Growth Of Interstitial Clusters Research Articles

Multi-scale simulation of the experimental response of ion-irradiated zirconium carbide: Role of interstitial clustering

The response of zirconium carbide to heavy-ion irradiation at room temperature has been studied by X-ray diffraction, ion channeling and transmission electron microscopy. Below 5 × 1014 cm−2, we observe a build-up of elastic strain with increasing fluences. At this threshold fluence the strain is released and important dechanneling appears as well as visible TEM damage. With increasing fluence, this damage is found to spread in the material deeper than the depth of direct damaging by the ion beam. These experimental observations are reproduced and explained by Density Functional Theory informed Rate Equation Cluster Dynamics simulations. Simulations show that the response of ZrC upon ion-irradiation is driven by the diffusion and clustering of interstitials. The two-step evolution seen in experiments stems from the growth of interstitial clusters with a concomitant starvation of the smallest clusters induced by the continuous accumulation of vacancies. The damaging of the material beyond the range of primary damage is driven by diffusion of interstitials.

Molecular dynamics simulation of the early stages of self-interstitial clustering in silicon

We have studied the early stages of self-interstitial clustering in silicon using molecular dynamics simulation techniques. We have generated silicon samples of over 200,000 atoms where we introduced a 0.5% extra concentration of self-interstitials. Then samples were annealed at several temperatures. During the simulations we observed the formation of interstitial clusters with different atomic structures, ranging from spherical and amorphous-like clusters, to highly ordered extended configurations such as (110) chains, {111} rod-like defects and dislocation loops, and {100} planar defects. This last type of defects, while common in germanium, have not been observed in silicon until very recently, in ultra-fast laser annealing experiments. The particular morphology of formed interstitial clusters is found to be related to the annealing temperature, as it is observed in the experiments. From the molecular dynamics simulations we have analyzed the atomic mechanisms leading to the formation and growth of interstitial clusters, with special attention to the newly found {100} planar defects.

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.

Accelerated dynamics simulations of interstitial-cluster growth

We apply parallel replica dynamics to simulate the growth of silicon interstitial clusters. Existing interstitial clusters are efficient traps for mobile interstitial and di-interstitial defects. For clusters involving more than four interstitials, many metastable structures are achieved by local bonding rearrangements. The shape of interstitial nuclei critically determines the final interstitial clusters. Once an elongated cluster is formed, additional captured interstitials diffuse in the chain direction and eventually settle at the chain ends, resulting in further elongation.

Interstitial cluster formation in metals under intense irradiation

A new mechanism of point defect clustering in irradiation environment is proposed. The process considered reflects a possibility for spontaneous (nondiffusional) clustering to exist in a highly enough supersaturated solution of point defects under certain conditions. The influence of the production of di-interstitial clusters by this mechanism on the kinetics of interstitial dislocation loop nucleation and growth is analyzed. It is shown that the intensity of irradiation must be high enough and the temperature of the sample must be correspondingly low enough for the effect of the spontaneous small interstitial clustering to prevail over the usual mechanism of diffusion driven nucleation and growth of interstitial clusters. Both analytical and numerical solutions of an extended version of conventional rate equations demonstrate the possibility of this phenomenon. This version takes into account the terms of higher order in an expansion of the chemical potential over the interstitial concentration. The new form of equations naturally follows from a canonical approach to chemical-type rate equations that is known in nonlinear nonequilibrium thermodynamics. When density effects are negligible, one gets the conventional kinetics of diffusion driven interstitial loop formation. A simple model of interstitial loop nucleation and growth through di-interstitial is used in the analysis of the clustering kinetics. A noticeable enhancement of interstitial loop number density with smaller radii is expected for temperatures below one third of the melting point, where the loop growth process due to diffusional processes is weak enough. For this novel phenomenon to be noticeable, a prolonged irradiation is necessary. Conditions and possibilities for experimental investigations of the predicted effect as well as the possible competitive processes are considered.

