Kazuto Arakawa

Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations

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.

Evolution of point defect clusters in pure iron under low-energy He+ irradiation

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...

Drastic Decrease in Dislocations during Liquid Phase Epitaxy Growth of GaN Single Crystals Using Na flux Method without Any Artificial Processes

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.

Formation process of dislocation loops in iron under irradiations with low-energy helium, hydrogen ions or high-energy electrons

Abstract Formation processes of interstitial-type dislocation loops (I loops) in high-purity Fe under irradiations with 5 keV H+ ions or 1000 keV electrons are examined by in situ transmission electron microscopy at temperatures below room temperature, and the results are compared with that obtained under He+ ion irradiation. For the electron irradiation, conventional model of I-loop nucleation based on the assumption that di-interstitial atoms are stable nuclei of I loops is questioned. The volume density of I loops by H+ ion irradiation is one-order of magnitude higher than that by electron irradiation, and several times lower than that by He+ ion irradiation. The temperature dependence of the volume density of I loops by H+ ion irradiation supports the idea that such enhancement of I-loop formation is due to trapping of self-interstitial atoms by gas atom-vacancy complexes.

Formation and migration of helium bubbles in Fe and Fe–9Cr ferritic alloy

Abstract Formation and migration of helium bubbles in high purity Fe and an Fe–9Cr ferritic alloy have been studied by in situ electron microscopy and scanning transmission electron microscope–electron energy loss spectrometry (STEM–EELS) analysis. The swelling under irradiation with 10 keV He + ions in the ferritic alloy was retarded in the low fluence region at 400 °C and was reduced at 600 °C even under heavy irradiation. These results suggest trapping effects of vacancies by Cr and its modulation with helium atoms in the bubble formation. Furthermore, it is found that the mean square of the bubble migration distance in both materials is proportional to time, which is a quantitative evidence of Brownian type motion and yields lower diffusivities of the bubbles in Fe–9Cr than in Fe. The retardation of the bubble mobility in Fe–9Cr should be caused by Cr segregation on the bubble surface which was revealed by STEM–EELS analysis.

Langevin model for real-time Brownian dynamics of interacting nanodefects in irradiated metals

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.

Spatial ordering of nano-dislocation loops in ion-irradiated materials

Defect microstructures formed in ion-irradiated metals, for example iron or tungsten, often exhibit patterns of spatially ordered nano-scale dislocation loops. We show that such ordered dislocation loop structures may form spontaneously as a result of Brownian motion of loops, biased by the angular-dependent elastic interaction between the loops. Patterns of spatially ordered loops form once the local density of loops produced by ion irradiation exceeds a critical threshold value.

Activation energy for long-range migration of self-interstitial atoms in tungsten obtained by direct measurement of radiation-induced point-defect clusters

The activation energy for the long-range intrinsic migration of self-interstitial atoms (SIAs) in metals, , is an important physical quantity closely associated with microstructural evolution upon energetic particle irradiation. The values for various metals have been widely investigated through recovery experiments on specimens irradiated at low temperatures upon thermal annealing, and the values have been estimated from the dependence of measured quantities of the specimens on the annealing temperature. On the other hand, the dependence of measured quantities on irradiation temperature is also expected to reflect the values. It is of importance to compare the values obtained by these two different kinds of experiments. However, no systematic studies have been carried out along the latter line. In this study, the number densities of SIA clusters formed in tungsten upon high-energy electron irradiation are directly measured as a function of the irradiation temperature using high-voltage electron microscopy. The analysis of the experimental data shows that the value is in the range from 0.088 to 0.102 eV or it is less than 0.046 eV. These values are consistent with those obtained in a recovery experiment and a theoretical study, respectively.

Quantitative analysis on size dependence of eutectic temperature of alloy nanoparticles in the Ag–Pb system

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.

Formation and migration of helium bubbles in Fe-16Cr-17Ni austenitic alloy at high temperature

Abstract 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.