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Void swelling, which induces the degradation of the original properties of nuclear materials under high-energy particle-irradiation, is an important problem. The incubation period, a transient stage before the steady void growth, determines the duration of service of nuclear materials. Several experimental studies have been performed on void observations by transmission electron microscopy (TEM), which, however, has a resolution limit for the size of defect clusters. Positron annihilation lifetime spectroscopy (PALS) enables the detection of small vacancy clusters, single vacancies, dislocations and precipitates. The use of these two methods provides complementary information toward detecting defect information in the incubation period. Here, defect structures during the incubation period in austenitic stainless steels, by means of PALS and TEM are reviewed. The role of alloying elements into determining the period is explained. Furthermore, the existing problems and research directions in this field are presented.

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 (99.9...

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.

In the slow positron beamline at the Kyoto University Research Reactor (KUR), positron creation was enhanced by increasing the gamma-ray intensity at the positron source via the reaction of 113Cd(n, γ )114Cd. To achieve this, a cadmium (Cd) cap was attached to the positron source, surrounding it, and thus, without intentional cooling, the temperature was able to reach near the melting point of Cd via nuclear heating. In this study, the degree to which the Cd cap contributes to the quantity of positron creation was estimated by using the Monte Carlo calculation code PHITS (Particle and Heavy Ion Transport code System), which simulates radiation transportation and interaction with matter. As a result, the number of positrons created was found to become 2.0 ± 0.1 times higher by using the Cd cap at the KUR slow positron beamline. The use of the Cd cap was confirmed to be significantly effective for enhancing positron creation.

To validate the Monte Carlo codes for prediction of radiation damage in metals irradiated by > 100 MeV protons, we developed a proton irradiation device with a Gifford–McMahon (GM) cryocooler to cryogenically cool two 0.25-mm-diameter wire samples of aluminum and copper. By using this device, the defect-induced electrical resistivity changes related to the displacement cross section of copper and aluminum were measured under irradiation with 200-MeV protons at 5 K at the beamline of the cyclotron facility at RCNP, Osaka University. After irradiation to a 3.89 × 1018 proton/m2 flux, the damage rate of the aluminum sample was 1.30 × 10−31 Ωm3/proton at 185 MeV and that of copper was 3.60 × 10−31 Ωm3/proton at 196 MeV. Based on measurements of recovery of the accumulated defects in aluminum and copper through isochronal annealing, which is related to the defect concentration in the sample, about 50% of the damage remained at 40 K, with the same tendency observed in other experimental results for reactor neutron, fusion neutron, and 125-MeV proton irradiations. A comparison of the measured displacement cross sections with the calculated results of the NRT-dpa and the athermal-recombination-corrected displacement damage (arc-dpa) cross sections indicates that arc-dpa with the defect production efficiencies provided by Almazouzi for aluminum and Nordlund for copper provide better quantitative descriptions of the displacement cross section than NRT-dpa.

We investigated the fluence dependence of irradiation-induced solute cluster, dislocation loop, and very small defect to reveal the hardening mechanism in surveillance test specimens from a reactor pressure vessel steel with low-Cu content (0.04 wt%) using atom probe tomography (APT), weak-beam scanning transmission electron microscopy (WB-STEM), and positron annihilation spectroscopy. A high number density (>1023 m−3) of solute clusters mainly composed of Ni, Mn, and Si atoms were found in highly neutron irradiated specimens (∼1024 neutrons m−2 (E > 1 MeV)) by APT. These solute clusters were one of the main sources of hardening as reported previously. On the other hand, it was also revealed that dislocation loops were formed with a number density of ∼1022 m−3 in the high-fluence specimens by WB-STEM. The estimated hardening due to dislocation loops was more than half of the actual hardening, showing that dislocation loops are also main source of irradiation hardening at high neutron fluence with the solid experimental evidences. Regarding specimens subjected to a low neutron fluence (∼1023 neutrons m−2), very small defects, not detected by either WB-STEM or APT, were formed by positron annihilation spectroscopy. This result suggested that, at a low neutron fluence, the defects were the initial hardening source and they may grow the dislocation loops observed by WB-STEM at high fluence range.

