Nb Pressure Tube Alloy Research Articles

The radial delayed hydride cracking behavior of Zr-2.5Nb alloy pressure tube at different temperatures

Cantilever beam samples with V-notches were employed to measure the radial delayed hydride cracking rate (DHCR) in Zr-2.5Nb alloy pressure tubes within the temperature range from 120 °C to 250 °C in this study. Both initiation and propagation of DHC were monitored using the direct current potential drop (DCPD) technique. The results show that radial DHCR and temperature fit the Arrhenius relationship within the temperature as mentioned above range, yielding a crack propagation activation energy of 38.66 kJ/mol. It was observed that the incubation periods for radial DHC initiation typically shortened with rising temperature from 180 °C to 250 °C, and the anisotropy of DHCR is also discussed in this study. Besides, the presence of undissolved circumferential hydrides appears to affect radial DHC behavior.

Microstructure and fracture toughness of 850°C quenched and aged Zr-2.5Nb alloy

The Zr-2.5Nb alloy pressure tubes manufactured by Water Quenched and Aged (WQA) fabrication route has reportedly exhibited superior resistance against in-reactor dimensional changes. WQA route comprises three consecutive thermo-mechanical treatments viz., solution heat treatment (SHT), cold working and vacuum aging, which govern the final microstructure and therefore fracture toughness of the pressure tube material. This work describes the influence of SHT soaking duration, degree of cold working and aging temperature on the microstructure and fracture toughness of WQA Zr-2.5Nb alloy. WQA pressure tubes are expected to have better resistance to in-reactor dimensional changes. However, if proper care of hydrogen absorption during thermo-mechanical processing is not taken, though resistance to creep will improve expectedly but fracture toughness at ambient temperature will deteriorate.

Effect of temperature on the delayed hydride cracking rate of Zr-2.5Nb alloy pressure tubes

Delayed hydride cracking (DHC) is a potential failure mechanism of Zr-2.5Nb alloy pressure tubes, employed in heavy-water reactors. This study focused on understanding the DHC behavior of the Zr-2.5Nb pressure tubes within the temperature range 393–523 K. Our findings demonstrate that within this temperature range, the delayed hydride cracking rate (DHCR) and temperature follow the Arrhenius relationship, with the activation energy of crack propagation is found to be 43.16 kJ/mol. The changes in the fracture morphologies at different temperatures are analyzed. Variations in the fracture hydride lengths with temperature and the stress intensity factor K are obtained. Furthermore, it is found that the influencing mechanism of temperature on DHC behavior changes as the temperature increases.

Anisotropy study of hydrogen diffusion along different directions of Zr-2.5%Nb alloy pressure tube using neutron imaging

Hydrogen diffusion studies in Zr-2.5% Nb alloy pressure tube material are important inputs for modeling localized embrittlement phenomenon manifested in pressure tubes. The microstructure of the pressure tube consists of lamellar α-Zr along with the β-phase present as stingers between two alpha laths as well as fine and coarse beta globules. This two-phase microstructure along with a vast difference in diffusivities for the alpha and beta phase necessitates the need for evaluating the diffusion parameters along the three orthogonal directions of the pressure tube. This work focuses on studying the diffusivity values for hydrogen in Zr-2.5%Nb alloy along different pressure tube directions in the temperature range is 250 ˚C -450 ˚C and also correlates it to the microstructure. FEM studies have also been carried out to identify the concentration region that is unaffected by stress imposed due to the presence of a hydride layer. Neutron radiography has been used to study diffusion along the axial, circumferential and radial directions of an as-received un-irradiated pressure tube. It was observed that the diffusion along the axial and circumferential directions is similar while it is slightly lower in the case of radial direction. A theoretical approach has also been followed for evaluating the diffusivity values for the α-Zr and β-Zr phases for the same temperature range.

