Hydride Transformation Research Articles

Limits of hydrogen analysis by atom probe tomography targeting Zr(Fe,Cr)2 second phase particles in Zr-based fuel cladding from reactor operation

We report results from atom probe tomography (APT) experiments capturing Zr(Fe,Cr)2 second phase particles (SPPs) in Zircaloy-2-type fuel cladding after reactor operation. In light of recent reports of H trapping around SPPs, we assess the feasibility of H analysis in modern commercial atom probe instruments on this system. To this end we employed voltage and laser pulsing APT on specimens prepared by focused ion beam (FIB) at room and cryogenic temperature. Room temperature FIB caused transformation of the α-Zr matrix into δ-hydride, but left SPPs mostly unaffected. This indicates that α-Zr has a higher affinity for H than SPPs. However, even under optimized conditions, we were not able to find evidence for H trapping near SPPs located within the α-Zr matrix in cryogenically FIB sharpened specimens, where no hydride transformation occurs.

Modeling hydride growth and strain-field evolution at a stress concentrator in zirconium alloys

The redistribution of hydrogen in zirconium-based components of nuclear reactors is strongly influenced by local stress states. Such a stress-state can be influenced by the stress concentration associated with crack tips or blunt flaws as well as the volumetric expansion resulting from the precipitation and growth of hydrides themselves. To date, the way the matrix material accommodates hydride transformation strain is not fully understood. In this study, two approaches were employed to accommodate the 17.2% volumetric hydride induced strain accompanying the phase transformation. In the first approach, this strain is assumed to accommodate along both a and c-axis of the hcp α-Zr crystal based on known crystallography, while the second approach assumed the entire strain was in the alloy material’s circumferential direction, with zero expansion in either the axial or radial directions. A finite element model fully coupling diffusion with stress analysis is used to gain insight into the progression of hydride precipitation and the associated effect on the stress/strain at the tip of a crack. The shapes and distributions of hydrided material when the entire transformation strain is confined to the circumferential direction of the model were found to be close to experimental observations. However, the impact of the hydride induced strain and matrix plasticity was found to be insufficient to drive the experimentally observed relaxation.

Hydrogen-phase naklep as a result of single isothermal hydride transformations

Hydrogen treatment of metallic materials is a promising area of controlled hydrogen action on metals and alloys in order to impart special properties to them. In this work, the changes in the mechanical properties of palladium during single hydride transformations, which are initiated in the initially non-nakleped palladium hydride, have been experimentally studied. Were investigated wire palladium samples with a purity of 99.98 %, a diameter of 0.5 mm and a length of 165 mm. Samples for research were preliminarily subjected to hydrogen treatment, and then mechanical tests were carried out. The mechanical properties of non-nakleped palladium hydride are presented. Hydrogen-phase naklep of palladium during hydrogen treatment has been studied. It is shown that palladium hydride is a plastic material with low strength characteristics. During the reverse hydride transformation, the hydrogen phase naklep of palladium develops. Experimental facts of the appearance of hydrogen-phase naklep during the interaction of hydrogen with other metals and alloys are discussed.

Nanoindentation study of hydride diffusion layer in commercial pure titanium

In this work, the mechanical properties of hydride diffusion layer were thoroughly researched by nanoindentation technique. For the first time, the clear section microstructure of hydride diffusion layer was revealed under scanning electron microscope with complicated interactions of hydride plates near the interface between hydride layer and titanium matrix. The hydride layer section has higher hardness and lower modulus towards titanium matrix due to the hard nature and easier crack nucleation of hydride precipitation. Besides, the hardness of titanium matrix shows considerable anisotropy. The anisotropic hardness of sample surface was studied before and after hydrogen charging. The hardness of titanium matrix decreases with the increase of deviation angle between c-axis and surface normal. The hydride layer formed in the titanium grain with more favorable orientation for hydride transformation tends to possess higher indentation hardness.

