Zr-Nb Alloys Research Articles

Effects of thermo-mechanical process on phase transitions, hydrogen solubility and corrosion of Ta-modified Zr-1Nb alloys

The feasibility of using Ta as a minor alloying element in Zr-1Nb alloys is addressed in terms of phase stability and resistance to corrosion of the microstructure obtained with the following two thermomechanical processes after a water quenching from βZr: a) annealed at 570°C for 3840 h and b) a combination of intermediate annealing temperatures and cold rolled steps. Hydrogen solubility was also measured, but only in alloys with the first process. It was found, through calorimetric measurements, that an increase of Ta replacing Nb in the nominal alloy composition increases the α→(α+β) phase transformation temperature in agreement with an estimated increase in the enthalpy of formation of the βNb/Ta phase, ΔHβNb/Taf. As expected, the mass fraction of βNb/Ta measured in the rolled alloys is lower. Further, this diminution is larger in the alloys containing Ta due to the lower mobility of Ta and higher ΔHβNb/Taf. The growth of the oxide layer in each alloy, at 400°C steam up to 14 days, follows the predictor calculated as the ratio between the atomic concentrations of Nb plus Ta in the alloy and ΔHβNb/Taf. A slight decrease in hydrogen solubility was observed at low Ta content in the alloy.

Enhancement of high temperature steam oxidation resistance of Zr–1Nb alloy with ZrO2/Cr bilayer coating

The ZrO2/Cr bilayer coating on Zr–1Nb alloy fabricated by plasma electrolytic oxidation and filtered cathodic vacuum arc deposition duplex treatments had good steam oxidation resistance. The mass gain of ZrO2/Cr-coated alloy was about 36 %, 44 % and 59 % of that of Cr-coated one after 3600 s steam oxidation at 900 °C, 1000 °C and 1100 °C, respectively. The top Cr layer for Cr-coated and ZrO2/Cr-coated alloys was oxidized into a Cr2O3 layer, and then a part of Cr2O3 was reduced by Zr to form metallic Cr layer again after the as-prepared Cr layer was completely consumed.

INFLUENCE OF ELECTRON IRRADIATION ON THE STRUCTURE EVOLUTION AND CREEP OF Zr1Nb ALLOY AT 380 °С E

The effect of electron irradiation on the evolution of the structure and creep of the samples Zr1Nb alloy of industrial production and samples obtained by the IPD method were studied. It was shown that irradiation with electrons with a dose not exceeding D = 5∙1019 сm–2 does not affect the plastic deformation mechanisms of the industrial Zr1Nb alloy, as a result of which its high thermomechanical and radiation stability is maintained. Irradiation by electrons with an energy of E = 10 MeV and doses of D = 6∙1017 and 5∙1019 сm–2 does not change the character of the deformation nanostructure of the Zr1Nb alloy, but initiates return processes in grain boundaries and boundary regions and cause the changes in crystalline anisotropy of GPU lattice, which leads to softening of the material during creep at 380 °C.

STRUCTURE AND PROPERTIES OF FeCr, CrAl AND FeCrAl COATINGS DEPOSITED BY CATHODIC ARC EVAPORATION

Protective coatings with different compositions of Fe, Cr, and Al were deposited by cathodic arc evaporation method on fragments of Zr1Nb alloy fuel claddings. The influence of the composition and structural state of the developed coatings on their physical and mechanical properties (microhardness, resistance to corrosion and cavitation and abrasive wear) has been studied. It is shown that the optimal combination of mechanical properties and high resistance to oxidation from a series of studied coatings FeCr, FeCrAl, and CrAl have coatings of the FeCrAl system with a concentration of Cr ~ 22 at.% and Al ~ 14.3 at.%. It was found that protective coatings such as FeCr, FeCrAl, and CrAl with a thickness of ~ 12 μm significantly increase the resistance to oxidation and prevent the destruction of fuel claddings under oxidation in air at a temperature of 1150 °C for 1 h.