Anisotropy and variability in thermal creep behaviour of Zr-2.5Nb pressure tube

The Zr-2.5Nb alloy pressure tubes of Indian Pressurized Heavy Water Reactors were manufactured from double melted ingots initially and subsequently from quadruple melted ingots to reduce impurities such as chlorine, phosphorus and carbon. In the current investigation, creep tests were carried out in the stress range of 0.7–0.9 times the yield strength and the temperature range of 350 °C–450 °C to characterize the thermal creep behaviour of Zr-2.5Nb alloy pressure tubes fabricated from double and quadruple melted ingots. The rupture time, minimum creep rate, stress exponent, activation energy and threshold stresses were obtained by carrying out the creep tests of the samples fabricated with their axis parallel to the axial and transverse directions of the pressure tubes. Experimental analysis revealed that the rupture time for double melted tube was 2–3 times lower than that of quadruple melted tubes, whereas no significant change in minimum creep rate was observed. Stress exponent values were obtained in the range of 3–5 signifying dislocation creep as the dominant creep mechanism. To predict the long-term creep properties, the master curves employing the Larson-Miller and Monkman-Grant methods were also evaluated. The effect of microstructural anisotropy, alloying elements and impurities on the thermal creep behaviour of Zr-2.5Nb pressure tube has been investigated.

Effect of manufacturing route on thermal creep behaviour of Zr-2.5Nb pressure tube alloy used in Indian PHWR

Indian Pressurized Heavy Water Reactors (IPHWRs) uses Zr-2.5Nb alloy pressure tube (PT) in cold work and stress relieved condition. These PTs are subjected to extreme conditions of stress, temperature and irradiation. Since past three decades of PHWR operation in India, fabrication routes of PTs for IPHWR has undergone several modifications to enhance its in-reactor performance. The deformation of PT in PHWR occurs due to irradiation creep, irradiation growth and thermal creep. In the present study, the different generations of PTs used in IPHWR are tested for its thermal creep behaviour. The accelerated creep rupture tests were carried out using flat tensile specimens with its axis parallel to the longitudinal direction of the PT in the temperature range of 350 to 450°C under stresses in the range of 0.7 to 0.9 times of yield strength at the respective temperatures. The minimum creep rates were evaluated along with the stress exponents and activation energies. The values of stress exponent and activation energy revealed the underlying creep mechanisms responsible for the creep deformation. Microstructural examination was carried out to understand the need and efficacy of the new fabrication route. The observed thermal creep behaviour of all the generations of pressure tubes were rationalised in terms of their respective texture and microstructures.

Analysis of Dimensional Change in Zr-2.5%Nb CANDU Pressure Tube Material aged at 300-400°C using Neutron Diffraction

The primary boundary of the CANDU reactor consists of Zr-2.5%Nb alloy pressure tubes and SA106 alloy feeder pipes. The pressure tubes are exposed to fast neutron (E>1Mev) irradiation during reactor operation, and experience dimensional changes. The actual dimensional change in the pressure tube during in-service operation is greater than the design estimate. It is not fully understood what is responsible for the dimensional changes in the pressure tubes. In this study, to accelerate the aging effect on the pressure tube material, aging was performed at 300-400°C for up to 20,000 hours and neutron diffraction was used to track variation in lattice spacing, thereby identifying the reason for the dimensional change. The analysis result showed that α-Zr, the main phase of Zr-2.5%Nb, had little dimensional change at 350°C or lower. On the other hand, the aging treatment at 400°C resulted in anisotropic expansion in the α-Zr of the HCP crystal, of 0.02% and 0.08% in the (1010) and (0002) directions, respectively. It was confirmed that above 350oC the β-phase in the Zr-2.5%Nb alloy was decomposed to precipitate β-Nb . The driving force of the lattice expansion may be due to the diffusing out tendency of the super saturated Nb in α-Zr and due to the residual cold working effect applied during the manufacturing process. The enhancing effects of fast neutron irradiation on lattice diffusion are discussed in detail.

