Radiation-induced glide motion of interstitial clusters in concentrated alloys

We propose a mechanism for glide motion, i.e. one-dimensional (1D) migration, of interstitial clusters in concentrated alloys driven by high-energy particle irradiation. Interstitial clusters are fundamentally mobile on their respective 1D migration tracks, but in concentrated random alloys they are stationary at the position where the fluctuating formation energy achieves a local minimum. Irradiation changes the microscopic distribution of solute atoms through atomic displacement and recovery of the produced Frenkel pairs, which causes cluster 1D migration into a new stable position. In molecular dynamics simulations of interstitial clusters up to 217i in Fe–Cu alloys, stepwise 1D migration was observed under interatomic mixing or shrinkage of the cluster: a single 1D migration was induced by two exchanges per atom or cluster radius change by two interatomic distances. The 1D migration distance ranged up to several nanometers. We compared the frequency and distance of 1D migration with those for in situ observation using high-voltage electron microscopy, allowing for the extremely large rate of interatomic mixing and cluster shrinkage in the present simulation.     

Vacancy effects on one-dimensional migration of interstitial clusters in iron under electron irradiation at low temperatures

We performed in situ observation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110–300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 10^?4–10^?2, using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 10^?3. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile.     

One-dimensional atomic transport by clusters of self-interstitial atoms in iron and copper

Atomic-scale computer simulation has been used to study the thermally activated atomic transport of self-interstitial atoms (SIAs) in the form of planar clusters in pure Cu and f-Fe. There is strong evidence that such clusters are commonly formed in metals during irradiation with high-energy particles and play an important role in accumulation and spatial distribution of surviving defects. An extensive study of the mobility of SIA clusters containing two to 331 interstitials has been carried out using the molecular dynamics simulation technique for the temperature range from 180 to 1200 K. The results obtained show that clusters larger than three to four SIAs are one-dimensionally mobile in both Cu and Fe. Large clusters of more than 100 SIAs in Cu and 300 SIAs in Fe have significantly reduced mobility. The problem of describing one-dimensional (1D) motion in three-dimensional space is discussed. An attempt is made to describe the mobility of SIA clusters within the approximation of 1D diffusion. For clusters in both metals the effective migration energy of 1D diffusion as estimated via the jump frequency of the cluster centre of mass is found to be independent of the number of SIAs in the clusters, although the cluster jump frequency decreases with increasing cluster size. Mechanisms of 1D mobility of interstitial clusters are discussed.     

One-dimensional migration of interstitial clusters in SUS316L and its model alloys under electron irradiation

The one-dimensional (1D) migration of interstitial clusters in austenitic stainless steel SUS316L and its model alloys, namely, Fe–18Cr–13Ni, Fe–18Cr–13Ni–0.012C, and Fe–18Cr–13Ni–1.7Mn (mass %), was examined using in situ observation by high-voltage electron microscopy. Such 1D migration was confirmed to occur along the ?110? direction at irregular intervals in all these alloys under 1250-kV electron irradiation at room temperature. The frequency of 1D migration was found proportional to electron beam intensity, and was about 1/10 that in high-purity iron under the same irradiation intensity. The distance of 1D migration in the four alloys was less than 10?nm, which was much shorter than that in high-purity iron. No clear difference in the frequency or distance of 1D migration was observed among the four alloys, suggesting that minor solute/impurity elements have no apparent effect on 1D migration in SUS316L.     

Structure of endohedral complexes of carbon nanotubes encapsulated with lithium and sodium

The article provides the results of ab initio calculations employing density functional theory of carbon nanotubes that contain clusters of lithium and sodium atoms. Stable positions of interstitial atoms, the electron density distribution in the system and the density of electronic states are determined. It is shown that the amount of charge transferred from the interstitial atoms in a cluster significantly differs from the corresponding value for a single atom. It is established that the density of electronic states of the system at low concentrations of atoms of the introduced element is determined by the electronic structure of a hollow nanotube, and as the concentration of interstitial atoms increases, this quantity becomes virtually independent on the type of alkali metal (lithium or sodium) and the initial type of the nanotube conductivity.     

