elongation of cladding tubes. Scatter in values of mechanical properties are observed in the case of 220 wppm radial hydrides, which could be attributed to irregular distribution of interconnected radial hydrides. Microstructure characterization reveals that microvoids initiate at the junction boundaries of hydride platelets under the tension effect of surrounding α-matrix deformation. These microvoids are proposed to propagate along the interior of brittle hydrides, rather than the interface, eventually leading to premature fracture failure given the alignment of radial hydrides perpendicular to the external tensile stress. The presented results are expected to shed light on microcrack initiation and propagation with respect to hydride orientation in zirconium-based nuclear fuel claddings.

Formation of pure zirconium islands inside c-component loops in high-burnup fuel cladding

High-burnup Zr-based nuclear fuel claddings exhibit accelerated irradiation growth, corrosion and hydrogen pick-up, all correlated with the emergence of c-component dislocation loops. We made use of sub-nm-resolution atom probe tomography to characterize the nanoscale chemistry of c-loops in fuel cladding from boiling water reactor operation. We found segregation of Fe, Ni and Sn to dislocation lines and depletion of Sn and O inside the loops, resulting in nearly pure Zr islands. We also observed nucleation of suboxide inside one c-loop, pointing to a possible mechanism of accelerated in-reactor corrosion. The Zr islands might also promote hydride precipitation and associated degradation.

Hydride-enhanced strain localization in zirconium alloy: A study by crystal plasticity finite element method

Hydride precipitation in zirconium (Zr) alloys leads to the deterioration of mechanical properties in key components of nuclear reactors. However, the underlying mechanisms governing the failure initiation of hydrogenated Zr alloys remain incompletely understood. This study focuses on investigating the effects of δ-phase hydride precipitation on the local deformation behavior of Zr alloys. A crystal plasticity finite element method is used to simulate micropillar compression tests of single-crystal samples and macroscale tensile tests of polycrystalline samples. Our results reveal that in the case of micropillar B containing δ-hydride (where the hydride is oriented at 45° from the loading direction), the shear along the hydride–matrix interface and the misfit strain induced by hydride precipitation synergistically contribute to strain localization of Zr alloys. However, for micropillar P containing δ-hydride (where the hydride is parallel to the loading direction), the introduction of hydrides does not enhance the strain localization. Instead, it impedes the local prismatic slip. On the other hand, examination of polycrystalline samples through simulations indicates that hydrides enhance the strain localization of Zr alloys. This effect arises from several mechanisms: shear stress along hybrid–matrix interface, promoting plastic slip in the direction close to this interface; hydride-induced hindrance to certain slip bands, leading to non-uniform local deformation; and misfit strains induced by hydride precipitation, contributing to localized deformation around the hydrides.

Phase-field simulations of the effect of temperature and interface for zirconium δ-hydrides

Hydride precipitation in zirconium cladding materials can damage their integrity and durability. Service temperature and material defects have a significant effect on the dynamic growth of hydrides. In this study, we have developed a phase-field model based on the assumption of elastic behaviour within a specific temperature range (613 K–653 K). This model allows us to study the influence of temperature and interfacial effects on the morphology, stress, and average growth rate of zirconium hydride. The results suggest that changes in temperature and interfacial energy influence the length-to-thickness ratio and average growth rate of the hydride morphology. The ultimate determinant of hydride orientation is the loss of interfacial coherency, primarily induced by interfacial dislocation defects and quantifiable by the mismatch degree q. An escalation in interfacial coherency loss leads to a transition of hydride growth from horizontal to vertical, accompanied by the onset of redirection behaviour. Interestingly, redirection occurs at a critical mismatch level, denoted as q c, and remains unaffected by variations in temperature and interfacial energy. However, this redirection leads to an increase in the maximum stress, which may influence the direction of hydride crack propagation. This research highlights the importance of interfacial coherency and provides valuable insights into the morphology and growth kinetics of hydrides in zirconium alloys.

Hydrogen diffusion in zirconium hydrides from on-the-fly machine learning molecular dynamics

Reactor pressure vessels and fuel cladding tubes have repeatedly failed due to zirconium hydrides. Zirconium hydride precipitation and growth are directly affected by hydrogen atom transport properties, which would make nuclear fuel storage less safe over long periods of time. Herein, we employ first-principles calculations to investigate the hydrogen diffusion mechanism in zirconium hydrides, utilizing on-the-fly machine learning force field molecular dynamics. It is verified that the machine learning force field can accurately describe the hydrogen atomic diffusion properties in zirconium hydrides at several temperatures and compositions. The atomic migration paths of hydrogen in zirconium hydrides as well as their barriers and pre-factors are also calculated. According to our results, the temperature and composition of zirconium hydrides affect the microscopic dynamical behavior of hydrogen.

