Hydride Nucleation Research Articles

Multiscale modeling of metal-hydride interphases—quantification of decoupled chemo-mechanical energies

The quantification of interphase properties between metals and their corresponding hydrides is crucial for modeling the thermodynamics and kinetics of the hydrogenation processes in solid-state hydrogen storage materials. In particular, interphase boundary energies assume a pivotal role in determining the kinetics of nucleation, growth, and coarsening of hydrides, alongside accompanying morphological evolution during hydrogenation. The total interphase energy arises from both chemical bonding and mechanical strains in these solid-state systems. Since these contributions are usually coupled, it is challenging to distinguish via conventional computational approaches. Here, a comprehensive atomistic modeling methodology is developed to decouple chemical and mechanical energy contributions using first-principles calculations, of which feasibility is demonstrated by quantifying chemical and elastic strain energies of key interfaces within the FeTi metal-hydride system. Derived materials parameters are then employed for mesoscopic micromechanical analysis, predicting crystallographic orientations in line with experimental observations. The multiscale approach outlined verifies the importance of the chemo-mechanical interplay in the morphological evolution of growing hydride phases, and can be generalized to investigate other systems. In addition, it can streamline the design of atomistic models for the quantitative evaluation of interphase properties between dissimilar phases and allow for efficient predictions of their preferred phase boundary orientations.

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.

AB-type dual-phase high-entropy alloys as negative electrode of Ni-MH batteries: Impact of interphases on electrochemical performance

High-entropy alloys (HEAs) and their corresponding high-entropy hydrides are new potential candidates for negative electrode materials of nickel-metal hydride (Ni-MH) batteries. This study investigates the cyclic electrochemical hydrogen storage performance of two AB-type HEAs (A: hydride-forming elements, B: non-hydride-forming elements) in Ni-MH batteries. TiV2ZrCrMnFeNi with a dual-phase structure shows a fast activation and a low charge transfer impedance with a discharge capacity of 150 mAhg−1, while TiV1.5Zr1.5CrMnFeNi with a single phase shows a slow activation and a capacity of only 60 mAhg−1. The better electrochemical performance of TiV2ZrCrMnFeNi was attributed to its higher vanadium/zirconium ratio and abundant interphase boundaries, which act as hydrogen paths and heterogeneous hydride nucleation sites. These results suggest the high potential of dual-phase HEAs as new active electrode materials for Ni-MH batteries.

Role of induced elastic deformations at the Mg/MgH2 transformation

The kinetics of magnesium hydride formation depends on many factors such as thermodynamic conditions, the presence/absence of additives and the mechanical treatments applied to the starting material. As a result, the impact on the reaction of the material is complex. Additionally, during repeated hydrogenation/dehydrogenation cycles, the microstructure of the material becomes significantly different from the original one. In such case, it is not so simple to order the relative importance of the different experimental contributions. In fact limited research efforts have been devoted to the impact of elastic deformation and its direct consequences on the hydride nucleation process. This motivates the present theoretical analysis of the mechanical energy balance between the Mg and MgH2 entities. Here the formation and interaction of a hydride nucleus with structural defects as a pore or a free surface is carried out in the Mg-MgH2 system.The present work evidences that calculation makes it possible to underline and unequivocally analyze the importance of a single parameter among a set of multiple factors that potentially have an impact on the kinetic processes. It is demonstrated here that due solely to the change in volume during hydride nucleation, conditions develops in the magnesium matrix for predominant release of hydride onto free surfaces, including pores.The calculation of the energy balance made it possible to classify the local impact on the nuclei for different typical configurations such as the vicinity of a free surface, a boundary and the vicinity of low rigidity defects (e.g. pores, vacancies, etc.). Hydride formation near a surface (external or internal, a twin plane or a grain boundary, etc.) results in a reduction in mechanical energy terms, which favors the easy nucleation step in magnesium. Nucleation occurs primarily on free surfaces rather than in bulk.

