Zircaloy-4 Samples Research Articles

The effect of substrate texture and oxidation temperature on oxide texture development in zirconium alloys

During corrosion of zirconium alloys a highly textured oxide is formed, the degree of this preferred orientation has previously been shown to be an important factor in determining the corrosion behaviour of these alloys. Two distinct experiments were designed in order to investigate the origin of this oxide texture development on two commercial alloys. Firstly, sheet samples of Zircaloy-4 were oxidised between 500 and 800 °C in air. The resulting monoclinic oxide texture strength was observed to decrease with increasing oxidation temperature. In a second experiment, orthogonal faces of Low Tin ZIRLO™1 were oxidised in 360 °C water, providing different substrate textures but identical microstructures. The substrate texture was observed to have a negligible effect on the corrosion performance whilst the major orientation of both oxide phases was found to be independent of substrate orientation. It is concluded that the main driving force for oxide texture development in single-phase zirconium alloys is the compressive stress caused by the ZrZrO2 transformation.

Evaluating Fracture Toughness of Rolled Zircaloy-2 at Different Temperatures Using XFEM

Fracture toughness and mechanical properties of the zircaloy-2 processed by rolling at different temperatures have been investigated, and simulations have been performed using extended finite element method (XFEM). The solutionized alloy was rolled at different temperatures for different thickness reductions (25–85%). Fracture toughness has been investigated by compact tension test. The improved fracture toughness of the rolled zircaloy-2 samples is due to high dislocation density. SEM image of the fractured surface shows the reduction in dimple sizes with the increase in dislocation density due to the formation of microvoids as a result of severe strain induced during rolling. Compact tension test, edge crack, center crack and three-point bend specimen simulations have been performed by XFEM. In XFEM, the cracks are not a part of finite element mesh and are modeled by adding enrichment function in the standard finite element displacement approximation. The XFEM results obtained for compact tension test have been found to be in good agreement with the experiment.

On the effects of reorientation and shear transfer during twin formation: Comparison between high resolution electron backscatter diffraction experiments and a crystal plasticity finite element model

The effects of reorientation and twin shear transfer on the load sharing between twin and parent pairs in hexagonal closed-pack (HCP) polycrystals have been examined by combined experimental and numerical methods. A highly textured Zircaloy-2 sample was uniaxially strained in a direction that favours twin formation and then unloaded to measure variations in residual elastic strains, lattice rotations, and stresses within twin and parent grains by the use of high resolution electron backscattered diffraction (HR-EBSD). The measured grain structures were imported into a finite element solver to study local stresses within each grain and their evolution as twins form. A crystal plasticity finite element code was modified to integrate the effect of twin shear strain into the constitutive equations. Results show that between reorientation and twin transformation strain, the later plays the more important role on determining the state of the stress in the parent, twin and the surrounding environment. The elastic energy of the parent grain was shown to reduce upon twin formation but then stay constant after the early stages of twin shear transfer. This can promote the formation of the next twin in preference to increasing the size of the current one. Comparison of the model with HR-EBSD measurements took into account that the residual stress variations were measured relative to the (unknown) stress state at the reference point within each grain.

Hydrogen motion in Zircaloy-4 cladding during a LOCA transient

Hydrogen and oxygen are key elements influencing the embrittlement of zirconium-based nuclear fuel cladding during the quench phase following a Loss Of Coolant Accident (LOCA). The understanding of the mechanisms influencing the motion of these two chemical elements in the metal is required to fully describe the material embrittlement. High temperature steam oxidation tests were performed on pre-hydrided Zircaloy-4 samples with hydrogen contents ranging between 11 and 400 wppm prior to LOCA transient. Thanks to the use of both Electron Probe Micro-Analysis (EPMA) and Elastic Recoil Detection Analysis (μ-ERDA), the chemical elements partitioning has been systematically quantified inside the prior-β phase. Image analysis and metallographic examinations were combined to provide an average oxygen profile as well as hydrogen profile within the cladding thickness after LOCA transient. The measured hydrogen profile is far from homogeneous. Experimental distributions are compared to those predicted numerically using calculations derived from a finite difference thermo-diffusion code (DIFFOX) developed at IRSN.

