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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.

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.

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.

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.

Void formation and structure change by heavy ion irradiation were investigated in GaSb and InSb thin films. The voids were formed after irradiation in both materials. The average diameter of the voids was about 15 nm in GaSb and 20 nm in InSb irradiated with 60 keV Sn þ ions to a fluence of 0:25 � 10 18 ions/m 2 at room temperature. The void size in InSb is larger than that in GaSb. The large void size is quantitatively explained by the amount of induced vacancies obtained by the SRIM code simulation. The Debye-Scherrer rings were observed in the SAED patterns on both materials. The structure changes into a polycrystal by ion irradiation. Additionally, the 200 superlattice reflections in the [001] net pattern were almost absent, and the streak pattern along the h110i direction was observed in InSb. It is considered that the anti phase domains of different lengths are formed by ion irradiation. Ion irradiation transforms the structure of InSb from chemical ordering to chemical disordering via the formation of anti phase boundaries. [doi:10.2320/matertrans.M2010037]

The correlation of microstructure evolution and hardening was studied in two kinds of A533B-1 steel with high and low levels of Cu irradiated in a range of dose from 0.32 to 9.9 × 10 19 n cm −2 ( E > 1 MeV) under a high flux of about 1.7 × 10 13 n cm −2 s −1 using three-dimensional local electrode atom probe (3DAP), positron annihilation (PA) techniques, and Vickers microhardness. The early rapid hardening was found to be caused by mainly matrix defects such as mono- or di-vacancies ( V 1 − V 2 ) and/or dislocations indicated by the PA analysis. The 3DAP analysis showed that dense dispersion of dilute Cu rich clusters and lean distribution of Mn–Ni–Si rich clusters, which were identified to possess the same dislocation-pinning effect by applying a Russell and Brown model, were responsible for large and small hardening in high- and low-Cu steels irradiated above 0.59 × 10 19 n cm 2 , respectively.

Helium retention in metals is related to their atomic structure and the type of defects they contain. In order to investigate the dependence of helium retention on structure, amorphous Fe79B16Si5 and crystalline FeBSi alloys were irradiated by helium ions at room temperature. In the crystalline alloy irradiated with 5 keV He+ ions, three types of helium trapping sites were found: surface defects produced by the irradiation, interstitial-type dislocation loops, and voids. Although these defects did not exist in the amorphous FeBSi alloy, we did observe thermal desorption peaks related to all three types. In addition, helium was released during the crystallization of amorphous FeBSi that had been irradiated by He+ ions.

A533B steels irradiated at 290 °C up to 10 mdpa in the Kyoto University Reactor were examined by hardness, positron annihilation and atom probe measurements. Dose dependent irradiation hardening and formation of Cu-rich clusters were confirmed in medium Cu (0.12% and 0.16%Cu) steels whereas neither hardening nor cluster formation was detected in low Cu (0.03%Cu) steel. No microvoids were formed in any of the steels. Post-irradiation annealing in medium Cu steels revealed that the hardening recovery at temperatures above 350–400 °C could be attributed to compositional changes and dissociation of the Cu-rich clusters. Compared to electron irradiation at almost the same dose and dose rate, KUR irradiation caused almost the same hardening and produced Cu-rich clusters, more solute-enriched with larger size and lower density. Considering lower production of freely-migrating vacancies in neutron irradiation, the results suggested that cascades enhance the formation of Cu-rich clusters.

Body-centered cubic (bcc) Cu precipitates were investigated using empirical N -body potentials fitted to experimental values of face-centered cubic (fcc) Cu and using first-principle calculations, and the validity of applying these fcc empirical potentials to bcc Cu precipitates was examined. The atomic distance, cohesive energy and atomic volume in bcc Fe, fcc Cu and bcc Cu for the two calculations were compared. The magnitude correlation of the atomic distance was different for the two calculations. The vacancy formation energy in an Fe–Cu alloy system for the two calculations was compared and the dependence of the vacancy formation energy on the vacancy position was found to be the same for both calculations when the defect structure was fully relaxed.

