Growth Rate Of Loops Research Articles

Effect of dose rate on the characteristics of dislocation loops in palladium: In-situ TEM analysis during 30 keV H2+ irradiation

Ion irradiation is usually used to simulate neutron irradiation to accelerate the evaluation of the irradiation behavior of reactor materials. However, the validity of using a high damage rate of ion irradiation to simulate a low damage rate of neutron irradiation has always been a controversial topic. Here, the effect of two dose rates (2.94 × 10–6 and 7.35 × 10–5 dpa s–1) on the characteristics and evolution of dislocation loops in palladium was studied in situ during 30 keV H2+ irradiation using transmission electron microscopy. The dose rate obviously affected the nucleation rate and growth rate of dislocation loops, the types (Frank loops or perfect loops) of dislocation loops, and the irradiation hardening and total damage obtained from the product of average loop size and loop density. At the same irradiation dose, a high dose rate would lead to high loop density, small average loop size, low loop growth rate, and low irradiation hardening and damage induced by loops in pure Pd. Meanwhile, it was found for the first time that a high dose rate was beneficial to the generation of perfect dislocation loops. The effect of dose rate was attributed to the different dynamic equilibrium results between the effective generation rate of point defects and their absorption rate by existing sinks. The present results show that the effect of dose rate should be considered when using ion irradiation to simulate neutron irradiation to evaluate the irradiation damage to materials.

Object kinetic Monte Carlo modelling of irradiation microstructures with elastic interactions

Elastic interactions between point defects and sinks, such as dislocations and cavities, affect the diffusion of point defects and are responsible for some of the features observed in microstructures under irradiation. It is therefore necessary to include elastic interactions in kinetic simulations for a quantitative prediction of material properties. In this work a method is presented to accurately and efficiently evaluate the strain field in object kinetic Monte Carlo simulations. It can handle any strain field which is biharmonic, such as the one generated by a dislocation segment or a cavity in isotropic elasticity. A speed-up of several orders of magnitude is obtained compared to the direct summation over strain sources, so that simulations over experimental time scales can be performed within reasonable computation times. The case of a thin foil containing a high density of loops under irradiation is investigated. Loop growth rates are found to depend on the loop radius, as shown experimentally, but more complex effects due to the surrounding microstructure are also highlighted.

Modeling of healing pores of cylindrical form under the action of shock waves in a crystal subjected to shear deformation

Volumetric defects in crystals worsen operational properties of structural materials; therefore, the problem of reducing discontinuities in solid is one of the most important in modern materials science. In the present work, the results of computer simulation are presented that demonstrate possibility of collapse of pores in a crystal in state of shear deformation under the influence of shock waves. Similar waves can occur in a solid under external high-intensity exposure. For example, in the zone of propagation of displacement cascade, there are regions in which occurs a mismatch between the thermalization times of atomic vibrations and the removal of heat from them. As a result of the expansion of such a region, a shock after cascade wave arises. The simulation was carried out based on molecular dynamics method using the potential calculated by means of mmersed atom method. As a bulk defect, we considered extended pores of cylindrical shape, which can be formed after passing of high-energy ions through a crystal, or, for example, when superheated closed fluid inclusions (mother liquor) reach the surface. The study has shown that such defects are the source of heterogeneous nucleation of dislocation loops, contributing to a decrease in the shear stresses in simulated structure. Dependences of the average dislocation density on the shear angle and temperature of the designed cell were established, and the loop growth rate was estimated. Generated shock waves create additional tangential stresses that contribute to the formation of dislocation loops; therefore, in this case, dislocations are observed even with a small shear strain. If during simulation the thermal effect increases, the pore collapses.

Effect of pulse irradiation on the evolution of damage structure

Charged particle beams from accelerators driven by radiofrequency or with beam scanners possess time structures (i.e., pulse beam operations). However, it is not clear whether the irradiation effects of these pulse beam operations are the same as those of continuous beams, although the average damage rate is the same in both cases. In this study, the difference between continuous beams and pulse beams, and the effect of the repetition frequency and pulse duration on the defect accumulation are evaluated by the rate equation analysis for Al and Fe. The growth rate of interstitial type dislocation loops in Al is simulated with fixed duty ratios (constant pulse duration over the irradiation period) at 453 K. The effect of pulse duration on a constant period of 1 s is simulated in Fe, changing the pulse duration from 1 to 10−6 s, while the damage (dpa) caused by one pulse remains the same. The results are analyzed and the effects of pulse duration and repetition rate on the damage structure evolution are demonstrated in relation to materials parameters.