Isothermal stress reorientation of hydrides in Zr-2.5Nb pressure tube alloy

• For the first time, in-situ hydride reorientation phenomenon in Zr-2.5Nb alloy has been investigated. • Threshold stress for reorientation of hydrides was determined under isothermal condition using a specially designed setup. • Threshold stress values determined using in-situ and ex-situ methods were comparable. The precipitation of hydrides normal to the transverse direction in Zr-2.5%Nb pressure tube alloy when cooled under stress above a threshold value, is known as stress reorientation of hydrides. The ex-situ threshold stress values reported in the literature were determined using pre-charged samples heated to a peak temperature to dissolve all the hydrides, cooled to reorientation temperature followed by cooling under stress from reorientation temperature. Leger and Donner performed in-situ reorientation experiments using samples having hydride deposited on its surface. In both cases, hydride reorientation occurred during cooling. In the present investigation, for the first time, experiments were done to study the reorientation phenomenon using in-situ gaseous charging of samples under stress at a constant temperature using a specially designed set up. It is expected that the threshold stress determined by in-situ method used in the present investigation will more closely resemble the formation of radial hydrides under reactor operating conditions. In this work, the threshold stress values for reorientation of hydrides in Zr-2.5%Nb pressure tube material were determined using both in-situ and ex-situ methods. The precipitation of hydrides normal to the transverse direction in Zr-2.5%Nb pressure tube alloy when cooled under stress above a threshold value is known as stress reorientation of hydrides. In the present investigation, for the first time the threshold stress (σ th ) for reorientation of hydrides in Cold worked and stress relieved (CWSR) Zr-2.5Nb alloy pressure tube material was determined by using a specially designed fixture shown below consisting of load train comprising sample, pull rod and load cell connected in a series, mounted between two rods welded to a flange. A flexible bellow is welded to flange and pull rod to accommodate the displacement of pull rod and to ensure vacuum leak tightness. Ports are provided in the flange to measure the temperature of sample and to connect the system to vacuum pump. The entire setup after sample loading is inserted into the glass tube and loaded into a resistance heated furnace samples were charged with hydrogen under stress at constant temperature. It is expected that the threshold stress determined by in-situ method will more closely resemble the formation of radial hydrides under reactor operating conditions.

Transformation behavior of hydrides precipitated with or without stress in Zr-2.5Nb investigated by in-situ S/TEM thermal cycling

S/TEM imaging has shown that the hydrides precipitated in Zr-2.5Nb pressure tube alloy under externally applied hoop tensile stress or under stress-free conditions show different morphologies. When precipitating under stress, at the bulk scale the hydrides tend to align towards the radial direction , forming so-called ‘re-oriented’ or radial hydrides. At the grain level, the observed re-oriented hydrides consisted of hydride platelets growing along the tube radial direction. In contrast, the hydrides precipitated under stress-free conditions were orientated along the transverse direction, and consisted of platelets that had grown along the tube transverse direction, and exhibited almost full transformation of the parent Zr grain. Upon heating, TEM samples where both transverse and radial oriented platelets were present, the radial platelets were shown to be more resistant to heating, i.e., dissolving at a higher temperature than the transverse hydrides. The migration of the hydride/Zr phase boundary was observed to follow different directions during dissolution, depending on the morphology of the individual hydrides. Lastly, the texture of the Zr grains where hydrides precipitated was analyzed by electron diffraction . Results showed that hydrides preferred to form in grains with (0002) pole aligned in/or close to the TD (transverse direction)-RD (radial direction) plane when external hoop tensile stress was applied, and the applied stress showed a profound impact on where hydrides precipitated within the available population of differently oriented Zr grains.

