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Dynamical process of helium bubbles in face‐centred cubic (fcc) metals has been studied by in situ irradiation and electron microscopy with a video recording system. Brownian type motion of helium bubbles was demonstrated quantitatively by establishing a proportional relation between the mean square of the bubble migration distance and time. This relation yielded the diffusion coefficient of the bubble. Examples of Brownian motion in the matrix and along general grain boundaries in pure aluminium are shown comparatively. Similar Brownian motion was also observed in an Fe‐16Cr‐17Ni austenitic alloy. It was revealed from scanning transmission electron microscopy–electron energy‐loss spectroscopy that Cr was depleted around the bubble surface, but Ni was enriched. Response of the bubble motion to irradiation with high‐energy self‐ions was examined in pure aluminium and both effects of enhancement and retardation of the bubble mobility were found.

High-speed, high-strain tensile deformation of thin foils of fcc metals has recently been found to result in the formation of an anomalously high density of small vacancy clusters, probably in the absence of dislocations. In the present study, deformation of secondary-defect-pre-introduced pure Al is performed; at strain rates ranging from 10 m 4 to 10 5 /s and a deformation temperature of m 196 or 25°C. Microstructures in the deformed regions are examined by transmission electron microscopy. Vacancy-type dislocation loops and voids are eliminated by the deformation. He-bubbles are observed to elongate under a tensile stress, mainly in the absence of dislocations. Rows of bubbles oriented close to directions of elongation are found, and might result from extreme elongation and division of bubbles. The bubble rows are parallel to particular crystallographic directions, either d 001 ¢ or d 112 ¢ . This indicates that displacement of atoms during high-speed, high-strain tensile deformation progresses while c...

Sano T., Eimura T., Kashiwabara R., et al. Femtosecond laser peening of 2024 aluminum alloy without a sacrificial overlay under atmospheric conditions. Journal of Laser Applications, 29, 1, 012005. https://doi.org/10.2351/1.4967013., The authors have successfully performed femtosecond laser peening on a 2024 aluminum alloy without any sacrificial overlays. Laser pulses were directly irradiated to the surface of specimens in the air without water film as a plasma confinement medium during the peening treatment. The fatigue life was improved as much as 38 times in comparison with base material at a stress amplitude of 195 MPa. The fatigue strength of the peened specimen after 2 × 106 cycles was 58 MPa larger than that of the base material. The femtosecond laser peening process has a great potential to be applied in various fields where conventional peening methods cannot be used, as this process can be performed under ambient conditions without the use of a plasma confinement medium such as water or transparent materials.

The formation process of point defect clusters in high-purity (99.999%) iron as a typical bcc metal under the irradiation with low-energy (5 keV)He+ is studied by in situ transmission electron microscopy (TEM). Using conventional TEM techniques, clusters induced by the irradiation are determined to be interstitial-type dislocation loops (I loops) at temperatures ranging from 85 to 770 K and cavities from 300 to 770 K. Most of the I loops are determined to lie on {100} planes, and their Burgers vector are determined to be a[100]. The temporal variation in the volume density of I loops is measured by stereomicroscopy at several temperatures. The volume density of I loops is about two orders of magnitude higher in comparison with the case of high-energy electron irradiation at the same dpa rate; hence, it is deduced that helium atoms have an effect enhancing the nucleation of I loops. The depth distribution of the loop densities becomes broad above around 235 K at which vacancies become thermally mobile, and...

The formation and migration of helium bubbles in Fe–16Cr–17Ni austenitic model alloy at temperatures 400–1250°C have been studied by in situ electron microscopy, energy dispersive spectrometry (EDS) and EELS analysis. The nucleation of bubbles under 10 keV He+ ion irradiation was dominant at 400°C, while the growth was dominant above 600°C. It was revealed that the mean square of the migration distance was proportional to time, which is quantitative evidence of Brownian type motion and yielded diffusivities from 10−18 to 10−20 m2/s at 1185°C, depending on the diameter from 1 to 3 nm. These facts suggest that the mobility of helium–vacancy complexes or bubbles is an important factor governing the formation process of helium bubbles. The analysis by scanning transmission electron microscope-electron energy loss spectrometry (STEM-EELS) elucidated Ni precipitation even around small helium bubbles.