Effects of Helium Implantation and Subsequent Electron Irradiation on Microstructures of Fe-11 wt.% Cr Model Alloy**Supported by the National Natural Science Foundation of China under Grant Nos U1832133, 11475229 and 91426301.

Helium effects on dislocation and cavity formation of Fe-11 wt.% Cr model alloy are investigated. Single-beam (electron) and dual-beam (He+/e−) irradiations are performed at 350°C and 400°C using an ultra-high voltage electron microscope combined with ion accelerators. In-situ observation shows that the growth rate of dislocation loops is reduced in the helium pre-injected specimen. The mean size of cavities decreased in the helium pre-injected specimen. The possible mechanisms are discussed.

Effect of helium and hydrogen synergy on vacancy migration energy in Fe-10Cr model alloy

Effect of helium and hydrogen synergy on vacancy migration energy (Emv) and microstructural evolution of Fe-10Cr model alloy were investigated by single-He, H and sequential-(He + H) ion pre-implantation and subsequent in-situ electron irradiation at different temperature. Growth rates of irradiation induced dislocation loops were experimentally measured at temperatures ranging from 573 to 773 K to calculate Emv. The loop's size increased as a function of irradiation dose. Moreover, pre-implantation with sequential-(He + H) ion showed an increase in the temperature dependence of the loop growth rate compared to implantation with single He and H. The Emv were determined to be 1.4–1.5 eV in single He and H implanted sample. In-situ electron irradiation experimental results indicated that the Emv be increased due to He and H synergy. Foil surface effect on the calculation of the migration energy has been considered also by discussing a new derivation. This synergic effect of He and H on the evolution of dislocation loop and the mobility of vacancy have been discussed.

Effect of precipitates on high-temperature strength and irradiation behavior of vanadium-based alloys

The formation of precipitates and their effect on mechanical properties and irradiation damage in V–4Cr–4Ti alloys have been investigated using transmission electron microscope, high-temperature tensile tests and high-voltage electron microscope (HVEM). V–4Cr–4Ti alloys were aged for 20–40 h at 873 K, and extremely fine precipitates were produced during the aging process, which coarsened with aging time. The results of high-temperature (773, 873 and 973 K) tensile tests showed an increase in strength with increasing aging time (i.e., increasing size of precipitates) while a decrease in the uniform elongation. There was evidence that presence of precipitates strengthened V–4Cr–4Ti alloys. In situ HVEM observations showed that the precipitates restricted the growth rate of dislocation loops induced during electron irradiation at 773 K. The interactions between precipitates and irradiation-induced dislocation loops were discussed. The presence of particles alleviated the increase in irradiation hardening up to an irradiation dose of 3.42 dpa (displacement per atom). The precipitates shrank in size during electron irradiation, which was due to the dissolution of constituent atoms of the precipitates into the alloy matrix.

Undersize solute element effects on defect structure development in copper under electron irradiation

We conducted systematic experiments of defect structure development in Cu base binary alloys under 1000 kV electron irradiation at temperatures higher than 300 K, using in situ observations with high voltage electron microscopy. This report describes the effects of undersize elements: Co (–3.78%), Ni (–8.45%) and Be (–26.45%). The volume size factors are given in parentheses. The amounts of the respective elements were 2, 0.3, 0.05 at.%, or less. In Cu–Ni and Cu–Co and in the reference Cu, temperature dependence of the number density of interstitial-type dislocation loops had a down peak (i.e. loops hardly formed) at approximately 373 K, attributed to unexpected impurity atoms. Above the down-peak temperature, the addition of Co or Ni increased the loop number density through continuous nucleation of loops, extended the loop formation to higher temperatures, and decreased the apparent activation energy of loop growth rate. The addition of Be for 0.3 at.% or more delayed loop formation after formation of stacking fault tetrahedra (SFTs) around 300 K. The apparent mobility of self-interstitial atoms is expected to be smaller than that of vacancies because of strong binding with Be. Loop formation at temperatures higher than 373 K was enhanced by Be for 0.3 or 2 at.%, although it was suppressed greatly for 0.05 at.% or less. All undersize atoms increased the stability of SFTs under irradiation. Mechanisms of those effects were discussed and were briefly compared with earlier results found for oversize elements in Cu.