Radiation-enhanced diffusion (RED) of copper (Cu) in iron (Fe) is essential for understanding solute/impurity diffusion in nuclear materials, especially reactor pressure vessel steel, but has been rarely reported experimentally. In this study, we performed a high-precision investigation of RED using well-controlled electron irradiation and three-dimensional atom probe (3D-AP). Cu-Fe diffusion pairs were created using high-purity Fe and Cu as base materials, and irradiated by 2 MeV electron at a temperature of 773 – 893 K controlled to within ±3 K. Cu diffusion into the Fe matrix was observed at the atomic level using 3D-AP, and the diffusion coefficient was obtained directly using Fick's law. RED was clearly observed, and the ratio of diffusion under irradiation to thermal diffusion was increased as the irradiation temperature decreased. RED was quantitatively evaluated using the reaction kinetics model, and the model which consider only vacancies gave a good agreement. This gave experimental clarification that RED was dominated by irradiation-induced vacancies. In addition, the direct experimental results on the effect of irradiation on the solubility limits of Cu in Fe was obtained; solubility limits under irradiation were found to be lower than those under thermal aging.

The change in the positron annihilation lifetime (PAL) of vacancies containing hydrogen atoms was investigated by taking PAL measurements in tungsten. The vacancies were introduced by electron irradiation, and hydrogen atoms were charged under a high-pressure hydrogen atmosphere (5.8 MPa). The PAL of single vacancies was measured to be approximately 175 ps, which decreased to approximately 155 ps after hydrogen charging. The PAL of single vacancies containing hydrogen atoms was calculated using the electron density obtained by a first principles calculation. The change in the PAL of vacancies containing hydrogen atoms was measured in experiments and calculated in simulations, and then, the two values were compared. It was found that one vacancy captured one or two hydrogen atoms (an average of 1.6 atoms). The binding energy of hydrogen to vacancies of 1.19 eV was obtained under the assumption of a thermal equilibrium state, which was slightly higher than the value determined by a previous study (1.06 eV). It is believed that the effect of impurities causes this discrepancy. The elucidation of the effect of impurities on the change in the PAL and the binding energy leads to greater accuracy in the quantitative evaluation of the hydrogen atoms trapped at single vacancies.

The effect of the volume size factor of solute atoms and their clusters on the one-dimensional motion of interstitial clusters was studied by computer simulations. Solute atoms were placed on the migration path of the interstitial clusters, and the change in the migration energy of the interstitial clusters was obtained by the static method. The volume size factor of Au atoms in a Ni-Au binary alloy was systematically changed from −30% to +100%. When the volume size factor of the solute atoms was larger or smaller than Ni atoms, the migration energy of the interstitial clusters increased. The average migration energy required for the interstitial clusters to pass through one, two, three, and four solute atoms (volume size factor: +63.7%) was 0.57, 1.28, 2.09, and 2.47 eV, respectively. If the solute atom clusters grow and their density is sufficiently high, the jump frequency of the interstitial clusters is so low that the interstitial clusters cannot move through the solute atom clusters under an irradiation temperature of 573 K.

Thermal desorption spectroscopy (TDS) of tungsten implanted with D2+ ions was performed after irradiation with 8 MeV-electrons, 5.0 keV-D2+, and 6.0 MeV-Fe3+. The release peak temperatures of the TDS spectra are discussed. Positron annihilation lifetime (PAL) measurements of electron-irradiated tungsten were also performed, showing that single vacancies migrate a sufficient distance to arrive at a sink or meet interstitial-type defects during annealing at 673 K. A decrease in the PAL was detected for single vacancies that contain deuterium atoms. The peak temperature of deuterium release from dislocations was lower than that from single vacancies. In samples irradiated with 6.0 MeV-Fe3+, the effect of Fe contamination on deuterium trapping and the deuterium release from voids were detected. These tendencies correspond to previous works.

The effects of Cr and W solute atoms on the evolution of irradiation-induced defects in Fe-8Cr and Fe-8Cr-2W alloys upon annealing after low-temperature neutron and electron irradiation were investigated by using positron annihilation lifetime measurements. Vacancy defects behavior at elevated temperatures showed that added solute Cr and W in Fe clearly suppress void growth. The void swelling suppression by Cr was explained using configuration-trapping model. For the W solute effects, the results of two types of irradiation support the solute and dislocation interaction model. Related results are important for the fabrication of improved radiation-resistant materials for fission or fusion reactors.