Assessment of residual stress fields at deformation twin tips and the surrounding environments

Stress fields close to twin tips and the associated local neighbourhoods of a hexagonal close-packed (HCP) polycrystal were studied in detail. For this purpose, a coarse grain textured Zircaloy-2 sample was firstly strained uniaxially in a macroscopic direction that favours tensile twin formation. The sample was then unloaded and residual elastic strains and lattice rotations measured using the high-resolution electron backscatter diffraction (HR-EBSD) technique. Measured elastic strain maps of various clusters of grains including parent and twin pairs were then analysed. Stress, dislocation density, and their associated concentrations close to twin tips, within twins, in the immediate neighbouring grain, at the intersection of two twins, and within parent grains were investigated. It is shown that the stress field at the twin tips varies as a function of local neighbourhood. High stress, lattice rotation, and dislocation density concentrations were generally observed close to twin tips both within twins and within the immediate neighbouring grains. It is shown that dislocation density concentration is maximum at the intersection of two twins which can potentially provide susceptible site for crack nucleation.

Influence of hydrogen on the oxygen solubility in Zircaloy-4

Despite the influence of hydrogen on the behavior of zirconium fuel cladding in many nuclear safety issues, the pseudo-binary Zircaloy-4 – oxygen phase diagram still lacks of data, especially above 1000 °C. The aim of this study was to provide experimental data to better assess the influence of hydrogen on the oxygen solubility in Zircaloy-4. Homogenized two-phase Zircaloy-4 samples were elaborated from 300 to 1000 wppm pre-hydrided samples. Local distributions were characterized thoroughly using Electron Probe Micro-Analysis (EPMA) for oxygen and Elastic Recoil Detection Analysis (ERDA) for hydrogen. The data obtained in this work were included in the pseudo-binary Zircaloy-4 – oxygen phase diagram and have shown that hydrogen has limited influence on the α + β → β transus. Regarding the α → α + β transus, no influence of hydrogen concentration in the α phase below 400 wppm was evidenced.

Characterization of Zircaloy-4 corrosion films using microbeam synchrotron radiation

A study of the oxide layers formed in 360°C and 316°C water on Zircaloy-4 samples has been performed in an attempt to help answer fundamental questions about oxide protectiveness, growth mechanisms, and the nature of oxide growth during autoclave corrosion. Two different oxide thicknesses – 12 and 39.5μm – were investigated. Microbeam synchrotron radiation diffraction and fluorescence techniques with an X-ray beam size of 0.2μm were used to characterize oxide in cross sections to determine the oxide phase content, grain size, texture, and orientation relationships as a function of through-thickness from the oxide–metal interface.The results confirm that the oxide is comprised primarily of monoclinic ZrO2, with tetragonal ZrO2 present in small amounts. The observed diffraction peaks are consistent with monoclinic phases having a strong fiber texture with the 200m plane aligned with the oxide–metal interface, and with the 011m plane closely aligned with the transverse-normal (T) plane.The fraction of bulk tetragonal phase increased in the region located within one transition thickness near the oxide–metal interface. A strong periodicity was seen in oxide intensity from both the monoclinic and tetragonal phases corresponding to an oxide transition thickness of 1.8–1.9μm. The grain size of the tetragonal phase was determined to be smaller than the monoclinic phase, and the grain size for the monoclinic phase decreased starting at a distance of approximately one transition layer from the oxide–metal interface. The relative amounts of monoclinic peak broadening due to strain and grain size were calculated, the former being approximately constant, while the latter decreased with increasing distance from the oxide–metal interface, corresponding to an increase in grain size. These findings are compared to previously performed microbeam diffraction experiments on Zircaloy-4 and other Zr-based alloys.

