Grain boundary influence on displacement cascades in UO2: A molecular dynamics study

The influence of grain boundaries on the primary damage state created by a recoil nucleus in UO₂ matrix is studied here by molecular dynamics simulations. This study is divided in two steps: (1) the study of the structural properties of several symmetrical tilt boundaries for different misorientation angles ranging from 12.7° to 61.9°; and (2) the study of displacement cascades near these grain boundaries. For all the grain boundaries studied, the structure around the interface up to about 2 nm presents a perturbed but stable fluorite lattice. The type of defect at the interface depends directly on the value of the misorientation angles. For the small angles (12.7° and 16.3°) the interface defects correspond to edge dislocations. For higher misorientation angles, a gap of about 0.3 nm exists between the two halves of the bicrystal. This gap is composed of Schottky defects involving numerous vacancies along the interface. About 10 keV displacement cascades were initiated with an uranium projectile close to the interface. In all the cases, numerous point defects are created in the grain boundary core, and the mobility of these defects increases. However, cascade morphologies depend strongly on the grain boundary structure. For grain boundaries with edge dislocations, the evolution of the displacement cascades is similar to those carried out in monocrystals. On the other hand, cascades initiated in grain boundaries with vacancy layer defects present an asymmetry on the number of displaced atoms and the number of point defects created.     

Self-diffusion near symmetrical tilt grain boundaries in UO2 matrix: A molecular dynamics simulation study

Molecular dynamics simulations have been carried out to study the influence of grain boundaries in stoichiometric UO₂ on uranium and oxygen self-diffusions over a large range of temperature varying from 300 K to 2100 K. The study was carried out on two symmetrical tilt grain boundaries, Σ5 and Σ41, which have respectively two different atomic structures. Firstly, the study of the temperature effect on the grain boundary core structure is presented. With the raise of temperature, the grain boundary core grows with an increase of disorder. Secondly, self-diffusion near both grain boundaries is studied. It has been found that grain boundaries accelerate the uranium and oxygen self-diffusion rates over several nanometres from the grain boundary interface. Uranium and oxygen self-diffusion are anisotropic, with a high acceleration along the grain boundary interface. Using the self-Van Hove correlation functions, hopping mechanisms were identified for Σ41 in all directions while for Σ5 hopping mechanism takes place along the grain boundary interface and random diffusion appears in the perpendicular direction of the grain boundary plane.     

Effects of fabrication process on microstructure and texture of Inconel 690 tubes for steam generator

The main goal of this research was to investigate the relationship between the grain boundary misorientation and the precipitation of intergranular M₂₃C₆ carbides during the pilgering process and the heat treatment of Inconel 690 tubes for steam generators. The M₂₃C₆ carbides behavior is obviously influenced by the grain boundary character and interfacial energy. The grain boundary misorientation of the Inconel 690 tubes was investigated by electron backscattered diffraction of carbide precipitates at these grain boundaries. Numerous M₂₃C₆ carbide precipitate at the large angle grain boundaries with high interfacial energy.     

α-Fe中〈110〉倾斜晶界的分子动力学研究

采用分子动力学结合不同原子间相互作用势函数研究了α-Fe中具有不同位相差（26°～140°）的〈110〉倾斜晶界的强度、γ面及滑移系,并通过模拟准静态拉伸试验研究了各晶界解理断裂过程。研究结果表明,随着位相差的增加,晶界能量在110°附近出现极小值,Σ3{112}晶界是α-Fe〈110〉倾斜晶界中最稳定的晶界之一;Σ3{111}晶界解理断裂能量计算结果与实验和第一性原理计算结果符合得很好;Σ3{112}晶界解理断裂过程中伴随着孪生与滑移,从而导致其解理断裂能量高于晶界形成能。     

Radiation tolerance in a nanostructure: Is smaller better?

