Zr Alloy Tube Research Articles

Microstructure characteristics, yield asymmetry and fracture mechanism of the fine grained thin-wall Mg-6.03Zn-0.55Zr tubes fabricated by hot spinning

In this study, thin-wall Mg-6.03Zn-0.55Zr alloy tubes consisted of fine dynamic recrystallized (DRXed) grains and ultrafine grains were successfully obtained by hot spinning process. The results showed that the microstructures of the initial extruded tubes were greatly refined with average grain sizes of 4.46–10.32 µm (300–450 ℃) due to the DRX process, especially at low spinning temperatures. Moreover, the basal texture was transformed from extruded fiber texture to spun tilted plane texture with an angle of ∼10° deviated from the axial direction (AD). More specifically, the basal texture was weakened along AD but was strengthened along the transverse direction (TD) compared with the extruded texture due to the reduced difficulty in pyramidal <c+a> slip activation and the shear deformation induced by the rollers. Consequently, the spun Mg-6.03Zn-0.55Zr alloys showed significant yield asymmetry along AD and TD at room temperature and the increase in spinning temperature had a negative influence on yield strength (YS) that the YS along AD decreased from 290 ± 5 MPa (300 ℃) to 232 ± 2 MPa (450 ℃) and the YS along TD decreased from 236 ± 6 MPa (300 ℃) to 198 ± 4 MPa (450 ℃). Both texture and grain refinement were the important factors in the mechanical properties according to the Hall-Petch relationship due to the varied activation fractions of deformation modes along AD and TD, especially the activation of prismatic <a> slip. Moreover, the fracture mechanism of the spun alloys consisting of fine DRXed grains and ultrafine grains was unusual. The aggregated ultrafine grains played a negative role in the fracture mechanism, because the ultrafine grains were transformed from the dislocation cells and the micro-crack easily occurred in the ultrafine grains.

Optimizing processing performance of Fe-6.5 wt%Si–SiC magnetic abrasive particles by in-situ formation of FexSiy interlayer

The Magnetic Abrasive Finishing (MAF) technology has demonstrated significant potential for precision machining due to its adaptable processing characteristics. However, the current methods for preparing Magnetic Abrasive Particles (MAPs) suffer from issues such as high cost, uneven particle size distribution, and unsatisfactory structure. In this paper, a two-step ball milling method is proposed to address these problems and produce MAPs with a uniform core-shell structure. During the ball milling process, nanoscale iron powder (NIP) is added and forms a coated intermediate bonding layer on the Fe-6.5 Si powder core, increasing the adhesion between the ferromagnetic phase core and the abrasive hard phase. The study found that the addition of 10 wt% NIP as a mass percentage can optimize the MAP structure. During the sintering process at 1169.7 °C, the NIP interface transforms into a metallic interface layer, namely FexSiy (Fe5Si3 and Fe3Si), between the hard phase and the ferromagnetic phase. This intermediate layer combines the hard phase with the ferromagnetic phase, enhancing the magnetic saturation intensity while reducing the coercivity. The developed MAPs were applied to the polishing of Zr alloy tubes and effectively removed localized defects on the outer surface of the tubes. After six polishing cycles, the surface roughness decreased from 0.361 µm to 0.085 µm. This study establishes the correlation between interface behavior and magnetic properties and proposes a new strategy for the preparation of high-performance MAPs.

Establishment of fretting maps of Zr alloy cladding tube mated with Zr alloy dimple in simulated primary water of pressurized water reactor

Fretting maps of Zr alloy tube under grid-to-rod configuration in high temperature pressurised water were established. Running condition fretting map shows three fretting regimes, namely, partial slip regime, mixed fretting regime and gross slip regime. According to material response fretting map, the wear mechanism under partial slip regime is adhesive wear with the character of delamination of local regions, both adhesive wear and cracking are found for mixed fretting regime, while abrasive wear and severe delamination for gross slip regime. Compared with the results at room temperature, the mixed fretting regime range increases obviously at high temperature, while friction coefficient at high temperature is lower. In addition, fretting wear behaviour of Zr alloy tube under different fretting regimes is discussed.

An Orthogonal Experimental Study on the Preparation of Cr Coatings on Long-Size Zr Alloy Tubes by Arc Ion Plating.