Effect of Specimen Thickness on Threshold Stress Intensity Factor (K IH) Associated with DHC in Zr-2.5 Nb Alloy Pressure Tube Material

Abstract Delayed hydride cracking (DHC) behavior for zirconium alloys is characterized in terms of DHC velocity, VDHC, and Threshold Stress Intensity factor, KIH. The effect of specimen thickness on KIH associated with DHC of unirradiated Zr-2.5 Nb pressure tube material was investigated. For this purpose, KI at crack arrest for 1 mm–, 2 mm–, and 4.5 mm–thick specimens was determined using the load drop technique. Significant tunneling of the crack was observed for 1 mm– and 2 mm–thick specimens in the load drop test at 250°C. The KI at crack arrest was found to be increasing with the decrease in specimen thickness. However, extended finite element model (FEM) simulation revealed that KI at crack arrest in the center of the 1-mm specimen and 2-mm specimen is comparable to the values 9.2 ± 0.5 MPa.m1/2obtained for the 4.5 mm–thick specimen. Hence, if tunneling is not present in the specimen, the KIH was found to be independent of specimen thickness.

Analysis of texture and grain shape effects on the yield anisotropy of Zr-2.5wt%Nb pressure tube alloy using crystal plasticity finite element method

After annealing the Zr-2.5wt%Nb pressure tube material at 700 ∘ C for 24h, the largest grain aspect ratio reduced from 3.1 to 1.8, and the average minimum grain size increased from 0.2 μ m to 0.9 μ m . In addition, Kearns’ factors characterizing texture along specific directions became more anisotropic by the annealing. While the annealed microstructure resulted in the decreases of all compressive strengths along three directions—transverse, axial, and radial, the anisotropy in the yield strengths displays uneven changes. The ratio of the transverse and axial strengths remains nearly constant. On the other hand, the ratio of the radial and the axial becomes lower. In the study, the origin of the changes in the strength anisotropy was investigated by the crystal plasticity finite element method (CPFEM). To consider the alteration of grain morphology, the grain shape model is included in the material constitutive model using a well-known Hall–Petch type relation. Through iteratively comparing the calculated yield strengths to the compressive yield strengths, the best-estimated critical resolved shear stresses (CRSSs) of the deformation systems were obtained without and with the grain model. In the latter case, the dependencies of CRSSs on the grain size were found as well. Although the effect of the grain model on the yield strengths was apparent, its influence on determining the anisotropic ratio was insignificant. On the other hand, changing texture information led to a quite definite variation in the yield anisotropy. It implies that the decrement in the ratio of the radial and the axial yield strengths could be caused mainly by the texture modification after annealing. In summary, the anisotropy in the yield strengths of the Zr-2.5wt% pressure material is mainly controlled by the texture and partly affected by the grain morphology.

Investigation of hydrogen diffusivity in Zr-2.5%Nb alloy pressure tube material using Metallography and Neutron Radiography

Zr-2.5%Nb alloy is used as pressure tube material in Indian pressurised heavy water reactors (PHWR). Hydrogen diffusion studies have been carried out in this alloy in the temperature range 523 K-723 K. Samples were prepared by electrolytic hydrogen charging at one end and then subsequent annealing at said temperatures to form a concentration gradient. Neutron radiography and for the first time, metallography techniques have been used for hydrogen content profiling. Calibration studies for hydrogen in Zr-2.5%Nb alloy have also been carried out at Dhruva imaging beamline to determine the absolute hydrogen content from neutron transmission values. The hydride area fraction has been used as the measure of hydrogen content in metallography studies. The absolute hydrogen content for radiography and hydride area fraction for metallography; vs distance data were fitted with error function to obtain diffusivities at various temperatures. The diffusivity data were further processed to obtain the temperature-dependent diffusion coefficients for both techniques in the temperature range of 523 K-723 K. The results obtained from both techniques are consistent, establishing the fact that a simple lab-based technique like metallography can also be used for diffusion studies.