The mechanical property evolution and grain boundary accommodation during hydride transformation in commercial pure titanium

The mechanical property evolution of titanium surface during hydrogen charging was investigated by nanoindentation tests. The nanohardness increases while elastic modulus decreases due to the formation of hydride precipitation. By mean of interrupted in-situ electron backscatter diffraction (EBSD) measurements, the microstructure evolution of hydrogenated surfaces with different grain sizes was examined. The more prevalent grain-boundary accommodation in fine grain sample leads to the faster hydride transformation from metastable γ to stable δ hydride than coarse grain sample. The formation of intergranular hydride pair is regarded as one type of grain-boundary accommodation mode. Adjacent hydrides prefer to be induced at grain boundaries with both low c-axis misorientation angle θc (θc < 30o) and high θc (75o < θc < 85o). The synergistic shear of Shockley partial dislocation and the c-axis dilatation misfit of hydride transformation are the fundamental reasons for the above two types of favorable grain boundaries, respectively.

Design of Nanomaterials for Hydrogen Storage

The interaction of hydrogen with solids and the mechanisms of hydride formation experience significant changes in nanomaterials due to a number of structural features. This review aims at illustrating the design principles that have recently inspired the development of new nanomaterials for hydrogen storage. After a general discussion about the influence of nanomaterials’ microstructure on their hydrogen sorption properties, several scientific cases and hot topics are illustrated surveying various classes of materials. These include bulk-like nanomaterials processed by mechanochemical routes, thin films and multilayers, nano-objects with composite architectures such as core–shell or composite nanoparticles, and nanoparticles on porous or graphene-like supports. Finally, selected examples of recent in situ studies of metal–hydride transformation mechanisms using microscopy and spectroscopy techniques are highlighted.

Magnesium Nanoparticles With Pd Decoration for Hydrogen Storage.

In this work, Magnesium nanoparticles with Pd decoration, ranging from 40 to 70 nm, were successfully coprecipitated from tetrahydrofuran (THF) solution, assigned as the Mg–Pd nanocomposite. The Mg–Pd nanocomposite exhibits superior hydrogen storage properties. For the hydrogenated Mg–Pd nanocomposite at 150°C, the onset dehydrogenation temperature is significantly reduced to 216.8°C, with a lower apparent activation energy for dehydrogenation of 93.8 kJ/mol H2. High-content γ-MgH2 formed during the hydrogenation process, along with PH0.706, contributes to the enhancing of desorption kinetics. The Mg–Pd nanocomposite can take up 3.0 wt% hydrogen in 2 h at a temperature as low as 50°C. During lower hydrogenation temperatures, Pd can dissociate hydrogen and create a hydrogen diffusion pathway for the Mg nanoparticles, leading to the decrease of the hydrogenation apparent activation energy (44.3 kJ/mol H2). In addition, the Mg–Pd alloy formed during the hydrogenation/dehydrogenation process can play an active role in the reversible metal hydride transformation, destabilizing the MgH2.

Switching between Stepwise and Concerted Proton-Coupled Electron Transfer Pathways in Tungsten Hydride Activation.

Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal-hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energy-efficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)2(PMe3)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton-electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.

Controlling the Phase Composition, Structure, and Complex of Properties of the High-Modulus Titanium Alloy by Thermohydrogen Processing

