Zircaloy-4 Sheet Research Articles

Hydride-Induced Responses in the Mechanical Behavior of Zircaloy-4 Sheets

This study aimed to investigate the impact of hydrogen content, up to 1217 ppm, on the mechanical properties of Zircaloy-4, with a particular focus on the formation and impact of hydrides. Tensile specimens were tested across a range of temperatures and hydrogen concentrations. The results revealed a pronounced ductile-to-brittle transition associated with hydride formation. When the hydrogen content in the specimens ranged between 700 and 850 ppm, a ductile-to-brittle transition was observed at temperatures of 25 °C, 50 °C, and 75 °C. At 25 °C, the ultimate tensile strength (UTS) of Zircaloy-4 linearly increased as the hydrogen concentration rose from 0 to 1217 ppm H. However, at higher temperatures, the behavior of UTS became more complex, especially in the hydrogen concentration ranges of 500–850 ppm H. Elongation (EL) in the hydrided specimens was affected by both temperature and hydrogen concentration. As hydrogen concentration increased, there was a noticeable decline in uniform EL, while non-uniform EL showed even more significant reductions. Scanning electron microscopy (SEM) analysis of the fracture surfaces revealed that quasi-cleavage features became evident when the hydrogen content reached 850 ppm H, across all tested temperatures. These findings not only provide a quantitative assessment of the safety implications of Zircaloy-4 in nuclear reactor applications but also highlight the importance of the hydrogen charging process and mechanical testing in understanding its mechanical behavior.

Comparative Study of Flow Stress Modeling of Zircaloy-4 Sheets Manufactured from Different Routes

Zircaloy-4 is proven to be an excellent structural material in the nuclear industry. In the present work, three types ofzircaloy-4 sheet materials are subjected to tensile tests at strain rates of 0.001s-1, 0.005s-1, 0.01s-1, and at four differenttemperatures of 298K(room temperature), 348K, 423K, and 498K.The tensile stress-strain data has been used to modelthe flow stress using three different constitutive models, namely the Johnson-Cook (JC) model, modified Zerilli-Armstrong(m-ZA), and modified Arrhenius (m-Arr) model. The models' prediction capability for all the materials is compared usingthe Coefficient of correlation (R), average absolute error (Δ

Virtual design of formability for Zircaloy-4 sheet through texture control

Zircaloy-4 (Zr-4) sheet is commonly used in fuel assembly structural parts as spacer grids which are subjected to high temperature and stresses, corrosive and radiation environment. The knowledge regarding the properties and the formability with respect to initial texture of Zr-4 sheet is not enough. In this paper, finite element method was used to simulate the forming process of Zr-4 sheet, and effect of different initial hardening exponent (n value) on the formability of sheet is studied. The results show formability of sheet reveals better with larger n value. However, the hardening exponent (n value) is closely related to the features of initial texture of sheet. Thus, the well-established polycrystal plasticity method, visco plastic self-consistent (VPSC) model, was used to predict the effect of different initial textures on the hardening exponent of Zr-4 sheet, and plastic deformation mechanism of different initial textures in stamping process was analyzed. The results show that a larger the hardening exponent is obtained under a smaller normal Kearn factor (Fn) of the initial texture, and the activation of prismatic slip decreases with increasing the Fn.

In-situ tensile deformation of Zircaloy-4 sheets with different hydrogen contents at RT and 300 °C