While previous experimental studies suggest that the presence of nanostructure is detrimental to the phase-transition resistance under an irradiation environment, we have recently found an opposite effect - nanostructure enhances phase-transition resistance. Here we analyze the change in free energy of an irradiated single-phase nanocrystalline material and explain the radiation tolerance (characterized by the resistance to phase-transition) in terms of two competing effects: (i) a smaller grain size tends to lower the free energy because the accumulation of point defects (mainly vacancies) in the grain interior is suppressed and (ii) a smaller grain size tends to increase the free energy because the area fraction of grain boundary is larger. For a two-phase nanocrystalline material, the heat of mixing between the two-phases often needs to be sufficiently positive so that the ion-beam mixing is avoided. Our analysis explains all previous experimental results where a nanostructure is found to either enhance or lower the phase-transition resistance.     

Temperature accelerated dynamics study of migration process of oxygen defects in UO2

We studied the migration dynamics of oxygen point defects in UO₂ which is the primary ceramic fuel for light-water reactors. Temperature accelerated dynamics simulations are performed for several initial conditions. Though the migration of the single interstitial is much slower than that of the vacancy, clustered interstitial shows faster migration than those. This observation gives us important insight on the formation mechanism of high-burnup restructuring, including planar defects and grain sub-division (the rim structure), found in UO₂.     

Application of dislocation dynamics to assess irradiation effect on materials with different grain sizes

Irradiation of materials by energetic particles produces defect clusters like vacancies, self-interstitial atoms and stacking-fault tetrahedra. These defect clusters form loops around existing dislocations, leading to their decoration and immobilization, which ultimately leads to radiation hardening in most of the materials. Effect of irradiation on material shear yield strength is analyzed using two-dimensional poly-crystal dislocation dynamics (DD) modelling. The plastic flow in the material is represented as collective behavior of a large number of edge dislocations distributed among many grains. The unit cell is assumed to have grains of hexagonal shape with uniform size. Grain boundaries are considered to be impenetrable to dislocations. The irradiation effects are modelled by taking all dislocations being locked by irradiation defects thus characterizing the fluence. When the total stress on the dislocations exceeds a critical stress value, they get unlocked and become free to move on their glide planes. Typical stress-strain curves for various critical values are obtained for irradiated Aluminium with different grain sizes, which reveal the effect of dislocation loops on increased yield stress as a function of both fluence and grain size. Critical locking stress is correlated to irradiation fluence by using single crystal yield stress values of irradiated Aluminium from DD analysis and corresponding experimentally available yield stress values.     

Advanced TEM characterization of stress corrosion cracking of Alloy 600 in pressurized water reactor primary water environment

Advanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr₂O₃ oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations.     

Study of microstructure and microtexture of modified 9Cr–1Mo steel subjected to high deformation

The Nb–V modified 9Cr–1Mo ferritic steel is an extremely important structural material in the power industry. Present work is focused on how the microstructure, microchemistry and microtexture of this steel evolve during high degree of deformation, starting from the normalized and tempered condition. While the microstructure and microchemistry are analyzed using analytical transmission electron microscopy, the microtexture analysis is carried out using the electron back-scatter diffraction (EBSD) technique. High degree of cold rolling leads to the formation of forests of dislocations and cellular structure of ferritic matrix. The carbides were found to retain their morphology and chemistry after 88% cold work. Analysis of misorientation angles was used to derive the grain boundary character of the normalized and tempered steel and the deformed steel. Analysis of fiber texture reveals that deformation in the material does not lead to predominance of any particular fiber which is useful from fabrication point of view.     

Microstructural evolution of the China Low Activation Martensitic (CLAM) steel irradiated by H and He ion beams

China Low Activation Martensitic (CLAM) steel was irradiated at room temperature with different doses of He⁺ and H⁺ ion beams. TEM indicated that the microstructure of unirradiated CLAM steel consisted of laths, grain boundaries, dislocations and carbides. Electron diffraction patterns revealed that the microstructure of carbides at grain boundaries was primarily dominated by M₂₃C₆ carbide. Vacancy clusters were induced into the matrix after irradiation. TEM-EDX of carbides and matrices of unirradiated and post-irradiated samples were performed to investigate the composition of carbides and the effect of irradiation on the composition of carbides. Carbides from unirradiated and irradiated specimens at grain boundaries were found to be enriched with Cr. For irradiated specimens, concentrations of Cr increased as the irradiation dose was increased. Cr enrichment could lead to precipitation of additional phase.     