Solubility and vacancy-mediated inter-diffusion in the Zr-Nb-Cr system

To better understand the microstructural evolution of Cr-coated Zr alloy cladding, we investigate, using density functional theory calculations, the vacancy-mediated diffusion behaviour of Zr and Nb solutes in BCC-Cr as well as Zr and Cr solutes in BCC-Nb. The calculated vacancy formation and migration energies are in good agreement with available literature, as are the self-diffusivities. We show that in BCC-Cr both Nb and Zr are faster diffusers than Cr (vacancy-mediated self-diffusion). It was also found that both Zr and Nb segregate towards vacancy sinks in BCC-Cr at normal reactor operating temperatures, but at elevated temperatures their flux is expected to be in opposite directions. In BCC-Nb, we show that Cr is a slower diffuser than Nb self-diffusion, while Zr is faster; and both Zr and Cr are expected to decorate vacancy sinks in BCC-Nb at all temperatures relevant to reactor operation. A similar behaviour is likely to occur in β-Nb phase found in Zr-Nb alloys. The implications of these findings for Cr-coated Zr alloy cladding are discussed.

An effective approach for bonding of TZM and Nb-Zr system: Microstructure evolution, mechanical properties, and bonding mechanism

A novel method of liquid metallic film (LMF) bonding was developed to join titanium zirconium molybdenum alloy (TZM) and Nb-Zr alloy with a Ni interlayer. Using this method, a Ni-Zr liquid phase was formed by the eutectic reaction and then squeezed out from the gap due to a transient pressure, leaving an LMF. It not only achieved a reliable metallurgical bonding but also served as a transition layer between TZM and Nb-Zr alloy to reduce the mismatch between them thus further improving its performance. The bonding mechanism of the TZM and Nb-Zr system was discussed based on theoretical calculation and high–resolution microscopy analysis. The advantages of this method were established by comparing the microstructure and mechanical properties of LMF bonded joints with that of traditional contact-reaction brazing and direct diffusion bonding. Additionally, the feasibility of the LMF bonding method was also demonstrated by the reliable joining of other high-temperature and immiscible systems.

Stability of β-Nb phase in Zr–Nb alloys under neutron irradiation: Phase field modeling

A phase field approach to study stability of β-Nb precipitates in Zr–Nb alloys is extended by taking into account local rearrangement of point defects and misfit dislocations. Kinetic properties of β-Nb phase formation are discussed at a heat treatment stage. Stability of secondary phase precipitates is studied at different irradiation temperatures and dose rates. It is shown that processes of dissolution/growth of precipitates are governed by the competition of ballistic mixing and thermal diffusion. It is found that at large values of dose rates and low temperatures, precipitates are dissolved due to a major role of ballistic mixing, whereas at low dose rate and elevated temperatures, β-niobium particles grow slowly by an Ostwald ripening scenario up to 1–2 nm due to the dominant role of thermal diffusion. Misfit dislocations sustain the existence of Nb-enriched domains of mixed symmetry. It is shown that growing dislocation loops and dissolving/growing precipitates result in the hardening change up to 0.01% at a dose rate of 10−5dpa/s and temperatures of 550–575 K. Obtained theoretical results are verified by experimental data.

Effects of Nb concentration and temperature on generalized stacking fault energy of Zr–Nb alloys by molecular dynamics simulations

The effects of Nb concentration and temperature on the generalized stacking fault energy (GSFE) of basal, prismatic I, pyramidal I and II plane for Zr-Nb alloys are investigated by molecular dynamics simulations (MD). The stable and unstable SFEs of different slip systems show no significant change with the increasing Nb concentration (0, 0.5, 1.0, 1.5, 2.0, and 2.5 at.%) in Zr-Nb alloys at 0 K. Basal, pyramidal I and II planes slip of Zr-Nb alloys prefer to deform by full dislocation with the temperature increases. Additionally, plastic deformation anisotropy of Zr-Nb alloy is improved with the increasing temperature using both embedded atom method (EAM) and angular-dependent potentials (ADP). The present work provides a theoretical basis for understanding enhanced plasticity of Zr-Nb alloys under finite temperature.