In this paper, the in-situ tensile deformation behaviors of Zircaloy-4 sheets with different hydrogen contents were investigated at room temperature (RT) and 300 °C. The results showed that at RT, the ultimate tensile strength (UTS) increased from 407 MPa to 508 MPa and the reduction of area (RA) decreased from 51% to 29% with the hydrogen content increased from 0 ppm to 850 ppm. When the hydrogen content reached 1300 ppm, both the UTS and RA dropped abruptly, showing an obvious ductile-brittle transition. The in-situ observations showed that for the un-hydrided samples, with the increase of tensile deformation, there first appeared slip deformation, then grain distortion and elongation, and finally crack initiation at the grain boundaries. For the hydrided samples, due to the precipitation of hydrides, the slip deformation and grain distortion were reduced, and the cracks trended to initiate around the hydrides. At 300 °C, the UTS increased from 246 MPa to 318 MPa and the RA decreased from 66% to 55.7% with the hydrogen content increased from 0 ppm to 850 ppm. When the hydrogen content reached 1300 ppm, no ductile-brittle transition was observed. The in-situ observations showed that for the un-hydrided and hydrided samples, with the increase of tensile deformation, they both exhibited good plastic deformation except more cracks initiated around the hydrides in the hydrided samples. The transmission electron microscope observations showed that at RT, with the increase of hydrogen content, the number of hydrides increased, and when the hydrogen content reached to a high level, these hydrides would intersect or collide with each other. However, at 300 °C, these hydrides were observed to be twisted or kinked after tensile deformation, which showed a good deformation ability. Based on these observations, the tensile deformation behavior of the hydrided samples was discussed. • A ductile-brittle transition occurs at hydrogen content of 850 ppm at RT. • Plastic deformation is reduced with the increase of hydrogen content at RT. • Intersect between hydrides is the reason for the ductile-brittle transition. • The ductile-brittle transition can be inhibited below 1300 ppm hydrogen at 300 °C.

Effect of heat input on microstructure, mechanical and corrosion properties of electron beam welded zircaloy-4 sheets

The present study reports the effect of heat input on the evolution of microstructure, mechanical properties, and corrosion behavior of electron beam butt welded zircaloy-4 sheet. Joints prepared with lower heat input demonstrated lower peak temperature and higher cooling rate that resulted in narrow fusion zone and heat-affected zone. Fine basket weave Widmanstatten type structure in fusion zone was observed at lower heat input, while at higher heat input, a large columnar grain with coarse parallel plate Widmanstatten type structure was observed. Traces of second phase particles Zr(Cr,Fe)2 were also found in the fusion zone. The tensile strength of all welds was better than the base metal, irrespective of heat inputs. However, the joint prepared with lower heat input demonstrated higher ductility compared to other joints. The dominant mechanism of corrosion was found to be pitting type and it was supposed to be caused by intermetallic precipitation. The extent of corrosion decreased with decrease in heat input. Adhesive wear was found to be the dominant mechanism of wear and the coefficient of friction decreased with the decrease in heat input.

Breakaway characterization of Zircaloy-4 oxidized in steam and in oxygen at high temperatures using HT- XRD analysis

HT-XRD analyses were performed on Zircaloy-4 sheet samples in steam and oxygen mixtures at high temperatures in order to investigate any possible relation between zirconia phase transformation and the breakaway oxidation. The tetragonal phase fraction decreases for temperatures less than 1000°C. At 1000°C and 1030°C, breakaway oxidation occurred, however, the tetragonal phase fraction remained constant during the isothermal oxidation showing no sudden decrease that could be attributed to the onset of breakaway oxidation. These results demonstrate that the tetragonal-to-monoclinic phase transformation in the external surface of the oxide could not be correlated to the occurrence of the breakaway oxidation.

Effect of texture on crack initiation toughness and the corresponding anisotropy in ductile fracture resistance of thin Zircaloy-4 sheets of Indian PHWR core component

Zirconium alloy core components of nuclear reactors are manufactured following a series of cold-working and heat treatment processes which alter the orientation of grains and hence, their mechanical and fracture properties. Due to the hexagonally close-packed nature of crystal lattice in these alloys, the plastic deformation and the corresponding development of crack-tip constraint is dependent upon the orientation of grains and the major texture. In this work, a thin sheet of Zircaloy-4 has been used to fabricate deeply-cracked compact tension specimens with initial cracks orientated along the rolling and transverse directions respectively. The fracture specimens have been tested at 25 and 300 deg. C, which is the operating temperature of pressurized heavy water type reactors. The crack initiation toughness values have been measured through a modified blunting line expression as well as through measurement of width of stretched zone prior to stable crack growth from scanning electron microscope images. The values of crack initiation toughness as measured through the later method is lower compared to the former signifying physical initiation of crack unlike the approximation associated with 0.2 mm offset line. In addition, the initiation toughness as well as the fracture resistance for specimens with cracks oriented along the rolling direction of the strip are higher when compared to corresponding data for specimens with cracks oriented along the transverse direction. These differences have been explained from point of view of orientation of grains along the rolling direction during the fabrication process of these sheets. The grains plastically deform easily for loading along the transverse direction which is the mode-I loading direction for cracks oriented along rolling direction. The associated loss of crack-tip constraint raises the fracture resistance for rolling direction cracks.

