Annihilation Of Point Defects Research Articles

On corrosion and passivity of Inconel alloy 625 in HNO3 solution

Exploring the passivity and electrochemical behavior of Inconel alloy 625 in nitric acid solution could open up possibilities for potential applications, either in bulk form or as claddings. Hence, this research investigates corrosion behavior and passivity of Inconel alloy 625 in nitric acid solutions of varying concentrations (0.01–1.00 mol/L) using a range of analytical techniques, including potentiodynamic polarization testing, chronoamperometry measurement, Mott-Schottky (MS) analysis, X-ray diffraction (XRD) approaches, and point defect modeling (PDM). Results demonstrate that corrosion current density does not necessarily decrease with increasing electrolyte concentration (Please check it out). Additionally, the alloy exhibits higher passive film transpassive potentials in more concentrated solutions. Potentiostatic polarization tests reveal an increase in steady-state current densities attributed to the passive films formed in more concentrated solutions. Lower electrolyte concentrations lead to the formation of more intact bilayered passive films. In this context, the formation of passive films is elucidated by examining the generation and annihilation of point defects. The semiconducting behavior of passive layers is further examined through XRD patterns and electrochemical properties. According to PDM, the diffusivity of point defects within the passive films and their respective donor densities can be closely correlated.

Exploring defect behavior in helium-irradiated single-crystal and nanocrystalline 3C-SiC at 800°C: A synergy of experimental and simulation techniques

In this study, single crystal (sc) and nanocrystalline (nc) 3C-SiC samples were subjected to 30 keV He ion irradiation across various doses while maintaining a temperature of 800 °C. Employing techniques including Raman spectroscopy, transmission electron microscopy (TEM), and nanoindentation, the alterations in microstructure and hardness resulting from He irradiation with various fluences were examined.In sc-SiC, irradiation prompted the formation of He platelets, resulting in a hardness increase of 7 GPa. In contrast, nc-SiC, characterized by a higher stacking fault density, exhibited the formation of bubbles, primarily at grain boundaries (GBs), with fewer occurrences within the grain interior, leading to a hardness increase of 1 GPa. Notably, in both sc- and nc-SiC, hardness reached saturation and subsequently stabilized or declined with increasing He fluence.Through molecular dynamics (MD) cascade simulations, we discerned that various planar defects do not uniformly contribute to enhancing radiation resistance. For example, intrinsic stacking faults (ISF) and twins in SiC played a substantial role in altering defect density and configurations, thereby facilitating point defect annihilation. Conversely, extrinsic stacking faults (ESF) and Σ3 GBs had a limited impact on defect production during a cascade.Furthermore, calculations of cluster diffusivity revealed an accelerated movement of He-vacancy towards GBs compared to bulk material and other planar defects. Moreover, the scarcity of point defects and constrained mobility of He atoms towards stacking faults in nc-SiC elucidated the marked tendency of He to form platelets in sc-SiC.Additionally, our findings established a correlation between the calculated indentation hardness and the geometry of He defects, consistent with experimental results from nanoindentation. These results significantly contribute to ongoing efforts to design SiC materials with heightened radiation tolerance.

Minimizing the diffusivity difference between vacancies and interstitials in multi-principal element alloys

Interstitial atoms usually diffuse much faster than vacancies, which is often the root cause for the ineffective recombination of point defects in metals under irradiation. Here, via ab initio modeling of single-defect diffusion behavior in the equiatomic NiCoCrFe(Pd) alloy, we demonstrate an alloy design strategy that can reduce the diffusivity difference between the two types of point defects. The two diffusivities become almost equal after substituting the NiCoCrFe base alloy with Pd. The underlying mechanism is that Pd, with a much larger atomic size (hence larger compressibility) than the rest of the constituents, not only heightens the activation energy barrier (Ea) for interstitial motion by narrowing the diffusion channels but simultaneously also reduces Ea for vacancies due to less energy penalty required for bond length change between the initial and the saddle states. Our findings have a broad implication that the dynamics of point defects can be manipulated by taking advantage of the atomic size disparity, to facilitate point-defect annihilation that suppresses void formation and swelling, thereby improving radiation tolerance.

