Neutron Irradiated Iron Research Articles

Mechanical Properties of Neutron-Irradiated Armco Iron upon Plastic Deformation at Elevated Temperatures

Mechanical Properties of Neutron-Irradiated Armco Iron upon Plastic Deformation at Elevated Temperatures

Irradiation dose-rate effect in Fe-C system: An Object Kinetic Monte Carlo simulation

Herein, the defect features under irradiation in the Fe-C system are studied by an Object Kinetic Monte Carlo (OKMC) model. The model was based on recent parameters and was validated by comparing the numerical estimates with the experimentally obtained defect features in neutron-irradiated iron. Systematic simulations of defects evolution at 70 °C in a wide range of dose rates from 10−8 to 10−4 dpa/s were carried out. The simulation results demonstrate that at the lower dose range (< 0.02 dpa), a higher dose rate irradiation leads to a higher interstitial-loop density and irradiation hardening. In contrast, the dose rate does not obviously influence the defect features at the high dose range. Defect features under two carbon concentrations 50 and 100 appm show a similar dose-rate effect, and a higher carbon concentration leads to a higher interstitial loop density and irradiation hardening. We attribute the dose rate effect change with dose to the competition between sinks absorption and SIAs-vacancies annihilation. Based on the interaction model of <100> production, the ratio of <100>-type interstitial loops is higher at the higher dose rate irradiation, consistent with the experimental results.

Iron nitride, α″-Fe16N2, around interstitial type dislocation loops in neutron-irradiated iron

ABSTRACT Atom probe tomography (APT) and selected area electron diffraction (SAED) by transmission electron microscopy (TEM) were applied to nano-scale precipitates formed in a neutron-irradiated iron (99.99% pure) at high temperatures. In the conventional TEM, we discovered donut-type precipitates on the {100} planes that formed at the dilatational side of interstitial type dislocation loops with the Burgers vector of <100> on the habit planes of {100} in the bcc iron lattice. The precipitates were identified as nitride, bct α″-Fe16N2, by chemical composition and lattice structural analyses using APT and SAED. Image contrasts of the α″-Fe16N2 in a weak beam dark-field electron microscopy study were carefully analysed with the diffraction vector and the sign of deviation parameter from the Bragg condition, and it was concluded to be a selective visualisation of α″-Fe16N2 on one of the {100} planes. These results were in agreement with the crystallographic orientation between the α″-Fe16N2 and matrix iron, thereby bridging a knowledge gap in α″-Fe16N2 formation during neutron irradiation using a sodium-cooled experimental fast reactor, JOYO, or a 14 MeV D-T neutron source, RTNS-II.

Re-evaluating ion and neutron irradiation data with calculated applicable arc-dpa

ABSTRACT To develop advanced nuclear materials, lots of irradiation experiments have been done to various materials in different conditions. Accurate quantification of the defects induced by irradiation is important for researches in nuclear materials, especially when evaluating the damage caused by different particles. An advanced model – athermal recombination corrected dpa (arc-dpa) is recently introduced. In order to apply arc-dpa on experimental data, overall arc-dpa in ion and neutron irradiated iron, copper and tungsten are calculated based on the arc-dpa function for single cascade and the calculated PKA energy spectra. Data of irradiation experiments are re-evaluated with our applicable arc-dpa. The use of arc-dpa on experimental data showed good fit, giving new insights into the evolution of irradiation damage done by different particles.

Cluster dynamics simulation of the flux effect for neutron irradiated pure iron

ABSTRACTThe effect of flux in a wide range from 1.39×10−10 to 1.39 ×10−3 dpa/s has been studied by the cluster dynamics (CD) simulation for pure iron neutron irradiated at a temperature of 300°C for the two fluences. The critical flux values which mark the regimes of flux dependence of average size, number density and irradiation induced hardening for vacancy clusters (VC) and self-interstitial atoms clusters (SIAC) are found. It was found that the effect of neutron irradiation on average size of SIAC was similar for low and high fluxes in the case of the same fluence.

Impact of neutron irradiation on thermal helium desorption from iron

The synergistic effect of neutron irradiation and transmutant helium production is an important concern for the application of iron-based alloys as structural materials in fission and fusion reactors. In this study, we investigated the impact of neutron irradiation on thermal helium desorption behavior in high purity iron. Single crystalline and polycrystalline iron samples were neutron irradiated in HFIR to 5 dpa at 300 °C and in BOR-60 to 16.6 dpa at 386 °C, respectively. Following neutron irradiation, 10 keV He ion implantation was performed at room temperature on both samples to a fluence of 7 × 1018 He/m2. Thermal desorption spectrometry (TDS) was conducted to assess the helium diffusion and clustering kinetics by analyzing the desorption spectra. The comparison of He desorption spectra between unirradiated and neutron irradiated samples showed that the major He desorption peaks shift to higher temperatures for the neutron-irradiated iron samples, implying that strong trapping sites for He were produced during neutron irradiation, which appeared to be nm-sized cavities through TEM examination. The underlying mechanisms controlling the helium trapping and desorption behavior were deduced by assessing changes in the microstructure, as characterized by TEM, of the neutron irradiated samples before and after TDS measurements.

