In situ crack growth observation and fracture toughness measurement of hydrogen charged Zircaloy-4

The effect of hydrogen on the fracture behaviour of a Zircaloy-4 alloy was analysed performing simultaneous fracture mechanics tests of small SE(B) specimens and in situ observation of crack initiation and propagation inside the chamber of a scanning electron microscope. Load and displacement were continuously measured and J_IC, J-R curves and CTOD determinations were obtained. Detailed images of the zone close to the crack tip were taken and the resistance to crack growth was correlated with hydrogen content and hydride morphology. The size and orientation of hydride precipitates showed an important influence on the fracture process. A good agreement with results obtained using standard CT specimens was met.     

In situ study of the oxidation of Zircaloy-4 by ESEM

Zircaloy-4 has been oxidised in environmental scanning electron microscope (ESEM), at a temperature of 700 °C and the surface of the material has been observed during the oxidation, directly in the ESEM. The material has been subsequently analysed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Several aspects of the oxidation were revealed in these experiments. Zircaloy-4 oxidises at a different rate depending on the crystallographic orientation of the grains at the surface. The iron and chromium in the secondary phase particles (SPP) diffuse to the surface and oxidise to produce a profile at the surface of the material. The in situ observations reveal no cracks at the surface during the oxidation. Subsequent examination of the surface (ex situ) by AFM reveals the presence of pores in the oxide. TEM examinations indicate the presence of weak internal planes in the oxide layer, inducing cracks parallel to the metal-oxide interface.     

Characterization and modeling of creep mechanisms in Zircaloy-4

The deformation microstructure and creep mechanisms of Zircaloy-4 have been investigated. Four Zircaloy-4 specimens were tested at different temperatures and stress levels and the deformation microstructures of these specimens were analyzed using transmission electron microscopy. On the basis of microstructural observation of a-type screw dislocations in prismatic slip systems, the modified jogged-screw model has been applied as a rate controlling mechanism for creep of Zircaloy-4. In addition, the stress dependency of dislocation density, jog spacing, and jog height has been evaluated via modeling and experimental observations. The purpose of this study is to provide a detailed understanding of the creep deformation of Zircaloy-4 and prediction of creep rates in this alloy based on the microstructural information obtained from TEM analysis.     

The influence of hydride blisters on the fracture of Zircaloy-4

The fracture behavior under near plane-strain deformation conditions of Zircaloy-4 sheet containing solid hydride blisters of various depths has been examined at 25 and 300 °C. The study was based on material with either model ‘blisters’ having diameters of 2 and 3 mm or a continuous layer of hydride; in all cases, the substrate material contained discrete hydride precipitates. The fracture strains decrease rapidly with increasing hydride blister/layer depth to levels of about 100 μm deep, and then remain roughly constant. For a given blister depth, the material is significantly more ductile at 300 °C than at room temperature although measurable ductility is retained even at 25 °C and for large blister depths. The material is somewhat more ductile if the hydride is in the form of a blister than in the form of a continuous layer (rim). The hydride blisters/layers are brittle at all temperatures, and crack shortly after yielding of the ductile substrate. Consequently, both experimental evidence and analytical modeling indicate that fracture of the sheet is controlled by the crack growth resistance of the substrate at 25 °C. At elevated temperatures, the hydride particles within the substrate are quite ductile, inhibit crack growth, and failure eventually occurs due to a shear instability.     

Influence of the crack-tip hydride concentration on the fracture toughness of Zircaloy-4

The influence of a hydrogen concentration gradient at the crack-tip and hydride platelet orientation on the fracture toughness, fracture mode and micromechanisms of a Zircaloy-4 commercial alloy was studied. Fracture toughness was measured on CT specimens and the analysis was performed in terms of J-integral resistance curves at temperatures ranging from 293 to 473 K. Fracture toughness results of specimens containing higher hydrides concentration near the crack-tip region, preferentially orientated in the crack plane, were compared to those obtained from specimens with a homogeneous hydrogen distribution and different platelet orientation; specimens were obtained by charging them in loaded and unloaded condition, respectively. Changes on both macroscopic and microscopic fracture behaviour were observed at temperatures ranging from 293 to 343 K, and the results show the relevance of both hydride concentration and platelet orientation. The existence of a ductile-to-brittle transition is discussed at the light of these new results.     