Enhanced hydrogen storage properties of Ti40-xV45Zr15Crx alloys by dual-phase nanostructures of eutectic and spinodal decomposition

Multi-principal elements alloys (MPEAs) with body center cubic (BCC) structure have shown great potential in hydrogen storage. However, the multi-cycle activation process and the high desorption temperature have inhibited their applications. In this study, the Ti40-xV45Zr15Crx (x = 1, 2, 3 at. %) alloys with three different BCC phases are synergized, and high hydrogenation capacities of over 2 wt% are achieved in the first hydrogen absorption. Owing to the formation of spinodal decomposition nanostructure in Ti38V45Zr15Cr2 alloy, 90 % of total hydrogen capacity is reached in 30s at 250 °C under 1 MPa H2, which indicates no incubation time is needed for activation. The Ti38V45Zr15Cr2 alloy also shows the highest reversible hydrogen storage capacity with fastest kinetics at 200 °C. The hydrogenation reaction is controlled by the 1D diffusion process of H atoms according to JMAK model. The spinodal decomposition structure can accelerate the decomposition of H2 molecules and the diffusion of H atoms by providing extra energy from large lattice distortion. The high-density interfaces in nano-eutectic structure can serve as active sites for the nucleation of hydrides and fast paths for the diffusion of H atoms.

Determining the habit plane for radial δ-hydrides in a textured zirconium-alloy tube under hoop tensile stress: A theory-experiment approach

The orientation of hydrides with respect to externally applied stress affects the susceptibility of Zirconium (Zr) alloy fuel cladding tubes to through-wall failure. Previous studies have mainly focused on the situation in the absence of external stress, with little attention paid to the situation under applied stress. Hoop tensile stress can cause the reorientation of hydride platelets from the circumferential to the radial direction in the radial textured Zr fuel clad tubes, substantially degrading its hoop ductility. In this study, we used a theory-experiment approach to investigate the radial hydride-matrix orientation relationship. A microstructure criterion was proposed to determine the habit plane of the radial δ-hydrides. Our results demonstrate that radial δ-hydrides predominantly form on the pyramidal 101¯1α plane of α-Zr with 101¯1α−Zr//111δ−hydride. The electron backscatter diffraction study of hydrided Zircaloy-4 tube provides experimental evidence supporting theoretical prediction. The underlying mechanism for this phenomenon is attributed to a competition between tensile stress-assisted hydride nucleation and strain-induced resistance to nucleation.

Direct Observation of Vacancy-Cluster-Mediated Hydride Nucleation and the Anomalous Precipitation Memory Effect in Zirconium.

Controlling the heterogeneous nucleation of new phases is of importance in tuning the microstructures and properties of materials. However, the role of vacancy-a popular defect in materials that is hard to be resolved under conventional electron microscopy-in the heterogeneous phase nucleation remains intriguing. Here, this work captures direct in situ experimental evidences that vacancy clusters promote the heterogeneous hydride nucleation and cause the anomalous precipitation memory effect in zirconium. Both interstitial and vacancy dislocation loops form after hydride dissolution. Interestingly, hydride reprecipitation only occurs on those vacancy loop decorated sites during cooling. Atomistic simulations reveal that hydrogen atoms are preferentially segregated at individual vacancy and vacancy clusters, which assist hydride nucleation, and stimulate the unusual memory effect during hydride reprecipitation. The finding breaks the traditional view on the sequence of heterogeneous nucleation sites and sheds light on the solid phase transformation related to vacancy-sensitive alloying elements.

Hydride precipitation dependence on α-parent orientation and plastic deformation in zirconium alloys

Hydrides are at the center source of mechanical degradation affecting Zr-based nuclear fuel cladding integrity during in-reactor service and even spent fuel storage. This study presents investigations on hydride precipitation in response to thermo-mechanical treatment in a hydrogenated Zircaloy-4 plate. Correlated EBSD and ECCI characterizations demonstrate stress-induced variations in α-parent orientation and stacking structure between the in-plane and reoriented hydrides. Based on the hydride strains quantified by HR-EBSD, external tensile stress is found to favor the α-grains submitted to significant {0001}<10-10> shear stress as preferential nucleation sites for reorientated hydrides. Regardless of elastic or plastic stress, the interaction between external stress and hydride strains, particularly the shear component, is expected to govern hydride nucleation probability as a function of α-grain orientation and eventually induce hydride reorientation. Influence of plastic deformation is mainly manifested in hydride distribution, where the {10-10}<11-20> slipping promotes reoriented hydride interconnection up to hundreds of microns while the {10-11}<10-1-2> twining results in a fragmented distribution, as visualized by X-ray tomography. The presented results provide solid experimental evidences for the auto-catalytic nucleation and martensitic transformation of hydride precipitation, as well as new insights into stress-induced hydride reorientation in the light of stress interaction with hydride strain.