High-speed heavy plastic deformation of thin foils of fcc metals, including aluminium, is found to produce a high density of small vacancy clusters, in the form of stacking-fault tetrahedra. The dependences of the density of the clusters on the deformation temperature and deformation rate indicate the production of vacancy clusters from deformation-induced dispersed vacancies. Neither dislocations nor any indication of the reaction of dislocations are present in the regions containing a high density of vacancy clusters. A possible model is proposed that describes, at extremely high strain rates where dislocation generation is difficult, how a high concentration of point defects is produced by a large number of parallel shifts of atomic planes without dislocations.

Dynamical process of defects clustering in pure Ni under the irradiation with low energy (typically 5 keV) He+ ions has been studied by in situ electron microscopy. Effects of the ion energy (0.5–20 keV), fluence, flux, depth and irradiation temperature on the formation of interstitial type dislocation loops (I-Loops) and bubbles were examined. The density of I-Loops sharply increased with the fluence, but was slightly dependent on the ion flux. It is demonstrated that the formation of I-Loops at room temperature is promoted 3–4 orders magnitude higher than that expected in the case of no assistance of helium atoms on the nucleation. These results suggest a possible nucleation mechanism where helium–vacancy complexes trap the self-interstitial atoms and act as nucleation sites of I-Loops. By irradiation with 20 keV He+ ions, SFT were formed even at room temperature, coexisting with I-Loops. Bubbles are formed preferentially inside of I-Loops, and by more heavy irradiation, they coalesced or interconnected, leading to a characteristic channel structure.

The behavior of dislocations of GaN single crystals grown using the liquid phase epitaxy (LPE) technique in a Na flux was investigated using a transmission electron microscope (TEM). Almost all dislocations are concentrated around the interface between LPE-GaN and metal organic chemical vapor deposition (MOCVD)-GaN, which was used as a homoepitaxial substrate. It was revealed that the reduction of dislocation is due to an unusual phenomenon in which many (1011) facets are formed in the initial LPE growth period, followed by the development of the GaN(0001) face with further dislocation reduction. On the basis of our TEM observations and an investigation of the surface morphology at each growth stage, we propose a dislocation reduction mechanism during LPE growth.

The dynamical response of helium bubbles to irradiation with high-energy self-ions in pure gold has been studied by in situ electron microscopy. It is found that sporadic movement of small helium bubbles is induced in random directions within a few nm or less under the irradiation with 400 keV Au + ions at room temperature, where bubbles are thermally immobile. Computer simulation indicates that these facts are consistent with the bubble movement interacting with the collision sub-cascades.

We quantitatively analyzed the contrast degradation and blur of 20-nm gold nanoparticles adsorbed on the top of amorphous silicon films of thicknesses of 0.54, 1.09, 1.63 and 2.2 μm in bright-field transmission electron microscope (TEM) images taken at accelerating voltages of 0.5, 1, 2 and 3 MeV. The thickness dependence of the transmission was well explained and consistent with our calculations. The blur function, derived by assuming that the TEM image of a thick specimen can be reproduced by convolving the TEM image of a very thin specimen with it, was found to be expressed by a two-dimensional Lorentzian function. Considering the two characteristics of the Lorentzian function, a sharp peak around the center and a long tail, we concluded that, for TEM observations of thick specimens, the image contrast is degraded predominantly by inelastic scattering and the image is blurred predominantly by multiple elastic scattering.