Effect of β Phase Decomposition on Recrystallization in α-Zr Region in Zr-2.5%Nb Pressure Tube Material

The effects of β phase decomposition on recrystallization and texture variation in Zr-2.5% Nb alloy pressure tube material were investigated. Isothermal annealing was conducted at 350 to 550 <sup>o</sup>C for 240 hours, and isothermal annealing was performed at 500 <sup>o</sup>C for 240 to 3,000 hours. The recrystallization and texture variation were analyzed by inverse pole figure variation using the XRD and EBSD methods. Annealing in α-Zr region at below 610 <sup>o</sup>C induced recrystallization and texture variation in the α-Zr. These results differ from those from a previous annealing study of the α+β region at 750-830 <sup>o</sup>C. Annealing above 400 <sup>o</sup>C for 240 hours caused β-Zr decomposition into β-Nb. The decomposition of the β-phase by annealing above 475 <sup>o</sup>C caused a contraction of 7.5% in the d(110) spacing in the β-phase, and a reduction in the volume fraction of the β phase by about 80%. It seems that the stress internally formed by the lattice contraction of the β-phase provides the driving force for recrystallization. In addition, it suggests that the newly formed α-Zr produced by β phase decomposition provides new nucleation sites for recrystallization and causes texture variation in the α-Zr. The reason why the recrystallization and the texture variation occurs only in the α-Zr stable region at below 610 <sup>o</sup>C is discussed.

Theory model combined with XFEM of threshold stress intensity factor and critical hydride length for delay hydride cracking

In order to determine the threshold stress intensity factor and critical hydride length for delayed hydride cracking in Zr-2.5Nb pressure tube alloy, the distribution of normal stress in the plastic zone of crack tip by the developed method that combines theory calculation with extended finite element method (XFEM) was improved. The fracture process of two-phase composites containing Zr-2.5Nb and hydride precipitate was simulated by XFEM. Based on that, critical hydride length LC corresponding to the theoretical model for K1H was estimated. Meanwhile, to illustrate the validity of theoretical and numerical methods, recent theoretical models and experimental measurements were utilized to verify the results of this paper. The theoretical model of DHC was improved to estimate the critical hydride length corresponding to threshold stress intensity factor. The predicted value of critical hydride length is close to the experimental values.

Localized oxidation of Zr-2.5Nb alloy in high temperature carbon dioxide environment containing hydrochloric acid

Zr-2.5Nb pressure tube alloy with uniform oxide layer was shown to undergo extensive nodular corrosion when exposed to a gas mixture of CO2 and O2 at high temperature in presence of hydrochloric acid. Substantial hydrogen pick-up with radial hydrides at the nodule base was observed. A bare metal surface, however, did not undergo nodular corrosion. A thicker initial oxide was less prone to nodular corrosion. A global mechanism was proposed which postulates that a site with localized damage (chemical/mechanical) of the uniform oxide layer, yet surrounded by the remaining oxide, tends to oxidize at an enhanced rate resulting in nodules.

Effects of cold work, heat treatment and irradiation on the state of iron in β and ω phases of Zr-2.5Nb pressure tube alloy investigated with Mössbauer spectroscopy

The properties of Fe additions in Zr-2.5Nb pressure tube alloy are investigated after plastic deformations, heat treatments below its monotectoid temperature and severe radiation damage by neutrons and ions. Mössbauer spectra showed that in the β-quenched and hot-extruded Zr-2.5Nb about 80% of Fe atoms is located in βZr and the reminder in a metastable ω-phase formed within βZr filaments retained in αZr grain boundaries. The fraction of Fe in ω-phase increased to ∼40% after heat treatment at 400 °C for 24 h in vacuum, and also after intense cold rolling, service in the reactor core and ion implantation. In samples with β-phase fully transformed at ∼560–580 °C to βNb, Fe in ω-phase didn't appear after any cold work, while in untransformed samples the fraction of Fe in ω-phase clearly increased with the extent of cold-rolling. A large s-electron density observed at 57Fe nuclei in ω-phase suggests that iron atoms in this phase are mostly located in low-volume B-sites, with hybridized sd2 trigonal, partially covalent orbitals.