The possibilities and efficiency of applying the thermohydrogen processing (THP) of the high-modulus Ti–8.7Al–1.5Zr–2.0Mo titanium alloy with an aluminum content exceeding the solubility limit in α-titanium are considered. Experimental data on the influence of hydrogen on the alloy phase composition and structure are acquired. Regularities of phase transformations in the hydrogen-containing alloy under various thermal impacts are analyzed. The phase diagram of the alloy–hydrogen system in the hydrogen concentration range from the initial one to 1.0 wt % and the temperature range from 20 to 1100°C is constructed. It is shown that a single-phased β-structure is fixed with the concentration of introduced hydrogen of 0.6 wt % and higher by means of quenching from the β-region. Hydrogen saturation up to 0.8–1.0% leads to the implementation of the β → δ shear hydride transformation during quenching from temperatures below 750°C, and to the partial eutectoid transformation of the β-phase under slow cooling. It is established that hydrogen extends the stability region of the β-phase, lowering the temperature of the β/(α + β) transition by 210°C (at 1.0% H), and increases the stability temperature of the α2-phase by 50°C. The process flowsheets and THP modes forming two structural types—submicrocrystalline and bimodal—are developed and approved for alloy samples. The formation mechanisms of these structures during THP are analyzed and the mechanical properties of the alloy are determined. It is established that THP leads to an increase in strength and hardness when compared with the initial state. The THP forming the microcrystalline structure causes a decrease in plasticity characteristics at the maximal hardness.

Reversible metal–hydride phase transformation in epitaxial films

Metal–hydride phase transformations in solids commonly proceed with hysteresis. The extrinsic component of hysteresis is the result of the dissipation of energy of internal stress due to plastic deformation and fracture. It can be mitigated on the nanoscale, where plastic deformation and fracture are suppressed and the transformation proceeds through formation and evolution of coherent phases. However, the phase coherency introduces intrinsic thermodynamic hysteresis, preventing reversible transformation. In this paper, it is shown that thermodynamic hysteresis of coherent metal–hydride transformation can be eliminated in epitaxial film due to substrate constraint. Film–substrate interaction leads to formation of heterophase polydomain nanostructure with variable phase fraction which can change reversibly by varying temperature in a closed system or chemical potential in an open system.

Ti(Ni,Cu) pseudobinary compounds as efficient negative electrodes for Ni–MH batteries

The effect of Ni by Cu substitution on the structural, solid–gas and electrochemical hydrogenation properties of TiNi has been investigated. Pseudo-binary TiNi1−xCux (x ≤ 0.5) compounds have been synthesized by induction melting. They crystallize in B2 structure above 350 K and either in B19′ (x < 0.1) or B19 (0.2 ≤ x ≤ 0.5) at room temperature (RT). For all compounds, Pressure-Composition Isotherms at 423 K exhibit a single slopping plateau pressure within the range 10−3–1 MPa of hydrogen pressure revealing a metal to hydride transformation. Both the hydrogenation capacity and the hydride stability decrease with Cu-content. The hydrided pseudobinary compounds crystallize in the tetragonal S.G. I4/mmm structure as for TiNi hydride. The electrochemical discharge capacity increases with Cu content from 150 mAh g−1 for TiNi up to 300 mAh g−1 for TiNi0.8Cu0.2 and then decreases again for larger Cu amounts. Electrochemical isotherms and in-situ neutron diffraction measurements at RT demonstrate that such a capacity increase results from a metal to hydride phase transformation in which the hydride phase is destabilized by Cu substitution. The TiNi0.8Cu0.2 compound exhibits interesting cycling stability for 30 cycles and good high-rate capability at D/2 rate. This compound has promising electrochemical properties as compared to commercial LaNi5-type alloys with the advantage of being rare-earth metal free.

Hydrogen storage properties of Mg–Ni nanoparticles

The aim of this work is to investigate metal–hydride transformation in Magnesium (Mg) nanoparticles decorated by Nickel (Ni). The samples were synthesized by Inert Gas Condensation: Mg single crystal nanoparticles were deposited on a metal substrate and subsequently their surface was exposed to evaporation of Ni. Structural analysis was made by Synchrotron Radiation Powder X-ray Diffraction and thermodynamic measurements by Sieverts apparatus. Ni decoration significantly improves the hydrogen release and uptake kinetics of the nanoparticles. The results connect the formation of Mg2Ni and Mg2NiH4 phases to the enhancement of hydrogen sorption properties.