Cellular defect structure forms on GaSb, InSb and Ge surfaces irradiated with energetic ions. These structures are generated after void formation via the movement of point defects induced by ion irradiation. In this transmission electron microscopic study, void formation by thin foil irradiation (irradiation after polishing for transmission electron microscopy (TEM)) was compared with void formation by bulk irradiation (cross-sectioning after ion irradiation), in order to investigate the effect of surface sinks for the point defects. The voids formed in GaSb and InSb by thin foil irradiation with 60 keV Sn þ at a temperature of 100–150 K were smaller than those formed by bulk irradiation. The diameters and densities of the voids increased rapidly as the ion dose increased from 1 � 10 14 to 2 � 10 14 ions/cm 2 under both types of irradiation. Amorphous halos were observed in the selected area diffraction patterns (SAED) of the thin foil irradiation specimens in addition to the main spots of the zincblende structure at a dose of 1 � 10 14 ions/cm 2 . This finding contrasts with the polycrystalline ring patterns observed in the SAED of the bulk specimen under the same dose. It was concluded that the easy escape of interstitials to the surface assists both void formation and amorphization in thin foil irradiation. [doi:10.2320/matertrans.MBW200729]

As an image recording medium for transmission positron microscopes, imaging plates are quite useful and powerful. Imaging plates are also quite sensitive and the photon-stimulated luminescence (PSL) is linearly proportional to the positron intensity in six digits (106). No bulky or expensive equipment is necessary to accommodate in vacuum. Imaging plates can be set under bright lights, this is different from the photographic films. Darkness is only required during exposure and transfer to a reader. Slow Positron Facility at KEK, Japan was used to study the effect of “mono-chromatic” positron beam. Specimens were set just in front of an imaging plate. After a certain time of exposure, the imaging plates were processed by a reader. Used imaging plates can be used repeatedly after erased by ultra-violet lights. Images through samples can be obtained. Similar experiments using non-monochromatic (white) positrons and electrons have been performed at Teikyo University of Science and Technology (TUST) and Research Reactor Institute, Kyoto Univ. (RRI). Sealed 22Na positron source can be conveniently used for non-destructive tests.

The characteristics of defects in chromium containing hydrogen or helium atoms have been investigated by positron lifetime quantum calculations. On the basis of calculated results, the behaviors of empty nano-voids and nano-voids with hydrogen or helium were discussed. It was found that hydrogen and helium in larger three-dimensional vacancy clusters change the annihilation characteristics dramatically. The hydrogen and helium atoms are trapped by lattice vacancies. Helium bound with vacancies can form larger size cluster in chromium. These results provide physical insight for positron interactions with defects in chromium and can be used for prediction of hydrogen or helium generation for the design of fission and fusion reactors. The positron lifetime calculations were performed by the standard DFT density functional theory method. The electron wave functions have been obtained in the local density approximation LDA to the DFT.

The authors have found that a cellular defect structure is formed on the surface of Sn + ion implanted GaSb at a low temperature and proposed its formation mechanism based on the movement of the induced point defects. This research was carried out in order to examine the validity of the mechanism by clarifying the effect of the mobility of the point defects on the defect formation. The defect structure on the GaSb surfaces implanted at cryogenic temperature and room temperature was investigated by scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM) observation. In the sample implanted at room temperature, the sponge-like structure (a pileup of voids) was formed and the cellular structure, as observed at a low temperature, did not develop. This behavior was explained by the high mobility of the vacancies during implantation at room temperature, and the proposed idea that the defect formation process is dominated by the induced point defects was confirmed.