Modeling hydrogen localization in Zircaloy cladding subjected to temperature gradients

In light water reactors, Zr-based alloys used for nuclear fuel cladding undergo oxidation and absorb hydrogen, which can precipitate as brittle zirconium hydrides. During reactor normal operation conditions, hydrogen concentration varies locally due to various factors, where local high concentration can significantly degrade cladding mechanical properties and, hence, its service life. This study aims to quantify the redistribution of hydrogen within the fuel rods caused by temperature gradients, considering geometric irregularities such as inter-pellet regions, oxide spallation, and missing pellet surfaces. Through simulations, we have discovered that these temperature variations can lead to significant local hydrogen enrichment, even under normal operational conditions. Consequently, the Zircaloy cladding may suffer from critical weakening in its mechanical performance due to excessive hydride precipitation at specific locations. These findings underscore the importance of accounting for local hydrogen concentrations when evaluating the overall reliability of the cladding.

Investigation of fine-scale dislocation distributions at complex geometrical structures by using HR-EBSD and a comparison with conventional EBSD

Dislocation distribution is one of the elementary factors that directly influence deformation processes in structural components. Electron backscatter diffraction (EBSD) analysis is one of the experimental techniques that allow for a semi-quantitative evaluation of the Geometrically Necessary Dislocation (GND) distribution in polycrystals. Owing to its noise floor the accuracy of conventional Hough transformed-based GND analysis is restricted to relatively large plastic strains. This problem can be overcome by using the cross-correlation-based high angular resolution electron backscatter diffraction (HR-EBSD) method. The present study demonstrates the improvements in characterizing localized dislocation distribution using the HR-EBSD method compared to the conventional approach. To demonstrate the efficacy, we investigated two more extreme examples of deformation conditions in polycrystalline zirconium (Zr), 1) a plastically deformed crack tip in presence of applied strain, and 2) a misfit plastic field around a hydride precipitate. The edge, screw and total GND components are compared for both cracked and hydride specimens. Dissimilarities in the identification of screw dislocations and subtle features with misorientation uncertainties are reported using HR-EBSD and conventional EBSD. A direct correlation between slip bands and the HR-EBSD estimated GNDs is presented, enhancing the scope of this approach to identify individual slip bands, which is traditionally challenging for any misorientation-based characterization technique.

Exploring the hydride-slip interaction in zirconium alloys

Hydrogen pick-up and hydride precipitation can lead to embrittlement and fracture strength reduction of nuclear fuel cladding tubes made of Zircaloy. Plastic deformation of hydride packets and its interaction with local plasticity in the zirconium matrix is a key linkage of microstructure feature to structural integrity of hydrided polycrystalline bulk Zircaloy. This work focuses on explicit representation of hydride packets from high spatial resolution electron backscatter diffraction onto a crystal plasticity finite element model for capturing and understanding slip localisation near hydride-matrix phase boundaries, based on the extracted material property of hydrides. The mechanisms behind slip evolution including slip nucleation, slip transfer, and slip inhibition are studied by combined high-resolution digital image correlation and crystal plasticity results. Through assessing various slip transfer parameters, new slip transfer criterion is proposed for α/δ phase boundaries. Prior to slip transfer criterion, local micromechanical quantities, specifically shear stress and stored energy density, are necessary to drive and provide pathway for subsequent slip transfer at α/δ phase boundaries.

Identification, classification and characterisation of hydrides in Zr alloys

Hydride precipitation in zirconium alloys leads to embrittlement, making it essential to understand their prevalence and stability in the microstructure. Dictionary indexing of Kikuchi patterns, along with orientation relationship analysis and x-ray diffraction, confirmed the presence of both delta and gamma hydride phases in Zircaloy-4. Both phases were found to be stable in recrystallised zirconium, with the gamma phase exhibiting a distinct orientation relationship with the matrix. Delta hydride morphology and orientation were influenced by local stresses, resulting in a change in orientation during precipitation. By analysing the orientation relationships, the evolution of hydride phases could be visualised, providing insights into the room temperature stability of both delta and gamma hydrides.

On the effects of transformation strain induced by hydride precipitation

One of the main degradation mechanisms of the zirconium alloys used in nuclear reactors is hydrogen embrittlement and hydride formation. The formation of zirconium hydrides is accompanied by a transformation strain, the effects of which on the development of localized deformation zones are not well-understood. This study uses a crystal plasticity finite element model that is coupled with diffusion subroutines to quantify such effects. For this purpose, a zirconium specimen was hydrided in the absence of any external mechanical loads. With the use of electron backscatter diffraction, the rotation fields around interacting intragranular hydrides as well as those located at grain boundaries or triple points were measured at a high spatial resolution. The as-measured zirconium and hydride morphologies were mapped to the model for numerical simulation. Both numerical and experimental results show that hydride precipitation induces large rotation fields within the zirconium matrix, where such rotations are at their maximum in the vicinity of hydride tips. While the crystallographic orientations and shapes of hydrides affect the magnitude of rotation fields, both experimental and modeling results revealed the development of discrete and parallel geometrically necessary dislocation fields and a strong interaction among neighboring hydrides. It is shown that the stress field resulting from hydride precipitation affects the patterning of hydrogen distribution, which in return affects further hydride interlinking.