Spatial distributions of radiation induced point defect clusters were analyzed with comparison of the distribution function of the separation of 1st-nearest-neighbor cluster pairs with that of the random distribution and the radial distribution function. In electron irradiation, radiation induced interstitial type dislocation loops nucleate at random. With the increase of the number of loops, the spatial distribution deviates towards an homogeneous one owing to the suppression of the formation of new loops near the existing loops. In aluminum, the radius of the effective repulsive interaction between neighbor loops is about 20 nm. In neutron irradiation, the spatial distribution of radiation induced vacancy clusters is heterogeneous owing to the collision cascade from randomly generated primary knock-on atoms. In D-T neutron irradiated silver, the size of the cascade is estimated to be about 30 run from their heterogeneous distribution.

The size limitation of ab initio calculations impedes first-principles simulations of crystal defects at nanometer sizes. Considering clusters of self-interstitial atoms as a paradigm for such crystal defects, we have developed an ab initio--accuracy model to predict formation energies of defect clusters with various geometries and sizes. Our discrete-continuum model combines the discrete nature of energetics of interstitial clusters and continuum elasticity for a crystalline solid matrix. The model is then applied to interstitial dislocation loops with $\ensuremath{\langle}100\ensuremath{\rangle}$ and $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ Burgers vectors, and to C15 clusters in body-centered-cubic crystals Fe, W, and V, to determine their relative stabilities as a function of size. We find that in Fe the C15 clusters were more stable than dislocation loops if the number of self-interstitial atoms involved was fewer than 51, which corresponds to a C15 cluster with a diameter of $1.5$ nm. In V and W, the $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ loops represent the most stable configurations for all defect sizes, which is at odds with predictions derived from simulations performed using some empirical interatomic potentials. Further, the formation energies predicted by the discrete-continuum model are reparametrized by a simple analytical expression giving the formation energy of self-interstitial clusters as a function of their size. The analytical scaling laws are valid over a very broad range of defect sizes, and they can be used in multiscale techniques including kinetic Monte Carlo simulations and cluster dynamics or dislocation dynamics studies.

Y.Shiratsuchi, H.Noutomi, H.Oikawa, et al. Detection and in situ switching of unreversed interfacial antiferromagnetic spins in a perpendicular-exchange-biased system. Physical Review Letters 109, 077202 (2012); https://doi.org/10.1103/PhysRevLett.109.077202., By using the perpendicular-exchange-biased Pt/Co/α-Cr₂O₃ system, we provide experimental evidence that the unreversed uncompensated Cr spins exist at the Co/α-Cr₂O₃ interface. The unreversed uncompensated Cr spin manifests itself in both the vertical shift of an element-specific magnetization curve and the relative peak intensity of soft-x-ray magnetic circular dichroism spectrum. We also demonstrate an in situ switching of the interfacial Cr spins and correspondingly a reversal of the exchange bias without interfacial atomic diffusion. Such switching shows the direct relationship between the interfacial antiferromagnetic spins and origin of the exchange bias. The demonstrated switching of exchange bias would likely offer a new design of advanced spintronics devices, using the perpendicular-exchange-biased system, with low power consumption and ultrafast operation.

Topics of in-situ transmission electron microscopy studies on the dynamic behaviors of materials are described. Estimation of the activation energy for the migration of self-interstitial atoms is successfully carried out. The coalescence between nm-scale prismatic dislocation loops with different Burgers vectors, as well as the one-dimensional glide diffusion of the loops, has been confirmed. A new electron-beam-induced deposition method using no precursor gas, has been developed for the fabrication of tungsten nanostructures. Size dependence of the eutectic temperature of alloy particles was examined. A linear relationship between the eutectic temperature and the inverse radius of the particle was obtained. In-situ experiments with TEM are useful in revealing the dynamic behaviors of materials.

In situ real-time electron microscope observations of metals irradiated with ultrahigh-energy electrons or energetic ions show that the dynamics of microstructural evolution in these materials is strongly influenced by long-range elastic interactions between mobile nanoscale radiation defects. Treating long-range interactions is also necessary for modeling microstructures formed in ex situ high-dose-rate ion-beam irradiation experiments, and for interpolating the ion-beam irradiation data to the low-dose-rate limit characterizing the neutron irradiation environments of fission or fusion power plants. We show that simulations, performed using an algorithm where nanoscale radiation defects are treated as interacting Langevin particles, are able to match and explain the real-time dynamics of nanodefects observed in in situ electron microscope experiments.

