Formation Of Zirconium Hydride Research Articles

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.

The bonding of H in Zr under strain

Accurate computer simulation is important for understanding the role of irradiation-induced defects in zirconium alloys found in nuclear reactors. Of particular interest is the distribution and trapping of hydrogen, and the formation of zirconium hydride. These simulations require an accurate representation of Zr-H bonding in order to predict the behaviour of H around atomic-scale defects, dislocation lines, and dislocation loops. Here we explore the bonding of H in Zr under strain, how well it is represented by state-of-the-art Embedded Atom Method (EAM) potentials, and what physics is needed for an accurate representation in a Linear Combination of Atomic Orbitals (LCAO) Density Functional Theory (DFT) framework. For H in dilute solution under hydrostatic strain in the range -10% to +10%, solution energies and Zr-H bond lengths computed using EAM potentials are found to be in poor agreement with plane-wave DFT results. We note that the bond lengths are in a poor agreement even in equilibrium. LCAO basis sets are used to explore the importance of electron distribution around H atoms, and the transfer of electrons between H and Zr. The electron distribution around H atoms is found to be important to the explanation of the difference between octahedral and tetrahedral interstitial sites for H, with H in a tetrahedral site having very similar bonding to H in zirconium hydrides. The interatomic electron transfer has a smaller impact but is needed for maximum accuracy.

Interaction of zirconia with magnesium hydride and its influence on the hydrogen storage behavior of magnesium hydride

This study demonstrates how zirconia additive transforms to zirconium hydride and substantially lowers the dehydrogenation temperature of magnesium hydride. We prepared MgH2+xZrO2 (x = 0.125 and 0.5) powder samples reacted for 15 min, 1 h, 5 h, 10 h, 15 h, 20 h and 25 h, and monitored the phase changes at each stage of the reaction. Differential scanning calorimetry (DSC) study provides the first crucial evidence regarding the chemical transformation of zirconia. Subsequently, detailed additional sample testing by X-ray diffraction (XRD), energy dispersive x-ray spectroscopy and confocal Raman microscopy provide strong supports that low temperature dehydrogenation of magnesium hydride is a result of formation of an active in situ product (zirconium hydride). This observation is validated by the negative Gibbs free energy values obtained for the formation of zirconium hydride over a broad working temperature range of 0–600 °C. Scanning electron microscopy (SEM) results prove the high dispersion of tiny nanoparticles all across the surface after the chemical interaction between MgH2 and ZrO2 and atomic force microscopy (AFM) study further proves that objects with grain sizes of ∼10 nm are abundant throughout the scanned surfaces. These observations reiterate that better metal oxide additives interact with MgH2 and results to the evolution of highly active insitu nanocatalysts.

Quantifying hydrogen concentration in the vicinity of zirconium hydrides and deformation twins

This study focuses on investigating the effects of the localized stresses that develop during formation of zirconium hydrides on the redistribution of hydrogen atoms in the zirconium matrix. A coupled mass diffusion and crystal plasticity finite element approach is used. The effects of the transformation strain associated with the formation of hydrides within various combinations of soft-hard grains and tri-crystals are studied. In addition, the interaction between intergranular and intragranular hydrides with grain boundaries is investigated. It is shown that hydrogen atoms tend to diffuse towards the tips of hydrides and away from their sides due to the tensile and compressive hydrostatic stresses that respectively develop at these two locations. This results in the propagation of a hydride along its axial direction. It is further shown that under a tensile applied stress, hydrides tend to form and propagate within hard grains, where stress hotspots with high tensile hydrostatic stresses develop. Lastly, it is shown that hydrogen atoms tend to diffuse towards twin tips, particularly when the twin tip is located near a harder grain.

A Review of Hydride Reorientation in Zirconium Alloys for Water-Cooled Reactors

This review summarizes the results of the available to the moment papers devoted to the precipitation of the hydride phase in zirconium alloys—the core structural materials of water-cooled reactors. The main parameters affecting the formation of zirconium hydrides having a certain orientation are analyzed. Particular attention is paid to the hydride reorientation at applied stresses and the effect of radial hydrides on both the ductility and fracture toughness of the material.