Numerical Study of the Hydride Embrittlement in Zirconium Alloy using XFEM

A numerical model for hydride embrittlement in Zirconium alloy (Zr–2.5Nb) is developed utilizing the extended finite element method (XFEM). Hydride embrittlement reduces the ductility and failure time of a metal/alloy. During hydride embrittlement, stress-directed hydrogen diffusion, metal-hydride phase transformation, mechanical deformation, and hydride precipitation occur simultaneously. The present model incorporates all these processes and is able to predict the hydrogen concentration and the hydride fraction distribution under any externally applied stress field. In this work, both the steady and transient hydrogen diffusion cases are evaluated. Further, the XFEM is utilized to develop a model of hydride embrittlement in the presence of a crack. The first step of the hydride embrittlement process is the diffusion of hydrogen. According to Fick’s law of diffusion, hydrogen diffusion is directly dependent on hydrostatic stresses and hydrogen concentration gradient under external stresses. The next step is the hydride precipitation in hydride embrittlement, where the expansion of material takes place that changes the hydrostatic stress field. Thus, studying the effect of precipitation of hydride on hydrostatic stresses is essential. Moreover, the process of hydride embrittlement is highly influenced by residual stresses in the structure. Hence, the effect of residual stress present in the zirconium alloy pressure tube (PT) is also evaluated. The results indicate that the residual tensile stresses contribute to the growth of hydride, which will reduce the material failure time.

Hydride reorientation in fuel cladding under interim storage conditions with low hoop stress

Abstract. The restart of the search for a final repository for high-level nuclear waste in Germany and the resulting so-called white map lead to a reassessment of the safety of interim storage as well. Due to the resulting increase in interim storage periods with a prospected timescale until 2068, among other things, renewed consideration of cladding tube integrity is required for the handling of the fuel assemblies during repackaging into the final storage containers. Besides oxide formation during operation, hydrides can have a negative influence on stability during interim storage and handling later on. The hydrides, which are initially precipitated tangentially in the tube material during operation, can reorient during interim storage as a result of the temperature and load history and precipitate again in a radial direction during cooling under load. In recent years, a number of investigations have already been carried out with respect to this topic, with cladding conditions that are present in high burn-up fuel assemblies with corresponding high hydrogen contents. In these cases, reorientation effects did occur with stresses that can be encountered in interim storage, but the proportions of radial hydrides were very low and were classified as not being harmful. In addition to the tests with cladding tubes containing high hydrogen contents, sporadic tests with low hydrogen contents were also carried out, for example, by Hardie and Shanahan (1975) and later by Chu et al. (2008). These indicated that strong reorientation effects could occur because of the complete dissolution of hydrogen under high temperature and tensile stress. In recent years, Kaufholz et al. (2018) also formulated a hypothesis that describes this effect as being the consequence of free nucleation without the presence of precipitates. This results in the possible formation of extremely pronounced radial hydrides, which can even extend through the tube wall. This is possible because residual hydrides that can hinder free precipitation or represent preferential points for nucleation are absent. The results of the present investigations confirm this behaviour. They show that massive reorientations can already occur at stresses at the lower end of the literature values and that, until now, these reorientations were interpreted as being uncritical. Considering the hysteresis in solubility between terminal solid solubility for dissolution (TSSD) and precipitation (TSSP), it becomes apparent that precipitation at hydrogen contents of 100 ppmw (parts per million by weight) already starts at approx. 538 K and that the hydrides precipitate radially under a stress of 52 MPa and form extremely pronounced hydrides. This results in a need for further research, since the thermal and mechanical loads under which the tests were carried out are well below the permissible limit loads in interim storage and below the values expected so far. This applies in particular to materials and cladding tubes that either absorb less hydrogen during operation or have only low burn-ups. The study provides an overview of the status quo, presents part of the investigations carried out during the project MUDZ (Modellierung und Untersuchung der Degradation von Hüllrohrmaterialien aus Zr-Legierungen durch Hydridbildungs- und Hydridverteilungsprozesse im Hinblick auf die Langzeitzwischenlagerung) in the KEK (Kompetenzerhalt in der Kerntechnik) initiative and the results obtained, and discusses the ramifications of the results.