Significance of interphase boundaries on activation of high-entropy alloys for room-temperature hydrogen storage

The ability of high-entropy alloys (HEAs) for hydrogen storage is a rather new topic in the hydrogen community. HEAs with the C14 Laves phase have shown a high potential to reversibly store hydrogen at room temperature, but most of these alloys require a high-temperature activation treatment. This study explores the role of interphase boundaries on the easy activation of HEAs at room temperature. Two chemically similar HEAs with single and dual phases, TiV1.5ZrCr0.5MnFeNi (C14 + 4 vol% BCC phases) and TiV1.5Zr1.5CrMnFeNi (single C14 phase), are designed and synthesized. While the dual-phase alloy readily absorbs hydrogen at room temperature without any activation treatment, the single-phase alloy requires a high-temperature activation. It is suggested that interphase boundaries not only provide pathways for easy hydrogen transport and activation of HEAs at room temperature but also act as active sites for heterogeneous nucleation of hydride. This study introduces interphase-boundary generation as an effective strategy to address the activation drawback of HEAs.

Mechanisms of Hydride Nucleation, Growth, Reorientation, and Embrittlement in Zirconium: A Review.

Zirconium (Zr) hydrides threaten the reliability of fuel assembly and have repeatedly induced failures in cladding tubes and pressure vessels. Thus, they attract a broad range of research interests. For example, delayed hydride cracking induced a severe fracture and failure in a Zircaloy-2 pressure tube in 1983, causing the emergency shutdown of the Pickering nuclear reactor. Hydride has high hardness and very low toughness, and it tends to aggregate toward cooler or tensile regions, which initiates localized hydride precipitation and results in delayed hydride cracking. Notably, hydride reorientation under tensile stress substantially decreases the fracture toughness and increases the ductile-to-brittle transition temperature of Zr alloys, which reduces the safety of the long-term storage of spent nuclear fuel. Therefore, improving our knowledge of Zr hydrides is useful for effectively controlling hydride embrittlement in fuel assembly. The aim of this review is to reorganize the mechanisms of hydride nucleation and growth behaviors, hydride reorientation under external stress, and hydride-induced embrittlement. We revisit important examples of progress of research in this field and emphasize the key future aspects of research on Zr hydrides.

Correlative high-resolution imaging of hydrogen in Mg2Ni hydrogen storage thin films

Nanometer scale imaging of hydrogen in solid materials remains an important challenge for the characterization of advanced materials, such as semiconductors, high-strength metallic alloys, and hydrogen storage materials. Within this work, we demonstrate high-resolution imaging of hydrogen and deuterium within Mg2Ni/Mg2NiH4 hydrogen storage thin films using an in-house developed secondary ion mass spectrometer (SIMS) system attached to a commercially available dual-beam focused ion beam - scanning electron microscope (FIB-SEM) instrument. We further demonstrate a novel approach to measure the size, shape, and distribution of the hydride phase in partially transformed films using laser scanning confocal microscopy (LSCM) to measure surface topography changes from the hydride phase volume expansion. Combining these techniques provides new insights on hydride nucleation and growth within the Mg2NiHx system. Finally, we demonstrate the efficacy of tracking deuterium as a hydrogen analog to reduce the background for SIMS imaging of hydrogen in high-vacuum chambers (∼10−6 mbar).