Effect of Processing Routes on Orientation-Dependent Tensile Flow Behavior of Zircaloy-4 at Elevated Temperatures

The present work describes the correlation between orientation-dependent tensile flow behavior and texture of Zircaloy-4 sheet at elevated temperatures. The sheets are manufactured from three different routes, namely slab route, tube route, and pilgering. The material is tested at elevated temperatures raising from 75 to 225 °C with an increment of 75 °C. The XRD results show that the texture intensity is more in LOS than that of the other two. In all the three alloys, irrespective of temperature, the yield strength is maximum in transverse direction and minimum in longitudinal direction. The yield strength dependent in-plane anisotropy (AIP) and percent of elongation-dependent anisotropy (δ) values show presence of moderate textures. The true stress and true plastic strain values are best fitted with the Holloman equation, and the coefficient of determination (R2) ranges between 0.99039 and 0.99959. The differential curves are plotted by numerically differentiating true stress with respect to true plastic strain for quantitative description of work-hardening rate of materials. The differential curves of all the three alloys exhibit the three typical work-hardening regions (i.e., elasto-plastic transition, easy glide stage, dynamic recovery).

Studies on texture and formability of Zircaloy-4 produced by pilgering route

Zircaloy-4 is a widely used material in nuclear industries due to its excellent corrosion resistance at elevated temperatures, low neutron absorption cross section and balanced strength and ductility. In the present study, the formability of Zircaloy-4 sheets produced through pilgering route has been investigated. Nakajima dome tests have been conducted on sheet samples taken along rolling, transverse and diagonal directions with respect to the rolling direction of the sheet to construct the forming limit diagrams (FLDs). The strain signature of deformed samples has also been studied to determine the peak limiting strains. Further, limiting dome height values are calculated and all the results are validated with FE-Simulations. Finally, XRD studies have been performed to understand the relationship between the texture and formability. The FLD and texture studies reveal that the Zircaloy-4 possesses higher drawability in comparison to that of the stretchability. The orientation of the specimen has negligible influence on FLD in tension–tension region and marginal influence in tension–compression region.

The challenge of acquiring a satisfactory EBSD result of CWSR Zircaloy-4 cladding tube.

We present a methodology combining multiple microscopic methods to analyse a recrystallised Zircaloy-4 sheet and cold-worked stress-relieved (CWSR) Zircaloy-4 cladding, important alloys of structural materials widely used in nuclear application, and emphasis on the challenge of acquiring a satisfactory electron backscatter diffraction (EBSD) result of CWSR Zircaloy-4 cladding material in great details. EBSD is a powerful technique to characterise the crystallographic distribution and lattice type of conductive crystalline materials, especially for a highly textured material like CWSR Zircaloy-4 alloy. However, zirconium alloys are known to be one of the most difficult materials to prepare for EBSD characterisation. We point out that the configuration of the microstructure of the specimen cause the challenge in the EBSD sample preparations. Moreover, the occurrence of tiny zirconium hydride precipitates in Zircaloy-4 increases the difficulty. We believe that the information of the EBSD sample preparation related results in this paper can provide researchers and scientists in this community a useful reference to speed up the EBSD sample preparation of CWSR Zircaloy-4 cladding material and to expect the corresponding EBSD results.