Four types of high purity tungsten were irradiated with 2 MeV electrons to 5 dpa using a high voltage electron microscope, and defect structural evolutions were examined as a function of the irradiation temperature and the concentration of impurity atoms. Three of materials were made by sintering of tungsten powder with purity of 99.999% (5N-W), 99.99% (PF-W) and 99.95% (N-W), and one was a chemical vapor deposited tungsten of 99.9999% (CVD-W) purity. The formation of interstitial type dislocation loops is observed above room temperature by electron irradiation. In sintered tungsten, the number density of loops increases with increasing density of impurity atoms, i.e., N-W > PF-W > 5N-W. The density of loops in CVD-W is relatively high, contrary to its purity. In CVD-W, a heterogeneous formation of loops is observed at above 573 K. Loops are aligned on layers, and no loops are formed between the layers. All four types of specimens have a change in slop of the temperature dependence of loop number density at around 500 K which is caused by impurity atoms. Results of radioactivation analysis and hardness testing are also presented.

Isochronal annealing experiments for Fe irradiated with H or He ions were carried out to study interactions between gas atoms and vacancy clusters. The accelerating energy of H and He ions was 1.0 MeV and 3.3 MeV, respectively. The total dose was 1:0 10 21 H ions/m 2 (0.2 dpa) and 9:6 10 19 He ions/m 2 (0.3 dpa), and the irradiation temperature was 300C. Positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements were carried out after irradiation and annealing for 1 hour up to the complete recovery temperature. Microvoids were detected after H or He ion irradiation. The characteristic difference was mainly shown in the recovery of vacancy type defects. The recovery of vacancy type defects introduced by H ion irradiation finished completely at 450C, while that by He ion irradiation finished at 1000C. It suggests that H-vacancy complexes were less stable than He-vacancy complexes. In addition, vacancy clusters formed by He ion irradiation grew after annealing at 600C. This was caused by the emission of He atoms from small He-vacancy clusters and the coalescence between free vacancies and vacancy clusters, and by the migration and coalescence of He bubbles.

Thin foil specimens of Cu and Au were irradiated with fission neutrons at 285–295 K to neutron fluences of 7 × 10 21 n m −2 (>0.1 MeV), using the Kyoto University Reactor. The structure of cascades and subcascades was examined by fitting the experimentally observed distribution of vacancy-type defect clusters to the calculated primary recoil energy spectrum, and was compared directly with that for fusion neutron irradiation. In Cu, 70% of cascades consist of only one defect cluster and 2% of more than four clusters. This is in contrast to larger cascades produced by 14 MeV neutrons that contain more than 10 clusters and extend 60 nm in diameter. The subcascade energy was 18 and 15 keV for Cu and Au, respectively. These values are about 30–60% higher than those obtained for fusion neutron irradiation.

The effects of aging and ion irradiation on microstructure stability and hardness change in the joint materials of CuNiBe/316L and CuAl25/316L have been investigated in the present study. The aging at 673 K for 1000 h or Ni ion irradiation at 573 and 673 K to 10 dpa did not promote the interdiffusion and void swelling at the interface. The hardness in both Cu alloys and stainless steel was increased by irradiation, however, it was decreased by aging except for CuNiBe alloy. The hardness change in CuNiBe alloy was larger than that in CuAl25 alloy. The hardness changes would have a significant effect on the mechanical properties of joint materials.

Based on computer simulations, we examined a new mechanism of plastic deformation that has been proposed to operate in tensile fracture of metal foils. We constructed a Au crystal containing high concentration of vacancies and/or one subjected to large elastic tensile strain using embedded atom method (EAM) potential, and then calculated transmission electron microscopy (TEM) images of the crystal based on multislice method. Randomly distributed vacancies did not cause a large distortion in the crystal lattice, and did not affect the TEM image intensity appreciably unless the vacancy concentration exceeded several percent. Under a large elastic tensile strain of about 10% along 〈100〉, a periodic displacement of atoms whose amplitude was 10% of the atomic distance was induced, reducing the intensity of equal thickness fringe by about half. At around 15% tensile strain along 〈110〉, the crystal transformed from fcc to bcc structure, releasing the distortion of crystal lattice.

