Effects of dose rate change under irradiation on hardening and microstructural evolution in A533B steel

Iron-ion irradiations were carried out for 0.09wt%Cu A533B steel specimens at 290°C to investigate effects of dose rate change during irradiation; the irradiations consisted of the base irradiation (with an unchanged dose rate) and an additional one with changed dose rates from 1 to 50 times that of the base one. Nano-indentation hardness measurements showed that the increase in hardness was higher for lower dose rate of the base irradiation. A similar trend was identified during the additional irradiation. Transmission electron microscope (TEM) and three-dimensional atom probe (3DAP) analyses were carried out for the quantitative characterization of defect features. Mn/Ni/Si/Cu-enriched clusters and dislocation loops were observed in all specimens. The increase in hardness mainly depended on the formation of the solute atom clusters. The square root of the volume fraction of the solute atom clusters provided a good correlation with the increase in hardness. The effects of dose rate and dose rate change during irradiation were explained by the formation of solute atom clusters.     

Loop formation by ion irradiation in yttria stabilized zirconia

Yttria stabilized zirconia (YSZ) is a candidate material focused as optical and insulating materials in nuclear reactors. Therefore, it is useful to investigate defect formation during irradiation, in order to assess YSZ resistance to radiation damage. In the present study, in situ transmission electron microscopy (TEM) observations were performed on YSZ during 30 keV Ne⁺ ion irradiation in the temperature range of 723–1123 K (using 100 K intervals). Results revealed that damage evolution morphology depends on irradiation temperature. For irradiations below 1023 K, defect clusters and bubbles were formed simultaneously. On the other hand, at 1123 K, only bubbles were formed in the initial stage of irradiation. Loops formed later following the bubble formation. It was also observed that, in the early stage of irradiation above 923 K, larger bubbles were formed along the loop planes compared with other areas.

TEM observations indicated that dislocation loops formed on three kinds of crystallographic planes: namely, {1 0 0}, {1 1 1} and {1 1 2} planes.     

Formation and Growth Process of Dislocation Loops in Zircaloys under Electron Irradiation

We have investigated the formation and growth process of dislocation loops in Zircaloys (Zrys) under electron irradiation using a high voltage electron microscope (HVEM). Dislocation loops are of great importance to degradation phenomena in fuel claddings, such as irradiation growth and reduced ductility in light water reactors. TEM specimens of three kinds of Zircaloys (Zry-2, Zry-4 and improved Zry-2) were irradiated with 1MeV electrons at temperatures from 320 K to 970K in the HVEM. Interstitial-type dislocation loops with the Burgers vector b = 1/3(1120) were formed on the pyramidal or the prismatic planes at the beginning of irradiation. It was found that the nucleation and growth process of those loops follows the kinetics based on the &-interstitial model where di-interstitials act as the nuclei of interstitial dislocation loops. Based on this model, migration energies of interstitials and vacancies were determined to be 0.15–0.22eV and 1.0–1.2eV, respectively, from the irradiation temperature dependence of the density and the growth rate of loops.     

电子辐照条件下高纯铁中位错环演化的多尺度模拟

辐照诱导材料微观结构演化导致的材料力学性能降级或尺寸不稳定性是限制反应堆安全与经济性的关键因素之一。本文基于速率理论建立了辐照诱导材料微观结构演化的物理模型,并开发了模拟程序Radieff。采用分子动力学计算了高纯铁中缺陷的形成能、结合能、迁移能以及间隙原子位错环的构型,在此基础上模拟了电子辐照诱导高纯铁内位错环的演化过程,并与实验结果进行了对比。基于分子动力学的计算结果表明,当间隙原子团簇包含3个间隙原子时,团簇的排列方式为〈110〉构型,间隙原子团簇包含4个以上间隙原子时,团簇排列方式变为〈111〉构型。此外基于Radieff研究了400~600K温度范围内,损伤速率为1.5×10-4 dpa/s电子辐照条件下,位错密度对位错环演化的影响,位错密度对位错环数密度及其平均尺寸的影响取决于位错以及间隙原子团簇对间隙原子的阱强度;在464K和550K温度下辐照,位错环数密度及其平均尺寸分别在位错密度增加到1011 cm-2和1010 cm-2后急剧减小,这是由于此时位错对间隙原子的阱强度会大于间隙原子团簇对间隙原子的阱强度。     