Fractal disperse hydrogen sorption kinetics in spark discharge generated Mg/NbOx and Mg/Pd nanocomposites

Isothermal hydrogen desorption of spark discharge generated Mg/NbOx and Mg/Pd metal hydride nanocomposites is consistently described by a kinetic model based on multiple reaction rates, in contrast to the Johnson-Mehl-Avrami-Kolmogorov [M. Avrami, J. Phys. Chem. 9, 177 (1941); W. A. Johnson and R. F. Mehl, Trans. Am. Inst. Min., Metal. Eng. 135, 416 (1939); A. N. Kolmogorov, Izv. Akad. Nauk SSSR, Ser. Mat. 3, 355 (1937); F. Liu, F. Sommer, C. Bos, and E. J. Mittemeijer, Int. Mat. Rev. 52, 193 (2007)] model which is commonly applied to explain the kinetics of metal hydride transformations. The broad range of reaction rates arises from the disperse character of the particle size and the dendritic morphology of the samples. The model is expected to be generally applicable for metal hydrides which show a significant variation in particle sizes, in configuration and/or chemical composition of local surroundings of the reacting nanoparticles.

Characterization of Zirconium Hydrides and Phase Field Approach to a Mesoscopic-Scale Modeling of Their Precipitation

Abstract Zirconium alloys are currently used in nuclear power plants where they are submitted to hydrogen pick-up. Hydrogen in solid solution or hydride precipitation can affect the behavior of zirconium alloys during service but also in long term storage and in accidental conditions. Numerical modeling at mesoscopic scale using a “phase field” approach has been launched to describe hydride precipitation and its consequences on the mechanical properties of zirconium alloys. To obtain realistic results, it should take into account an accurate kinetic, thermodynamic, and structural database in order to properly describe hydride nucleation, growth, and coalescence as well as hydride interaction with external stresses. Therefore, an accurate structural characterization was performed on Zircaloy-4 plates and it allowed us to identify a new zirconium hydride phase called ζ. The ζ phase has a trigonal symmetry and is fully coherent with hcp αZr. The consequences of this new zirconium hydride phase on hydride transformation process and stress-reorientation phenomenon are discussed. A first attempt to numerically model the precipitation of this new zirconium hydride phase has been undertaken using the phase field approach.

Study of Thermal Desorption of Helium from HydrogenatedZirconium

A new method prepared for helium and hydrogen co-containing Zr films ispresented to simulate aging metal tritides, in which direct currentmagnetron sputtering with a He/H/Ar mixture is used. The retainedamount and depth profiles of helium and hydrogen are determined byelastic recoil detection analysis. Thermal desorption spectrometry isapplied to investigate He thermal release and the effect of hydrogen.It is found that the high-temperature peaks with a large mount ofhelium release obviously shifted toward lower temperature at highhydrogen concentration, especially at the hydride transformation region,and that the shapes of the release peaks also changed due to theadditional hydrogen. However, at the low-temperature releasing region thepeak intense decreases when phase transformation takes place. Themechanism of helium thermal release and the effect of hydrogen arealso discussed.

Reverse hydride transformations in the Pd–H system

In the present review, the development of notions about hydride transformations is described. It is shown that hydride transformations were classified as a special type of phase transition in a number of classic phase transformations. Then, the available information about reverse hydride transformations in the Pd–H system is summarized and it is shown that reverse hydride transformations in this system proceed by the mechanism of generation and growth. The C-shaped kinetic isothermal diagrams describe kinetics of direct α → β HT. Another type of kinetic diagram is typical for reverse β → α hydride transformations, and the rate of these hydride transformations just accelerates with temperature increase or hydrogen pressure decrease. Morphological peculiarities of reverse hydride transformations are described in detail. In the conclusion, a discussion of the unique role of hydrogen concentrational and hydrogen phase stresses in processes of hydride transformations is given. It has been found that these stresses are the most important thermodynamic and kinetic factors in the hydride transformation development.

