Radiation-induced Point Defects Research Articles

Evolution of precipitate and its effect on dislocation loops during in-situ He+ irradiation and annealing

The microstructural evolution of irradiation-induced point defects around the precipitates during in-situ 30 keV He+ irradiation was systematically investigated in Mo3Nb alloy using transmission electron microscopy (TEM) at various temperatures: room temperature (RT), 573 K, and 1073 K. The average size and volume number density of dislocation loops were obtain under the influence of irradiation temperature, fluence and different types (sizes and shapes) of precipitates. The irradiated defects showed distinct characteristics that correlated with the precipitate types. Dislocation loops of larger size and lower density were observed around the larger precipitates, which were also influenced by the precipitate morphology. Temperature had a great effect on defect migration, resulting in a decrease in loop density and an increase in loop size. Furthermore, the hardening effect attributed to irradiation-induced loops decreased with the increase of temperature and precipitate size. The dissolution of precipitates became increasingly pronounced with the increase of temperature, and irradiation could accelerate the process. At 573 K, the dissolution was only found in the large non-spherical precipitate, while at 1073 K, all the precipitates underwent dissolution. The annealing experiment conducted at 1073 K showed that the dissolution of large-sized precipitates would lead to the dispersion of small-sized precipitates, which was beneficial to increase the interfaces and potentially improve the radiation tolerance and mechanical properties of the alloy.

The capacity of disclinated non-equilibrium GBs to accommodate point defects in tungsten

Experimental observations reveal that disclinated non-equilibrium grain boundaries (GBs) exist extensively in polycrystalline metals; however, the interaction between these GBs and irradiation-induced point defects is rarely reported. In the present work, Molecular statics simulation was used to evaluate the capacity of disclinated non-equilibrium GBs to accommodate point defects in tungsten. Simulation results showed that disclinated non-equilibrium GBs are more efficient sinks for point defects than their equilibrium counterparts because of long-range stress field around them. Continuous segregation of point defects will change the structure of the disclinated non-equilibrium GBs and leads to the GBs tending to relax to equilibrium state. According to theoretical calculation, the disclinated non-equilibrium GBs can absorb a large number of point defects before transforming into equilibrium state; therefore, disclinated non-equilibrium GBs have very strong capacity to accommodate point defects and can be used as strong defect sinks for developing radiation resistance materials.

Unusual nanoscale amorphization of metallic chromium interfacing with SiC under high energy irradiation

Layered metal/silicon carbide (SiC) materials systems are becoming increasingly relevant to solve materials challenges in advanced fission and fusion nuclear energy systems, necessitating a deeper understanding of how interfaces between dissimilar materials behave under high energy irradiation. In this work, we use a Cr-SiC bilayer system as a model to study the behavior of such interfaces under high energy ion irradiation (80 MeV Xe26+ ions) at elevated temperatures (∼350 °C). Through high resolution characterization of the interface, we observed the formation of a nanoscale Cr-rich amorphous layer adjacent to crystalline SiC. We explain this phenomenon through a multi-scale computational approach incorporating ballistic mixing simulations, density functional theory calculations, and CALPHAD-based non-equilibrium modeling that shows the localized amorphization of Cr to be driven by the synergistic action of irradiation-induced point defects within Cr and transport of Si and C atoms across the interface. In particular, the accumulation of radiation damage results in the thermodynamic destabilization of the point-defect containing, metastable Cr-Si-C solid solution with respect to an amorphous phase of identical composition. This study advances the understanding of how metal/SiC interfaces behave under irradiation and establishes a modeling framework that can be applied to interfacial systems to understand irradiation-induced amorphization.