An on-line matrix separation and preconcentration procedure for ICP OES determination of Cd, Co, Cu, Mn and Pb traces in Zr and Zr–Nb alloys using a cation-exchange resin microcolumn

Development of a highly efficient on-line matrix separation system for the determination of several trace metal ions in nuclear grade zirconium alloy by inductively coupled plasma optical emission spectrometry.

Molecular dynamics study of special quasirandom structure of Zr-Nb alloys

Irradiation damage to zirconium alloys (e.g., zirconium niobium (Zr-Nb) alloy) is the key to the design of fission-reactor structural materials and fuel rod cladding materials. Atomic scale computational simulations such as molecular dynamics and first principles are often needed to understand the physical mechanism of irradiation damage. For the simulation of randomly substitutional solid solution, it is necessary to construct large-sized supercells that can reflect the random distribution characteristics of alloy elements. However, it is not suitable to use large-size supercells (such as ≥ 200 atoms) for first principle calculation, due to the large computational cost. Special quasirandom supercells (SQS) are usually used for first principles calculation. The SQS can partly reflect the random distribution characteristics of alloy elements, but it only corresponds to one configuration for specific components, hence whether this model can reflect the statistical average of multiple local configurations in a real randomly substitutional solid solution is still an open question, and needs further studying and verifying. Molecular dynamics (MD) simulation can be carried out on the randomly substitutional solid solution with a larger scale based on random substitution (RSS) method, these supercells include more local configurations. Therefore, the MD studies of Zr-Nb alloy are carried out for the RSS and SQS-extended supercells. The critical size of RSS supercell which can truly reflect the statistical properties of solid solution alloy is determined. Then the lattice constant, formation energy and energy-volume relationship of SQS-extended supercell of Zr-Nb alloy and a series of RSS supercells are calculated and compared. The results show that the lattice constants, the formation energy and energy volume curves of the solid solution obtained by SQS supercell simulation are close to a series of corresponding statistical values of the physical properties of RSS supercells, so the SQS supercells can be used to study the random substitution of solid solution alloys.

Comparative Study of Superelastic Ti–Zr–Nb and Commercial VT6 Alloy Billets by QForm Simulation

A comparative simulation of hot radial shear rolling (RSR) of billets made of a superelastic Ti–Zr–Nb and a commercial VT6 alloy was performed using the QForm finite element modeling program. Rolling in 48 modes with a variable feed angle and elongation ratio at 4 levels and initial rolling temperature at 3 levels was investigated for each alloy. The Ti–Zr–Nb alloy rheology during hot deformation was determined experimentally by hot upset forging and imported into the QForm program. The presence of maxima on the flow curves at the initial stage of deformation, which are absent in the VT6 alloy, is revealed. Simulation results are presented in the form of fields of the stiffness coefficient, strain rate intensity, cumulative strain degree in the maximum reduction section depending on the rolling mode. General regularities of the Ti–Zr–Nb and VT6 behavior in RSR are similar. The gradient of the fields studied decreases, and the roll pressure and torque increase with an increase in the feed angle and elongation ratio. The initial rolling temperature does not significantly affect the deformation pattern, but it significantly affects the roll pressure and torque. At the same time, the experimental alloy demonstrated the greater tendency to localize deforming forces in the near-contact zone and to increase the gradient of stress-strain state parameters over the billet section. The study of the tightening shape and depth of rolled billet ends showed that the Ti–Zr–Nb alloy has a 3.5–9.6% greater tightening depth. It is shown that experimental alloy rolling requires 1.6–2.4 times higher roll pressure and torque as compared to the commercial alloy.

Co-deformation and dynamic annealing effects on the texture development during alpha–beta processing of a model Zr-Nb alloy