Cr-coated Zr alloys are widely considered the most promising accident-tolerant fuel (ATF) cladding materials for engineering applications in the near term. In this work, Cr coatings were prepared on the surfaces of 1400 mm long N36 cladding tubes using an industrial multiple arc source system. Orthogonal analyses were conducted to demonstrate the significance level of various process parameters influencing the characteristics of coatings (surface roughness, defects, crystal orientation, grain structure, etc.). The results show that the arc current mainly affects the coating deposition rate and the droplet particles on the surface or inside the coatings; however, the crystal preferred orientation and grain structure are more significantly influenced by the gas pressure and negative bias voltage, respectively. Then, the underlying mechanisms are carefully discussed. At last, a set of systemic methods to control the quality and microstructures of Cr coatings are summarized.

Modeling of flux-dependent fluence effect on in-reactor deformation in Zr-alloy tubes

Zr-alloy tubes in reactors suffer from harsh thermal-mechanical-irradiation coupling conditions. Abundant modeling efforts have been devoted to predicting in-reactor deformation but the flux-dependent fluence effect was rarely considered. In this work, a polycrystalline model which considers this effect is proposed, by introducing a flux-dependent factor α which characterizes the trend that higher flux results in more accumulated irradiation defects and therefore higher obstruction on dislocation movement. With this model, the flux-dependent anisotropic deformation behaviors as a function of fluence can be well captured. Particularly, a linear relationship between α and flux is revealed from the experimental results in CANDU reactors.

Effect of surface nanocrystallization on high-cycle fatigue behavior of Ti–2Al–2.5Zr alloy tube

Gradient nanostructured surface (GNS) can enhance the properties of metals and alloys. Here, the effect of GNS layers on the lifetime of a Ti-based alloy were investigated. Stress-controlled high-cycle fatigue (HCF) tests were carried out to study the fatigue behavior of Ti–2Al–2.5Zr alloy tubes with different GNS layers obtained via deep rolling (DR) treatment. The fracture failure mechanism was analyzed using the quantitative tilt technique, continuous-slice method, and electron backscatter diffraction. The DR samples had lower surface roughness, an added hardening layer, and better comprehensive mechanical properties than the as-received samples. Furthermore, after 107 cycles of the DR treatment, the fatigue strength of the samples was enhanced from 166.5 to 189 MPa, and the initiation sites of the fatigue cracks moved from the surface to the subsurface. The facets of crack initiation along the slip system in the as-received samples were revealed. The nanocrystals and twins generated by DR treatment were considered to retard crack initiation and propagation, thus increasing HCF life.

Microstructure evolution and strengthening mechanisms of Mg-6Gd-4Y-0.5Zn-0.5Zr alloy during hot spinning and aging treatment

The work aims to investigate the optimized spinning temperature of Mg-6Gd-4Y-0.5Zn-0.5Zr alloy and reveal the microstructure evolution and strengthening mechanism of Mg-6Gd-4Y-0.5Zn-0.5Zr alloy during hot spinning and aging treatment. The study shows that the optimized spinning temperature window of Mg-6Gd-4Y-0.5Zn-0.5Zr alloy ranges from 420 °C to 460 °C, in which the high-performance Mg-6Gd-4Y-0.5Zn-0.5Zr alloy tube with a thickness of 1.5 mm can be successfully fabricated. The multi-pass heating and deformation during hot spinning lead to the precipitation, kinking and break-up of γ′ phase. While the precipitation of γ′ phase aggravates the microstructure inhomogeneity through the thickness of spun tube by restraining dynamic recrystallization. Solid solution strengthening, grain boundary strengthening and shearing strengthening of β′ phase are the dominant strengthening mechanisms of spun Mg-6Gd-4Y-0.5Zn-0.5Zr alloy tube. It is worth noting, finally, that the precipitation of γ′ phase decreases the precipitation density of β’ phase, and thus reduces the age hardening response in the spun tube compared to the extruded Mg-6Gd-4Y-0.5Zn-0.5Zr alloy.

Study on deformation mechanism of Ti-2Al-2.5Zr alloy tube in the flattening test

The flattening test, using one pair paralleled plates to flatten tube structure radially, is generally employed to examine the ductility of tube used in once through steam generator (OTSG). This study focuses on the stress condition and deformation mechanism analysis on the concentrated stress regions of Ti-2Al-2.5Zr alloy tube during the flattening test. Firstly, the finite element analysis was performed using the commercial software ABAQUS to determine the stress condition. Secondly, the Electron Back-Scattered Diffraction (EBSD) was implemented to observe the microstructure evolution of Ti-2Al-2.5Zr alloy. Finally, Schmid law was employed to analyze the activated deformation mechanism under condition of the complex stress. It was found that the condition of the complex stress in stress concentration regions, including tension and compression regions, can be simplified into two directional stresses condition. In grains whose c-axis is nearly towards TD and ND, the strain is mainly accommodated by prismatic slip, whereas in grains whose c-axis deviates about 45° from ND to TD mainly by basal slip. The {10–12} extensive twin and corresponding parent grain orientation mainly relied on the stress condition. Additionally, the intergranular twin pairs connected at common grain boundaries (GBs) with high strain compatibility were found at low angle GBs, which were activated under a combination of macro stress and strain compatibility effect.