注氘低活化马氏体钢在电子辐照下的缺陷行为

低活化铁素体/马氏体(RAFM)钢被视为国际热核聚变反应堆以及聚变反应堆的第1壁候选结构材料之一,很多国家均在研究不同的RAFM钢,中国低活化马氏体(CLAM)钢的研究亦正在进行。核聚变会产生氢、氦、氘及氚,这些气体元素与辐照缺陷结合在一起,对材料的辐照性能会产生较大影响。本文对注氘后不同温度下的辐照后微观结构进行研究。试验利用日本北海道大学的JEOL-1300高压电子显微镜研究注氘CLAM钢从室温到873K在1250keV电子辐照下的微观结构变化。研究结果表明,在电子辐照下,注氘产生的缺陷团会出现消失和长大两种现象,意味着间隙型与空位型位错环在注氘过程中同时产生。并研究了注氘产生的空洞。     

非均匀形核对辐照诱导钨内微结构演化行为影响的团簇动力学模拟

金属钨(W)及其合金作为未来聚变堆最具应用前景的面向等离子体结构材料(PFMs),其服役性能直接影响聚变堆长期服役的安全性,辐照诱导W及其合金内微结构演化导致的辐照脆化现象始终是限制其工程应用的关键因素。本文基于分子动力学计算结果,进一步完善了辐照诱导材料微结构演化行为的团簇动力学模型,采用更加完备的物理模型描述材料内辐照缺陷的产生行为,并进一步探讨了W基体内辐照缺陷产生过程对微结构演化行为的影响。模拟结果表明,高能初始离位原子(PKA)诱发级联碰撞直接产生的缺陷团簇是W内位错环、空洞演化中最重要的形核机制;非均匀形核所产生的间隙团簇的扩散行为对位错环的长大行为有重要影响,会导致位错环尺寸分布中出现亚尖峰与台阶状形貌。     

Evaluation of Neutron Cross Sections of 241Pu in Unresolved Resonance Region

Abstract

Damage structure produced in stainless steel by “in-situ” irradiation with 1 MV electron in a high voltage electron microscope has been observed as a function of irradiation temperature. Small hardly resolvable dislocation loops were produced by the irradiation at 300°C and below. Hexagonal-, diamond- and irregular-shaped loops were observed at 400°C irradiation. During prolonged irradiation the diamond-shaped loops directly developed into dislocation network, while the other faulted hexagonal and irregular shaped loops grew to be dislocation networks after unfaulting. At 500°C irradiation, dislocation loops grew rapidly to be dislocation lines and void nuclei appeared with strain field images. On further irradiation, the void grew larger with little increase in the density. Grain boundary migration toward or away from the highly damaged region was observed at 500°C irradiation.     

Understanding silicon-rich phase precipitation under irradiation in austenitic stainless steels

Atom probe samples have been Fe⁺ ion irradiated at different doses (from 0.5 to 10 dpa) and different temperatures (between 300 and 400 °C) in order to understand the mechanism of formation, under irradiation, of Si-rich phases in austenitic stainless steels. Atom probe results show the presence of Si-enriched clusters which can also be enriched in Ni and depleted in Cr. Number densities of solute clusters can be linked to number densities of dislocation loops already observed by transmission electron microscopy in a previous work. This suggests that solute clusters are formed by heterogeneous precipitation on dislocation loops. Furthermore, the evolution of the composition of solute clusters as a function of the irradiation temperature is consistent with a radiation-induced mechanism. Results are also compared with previous results obtained after neutron irradiation at lower dose rate (in term of dpa s⁻¹). The comparison is, here again, consistent with the radiation-induced mechanism. Thus, Si-rich clusters may be formed by radiation-induced segregation to dislocation loops. Results also show that Si is probably dragged to sinks via the interstitial mechanism.     