Point defects and their clusters formed by high energy particle irradiation, plastic deformation, and so on, often affect mechanical and/or electric properties of materials severely. Therefore, understanding of dynamic processes such as formation, growth, shrinkage, motion, and transformation of these defects is important in materials science. We briefly review our recent results related to the dynamics of point‐defect clusters in metals, which have been obtained by using in‐situ transmission electron microscopy.

We report the observations of a new type of changing process in the Burgers vector of dislocations by in situ transmission electron microscopy. Small interstitial-type perfect dislocation loops in bcc iron with diameters less than approximately 50 nm are transformed from a 1/2111loop to another 1/2111one or an energetically unfavorable100one; furthermore, a100loop is transformed to a 1/2111one. These transformations occurred on high-energy electron irradiation or simple heating without contact with external dislocations. The origin of these phenomena is discussed.

We found that a femtosecond laser-driven shock wave induces marked changes in dislocation structure in iron over a fluence range from 1.3 to 8.3 J/cm2. Transmission electron microscopy observations showed a change in dislocation structure from lath structures with twist boundaries to only laths, and an increase in depth where laths begin to appear, with increasing fluence. X-ray diffraction results showed the distribution of crystallite sizes corresponding to the change in dislocation structure. We proposed that the dislocation structure is determined by the laser fluence, through the change in the duration of the shock wave.

We propose new techniques of electron irradiation for studying the kinetics of simple point defects and their agglomeration, by making the most use of the intrinsic function of an ultrahigh-voltage scanning transmission electron microscope. The methods enable us to realize a very high defect production rate at a localized area and create a spot or band-shape source of point defects. The present methods were applied to an attempt to detect the point defects flowing out of a very localized area, examining the growth and shrinkage of pre-introduced interstitial clusters. At low temperatures where vacancies are immobile, outflow of the interstitials was observed. In contrast, at an elevated temperature where vacancies are mobile, outflow of the vacancies seemed to play a dominant role in the apparent secondary defect reactions. A possible explanation to aid understanding of the present results is proposed.

Matsuda T., Sano T., Arakawa K., et al., Applied Physics Letters, 105(2), 021902 (2014) https://doi.org/10.1063/1.4890389, We found that multiple shots of femtosecond laser-driven shock pulses changed coarse crystalline iron grains with a size of 140 μm into nanocrystals with a high density of dislocations, which had never been observed in conventional shock processes. We performed metallurgical microstructure observations using transmission electron microscopy (TEM) and hardness measurements using nanoindentation on cross-sections of shocked iron. TEM images showed that grains with sizes from 10 nm through 1 μm exist within 2 μm of the surface, where the dislocation density reached 2 × 10 15m-2. Results of the hardness measurements showed a significant increase in hardness in the nanocrystallized region. We suggest that the formation of a high density of dislocations, which is produced by a single shock, induces local three-dimensional pile-up by the multiple-shocks, which causes grain refinement at the nanoscale. © 2014 AIP Publishing LLC.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

The stochastic fluctuation of point defect reaction was studied by observing atomistic level of random growth and shrinkage of small vacancy clustered defects under electron irradiation with a high voltage electron microscope. The phenomena of the stochastic fluctuation was detected in all three representative materials; copper with much higher mobility of interstitials than vacancies, Fe-15Cr-20Ni alloy with less difference of the mobilities, and Ag-9at.%Zn alloy with higher mobility of vacancies than interstitials. A Fourier analysis of the time-wise fluctuation of the size of small vacancy clusters showed no particular frequency peak. A Monte Carlo simulation succeeded to reproduce the experimentally observed fluctuation of the size of small vacancy clusters. The randomness of point defect reactions was proved.

