Interstitial Defect Clusters Research Articles

The influence of solute elements on grain size, hardness and irradiation-induced defect clusters in dilute Fe-based alloys

The precipitation of solutes is the primary factor that leads to the occurrence of irradiation hardening and embrittlement, thus limiting the service life of reactor pressure vessel (RPV) steels. In this study, four model alloys (Fe-1.35Mn-0.75Ni, Fe-1.35Mn-0.20Si, Fe-0.75Ni-0.20Si and Fe-1.35Mn-0.75Ni-0.20Si) were used to investigate the influence of solute elements on grain size, hardness, irradiation hardening, and defect clustering behavior. The materials were irradiated with 3.5 MeV Fe ions to doses of 0.1, 0.3, 1.0, and 3.0 dpa at 290 °C. Results indicated that the addition of solute Mn resulted in grain refinement while the addition of solutes Ni and Si led to grain coarsening. The strength of the model alloys was affected by the change in grain size. A definite dose rate effect was found in all four model alloys under irradiation, where irradiation with a higher dose rate led to a lower irradiation hardening effect and irradiation with a lower dose rate induced a higher hardening effect. Atom probe tomography (APT) results revealed that the precipitated solutes were always coupled with interstitial defect clusters. Solute Mn had a strong influence on cluster nucleation and a weak influence on cluster size. Solutes Si and Ni significantly promoted the nucleation and growth of clusters, and Ni had a greater influence on the nucleation and growth of clusters than Si. Meanwhile, solute Si consistently exhibited a low fraction in the clusters when the solute Mn element was presented in the alloys.

Energy exchange in M-crowdion clusters in 2D Morse lattice

Dynamics of new class of M-solitons and M-crowdions, here M = 3 is the number of adjacent close-packed atomic rows where the atoms move, are studied in two-dimensional triangular Morse lattice using classical molecular dynamics simulations. 3-solitons/3-crowdions are excited by giving initial velocities to the three atoms in three neighboring close-packed atomic rows along the rows. Different relations between the initial velocities are considered: all three initial velocities are equal, initial velocity for the middle atom is lower than for the outermost atoms, and all three velocities are different. During propagation of a 3-soliton the atoms do not overcome potential barrier and relax back to their original lattice sites. Propagation of a 3-crowdion results in the shift of the atoms to the neighboring lattice sites along the direction of movement. It is found that propagation of 3-soliton/3-crowdion clusters is associated with the energy exchange between the adjacent atomic rows. The ratio between the initial energies, at which the maximum energy exchange occurs, is determined. The energy is transferred from the high-energy atomic rows to the low-energy one. In the case when initial velocities in all three rows are different, the dynamics of 3-soliton/3-crowdion clusters is unstable. Obtained results allow to better understand the dynamics of interstitial defect clusters.

Evaluation of tungsten as divertor plasma-facing material: results from ion irradiation experiments and computer simulations

The effect of the primary knock-on atom (PKA) spectrum in radiation damage and the subsequent defect structure formation and their impact in deuterium (D) trapping has been investigated using computer simulations and surrogate ion irradiation experiments. The neutron spectrum for an ‘ITER-like’ divertor shape and parameters has been generated using ATTILA and SPECTER codes to identify the relevant PKA energies. It has been observed that 10 MeV boron (B) produces a PKA spectrum similar to that obtained from a reactor-like neutron spectrum. Experiments have been carried out with ions of gold (Au), B, helium (He) and D with energies ranging from 0.1 MeV–80 MeV for a fluence range of ions m−2– ions m−2, and distinctly different PKA spectra have been produced. While 80 MeV Au ions produced dense and small clusters of interstitial defects (<10 nm), B produced large dislocation loops up to 60 nm in size. At room temperature, the imprint of the cascade is well captured by the vacancies due to their low mobility, and the vacancy defects observed in Au and B irradiation showed significant differences. Molecular dynamics simulations show that at PKA energies exceeding 150 keV, the fragmentation of the cascades takes place, which tends to limit the size of individual defects in the case of 80 MeV Au irradiation. A mechanism based on the competitive capture of mobile interstitials has been proposed to explain the observed large dislocation loops as well as dislocation lines in different irradiation experiments.

Interstitial migration behavior and defect evolution in ion irradiated pure nickel and Ni-xFe binary alloys

Transition from long-range one-dimensional to short-range three-dimensional migration modes of interstitial defect clusters greatly reduces the damage accumulation in single-phase concentrated solid solution alloys under ion irradiation. A synergetic investigation with experimental, computational and modeling approaches revealed that both the resistance to void swelling and the delay in dislocation evolution in Ni-Fe alloys increased with iron concentration. This was attributed to the gradually increased sluggishness of defect migration, which enhances interstitial and vacancy recombination. Transition from long-range one-dimensional defect motion in pure nickel to short-range three-dimensional motion in concentrated Ni-Fe alloys is continuum, not abrupt, and within an iron concentration range up to 20%. The gradual transition process can be quantitatively characterized by the mean free path of the interstitial defect clusters.