The growth process of He-filled vacancy clusters during annealing was investigated with positron annihilation lifetime spectroscopy and coincidence Doppler broadening (CDB) measurements. The reduced activation ferritic/martensitic steel F82H was irradiated with high-energy protons and spallation neutrons. The He-filled vacancy clusters absorbed more He atoms when annealed below 673 K, and the long and mean positron lifetimes decreased. When annealed above 873 K, the vacancies (V)–Hem or Vn–Hem complexes dissociated (n and m are the number of vacancies and He atoms, respectively). The He-filled vacancy clusters then absorb these dissociated vacancies and He atoms. Therefore, the size of the He-filled vacancy clusters increased, and the He-to-vacancy ratio decreased. These annealing-induced phenomena increased the long positron lifetime in addition to the higher positron trapping rates of the He-filled vacancy clusters. By comparing electron-irradiated samples that did not contain He atoms to the proton- and neutron-irradiated samples containing He atoms, the effects of He atoms on the CDB ratio curves were studied. The results agreed with the previous study of He-ion-implanted Fe–Cr alloys.

Nuclear reactions in accelerator-driven systems (ADS) result in the generation of helium within the ADS materials. The amount of helium produced in this way is approximately one order of magnitude higher than that generated by nuclear fusion. As helium is well-known to induce degradation in the mechanical properties of metals, its effect on ADS materials is an important factor to assess. The results obtained in this study show that low-dose proton irradiation (11 MeV at 573 K to 9.0 × 10−4 dpa and 150 MeV at room temperature to 2.6 × 10−6 dpa) leads to a decrease in yield stress and ultimate tensile strength in a Fe–9Cr alloy. Moreover, interstitial helium and hydrogen atoms, as well as the annihilation of dislocation jogs, were identified as key factors that determine the observed softening of the alloy.

In reactor pressure vessel surveillance data, it was found that the concentration of matrix defects was very low even after nearly 40 years of operation, though a large number of precipitates existed. In this paper, defect structures obtained from surveillance data of A533B (high Cu concentration) were simulated using reaction kinetic analysis with 11 rate equations. The coefficients used in the equations were quite different from those obtained by fitting a Fe-0.6 wt%Cu alloy irradiated by the Kyoto University Reactor. The difference was mainly caused by alloying elements in A533B, and the effect of alloying elements was extracted. The same code was applied to low-Cu A533B irradiated with high irradiation damage rate, and the formation of voids was correctly simulated., Available online 30 January 2015

Positron beam facilities are widely used for solid state physics and material science studies. A positron beam facility has been constructed at the Kyoto University Research Reactor (KUR) in order to expand its application range. The KUR is a light-water-moderated tank-type reactor operated at a rated thermal power of 5 MW. A positron beam has been transported successfully from the reactor to the irradiation chamber. The total moderated positron rate was greater than 1.4 × 10 6 /s while the reactor operated at a reduced power of 1 MW. Special attention was paid for the design of the in-pile position source to prevent possible damage of the reactor in case of severe earthquakes.

A series of irradiation experiments with 150 MeV protons was performed. The relationship between target depth (or shield thickness) and displacement damage during proton irradiation was obtained by in situ electrical resistance measurements at 20 K. Positron annihilation lifetime measurements were also performed at room temperature after irradiation, as a function of the target thickness. The displacement damage was found to be high close to the beam incident surface area, and decreased with increasing target depth. The experimental results were compared with damage production calculated with an advanced Monte Carlo particle transport code system (PHITS).

The effects of neutron irradiation on F82H ferritic/martensitic stainless steel and its model alloys were studied by Mossbauer spectroscopy. The microstructural damage mechanisms of these alloys, during the void incubation period were interpreted using the short range order (SRO) parameters. Results show that within Fe–8Cr alloy, the atoms in the nearest neighbor (NN) of the Fe nuclei were inhomogeneous, prior to irradiation. A configuration trapping model of Cr supported the negative average SRO observed for the NN shells in our Fe–Cr alloys. We found that irradiation also accelerated the SRO in Fe–8Cr through a diffusion mechanism, where Cr atom repulsion was concentration dependent. Finally, comparative studies were conducted on F82H model alloys using the present Mossbauer measurements and our previously reported work on positron annihilation spectroscopy, which further established that irradiation of the alloys promoted the growth of a M 23 C 6 complex.