Sensitivity Analysis and Uncertainty Quantification on Point Defect Kinetics Equations with Perturbation Analysis

The concentration of radiation-induced point defects in general materials under irradiation is commonly described by the point defect kinetics equations based on rate theory. However, the parametric uncertainty in describing the rate constants of competing physical processes, such as recombination and loss to sinks, can lead to a large uncertainty in predicting the time-evolving point defect concentrations. Here, based on perturbation theory, we derive up to the third-order correction to the solution of point defect kinetics equations. This new set of equations enables a full description of continuously changing rate constants and can accurately predict the solution up to 50% deviation in these rate constants. These analyses can also be applied to reveal the sensitivity of the solution to input parameters and aggregated uncertainty from multiple rate constants.

A mechanistic study of grain boundary behaviour during irradiation-induced growth in zirconium

Grain boundaries play a significant role during deformation and can exhibit both beneficial and adverse effects on deformation behaviour during irradiation. For instance, macroscopic irradiation growth is increased by the presence of a high density of grain boundaries. Grain boundaries can act as sinks for point defects and barriers to dislocation motion. Thus, a mechanistic assessment of the impact of grain boundaries is essential for better understanding irradiation-induced deformation. The interplay between irradiation-induced microstructure and mechanical properties has been an area of research for many decades, but many uncertainties remain. In the present work, the deformation fields adjacent to grain boundaries in pure zirconium are investigated by using high angular resolution electron backscatter diffraction (HR-EBSD) after a period of irradiation growth. It is observed that the extent of grain boundary deformation is associated with the crystallographic orientation difference between adjacent grains. High-angle grain boundaries function as a deformation constraint, whereas a significant grain boundary migration occurs at some low-angle grain boundaries, which likely act as an active diffusion path for the irradiation-induced point defects. In addition, the change in residual strain fields associated with the irradiation damage is quantified with respect to a non-irradiated reference. A net change in the state of residual elastic strain in the order of ∼10−3 has been estimated for ∼8 dpa proton irradiation. Furthermore, the origin of residual elastic strain and lattice misorientation concentration along (0002) traces are discussed.

Properties of radiation-induced point defects in austenitic steels: a molecular dynamics study

Austenitic steels are recognized as excellent structural materials for pressurized water reactors due to their outstanding mechanical properties and radiation resistance. However, compared to the widely studied FeCrNi series of steels, little is known about the radiation resistance of FeCrNiMn steel. In this study, the generation and evolution of radiation-induced defects in FeCrNiMn steel were investigated by molecular dynamics simulations. The results showed that more defect atoms were produced in the thermal spike stage, but fewer defects survived at the end of the cascades in FeCrNiMn compared to pure Fe. Point defect properties were analyzed by molecular statics, and the formation energies of defects in FeCrNiMn were lower than those of pure Fe, while the migration energies were higher. Compared to FeCrNi, FeCrNiMn had smaller migration energies and a larger overlap of vacancy and interstitial migration energies. The low vacancy formation energies and widely overlapping migration energies suggested that the number of point defects in the thermal spike stage was higher, but the possibility of recombination was greater. Additionally, Mn exhibited the smallest interstitial formation energies and migration energies. The difference in defect migration energies revealed that vacancy and interstitial defects migrate through different alloy constituent elements. This study revealed the underlying mechanism for the excellent irradiation resistance of FeCrNiMn.

Investigation of radiation tolerance of yttria stabilized zirconia in the ballistic collision regime: Effect of grain size and environmental temperature

The irradiation response of yttria-stabilized zirconia, having different grain sizes, subjected to 900 keV iodine ions at room temperature and 1000 K is reported. GIXRD and Raman spectroscopy indicate superior radiation tolerance of the fine-grained samples when compared to their coarse-grained counterparts which is attributed to the abundance of grain boundaries at relatively shorter distances. A reduction in the residual strain is observed in the samples irradiated at 1000 K. This is attributed to the clustering of the irradiation induced point defects thereby resulting in a decrease in their density, but with a corresponding increase in the extended defects density.