It is well known that the minor elements Si and Sn can suppress the formation of voids in Ni alloys. In the present study, to investigate the effects of Si and Sn on the retention of helium in Ni alloys, Ni, Ni–Si, and Ni–Sn alloys were irradiated by 5 keV He ions at 723 K. Thermal desorption spectroscopy (TDS) was performed at up to 1520 K, and microstructural observations were carried out to identify the helium trapping sites during the TDS analysis. Two peaks, at 1350 and 1457 K, appeared in the TDS spectrum of Ni. On the basis of the microstructural observations, the former peak was attributed to the release of trapped helium from small cavities and the latter to its release from large cavities. Small-cavity helium trapping sites were also found in the Ni–Si and Ni–Sn alloys, but no large cavities were observed in these alloys. In addition, it was found that the oversized element Sn could trap He atoms in the Ni–Sn alloy.

To investigate the migration behavior of interstitials and vacancies in vanadium alloy V–4Cr–4Ti, high purity V alloys were irradiated with 2 MeV electrons using a high voltage electron microscope (HVEM) and 28 MeV electrons using a electron linear accelerator. Interstitial type dislocation loops formed quickly in all cases of HVEM irradiation. The saturation density of the dislocation loops was independent of irradiation temperature and defect production rate at temperatures in the range of 323–448 K. However, the logarithm of saturated dislocation loop density was inversely proportional to the irradiation temperature, and dislocation loop density was directly proportional to the square root of the defect production rate above 448 K. The dissociation energy of the interstitial-impurity complex in V–4Cr–4Ti alloy was estimated to be 1.1 eV from the HVEM experiment. It was found from positron lifetime measurements that the vacancies form vacancy clusters at 373 K.

One dimensional (1-D) motion of interstitial clusters is important for the microstructural evolution in metals. In this paper, the effect of 2 at.% alloying with elements Si (volume size factor to Ni: −5.81%), Cu (7.18%), Ge (14.76%) and Sn (74.08%) in Ni on 1-D motion of interstitial clusters and void growth was studied. In neutron irradiated pure Ni, Ni–Cu and Ni–Ge, well developed dislocation networks and voids in the matrix, and no defects near grain boundaries were observed at 573 K to a dose of 0.4 dpa by transmission electron microscopy. No voids were formed and only interstitial type dislocation loops were observed near grain boundaries in Ni–Si and Ni–Sn. The reaction kinetics analysis which included the point defect flow into planar sink revealed the existence of 1-D motion of interstitial clusters in Ni, Ni–Cu and Ni–Ge, and lack of such motion in Ni–Si and Ni–Sn. In Ni–Sn and Ni–Si, the alloying elements will trap interstitial clusters and thereby reduce the cluster mobility, which lead to the reduction in void growth.

To investigate the microstructure evolution during temperature change irradiation, Ni and its binary alloys were irradiated in the Japan Materials Testing Reactor to doses up to 0.46 dpa under two cycles of temperature change of 533/693 K, i.e., 533 K, then 693 K, then 533 K, then 693 K. The irradiation dose at each temperature stage was about the same. The specimen sets were pulled out five times during irradiation. In the beginning of the first irradiation at the high temperature of 693 K after irradiation at 533 K, the densities of interstitial type dislocation loops and voids decreased in pure Ni, Ni–2at.%Cu and Ni–2at.%Ge alloys. In the second irradiation at 533 K, however, the formation of dislocation loops and voids was promoted. The void density continued to increase even after the second irradiation at 693 K. On the other hand, in Ni–2at.%Si and Ni–2at.%Sn alloys, where the formation of voids was suppressed by addition of the minor elements Si or Sn, the microstructure evolution was almost independent of irradiation temperature.