Effects of hydrogen content and temperature on fracture toughness of Zircaloy-4

The influence of hydrogen content and temperature on the fracture toughness of a Zircaloy-4 commercial alloy was studied in this work. Toughness was measured on CT specimens obtained from a rolled material. The analysis was performed in terms of J-integral resistance curves. The specimens were fatigue pre-cracked and hydrogen charged before testing them at different temperatures in the range of 293–473 K. A negative influence of the H content on material toughness was important even at very small concentrations, being partially restored when the test temperature increased. Except for some specimens with high H concentration tested at room temperature, the macroscopic fracture behaviour was ductile. The role of Zr-hydrides and Zr(Fe,Cr)₂ precipitates in the crack growth and the dependence with hydrogen content were analysed by observation of the fracture surfaces and determination of the Zr(Fe,Cr)₂ precipitates density on them.     

Effect of hydride orientation on fracture toughness of Zircaloy-4 cladding

Hydrogen embrittlement is one of the major degradation mechanisms for high burnup fuel cladding during reactor service and spent fuel dry storage, which is related to the hydrogen concentration, morphology and orientation of zirconium hydrides. In this work, the J-integral values for X-specimens with different hydride orientations are measured to evaluate the fracture toughness of Zircaloy-4 (Zry-4) cladding. The toughness values for Zry-4 cladding with various percentages of radial hydrides are much smaller than those with circumferential hydrides only in the same hydrogen content level at 25 °C. The fractograghic features reveal that the crack path is influenced by the orientation of zirconium hydride. Moreover, the fracture toughness measurements for X-specimens at 300 °C are not sensitive to a variation in hydride orientation but to hydrogen concentration.     

Comparative studies on high-temperature corrosion of ZrNb1 and Zircaloy-4

A comparative study of the oxidation behaviour of ZrNb1 and Zircaloy-4 was carried out in a steam atmosphere in the temperature range from 700–1100°C. ZrNb1 and Zircaloy-4 are oxidizing approximately according to similar oxidation kinetics. The oxidation rate of ZrNb1 is somewhat lower. It can be described by the equation Δm = 0.4873√t exp(−10261/T). However, remarkable differences are observed in respect to morphology of the oxide and the O-stabilized α-layer, hydrogen uptake, and both the fraction and distribution of the oxygen dissolved in the metal. Above all the rapid drop in ductility by exposure to steam is of significance under safety aspects. Differences in the thermodynamic conditions for equilibrium of the ternary systems Zr---O---Nb and Zr---O---Sn may provide an appropriate explanation of these differences.     

Zr-4板材拉伸性能的研究

研究了变形温度(室温—773K)、晶粒大小对 Zr—4 板机械性能的影响。观察了拉伸后的组织随变形温度及变形量的变化后,认为在673K 拉伸时延伸率最低,是由于动态回复造成变形集中在缩颈区的结果。强度随晶粒减小而增加,但 σ-d-1/2之间不符合 Hall-Petch关系式,这可能与板材中存在织构有关。     

Thermally activated deformation of Zircaloy-4

Tensile tests were carried out on Zircaloy-4 over the temperature range 298–798 K. Yield stress values at the strain rates $\text{1.33 × 10}{}^{\mathrm{?4}}\text{s}{}^{\mathrm{?1}}$ and $\text{6.67 × 10}{}^{\mathrm{?4}}\text{s}{}^{\mathrm{?1}}$ were used to determine the activation parameters. A peak in activation volume ($\text{V}{}_{\mathrm{app}}\text{= 3100 b}{}^3$) was observed at about 690 K; outside this temperature range the activation volumes became almost independent of temperature ($\text{V}{}_{\mathrm{app}}\text{= 200?300 b}{}^3$). The peak in activation volume was explained in terms of a basic rate controlling mechanism and dynamic strain aging. This analysis indicated that the peak could be ascribed to the negative value of the strain rate sensitive solute strengthening term $\text{M}$ and that the mechanism based on the non-conservative motion of jogs appeared to be more favored as the basic rate controlling mechanism of Zircaloy-4 than an impurity mechanism     