Fe ion irradiation-induced defects in F82H model alloys were studied using positron annihilation lifetime measurements. Irradiation was performed with an accelerating voltage of 6.0 MeV to a dose of 22 dpa at 593 K. In addition, a comparative study was conducted with results from our previous neutron irradiation measurements for F82H model alloys. The results showed an apparent decrease of the mobility of interstitial clusters in Fe by addition of Cr. This finding supports the configuration model of Cr on void swelling resistance in Fe–Cr alloys. A remarkable delay of defect structural evolution was also observed by ion irradiation compared to neutron irradiation because of a high damage rate.

The effect of neutron irradiation on recovery (annealing) of irradiation damage has been investigated for self-ion implanted Si. A damage layer was introduced by 200 keV Si+ implantation to a fluence of 5 × 1014 Si/cm2 at room temperature. The damaged samples were neutron-irradiated to 3.8 × 1019 n/cm2 (fast neutron), without intentional heating, in the core of the Kyoto University Reactor. During neutron irradiation, the samples were heated only by nuclear reactions, and the irradiation temperature was estimated to be less than 90 °C. The damage levels of the samples were characterized by Rutherford backscattering with channeling. Reduction of damage peaks as a result of neutron irradiation was clearly observed in the samples. The annealing efficiency was calculated to be 0.44 defects/displacement.

Iron (Fe) based alloys are widely used in the nuclear industry because the activation of Fe is low and the irradiation data for Fe alloys is abundant. The interaction between helium (He) atoms and defects in Fe alloys is an important issue in fusion reactors, since the rate of He generation is high. In the present study, well-annealed Fe specimens were cold-rolled until the original thickness was reduced by 10% or 40%, and then annealed at 673 or 723 K for 1 h, respectively, to annihilate the vacancy clusters formed during cold rolling. He atoms were implanted at 100 eV without causing displacement damage in the annealed cold-rolled samples, which contained only dislocations. Upon He implantation, the yield stress (lower yield stress) and tensile strength decreased, and the total elongation increased. The former effect became stronger with increasing deformation (elongation induced by cold rolling from 10% to 40%), whereas the latter became weaker.

For developing an accelerator-driven system (ADS), many studies, such as those on accelerator development, subcritical system neutronics, materials irradiation effects, and thermal hydraulics, are required. In the Research Reactor Institute at Kyoto University (KURRI), ADS is planned as a future neutron source. Recent activities of relevant ADS studies at KURRI are introduced herein.

Reduced activation ferritic/martensitic steels F82H and T91 irradiated with high-energy protons and spallation neutrons were investigated using positron annihilation spectroscopy. Subnanometer-sized He-filled vacancy clusters, which cannot be observed by transmission electron microscopy, were detected. In the low dose region, positrons were trapped at relatively large He-filled vacancy clusters, small He-filled vacancy clusters and other defects (dislocations, precipitates and interfaces). In the high dose region, almost all positrons were trapped at vacancy clusters containing a lot of He atoms and at other defects. Large He-filled vacancy clusters absorb He atoms produced by the irradiation, and the long and mean lifetimes decreased. Conspicuous peaks related to He atoms could not be obtained in CDB ratio curves, and it was also difficult to detect the effect of He atoms in S – W plots. However, almost all data for irradiated samples lay on one line in the S – W plots, indicating that the change in the S - and W -parameter was caused by growth process of one type of defects. This may be caused by absorption of He atoms leading to the lifetime change. To prove this, further experiments and analysis are necessary.

Neutron irradiation-induced vacancy defects in F82H model alloys were studied using positron lifetime measurements. A comparison between the results from neutron irradiation and our previous electron irradiation in F82H model alloys was conducted. The results showed that Cr has a consistent effect on void swelling resistance. The void swelling suppression by Cr was explained using the configuration trapping model. However, for the W, V and Ta solute effects, the results of two types of irradiation support the solute and dislocation interaction model. The effects on metal carbides (M23C6) were investigated using neutron irradiation at 363 K and electron irradiation. For neutron irradiation, the solute effect may be the dominating factor compared to the effects of the M23C6. Solute depletion caused by metal carbides can explain the difference.