Finite element modeling of hydrogen migration and hydride precipitation in Zr alloys

The Finite Element software, ABAQUS® together with subroutine codes written in FORTRAN is used to fully couple the mass diffusion – displacement analysis of hydrogen redistribution and hydride precipitation in a stress gradient. Since the redistribution of hydrogen in solid solution is driven by gradients in H concentration, stress, and temperature, two different precipitation models were investigated in specimens subjected to such gradients, and the results validated against available experimental data and analytical solutions where applicable. Simulation results obtained with the new Hydride Nucleation Growth and Dissolution (HNGD) model that considers growth in the hysteresis region show closer agreement with experimental observations, signifying that the previous hydride precipitation model may be underestimating the amount of hydride precipitation.

A new perspective on the initial hydrogenation of TiFe0.9M0.1 (M = V, Cr, Fe, Co, Ni) alloys gained from surface oxide analyses and nucleation energetics

Despite the promise of TiFe-based alloys as low-cost solid-state hydrogen storage materials with mild operating conditions and reasonable hydrogen capacity, their initial hydrogenation process is difficult, hindering broad utilization. The effect of alloying element on the initial hydrogenation kinetics of TiFe alloys, TiFe0.9M0.1 (M = V, Cr, Fe, Co and Ni), was evaluated by analyzing changes to the passivating surface oxide layer that inhibits hydrogen permeation, as well as the ease of initial-stage hydrogen absorption into the underlying matrix. X-ray photoelectron spectroscopy and atom probe tomography revealed key variations in surface oxide compositions and thinning of the passivating oxide layer compared to pure TiFe, which suggests suppressed oxide growth by alloying-induced elemental redistribution. At the same time, density functional theory calculations predicted exothermic formation of hydride nuclei when alloying with V or Cr, as well as a reduced nucleation barrier when alloying with Co or Ni. Overall, these results are consistent with the observed experimental trend of the activation kinetics. We propose that improvements in activation kinetics of TiFe with alloying arises from the combined effect of reduced passivating oxide thickness and easier hydride nucleation, offering a starting point for alloy design strategies towards further improvement.

Hydrogen adsorption and diffusion on doped Zr(0001) surfaces: A first-principles study

The hydrogen adsorption and diffusion behaviors on the clean and a series of element doped Zr(0001) surfaces are studied through first-principles calculations. Among the studied doping elements, Cu, Co, Y, and Mg prefer to substitute Zr on the topmost surface layer, Al, Pd, Ir, and Si are favored from topmost layer to several surface layers down, while Mo is not favored. Independent of the substitution energies, Mo, Co, and Ir induce a symmetry-breaking local distortion surface structure. Based on the obtained geometries, it is found that most dopants promote the hydrogen adsorption on their next nearest neighbor sites but hinder it on the nearest neighbor sites. Most of the dopants also promote both the hydrogen diffusion on the surface plane and the hydrogen penetration into the subsurface layers. The results indicate that element doping may facilitate the hydride nucleation in Zr alloys.

Activation energy of homogeneous nucleation of Zr hydride: Density functional theory calculation

Considering the nucleation process of Zr hydrides as phase transformation from hexagonal closed-packed (HCP) to face-centered tetragonal (FCT) structure, we calculated the activation energy of the homogeneous nucleation process of Zr hydrides and atomic rearrangement during nucleation for Zr4H, Zr2H, ZrH and ZrH2 using density functional theory calculations and minimum energy path detection. At 0 K limit, although ZrH and ZrH2 have lower chemical potentials and are more energetically stable than Zr4H and Zr2H, the latter have lower activation energies for nucleation. At finite temperatures, the crossover of activation energies occurs around 300 K, where ZrH becomes the most possible candidate with the lowest activation energy. This was explained by the difference in the atomic rearrangement and change in phonon frequency during phase transformation.

Effect of external stress on hydride reorientation in zirconium

The usage and long-term storage of nuclear fuel cladding motivate efforts to understand the effect of external stress on hydride precipitation. Here, by using in situ bending device in electrolytic hydrogen charging, the nucleation and precipitation behaviors of subsurface hydrides under different degrees of tensile stress were obtained. The number density of surface hydride bumps increases with the elastic tensile stress and reaches a peak before plastic yielding. This is related to the statistical result that the reorientation of hydride bumps reaches maximum at the end of elastic stage. Morphologies of hydride bumps varies from needle- or circular-shape to faceted, indicating hydride reorientation. The faceted hydrides have special crystallographic characteristics. Under tensile stress, the habit planes of faceted hydrides gradually transform from {0001} to {101¯i} (i = 1–7), even to the {101¯0} planes. Because of the local compressive stress, faceted hydrides growing along one <112¯0> direction are suppressed. External stress-stimulated tensile or compressive lattice stress on specific crystallographic planes impacts the choice of faceted/habit planes in hydride precipitation, which plays an important role in the long-term performance of Zr cladding tubes.