Factors to influence the nucleation and growth of interstitial clusters during cascade damage

Small interstitial clusters produced by cascade collisions are known to serve as the nucleation sites of dislocation loops. However, their density is quite low compared with the number of large cascades. Factors which suppress the nucleation of interstitial type dislocation loops are explained and their relative importance is discussed. The first is the positional instability of small interstitial clusters. This effect is expected to enhance the dislocation structure development by agglomeration of the embryos, but it is also expected to suppress the formation of loops by easy escape to sinks. The second is the stochastic fluctuation of point defect reactions with embryos. This effect plays an important role when the point defect concentrations of a quasi-steady state are high. The third is the thermal instability of embryos of loops. The fourth is the cascade localization induced bias (CLIB) effect, which leads a vacancy dominant atmosphere in a cascade. At a low temperature at which vacancies are immobile and embryos are thermally stable, only the first factor is effective. At a very high temperatures on the other hand, the third and fourth factors are the main factors. In the intermediate temperature range, four factors are competitive.

Diffuse X-ray scattering studies of defect reactions in electron-irradiated dilute nickel alloys. II. Ni-Ge

For pt.I see ibid., vol.14, p.1347 (1984). Defect properties in dilute Ni-Ge alloys are investigated by Huang diffuse scattering and the results for this alloy (oversized impurity) are compared with those for Ni-Si (undersized impurity). Ni alloys containing 1 at.% and 0.05 at.% Ge were irradiated with electrons at 4.2K and were subsequently isochronally annealed. It was found that, prior to annealing, the Frenkel-pair resistivities and self-interstitial atom configurations were the same in the alloys as in pure Ni. The independence of the Frenkel-pair resistivity to Ge concentration indicates that the resistivities due to Frenkel pairs and Ge solute are linearly additive in Ni. After annealing through stage I to 85K, the average defect cluster size grew to 1.8, 3.0 and 3.0 interstitial atoms for the 1 at.% Ge, 0.05 at.% Ge and pure Ni specimens, respectively. These results demonstrate that self-interstitial atoms are not immobilised by single Ge atoms in Ni, but rather complexes involving several Ge atoms and/or two interstitial atoms are the stable defects at the end of annealing stage I. There is a strong suppression of interstitial cluster growth during stage II only for the 1 at.% Ge alloy. Although these results are qualitatively very similar to those for Ni-Si, there are three characteristic differences: (i) interstitial-atom clusters of equal size are mobile in Ni-Ge at lower temperatures than in Ni-Si; (ii) there appear to be different mechanisms for the transport of solute by interstitial-atom motion in the Ni-Si and Ni-Ge alloys; (iii) in annealing stage III there is a strong interaction of small vacancy agglomerates with Ge atoms but not with Si atoms.

Self-interstitial atoms, vacancies and their agglomerates in electron-irradiated nickel investigated by diffuse scattering of X-rays

Frenkel defects of concentrations varying between 200 and 600 PPM were produced in single crystals of Ni by irradiation with 3 MeV electrons at 4.5K. These radiation-induced defects and their reactions during thermal annealing were investigated by measuring the Huang diffuse scattering of X-rays, the change of the lattice parameter and the electrical resistivity. At 6K the authors observed statistically distributed Frenkel pairs. The configuration of the self-interstitial atoms (SIA) agrees with the (100) split configurations. The relaxation volume of the SIA was determined to be 1.8 (+or-0.2) atomic volumes and the Frenkel defect resistivity was rho F=7.1(+or-0.8) mu Omega cm at.%-1. This set of values implies a relaxation volume for the vacancy of -0.22 atomic volumes. The annealing behaviour of Ni is characterised by interstitial motion during stage I and growth of interstitial type clusters during stage II that eventually form dislocation loops on (111) planes. Impurities of Si (undersized) and Al (oversized) in concentrations comparable to the concentration of the Frenkel defects cannot effectively suppress the cluster growth. Vacancy agglomeration has been observed directly after annealing in stage III. The additional diffuse scattering from the vacancy agglomerates can be distinguished from the scattering from interstitial type loops by its different asymmetry relative to the Bragg reflections. There is a simultaneous decrease of rho F with the formation of the vacancy agglomerates. All of these reactions are similar to those in Cu although the growth of the interstitial clusters is retarded for Ni.