The in-service properties and performance of dual-phase Zr and Ti alloys depend on their crystallographic texture, which develops during hot-working and is affected by interactions between the α and β phases during deformation, annealing and phase transformation. Recent work on hot-rolled Zr-2.5Nb has shown that the texture of the two phases are related, with coupled strengthening of the α near {112¯0}〈101¯0〉, which produces strong 0002 pole intensities along the transverse direction, and β with {001}〈110〉 rotated cube, particularly when the relative volume fraction is around 50:50. To investigate the origin of this texture coupling, we studied a hot-rolled model Zr alloy with 7 wt.% Nb, in which the as-deformed α + β microstructure is preserved on cooling. The alloy was hot-rolled to different reductions at 725∘C, which corresponds to a relative α:β volume fraction of 30:70, where the characteristic textures are known to develop quickly at first and then weaken with further reduction. The rolled material was characterised using both 2D and 3D electron backscatter diffraction (EBSD). This analysis uncovered evidence that both recrystallization and phase transformation cause the disappearance of specific α variants during rolling, favouring the formation of “soft” primary α grains flattened in 〈112¯0〉 and elongated along 〈101¯0〉 during rolling, which in turn has an effect on surrounding β orientations, promoting the stronger rotated cube component. At higher reductions, these elongated α-grains start to break up, as does the β surrounding it, forming bands of characteristic coupled textures. These observations imply that non-plasticity effects should be included in models of texture evolution during processing of α + β Zr and Ti alloys.

High temperature oxidation of Zr[sbnd]1Nb alloy with plasma electrolytic oxidation coating in 900–1200 °C steam environment

A continuous ceramic coating of 8 μm thick on Zr1Nb alloy was fabricated by plasma electrolytic oxidation (PEO) in alkaline phosphate electrolyte. The high-temperature steam oxidation behavior of bare and PEO coated Zr1Nb alloys was evaluated using a thermogravimetric analyzer (TGA) in 900–1200 °C water vapor environment. Their morphologies, compositions, phase constituents before and after the steam oxidation were characterized by scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD). It was found that the mass gain of PEO coated Zr alloy was about 38%, 56%, 93% and 100% of bare Zr alloy after 3600 s steam oxidation at 900 °C, 1000 °C, 1100 °C and 1200 °C, respectively. The steam oxidation resistance of Zr1Nb alloy was obviously improved after PEO treatment below 1000 °C, but was hardly enhanced above 1100 °C. The mass gain rate for Zr1Nb alloy samples in high-temperature steam is related to the oxygen diffusion coefficients in β-Zr, α-Zr(O) and ZrO2 phases. After steam oxidation at 900–1200 °C, the oxide layers for bare and PEO coated Zr1Nb alloys contain monoclinic zirconia (M-ZrO2) and a little tetragonal zirconia (T-ZrO2) phases below 1100 °C, but the T-ZrO2 phase disappears at 1200 °C.

Comparative study of superelastic Ti–Zr–Nb and commercial VT6 alloy billets by QForm simulation

A comparative simulation of hot radial shear rolling (RSR) of billets made of a superelastic Ti–Zr–Nb and a commercial VT6 alloy was performed using the QForm finite element modeling program. Rolling in 48 modes with a variable feed angle and elongation ratio at 4 levels and initial rolling temperature at 3 levels was investigated for each alloy. The Ti–Zr–Nb alloy rheology during hot deformation was determined experimentally by hot upset forging and imported into the QForm program. The presence of maxima on the flow curves at the initial stage of deformation, which are absent in the VT6 alloy, is revealed. Simulation results are presented in the form of fields of the stiffness coefficient, strain rate intensity, cumulative strain degree in the maximum reduction section depending on the rolling mode. General regularities of the Ti–Zr–Nb and VT6 behavior in RSR are similar. The gradient of the fields studied decreases, and the roll pressure and torque increase with an increase in the feed angle and elongation ratio. The initial rolling temperature does not significantly affect the deformation pattern, but it significantly affects the roll pressure and torque. At the same time, the experimental alloy demonstrated the greater tendency to localize deforming forces in the near-contact zone and to increase the gradient of stress-strain state parameters over the billet section. The study of the tightening shape and depth of rolled billet ends showed that the Ti–Zr–Nb alloy has a 3.5–9.6 % greater tightening depth. It is shown that experimental alloy rolling requires 1.6–2.4 times higher roll pressure and torque as compared to the commercial alloy.