Microstructure evolution of recrystallized Zircaloy-4 under charged particles irradiation

Recrystallized zirconium alloys are used as nuclear fuel cladding tubes of Pressurized Water Reactors. During operation, these alloys are submitted to fast neutron irradiation which leads to their in-reactor deformation and to a change of their mechanical properties. These phenomena are directly related to the microstructure evolution under irradiation and especially to the formation of <a>-type dislocation loops. In the present work, the radiation damage evolution in recrystallized Zircaloy-4 has been studied using charged particles irradiation. The <a> loop nucleation and growth kinetics, and also the helical climb of <a> linear dislocations, were observed in-situ using a High Voltage Electron Microscope (HVEM) under 1 MeV electron irradiation at 673 and 723 K. In addition, 600 keV Zr+ ion irradiations were conducted at the same temperature. Transmission Electron Microscopy (TEM) characterizations have been performed after both types of irradiations, and show dislocation loops with a <a> Burgers vector belonging to planes close to {101¯0} first order prismatic planes. The nature of the loops has been characterized. Only interstitial <a> dislocation loops have been observed after ion irradiation at 723 K. However, after electron irradiation conducted at 673 and 723 K, both interstitial and vacancy loops were observed, the proportion of interstitial loops increasing as the temperature is increased. The loop growth kinetics analysis shows that as the temperature increases, the loop number density decreases and the loop growth rate tends to increase. An increase of the flux leads to an increase of the loop number density and a decrease of the loop growth rate. The results are compared to previous works and discussed in the light of point defects diffusion.

Effect of impurities on vacancy migration energy in Fe-based alloys

Effects of impurities, such as carbon, nitrogen, helium and hydrogen, on microstructural evolution in pure iron were investigated by means of a multi-beam electron microscope. Growth rate of dislocation loops were measured to calculate vacancy migration energies. In all irradiation temperature conditions, both the size and the number density of dislocation loops were increased as a function of dose. Irradiation with more impurities showed an increase in the temperature dependence of the dislocation loop growth rate compared to irradiation with little impurities. The in situ experiment indicated that the net migration energy of vacancies could be increased due to trapping by impurities, and the effect of C and N on the migration energy of vacancy would be larger than that of W, V, Ta. Furthermore, H and He would increase vacancy migration energy greater than C and N, as well as W, V, Ta. The density functional theory (DFT), applied to the atomic models of BCC iron, indicated an increase in vacancy migration energy by the trapping of impurity atoms, that is a good agreement with this in situ experiment.

The evolution mechanism of the dislocation loops in irradiated lanthanum doped cerium oxide

Cerium dioxide, a non-radioactive surrogate of uranium dioxide, is useful for simulating the radiation responses of uranium dioxide and mixed oxide fuel (MOX). Controlled additions of lanthanum can also be used to form various levels of lattice oxide or anion vacancies. In previous transmission electron microscopy (TEM) experimental studies, the growth rate of dislocation loops in irradiated lanthanum doped ceria was reported to vary with lanthanum concentration. This work reports findings of the evolution mechanisms of the dislocation loops in cerium oxide with and without lanthanum dopants based on a combination of molecular statics and molecular dynamics simulations. These dislocation loops are found to be b=1/3〈111〉 interstitial type Frank loops. Calculations of the defect energy profiles of the dislocation loops with different structural configurations and radii reveal the basis for preference of nucleation as well as the driving force of growth. Frenkel pair evolution simulations and displacement cascade overlaps simulations were conducted for a variety of lanthanum doping conditions. The nucleation and growth processes of the Frank loop were found to be controlled by the mobility of cation interstitials, which is significantly influenced by the lanthanum doping concentration. Competition mechanisms coupled with the mobility of cation point defects were discovered, and can be used to explain the lanthanum effects observed in experiments.

Analysis of helium and hydrogen effect on RAFS by means of multi-beam electron microscope

Effects of helium and hydrogen gas on microstructural evolution in pure iron, Fe–8Cr, and F82H IEA were investigated by means of a multi-beam electron microscope. Growth rate and saturated number density of dislocation loops were measured to calculate migration energies of point defects. In all irradiation temperature conditions, both the size and the number density of dislocation loops were increased as a function of dose. Irradiation with helium and hydrogen showed an increase in the temperature dependence of the dislocation loop growth rate compared to irradiation without helium and hydrogen. On the other hand, little difference was observed in the saturated loop number density at each irradiation temperature. The migration energy of interstitials was determined to be 0.3eV for both types of irradiation. The migration energy of vacancies was determined to be 0.9eV for electron irradiation. This in situ experiment indicated that the net migration energy of vacancies could be increased due to trapping by helium and hydrogen. Irradiation experiments indicated that net migration energy of vacancy in F82H IEA irradiated with helium or hydrogen tended to be higher compared to that in pure iron and Fe–8Cr, which could relate not only to helium and hydrogen, but also to carbon and nitrogen concentration in matrix.