Nested loops explain low irradiation-induced swelling rate

In the quest of radiation and oxidation-resistant materials for the fuel claddings of nuclear power stations, chromium (Cr) emerges as a good candidate. It shows a significantly lower irradiation-induced swelling rate of nearly an order magnitude lower than other body-centered cubic materials, such as W and Fe. This phenomenon indicates that there is an unsuspected aspect, beyond the crystal structure that affects the swelling of materials. To understand it, the structure and distribution of irradiation-induced dislocation loops in Cr were investigated after ion irradiation at 550 ℃ from 0.1 to 15 dpa. It appears that after irradiation Cr exhibits nested loops, either vacancy-type loops called ‘trough’ or interstitial-type loops called ‘island’, which are composed of a large dislocation loop containing multiple small and coplanar loops with inverse nature. Our results indicate that the nested trough and island dislocation loops stem from the 1D migration of dislocation loops. This loop formation mechanism is an alternative mechanism of recombination and annihilation of point defects, which in turn can explain the low swelling rate in Cr. This mechanism provides a new insight into the irradiation-induced swelling rate of metals.

Statics and dynamics of point boojums, line and modified Saturn ring topological defects in nematic confined geometry.

In this paper we study the static structure and the dynamics of topological defects associated with isotropic droplets in nematic environment. Investigations were made in confined geometry of optical cells when the droplet size was of the order of or larger than the gap of the cell. We observed the coexistence of point boojums and Saturn ring or modified Saturn ring defects. We found transformation of the Saturn ring defect to two localized broad defects at increasing the droplet size. At droplet coalescence antipodes of point and localized broad defects were born and the dynamics of their annihilation with existing defects was investigated. We found strong difference in the process of annihilation of point and localized broad defects. Microscope images of isotropic droplets in nematic environment in a planar cell. The director orientation far from the droplets is in horizontal direction. The photographs were taken with crossed vertical and horizontal polarizers (a) and with a single horizontal polarizer (b). The cell thickness is 100μm. Droplet diameter is less than the cell thickness. 1 and 2 are point boojums, L is the Saturn ring defect.

Engineering the formation of spin-defects from first principles

The full realization of spin qubits for quantum technologies relies on the ability to control and design the formation processes of spin defects in semiconductors and insulators. We present a computational protocol to investigate the synthesis of point-defects at the atomistic level, and we apply it to the study of a promising spin-qubit in silicon carbide, the divacancy (VV). Our strategy combines electronic structure calculations based on density functional theory and enhanced sampling techniques coupled with first principles molecular dynamics. We predict the optimal annealing temperatures for the formation of VVs at high temperature and show how to engineer the Fermi level of the material to optimize the defect’s yield for several polytypes of silicon carbide. Our results are in excellent agreement with available experimental data and provide novel atomistic insights into point defect formation and annihilation processes as a function of temperature.

Spatial inhomogeneity of point defect properties in refractory multi-principal element alloy with short-range order: A first-principles study

Short-range order can be developed in multi-principal element alloys and influences the point defect behavior due to the large variation of the local chemical environment. The effect of short-range order on vacancy and interstitial formation energy and migration behavior was studied in body-centered cubic multi-principal element alloy NbZrTi by first-principles calculations. Two short-range order structures created by density functional theory and Monte Carlo method at 500 and 800 K were compared with the structure of random solid solution. Both vacancy and interstitial formation energies increase with the degree of short-range order. Point defect formation energies tend to be higher in regions enriched in Nb and lower in regions enriched in Zr and Ti. Both vacancies and interstitials prefer to migrate toward Zr,Ti-rich regions and away from Nb-rich regions, suggesting that Zr,Ti-rich regions can potentially act as recombination centers for point defect annihilation. Compared to an ideal random solid solution, the short-range order increases the spatial inhomogeneity of point defect energy landscape. Tuning the degree of short-range order by different processing techniques can be a viable strategy to optimize the point defect behavior to achieve enhanced radiation resistance in multi-principal element alloys.