Oxidation-resistant coating of FeCrAl on Zr-alloy tubes using 3D printing direct energy deposition

Zr-alloys are used as nuclear fuel cladding tubes in nuclear reactors. Recently, surface coating of conventional Zr-alloy tubes has been performed to enhance their resistance to corrosion and high-temperature oxidation. FeCrAl is a candidate coating material because of its extremely low oxidation rates and desirable mechanical properties. In this study, FeCrAl was deposited directly on a Zr-alloy using 3D printing technology. The processing conditions, such as laser beam power, scanning speed, and powder/gas feed rates, were optimized to attain a dense and adhesive FeCrAl layer. The minimum beam power required to melt the FeCrAl source powder was found to be 160 W. The FeCrAl layer coagulated when the scanning speed was low and the powder feed rate was high, and it became sparse and porous when the scanning speed was high and feed rate was low. The FeCrAl-coated Zr samples were tested for oxidation at 1200 °C. A thin Al2O3 (2–3 μm) formed on the FeCrAl surface, and a diffusion layer developed between the FeCrAl/Zr interfaces. By comparison, a thick ZrO2 (approximately 120 μm) layer formed in the Zr-alloy without coating. The oxidation resistance of the Zr-alloys increased by approximately 50 times by the formation of FeCrAl layer on their surfaces. This 3D printing method is expected to improve the safety of Zr fuel cladding tubes.

Drastic improvement in elongation and impact toughness of Ti–Al–V–Mo–Zr alloy tube via three-step heat treatment

The limited ductility and impact toughness of titanium alloys is a critical issue that limits the use of titanium alloys in the oil and gas industry. In this study, we obtained the highest impact toughness (55.7 J/cm2) for an extruded Ti–Al–V–Mo–Zr alloy tube using a three-step heat treatment. The microstructure and corresponding mechanical properties of the Ti–Al–V–Mo–Zr alloy tube were studied. We achieved significantly improved elongation (69%) and impact toughness (51%) of the Ti–Al–V–Mo–Zr alloy, contribute to the mixing coarse equiaxed and lamellar microstructure. The special heat treatment and bending lamellar microstructures play an important role in this significant phenomenon.

Crack propagation during Charpy impact toughness testing of Ti−Al−V−Mo−Zr alloy tubes containing equiaxed and lamellar microstructures

Numerous reports have elucidated the importance of impact toughness measurements for investigating the damage tolerance of a material. However, the effect of heat treatment parameters on impact toughness and the crack propagation mechanism in titanium alloys has rarely been addressed. Herein, the impact toughness of extruded Ti−Al−V−Mo−Zr alloy that comprises equiaxed and lamellar α microstructures has been investigated by conducting Charpy impact tests. The resulting fracture surface and alloy microstructure were examined by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The obtained results revealed that the colony size decreased after a solid solution and aging treatment, whereas subcritical annealing and slow cooling led to the globularization of lamellar α grains. After globularization, the impact toughness of the alloy increased by 69.2% and 50.0%, respectively; however, the impact toughness of the sample after the solid solution and aging treatment decreased by 61%. During impact testing, the direction of crack propagation was oriented toward the low-angle boundary region. Furthermore, a dissolved secondary phase was formed in the vicinity of the fracture surface, which increased the resistance of the alloy to crack propagation. The present study provides significant guidance for the preparation of titanium alloys with superior impact toughness properties.

Modeling of Annealing Heat Treatment Parameters for Zr Alloy Tube by ANN-Ga

In this paper, there are tow part of module for predicting the Annealing heat treatments of Zr tube. The artificial neural network (ANN) were used for relationship between mechanical property and annealing parameters. The genetic algorithm (GA) were used for Annealing heat treatments of Zr tube. The best ANN network architecture is 2-8-3, and the optimum values of momentum factor is 0.8 while the Crossover is also 0.8 by ANN-GA, which can be efficiently track the effect of annealing Heat treatment on properties for Zr-4 alloy. Keywords: Zr alloy, Heat Treatment, mechanical propert

From Micro to Nano: Material Characterization Methods for Testing of Nuclear Core and Structural Materials