Positron lifetime of defects in nickel containing hydrogen or helium has been calculated, using Monte-Carlo, density-functional theory and local-density approximation. It is found that the values of positron lifetime in voids are lowered by the presence of hydrogen or helium. The calculations show a correlation between the microvoid size in nickel and the value of the positron lifetime. With the growth of the void size the difference in the positron lifetime for clusters containing hydrogen or helium becomes significant. Calculations show a saturation of the positron lifetime for microvoids containing more than 45 hydrogen atoms and around 30 helium atoms. A dependence between the positron lifetime and the ratio of hydrogen or helium atoms to the number of vacancies in nickel voids is established.

Defect structures in iron under varying temperature neutron irradiation and those under constant-temperature irradiation were examined by a transmission electron microscope (TEM). For irradiation at a low temperature (473 K), microvoids were detected by positron annihilation lifetime (PAL) measurement. For irradiation at a high temperature (673 K), with increasing the irradiation dose a few interstitial (I)-type dislocation loops grew and the total number density of the loops decreased. In a varying temperature irradiation between 473 and 673 K, after a shift to the high temperature the absorption of interstitials to I-type dislocation loops was suppressed by microvoids introduced at the low temperature and the loops shrank or disappeared by vacancies released from microvoids. As the cycle of varying temperature irradiation became short, the effect of the suppression of the defect development became small.

Systematic experiments of 1 MeV electron irradiation were made on Cu-based binary alloys above 300 K using a high-voltage electron microscope in order to study the effects of solute atoms on defect structure development. The solute elements examined were Si (+5.08%), Ge (+27.77%) and Sn (+83.40%), the volume size factors of which are given in parentheses; the amounts of these were 0.05, 0.3 and 2 at.% respectively. Interstitial-type dislocation loops and stacking-fault tetrahedra (SFTs) were formed in pure Cu and all the alloys. In pure Cu, the temperature dependence of the loop number density had a ‘down peak’ (i.e. loops hardly formed) around 373 K; below this temperature the majority of the loops shrank and disappeared during irradiation, while all the loops grew larger above it; and SFTs were unstable and repeated the formation and disappearance. In the alloys, the loop number density decreased monotonically with increasing temperature (no down peak was observed); loop formation was greatly e...

Kinetic Monte Carlo (kMC) computer simulations were used to study the effects of temperature change during low-dose 14-MeV fusion neutron irradiations on defect accumulation in copper. Instantaneous temperature changes from 300 to 473 K, from 300 to 673 K and from 373 to 573 K were simulated. All the high temperatures are above the Stage-V annealing temperature for copper, approximately 400 K. The total simulated dose was 0.01 dpa for all temperature-change simulations, at a dose rate of 10 −6 dpa/s. In general, the concentrations of both self-interstitial atom (SIA) clusters and vacancy clusters decrease when the temperature is changed from low to high. The relative decrease in concentration of defect clusters when the temperature is increased is determined by the level of accumulation of vacancy clusters in the initial low-temperature part of the irradiation as well as by the specific temperatures involved. If the vacancy clusters are small and their concentration is high, the decrease of cluster density is prominent when the temperature increases, and the higher the second temperature, the easier it is for defect clusters to dissolve.

The effect of alloying elements, Si (-5.8%: the volume size factor in Ni), Ge (+14.76%) and Sn (+74.08%), on void swelling in neutron irradiated Ni at 573 K was studied by transmission electron microscope (TEM) observation and positron annihilation lifetime measurement. Neutron irradiation dose was changed widely from 0.001 to 0.4 dpa using two reactors, the Kyoto University reactor (KUR) and the Japan materials testing reactor (JMTR). Voids were observed in pure Ni by TEM even after very small irradiation dose of 0.001 dpa. With increasing dose, the density of voids did not change much while their size increased. The same tendency was observed in Ni-2at.%Ge. In Ni-2at.%Sn and Ni-2at.%Si, however, no voids were observed by TEM at a damage dose of 0.4 dpa. But positron lifetime measurement revealed the existence of microvoids at a medium dose of irradiation. When irradiation dose increased to 0.4 dpa in Ni-2at.%Si and 0.13 dpa in Ni-2at.%Sn, their existence was not detected. Suppression of microvoids in these alloys is discussed from the standpoint of solute point defect interactions.