The MRO-accompanied modes of Re-implantation into SiO2-host matrix: XPS and DFT based scenarios

The following scenarios of Re-embedding into SiO2-host by pulsed Re-implantation were derived and discussed after XPS-and-DFT electronic structure qualification: (i) low Re-impurity concentration mode → the formation of combined substitutional and interstitial impurities with Re2O7-like atomic and electronic structures in the vicinity of oxygen vacancies; (ii) high Re-impurity concentration mode → the fabrication of interstitial Re-metal clusters with the accompanied formation of ReO2-like atomic structures and (iii) an intermediate transient mode with Re-impurity concentration increase, when the precursors of interstitial defect clusters are appeared and growing in the host-matrix structure occur. An amplification regime of Re-metal contribution majority to the final Valence Band structure was found as one of the sequences of intermediate transient mode. It was shown that most of the qualified and discussed modes were accompanied by the MRO (middle range ordering) distortions in the initial oxygen subnetwork of the a-SiO2 host-matrix because of the appeared mixed defect configurations.

Ab initioscaling laws for the formation energy of nanosized interstitial defect clusters in iron, tungsten, and vanadium

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.

The non-Arrhenius migration of interstitial defects in bcc transition metals

Thermally activated migration of defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-interstitial defects, and particularly self-interstitial atom clusters, is very low. In this case a trajectory of a defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual interstitial defects, as well as clusters of interstitial defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of interstitial defects and interstitial defect clusters is associated with the interaction between a defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008).

Diffuse X-ray scattering measurements of point defects and clusters in iron

The nature of residual damage from atomic displacement cascades in iron has been investigated by diffuse X-ray scattering. A single crystal iron sample was irradiated at ∼60 °C in the High Flux Isotope Reactor at the Oak Ridge National Laboratory to a fluence of 1 × 10 23 n/m 2 ( E > 0.1 MeV), or ∼0.01 dpa. Diffuse X-ray scattering measurements were carried out at the Advanced Photon Source at the Argonne National Laboratory on three specimens: unirradiated, as-irradiated, and irradiated and annealed for one hour at 450 °C (IA). The specimens were cooled to ∼40 K to minimize thermal diffuse scattering. Subsequent measurements of the specimen lattice parameter were completed at ORNL. The diffuse scattering in the as-irradiated specimen was dominated by interstitial defect clusters exhibiting a tetragonal distortion consistent with a 〈1 0 0〉 type defect. Substantial recovery of this defect component was observed following the anneal at 450 °C. A lattice parameter increase was observed following irradiation, which is also consistent with a substantial population of interstitial type defects. A net lattice parameter decrease was observed in the IA specimen, indicating loss of the interstitial defects with a residual population of vacancy-type defects.

Damage accumulation in neon implanted silicon

Damage accumulation in neon-implanted silicon with fluences ranging from 5×1014to5×1016Necm−2 has been studied in detail. As-implanted and annealed samples were investigated by Rutherford backscattering spectrometry under channeling conditions and by transmission electron microscopy in order to quantify and characterize the lattice damage. Wavelength dispersive spectrometry was used to obtain the relative neon content stored in the matrix. Implantation at room temperature leads to the amorphization of the silicon while a high density of nanosized bubbles is observed all along the ion distribution, forming a uniform and continuous layer for implantation temperatures higher than 250°C. Clusters of interstitial defects are also present in the deeper part of the layer corresponding to the end of range of ions. After annealing, the samples implanted at temperatures below 250°C present a polycrystalline structure with blisters at the surface while in the other samples coarsening of bubbles occurs and nanocavities are formed together with extended defects identified as {311} defects. The results are discussed in comparison to the case of helium-implanted silicon and in the light of radiation-enhanced diffusion.

Characterization of neon implantation damage in silicon

The damage accumulation in neon-implanted silicon, with doses ranging from 1 × 10 16 to 5 × 10 16 Ne cm −2, was explored. To avoid amorphization, samples were implanted at 250 °C. As implanted and annealed samples were investigated by Rutherford backscattering spectrometry under channeling conditions (RBS/C) and by transmission electron microscopy (TEM) to quantify and characterize the lattice damage. Anneals were performed at 800 °C during 30 min. In the as-implanted samples, a high density of small bubbles is observed all along the ion distribution, forming a uniform and continuous layer. Clusters of interstitial defects are also present in the deeper part of the layer corresponding to the end of range of ions. After annealing, cavities are formed together with extended defects characterized as {3 1 1} defects. The results are discussed in comparison to the case of helium implantation and in the highlight of the bubble formation mechanisms.