Athermal migration of vacancies in iron and copper induced by electron irradiation

Abstract

Irradiation with high-energy particles induces athermal migration of point defects, which affects defect reactions at low temperatures where thermal migration is negligible. We conducted molecular dynamics simulations of vacancy migration in iron and copper driven by recoil energies under electron irradiation in a high-voltage electron microscope. Minimum kinetic energy required for migration was about 0.8 and 1.0 eV in iron and copper at 20 K, which was slightly higher than the activation energy for vacancy migration. Around the minimum energy, the migration succeeded only when a first nearest neighbour (1NN) atom received the kinetic energy towards the vacancy. The migration was induced by higher kinetic energies even with larger deflection angles. Above several electron-volts and a few 10s of electron-volts, vacancies migrated directly to 2NN and 3NN sites, respectively. Vacancy migration had complicated directional dependence at higher kinetic energies through multiple collisions and replacement of atoms. The probability of vacancy migration increased with the kinetic energy and remained around 0.3–0.5 jumps per recoil event for 20–100 eV. At higher temperatures, thermal energies slightly increased the probability for kinetic energies less than 1.5 eV. The cross section of vacancy migration was 3040 and 2940 barns for 1NN atoms in iron and copper under irradiation with 1.25 MV electrons at 20 K: the previous result was overestimated by about five times.     

Effect of impurities on the growth of {113} interstitial clusters in silicon under electron irradiation

The growth and shrinkage of interstitial clusters on {113} planes were investigated in electron irradiated Czochralski grown silicon (Cz-Si), floating-zone silicon (Fz-Si), and impurity-doped Fz-Si (HT-Fz-Si) using a high voltage electron microscope. In Fz-Si, {113} interstitial clusters were formed only near the beam incident surface after a long incubation period, and shrank on subsequent irradiation from the backside of the specimen. In Cz-Si and HT-Fz-Si, {113} interstitial clusters nucleated uniformly throughout the specimen without incubation, and began to shrink under prolonged irradiation at higher electron beam intensity. At lower beam intensity, however, the {113} interstitial cluster grew stably. These results demonstrate that the {113} interstitial cluster cannot grow without a continuous supply of impurities during electron irradiation. Detailed kinetics of {113} interstitial cluster growth and shrinkage in silicon, including the effects of impurities, are proposed. Then, experimental results are analyzed using rate equations based on these kinetics.     

Atomistic simulation of the point defects in B2-type MoTa alloy

The formation and migration mechanisms of three different point defects (mono-vacancy, anti-site defect and interstitial atom) in B₂-type MoTa alloy have been investigated by combining molecular dynamics (MD) simulation with modified analytic embedded-atom method (MAEAM). From minimization of the formation energy, we find that the anti-site defects Mo_Ta and Ta_Mo are easier to form than Mo and Ta mono-vacancies, while Mo and Ta interstitial atoms are difficult to form in the alloy. In six migration mechanisms of Mo and Ta mono-vacancies, one nearest-neighbor jump (1NNJ) is the most favorable due to its lowest activation and migration energies, but it will cause a disorder in the alloy. One next-nearest-neighbor jump (1NNNJ) and one third-nearest-neighbor jump (1TNNJ) can maintain the ordered property of the alloy but require higher activation and migration energies, so the 1NNNJ and 1TNNJ should be replaced by straight [1 0 0] six nearest-neighbor cyclic jumps (S[1 0 0]6NNCJ) or bent [1 0 0] six nearest-neighbor cyclic jumps (B[1 0 0]6NNCJ) and [1 1 0] six nearest-neighbor cyclic jumps ([1 1 0]6NNCJ), respectively. Although the migrations of Mo and Ta interstitial atoms need much lower energy than Mo and Ta mono-vacancies, they are not main migration mechanisms due to difficult to form in the alloy.     

Evolution of ion-irradiated point defect concentration by cluster dynamics simulation

The relationship between ions irradiation and the induced microstructures(point defects, dislocations, clusters, etc.)could be better analyzed and explained by simulation. The mean field rate theory and cluster dynamics are used to simulate the effect of implanted Fe on the point defects concentration quantitatively. It is found that the depth distribution of point defect concentration is relatively gentle than that of damage calculated by SRIM software. Specifically, the damage rate and point defect concentration increase by 1.5 times and 0.6 times from depth of 120 nm to 825 nm, respectively. With the consideration of implanted Fe ions, which effectively act as interstitial atoms at the depth of high ion implantation rate, the vacancy concentration C_v decreases significantly after reaching the peak value, while the interstitial atom concentration C_i increases significantly after decline of the previous stage. At the peak depth of ion implantation, C_v dropped by 86%, and C_i increased by 6.2 times. Therefore, the implanted ions should be considered into the point defects concentration under high dose of heavy ion irradiation, which may help predict the concentration distribution of defect clusters, further analyzing the evolution behavior of solute precipitation.     

Low density approximation for diffusion annealing

The problem of annealing of Frenkel-pairs in electron irradiated fee metals due to diffusion of interstitials is treated starting from exact equations for single particle densities, pair densities, etc. The mobile interstitials are considered to interact with vacancies (leading to recombination), impurity atoms (leading to interstitial-impurity complexes) and with each other (leading to interstitial clusters).