Investigation of Textures of α-Zirconium and Zirconium Alloy by Neutron and X-Ray Diffraction

The textures of three samples of zirconium alloys were compared with the textures of seven samples of alpha-zirconium deformed at 300°C under uniaxial tension from 5 to 30% (strain step 5%). Texture measurements by neutron diffraction were performed on a KSN-2 neutron diffractometer located at the LVR-15 research reactor of the Research Centre Řež, Czech Republic. X-ray measurements were performed on a theta/theta X'Pert PRO diffractometer with a Cr tube. The observed data were processed by the GSAS and X'Pert Texture software packages. The (100) and (110) plane poles are found to be oriented parallel to the rolling direction for samples of both types. In all samples, the (102) and (103) planes are predominantly perpendicular to the normal direction. A more significant tilt of the basal poles from the normal direction to the transverse one was observed in α-zirconium. Consequently, zirconium alloying creates an ideal texture with respect to zirconium hydride formation. Zirconium alloying is reflected in a decrease in the overall texture sharpness. Doping of zirconium is reflected in a decrease in the overall sharpness of the texture. The results obtained are characteristic of zirconium.

Atomic scale analysis of grain boundary deuteride growth front in Zircaloy-4

Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%) was electrochemically charged with deuterium to create deuterides and subsequently analysed with atom probe tomography and scanning transmission electron microscopy to understand zirconium hydride formation and embrittlement. At the interface between the hexagonal close packed (HCP) α-Zr matrix and a face centred cubic (FCC) δ deuteride (ZrD1.5–1.65), a HCP ζ phase deuteride (ZrD0.25–0.5) has been observed. Furthermore, Sn is rejected from the deuterides and segregates to the deuteride/α-Zr reaction front.

Numerical prediction of mechanical properties of zirconium alloy with hydrides using finite element method

During nuclear power plant (NPP) operation, degradation effects like ageing, corrosion, fatigue, and others may significantly impact component integrity. One of the degradation mechanisms is hydrogen absorption. High levels of hydrogen in zirconium alloys can lead to the formation of zirconium hydrides and that can influence material properties. Therefore, determination of material properties under different levels of hydrogen concentration in zirconium alloys is important. It is not always possible to conduct an experimental testing. Therefore, there is a need for alternative methods for determination of material properties. This article presents the numerical prediction of material properties of zirconium 2.5% niobium alloy with hydrides. According to the objective of the work, numerical prediction was performed using the finite element (FE) method. This was done by creating a finite element model of zirconium hydride embedded in zirconium alloy. The geometry and size of hydride were measured from a real specimen. The size of zirconium alloy surrounding the hydride was selected in such a way that hydride volume part in the model would match experimental measurements. The prognosis results were compared with the experimental data.

Synergy between Two Metal Catalysts: A Highly Active Silica-Supported Bimetallic W/Zr Catalyst for Metathesis of n-Decane.

A well-defined, silica-supported bimetallic precatalyst [≡Si-O-W(Me)5≡Si-O-Zr(Np)3] (4) has been synthesized for the first time by successively grafting two organometallic complexes [W(Me)6 (1) followed by ZrNp4 (2)] on a single silica support. Surprisingly, multiple-quantum NMR characterization demonstrates that W and Zr species are in close proximity to each other. Hydrogenation of this bimetallic catalyst at room temperature showed the easy formation of zirconium hydride, probably facilitated by tungsten hydride which was formed at this temperature. This bimetallic W/Zr hydride precatalyst proved to be more efficient (TON = 1436) than the monometallic W hydride (TON = 650) in the metathesis of n-decane at 150 °C. This synergy between Zr and W suggests that the slow step of alkane metathesis is the C-H bond activation that occurs on Zr. The produced olefin resulting from a β-H elimination undergoes easy metathesis on W.

Hydrogenation of the intermetallic compound Zr2Ni

We have determined conditions for the preparation of hydride phases with the composition Zr2NiH∼5 by reacting the intermetallic compound Zr2Ni with hydrogen or ammonia and identified the products of the reaction between the intermetallic compound and ammonia in the temperature range 150–500°C in the presence of NH4Cl as an activator. The results demonstrate that the use of ammonia at 500°C leads to decomposition of the intermetallic compound and formation of zirconium hydride, zirconium nitride, and metallic nickel.

Formation of zirconium hydride in the vicinity of stereo disclinations

The kinetics of phase transformations in the vicinity of stereo dislocations is considered. An exact analytic solution to the diffusion kinetics equation is obtained taking into account the field of internal stresses of a stereo disclination. The simplicity of the resultant solution is due to the logarithmic coordinate dependence of the first invariant of the internal stress tensor of the structural defect under investigation. The results of theoretical analysis are used for studying the kinetics of the formation of zirconium hydride.