Instability of interstitial clusters under ion and electron irradiations in ceramic materials

One type of candidate materials for future fusion reactors is ceramics, which can be applied as radio frequency windows, toroidal insulating breaks and diagnostic probes. The degradation of physical and mechanical properties of these materials under neutron irradiation is determined by the kinetics of radiation defects including a point defect cluster formation (dislocation loops, voids and so on). The physical mechanisms of defect structure development in ceramic materials, where point defects and their clusters can have an effective charge, are completely different from those in metals. We have investigated the physical mechanisms of instability of extended interstitial defect clusters (charged dislocation loops), which were formed in stabilized cubic zirconia under electron irradiation with 100–1000 keV due to the selective displacement damage in oxygen sublattice. A new theoretical model is suggested for the explanation of the growth process and instability of the interstitial clusters. The suggested model takes into account an accumulation of effective charge on growing dislocation loops due to the trapping of electrons in dislocation cores. Our calculations show that the elastic stress and strain fields are much intense around charged dislocation loops than non-charged dislocation loops, due to an additional stress and strain fields driven by an electric field of accumulated charge. The stress induced by the charged dislocation loops with the density of trapped electrons per atom n=0.4 is found to be comparable with the theoretical yield stress of zirconia, which explains the multiplication of dislocation network at a critical size of the defect clusters observed by experiments.

Heterogeneous void swelling near grain boundaries in irradiated materials

Department of Mechanical Engineering, The Hong-Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong~Received 25 July 2002; revised manuscript received 6 November 2002; published 5 March 2003!We found that by assuming that the density of dislocations in an irradiated material varies as a function ofthe distance to grain boundaries and that mobile interstitial defect clusters perform three-dimensionaldiffu-sional motion it is possible to achieve significantly better agreement with experimental observations of profilesof heterogeneous void swelling than in the model where defects diffuse purely one-dimensionally. This ap-proach explains the origin of several distinct features characterising the effect of heterogeneous void swelling,including the variation of the shape of swelling profiles as a function of irradiation dose, the formation of peaksof swelling and void denuded zones, and the occurrence of anomalously large voids in the regions adjacent tograin boundaries.DOI: 10.1103/PhysRevB.67.094103 PACS number~s!: 61.72.Mm, 61.80.Az, 61.82.Bg, 28.52.FaI. INTRODUCTION

Characterization of interstitial defect clusters in ion-implanted Si

ABSTRACTWe have investigated the properties of Si interstitial clusters in ion implanted crystalline Si. Deep Level Transient Spectroscopy measurements have been used to characterize the residual damage in Si samples implanted with Si ions at fluence in the range 1×109-1×1012/cm2 and annealed at temperatures of 100–700 °C. We have found that, in the fluence and annealing temperature range where extended defects are not formed, the residual damage is dominated by Si interstitial clusters which introduce deep levels at Ev+0.36 eV and at Ev+O-53 eV. By using Si substrates with a different impurity and dopant content, we have found that C, O and B play a role in determining the defect growth kinetics but are not the main constituents of these clusters. We estimate that 40 to 125 Si self intersti-tials are stored in these clusters and believe that they are the main source of Si self-inter-stitials in transient enhanced diffusion phenomena occurring in the absence of {311} or extended defects.

Study of point defect clusters in high purity single crystals of silicon grown by Czochralski and float-zone methods by diffuse X-ray scattering technique

Results of study of point defect clusters in high purity ( ∼ 10 kΩ · cm ) dislocation-free Czochralski (Cz) and float-zone (FZ) grown crystals (∼ 50 mm diameter, 11 mm thick, p-type) are reported. A multicrystal X-ray diffractometer set in (+, −, −, +) geometry and employing a well-collimated Mo K α 1 beam was used. High resolution X-ray diffractometry, absolute integrated intensity measurements and diffuse X-ray scattering (DXS) measurements were carried out. The specimens gave sharp diffraction curves and the integrated intensities were found to be close to the theoretical value for an ideally perfect silicon crystal. DXS measurements were made around 111 reciprocal lattice point (RLP) along four directions of K ∗ : [111], [1̄1̄1̄], [01̄1] and [011̄]. K ∗ is the vector which connects the elemental reciprocal volume under investigation with the nearest RLP. From the analysis of the DXS intensity versus K ∗ plots, it was found that the source of DXS is interstitial defect clusters in Cz-grown crystals and vacancy defect clusters in FZ-grown crystals. By using a phenomenological model, the values of cluster size R cl, cluster volume A cl and number of defects per cluster N cl, respectively, were found to be: 0.8 μm, 2.02 × 10 −16 cm 3, 1.26 × 10 6 for Cz specimen and 0.6 μm, 1.32 × 10 −16 cm 3, 8.3 × 10 5 for the FZ specimen.