The activation energy for the migration of vacancies in Cu under high strain was calculated by computer simulation using static methods. The migration energy of vacancies was 0.98 eV in the absence of deformation. It varied with the migration direction and stress direction because the distance between a vacancy and its neighboring atoms changes by deformation. For example, the migration energy for the shortest migration distance was reduced to 9.6 and 39.4% of its initial value by 10% compression and 20% elongation, respectively, while that for the longest migration distance was raised to 171.7 by 20% elongation. If many vacancies are created during high-speed deformation, the lowering of migration energy enables vacancies to escape to sinks such as surfaces, even during the shorter deformation period. The critical strain rate above which the strain rate dependence of vacancy accumulation ceases to exist increases with the lowering of vacancy migration energy.

Stacking-fault tetrahedra (SFT) are typical vacancy clusters in fcc metals. SFT have been, however, considered to be unstable in aluminum because of its high stacking-fault energy, until the recent finding of SFT in thin aluminum foils subjected to a tensile fracture. This study confirmed that SFT form in aluminum following irradiation with high-energy particles. In electron irradiation, SFT with an average size of 2 nm formed below 203 K, while a larger irradiation intensity at lower temperature induced SFT with a larger number density. Irradiation with 60-keV Al + ions induced SFT below room temperature, although the defect yield (the ratio of the number of defect clusters to the number of incident ions) was about 10 3 , considerably smaller than that in the other fcc pure metals. With neutron irradiation below 15 K to a fluence of 2 × 10 2 1 neutrons m - 2 , SFT were not observed at room temperature. Instead, dislocation loops were observed to form and to disappear during observation with 120-kV electrons that do not cause atomic displacements. Tensile fracture of aluminum thin foil induced SFT at up to 400 K, which is close to the temperature at which SFT become unstable in isochronal annealing experiments. These results suggest that a high concentration of vacancies at lower temperature tends to cause SFT to form rather than vacancy-type dislocation loops in aluminum.

α-Al2O3, AlN, β-Si3N4, and β-SiC ceramic specimens were irradiated by a 30 MeV electron beam up to 0.01 dpa at 80°C, and they started to recover thermal diffusivity with annealing from 300{500°C. In previous studies, similar ceramics were heavily neutron-irradiated in a fast reactor, JOYO, and the neutron-irradiated β-Si3N4 and β-SiC also showed recovery above 400°C, though they were irradiated at 502°C or 738°C. This result is explained by the hypothesis that a ‘primary’ irradiation occurred at the set temperature, and after that, a ‘secondary’ irradiation occurred at a lower temperature during the shutdown process of the reactor. In contrast, the neutron-irradiated β-Al2O3 and AlN did not show such unexpected recovery, but showed recovery above 800-900°C. This difference is explained on the basis of structural models of dislocation loops namely, the ‘pileup’ model and the ‘nano-partition’ model.

Numerical calculations of radiation damages in beryllium, alpha-iron and tungsten irradiated by fusion neutrons were performed using molecular dynamics (MD) simulations. The displacement cascades efficiency has been calculated using the Norgett-Robinson-Torrens (NRT) formula, the universal pair-potential of Ziegler-Biersack-Littmark (ZBL) and the EAM inter-atomic potential. The pair potential overestimates the defects production by a factor of 2. The ZBL pair potential results and the EAM are comparable at higher primary knock-on atom (PKA) energies (E > 100 keV). We found that the most common types of defects are single vacancies, di-vacancies, interstitials and small number of interstitial clusters. On the bases of calculated results, the behavior of vacancies, empty nano-voids and nano-voids with hydrogen and helium were discussed.