Nuclear grade fuel claddings act as a barrier against release of fuel particles into the coolant water during nuclear power plant operation, handling, and wet or dry storage of the spent fuel rods. The integrity of claddings is always a critical issue during the reactor operation, storage, or loss of coolant accidents (LOCA). After Fukushima disaster, cladding materials are widely studied, as the neutron transparent zirconium (Zr)-based alloys or accident tolerant fuel claddings, which should reduce the high-temperature oxidation rate of Zr-based alloys and enhance the accident tolerance. One part of the long-term investigation of Zr-based claddings in the Research Center Řež contributes to the characterization of material behavior after creep testing and high-temperature oxidation, i.e., conditions that may occur during reactor operation or storage of the spent fuel. Then, the microstructure response can be investigated by means of standard as well as advanced microscopy methods. In this study, we introduce the methodology of the cladding microstructure complex characterization demonstrated on the reference (nonirradiated) Zr-1%Nb alloy after creep testing. In the Zr-1%Nb microstructure, the hydrogen is present under some conditions as the hydride phases ZrH or ZrH2 that cause the mechanical properties degradation. As well, the hydride reorientation can pose a risk. Therefore, currently used methods of hydrides evaluation are presented. Furthermore, we introduce the microstructure analysis of the zircalloy-4 cladding after high-temperature oxidation at the temperature 1300 °C simulating LOCA. For this study, the new automatic chemical analysis procedure was developed. The main issue is the determination of the oxygen concentration and alloying elements behavior after critical increase of oxygen in the Zr-alloy tube at LOCA, also concerning the hydrogen content in the microstructure. Based on the element concentration gradients, the phase transitions from the oxide ZrO2 to the prior β-Zr can be determined and contributes to the phase diagrams experimental verification. In our contribution, the significant role of the electron microscopy methods “from micro to nano-scale” in the nuclear research is emphasized in these two research topics.

On Technological Uses of Local Strain Patterns of the Commercial Zr-Nb Alloys

Plastic flow localization behaviour has been investigated in commercial Zr alloys used in the nuclear power industry. The type of deformation localization pictures corresponding with the stage preceding material fracture is described. Proposed is a method for assessing the reserve ductility of the metal subjected to deformation in rolling of thin-wall Zr alloy tubes by examining the local strain patterns.

원자력산업에서 지르코늄 스크랩 재활용을 위한 세정기술에 관한 연구

본 연구에서는 지르코늄 피복관 제조공정에서 발생되는 스크랩을 원전급(nuclear grade)으로 재활용하기 위해 스크랩 표면에 부착되어 있는 오염물 제거조건을 최적화하였다. 주 오염물은 피복관 제조시 필거링 공정에서 사용하고 있는 수용성 냉각윤활제 잔류물로서 튜브 표면에 압착 및 탄화된 것으로 가정된다. 스크랩 발생 빈도가 높은 <TEX>${\phi}9.50mm$</TEX>, zirlo 합금 튜브를 피 세정 대상물로 선정하여 세정 후 피 세정물 표면에 잔존하고 있는 오염물의 특성분석과 피 세정물의 표면 성분분석으로 세정성을 평가하였다. 세정제별 세정능력을 평가하기 위하여 수산화나트륨(sodium hydroxide) 계열 2종과 수산화칼륨(potassium hydroxide) 계열 3종을 선정하여 비교하였다. 또한 온도 및 초음파 강도에 따른 세정 효과 분석을 위해 상온, <TEX>$40^{\circ}C$</TEX>, <TEX>$60^{\circ}C$</TEX>에서 각각 세정한 결과, 세정온도 및 초음파 강도가 높을수록 세정효과도 높은 것으로 나타났다. 육안검사 결과 수산화나트륨 계열은 초음파 강도와 무관하게 모두 양호한 것으로 나타났으나 수산화칼륨 계열은 초음파 강도 120 W 이상에서 피 세정물의 표면상태가 양호한 것으로 나타났다. 중량측정법에 의한 세정효과 분석결과 수산화나트륨 계열은 세정효율이 97.6% (<TEX>$60^{\circ}C$</TEX>, 120 W)까지 나타났으나 수산화칼륨 계열은 피 세정물의 표면상태 불량으로 중량측정 방법을 적용하는 것이 부적합한 것으로 나타났다. 피 세정물의 표면 오염물 분석 결과 C, O, Ca, Zr 성분이 검출되었으며 그 중 C, O의 성분이 대부분을 차지하였음을 알 수 있었다. 피 세정물의 세정 정도에 따라 C, O 구성 비율의 변화가 큰 것으로 나타났으며 세정이 잘될수록 C의 구성비율이 감소되며 상대적으로 O의 구성 비율이 증가되었다. 본 연구 결과를 바탕으로 산업현장에 적용하기 위하여 세정공정을 알카리세정, 수세, 건조의 3단계로 구분하고 각 단계별로 세정변수를 조정함으로써 세정효과의 극대화를 기대할 수 있다. In this study, we optimized the removal condition of contaminants attached on the scrap surface to recycle the scrap generated from the Zr alloy tube manufacturing process back to the nuclear grade. The main contaminant is remnant of watersoluble cooling lubricant that is used in the pilgering manufacture during the tube production, and it is assumed to be compressed and carbonized on the surface of tube. Zirlo alloy tube of <TEX>${\phi}9.50mm$</TEX>, which has high occurrence frequency of scrap, was selected as the object to be cleaned, and cleaning abilities of reagents were evaluated by measuring the characteristics of contaminants remained and by analyzing the surface of the tube after cleaning process. For evaluation of each cleaning agent, we selected two types of sodium hydroxide series and three types of potassium hydroxide series. Furthermore, to confirm dependence on tempe-rature and ultrasonic intensities, cleaning at the room temperature, <TEX>$40^{\circ}C$</TEX>, and <TEX>$60^{\circ}C$</TEX> was conducted, and results showed that higher the cleaning temperature and higher the ultrasonic intensity, better the cleaning effect. As a result of the bare-eye inspection, while the use of sodium hydroxide provided satisfactory condition on the tube surface, the use of potassium hydroxide series provided satisfactory condition on the tube surface only when the ultrasonic intensity was over 120 W. In the cleaning effect analysis using the gravimetric method, cleaning efficiency of sodium hydroxide series was as high as 97.6% (<TEX>$60^{\circ}C$</TEX>, 120 W), but since the tube surface condition was poor after the use of potassium hydroxide, the gravimetric method was not appropriate. In the analytical result of surface contaminants on the tube surface, C, O, Ca, and Zr were detected, and mainly C and O dominated the proportion of contaminants. It was also found that the degree of cleaning on the tube affected the componential ratio of C and O; if the degree of cleaning is high, or if cleaning is well-conducted, the proportion of C is decreased, and the proportion of O is increased. Based on these results, optimal cleaning for application in the industry can be expected by categorizing cleaning process into three steps of Alkali cleaning, Rinsing, and Drying and by adjusting cleaning parameters in each step.