TEM investigation of irradiation damage in single crystal CeO2

In order to understand the evolution of radiation damage in oxide nuclear fuel, 150-1000 keV Kr ions were implanted into single crystal CeO₂, as a simulation of fluorite ceramic UO₂, while in situ transmission electron microscopy (TEM) observations were carried out. Two characteristic defect structures were investigated: dislocation/dislocation loops and nano-size gas bubbles.The growth behavior of defect clusters induced by 1 MeV Kr ions up to doses of 5 × 10¹⁵ ions/cm² were followed at 600 °C and 800 °C. TEM micrographs clearly show the development of defect structures: nucleation of dislocation loops, transformation to extended dislocation lines, and the formation of tangled dislocation networks. The difference in dislocation growth rates at 600 °C and 800 °C revealed the important role which Ce-vacancies play in the loop formation process. Bubble formation, studied through 150 keV Kr implantations at room temperature and 600 °C, might be influenced by either the mobility of metal-vacancies correlated with at threshold temperature or the limitation of gas solubility as a function of temperature.     

金属钨级联碰撞中势函数的影响

原子间相互作用势函数的精确性会影响分子动力学级联碰撞模拟结果的精确性。本文选取5种典型的金属钨势函数进行比较测试,通过分子动力学方法,用能量为10 keV和50 keV的初级碰撞原子进行级联碰撞模拟,讨论和分析了辐照过程中缺陷的产生、缺陷团簇和位错环的分布。结果表明：对于最终稳定状态下的弗兰克尔对的数目,不同势函数的模拟结果没有明显差别,而对于缺陷的空间分布、缺陷团簇分数及位错环的分布,不同势函数的模拟结果各有特点。本文结果为用于辐照级联模拟势函数的选择提供了参考,也为钨基势函数的进一步优化提供了指导。     

双束（H+/e-）辐照下氢对12Cr-ODS铁素体钢组织损伤影响研究

利用氢离子（H⁺）束和电子（e^-）束双束（H⁺/e^-）同时辐照用化学浸润法制备的新型12Cr-ODS铁素体钢,研究其辐照损伤效应及组织变化。实验结果表明：由于氧化物的钉扎,基体内保持低密度位错网络;辐照初期随辐照剂量的增加,缺陷团在位错线上及其周围形成,尺寸增加,密度不断增大,并形成间隙型位错环;不同温度下辐照均产生小尺寸高密度的空洞,随辐照剂量的增大,空洞长大速度降低,空洞密度缓慢减小;不同温度下,辐照剂量达15dpa时,空洞肿胀均小于0.15%。对辐照产生的点缺陷与氢相互作用进行理论分析,12Cr-ODS铁素体钢在623~823K经双束辐照后,表现出良好的抗辐照损伤性。     

Understanding of copper precipitation under electron or ion irradiations in FeCu0.1 wt% ferritic alloy by combination of experiments and modelling

This work is dedicated to the understanding of the basic processes involved in the formation of copper enriched clusters in low alloyed FeCu binary system (FeCu0.1 wt%) under irradiation at temperature close to 300 °C. Such an alloy was irradiated with electrons or with ions (Fe⁺ or He⁺) in order to deconvolute the effect of displacement cascades and the associated generation of point defect clusters (ion irradiations), and the super-saturation of mono-vacancies and self-interstitial atoms (electron irradiation). The microstructure of this alloy was characterised by tomographic atom probe. Experimental results were compared with results obtained with cluster dynamic model giving an estimation of the evolution of point defects (free or agglomerated) under irradiation on the one hand and describing homogeneous enhanced precipitation of copper on the other hand. The comparison between the results obtained on the different irradiation conditions and the model suggests that the point defect clusters (dislocation loops and/or nano-voids) created in displacement cascades play a major role in copper clustering in low copper alloy irradiated at 573 K.     