Recovery at room temperature annealing on 4H–SiC SBDs by gamma irradiation

The impact of gamma irradiation and subsequent recovery at room temperature on the device performance of commercial 4H–SiC Schottky barrier diodes (SBDs) was investigated through the analysis of the electrical properties and the deep level transient spectroscopy (DLTS). The ideality factor (n) of the SBDs increased from 1.01 to 1.13 with an increasing irradiation dose, but recovered after 7 days of room temperature annealing. To determine Schottky barrier heights (ΦB), both current-voltage (I–V) and capacitance-voltage (C–V) measurements were performed. The results of the combined I–V and C–V analysis showed that a little variations in ΦB were consistent with the variations in irradiation dose, except for the ΦB calculated by C–V measurement at 30 kGy. Similarly, the ΦB recovered after 7 days at room temperature. DLTS analysis revealed the presence of Z1/Z2 traps in the samples after 7 days at room temperature annealing. These traps exhibited activation energies ranging from 0.46 eV to 0.55 eV. The trap concentration (NT) increased from 9.48 × 1012 cm−3 to 2.23 × 1013 cm−3 with the increasing irradiation dose. These findings indicate that gamma irradiation-induced point defects were the primary cause of the degradation of 4H–SiC SBDs.

Paramagnetic Point Defect in Fluorine-Doped Silica Glass: The E^{'}(F) Center.

Fluorine-doped silica is a key material used in all low-loss and/or radiation-resistant optical fibers. Surprisingly, no fluorine-related radiation-induced point defects have been identified. By using electron paramagnetic resonance, we report the first observation of F-related defects in silica. Their fingerprint is a doublet with 10.5mT splitting due to hyperfine coupling (hfc) to ^{19}F nuclear spins. An additional 44.4mT hfc to the ^{29}Si nucleus indicates that this defect belongs to the "E^{'} center" family and has a structure of a fluorine-modified Si dangling bond: 3-coordinated Si atoms with an unpaired electron in an sp^{3} orbital, bonded to a glass network by 2 bridging oxygen atoms and to a F atom.

Influence of irradiation-induced point defects on nanotribological properties of m-plane GaN investigated using molecular dynamics simulation

Molecular dynamics (MD) simulation toward the irradiation cascade collision of GaN (10−10) by using different primary knock-on atom (PKA) energies was performed. Simulation results showed that point defects comprising vacancies and interstitials in pairs increased with increased PKA energy. The number of stable defects after cascade collision was linearly related to the PKA energy, which well agreed with the Norgett–Robinson–Torrens equation. Then, we investigated the effect of irradiation-induced point defects on the nanotribological properties and subsurface damage of the GaN (10−10) workpiece by MD simulation of nanoscratching. The irradiation-induced point defects induced a significant change in the wear morphology, as well as more slight wear and a lower coefficient of friction, compared with non-irradiated GaN. The irradiated GaN also exhibited fewer chips and smoother surfaces than the non-irradiated one after scratching. The number of deformed atoms of GaN increased with increased PKA energy, suggesting that irradiation can suppress the atomic/near-atomic scale deformation of GaN crystal under certain conditions. The presence of point defects can, to some extent, hinder the development of dislocations and differentiate the scratching-induced subsurface damage. This study provided atomic-scale insight into the effect of irradiation point defects on the nanotribological behavior of GaN.

A steady-state irradiation creep and thermal creep model for zirconium alloys

In this work, a mechanistic steady-state creep model is developed to characterize the macroscopic strain rate of metallic materials affected by irradiation flux, testing temperature and applied stress. Thereinto, the steady-state strain rate is obtained by considering the density evolution of mobile dislocations, which involve the mechanisms of dislocation multiplication, dynamic annihilation and time-dependent dislocation annihilation. In order to effectively analyze the irradiation creep behavior in the low and high stress region, main attentions are focused on the influence of dislocation mobility on the process of time-dependent dislocation annihilation. At low stress, dislocation mobility is affected by dislocation climb and thermally activated glide. For the former, the absorption of irradiation-induced point defects can promote dislocation climb; whereas, thermally activated dislocation glide might be inhibited by the existing defect clusters. At high stress, the dominant deformation mechanism changes from dislocation climb to displacement cascade unpinning. For the latter, an explicit formula for dislocation mobility is deduced to address the influence of dislocation unpinning from the barriers on irradiation creep. To further verify the established model, thermal creep and irradiation creep data of zirconium alloys are considered to compare with the theoretical results. A good agreement is achieved over a wide range of temperature and stress indicating that the model can well describe the deformation behavior of steady-state creep. In addition, contribution of the dominant dislocation mobility components and dislocation density evolution components is compared under both thermal and irradiation creep, which can facilitate the comprehension of fundamental creep mechanisms of metallic materials.