By using the superposition approximation, i.e. replacing three-particle probabilities by products of two-particle probabilities we obtain generalized Waite equations. For low defect densities the annealing is at different times governed by different processes. For short times the important process is the recombination of an interstitial with the near-by correlated vacancy generated by the same electron impact event, the so-called correlated recovery. For long times the remaining interstitials undergo long range migration and interact with uncorrelated sinks. During this process interstitial impurity complexes and interstitial clusters are formed.

The time dependence of the defect densities, the remaining fraction of defects after completion of diffusion annealing and the size distribution of interstitial clusters are calculated. Detailed comparison with experiments in Cu and Pt will be made.     

Metallic-like droplets produced by irradiating rare-gas clusters with free electron laser pulses

Unlike metallic elements, rare-gas atoms remain non-metallic even if they are condensed. However, once rare-gas clusters are irradiated by extreme ultraviolet free electron laser (EUV-FEL) pulses with slightly higher energy than their ionization potential, they exhibit metallic-like behaviors because the ionization takes place sequentially within the clusters owing to the huge valence electron ionization cross section in the EUV regime. In this work Ar and Xe clusters were produced by pulsed supersonic jets, and the cluster beams were crossed synchronously with focused EUV-FEL beams ejected from the SCSS test accelerator in Japan. We measured time-of-flight (TOF) mass spectra and kinetic energy distribution (KED) of daughter ions produced via Coulomb explosion by using three dimensional (3D) momentum imaging spectrometer. The metallic-like nature is evidenced not only by widely spread charge distributions within the cluster but also by enhanced positive surface charges surrounded by quasi-free electron clouds.     

Mass transfer of indium in the In-CdTe structure under nanosecond laser irradiation

The features of mass transfer of indium in high-resistance CdTe crystals under nanosecond irradiation of the In-CdTe structure by laser pulses have been studied and analyzed. The mass transfer coefficients are determined and the average transfer rate of the indium atoms in CdTe under irradiation is evaluated. It is assumed that the concentration diffusion of indium in CdTe and the transfer of impurity atoms by the front of the laser-induced shock wave are not dominant mechanisms of mass transfer under dynamic doping. The most probable process, which provides the formation of the inverse layer in the surface region of the CdTe crystal under nanosecond laser irradiation of the In film, is an transfer of interstitial In atoms due to the thermal fluctuation jumps under the action of the driving force of the thermoelastic stresses and temperature gradient.     

嵌入原子法计算金属钚中点缺陷的能量

钚因放射性衰变而出现老化效应.钚中点缺陷的性质和行为是理解钚老化效应的一个基础和前提.运用分子动力学模拟技术,计算了金属钚中点缺陷和点缺陷团簇的形成能和结合能.其中钚-钚、钚-氦和氦-氦相互作用势分别采用嵌入原子多体势、Morse对势和Lennard-Jones对势.计算结果表明,单个自间隙原子易以〈100〉哑铃状形态存在;间隙氦原子在理想晶格的八面体间隙位置相对较为稳定;氦原子与空位的结合能较大,在钚的自辐照过程中两者易于结合并形成氦-空位团簇;氦-空位团簇的形成能随氦原子数的增加而增大,当氦与空位的数     

One-dimensional migration of interstitial clusters in SUS316L and its model alloys at elevated temperatures

For self-interstitial atom (SIA) clusters in various concentrated alloys, one-dimensional (1D) migration is induced by electron irradiation around 300 K. But at elevated temperatures, the 1D migration frequency decreases to less than one-tenth of that around 300 K in iron-based bcc alloys. In this study, we examined mechanisms of 1D migration at elevated temperatures using in situ observation of SUS316L and its model alloys with high-voltage electron microscopy. First, for elevated temperatures, we examined the effects of annealing and short-term electron irradiation of SIA clusters on their subsequent 1D migration. In annealed SUS316L, 1D migration was suppressed and then recovered by prolonged irradiation at 300 K. In high-purity model alloy Fe-18Cr-13Ni, annealing or irradiation had no effect. Addition of carbon or oxygen to the model alloy suppressed 1D migration after annealing. Manganese and silicon did not suppress 1D migration after annealing but after short-term electron irradiation. The suppression was attributable to the pinning of SIA clusters by segregated solute elements, and the recovery was to the dissolution of the segregation by interatomic mixing under electron irradiation. Next, we examined 1D migration of SIA clusters in SUS316L under continuous electron irradiation at elevated temperatures. The 1D migration frequency at 673 K was proportional to the irradiation intensity. It was as high as half of that at 300 K. We proposed that 1D migration is controlled by the competition of two effects: induction of 1D migration by interatomic mixing and suppression by solute segregation.     