Tensile specimen geometry and the constitutive behavior of Zircaloy-4

The influence of tensile specimen geometry on the deformation behavior of flat Zircaloy-4 tensile specimens has been examined for gauge length-to-width ratios that range from 1:1 to 4:1. Specimen geometry has only minor effects on the values of the yield stress, tensile strength, apparent uniform strain at maximum load, and strain-hardening exponent. However, in all geometries but the 4:1 configuration, diffuse necking occurs before maximum load. As a result, strain distributions at maximum load are uniform only in the 4:1 geometry. The elongation to failure is also affected by specimen geometry with the shorter gauge sections exhibiting much higher total elongation values, due in large part to the concomitant specimen necking behavior.     

Cyclic stress-strain response of textured Zircaloy-4

In this paper, the cyclic stress-strain response of textured Zircaloy-4 is investigated at room temperature in an incremental step test using fully reversed tension-compression loading under strain control. The material exhibits an asymmetry of stress response in both rolling and transverse directions, and the corresponding cyclic stress-strain curves can be expressed by a power law relation. Furthermore, phenomenological friction and back stresses are derived from an analysis of hysteresis loop shapes using the Cottrell scheme. It has been shown that the magnitude of the phenomenological friction stress in compression is always higher than that in tension for either rolling or transverse direction. While the magnitude of the phenomenological back stress, being independent of the loading direction, increases much more rapidly in transverse direction than that in rolling direction with increasing the plastic strain amplitude, and the trend in both directions can be expressed by a logarithmic relation. A further discussion suggests that (i) the intergranular thermal stress in the material is responsible for the difference in the phenomenological friction stress between tension and compression, thus leading to the asymmetry of stress response; (ii) the increase of the saturated stress with the plastic strain amplitude stems from the back stress that is primarily a direct consequence of the plastic strain incompatibilities between grains; (iii) the different performance between rolling and transverse directions results from the texture effect.     

Effects of UO2/Zircaloy-4 Mole Ratios on Reaction of UO2 Fuel with Molten Zircaloy-4

Dissolution of UO₂ crucibles by molten Zircaloy-4 (Zry) was investigated in the temperature range of 2,223-2,373 K and for specimens having UO₂/Zry mole ratios between 7 and 18.2. The uranium concentration in the Zry melt rapidly increased during initial reaction time and approached saturated values, depending on reaction temperature and UO₂/Zry mole ratio. Kinetics of uranium concentration increase in the melt was analyzed based on a natural convection mass transfer model that takes into account the change of contact surface area/melt volume ratio with reaction time. The saturated uranium concentration in the Zry melt was inversely proportional to the U0₂/Zry mole ratio. An empirical correlation of saturated uranium concentration in the Zry melt was obtained as a function of UO₂/Zry mole ratios and reaction temperature. This study of the empirical correlation was intended to estimate maximum UO₂ fuel dissolution by molten Zry cladding during severe fuel damage accidents for three different reactor type fuels.     