Computer simulations of interstitial loop growth kinetics in irradiated bcc Fe

The growth kinetics of (001) [001] interstitial loops in bcc Fe is studied by phase-field modeling. The effect of defect (vacancy/interstitial) concentration, generation, recombination, sink strength, and elastic interaction on the growth kinetics of interstitial loops is systematically simulated. Results show that the elastic interaction between the defects and interstitial loops speeds up the growth kinetics and affects the morphology of the interstitial loops. Linear growth rate, i.e., the loop average radius is linear to time, under both aging and irradiation are predicted, which is in agreement with experimental observation. The results also show that the interstitial loop growth rate, which is directly related to the sink strength of the interstitial loop for interstitials, increases linearly with the initial interstitial concentration during aging while changing logarithmically with the interstitial generation rate under irradiation.

Multiple-beam irradiation effects in electron-beam-welded F82H joint

Electron-beam-welded reduced-activation ferritic/martensitic steel F82H, which is one of prime candidate materials for experimental fusion reactors, have been irradiated by electron and helium ion beams using a High Voltage Electron Microscope (HVEM) as the experimental evaluation of the modeling and simulations. Growth rate and saturated number density of dislocation loops have been measured to calculate the migration energies of point defects. Electron irradiation experiment indicated that net migration energy of vacancy in the welded metal of F82H tended to be lower compared to that in base metal, which could be relating to difference of carbon concentration in matrix. In the case of dual beam (electron and helium) irradiation, net migration energies of vacancy were slightly higher than that in electron beam irradiation. This result indicates that vacancy would be trapped by implanted helium due to their strong interaction and appeared to have higher migration energy.

Microstructural defects in SiC neutron irradiated at very high temperatures

Microstructures in high purity β-SiC irradiated with fast neutrons (up to ∼9.6 × 10 25 n/m 2, in HFIR) at very high temperatures (1130, 1300, and 1460 °C) were studied by transmission electron microscopy. Cavities and dislocation loops were generally observed in irradiated samples. The cavities were preferentially formed at grown-in stacking faults, and were spherical in shape below 1300 °C and mainly faceted with {1 1 1} planes at 1460 °C. Estimated volume fractions of observed cavities were much smaller than macroscopic densitometer swelling recently reported, which implies other defects cause the swelling in this temperature regime. Larger Frank loops (>25 nm in radius) formed at 1460 °C were identified as interstitial type using the inside/outside method. Unfaulting of the loops was not observed or was very rare. Rapid loop growth and density decrease were observed in the temperature range of 1300–1460 °C concurrently with the rapid cavity growth. The limited growth rate of dense loops at lower temperature was discussed in terms of high sink density estimated from a grain-boundary-loop-denuded zone formed at 1130 °C.

Anisotropic evolution of Frank loops in ion-irradiated silicon carbide

Frank loop evolution in highly damaged polycrystalline cubic silicon carbide irradiated with 5.1 MeV Si 2+ ions at 1673 K was studied by transmission electron microscopy (TEM). Individual TEM images of Frank loops formed on each {1 1 1} plane revealed that their population strongly depended upon their orientation with respect to the incident beam direction. However, no significant difference in loop growth rates was observed between each habit plane. The anisotropic loop evolution has not been reported for neutron-irradiated SiC. Our examination shows that a grain, containing {1 1 1} planes nearly parallel to the irradiated surface, was largely strained toward the free surface by the preferential formation of Frank loops on the plane. Compressive stress following the anisotropic swelling in ion-irradiated specimen may affect the loop evolution. The possible mechanism of the anisotropic loop evolution observed here is accounted for by the stress induced preferential nucleation of Frank loops.

Nucleation and growth of defect clusters in CeO2 irradiated with electrons

Nucleation and growth process of defect clusters in cerium dioxide (CeO2) with fluorite-type crystal structure has been investigated in situ under electron irradiation by using high voltage transmission electron microscopy. Planar defect clusters were formed with electron irradiation ranging from 200 to 1000keV at temperatures below 450K. The defect clusters were determined to be faulted-interstitial type dislocation loops lying on {111} planes. The growth rate of dislocation loops was found to increase with decreasing electron energy. An analysis of the fluence dependence of the growth process of dislocation loops suggests an increase in the vacancy mobility with decreasing electron energy. The rate of the electronic excitation is discussed in terms of the radiation-induced diffusion of oxygen-ion vacancies.