500℃下中国低活化马氏体钢电子辐照缺陷行为的研究

利用超高压透射电子显微镜在500℃对两种不同成分的CLAM钢进行了小剂量的辐照实验,研究CLAM钢的辐照效应.发现在高能电子的辐照下,1号CLAM钢的微观结构发生了变化,产生了大量的位错环,研究了位错环形核和长大的规律;2号CLAM钢的微观结构也发生了变化,先产生位错环,而后形成了位错网络.     

Low temperature neutron irradiation effects on microstructure and tensile properties of molybdenum

Polycrystalline molybdenum was irradiated in the hydraulic tube facility at the High Flux Isotope Reactor to doses ranging from 7.2 × 10⁻⁵ to 0.28 dpa at 80 °C. As-irradiated microstructure was characterized by room-temperature electrical resistivity measurements, transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). Tensile tests were carried out between −50 and 100 °C over the strain rate range 1 × 10⁻⁵ to 1 × 10⁻² s⁻¹. Fractography was performed by scanning electron microscopy (SEM), and the deformation microstructure was examined by TEM after tensile testing. Irradiation-induced defects became visible by TEM at 0.001 dpa. Both their density and mean size increased with increasing dose. Submicroscopic three-dimensional cavities were detected by PAS even at 0.0001 dpa. The cavity density increased with increasing dose, while their mean size and size distribution was relatively insensitive to neutron dose. It is suggested that the formation of visible dislocation loops was predominantly a nucleation and growth process, while in-cascade vacancy clustering may be significant in Mo. Neutron irradiation reduced the temperature and strain rate dependence of the yield stress, leading to radiation softening in Mo at lower doses. Irradiation had practically no influence on the magnitude and the temperature and strain rate dependence of the plastic instability stress.     

The collapse of defect cascades to dislocation loops

We describe a number of experiments that we have recently performed to investigate the collapse of defect cascades to dislocation loops. This important ion and neutron irradiation phenomenon has been studied with in situ ion bombardment using the High Voltage Electron Microscope-Ion Accelerator Facility at Argonne National Laboratory in Cu₃Au, Cu, and Fe at temperatures of 30 and 300 K and in Ni at 30, 300 and 600 K. These experiments have demonstrated that individual defect cascades collapse to dislocation loops athermally at 30 K in some materials (Ni, Cu and Cu₃Au), while in Fe overlapping of cascades is necessary to produce dislocation loops. A slight sensitivity to the irradiation temperature is demonstrated in Cu₃Au and Fe, and a strong dependence on the irradiation temperature is seen in Ni. This phenomenon of cascade collapse to dislocation loops in metals at 30 K provides an understanding for previous neutron irradiation data. The more detailed dependencies of the collapse probability on material, temperature, bombarding ion dose, ion energy and ion mass contribute much information to a thermal spike model of the collision cascade which we will describe.     

Effect of ion irradiation and indentation depth on the kinetics of deformation during micro-indentation of Zr-2.5%Nb pressure tube material at 25 °C