Anisotropic diffusion of radiation-induced self-interstitial clusters in HCP zirconium: A molecular dynamics and rate-theory assessment

Under irradiation, Zr and Zr alloys undergo growth in the absence of applied stress. This phenomenon is thought to be associated with the anisotropy of diffusion of either or both radiation-induced point defects and defect clusters. In this work, molecular dynamic simulations are used to study the anisotropy of diffusion of self-interstitial atom clusters. Both near-equilibrium clusters generated by aggregation of self-interstitial atoms and cascade-induced clusters were considered. The cascade-induced clusters display more anisotropy than their counterparts produced by aggregation. In addition to 1-dimensional diffusing clusters, 2-dimensional diffusing clusters were observed. Using our molecular dynamic simulations, the input parameters for the “self-interstitial atom cluster bias” rate-theory model were estimated. The radiation-induced growth strains predicted using this model are largely consistent with experiments, but are highly sensitive to the choice of interatomic interaction potential.

Paramagnetic radiation-induced radicals in calcium pyrophosphate polymorphs

Calcium pyrophosphate (Ca2P2O7) is a promising material for biomedical and optical applications; however, relatively little is documented on the radiation-induced point defects of the material. This study reports on the formation, identity, and properties of X-ray-induced radicals in γ-, β-, and α-polymorphs of Ca2P2O7. Complementary thermally stimulated luminescence (TSL), electron paramagnetic resonance (EPR), and solid-state nuclear magnetic resonance (NMR) spectroscopy techniques were combined to reveal the effects of ionising radiation on each polymorph. The high temperature polymorphs β-, and α-Ca2P2O7 exhibited diminishing variety and intensity of EPR signals, whereas the opposite trend was observed in the TSL response. TSL glow curves consisted of multiple peaks in the temperature range of 20–350 °C, with maximum emission intensity located at approximately 600 nm. Multiple electronic spin 1/2 paramagnetic centres were identified on the basis of spin-Hamiltonian parameters determined from EPR spectra simulations. Annealing kinetics of the individual paramagnetic centres were determined to provide an interpretation of the complex nature of TSL glow curves. A decrease of the 31P spin-lattice relaxation T1 times after irradiation was detected for all Ca2P2O7 polymorphs. The results highlight the importance of structure, morphology, and synthesis conditions on the formation of charge traps in Ca2P2O7 and expand the knowledge base on the variety of radiation-induced radicals in phosphate materials.

Quantification of 1.75 MeV Xe5+ induced defects in zirconia doped ceria (Ce0.8Zr0.2O2)

This study investigates the structural stability of the CeO2-ZrO2 system when subjected to high doses of irradiation (a few hundred displacements per atom). The goal is to explore their potential use in safe immobilization of spent nuclear fuel and development of accident-tolerant fuels for next-generation nuclear reactors. Highly dense pellets were synthesized using a solid-state reaction and then irradiated with 1.75 MeV Xe5+ ions at ion fluences ranging from 1 × 1015 to 1 × 1017 ions/cm2. Structural and microstructural analyses were conducted using glancing angle x-ray diffraction, micro-Raman spectroscopy, and scanning electron microscopy. The results indicate that Ce0.8Zr0.2O2 has a high tolerance against irradiation-induced phase transformation or amorphization but does generate irradiation-induced point defects. Each energetic ion produced a deformed region with a voluminous swelling of ∼0.61 ± 0.09 nm and a damaged zone of ∼0.09 ± 0.02 nm, as calculated from the irradiation-caused peak broadening that is explained by a three-step damage accumulation model. The electron microscopy studies show that grain boundaries serve as a sink for defects, and an increase in grain size was observed due to defect accumulation inside the grain's volume. Overall, the study shows that polycrystalline fluorite-structured Ce0.8Zr0.2O2 is a promising nuclear material for advanced energy systems as it did not show significant structural damage such as amorphization and grain fragmentation, even on irradiation at a high dose of ∼428 dpa.