Dislocation structure in different texture components determined by neutron diffraction line profile analysis in a highly textured Zircaloy-2 rolled plate

A novel diffraction-based method has been developed to determine the substructure in terms of the fraction of prevailing slip systems and dislocation densities in multiple individual texture components of strongly textured materials. The method was applied to a strongly textured cold-rolled Zircaloy-2 sample, compressed along the normal direction of the rolled plate. Fourteen diffraction patterns were collected by time-of-flight neutron diffraction experiments at seven different purposely selected sample orientations. The diffraction peaks corresponding only to one single texture component out of the four prevailing components were identified and used to construct texture-specific diffraction patterns. The best five such patterns were evaluated by the convolutional multiple whole profile procedure of line profile analysis for the fractions of the prevailing 〈a〉-, 〈c + a〉- and 〈c〉-type slip systems and the corresponding dislocation densities. It is found that in the three texture components with thecaxis not parallel to the normal direction of rolling the average dislocation densities and fractions of slip-system types are the same. However, in the texture component with thecaxis parallel to the normal direction of rolling the fraction of the 〈a〉-type slip system is somewhat smaller than in the other components and that of the 〈c〉-type slip system is somewhat larger than zero. The fractions of the prevailing slip systems are in good correlation with literature data of self-consistent polycrystal plasticity modeling of Zircaloy-2.

Nanomechanical Characterization and Protein Adsorption of Cold-Rolled Zirconium Alloy

The success of the implants used for bone regeneration and repair is highly dependent on the cell material interaction, which is further influenced by interaction of body proteins with the implant materials. In this study, a novel nanostructured Zircaloy-2 has been developed using cold rolling (reduction of 25%, 50%. 75%, and 85%), which resulted in the refinement of grains by several orders of magnitude. Phase analysis was done using x-ray diffraction, and the microstructures of room-temperature rolled Zircaloy-2 were obtained using optical microscopy. The adsorption behavior of bovine serum albumin as a function of protein concentration on Zircaloy-2 was performed, and the wettability before and after protein adsorption on Zircaloy-2 samples was estimated by measuring the contact angle of a water droplet on the processed samples. Nanoindentation studies were performed, which indicated higher hardness (of 2.15 GPa) and reduced elastic modulus (47.4 GPa) when compared with that of as-received Zircaloy-2 (hardness of ~0.12 GPa and reduced elastic modulus of 5.6 GPa). Also, it was found that protein adsorption increases to 0.59 mg/cm2 for maximum deformed samples when compared with that of 0.38 mg/cm2 for as-received samples. Contact angle decreased with increasing reduction from 62° (of the as-received sample) to 44° (at a reduction of 85%). But after protein adsorption, a further decrease in the contact angle from 43° (as received) to 26° (for 85% reduction) was observed. Thus, the engineered cold rolling can allow tailoring the nanomechanical properties of Zircaloy-2 while rendering the required protein adsorption as potential implant material.

In-situ X-ray diffraction analysis of zirconia layer formed on zirconium alloys oxidized at high temperature

In the case of a hypothetical loss of primary coolant accident (LOCA) in a light water reactor, the zirconium alloys fuel cladding would be oxidized in steam at high temperature, typically in the range 800–1200°C. The monoclinic to tetragonal phase martensitic transition of zirconia occurs within this temperature range and complex phenomena possibly having an impact on the oxidation kinetics are then to be expected. In order to provide an accurate description of the structure and microstructure of the oxide layers, systematic X-ray diffraction analyses have been performed in-situ under oxidizing atmosphere at high temperature (between 800 and 1100°C) on Zircaloy-4 and M5™ sheet samples. It was confirmed that the volume fraction of the tetragonal and monoclinic zirconia phases formed during oxide growth drastically depends on the oxidation temperature. For example, the few outer microns of the oxide are fully tetragonal above 1050°C and contain only 20% of tetragonal phase at 800°C. It was also shown that cooling after oxidation induces irreversible phase transitions within the oxide. As a consequence, both the structure and the microstructure of the growing oxide cannot be observed post-facto, neither at room temperature nor after reheating at the prior oxidation temperature. It has been deduced from microstructural analyses that the grain size of the tetragonal zirconia phase is nanometric, about 100nm during oxidation at 1100°C down to 20nm after cooling down to room temperature. This small grain size allows the stabilization of the tetragonal phase. The lattice parameters of the monoclinic and tetragonal zirconia phases have been analyzed, during both high temperature oxidation and cooling. In both cases, it appears the ‘a’ and ‘b’ cell parameters of the monoclinic phase are strongly constrained by the tetragonal ‘a’ one. The structural characteristics of the oxide formed at high temperature on Zircaloy-4 and M5™ are quite similar. All those results can be interpreted in the frame of the classical description of the monoclinic–tetragonal martensitic transition of zirconia.