Stress-controlled cyclic deformation experiments under 150MeV proton irradiation (5.2 © 10 ¹11 dpa/s) were performed on Ni at room temperature, where a single deformation period was approximately 30s. After deformation, defect structures were studied using positron annihilation lifetime spectroscopy and transmission electron microscopy. In deformations of 300 cycles and 900 cycles, residual vacancies were high in in-situ cyclic deformed Ni and low in nonirradiated Ni. Although dislocation cell structures were observed in all specimens, the development process and the cell size were different in each deformation condition. Well-developed large cells in nonirradiated Ni and small cells in deformed Ni after irradiation were observed. The cell structure was not well developed in the in-situ deformed Ni under irradiation. The relationship between these defect evolutions and fatigue life is discussed. [doi:10.2320/matertrans.MD201305]

It is well known that helium (He) generated by (n, α) nuclear reactions in metals and alloys irradiated with high-energy particles prompts not only the nucleation of interstitial-type dislocation loops, but also the nucleation of voids, which degrades mechanical properties. In the present study, He atoms were implanted at 150 eV without causing displacement damage in Ni containing either dislocations, vacancies or a combination of both. The results showed that upon He implantation, the ultimate tensile strength increased and total elongation decreased in samples containing both dislocations and vacancies. However, it was also found that in samples with He trapped by dislocations, where the He concentration decreased with increasing sample depth, nickel increased both the ultimate tensile strength and total elongation at 300 K, but that the effect of He disappeared at 573 K. In addition, He trapped by vacancies did not strongly affect the ultimate tensile strength or total elongation in samples containing only vacancies.

The microstructure of fatigued and unfatigued Ni in the presence or absence of neutron irradiation was investigated using positron annihilation lifetime measurements and transmission electron microscopy (TEM). In irradiated unfatigued Ni, vacancy clusters were directly formed from cascades at doses of less than 10 −4 dpa, and started to grow at higher doses. In irradiated fatigued samples, vacancy clusters were directly formed from cascades at 1.5 × 10 −4 dpa. In unirradiated fatigued samples, single vacancies were produced. In all of the fatigued samples, dislocations introduced by the deformation acted as sinks and suppressed the growth of vacancy clusters. Following annealing at 373 K, vacancy clusters were observed in irradiated fatigued Ni, but vacancies were trapped at dislocations in unirradiated fatigued Ni. No change in dislocation structure was brought about by annealing and no vacancy clusters could be observed by TEM.

Fcc pure metals were irradiated with spallation neutrons (energies up to 500 MeV) at room temperature to a neutron fluence of 1 × 1018 n m−2 at KENS, High Energy Accelerator Research Organization (KEK). Defect clusters induced by large collision cascades were examined using transmission electron microscopy (TEM). In Au, large groups of defects included more than 10 clusters, and the damage zone extended over 50 nm, which was larger than that induced by fusion neutron irradiation ( 10 nm) were identified as vacancy type by the conventional inside–outside contrast method. Because of the low neutron fluence, spatial overlapping of collision cascades was ignored. Large vacancy loops are formed through cooperative reactions among subcascades in a single collision cascade with large recoil energy.

Defect structures in compressed and fractured pure Ni and Ni–2 at.%Sn alloy were investigated. The dislocation cell size of high-speed compressed Ni was smaller than that of low-speed compressed Ni. In Ni–Sn, the cell structure was small and independent of compression speed. The effect of solute Sn was very strong, and Sn trapped a high density of dislocations. At the saw-tooth-like fracture tips formed by high-speed elongation, the observed dislocation density was low in both metals, but was lower in Ni than in Ni–Sn. The number of stacking fault tetrahedra (SFTs) formed at the tips was almost the same in Ni as in Ni–Sn. Because the density of the dislocations was very low despite the strong effect of solute Sn, and because the density of formed SFTs was almost the same in both metals at the saw-tooth-like fracture tips, the formation of SFTs likely involves a dislocation-free mechanism during high-speed deformation.

Irradiation-induced vacancy defects in F82H model alloys were studied using positron lifetime measurements. It is shown that the presence of Cr and carbides in iron-based alloys prevents the formation of vacancy clusters. Divacancies are stabilized at room temperature by adding Cr to Fe. The suppression of void swelling by Cr can be explained by existing models. During the incubation period, elements such as W, V, and Ta have little effect on the evolution of point defect clusters in Fe–Cr based alloys those do not contain C. M23C6 plays an important role on void swelling resistance.