Transfer of molecular dynamics data to dislocation dynamics to assess dislocation–dislocation loop interaction in iron

We propose a computationally fast and physically justifiable method to treat dislocation loops as stochastic thermally activated finite-size obstacles in discrete dislocation dynamics simulations. The method was parameterized using molecular dynamics data for the interaction of dislocations with a0/2〈1 1 1〉 dislocation loops. As demonstration, the method is applied to rationalize experimental hardening of neutron-irradiated iron. The obtained results show good agreement with experimental data.

Effect of Carbon Impurity Content on Microstructural Evolution in Neutron-Irradiated Alpha Iron: Cluster Dynamics Modeling

ABSTRACTCluster dynamics (CD) modeling has been used to estimate the long-term evolution of point defect (PD) clusters. However, previous studies have often simplified the governing equations by assuming the maximum size of mobile self-interstitial atom (SIA) clusters and by ignoring the one-dimensional (1D) reaction kinetics of SIA loops. They have also conducted parameter fittings, such as the clustered fraction and the maximum size of clusters produced by collision cascade, to reproduce experimental data. In this study, in addition to modeling the 1D motion of SIA loops in the framework of the production bias model (PBM), reaction rates associated with carbon impurity atoms present in alpha iron were formulated to consider the trapping effect of one-dimensionally migrating SIA loops by a vacancy-carbon (V-C) complex that was shown to have strong bindings with SIA loops by previous atomistic simulations. Calculations results for neutron-irradiated alpha iron showed that the developed CD model can successfully reproduce the saturation trend of the number density of immobile SIA loops in contrast to the prediction using a model without the trapping effect.

Internal friction study of dislocation dynamics in neutron irradiated iron, and iron–copper alloys

The temperature dependent internal friction spectra of cold-worked and neutron irradiated iron and iron–copper binary alloys are investigated. By increasing dose, both γ - and Snoek-Köster-relaxation peaks exhibit strong shift towards low temperatures, as a consequence of the reduction of double kink activation energy. This shift is found to be the largest in alloys with the highest copper content. Besides, new modes appear in the spectra at energies of about 410 and 540 K. The 410 K peak intensity increases at the expense of Snoek-Köster peak intensity, indicating that redistribution of carbon takes place under irradiation, most probably as a result of grain boundary segregation. The presence of copper impedes the carbon redistribution by influencing the formation of carbon-vacancy complexes, which causes the grain boundary segregation, and activation of the 410 K relaxation process at larger neutron fluence in comparison with pure iron.

Cluster Dynamics Simulation of Reactor Pressure Vessel Steels under Irradiation

The coupling between copper rich precipitates (CRP) and point defects in neutron irradiated iron alloys and VVER steels was investigated by means of cluster dynamics (CD) simulations. The consideration of the strain energy effect on CRP kinetics as well as the application of the regular solution model for the case of different fixed copper contents of CRP provides a good agreement between the simulation results and experimental data for complex iron based alloys with small (0.015 wt%) and high (0.42 wt%) copper content. It was found that the CD simulation is applicable to irradiated VVER steel with 0.07 wt% of copper.

Microstructure of neutron-irradiated iron before and after tensile deformation

Tensile specimens of pure Fe were neutron irradiated at 50–70 °C in three fission reactors to displacement dose levels of 0.0001–0.79 displacements per atom (dpa). Irradiated specimens were characterized using transmission electron microscopy (TEM). Visible defect clusters were not detectable by TEM for doses below ∼0.001 dpa. Both the density and average size of the dislocation loops increased with increasing dose level. Spatially heterogeneous rafts of dislocation loops lying predominantly on {1 1 1} habit planes were visible for doses above ∼0.2 dpa. A high density of small cavities (number density >10 24 m −3, ∼1 nm diameter) was detected following irradiation to 0.79 dpa. The neutron irradiation led to an increase in the yield stress and a decrease in the uniform elongation as a function of increasing dose. Examination of specimens after tensile deformation revealed localized deformation in the form of cleared dislocation channels. Relatively few channels were formed in the uniform elongation region of the gage section. Dislocation channels on multiple slip systems were observed in the deformation region near the fracture surface, presumably due to the multiaxial stress state produced during necking and fracture of the tensile specimen.