Depth profile of oxide volume fractions of Zircaloy-2 in high-temperature steam: An in-situ synchrotron radiation study

To study the steam oxidation behavior of Zircaloy-2, a high-energy synchrotron X-ray diffraction technique was applied to perform an in-situ oxidation measurement. The depth profiles of oxide volume fractions were obtained at both 600 and 800°C. Multiple layers, including ZrO2 scale, (α+β) Zr matrix with ZrO2 incursions, and (α+β) Zr matrix, were mapped according to the volume fraction of each phase. The volume fractions of these phases were observed to change gradually with different distances to the surface, without a sharp edge distinguishing each of the layers. The ZrO2 consisted of tetragonal and monoclinic crystal structures, which were observed to coexist with different ratios of volume fractions in depth. The higher amount of tetragonal ZrO2 observed in the very inner region of the oxidizing Zircaloy sample indicates that the tetragonal crystal structure is the ab initio phase type, in which new oxide molecules form at the metal–oxide interface.

Fracture of Zircaloy-4 cladding tubes with or without hydride blisters in uniaxial to plane strain conditions with standard and optimized expansion due to compression tests

Two optimizations of the Expansion Due to Compression (EDC) test, which induces a near uniaxial loading, were proposed and developed to reach higher biaxiality ratios (ratio between mechanical quantities in axial and in circumferential direction). The first optimization, named HB-EDC for High-Biaxiality EDC, allowed to reach transverse plane strain conditions. The second optimization, named VHB-EDC for Very High Biaxiality EDC, was designed to reach higher loading biaxiality ratios. These optimized EDC tests were performed at 25°C, 350°C and 480°C on unirradiated hydrided Cold Worked Stress Relieved (CWSR) Zircaloy-4 samples. First, samples unhydrided or uniformly hydrided up to 1130wppm were tested. Second, samples hydrided at 310wppm with a hydride blister were tested. A large ductility reduction is induced by the increase in biaxiality level in the absence of a hydride blister or with small blisters (<50μm deep). The fracture strain decreases quickly with the blister depth at 25°C, but more progressively at higher temperature. An equation that quantifies the fracture strain reduction with the blister depth is proposed. Eventually, one of the tests developed in the present study, the HB-EDC test, was proven to be a good compromise between the test complexity and the stress state reached. It is a good candidate to characterize the mechanical behaviour of irradiated cladding.

Ductility Evaluation of As-Hydrided and Hydride Reoriented Zircaloy-4 Cladding under Simulated Dry-Storage Condition

ABSTRACTPre-storage drying-transfer operations and early stage storage expose cladding to higher temperatures and much higher pressure-induced tensile hoop stresses relative to normal operation in-reactor and pool storage under these conditions. Radial hydrides precipitate during cooling and could provide an additional embrittlement mechanism as the cladding temperature decreases below the ductile-to-brittle transition temperature. To simulate this behavior, unirradiated Zircaloy-4 samples were hydrided by a gas charging method to levels that encompass the range of hydrogen concentrations observed in current used fuel. Mechanical testing was carried out by the ring compression test (RCT) method at various temperatures to evaluate the sample’s ductility for both as-hydrided and post-hydride reorientation treated specimens. As-hydrided samples with higher hydrogen concentration (&gt;800 ppm) resulted in lower strain before fracture and reduced maximum load. Increasing RCT temperatures resulted in increased ductility of the as-hydrided cladding. A systematic radial hydride treatment was conducted at various pressures and temperatures for the hydrided samples with H content around 200 ppm. Following the radial hydride treatment, RCTs on the hydride reoriented samples were conducted and exhibited lower ductility compared to as-hydrided samples.