The strong influence of Ti, Zr, Hf solutes and their oxidation on microstructure and performance of Nb3Sn superconductors

Over the last few years a new type of Nb3Sn superconducting composite based on the internal oxidation approach has emerged and has demonstrated performance significantly superior to conventional Nb3Sn. It requires a supply of O and the use of a Nb alloy – Nb-X, where X is a solute element that can be selectively oxidized to form oxide particles within the Nb3Sn. Such oxide particles not only refine Nb3Sn grain size, but also have the proper size to act as artificial pinning centers (APC) directly restraining fluxon motion, and thus dramatically improve superconducting properties. In this article we show that the size and volume fraction of the oxide particles determine both the levels of grain refinement and the shift in the peak field of the flux pinning force (Fp-B) curve. We explore the factors influencing the microstructure and properties, which we find include: selection of the solute element X, solute content, O content, and heat treatment. For the selection of X, we searched the periodic table for all promising candidates but focused down on the group-IVB elements (Ti, Zr, Hf) here as the drawability of Nb3Sn wires made from Nb-Ti, Nb-Zr, and Nb-Hf alloys has been demonstrated in the past few decades. We found that while internally oxidizing Nb-1.5 at.%Ti led to negligible grain refinement and Fp-B peak shift, Nb-Zr alloys led to much more dramatic results, and internally oxidizing Nb-Hf alloys led to the strongest grain refinement and Fp-B peak shift. For Hf alloying in particular, we compared our internal oxidation method with another method for grain refinement, which uses Hf alloying itself without oxidation, and found that internal oxidation led to much stronger grain refinement and Fp-B peak shift. We also found that higher solute content and lower reaction temperature led to stronger grain refinement and Fp-B peak shift. We conclude with a discussion of the possible mechanisms for the influence of these factors.

A neural-network based framework of developing cross interaction in alloy embedded-atom method potentials: application to Zr–Nb alloy

Interaction potentials are critical to molecular dynamics simulations on fundamental mechanisms at atomic scales. Combination of well-developed single-element empirical potentials via cross interaction (CI) is an important and effective way to develop alloy embedded-atom method (EAM) potentials. In this work, based on neural-network (NN) models, firstly we proposed a framework to construct CI potential functions via utilizing single-element potentials. The framework contained four steps: (1) extracting characteristic points from single-element potential functions, (2) constructing CI functions by cubic spline interpolation, (3) evaluating the accuracy of CI functions by referring to first-principle (FP) data, and (4) searching for reasonable CI functions via NN models. Then with this framework, we developed a Zr–Nb alloy CI potential utilizing the MA-III (pure Zr potential developed by Mendelev and Ackland in 2007) and the Fellinger, Park and Wilkins (FPW) (pure Nb potential developed by FPW in 2010) potentials as single-element parts. The calculated results with this Zr–Nb alloy potential showed that: (1) the newly developed CI potential functions could simultaneously present the potential-function features of Zr and Nb; (2) the normalized energy–volume curves of L12 Zr3Nb, B2 ZrNb and L12 ZrNb3 calculated by this CI potential reasonably agreed with FP results; (3) the referred MA-III Zr and FPW Nb potentials can satisfactorily reproduce the priority of prismatic slip in Zr and the tension–compression asymmetry of 〈111〉{112} slip in Nb, while other ab initio developed Zr–Nb alloy potentials cannot. Our study indicates that, this NN based framework can take full advantage of single-element potentials, and is very convenient to develop EAM potentials of alloys; moreover, the new-developed Zr–Nb alloy EAM potential can reasonably describe the complicated deformation behaviors in Zr–Nb systems.

Electrochemical study of Zr-1Nb alloy with oxide coatings formed by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) coatings were formed on Zr-1Nb alloy in electrolyte containing 9 g/L sodium silicate, 5 g/L sodium hypophosphite, and 6 g/L submicron yttrium oxide powder at current densities 20, 30, and 40 A/dm2. The coatings surface morphology was studied by scanning electron microscope. The electrochemical behavior of coated and uncoated samples was investigated after 1-and 7-days exposure in 10 % HCl. The samples with PEO coatings formed at 30 A/dm2 current density had the best corrosion protective properties after 1-day exposure in 10 % HCl. After 7-days exposure in 10 % HCl the samples with PEO coatings formed at 30 and 40 A/dm2 current densities showed greater corrosion resistance then samples with PEO coatings formed at 20 A/dm2 current density and uncoated samples.