Towards the modeling of the interplay between radiation induced segregation and sink microstructure

Excess point defects created by irradiation in metallic alloys diffuse and annihilate at sinks available in the microstructure, such as grain boundaries, dislocations, or point defect clusters. Fluxes of defects create fluxes of alloying elements, leading to local changes of composition near the sinks and to a modification of the properties of the materials. The direction and the amplitude of this radiation-induced segregation, its tendency to produce an enrichment or a depletion of solute, depend on a set of transport coefficients that are very difficult to measure experimentally. The understanding of radiation-induced segregation phenomena has, however, made significant progress in recent years, thanks to the modeling at different scales of diffusion and segregation mechanisms. We review here these different advances and try to identify the key scientific issues that limit the development of predictive models, applicable to real alloys. The review addresses three main issues: the calculation of the transport coefficients from ab initio calculations, the modeling of segregation kinetics at static point defects sinks—mainly by kinetic Monte Carlo or diffusion-reaction models—and the more challenging task of modeling the dynamic interplay between radiation-induced segregation and sink microstructure evolution, especially when this evolution results from annihilation of point defects. From this overview of the current state-of-the-art in this field, we discuss still-open questions and guidelines for what constitutes, in our opinion, the desirable future works on this topic.

Mechanism of improved crystallinity by defect-modification in proton-irradiated GaAsPN photovoltaics: Experimental and first-principle calculations approach

This study presents a new model for point-defect modification in III-V-N alloys through first-principle calculations and several validation experiments conducted in our previous study, which explain the enhanced crystallinity of III-V-N alloys caused by proton irradiation and rapid thermal annealing (RTA). Validation experiments clarified that the conversion efficiency of the GaAsPN solar cell increased after proton irradiation followed by RTA, whereas that of the GaP solar cell decreased after the same process. Thus, the improved crystallinity of the GaAsPN alloy by this process is attributed to the decrease in nitrogen-related point defects in the crystal. The detailed annihilation mechanism of the nitrogen-related point defect was then studied using first-principle calculations demonstrating that the representative nitrogen-related point defects can change to a lower-energy state when a vacancy forms at its neighboring group V site, leading to the annihilation of the defects. It was concluded that vacancies created by proton irradiation enhance the annihilation of nitrogen-related point defects.

Insight into defect cluster annihilation at grain boundaries in an irradiated nanocrystalline iron

The design of radiation-tolerant polycrystalline materials has been mainly based on the control and manipulation of grain boundaries (GBs) that leads to annihilation of point defects at grain-boundary neighborhoods thus resulting in the formation of defect denuded zones. Nanocrystalline materials are potential candidates providing large density defect sinks for individual point defects and small highly mobile defect clusters (DCs). Using the in-situ irradiation transmission electron microscopy (TEM) technique, this study not only experimentally revealed the coalescence of small glissile DCs at/near GB and their subsequent annihilation at grain-boundary, but also provide insight into defect cluster dynamics. The small DCs were found to be transported to grain-boundary neighborhoods where they can annihilate by the one-dimensional loop hop or Burgers-vector rotation mechanism to GBs, considered as the main contribution to the long-range flux of interstitials to GB sinks. This process had marked effects on the morphology of the irradiated microstructure in nanocrystalline iron, limiting the length of DC strings and reducing the coalescence of DCs into large clusters (dislocation loops).