He and H can be produced by nuclear reaction. In addition, energetic particles of He and T and D, which are isotopes of H, in the plasma of fusion reactor induce the damage in the surface of materials, such as erosion, sputtering and blistering. To investigate the interaction of He and D, amorphous and crystalline FeBSi alloys were irradiated by He or D[2] or He + D[2] ions with 5 keV. The results of thermal desorption indicated that more He atoms were trapped in both the amorphous and crystalline alloy irradiated by He ions than D atoms in those alloys irradiated by D[2] ions. Although He and D atoms were trapped in FeBSi alloy irradiated by He + D[2] ions, desorption peaks of D[2] and He were separated. Absorption of D in an amorphous alloy was influenced by the presence of He; however, absorption of He was independent of D[2] irradiation in both alloys.

To obtain additional information for discussion on the origin of deep-level band photoluminescence (PL) in ZnO, hydrothermal ZnO wafers implanted with 60 keV Sn + ions at room-temperature (RT) and low temperature (LT) of approximately 120 K were examined by transmittance, absorbance, reflectance, AFM, and TEM techniques. The thickness of the implanted layer was approximately 45 nm. The sharpness of the characteristic feature normally observed in reflectance spectra gradually degraded with implantation dose. The tail observed in the absorbance spectra of the implanted layer red-shifted with implantation dose. This excludes the possibility that the red-shift of the deep-level PL band with implantation dose was caused by an increase in concentration of Li Zn centers with respect to that of oxygen vacancies.

In Fe–Cu alloys, Cu precipitates are formed during high-energy particle irradiation. If there exists energetic binding between vacancies and Cu atoms, vacancy clusters (voids) are formed in precipitates at an initial stage of irradiation, separate from voids in the matrix, because of the migration of Cu atoms with vacancies. In this paper, the damage rate dependence on the formation and annihilation of voids in the precipitates and in the matrix is simulated by reaction kinetic analysis. The initial formation of voids at precipitates, the annihilation of them with an increased dosage and new formation of voids in the matrix are simulated, and the results are compared with the experiments. In a high damage rate of 3.3 × 10^[−7] dpa/s, the formation of voids in Cu precipitates is not significant, but the formation of voids in the matrix is dominant, different from those in a low damage rate of 1.5 × 10^[−10] dpa/s.

The defect structures in Ni, Fe–15Cr–16Ni and Ti-added modified SUS316SS (modified SUS316) were examined after neutron irradiation below 0.3 dpa by the Japan Materials Testing Reactor and Belgian Reactor 2 to compare their defect structural evolution. The growth behaviour of interstitial-type dislocation loops (I-loops), stacking fault tetrahedra (SFTs) and voids was found to be quite different among these specimens. I-loops developed at lower temperatures in Ni than in Fe–15Cr–16Ni and modified SUS316, and more swelling occurred in Ni than in Fe–15Cr–16Ni. Finally, there were no voids in modified SUS316. These results were analysed in terms of the I-loop energy. A large discrepancy was found between the analytical results and experimental observations for Ni and modified SUS316, which suggests the formation of unfaulted I-loops directly from collision cascades. The growth of SFTs was detected in Fe–15Cr–16Ni and modified SUS316, and can be explained by a change in the dislocation bias of SFTs resulting f...

Beam flux dependence of ion-irradiation-induced porous structures in GaSb and InSb were studied by focused ion beam. The void and elevation structure were formed after irradiation. The average diameter of the void was approximately 42 nm in the sample irradiated to a flux of 4×1017 ions/m2s and 61 nm in the sample irradiated to a flux of 30×1017 ions/m2s in GaSb in a total fluence of 20×1018 ions/m2. It is considered that many vacancies are immediately induced in the sample of high-flux irradiation. Therefore, the diameter of the void in high-flux irradiation is larger than it is in low-flux irradiation.