Preferential damage at symmetrical tilt grain boundaries in bcc iron

The interaction of collision cascades with a (5 3 0) symmetrical tilt grain boundary in bcc iron has been studied using molecular dynamics simulations. Collision cascades were generated by a principle knock-on atom PKA of 1 keV at various distances from the boundary and with different velocity directions. Simulations reflect how the grain boundary acts as a barrier and also as a region where defects can be easily conducted along the boundary plane. Results show that there exist preferential sites in the periodic structure of the (5 3 0) symmetrical tilt boundary where defects are formed. These sites have been identified together with the atomic rearrangements produced in the grain boundary atomic structure have been analysed. The grain boundary energy increases after radiation. This increase in energy is proportional to the number of defects inside the grain boundary region.     

腐蚀疲劳断裂前氢与缺陷相互作用的正电子湮没研究

用正电子湮没谱和加热释氢研究了低合金高强钢人工海水腐蚀疲劳断裂前经若干小时疲劳的样品。实验结果表明,腐蚀疲劳早期阶段产生的位错、空位团以及钢中晶界是氢的捕获位,其捕获氢量随疲劳时间增加。氢与空位相互作用使空位团尺寸增加,数目增多,氢与位错相互作用促使位错增殖且随疲劳时间增加。晶界与氢相互作用能随疲劳时间、晶界捕获氢量增加而降低。碳化物沿晶析出促进了晶界捕获氢。较低的晶界-氢结合能有利于氢沿晶运动,促使预裂缝沿晶发展。     

The effect of grain boundary misorientation on the intergranular M23C6 carbide precipitation in thermally treated Alloy 690

The precipitation characteristics of chromium carbides on various types of grain boundaries in Alloy 690 thermally treated at 720 °C for 10 h were studied through transmission electron microscopy. Precipitation of the intergranular chromium carbides, identified as Cr-rich M₂₃C₆, was retarded on the low angle grain boundaries, compared to that on the random high angle grain boundaries on which coarse and discrete ones were found. They were rarely found on the coherent twin boundaries, however, needle-like ones were evolved on the incoherent twin and twin related Σ9 boundaries. Precipitation of the chromium carbides was also suppressed on the nearly exact coincidence site lattice boundaries such as Σ11 and Σ15, for which the Brandon criterion was fulfilled. The results of the intergranular M₂₃C₆ carbide precipitation were explained in terms of the influence of the grain boundary energy.     

氦对Fe-Cr-Ni合金和Fe-Cr-Mn合金辐照损伤的影响

利用超高压电镜与高能离子加速器连接装置 ,研究了氦 (He)对Fe Cr Ni和Fe Cr Mn两类奥氏体型合金辐照损伤行为的影响。观察了辐照过程中二次点缺陷的演变、空洞的形成以及辐照诱导晶界处溶质元素浓度的变化。实验结果表明 :He能促进两类合金空洞核心的增加 ,但空洞尺寸和密度不同 ;He能有效抑制辐照诱起晶界元素偏析 ,但对不同原子尺寸的溶质原子抑制效果不同。该差别是由于He的注入提高空位移动激活能和改变点缺陷与溶质原子相互作用的效果     

Measurement of plastic strain of polycrystalline material by electron backscatter diffraction