Orientation-Dependent Tensile Flow Behavior of Zircaloy-4 at Room Temperature

Present work describes a correlation between texture and sample orientation-based flow parameters of three different zircaloy-4 sheet materials. The three different sheet materials [slab route sheet (SRS), tube route sheet (TRS) and low-oxygen sheet (LOS)] corresponded to three different routes of fabrication and hence represented as variation in starting condition. All the three materials exhibited the presence of moderate texture. The intensity is more in TRS samples in comparison with that of the SRS and LOS. This in turn resulted in moderate values of anisotropy parameters. The strength parameters and elongation values have increased and decreased with increase in strain rate, respectively. The flow behavior of the alloys followed typical Holloman equation. The instantaneous work-hardening rate curves of the present alloys exhibited all the three typical regimes (i.e., regime I, regime II and regime III).

Numerical investigations on the effect of pit on two-step dimple forming of atomic fuel spacer grid

Surface defects, known as pits, due to intervention of impurities are difficult to control during the production of sheet metals for atomic fuel spacer grids. These surface defects can increase the fracture risk during sheet metal forming (SMF) processes and lead to failure due to cracking. In this research, an effect of pits on the formability of zircaloy-4 sheets in two-step stamping process is numerically investigated. Experimentally measured pits with averaged sizes are introduced into finite element (FE) models to investigate the effect pit size and the location on plastic strain distribution around weaker sections. By performing one-step and two-step stamping analyses, strains at the pit locations are compared with an experimental forming limit curve (FLC). Although the two-step stamping model reduces the susceptibility to cracking of the atomic fuel spacer grids over one-step stamping model, this study reveals that the fracture risk is higher, if a pit with relatively small radius and deep depth is located near the edge of the metal sheets. Therefore, it is recommended to identify the pits with critical size around the weaker sections before the SMF processes.

Strengthening of Zircaloy-4 using Y2O3 particles by a laser-beam-induced surface treatment process

The mechanical strength of Zircaloy-4 was enhanced by surface treatment consisting of laser beam scanning of an oxide-coated Zircaloy-4 sheet. An yttrium oxide (Y2O3) coating 7–55μm thick was applied to Zircaloy-4 sheets. Then, a laser beam was used to form a dispersed oxide layer on the Zircaloy-4 metal surface. The thickness of the dispersed oxide layer varied from 80 to 200μm depending on the process parameters. Tensile tests of samples having both surfaces treated were conducted at room temperature and 380°C. The tensile strength of Zircaloy-4 at room temperature was increased by up to 20% with the formation of a thin dispersed oxide layer with a thickness<10% of that of the Zircaloy-4 substrate. However, the tensile elongation of the samples decreased drastically. The decreased ductility became insignificant as the testing temperature increased to 380°C. The strengthening of the fabricated samples could be explained by two mechanisms: (i) oxide particles dispersed in the metallic matrix and (ii) the phase transformation in Zircaloy-4.

Room temperature mechanical properties of electron beam welded zircaloy-4 sheet

Resumption of operations at the Transient Reactor Test (TREAT) facility at Idaho National Laboratory was approved in 2014 to meet U.S. Department of Energy Office of Nuclear Energy objectives in transient testing of nuclear fuels. In parallel, the National Nuclear Security Administration, through the Office of Material Management and Minimization, is converting TREAT from its existing highly enriched uranium core to a low-enriched uranium core. This effort entails designing, fabricating, and qualifying a new TREAT low-enriched uranium fuel assembly, while maintaining TREAT’s experimental performance capabilities. Zircaloy-4 is being evaluated as TREAT low-enriched uranium assembly material. A preliminary study on the room temperature mechanical properties of as-received and electron beam welded Zircaloy-4 sheet (1.6mm) is presented. The sheet was high-vacuum electron beam welded using a three-pass process with varying heat input: (1) tack welding (10.5J/mm), (2) seams welded (36.7J/mm), and (3) sealed (15.7J/mm). As-received and electron beam welded specimens show comparable properties. Zircaloy-4 displays anisotropy between the transverse and longitudinal directions. Tensile properties measured for the transverse direction display higher yield strength, reduction of area, and slightly lower tensile strength and ductility than for the longitudinal (i.e., rolling) direction. Weld and base metal hardness are comparable, while hardness at the heat-affected-zone is slightly higher. Microscopic examinations show distinct microstructure morphology and grain size from weld to base metal. A correlation between welding parameters, mechanical properties, and microstructural features was established for electron beam welded Zircaloy-4 sheet.