Hydride Formation Process for the Powder Metallurgical Recycle of Zircaloy from Used Nuclear Fuel

A Zircaloy recycle process is being investigated based on the underlying principle that Zr reacts with H2 to form ZrH2, which is the same reaction that produces performance limiting ZrH2 formations in reactor cladding. However, in the proposed application, hydride formation is an enabling phenomenon that will embrittle and crumble Zircaloy as a precursor for milling and dehydration to form Zircaloy metal powder. Hydride formation experiments were performed to quantify the primary process variables of time and temperature. These experiments were performed by hydriding nuclear grade Zircaloy-4 tubes under flowing gas (Ar-5 pct H2) for various times and temperatures. The results were used to create a correlation for the formation of zirconium hydride as a function of time and temperature. Further, it was observed that it was much more effective to hydride the Zircaloy-4 tubes at temperatures below the α-β-δ eutectoid temperature of 818 K (545 °C), presumably related to the high hydrogen solubility of β-Zr. Samples treated below this temperature readily crumbled during the hydride formation reaction and were subsequently easily ground to powder, making this the ideal temperature range for the proposed recycle method. Hydrogen pickup was faster above 818 K (545 °C), but the samples were generally tougher.

Phase composition and corrosion resistance of magnesium alloys

The effects of phase composition of castable experimental and commercial alloys based on the Mg-Al, Mg-Al-Mn, Mg-Al-Zn-Mn, and Mg-Zn-Zr systems and of the form of existence of iron and hydrogen admixtures on the rate of corrosion of the alloys in 3% solution of NaCl are studied. The roles of heat treatment in the processes of hydrogen charging and phase formation in alloy ML5pch and of hydrogen in the process of formation of zirconium hydrides and zinc zirconides in alloys of the Mg-Zn-Zr system and their effect on the corrosion and mechanical properties of alloy ML12 are discussed.

Effects of electrochemical hydrogenation of Zr-based alloys with high glass-forming ability

Zr–(Ti)–Cu–Al–Ni metallic glasses exhibit a high thermal stability corresponding to a wide undercooled liquid region. Depending on their composition, the formation of metastable intermediate phases, e.g. a quasicrystalline phase is possible. The combination of early and late transition metals makes these alloys very interesting regarding their interaction with hydrogen. Amorphous Zr55Cu30Al10Ni5, Zr65Cu17.5Al7.5Ni10 and Zr59Ti3Cu20Al10Ni8 ribbons were prepared by melt spinning and their microstructure and thermal behaviour was checked by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in 1 vol.-% HF was investigated in 0.1 M NaOH by applying potentiodynamic polarisation techniques. Galvanostatically hydrogenated samples were characterised by XRD, DSC, TEM and thermal desorption analysis (TDA). For amorphous Zr59Ti3Cu20Al10Ni8 samples an increase in electrochemical surface capacity by two orders of magnitude is observed after pre-etching. Compared to the quasicrystalline and crystalline alloy, the hydrogen reduction takes place at significantly lower overpotentials. Zr-based alloys cathodically absorb hydrogen up to H/M=1.65 while keeping the amorphous structure. Already small amounts of hydrogen cause a significant decrease of the thermal stability and changes in the crystallisation sequence. The hydrogen desorption is a two-stage process: (T<623 K) hydrogen desorption from high interstitial-site energy levels and (T>623 K) zirconium hydride formation and subsequent transformation under hydrogen effusion. Hydrogen suppresses the oxygen-triggered formation of metastable phases upon heating and supports primary copper segregation. At very high H/M ratios, severe zirconium hydride formation causes the crystallisation of new compounds.

Effect of hydrogen on Zr 65Cu 17.5Al 7.5Ni 10 metallic glass

Glassy Zr 65Cu 17.5Al 7.5Ni 10 alloy samples with 0.13 and 0.46 at.% oxygen were prepared by single-roller melt-spinning. Subsequently, the ribbons were electrolytically charged to different hydrogen-to-metal ratios 0.035≤H/M≤1.6. The effect of hydrogen on the thermal stability and on the crystallisation behaviour of the alloy samples was studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). With increasing hydrogen content, a reduction of the undercooled liquid region and a significant change in the crystallisation process was observed. Hydrogen was found to suppress the oxygen-triggered formation of the metastable quasicrystalline phase, whereas the metastable f.c.c.-phase formation seems to be less affected. Furthermore, hydrogen-induced copper segregation was detected after long-term charging to high H/M ratios at room temperature and already for samples with low hydrogen contents upon heating. At very high H/M ratios, the thermal behaviour of the investigated Zr 65Cu 17.5Al 7.5Ni 10 alloy samples is determined by preferential zirconium hydride formation and successive decomposition causing the formation of new stable intermetallic phases with lower zirconium content.