Irradiation damage and corrosion performance of proton irradiated 304 L stainless steel fabricated by laser-powder bed fusion

The irradiation-induced microstructure and corrosion performance of proton irradiated 304 L stainless steel (SS) fabricated by laser powder bed fusion (LPBF) is investigated. Irradiation-induced dislocation loops are observed in both LPBF and traditionally manufactured (TM) 304 L SS following 2 MeV proton irradiation at 360 °C. A comparison analysis of the oxide scales formed on proton irradiated LPBF and TM 304 L SS is conducted along with the unirradiated counterparts. The irradiation-induced defects accelerate the corrosion by providing a rapid diffusion path to form a thicker and more porous film with a double-layer structure and herein the oxide film thickness of TM 304 L SS is approximately twice that of the LPBF 304 L SS. The unique microstructure of high dislocation densities, cellular sub-grains boundaries and dispersed nanoscale inclusions provides large amounts of interfaces in LPBF samples, which is benefit to absorb the irradiation-induced point defects. The irradiation induced fast diffusion paths and the nucleation points of oxides are thus reduced, resulting in a thinner oxide film thickness and a better corrosion resistance revealed by the more Cr enrichment of inner scale in LPBF 304 L SS.

The Primary Irradiation Damage of Hydrogen-Accumulated Nickel: An Atomistic Study.

Nickel-based alloys have demonstrated significant promise as structural materials for Gen-IV nuclear reactors. However, the understanding of the interaction mechanism between the defects resulting from displacement cascades and solute hydrogen during irradiation remains limited. This study aims to investigate the interaction between irradiation-induced point defects and solute hydrogen on nickel under diverse conditions using molecular dynamics simulations. In particular, the effects of solute hydrogen concentrations, cascade energies, and temperatures are explored. The results show a pronounced correlation between these defects and hydrogen atoms, which form clusters with varying hydrogen concentrations. With increasing the energy of a primary knock-on atom (PKA), the number of surviving self-interstitial atoms (SIAs) also increases. Notably, at low PKA energies, solute hydrogen atoms impede the clustering and formation of SIAs, while at high energies, they promote such clustering. The impact of low simulation temperatures on defects and hydrogen clustering is relatively minor. High temperature has a more obvious effect on the formation of clusters. This atomistic investigation offers valuable insights into the interaction between hydrogen and defects in irradiated environments, thereby informing material design considerations for next-generation nuclear reactors.

Interaction of irradiation defects with precipitates in CuCrZr and Cu-1Fe alloys

In this work, we employ transmission electron microscopy and helium ion irradiation to study the interactions between the radiation-induced point defects and the incoherent/semi-coherent precipitates in CuCrZr and Cu-1Fe alloys. Both thin foil and bulk implantation were used to explore the interactions of radiation defects with interfaces. The irradiated defects show a strong interface character dependence in both implanted CuCrZr and Cu-1Fe alloys after similar level of radiation damage. Large-sized voids appear around the incoherent Cr precipitates, while no visible cavities are formed around precipitates in Cu-1Fe alloy. Interestingly, three types of semi-coherent Fe precipitates exhibit various irradiation damage mitigation mechanisms, such as twin boundaries migration, short range elements redistribution and precipitate abnormal growth, which are caused by radiation defect-mediated dislocation glide, element diffusion and mass transportation. The high sink efficiency of the incoherent precipitates trap vacancies and leads to the generation of submicron-sized voids. In contrast, the semi-coherent precipitates can regulate the motion of point defects and effectively enhance the recombination rate of vacancies and interstitials.