Accumulation of point defects and their complexes in irradiated metals as studied by the use of positron annihilation spectroscopy – a brief review

During the last few years positron annihilation spectroscopy (PAS) has gained increasing recognition as a useful tool in studies of the accumulation of point defects and their complexes in irradiated metals. The technique is sensitive to both the size (in the range from monovacancies up to cavities containing 50–100 vacancies) and the density of vacancy clusters. In order to illustrate the potential of the PAS technique, we present a brief review of results obtained in our laboratory. One series of data show the distinct difference in microstructure between neutron irradiated bcc iron and fcc copper as a function of both irradiation and post-irradiation annealing temperature. Another set of results show the presence of sub-nanometre voids whose size and density increase with dose in neutron irradiated iron. The results show that PAS can be an important experimental tool for the validation of modelling of radiation produced microstructures.

Invisible and visible point defect clusters in neutron irradiated iron

Microstructure evolution in a low-dose neutron-irradiated iron was examined with a transmission electron microscope (TEM). The characteristics of the temperature dependence of defect structures in irradiated iron were as follows, (a) rather lower number density of defect clusters at 473 K than at 573 K, (b) development of dislocation loops and voids at an intermediate temperature of 623 K and (c) formation of irregularly shaped dislocation loops at 673 K. Comparing the defect structures produced by the irradiation with a conventional temperature control and that with an improved temperature control, nucleation of defect clusters was suppressed under neutron irradiation at 473 K. From the defect structures introduced by irradiations in which the irradiation temperature was cyclically changed between two temperatures (T-cycle irradiation), the accumulation of a large number of invisible vacancy type defect clusters by irradiation at 473 K was strongly suggested.

Physical model of the influence of oversize impurities on the radiation hardening of iron α-alloys

A physical model is proposed for the influence of oversize impurities on the characteristics of radiation-defect formation in neutron-irradiated iron alloys at low and high irradiation temperatures. A comparison is made between the theoretical expressions obtained for radiation hardening at different temperatures and the existing experimental data. For low temperatures, the agreement was satisfactory, so we can estimate one of the microscopic parameters of impurity atoms — the relative size of the region of influence of the impurity atoms.

Neutron and X-ray scattering studies of neutron-irradiated iron crystals

Neutron and X-ray scattering studies of neutron-irradiated iron crystals

Separation of carrier-free60Co and54Mn from neutron-irradiated natural iron target

A simple chromatographic method to separate carrier-free60Co and54Mn, produced in the neutron irradiation of Fe2O3, has been developed. The separation of carrier-free60Co was performed in the displacement mode using DTPA as a displacer and H+ as a barrier ion; carrier-free54Mn was separated in the form of54MnO 4 − from55+59Fe in the elution mode using HNO3 as an eluent.

Comment on the paper by Blackburn and Yassine (1989) “hot atom reactions in neutron irradiated solid iron group metallocenes”

Comment on the paper by Blackburn and Yassine (1989) “hot atom reactions in neutron irradiated solid iron group metallocenes”

Detection and analysis of microscopically invisible cascade defects in 14 MeV neutron irradiated aluminum and iron

Abstract Experiments to detect damage created by cascades in aluminum and iron, in which no defect structures have been observed by electron microscopy are designed by effectively combining 14 MeV neutron irradiation with electron irradiation damage by a high voltage electron microscope. The shrinkage and annihilation of interstitial loops formed by electron irradiation by subsequent neutron irradiation was most marked in aluminum and but occurred to some degree in iron. The result is explained by a simple model of diffusion of vacancies and interstitials based on the difference of the initial distribution profile between the two kinds of point defects in a cascade. The extent of the annihilation of neutron radiation induced defect clusters in iron formed at room temperature is detected by the variation of the interstitial cluster formation at 323 K by high energy electron irradiation. Two annealing stages of 523–573 K and 573–623 K are observed.

Intergranular segregation of phosphorus and sulfur in neutron irradiated iron alloys

Etude de l'effet d'une irradiation par neutrons sur la segregation intergranulaire de PetS dans du fer contenant les impuretes; S(0,005) P(0,04%) et C(0,0012 a 0,0942%)(% en poids). Cette segregation induit une fragilisation intergranulaire dans l'echantillon a faible teneur en C, mais pas dans celui a forte teneur. Pas d'effet appreciable de l'irradiation sur la segregation de P et S avec les conditions experimentales decrites

Point Defects and Induced Magnetic Anisotropy in Fast Neutron Irradiated Iron

Magnetic torque measurements have been made at 77 K for a single crystal disc of pure iron before and after a fast neutron irradiation at 5 K and after successive isochronal annealings at various temperatures between 77 K and 293 K. The observed torque curve after the irradiation contains both uniaxial and biaxial components. By analyzing the irradiation-induced torque and its change during isochronal annealings, the existence of 110-dumbbells has been confirmed, and the mechanism of the aggregation of interstitials and the change in their arrangements predicted theoretically by Bullough and Perrin have experimentally been proved.