Oxidation of Zircaloy-4 by oxygen and the production of water

The interaction of isotopic oxygen (¹⁸O₂) with Zircaloy-4 (Zry-4) at 150 and 300 K has been studied using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) methods. AES reveals the oxidation of the Zry-4 surface, reflected in shifts of the Zr(MNV) and Zr(MNN) features by about 5.5 and 3.0 eV, respectively, for both adsorption temperatures. The O(KLL)/Zr(MNN) Auger peak-to-peak height ratios as a function of exposure show the same trends at both temperatures. Following ¹⁸O₂ adsorption at 150 or 300 K, TPD experiments show hydrogen desorption near 400 K that is attributed to the presence of a surface-stabilized form of hydrogen. Additionally, water (H₂¹⁸O and H₂¹⁶O) desorption below 200 K and above 700 K is observed after 150 K oxygen adsorption. However, after oxygen adsorption at 300 K the only significant desorption features are from isotopic water (H₂¹⁸O). These findings indicate that mass transport involving the near-surface region contributes to the observed desorption, and that this behavior is dependent on the original adsorption temperature. Charging experiments using D₂ prior to and after ¹⁸O₂ adsorption were also performed and support our conclusions about the role of surface–subsurface mass transport in this system.     

Zr-4合金的表面晶粒细化研究

通过超声喷丸在Zr-4合金表面获得细晶组织,采用金相显微镜、X射线衍射(XRD)以及场发射扫描电镜(FEG-SEM)观察处理前后锆合金的晶粒组织变化.结果表明,喷丸后样品从表面到芯部依次为:纳米晶(厚度约为60 μm)、超细晶(厚度约为160 μm)和原始组织.喷丸处理的晶粒细化机制为剧烈塑性变形.主要过程包括:孪晶、位错萌生-位错纠缠-晶粒分割-取向随机化.     

Radial-hydride Embrittlement of High-burnup Zircaloy-4 Fuel Cladding

Prestorage drying operations of high-burnup fuel may make Zircaloy-4 (Zry-4) fuel cladding more susceptible to failure, especially during fuel handling, transport, and post-storage retrieval. In particular, hydride precipitates may reorient from the circumferential to the radial direction of the cladding during drying operations if a threshold level of hoop stress at or above a corresponding threshold temperature is exceeded. This study indicates that the threshold stress is approximately 75–80 MPa for both nonirradiated and high-burnup stress-relieved Zry-4 fuel cladding cooled from 400°C and, under ring compression at both room temperature and 150°C, that radial-hydride precipitation embrittles Zry-4. Specifically, the plastic tensile hoop strain needed to initiate unstable crack propagation along radial hydrides decreases dramatically from >8% to lt;1% as radial-hydride fraction increases. Lower hydrogen contents (lr;300wppm) appear to be more susceptible to radial-hydride embrittlement compared to higher contents (>600 wppm), like that found in high-burnup Zry-4.     

High-temperature internal friction in polycrystalline Zirconium and Zircaloy-4

The data on the high-temperature internal friction of zirconium and zirconium alloys are reviewed and new results on zirconium and Zircaloy-4, measured at low and at intermediate frequencies, are presented. It is shown that the damping spectrum of pure zirconium, for annealed polycrystals, shows a peak probably related to relaxation of grain or subgrain boundaries. The data on Zircaloy-4 show two peaks: one near the grain-boundary peak in the pure metal and another one at a higher temperature. Possible mechanisms for these peaks are discussed. Finally, the high-temperature internal friction background of zirconium and zirconium alloys is analyzed and, for Zircaloy-4, the apparent activation enthalpy is found to be related to the grain size.     

Evaluation of the thermal creep behaviors and microstructure of Zircaloy-4 strip

The thermal creep behaviors and the microstructure characteristics for the crept specimens of Zircaloy-4 strip were investigated under a constant load stress in the temperature range of 350-450 °C and a stress range from 110 to 230 MPa. A microstructure evaluation was carried out for the specimens before and after the creep test by using a TEM to understand the correlation between the creep mechanism and the microstructure. A variation of the crystal orientation with the creep deformation was investigated by using the electron back-scattered diffraction (EBSD) analysis. The stress exponent was in the range of 8-10 and the value of the stress exponent was varied by the applied stress. From the analysis of the stress exponent and a TEM microstructural observation of the crept specimens, the creep of Zircaloy-4 strip at a tested condition was mainly controlled by dislocations. From the EBSD results, it was observed that the crystal orientation in the crept sample strained about 10% was not changed as compared to the as-received sample.