To evaluate dislocations induced by neutron irradiation, we developed a weak-beam scanning transmission electron microscopy (WB-STEM) system by installing a novel beam selector, an annular detector, a high-speed CCD camera and an imaging filter in the camera chamber of a spherical aberration-corrected transmission electron microscope. The capabilities of the WB-STEM with respect to wide-view imaging, real-time diffraction monitoring and multi-contrast imaging are demonstrated using typical reactor pressure vessel steel that had been used in an European nuclear reactor for 30 years as a surveillance test piece with a fluence of 1.09 × 1020 neutrons cm-2. The quantitatively measured size distribution (average loop size = 3.6 ± 2.1 nm), number density of the dislocation loops (3.6 × 1022 m-3) and dislocation density (7.8 × 1013 m m-3) were carefully compared with the values obtained via conventional weak-beam transmission electron microscopy studies. In addition, cluster analysis using atom probe tomography (APT) further demonstrated the potential of the WB-STEM for correlative electron tomography/APT experiments.

This report describes the status review of the JSPS Grant Team to develop advanced materials for the spallation neutron sources and modeling of radiation damage. One of the advanced materials is a toughness enhanced, fine-grained tungsten material (W-TiC) having four-times larger fracture toughness than ordinary tungsten and appreciable RT ductility in the recrystallized state. The other is an intergranular crack (IGC)-resistant austenitic stainless steel which was processed by the grain-boundary engineering (GBE). The experimental results are devoted to corrosion in a lead–bismuth eutectic, arrest of corrosion of weld-decay, radiation damage and creep rupture as well as new technique of GBE using a laser and annealing procedure. New technique seems to be applicable to large or complicated-shaped components. A series of the multi-scale models is built up from nuclear reaction between incident particles and medium nuclei to material property change due to radiation damage. Sample calculation is made on 3 mm-thick nickel bombarded by 3 GeV protons.

Titanium-doped austenitic stainless steel (JPCA) and reduced activated ferritic/martensitic steel (F82H) irradiated with high-energy protons and spallation neutrons were investigated by positron annihilation lifetime measurements. Subnanometer-sized (

In the radiation damage process of austenitic stainless steels, there exists an incubation period before steady-state void growth, and the defect formation behaviors during that period strongly depend on alloy composition. Using the technique of positron annihilation lifetime measurement, the evolution of defect clusters during the incubation period in neutron, electron, and H-ion irradiations was studied for a variety of austenitic stainless steels including commercial and model alloys. The lifetime measurements indicated that in fission neutron irradiation to 0.2 dpa at 363 K, single vacancies were predominantly formed in the commercial alloys, SUS316L and Ti added, modified SUS316, while large voids were formed in Ni and Fe–Cr–Ni. After neutron irradiation at 573 K, stacking fault tetrahedra and/or precipitates were detected in the commercial alloys, while large voids were detected in the model alloys. In the 30 MeV electron irradiation to a dose of 0.012 dpa, the effect of alloying elements on lifetime data was less significant at 353 K, but a significant difference was found between model alloys and commercial alloys at 573 K. The H-ion irradiation at 2 MeV was also performed at room temperature. Defect evolution during the incubation period is discussed on the basis of the neutron, electron and H-ion irradiation results.

Post-irradiation annealing (PIA) behavior of irradiation-induced microstructural changes and hardening of an A533B (0.16 wt.% Cu) steel after neutron-irradiation of 3.9 × 1019 n cm−2 (0.061 displacement per atom (dpa)) at 290 °C was studied by positron annihilation spectroscopy (PAS), atom probe tomography (APT) and Vickers microhardness measurements. Coincidence Doppler broadening and positron lifetime measurements clearly reveal two recovery stages; (i) as-irradiated state to annealing at 450 °C and (ii) annealing from 450 to 600 °C. The first stage is due to annealing out of the most of irradiation-induced vacancy-related defects, while the second stage corresponds to dissolving of irradiation-induced solute nanoclusters (SCs). APT observations reveal that the SCs are enriched with Cu, Mn, Ni and Si and that their number densities decrease with increasing annealing temperature without coarsening to give almost complete recovery at 550 °C. The experimental hardening is almost twice the SC hardening estimated by the Russell–Brown model below 350 °C, whereas it is almost the same as that estimated in the range 400–550 °C.

Measurements of thermal diffusivity and positron annihilation lifetime were performed on neutron- and 30 MeV electron-irradiated ceramics. Thermal diffusivity of heavily neutron-irradiated ceramics decreased to a very low level and the difference between specimens was small. Positron annihilation lifetime of these specimens also showed a distinct change; however, the difference between specimens was also small. Accordingly, in this study, electron irradiation was performed to 0.01–0.02 dpa via the 30 MeV KURRI-Linac. The electron-irradiated specimens also showed obvious degradation in thermal diffusivity and an increment in positron annihilation lifetime, and it was clarified that positron annihilation lifetime showed some correlation with thermal diffusivity.