The role of β pockets resulting from Fe impurities in hydride formation in titanium

The corrosion potential of commercially pure titanium in NaCl solutions is dramatically affected by trace Fe additions, which cause the appearance of submicron pockets of β phase at grain boundary triple points. Furthermore, the low solubility of hydrogen in hexagonal close-packed α-Ti makes titanium alloys prone to subsequent hydride-associated failures due to stress corrosion cracking. We analyzed α-α and α-β sections of the abutting grain boundary of a β pocket in a Grade 2 CP-Ti, and the α-β phase boundary. Fe and H partition to β and segregate at the grain boundary, but no segregation is seen at the α-β phase boundary. In contrast, a significant Ni (>1 at%) accumulation is observed at the α-β phase boundary. We propose that the β-pockets act as hydrogen traps and facilitate the nucleation and growth of hydrides along grain boundaries in CP-Ti.

Cluster dynamics simulation of Zr hydrides formation on grain boundaries in Zr

The delta hydrides formation in zirconium cladding under irradiation and thermal aging is studied by the rate theory modeling. The precipitates may lead to cracking of the clad and are investigated both experimentally and theoretically. The rate theory studies published so far are based on the well-established classical nucleation theory, but use several assumptions, e.g. of homogeneous hydride nucleation mode and spherical shape of precipitates, which contradict observations. In this work we continue the development of the hydride precipitation model by taking into consideration the observed preferential nucleation of the platelet-shaped hydrides on grain boundaries, and analyze the importance of such modifications by comparing calculations performed with different approaches. We also extract information from the literature on the hydrogen solution energy and alpha Zr / delta hydride interface energy, which define the hydrogen-hydride binding energy as a function of precipitate size, and show that the observed hydride size and density are well described with the use of the capillary approximation. The difference in precipitation kinetics during thermal annealing of samples with pre-existing level of hydrogen and under irradiation to the same final hydrogen concentration is elucidated. This improves our understanding of the factors affecting hydride formation in the clad.

A first-principles study of hydrogen surface coverage on δ-Pu (100), (111), and (110) surfaces.

Hydriding corrosion of plutonium leads to surface cracking, pitting, and ultimately structural failure. Laboratory experiments demonstrate that hydriding begins on the surface or near the subsurface of plutonium. However, there has not yet been a systematic evaluation of hydrogen surface coverage on plutonium. In this work, we compute the surface energies of the low facet surfaces of face-centered cubic δ-Pu. The adsorption free energies of expected hydrogen structures at low and high coverage are presented along with the likely progression for filling sites as the H2 partial pressure increases. Implications for near-equilibrium pressure hydride nucleation and non-equilibrium millibar pressure hydriding are discussed.

Nucleation and recombination dependent desorption performance of Mg-based hydrides

High desorption temperature and sluggish desorption kinetics hinder the large-scale application of Mg-based hydrogen storage materials. Fundamental understanding of the dehydrogenation process and underlying mechanism is crucial for further modifying the desorption performance. Aiming at elucidating the dehydrogenation process of Mg-based hydrides, partially desorbed pure MgH2 and MgH2-Mg2NiH4-CeH2.73 composite have been prepared to investigate the effects of metal phase nucleation on overall desorption performance. It is found that metal Mg is formed on the surface of partially desorbed MgH2 and MgH2-Mg2NiH4-CeH2.73. Correspondingly, the desorption temperature of partially desorbed samples is significantly decreased, indicating an important role nucleation plays in desorption process. With minor Ni addition, the desorption temperature and kinetics of MgH2-Mg2NiH4-CeH2.73 are significantly improved. Promoting hydrogen recombination is another essential aspect to modify the desorption properties of Mg hydrides.