Niobium-Zirconium Oxynitride Nanotube Arrays Forphotoelectrochemical Water Splitting

We report on the first fabrication of vertically oriented niobium-zirconium oxynitride nanotube arrays and their useas an attractive and robust material for visible-light-driven water oxidation. These nanotube arrays with an average diameter of ∼120nm and very short length ∼90 nm were synthesized via one-step anodization of Nb-Zr alloy sheet in NH4F-containing electrolytes.Ammonolysis of the nanotubes resulted in narrowing the bandgap energy from 3.23 to ∼2.67 eV. The Nb-Zr oxynitride nanotubearrays showed approximately an enhancement of about 1900% over that reported for thin film electrodes made of niobiumoxynitride and 3700% greater than that recorded for nitrogen-doped mesoporous Nb2O5. Mott-Schottky and the valence band XPSanalyses revealed the favorable band positions of the fabricated oxynitride nanotubes with respect to the water redox potentials withvery high charge carriers density. The photocurrent transient measurements revealed the remarkable stability of the fabricatedoxynitride nanotubes.

The structure, mechanical, electronic and thermodynamic properties of bcc Zr-Nb alloy: A first principles study

First principles calculations based on different substitution models are performed to investigate the structural, elastic, thermodynamic, and electronic properties of body-centered cubic (bcc) Zr-Nb alloy which has great potential in biomedicine and nuclear power in the whole interval of concentration. The results show that the calculated lattice parameters decrease linearly with the increase of Nb concentration which are in good accordance with the experimental and other first principles calculations. The structure of bcc Zr is unstable at 0 K, and Nb additions can improve bcc Zr-Nb solid solution mechanical stability. Bcc Zr-Nb disordered solid solution with the relaxed structure deviate from the perfect bcc lattice position and transform into a more stable non-β phase, however, the atoms of the ordered solid solution occupy maintains the ideal sites of bcc structure. The elastic constants Cij, elastic modulus (B, G, E), Poisson’s ratio and elastic anisotropy also have been calculated. When the structure of bcc Zr-Nb alloy is stable, the Nb addition can increase the elastic modulus, B/G ration and Poisson's ratio which will improve the ductility of alloy. The electronic properties have been investigated based on density of states and charge density difference. The bcc Zr-Nb alloy electronic structure is usually composed of metal bonds and covalent bonds, and metal bonds is dominated. Zr50Nb50 has the highest covalent bond ratio. Finally, the thermodynamic properties are evaluated by quasi-Debye model.

Design and characterisation of AlCrFeCuNbx alloys for accident-tolerant fuel cladding

High entropy alloys (HEAs) have been considered as candidate materials used in the accident-tolerant fuel (ATF) cladding of reactor due to their promising oxidation and corrosion resistance. AlCrFeCuNbx (x = 0.3, 0.5, 0.7, 1) HEAs were designed to improve the accident tolerance of nuclear fuel in this paper. The microstructure, hardness, quasi-critical water corrosion resistance and high temperature water vapor oxidation of the AlCrFeCuNbx HEAs were systematically investigated. The results show that when x is 1.0, the alloy has optimal hardness, water corrosion and high temperature water vapor oxidation resistance. The X-ray diffraction (XRD) analyses showed that the AlCrFeCuNbx HEAs contained a base centered orthorhombic structure and intermetallic compound Cu9Al4. The AlCrFeCuNb HEA had the best performance of 16.7 mg/dm2 weight gain comparable with traditional Zr–1Nb alloy used for nuclear fuel cladding under operating conditions of nuclear power plant water environment of 360 °C and 18.6 MPa for three days keeping, and θ-Al2O3 and NbO formed on the surface of the HEA after water corrosion. The HEA with the same composition had also excellent oxidation resistance (36.77 mg/dm2 weight gain) better than other compositions, which was two orders of magnitude less than that of the Zr–1Nb alloy in the environment of 1200 °C water vapor. Protective α-Al2O3 and CrNbO4 formed on the surface of the AlCrFeCuNb HEA, which prevented further oxidation of the alloy.