It is interesting to investigate the formation of He bubbles in amorphous alloys because point defects do not exist in amorphous materials. In the present study, the microstructural evolution of amorphous Fe79B16Si5 alloy, either irradiated with 5 keV He^{+} ions or implanted with 150 eV He^{+} ions without causing displacement damage, and then annealed at a high temperature, was investigated using transmission electron microscopy (TEM). Vacancy-type defects were formed in the amorphous alloy after irradiation with 5 keV He^{+} ions, and He bubbles formed during annealing the irradiated samples at high temperature. On the other hand, for samples implanted with 150 eV He^{+} ions, although He atoms are also trapped in the free volume, no He bubbles were observed during annealing the samples even up to 873 K. In conclusion, the formation of He bubbles is related to the formation and migration of vacancy-type defects even in amorphous alloys.

Helium generated in materials by the nuclear reaction (n, α) is generally considered to be harmful. It is well-known that helium prompts not only the nucleation of interstitial-type dislocation loops, but also the nucleation of voids in metals and alloys irradiated with high-energy particles, which degrades their mechanical properties. In this study, however, we find that helium trapped by dislocations in Ni increases both the ultimate tensile strength and total elongation.

The dependence of microstructural evolution on solute atom concentration in Ni alloys was investigated by positron annihilation lifetime measurements. The positron annihilation lifetimes in pure Ni, Ni−0.05 at.%Si, Ni−0.05 at.%Sn, Ni−Cu, and Ni−Ge alloys were about 400 ps even at a low irradiation dose of 3 × 10 −4 dpa, indicating the presence of microvoids in these alloys. The size of vacancy clusters in Ni−Si and Ni−Sn alloys decreased with an increase in the solute atom concentration at irradiation doses less than 0.1 dpa; vacancy clusters started to grow at an irradiation dose of about 0.1 dpa. In Ni−2 at.%Si, irradiation-induced segregation was detected by positron annihilation coincidence Doppler broadening measurements. This segregation suppressed one-dimensional (1-D) motion of the interstitial clusters and promoted mutual annihilation of point defects. The frequency and mean free path of the 1-D motion depended on the solute atom concentration and the amount of segregation.

The defect structures in neutron-irradiated austenitic stainless steel, Ti-modified 316SS, were studied by using positron annihilation spectroscopy and transmission electron microscopy. By low dose irradiation of the order of 10−3 dpa, mono-vacancies and small microvoids were detected below 423 K by positron annihilation lifetime measurement. With increasing irradiation temperature, the positron lifetime was found to decrease, indicating no microvoid formation. Irradiation at doses above 4.2 dpa and temperatures above 673 K led to the formation of microvoids. These results indicated that microvoid formation below 423 K occurs even at low dose without incubation. Above 673 K, during the incubation period, irradiation damage changes the microstructure leading to vacancy clustering to form voids.

Interstitial-type dislocation loops were formed in Fe–9Cr alloys on irradiation with 1-MeV He ions at 673 K. However, with increasing irradiation dose, the dislocation loops shrunk. A molecular dynamics simulation was used to elucidate the mechanism of this unexpected phenomenon. The simulation shows that, although the binding energy of a self-interstitial atom to a dislocation loop is normally greater than that of a vacancy, the energy hierarchy is reversed when He atoms decorate the loop. This may indicates preferential absorption of vacancies, causing loop shrinkage at high doses, consistent with experimental observation.

The formation of Cu precipitates and point defect clusters was investigated in two Fe–Cu binary model alloys, Fe–0.3Cu and Fe–0.6Cu, irradiated at 573 K at three different damage rates, namely 3.8 × 10[−10], 1.5 × 10[−8] and 5 × 10[−8] dpa (displacements per atom)/s, up to about 1.6 × 10[−2] dpa. Results of positron annihilation experiments indicated that Cu precipitates were formed in these irradiations with different damage rates. The growth of Cu precipitates does not increase monotonously with increasing irradiation dose, but it rather depends on the nucleation and growth of microvoids. It is also clear that the nucleation and growth of microvoids are influenced by the irradiation dose rate.