Off-line sub-nanosecond laser conditioning on large aperture deuterated potassium dihydrogen phosphate crystal

The large aperture deuterated potassium dihydrogen phosphate (DKDP) is an important frequency conversion crystal in a large power laser device. There are many defects inside the DKDP bulk material, including the varying element impurities and electronic defects. Comparing with the defect-free material, these bulk defects can easily absorb incident laser energy and pose the risks of initiating damage sites when exposed to high-energy lasers. Beside bulk defects, there are surface defects originating from the DKDP machining process, including cracks, scratches and protuberances. These surface defects affect the damage performance of DKDP crystal by increasing light absorption and weakening local mechanical strength. Due to the defects from both bulk and surface, the actual damage threshold of DKDP crystal is much lower than the expected theoretical value. The lack of its laser damage resistance seriously restricts the laser output power. In this study, the off-line sub-nanosecond laser conditioning technology is proposed to effectively improve the laser damage performance of large aperture DKDP crystal. Its principle is to irradiate DKDP with a mild laser fluence in advance. Although the laser pretreatment cannot directly eliminate the impurities, dislocations, grain boundaries or other macro structural defects in crystals, it indeed changes the distribution and density of intrinsic point defects on a micro-scale. It suggests that the complicated reactions of electron-hole, atom-vacancy and the intrinsic point defect annihilation caused by the microstructural transformation of crystal materials under laser conditioning are the possible reasons for reducing absorption and improving the damage resistance. In this experiment, the result of the damage to high-power laser device shows that the mean surface damage density of DKDP crystal at 9 J/cm<sup>2</sup> decreases from 5.02 pp/cm<sup>2</sup> to 0.55 pp/cm<sup>2</sup> after sub-nanosecond laser conditioning. The laser conditioning can remove the protuberance defects on the surface, thus reducing the surface damage density. In addition, the damage size probably decreases after laser conditioning. There is a leftward shift in the damage size curve after laser conditioning, and its peak decreases from 25 μm to 18 μm–20 μm. In addition, due to the removal effect of laser conditioning on defects, the spatial distribution of damage points after sub-nanosecond laser irradiation turns more uniform. This study provides a foundation for the applications of off-line sub-nanosecond laser conditioning technology in high-power laser facility.

Investigation of the applicability of post-irradiation annealing to reduce the degree of disorder in AlN ceramic to proton irradiation

The work presents study results of the applicability of high-temperature heat treatment (500-700°C) of nitride ceramics irradiated with protons with an energy of 1.5 MeV and a dose of 10 16 cm−2 . It was found that heat treatment for 60 minutes at a temperature of 700°C allows us to significantly reduce the density of radiation-induced defects and distortions in ceramics structure due to partial annihilation and relaxation of point defects. Dependences of changes in the strength and mechanical characteristics of ceramics on the temperature of post-irradiation annealing are shown. Based on the data obtained, a conclusion was made about prospects of using post-irradiation annealing to maintain the strength of ceramics subjected to loading during operation.

Production, annihilation and migration of point defects in cyclic straining

Interactions of mobile dislocations in persistent slip bands with ladder-like structure are analyzed and production rates of point defects, preferably vacancies, are derived. Previous resistivity measurements in copper single crystals and polycrystals are used to derive the growth of vacancy concentration during cyclic loading at low temperatures. The parameters of the growth law are derived from the initial growth rate and saturated vacancy concentration. The consequences of vacancy production in localized cyclic straining on the surface relief formation are analyzed. At very low temperature vacancies are produced and annihilated only athermally and vacancy concentration saturates. Localized deformation in persistent slip bands produces high vacancy concentration which outcomes in formation of tiny extrusions. At ambient and slightly elevated temperatures vacancies are steadily produced and migrate from the persistent slip bands to the matrix. This results in formation of persistent slip markings consisting of extrusions and intrusions. The shape of extrusions and intrusions depends on the on the dislocation arrangement in the PSB and in the neighbor matrix and their history during cyclic straining. Fatigue cracks start from sharp-edged intrusions.