Effect of loading mode on lattice strain measurements via neutron diffraction

The study of lattice strain evolution during uniaxial deformation via in situ neutron diffraction is a well established technique for characterizing the deformation behavior of metals. However, the relatively low flux of neutron facilities results in count times on the order of several minutes, requiring experimenters to choose between either applying a very slow strain rate, or loading the sample incrementally rather than continuously. Here we investigate the effects on lattice strain data obtained by using stress, strain, and position controlled incremental loading, as well as continuous loading, on samples of Zircaloy-2 under uniaxial compression. It was found that both qualitative and quantitative differences arise in the lattice strain behavior of certain grain families, particularly {101¯0} and {112¯0}, while other grain families show no discernible effect. The differences in lattice strain evolution brought on by the variation in loading modes are believed to be the result of thermally activated dislocation motion.

Microstructural investigation of Zirconium alloys subjected to oxygen ions irradiations

In this study, samples of zircaloy-2 were irradiated with oxygen ion beam (O5+ ions with 116 MeV energy) to fluence varying from 1013 to 1015 ion/cm2. The irradiated samples were observed under a transmission electron microscope to identify the irradiation-induced defects and phase transformations. Microstructural investigations on these samples showed the presence of interstitial defects and ZrO2 precipitates in the α (hcp) matrix. The alignment of defects appeared to facilitate the formation of the ZrO2 phase in the irradiated samples. Orientation relationship between the oxide and the matrix phase was determined and a possible mechanism for the formation of the ZrO2 phase based on the entrapment of the oxygen ions has been proposed.

Determination of major to trace level elements in Zircaloys by electrolyte cathode discharge atomic emission spectrometry using formic acid

An electrolyte cathode discharge atomic emission spectrometry (ELCAD-AES) method using formic acid is described for the determination of major to trace level of the elements Ni, Co, Fe, Mn, Mg, Cu, Cr, Pb, Ca in Zircaloys. The influence of HCOOH and CH3COOH to improve the sensitivity of ELCAD-AES for analyte determination is investigated. Sensitivity of ELCAD-AES was significantly improved with HCOOH for the determination of these elements. 20% (v/v) HCOOH was found to produce a maximum enhancement in the sensitivity of ELCAD-AES for the analytes. With 20% (v/v) HCOOH, improved detection limits were determined for Ni (5 ng mL−1), Co (9 ng mL−1), Fe (16 ng mL−1), Mn (7 ng mL−1), Mg (5 ng mL−1), Cu (6 ng mL−1), Cr (65 ng mL−1), Pb (10 ng mL−1), and Ca (8 ng mL−1). The improvement is 2-fold for most elements, 5-fold for Pb, and essentially unchanged for Cr. The use of HCOOH enabled the direct determination of analytes in Zircaloys by ELCAD-AES. The method has been validated by determining the concentrations of analytes Ni, Co, Fe, Mn, Mg, Cu, Cr, Pb, Ca in a Zircaloy certified reference material (Zircaloy-4 NIST-360b) and also in a Zircaloy sample which was analysed by glow discharge quadrupole mass spectrometer (GD-QMS). The precision expressed as a percentage relative standard deviation of signals of multiple analyses (N = 4) of the ELCAD-AES method was found to be between 2 and 10% for major (>0.1 wt%) and minor level ( 100 μg g−1) elements and 10–15% for trace level ( 10 ng g−1) elements. Depending on the levels of the impurities the relative error in accuracy of the ELCAD-AES method was found to be between 3 and 20%.

Multi-scale modeling and experimental study of twin inception and propagation in hexagonal close-packed materials using a crystal plasticity finite element approach; part II: Local behavior

In-situ tensile tests are performed on Zircaloy-2 samples with various grain sizes to study twin inception and propagation. Orientation maps of some areas at the surface are measured before and after deformation, using the Electron BackScattered Diffraction (EBSD) technique. Strain fields of the same areas are determined using the digital image correlation technique and are compared with results from Crystal Plasticity Finite Element (CPFE) simulations. Different assumptions are made within the CPFE code to simulate twin propagation. It is observed that the predictions of different models does not really change from one model to another when statistical information on the twins is compared, yet local predictions for each grain, i.e. twin direction, twin variant selection, and twin inception site, do change. Also, it is shown that the twin Schmid factor can vary drastically within grains and that for those grains with a low tendency for twinning this variation may make them susceptible to twinning.