Dynamic studies of defect mobility using high voltage electron microscopy

The large variety of observed defect-structure development by electron irradiation in fcc and bcc metals is classified from the view point of point-defect mobilities. Vacancy mobility is obtained from the defect structure change caused by the annihilation of vacancies accumulated by irradiation, and their motion activation energy is obtained from the interstitial cluster growth at high temperatures. Mobility of interstitial atoms and their interaction with impurities are obtained from the variation of interstitial cluster formation. Various observed effects of electron radiation induced motion of point defects are explained by the displacement of atoms by the transfer of energy comparable with the migration activation energy of each point defect. The observed superficial temperature dependence of the induced diffusion of vacancies in fcc metals is attributed to divacancies created by the induced diffusion. An efficient method to obtain the self-diffusion energies is proposed with results for some metals.

Radiation-induced diffusion of point-defects during low-temperature electron irradiation

Abstract Electron irradiation damage in lead was studied in a high-voltage electron microscope at temperatures between 8 and 300 K. The formation and growth of interstitial clusters was observed during irradiation at temperatures between 8 and 165 K and of vacancy clusters between 50 and 165 K, i.e. at temperatures well below annealing stage III (at ∼170K in lead). The observation of vacancy cluster growth is interpreted as evidence for preferential absorption of interstitials at sample surfaces. By a combination of above-and sub-threshold irradiations and in situ annealing treatments it could be shown that the point-defect motion responsible for cluster formation below Stage III is of radiation-induced nature. The importance of radiation-induced diffusion for the low-temperature irradiation and the annealing behaviour of lead and gold is discussed. Various mechanisms which may be responsible for radiation-induced defect migration and its main characteristics are considered.

Mobility of interstitial defects in gold bombarded with 270 ev gold ions in stage III

Abstract Shrinkage of vacancy stacking-fault tetrahedra in gold foils bombarded with 270 ev gold ions at temperatures between -90°c and +100°c was studied using transmission electron microscopy. In this work interstitial defects were produced by replacement collision sequences initiated near the incident surface. The tetrahedra were partially annihilated by absorbing these defects and therefore served as indicators for long-range interstitial migration. By comparing the same tetrahedra in high resolution micrographs taken before and after bombardment, and after subsequent annealing, it was possible to determine unambiguously the tetrahedron shrinkage as a function of foil thickness. It was found that (1) the tetrahedron shrinkage was independent of bombardment temperatures over the range -90°c to +100°c and (2) no shrinkage of tetrahedra or intrinsic Frank loops, or growth of interstitial clusters, occurred when foils bombarded as low as -90°c were warmed to +30°c. Several additional specimens bombarded w...

The growth of interstitial clusters in graphite under irradiation

Abstract After high doses of fast neutron irradiation graphite is observed to contain circular clusters of interstitial atoms, each one atomic layer thick inserted between the existing hexagonal layers of the lattice. In the electron microscope these are visible as unsheared dislocation loops having Burgers vector c/2 in the c-axis direction. They grow by the diffusion to them of the mobile interstitial atoms produced by the irradiation. This diffusion-controlled growth is discussed in the present paper with particular reference to the macroscopic growth of graphite crystals in the c-direction. The fractional macroscopic growth ΔXc/Xc has been observed to exceed the fractional growth of the c-spacing, Δc/c, by an amount which increases rapidly with dose. The present paper shows that this behaviour is understandable for a model which contains, in addition to mobile interstitials, also numerous small immobile interstitial clusters. It is argued that there is strong evidence for such defects, of high stabili...