Control of the resistive wall mode with internal coils in the DIII–D tokamak

Internal coils, ‘I-Coils’, were installed inside the vacuum vessel of the DIII–D device to generate non-axisymmetric magnetic fields to act directly on the plasma. These fields are predicted to stabilize the resistive wall mode (RWM) branch of the long-wavelength external kink mode with plasma beta close to the ideal wall limit. Feedback using these I-Coils was found to be more effective as compared to using external coils located outside the vacuum vessel. Locating the coils inside the vessel allows for a faster response and the coil geometry also allows for better coupling to the helical mode structure. Initial results were reported previously (Strait E.J. et al 2004 Phys. Plasmas 11 2505). This paper reports on results from extended feedback stabilization operations, achieving plasma parameters up to the regime of Cβ ≈ 1.0 and open loop growth rates of γopenτw ≳ 25 where the RWM was predicted to be unstable with only the ‘rotational viscous stabilization mechanism’. Here Cβ ≈ (β − βno−wall.limit)/(βideal.wall.limit − βno−wall.limit) is a measure of the beta relative to the stability limits without a wall and with a perfectly conducting wall, and τw is the resistive flux penetration time of the wall. These feedback experimental results clarified the processes of dynamic error field correction and direct RWM stabilization, both of which took place simultaneously during RWM feedback stabilization operation. MARS-F modelling provides a critical rotation velocity in reasonable agreement with the experiment and predicts that the growth rate increases rapidly as rotation decreases below the critical. The MARS-F code also predicted that for successful RWM magnetic feedback, the characteristic time of the power supply should be limited to a fraction of the growth time of the targeted RWM. The possibility of further improvements in the presently achievable range of operation of feedback gain values is also discussed.

Effect of helium on dislocation loop formation and radiation swelling in SiC

SiC and SiC/SiC composites are considered as candidate materials for fusion reactors. In a fusion reactor environment, helium atoms will be produced in SiC up to very high concentrations (15 000–20 000 at. ppm) and therefore it is very important to understand how helium effects radiation swelling of SiC. In this paper a theoretical model of the helium effect on radiation swelling of SiC is suggested. This model is based on considering of kinetic growth of dislocation loops in the matrix taking into account the effect an internal electric field formed near dislocation loops has on diffusion processes of charged point defects. The trapping of helium atoms by vacancies results in an enhanced growth rate of dislocation loops and finally a swelling increase. The theoretical results for radiation swelling are compared with the existing experimental data. It is shown that helium atoms increase the radiation swelling of SiC, especially at high temperatures.

Effect of Helium on Radiation Swelling of SiC

Ceramic materials produced on the basis of SiC and SiC/SiC composites are considered, due to their high-temperature strength, pseudo-ductile fracture behavior and low-induced radioactivity, as candidate materials for fusion reactors. The radiation resistance of ceramic materials under neutron irradiation is one of the key problems, which determines the use of these materials in fusion reactor environment. The dimensional stability (radiation swelling) of SiC is an important problem for degradation of radiation resistance of SiC materials. In fusion reactor environment helium atoms will be produced in SiC in the first wall region up to very high concentrations (15,000–20,000 at.ppm) and therefore it is very important to understand the helium effect on radiation swelling of SiC.In the present paper the effect of helium on radiation swelling of SiC is investigated. Recent experimental results concerning the helium effect on radiation swelling of SiC under neutron, single-ion and dual-beam irradiation are presented. A new theoretical model is suggested for an explanation of the effect of helium on radiation swelling in ceramic materials. Point defects in ceramic materials can have an effective charge (e.g., an F+centre, vacancy with a single trapped electron). The suggested theoretical model is based on kinetic consideration of charged point defect accumulation and kinetic growth of dislocation loops in a matrix of ceramic material (SiC) taking into account the effect of internal electric fields formed under irradiation near dislocation loops on diffusion processes of charged point defects. In this theoretical model, vacancies and small vacancy clusters are considered as additional traps for helium atoms. This results in enhanced growth rate of dislocation loops and finally swelling increase. The obtained numerical and analytical theoretical results for radiation swelling in the presence of helium atoms are compared with the existing experimental data for irradiated SiC material containing helium atoms. It has been shown that helium atoms increase the radiation swelling of SiC.