The influences of migration behaviors on annihilation of point defects trapped into grain boundaries in Tungsten

ABSTRACT The nano-crystalline materials generally show better irradiation resistance than common poly-crystalline materials since grain boundaries (GBs) can usually enhance the annihilation of irradiation-induced defects. Although the defect annihilation mechanism has been thoroughly investigated in GBs in tungsten, the influences of defect migration behavior in GB-planes on the recombination capability between them are still not well understood. In view of this, several GBs with typical divergences of point defect migration behavior are selected to conduct this study. And then the evolutions of Frenkel pairs continuously introduced into these GB-planes are simulated with the molecular dynamic methods at 1200 K. Combined the point defect migration behaviors in GB-planes, it is found out that the GB satisfying the following two conditions may have more excellent recombination ability of point defects trapped into it. Firstly, the vacancy (V) and self-interstitial atom (SIA) migration abilities are similar in the GB-plane. Secondly, the V or SIA migration abilities along two orthogonal directions in the GB-plane are similar.

Production, Annihilation and Migration of Point Defects in Cyclic Straining

Interactions of mobile dislocations in persistent slip bands with ladder-like structure are analyzed and production rates of point defects, preferably vacancies, are derived. Previous resistivity measurements in copper single crystals and polycrystals are used to derive the growth of vacancy concentration during cyclic loading at low temperatures. The parameters of the growth law are derived from the initial growth rate and saturated vacancy concentration. The consequences of vacancy production in localized cyclic straining on the surface relief formation are analyzed. At very low temperature vacancies are produced and annihilated only athermally and vacancy concentration saturates. Localized deformation in persistent slip bands produces high vacancy concentration which outcomes in formation of tiny extrusions. At ambient and slightly elevated temperatures vacancies are steadily produced and migrate from the persistent slip bands to the matrix. This results in formation of persistent slip markings consisting of extrusions and intrusions. The shape of extrusions and intrusions depends on the on the dislocation arrangement in the PSB and in the neighbor matrix and their history during cyclic straining. Fatigue cracks start from sharp-edged intrusions.

FeCo– Fe2CoO4/Co3O4 nanocomposites: Phase transformations as a result of thermal annealing and practical application in catalysis

The results of the synthesis and subsequent phase transformations of FeCo nanowires depending on the annealing temperature are presented. An annealing temperature range of 200–800 °C was chosen to initialize the processes of annihilation of point defects at low temperatures, as well as phase transformations as a result of oxidation of nanostructures at high annealing temperatures. In the course of the research, a three-stage process of phase transformations was established, accompanied by oxidation of the structure followed by the formation of oxide phases of the spinel type Fe2CoO4 and Co3O4. It was found that at temperatures of 400 °C and 600 °C, the introduction of oxygen occurs nonuniformly along grain boundaries with the subsequent formation of oxide compounds, and in the case of annealing at a temperature of 800 °C, the oxygen distribution in the structure of nanowires becomes uniform, which indicates the complete oxidation of nanowires with isotropic the formation of oxide phases in the entire volume of nanowires. The prospects of using oxide biphasic nanowires for catalytic reduction reactions para-nitroaniline - para-phenyldiamine are shown.

Subsurface diffusion in crystals and effect of surface permeability on the atomic step motion

A new theoretical approach to characterize the diffusion of both surface and bulk point defects in crystals is presented. In our model, atomic steps are considered as sources and sinks not only for adatoms and advacancies but also for self-interstitials and bulk vacancies, providing a new mechanism for bulk point defect generation and annihilation. It is shown that the creation and annihilation of self-interstitials and vacancies occur at atomic steps and can be described by introducing a diffusive layer of the bulk point defects adsorbed just below the surface. The atomic step rate of advance is studied taking into account finite permeability of the surface for bulk and surface point defects. The surface permeability results in the appearance of the dependence of the total step rate of advance not only on the supersaturation in vapor phase but also on the supersaturation of point defects in the bulk.