The influence of stress state on the reorientation of hydrides in a zirconium alloy

Hydride reorientation can occur in spent nuclear fuel cladding when subjected to a tensile hoop stress above a threshold value during cooling. Because in these circumstances the cladding is under a multiaxial stress state, the effect of stress biaxiality on the threshold stress for hydride reorientation is investigated using hydrided CWSR Zircaloy-4 sheet specimens containing ∼180 wt ppm of hydrogen and subjected to a two-cycle thermo-mechanical treatment. The study is based on especially designed specimens within which the stress biaxiality ratios range from uniaxial (σ2/σ1 = 0) to “near-equibiaxial” tension (σ2/σ1 = 0.8). The threshold stress is determined by mapping finite element calculations of the principal stresses and of the stress biaxiality ratio onto the hydride microstructure obtained after the thermo-mechanical treatment. The results show that the threshold stress (maximum principal stress) decreases from 155 to 75 MPa as the stress biaxiality increases from uniaxial to “near-equibiaxial” tension.

Effects of hydrogen contents on the mechanical properties of Zircaloy-4 sheets

The purpose of this study was to investigate the effects of hydrogen contents on the mechanical properties of Zircaloy-4 sheets. Zircaloy-4 specimens were gaseously hydrided up to 832ppm H and then tested at 25, 100, 200, 300 and 400°C. Tensile specimens were chosen to understand the ductile-to-brittle transition associated with hydrogen content. Scanning electron microscopy was used to examine the fracture surface. The results showed that, at 25°C, a ductile-brittle transition occurred on the Zircaloy-4 specimen with hydrogen contents between 538 and 832ppm H. The behavior was not observed as the temperatures were above 100°C. In the temperature range of 100–300°C, the specimens charged with 712ppm H and below exhibited good ductility. The early occurrence of the strain localization for the specimens with hydrogen content larger than 456ppm H was found at temperatures higher than 200°C. Fractographic examinations indicated that quasi-cleavages features were observed as the hydrogen content was 712ppm H and above at the temperatures below 100°C, whereas ductile fracture occurred for all the hydrided specimens tested at temperatures higher than 200°C. Stress-strain curves suggested that yield points were observed on the unhydrided specimens while the phenomenon was suppressed on the hydrided ones.

Numerical approach to the evaluation of forming limit curves for zircaloy-4 sheet

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Pulsed laser induced mechanical behavior of Zircaloy-4

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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.

Dislocation cross-slip controlled creep in Zircaloy-4 at high stresses

Abstract Uniaxial creep tests were performed on Zircaloy-4 sheet in the temperature range of 500–600 °C at high stresses (>10−3E), to uncover the rate-controlling mechanism. A stress exponent of 9.3–11 and a stress-dependent activation energy in the range of 220–242 kJ/mol were obtained from the steady state creep data. TEM analyses revealed an extensive presence of hexagonal screw dislocation network on the basal planes indicating recovery of screw dislocations by cross-slip to be the dominant mechanism. The creep data was therefore analyzed in the light of Friedel׳s cross slip model for HCP metals according to which the stress-dependency of the activation energy determined from the creep data was written in the form, U = ( 150 ± 4 ) + ( 2236 ± 124 τ ) kJ / mol The constriction energy of screw dislocations of 150 kJ/mol is in agreement with the values reported in the literature for zirconium and other HCP metals. Further analysis of the yield strength and the activation volume data obtained from stress relaxation tests in the temperature range 500–600 °C favors cross-slip of screw dislocations as the rate controlling mechanism over the test conditions.