Structural damage in spinel after ion irradiation

Monocrystalline MgAl 2O 4 spinel crystals were irradiated at 100 K with overlapping ion implantations: an initial implantation of 185 keV He + ions to 1.7 × 10 15 ions/cm 2, followed by a second implantation of 50 keV Ne + ions to 5 × 10 12 ions/cm 2. An additional set of spinel crystals was exposed only to the He implantation. Each irradiated sample was characterized by ion channelling and cross-sectional transmission electron microscopy. Defect clusters and dislocation loops were observed in each irradiated sample. The size of microstructural defects in the doubly ion-implanted sample was not increased compared to the He only ion implantation, and the density of defects in the doubly implanted sample actually decreased. These observations indicate that preexisting irradiation-induced interstitial defect clusters and Frank dislocation loops do not serve as effective sinks for interstitial point defects in spinel. Vacancy clusters (if they exist) probably are effective sinks for interstitials and to some extent, are erased by interstitials created during the second irradiation. These observations have important implications for understanding the role of defects in determining the properties of irradiated spinel.

Thermodynamic model of UO2 + x

A thermodynamic model of hyperstoichiometric uranium dioxide, UO 2 + x is presented. The model takes four kinds of oxygen interstitial defect clusters into account and in addition treats the remaining excess enthalpy term as the molar volume dependent parts of both the defect formation energies and the phenomenological interaction energy between the defects. The model gives a consistent and satisfactory description of the full set of thermodynamic data of oxygen in UO 2 + x, g ̄ (O 2), h ̄ (O 2) and s ̄ (O 2) reported by the present authors.

Plastic deformation in nonstoichiometric UO2 + x single crystals—II. Deformation at high temperatures

Single crystals of UO 2 + x (5 × 10 −6 ⩽ x ⩽ 10 −1) have been deformed in equilibrium CO CO 2 mixtures at 1073, 1273 and 1673 K in compression. The primary glide plane is dependent on orientation, temperature and extent of non-stoichiometry, such that {001}〈1̄10〉 glide is most common at low O U ratios and high temperatures, while {111}〈1̄10〉 glide is prevalent at high O U ratios and low temperatures. The critical resolved shear stress (CRSS) for {001} glide is nearly independent of x, while the CRSS for {111} glide decreases with increasing x. At low values of x, slip is controlled by interactions between dislocations and extrinsic defects; the dislocation and defect mobilities are nearly equal (for the strain rates examined) at temperatures between 1350 and 1650 K. At high values of x, the interactions between dislocations and extrinsic defects still play a major role in controlling dislocation motion; however, it appears that the presence of oxygen interstitial defect clusters serves to diminish the hardening effects of impurities for {111} glide. When deformed specimens with high values of x are reduced to low values of x, dislocations apparently act as sinks for “exsolved” oxygen, resulting in increase in strength on further deformation.

Low-temperature heat transport in α-HgI 2 single crystals

The thermal conductivity of several samples from α-HgI 2 crystals grown by two different methods has been measured from 50 mK to 200 K. The thermal conductivity is found to be intrinsic but anisotropic above 15 K: it is smaller along c-axis than along a-axis, the anisotropy ratio being about 5 between 15 and 200 K. Below 15 K, the thermal conductivity is sample dependent and the calculated Casimir limit is not reached at the lowest temperatures. The results have been interpreted considering phonon scattering by structural defects. A simple quantitative analysis of the curves suggests that phonons are scattered mainly by large clusters of interstitial defects due to the lack of stoichiometry of the crystals; the typical dimensions of these clusters are not smaller than 1 μm perpendicular to c-axis and 0.3 /gmm along c-axis. The presence of plane defects is also detected. Point defect scattering is relatively small and explained by residual metallic impurities and carbon at interstitial sites. The intrinsic anisotropy is briefly discussed.

Interstitial defect clusters in non-stoichiometric NiAl

IntroductionAs part of an extensive study of the nature of defect structure in B2 alloys the nature of defects in NiAl is being studied in some detail. In earlier work Ball reported and interpreted observations of unusual lattice defects in Al-rich NiAl. He showed that these defects lay on {100} and considered several possible models to account for the intrinsic displacement contrast observed. The absence of such defects on the Ni-rich side of stoichiometry was taken as confirmation of his chosen model. In the present paper it is shown that similar defects do in fact occur in Ni-rich specimens and that the nature of the displacement fringe contrast is extrinsic on both sides of stoichiometry. Possible formation mechanisms and structures of these defects are discussed.ExperimentalSpecimens with compositions covering the whole composition range both sides of stoichiometry were prepared by argon arc melting appropriate weighed amounts of Ni and Al, and discs for examination by electron microscopy prepared from these as-cast specimens and from specimens homogenized at 1300°C for 72 hrs.

Interstitial defect clusters in gold after bombardment with 270 eV gold ions

Interstitial defect clusters in gold after bombardment with 270 eV gold ions