Non-nearest-neighbor jumps in 2D diffusion: Pd on W(110)

A variety of jumps has in the past been identified in diffusion of atoms on 1D channeled surfaces. To establish the jump processes important in diffusion on a 2D surface, the movement of individual Pd atoms has been examined on W(110). From the distribution of displacements of Pd at high temperatures, double jumps are found along the close-packed <111>. For the first time, sizable differences are also observed between the mean-square displacements along x and y, which demonstrate unexpected contributions from jumps along <110>, but not along <001>. These jumps proceed over activation barriers higher than for single jumps, under conditions predicted from previous work with Pd on the channeled W(211).     

Isolated molecules in metals

Isolated clusters consisting of a heavy radioactive impurity and a small number of light interstitial atoms can be produced in a metallic host and conveniently studied by hfi techniques. The heavy impurities are introduced by ion implantation using the radioactive impurities as probe atoms or in-situ irradiation of the host matrix. Light interstitials can be introduced by diffusion from the gas phase, by electrolytic charging or co-implantation. If a chemical interaction between the two types of atoms exists, these clusters can be regarded as molecules in metals. In reviewing the existing data, the following topics will be discussed: mechanisms of molecule formation, after-effects, stoichiometry, geometry and stability of the complexes formed. It will be shown that nuclear reaction analysis of the light interstitial atoms is a valuable complementary tool in the study of molecules in metals.     

Kinetics of the diffusion of an impurity atom on a fcc(111) surface

An analytical study of the migration of an embedded impurity atom over a solid surface under the influence of the diffusion of vacancies is performed. The case of small surface coverages of both vacancies and impurity atoms is considered. It is shown that the realization of multiple collisions of a single impurity atom with vacancies imparts a Brownian character to its motion. At long times, the dependence of the mean square displacement on the time differs little from the linear, whereas the spatial density distribution is close to the Gaussian, features that makes it possible to introduce a diffusion coefficient. For the latter, an analytical expression is derived, which differs from the product of the diffusion coefficient of vacancies and their relative concentration only by a numerical factor. The dependence of the diffusion coefficient of an impurity atom on the ratio of the frequency of its jumps to the frequency of jumps of vacancies is analyzed. In the kinetic mode, when the frequency of jump ω of the imurity atom is small, the diffusion coefficient of the impurity depends linearly on ω, whereas in the opposite case, a saturation occur and its dependence on the frequency of jumps of the impurity atom disappears.     

Investigation of radiation defects in Fe-Cr alloy

Abstract

The kinetics of radiation defect accumulation and subsequent recovery during/after electron irradiation below 273 K, at 323 K and 373 K were investigated for the Fe-15.7 at.% Cr using positron annihilation measurements at room temperature. Formation of vacancy clusters was observed at all of the irradiation temperatures. The formation of clusters and kinetics of their accumulation point to mobility of vacancies at least at room temperature. The cluster rearrangement and variations in the cluster configuration take place during annealing.     

Microstructure of high-pressure-synthesized diamonds under rapid quenching and electron irradiation

A type of synthetic diamond single crystal about 0.4–0.5 mm in dimension prepared under high pressure–high temperature (HPHT) in the presence of a FeNi molten catalyst was quenched from HPHT and irradiated with 300 keV electrons at room temperature. Transmission electron microscopy was employed to examine the microstructure of the diamond single crystal before and after electron irradiation. It was found that there exists a large amount of cellular interfaces in the quenched diamond sample, which indicates the growth condition of the diamond under HPHT. Hexagonal dislocation loops about several tens of nanometers in dimension were observed in the high-pressure-synthesized diamond single crystal before electron irradiation, which strongly suggests that a number of vacancies were quenched-in due to rapid quenching from high temperature at the end of diamond synthesis, and were aggregated in the synthetic diamond to form vacancy disks on the (111) plane, the collapse of such vacancy disks forming vacancy-type dislocation loops. After electron irradiation, it was found that defect clusters present as interstitial-character dislocation loops were formed in the electron-irradiated region of the diamond. The interstitial dislocation loops grow with increase of the irradiation time. The present study, in comparison to previous work on ion implantation on diamond, indicates that electron irradiation does not induce a phase transformation but produces interstitial dislocation loops due to the migration of interstitial atoms and vacancies. The result of the study directly indicates that interstitials and vacancies in diamond are mobile at room temperature under electron irradiation. Nitrogen, as the most important kind of impurity contained in the HPHT as-grown diamond, probably acts as nucleation of the interstitial loops. Received: 16 November 2001 / Accepted: 12 June 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. E-mail: yinlw@sdu.edu.cn