Hydrogenation and its effect on the crystallisation behaviour of Zr55Cu30Al10Ni5 metallic glass

Abstract Zr 55 Cu 30 Al 10 Ni 5 metallic glass exhibits high thermal stability, and as it contains early and late transition metal elements (ETM/LTM), it is of interest to study its hydrogenation properties. Charging melt-spun ribbons electrochemically to different hydrogen-to-metal (H/M) ratios and following the effusion of hydrogen by thermal desorption analysis (TDA) reveals hydrogen desorption from high interstitial-site energy levels at temperatures below 623 K. Zirconium hydrides are formed above 623 K. At higher temperatures partial desorption of hydrogen occurs. Simultaneously, transformation to different hydride phases takes place in the order tetragonal e-Zr-hydride, cubic δ-Zr-hydride and a mixture of (α+β)-Zr-hydride. Thermal stability investigations by differential scanning calorimetry (DSC) point out the exothermic peaks of formation/transformation to different Zr-hydride phases. The formation of zirconium hydride causes depletion in the number of free Zr atoms leading, in turn, to different crystalline phases upon crystallisation. X-ray diffraction (XRD) reveals the formation of different crystalline phases for different H/M ratios. For a H/M-ratio of 0.37 a hexagonal AlZr 2 phase forms at 753 K, whereas for high hydrogen contents of 0.7 2 Zr forms already at 713 K. In contrast, the uncharged ribbons crystallise at 771 K by formation of a mixture of metastable fcc NiZr 2 -type phase, orthorhombic NiZr and tetragonal CuZr 2 phases.

Observation of kinetics of γ zirconium hydride formation in Zr–2.5Nb by neutron diffraction

Neutron diffraction was employed to observe an isothermal transformation among zirconium hydrides in a commercial zirconium alloy. A specimen of Zr–2.5Nb, which contained about 200 mg deuterium/kg of alloy, was heated to 450°C for almost 17 h, then cooled directly to 17°C and held at this temperature for an extended time. A series of neutron diffraction patterns was collected during this thermal cycle. The diffraction patterns show that a small amount of the tetragonal γ-phase zirconium hydride appears soon after cooling, along with a predominant quantity of the cubic δ-phase hydride. Over three subsequent days at 17°C, the amount of γ-phase hydride increases while there is a corresponding reduction in the amount of δ-phase hydride.

Thermodynamics of the Zr‐H System

The consistencies among the reported univariant hydrogen pressures in the three two‐phase regions of the Zr‐H system, aZr/δZrH2‐xαZr/βZr, and βRZr/δZrH2–x, are evaluated with the thermodynamic constraint derived by Speiser.16 Hydrogen dissolution in αZr is evaluated by checking the consistencies among the Sieverts’law constant, the terminal solubility, and the corresponding two‐phase univariant pressures involving this phase. A modified Sieverts’law is proposed for the complicated behavior of hydrogen dissolution in βZr. An analytical compositiontemperature‐pressure relationship for δZrH2–x is established by using the quasi‐chemical model derived by Arita et al. The thermodynamic constraint of the free energy of formation of ZrH2 shows that the value measured by Fredricken et al. is the most reliable. In addition, a more accurate free energy of formation of zirconium hydride at the upper phase boundary of αZr/δZrH2–x two‐phase region is suggested.

Nonuniqueness of stationary states in combustion of mixtures of zirconium and soot powders in hydrogen

This article attempts to clarify the question of the existence of the nonuniqueness of regimes accompanying the combustion of a three-component system of zirconium-carbon-hydrogen. Values of the rates and temperatures of combustion, as well as the phase composition of combustion products of mixtures of zirconium with soot for three different cases of ignition, are presented. The results indicate that all the compositions investigated burn in the low-temperature regime with the formation of zirconium hydrides. It is concluded that the theoretically predicted nonuniqueness of stationary regimes of combustion and the critical transition from the high-temperature regime to the low-temperature regime using ignition sources with different temperature can be observed experimentally in the combustion of zirconium-carbon-hydrogen systems.

Formation of zirconium hydrides in supported organozirconium catalysts and their role in ethylene polymerization

On the basis of the IR-study of polymerization catalysts obtained by the interaction of tetrakis-π-allylzirconium with silica it has been concluded that when treating these catalysts with hydrogen, surface zirconium hydrides are formed. These hydride compounds are direct precursors of the propaga- tion centres for ethylene polymerization.