Effects of heavy ion irradiation on Zr-2.5Nb pressure tube alloy. I. Orientation dependent mechanical response

In this study, the orientation dependent hardness and creep properties of heavy ion irradiated Zr-2.5Nb pressure tube alloy are investigated by nanoindentation. The indentation tests are conducted along the axial direction (AD) and the transverse direction (TD) relative to the tube. TD samples demonstrate a dependence of the indentation size effect on irradiation damage, which is related to the decrease of the plastic zone size as irradiation damage increases. The hardness of AD and TD samples shows linear dependence on the square root of the irradiation damage density. The transition of the flow pattern from laminar to rotational flow happens in TD samples when the indentation is deeper than 1 μm; rotational flow is expected to be dominant after irradiation. AD samples exhibit laminar flow regardless of indentation depth or irradiation damage. The creep distance is increased for AD while it decreased for TD after irradiation. The creep process in the unirradiated materials and irradiated TD samples is found to be plasticity creep (dislocation glide plus climb). However, for AD samples, the mechanism is changed to power-law creep after 0.6 dpa irradiation. Both the hardness and creep results can be related to the anisotropic deformation mechanisms in the samples.

Study on the effect of Ar9+ ion irradiation of Zr–2.5 wt.% Nb alloy pressure tube

ABSTRACTResponse of Zr–2.5 wt.% Nb alloy pressure tube, used in PHWR nuclear reactors, to 315 keV Ar9+ ion irradiation at room temperature was investigated in the fluence range of 3.1 × 1015–4.17 × 1016 Ar9+ cm−2. Changes in microstructural parameters, viz., the size of coherently scattering domains, microstrain and dislocation density, upon irradiation were ascertained through grazing incidence X-ray diffraction. In general, a decrease in domain size was observed with fluence with a corresponding increase in microstrain and dislocation density. Residual stress measurement showed the development of compressive stresses in place of tensile after irradiation. Transmission electron microscopy showed the formation of dislocation loops of ⟨a⟩-type and ⟨c⟩-type during irradiation. The hardness of irradiated samples, probed through nanoindentation technique, was found to be higher in comparison with unirradiated samples. The above findings have been rationalised on the basis of the defects generated during the Ar9+ ion irradiation.

Orientation dependent evolution of plasticity of irradiated Zr-2.5Nb pressure tube alloy studied by nanoindentation and finite element modeling

The effect of heavy ion or neutron irradiation on the plasticity of Zr-2.5Nb pressure tube alloy is investigated by a combination of nanoindentation and numerical modeling, with indentation tests along the tube axial (AD) and transverse directions (TD). The true stress-strain curves are calculated from nanoindentation load-displacement curves based on an analytical model. The work hardening behaviors of samples loaded in different orientations are compared before and after irradiation. Heavy ion irradiation up to 1.2 × 1015 ion/cm2 significantly increases the hardness of both AD and TD samples. The analytical model can precisely capture the plastic properties of both unirradiated and irradiated material. The calculated true stress-strain curves, including elasticity, yield strengths, and work hardening behaviors of unirradiated and irradiated samples show reasonably good agreement with experimental stress-strain curves derived from uniaxial tensile tests. The change of work hardening exponent due to irradiation shows strong orientation dependency; it decreases from 0.22 to 0.11 in AD but stays almost the same at ∼0.09 in TD after ion irradiation. The work hardening exponent is about 0.16 and 0.08 for neutron irradiation AD and TD samples, respectively. These orientational differences are related to the anisotropic interactions between plastic-flow dislocations and irradiation induced prismatic <a> dislocation loops.

Micropillar compression study on heavy ion irradiated Zr-2.5Nb pressure tube alloy

In this paper, 40 MeV Zr ion irradiation of Zr-2.5Nb pressure tube alloy was carried out, delivering a damage layer 6.5 μm deep. Uniaxial micropillar compression experiments along the tube axial (AD) and transverse directions (TD) were conducted. Irradiation was found to alter the deformation mechanisms and to increase the yield strength more in the AD oriented pillars than TD pillars for irradiation up to 0.6 dpa. This anisotropy in response is discussed in terms of the limitations of the micropillar compression method and the different deformation mechanisms which are activated in each orientation. The anisotropic hardening effect of <a>-type dislocation loops on prismatic <a>, basal <a>, and pyramidal <c+a> slip systems is also discussed. The micropillar compression results were also compared with nanoindentation results conducted on the same material.