Hydride-induced degradation of hoop ductility in textured zirconium-alloy tubes: A theoretical analysis

Hydride-induced degradation of hoop ductility in Zr-alloy tubular components has been studied for many years because of its importance in the nuclear industry. In this paper the role of intergranular and intragranular δ-hydrides in the degradation of ductility of the textured Zr-alloy tubes is investigated. The correlation among hydride distribution, orientation and morphology in the tubes is formulated based on thermodynamic modeling, and then analyzed. The results show that the applied stress, the crystallographic texture of α-Zr matrix, the grain-boundary structure, and the morphology and size of Zr grains simultaneously govern the site preference and the orientation of hydrides. A criterion is proposed to determine the threshold stress of hydride reorientation. The hoop ductility of the hydrided Zr tubes is discussed using the concept of macroscopic fracture strain. It is shown that the intergranular hydrides may be more deleterious to ductility than the intragranular ones. This work defines a general framework for understanding the relation of the microstructure of hydride-forming materials to embrittlement.

軸方向にき裂を有するZr合金管のき裂伝播と破壊 第2報 き裂伝播速度におよぼす平均応力および板厚の影響とZr合金管の破壊実験

軸方向にき裂を有するZr合金管のき裂伝播と破壊 第2報 き裂伝播速度におよぼす平均応力および板厚の影響とZr合金管の破壊実験

軸方向にき裂を有するZr合金管のき裂伝播と破壊 第3報 き裂の急速伝播と圧力および歪応答の測定結果について

軸方向にき裂を有するZr合金管のき裂伝播と破壊 第3報 き裂の急速伝播と圧力および歪応答の測定結果について

Examination of residual stresses and texture in zirconium alloy cladding tubes after a large plastic deformation: Experimental and numerical study

X-ray diffraction is used to assess the development of residual stresses within the grains of Zr alloy tubes processed via cold pilgering. A modified elastoplastic self-consistent model was used to simulate the texture and the internal stresses developments. The influence and the role of elastoplastic anisotropy were also studied and explained in this work. The contribution and the magnitude of the first- as well as the second-order residual stresses were correctly evaluated using information from the model. Comparison between the X-ray diffraction results (texture and residual strains) and the simulations confirms that prismatic slip is the main active deformation mode in this alloy under large strain.

Crack Growth and Fracture of the Zr-alloy Tubes with an Axial Crack

Crack Growth and Fracture of the Zr-alloy Tubes with an Axial Crack