It is important to know the degree of plastic strain in order to evaluate the susceptibility and crack growth rate of stress corrosion cracking (SCC) in stainless steel and nickel based alloy, because SCC is enhanced by the cold work and causes many problems in nuclear power plant components. In this study, electron backscatter diffraction in conjunction with scanning electron microscopy is applied to measure the plastic strain imposed to stainless steel by tensile load. A new parameter, which quantifies the spread of the crystal orientation within individual grains arising due to dislocation accumulation during plastic deformation, is correlated with imposed plastic strain. The new parameter is called ‘crystal deformation’ and is determined from the spread in misorientation from the central grain orientation. It is confirmed that this parameter has a good correlation with plastic strain and is not affected by the data density of the crystal orientation map. The dislocation density distribution is also evaluated from the misorientation from the central orientation. Relatively high dislocation density was observed near grain boundaries and grain boundary triple points, which was consistent with the observed deterioration of EBSD pattern quality in those locations.     

充氚不锈钢中氦行为的PAL和TEM研究

对充氚和未充氚的抗氢-2(HR-2)不锈钢样品进行退火处理,利用正电子湮没寿命谱(PAL)以及透射电镜(TEM)等技术探讨不锈钢中氦和微缺陷的相互作用。未充氚样品中,退火温度对缺陷态的影响主要表现为偏聚物在晶界的析出。充氚样品实验中,退火温度小于300℃时,充氚不锈钢中的He原子主要通过自捕获机制在晶内缺陷处聚集成泡;热处理温度为300～600℃时,充氚不锈钢中的He原子主要通过热迁移的方式迁移至晶界导致晶界宽化,但晶界处无明显的He泡形成;热处理温度大于600℃时,热平衡空位开始发挥作用,与聚集在晶内缺陷处的He原子结合形成He泡,且随退火温度的升高,He泡有明显聚合长大的现象。     

Interfacial energies between uraniumdioxide and liquid metals

Interfaciai energies between solid uranium dioxide and liquid metals have been investigated for a number of systems. It was found, that at the metal melting point (1) the interfacial energies become approximately identical for all metals (γUO₂-liquid metals ≈ 1.676 ± 0.142 J/m²) with respect to an engineering approach, and (2) the respective contact angles (φ_s > 90°) point out no wettability. Linear temperature functions of interfacial energies in UO₂ -liquid metal systems have been derived. By appropriate extrapolation procedures it is possible to estimate temperature coefficients of interfacial energies for not yet measured solid UO₂ -liquid metal systems.     

Investigation on the formation of serrated grain boundaries with grain boundary characteristics in an AISI 316 stainless steel

The formation of serrated grain boundaries (GBs) depending on the GB characteristics has been investigated by using an electron backscattered diffraction (EBSD) technique and a transmission electron microscopy (TEM) in an AISI 316 stainless steel. It was observed that at the early stage of aging treatment, the GB morphology was changed from flat to wavy at random GBs without any indication of M₂₃C₆ carbide formation, and no GB serration at special GBs (lower than Σ29) was found. The comparison study on the misorientation angle between two neighboring grains indicated that the occurrence of GB serration at random GBs is attributed to the reduction of the total GB energy. Random GBs with high energy tend to be serrated, resulting in the formation of two segments with lower energies. On the other hands, the special GBs may be less likely to form serrated GBs due to their lower GB energy.     

On the rate determining step in fission gas release from high burn-up water reactor fuel during power transients

The radial distribution of grain boundary gas in a PWR and a BWR fuel is reported. The measurements were made using a new approach involving X-ray fluorescence analysis and electron probe microanalysis. In both fuels the concentration of grain boundary gas was much higher than hitherto suspected. The gas was mainly contained in the bubble/pore structure. The factors that determined the fraction of gas released from the grains and the level of gas retention on the grain boundaries are identified and discussed. The variables involved are the local fuel stoichiometry, the amount of open porosity, the magnitude of the local compressive hydrostatic stress and the interaction of metallic precipitates with gas bubbles on the grain faces. It is concluded that under transient conditions the interlinkage of gas bubbles on the grain faces and the subsequent formation of grain edge tunnels is the rate determining step for gas release; at least when high burn-up fuel is involved.