Microstructures with martensitic features induced by absorption of a large amount of hydrogen in a B2 single-phase Ti-22Al-27Nb alloy

In Ti-22Al -27Nb, the β→ γ hydride transformation which is a bcc→bct transition in crystal structure exhibits some of the characteristics that are considered typical of martensitic transformation. We investigated surface relief and microstructure of a pre-deformed β 0(B2) single-phase specimen hydrogenated to different hydrogen contents. When β hydride is formed, no distinctive changes are found on their electro-polished surfaces. However, surface relief with a thin plate-like morphology is found after the β→ γ hydride transition. The plates contain a high density of aligned striations which are determined to be <111> screw dislocations. Crystallographic features of the β→ γ hydride transformation analyzed by the Bowles–Mackenzie method agree well with the experimental observation. The role of aligned dislocations in improving the hydrogen absorption/desorption properties of Ti-22Al-27Nb is discussed based on a discovery of the similarities between the strain around <111> screw dislocations and that induced by the formation of hydrides. Possible ways to control the hydrogen absorption/desorption properties via defects engineering are proposed.

Hydrogen absorption/desorption behavior of Ti–22Al–27Nb alloys with β 0 single-phase and β/O two-phase microstructures

We investigated multiphase effects on the hydrogen absorption/desorption behavior at 100 °C of a ternary alloy (Ti–22Al–27Nb (at.%)) whose microstructure consists of β 0(B2) single-phase and β(bcc)/O(orthorhombic, based on Ti 2AlNb) two-phase structures. The β–γ hydride transformation is generally very sluggish in this alloy. However, when the O phase exists in the β matrix phase, the β–γ hydride transformation occurs and the reversible absorption/desorption of hydrogen between β- and γ-hydrides is observed. Roles of the O phase in the reversible hydrogen absorption/desorption behavior are discussed on the basis of a matrix analysis of lattice deformation commonly used for martensitic transformations.

Effects of cold-rolling on the hydrogen absorption/desorption behaviour of Ti–22Al–27Nb alloys

B2 single-phase alloys with a composition of Ti–22Al–27Nb (at.%) were deformed by compression or rolling to 5, 10.5 and 80% reduction in thickness at room temperature and the effects of deformation on their hydrogen absorption/desorption behaviour were investigated. P–C isotherm measurements were made at 50 and 100 °C. Similar to bcc hydrogen absorbing alloys, Ti–22Al–27Nb alloys with the B2 structure form β (bcc) hydride at a very low hydrogen pressure. However, the β hydride in undeformed specimens does not easily transform to γ (bct) hydride. In contrast, the β hydride in mildly deformed specimens transforms to the γ hydride with increasing hydrogen concentration and the γ hydride transforms back to the β hydride with decreasing hydrogen pressure. Heavy deformation does not show such beneficial effects. The strain field around dislocation is believed to be a kind of assistance to β–γ hydride transformation.

Video-investigation of reverse hydride transformations in the Pd–H system

In the present work, peculiarities of β→ α hydride transformation in subsurface layers of a β-hydride of palladium in the temperature range 270–100°C were studied. The transformation processes were fixed with video camera in real time and then video movies were analyzed with a Pentium computer. It is shown that a decrease in internal hydrogen phase stresses and hydrogen concentrational ones with increase in temperature strongly influences both morphological peculiarities and the rate of transformation. So, at temperatures of 100–190°C the generation and growth of nuclei can be seen clearly. The features of transformation change with decrease in temperature: at 150°C, nuclei appear one another, the incubation period in subsurface layers consists of 8 s . At 190–170°C, nuclei appear simultaneously without any incubation period. At temperatures of 230–270°C, the transformation proceeds without any relief on the sample surface. The transformation in subsurface layers of a β-PdH x accelerates with temperature increasing from 100°C up to 150°C more than 700 times.