A comparative study on the size dependence of the melting temperatures of pure metal and alloy nanoparticles has been carried out. It was found that the melting temperatures of Bi-Sn, In-Sn, and Pb-Sn alloy nanoparticles decreased more rapidly with decreasing particle size than those of the constituent metal nanoparticles (Bi, In, Pb, Sn). Namely, the size dependence of the melting temperature was stronger for the alloy nanoparticles than that for the constituent metal nanoparticles. Results calculated with a thermodynamic model were in good agreement with the experimental observations.

The eutectic transition of Ag–Pb alloy nanoparticles was investigated using in situ transmission electron microscopy. It was found that the eutectic temperature of Ag–Pb alloy nanoparticles decreased with decreasing particle size. A linear relationship was obtained for the eutectic temperature as a function of the reciprocal of the particle radius. Theoretical calculations based on the theory of thermodynamics also suggested a linear relationship between the eutectic temperature and the inverse radius. The calculated results were in good agreement with the experimental observations.

Vacancy-mediated climb models cannot account for the fast, direct coalescence of dislocation loops seen experimentally. An alternative mechanism, self climb, allows prismatic dislocation loops to move away from their glide surface via pipe diffusion around the loop perimeter, independent of any vacancy atmosphere. Despite the known importance of self climb, theoretical models require a typically unknown activation energy, hindering implementation in materials modeling. Here, extensive molecular statics calculations of pipe diffusion processes around irregular prismatic loops are used to map the energy landscape for self climb in iron and tungsten, finding a simple, material independent energy model after normalizing by the vacancy migration barrier. Kinetic Monte Carlo simulations yield a self climb activation energy of 2 (2.5) times the vacancy migration barrier for 1/2〈111〉 (〈100〉) dislocation loops. Dislocation dynamics simulations allowing self climb and glide show quantitative agreement with transmission electron microscopy observations of climbing prismatic loops in iron and tungsten, confirming that this novel form of vacancy-free climb is many orders of magnitude faster than what is predicted by traditional climb models. Self climb significantly influences the coarsening rate of defect networks, with important implications for post-irradiation annealing.

For global survival, we need to launch a rapid regeneration of the nuclear power industry. The replacement of the present fossil fuel industry requires a doubling time for alternative energy sources of 5–7 years and only nuclear energy has the capability to achieve this. The liquid metal cooled fast breeder reactors (LMFBR) have the best breeding criteria but the doubling time exceeds 20 years. Further, the use of plutonium in these systems has the potential of nuclear proliferation. The Thorium Molten-Salt Nuclear Energy Synergetic System [THORIMS-NES], described here is a symbiotic system, based on the thorium–uranium-233 cycle. The production of trans-uranium elements is essentially absent in Th–U system, which simplifies the issue of nuclear waste management. The use of 233 U contaminated with 232 U as fissile material, instead of plutonium/ 235 U makes this system nuclear proliferation resistant. The energy is produced in molten-salt reactors (FUJI) and fissile 233 U is produced by spallation in Accelerator Molten-Salt Breeders (AMSB). This system uses the multi-functional ‘‘single-phase molten-fluoride” circulation system for all operations. There are no difficulties relating to ‘‘radiation-damage”, ‘‘heat-removal” and ‘‘chemical processing” owing to the simple ‘‘idealistic ionic liquid” character of the fuel. FUJI is size-flexible, and can use all kinds of fissile material achieving a nearly fuel self-sustaining condition without continuous chemical processing of fuel salt and without core-graphite replacement for the life of the reactor. The AMSB is based on a single-fluid molten-salt target/blanket concept. Several AMSBs can be accommodated in regional centers for the production of fissile 233 U, with batch chemical processing including radio-waste management. FUJI reactor and the AMSB can also be used for the transmutation of long-lived radioactive elements in the wastes and has a high potential for producing hydrogen-fuel in molten-salt reactors. The development and launching of THORIMS-NES requires the following three programs during the next three decades