New insights in structural characterization of zirconium alloys oxidation at high temperature

Zircaloy-4 samples have been analyzed by grazing incidence X-ray diffraction during oxidation at high temperature (900–1100°C) in a He–O2 mixture simulating a LOCA atmosphere. We clearly show that the structure of the oxide layer strongly depends on the oxidation temperature, i.e. tetragonal at 1100°C, mixture of tetragonal and monoclinic at 1000°C, mainly monoclinic at 900°C. Moreover, the high temperature phases are not stable during cooling; at room temperature, the oxide is mainly monoclinic. Furthermore, re-heating the samples to the oxidation temperature in neutral atmosphere does not lead to the pristine structural composition. This behavior is explained by considering the martensitic character of the tetragonal to monoclinic transition of zirconia. As a conclusion, the structural and thermo-mechanical properties of the oxide layer as it forms during oxidation at high temperature cannot be deduced from post-mortem analysis.

Quantitative Analysis of Deuterium in Zircaloy Using Double-Pulse Laser-Induced Breakdown Spectrometry (LIBS) and Helium Gas Plasma without a Sample Chamber

A crucial safety measure to be strictly observed in the operation of heavy-water nuclear power plants is the mandatory regular inspection of the concentration of deuterium penetrated into the zircaloy fuel vessels. The existing standard method requires a tedious, destructive, and costly sample preparation process involving the removal of the remaining fuel in the vessel and melting away part of the zircaloy pipe. An alternative method of orthogonal dual-pulse laser-induced breakdown spectrometry (LIBS) is proposed by employing flowing atmospheric helium gas without the use of a sample chamber. The special setup of ps and ns laser systems, operated for the separate ablation of the sample target and the generation of helium gas plasma, respectively, with properly controlled relative timing, has succeeded in producing the desired sharp D I 656.10 nm emission line with effective suppression of the interfering H I 656.28 nm emission by operating the ps ablation laser at very low output energy of 26 mJ and 1 μs ahead of the helium plasma generation. Under this optimal experimental condition, a linear calibration line is attained with practically zero intercept and a 20 μg/g detection limit for D analysis of zircaloy sample while creating a crater only 10 μm in diameter. Therefore, this method promises its potential application for the practical, in situ, and virtually nondestructive quantitative microarea analysis of D, thereby supporting the more-efficient operation and maintenance of heavy-water nuclear power plants. Furthermore, it will also meet the anticipated needs of future nuclear fusion power plants, as well as other important fields of application in the foreseeable future.

Experimental study of the nucleation and growth of c-component loops under charged particle irradiations of recrystallized Zircaloy-4

Recrystallized zirconium alloys, used as structural materials for the Pressurized Water Reactor fuel assembly, undergo under neutron irradiation induced stress free growth which accelerates for high irradiation doses. This acceleration is correlated with the formation of c-component vacancy dislocation loops in the basal plane. Since these defects are responsible for breakaway growth of recrystallized zirconium alloys, it is of prime importance to know the various factors that can affect their nucleation and growth. In the present work, two types of charged particle irradiations were conducted on recrystallized Zircaloy-4 samples in order to study c-component loops. A 2MeV proton irradiation was performed up to a dose of 11.5dpa at 623K, and 600keV Zr ion irradiations were carried out at 573K up to 7dpa. For the first time after those charged particle irradiations, c-component loops were observed by Transmission Electron Microscopy. It has been shown that under Zr ion irradiation they start to nucleate and grow beyond a threshold dose as for neutron irradiation. The differences in the c-component loop microstructure are discussed for both ion irradiations and compared to the microstructure observed after neutron irradiation. Furthermore, it is shown that after proton irradiation the irradiated layer exhibits a misfit strain which is consistent with the irradiation induced growth of recrystallized zirconium alloys.