Micro-indentation creep tests were performed at 25 °C on radial-normal samples cut from Zr-2.5Nb CANDU pressure tube material in both the as-fabricated condition and after irradiation with 8.5 MeV Zr⁺ ions. The average indentation stress, and hence the yield stress, was found to increase with decreasing indentation depth and with increasing levels of ion irradiation. The activation energy of the indentation creep rate and hence the, activation energy of the obstacles that limit the rate of dislocation glide, was independent of indentation depth but increased from ΔG₀ = 0.185 to 0.215 μb³ with increasing ion irradiation damage. The magnitude of the activation energy indicates that ion irradiation introduces a new type of obstacle into the microstructure which reduces the low temperature indentation creep rate of Zr-2.5Nb pressure tubes. This is supported by TEM images showing that Zr⁺ ion irradiation produces small, nanometer size, dislocation loops which act as obstacles to dislocation glide and thus influence both the yield stress and the activation energy of the low-temperature thermal creep of Zr-2.5Nb pressure tube material. These findings suggest that neutron irradiation will have similar effect upon yield stress and low-temperature thermal creep as the Zr⁺ ion irradiation since both create similar crystallographic defects in Zr-2.5Nb pressure tubes.     

Radiation damage in vanadium

Transmission electron microscopy has been used to study the microstructure of neutron irradiated single crystal vanadium. These observations revealed the presence of small defect clusters which, upon annealing, grew into resolvable dislocation loops, analyzed as interstitial in nature. The radiation-induced clusters and loops were stabilized during annealing by interaction with impurity precipitates. Damage shells, ascribed to transmutation reactions, were found surrounding certain particles believed to contain boron.     

Formation and annealing behavior of defect clusters in electron or He-ion irradiated Ti-rich Ti–Al alloys

In order to clarify the effect of He atoms on the formation and annealing behavior of defect clusters in Ti–Al alloys, a Ti–47 at.% Al intermetallic compound has been irradiated with electrons and He-ions. Helium-ion irradiation enhances the nucleation of defect clusters, especially of interstitial loops, at temperatures from 623 to 773 K in both γ-TiAl and ₂-Ti₃Al grains of the sample. However, there is little difference between the annealing temperature ranges of defect clusters in TiAl grains formed by He-ion or electron irradiation at 623 K. The dot-shaped clusters and interstitial loops grow scarcely during annealing, but are annihilated by annealing up to 923 K. Cavities are formed after irradiation with He-ions below 10 dpa at 773 K, but no cavities are formed by electron irradiation up to 30 dpa. The cavities in γ-TiAl and ₂-Ti₃Al grains survive after annealing even at 1053 K for 1.8 ks, keeping their density and diameter to be nearly the same as those in the as-irradiated grains.     

Effects of transmutation elements on the defect structure development of W irradiated by protons and neutrons

A series of W-Re-Os alloys were fabricated by arc melting for investigating the effects of transmutation elements of tungsten on the defect structure development. Transmutation electron microscopy has been used to investigate the defect structure for proton-irradiated (E = 1 MeV) W, W-3Re, W-3Os and 0.15 dpa neutron-irradiated (E > 1 MeV) W-5Re-3Os and W, W-3Re, W-5Re and W-26Re. The irradiation-induced voids and dislocation loops which directly cause the irradiation hardening were observed. The results show the combination of W with Re or Os effectively restrains irradiation damage since the number density and radius of both voids and dislocation loops remarkably decrease with increasing Re or Os content.     

Irradiation damage of nickel in a high-voltage electron microscope

Electron irradiation damage in high-purity annealed and 20% deformed nickel has been studied using a high-voltage electron microscope (HVEM) operating at 650 kV. The effects of temperature of irradiation, electron dose and cold work on point-defect clustering in general and void formation in particular have been investigated. Both faulted and unfaulted dislocation loops were observed during irradiation at 240 to 500°C; multilayer dislocation loops were observed at the higher temperatures. Voids exhibited a cubic shape at low dose with a nearly homogeneous distribution in annealed and an inhomogenous distribution in 20% deformed nickel. The average void size for annealed nickel was larger than that for 20% deformed nickel and the void growth rate was found to be higher for annealed nickel. In annealed nickel, the void concentration increased up to ≈14 dpa and then decreased, while in 20% deformed nickel it increased up to ≈35 dpa. Swelling was considerably reduced by cold work compared to annealed nickel. These observations are discussed with emphasis on the role of dislocation density in the nucleation and growth of voids and swelling.