Microstructural Effects on Irradiation Creep of Reactor Core Materials

The processes that control irradiation creep are dependent on the temperature and the rate of production of freely migrating point defects, affecting both the microstructure and the mechanisms of mass transport. Because of the experimental difficulties in studying irradiation creep, many different hypothetical models have been developed that either favour a dislocation slip or a mass transport mechanism. Irradiation creep mechanisms and models that are dependent on the microstructure, which are either fully or partially mechanistic in nature, are described and discussed in terms of their ability to account for the in-reactor creep behaviour of various nuclear reactor core materials. A rate theory model for creep of Zr-2.5Nb pressure tubing in CANDU reactors incorporating the as-fabricated microstructure has been developed that gives good agreement with measurements for tubes manufactured by different fabrication routes having very different microstructures. One can therefore conclude that for Zr-alloys at temperatures < 300 °C and stresses < 150 MPa, diffusional mass transport is the dominant creep mechanism. The most important microstructural parameter controlling irradiation creep for these conditions is the grain structure. Austenitic alloys follow similar microstructural dependencies as Zr-alloys, but up to higher temperature and stress ranges. The exception is that dislocation slip is dominant in austenitic alloys at temperatures < 100 °C because there are few barriers to dislocation slip at these low temperatures, which is linked to the enhanced recombination of irradiation-induced point defects.

Effects of proton irradiation-induced point defects on Shockley–Read–Hall recombination lifetimes in homoepitaxial GaN p–n junctions

This work examined the intentional generation of recombination centers in GaN p–n junctions on freestanding GaN substrates. Irradiation with a 4.2 MeV proton beam was used to create a uniform distribution of vacancies and interstitials across GaN p+/n− and p−/n+ junctions through anode electrodes. With increasing proton dose, the effective doping concentrations were found to be reduced. Because the reduction in the doping concentration was much higher than the hydrogen atom concentration, this decrease could not be attributed solely to carrier compensation resulting from interstitial hydrogen atoms. In fact, more than half of the electron and hole compensation was caused by the presence of point defects. These defects evidently served as Shockley–Read–Hall (SRH) recombination centers such that the SRH lifetimes were reduced to several picoseconds from several hundred picoseconds prior to irradiation. The compensation for holes in the p−/n+ junctions was almost double that for electrons in the p+/n− junctions. Furthermore, the SRH lifetimes associated with p−/n+ junctions were shorter than those for p+/n− junctions for a given proton dose. These differences can be explained by variations in the charge state and/or the formation energy of intrinsic point defects in the p-type and n-type GaN layers. The results of the present work indicate the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes.

Elastic Strain Relaxation of Phase Boundary of α' Nanoscale Phase Mediated via the Point Defects Loop under Normal Strain.

Irradiation-induced point defects and applied stress affect the concentration distribution and morphology evolution of the nanophase in Fe-Cr based alloys; the aggregation of point defects and the nanoscale precipitates can intensify the hardness and embrittlement of the alloy. The influence of normal strain on the coevolution of point defects and the Cr-enriched α' nanophase are studied in Fe-35 at.% Cr alloy by utilizing the multi-phase-field simulation. The clustering of point defects and the splitting of nanoscale particles are clearly presented under normal strain. The defects loop formed at the α/α' phase interface relaxes the coherent strain between the α/α' phases, reducing the elongation of the Cr-enriched α' phase under the normal strains. Furthermore, the point defects enhance the concentration clustering of the α' phase, and this is more obvious under the compressive strain at high temperature. The larger normal strain can induce the splitting of an α' nanoparticle with the nonequilibrium concentration in the early precipitation stage. The clustering and migration of point defects provide the diffusion channels of Cr atoms to accelerate the phase separation. The interaction of point defect with the solution atom clusters under normal strain provides an atomic scale view on the microstructure evolution under external stress.