Analysis of plastic strain-enhanced diffusivity in nanocrystalline iron by atomistic simulation

Plastic deformation may affect bulk diffusion in nanocrystalline materials by altering the rates of point defect production and annihilation. In the present work, a detailed analysis of this phenomenon is given by a series of molecular dynamics (MD) simulations to clarify the effect of preplastic strain on the diffusivity of iron atoms. The embedded atom method interatomic potential was used to perform MD simulations. The self-diffusion coefficient of iron atoms in unstrained and prestrained samples was measured over temperatures ranging from 600 to 1000 K and at total strains of 5%–20%. The results reveal that the diffusivity is indeed enhanced as a result of plastic straining, especially at low temperatures. The calculated diffusion coefficient of iron atoms in the prestrained samples is 10–80 times higher than that in the unstrained samples. The atomic structure analysis results indicated that the generation of excess vacancies and unstructured region by preplastic deformation contributes to the enhancement of self-diffusion under plastic straining conditions. At low temperatures, preplastic straining has a considerable effect in the peak shifting and broadening of the radial distribution function, which probably lowers the activation barrier height for diffusion.

The electrochemical properties of alloy 690 in simulated pressurized water reactor primary water: Effect of temperature

By anodic potentiostatic polarization and electrochemical impedance spectroscopy (EIS) measurements, the effect of temperature (200 °C, 250 °C, and 300 °C) on the electrochemical behavior of Alloy 690 was investigated in oxygenated borate buffer solution containing H3BO3 (2000 ppm B) + LiOH (2 ppm Li). The steady state passive current density was independent of the film formation potential at each temperature, indicating that the passive film is n-type in electronic character based on the predictions of the Point Defect Model (PDM). The passive current density increased and the absolute value of impedance decreased with increasing temperature, indicating a corresponding decrease in corrosion resistance. A mixed potential model (MPM), containing the PDM for describing the anodic passive dissolution of the alloy and the generalized Butler–Volmer equation for the cathodic oxygen reduction reaction was optimized on the experimental EIS data in order to extract model parameters, including kinetic parameters for the generation and annihilation of point defects in the barrier layer of the passive film. Metal (Cr,Ni,Fe) interstitials, which were found to have a higher concentration than oxygen vacancies, dominate the point defect structure of the barrier layer of the passive film at each temperature, thereby accounting for the n-type behavior of the passive film. With increasing temperature, the film thickness and defect density also increased, resulting in the decreased corrosion resistance of film. This indicates that the defect density of passive film plays a more important role on corrosion behavior of alloy than the film thickness.

Self-healing ability assessment of irradiated multilayered composites: A continuum approach

Atomistic simulations have revealed an unconventional behavior of point defects at interfaces found in multilayer composites synthesized by physical vapor deposition but the observed mechanisms that involve point-defect annihilation are subject to time-scale limitations. So, a mathematical model that describes long-term evolution of point defects in such materials under irradiation is presented in this work. Firstly, the effect of interface point-defect trapping and recombination mechanisms on point-defect concentrations has been studied. In addition, the effect of interface self-interstitial atoms loading, which has been seen during collision cascades, and constitutional vacancies has been studied too. Two interface configurations have been considered between metals in a β-α-β three-layer system (α = Cu and β = Nb, or V), KSmin and KS1. These interfaces correspond to ground-state and defect-free KS structures respectively. The respond to irradiation of the systems investigated here, Cu/Nb and Cu/V, depends on both, interface characteristics and bulk properties. Nonetheless, the influence of the properties of one metal in the point-defect evolution of the other metal is only effective if there are constitutional vacancies at the interface, i.e., for KSmin. Especial attention has been paid to the behavior of the same metal (Cu) when it is surrounded by diverse metals (Nb, or V) with the aim of comparing quantitatively our model predictions with experimental results reported elsewhere. The lower concentration of vacancies in Cu layer of Cu/Nb system at steady state is due to the low mobility of vacancies in niobium.