Sinks For Point Defects Research Articles

Absence of random voids in the spatially ordered void ensemble

The void evolution in the presence of one-dimensionally (1-D) migrating self-interstitials is considered. No saturation of the void swelling due to 1-D self-interstitials is assumed. Void growth rate is considered to be driven by the dislocation bias, and undergoes the stochastic fluctuations caused by the random nature of point defect jumps and collision cascade initiation. Void nucleation and subsequent growth are investigated. It is shown that initially nucleated random voids start shrinking already when the spatially aligned voids make a small fraction of the void ensemble. Despite a continuous generation of small void embryos in the random spatial positions and negligible vacancy emission, development of the random void population becomes completely suppressed by the stochastic fluctuations when the aligned voids are the major sinks for point defects and the fraction of 1-D self-interstitials is small but not negligible.

The interstitial emission mechanism in a vanadium-based alloy

Under nuclear fusion environments, displacement cascades in the potential first protective wall material vanadium (V) and its alloys lead to a large number of point defects. As sink of point defects, grain boundaries are observed to significantly affect the radiation resistance of structured metals. By using potential energy surface searching tools, the interaction between the interstitial-loaded ∑3<110>{111} symmetric tilt grain boundary (STGB) and the point defects in V-based alloy is investigated. For the self-interstitial atoms (SIA)-loaded STGB, the vacancy located within certain distances from the STGB can be effectively annihilated within several nanoseconds via the interstitial emission (IE) mechanism. If an alloy doping interstitial as a Cr atom substitute the SIA, or the vacancy is stabilized by alloy solutes as Ti atoms forming a solute-vacancy complex near the STGB, IE works more effectively with lower activation energy barriers and less thermal activation time. The results indicate that the STGB-induced IE mechanism raises the radiation resistance of the V-based alloy. The results contribute to the micro-structure and constituent optimizations in designing an excellent V-based first protective wall material.

The effects of loop size on the unfaulting of Frank loops in heavy ion irradiation

When stainless steels are exposed to high irradiation dose, a fraction of Frank loops will unfault and transform into perfect loops. After unfaulting, perfect loops can glide and impinge into other loops to form dislocation networks, and dislocation networks will act as sinks for point defects and solute elements. Besides, unfaulting will influence the extent of irradiation hardening as well. Based on previous theories, the size of Frank loops was considered to be a key factor in unfaulting. To better understand the unfaulting process, 316L stainless steel model alloy was irradiated to 5dpa at 350 °C, 400 °C and 450 °C by 3 MeV Ni ions. Method was developed to quantitatively compare the size and density between Frank loops and perfect loops through relrod and weak-beam dark field techniques. Results showed that unfaulting barely occurred at 350 °C but was distinct at 400 °C and 450 °C. The majority of perfect loops were in the size range of 8–16 nm at 400 °C and were in the size range of 12–20 nm at 450 °C. The existence of a critical loop size for unfaulting was not supported by our results. Moreover, by our results, the energy of Frank loops does not play a decisive role in unfaulting, and the simple approximation of geometric constraint in the rate theory to calculate unfaulting diameter needs to be revisited. Irradiation hardening was also analyzed by nano-indention. It was found that loop unfaulting played an important role in the hardening decrease from 350 °C to 450 °C.

Modelling the influence of strain fields around precipitates on defect equilibria and kinetics under irradiation in ODS steels: A multi scale approach

Resistance to radiation damage is a crucial property for structural materials in fission or fusion reactors. The particle-matrix interface in oxide dispersion strengthened (ODS) steels is considered a sink for point defects created during irradiation. In this work we address the question, if elastic strain fields around oxide particles cause a long-range interaction between the precipitates and point defects. We use kinetic Monte Carlo simulations to simulate the diffusion of point defects under the influence of elastic strain fields caused by Y2O3 and Y2Ti2O7 precipitates. We show that there is essentially no interaction between vacancies and the strain fields, while the sink strength of precipitates for interstitials increases with misfit strain between precipitate and matrix. Consequently, the interstitial concentration decreases with misfit strain, while due to reduced interstitial-vacancy recombination the vacancy concentration increases with misfit strain. The total change of point defect concentration with misfit strain is, however, rather limited.

Point defect sink strength of low-angle tilt grain boundaries: A phase field dislocation climb model

Evaluating the point defect sink strength of grain boundaries is crucial for understanding the metal behavior of plasticity and damage under irradiation. In this paper, the point defect sink strength of low-angle symmetrical tilt grain boundaries is investigated by the phase field dislocation climb model under irradiation. The results indicate that sink strength of grain boundary is not only determined by the long-range point defect diffusion but also the short-range point defect absorption by the dynamic climbing of grain boundary dislocations. All of the study findings prove that the irradiation induced creep deformation of grain boundaries is essential for evaluating the radiation tolerance of materials.

Interplay Between Grain Boundaries and Radiation Damage

The need for enhanced radiation-tolerant materials for advanced nuclear energy designs has resulted in a growing number of investigations that have explored the effect of grain boundaries under irradiation. The key motivation for examining the role of grain boundaries in radiation environments is the ability to tailor grain boundary networks through either the introduction of specific grain boundaries or an increase in the grain boundary density. While traditionally thought to be efficient sinks for radiation-induced point defects, many recent experimental studies in model and pure systems have shown significant heterogeneity in grain boundary-defect interactions and associated sink efficiency as a function of grain boundary character. Furthermore, grain boundaries can migrate under irradiation, which creates an additional level of complexity. This article will provide a prospective on the experimental observations associated with defect evolution near grain boundaries including variation in sink efficiency and grain boundary mobility in radiation environments.

Modeling the Influence of Strain Fields Around Precipitates on Defect Equilibria and Kinetics Under Irradiation in ODS Steels: A Multi Scale Approach

Resistance to radiation damage is a crucial property for structural materials in fission or fusion reactors. The particle-matrix interface in oxide dispersion strengthened (ODS) steels is considered a sink for point defects created during irradiation. In this work we address thequestion, if elastic strain fields around oxide particles cause a long-range interaction between the precipitates and point defects. We use kinetic Monte Carlo simulations to simulate the diffusion of point defects under the influence of elastic strain fields caused by Y2O3 and Y2Ti2O7 precipitates. We show that there is essentially no interaction between vacancies and the strain fields, while the sink strength of precipitates for interstitials increases with misfit strain between precipitate and matrix. Consequently, the interstitial concentration decreases with misfit strain, while due to reduced interstitial-vacancy recombination the vacancy concentration increases with misfit strain. The total change of point defect concentration with misfit strain is, however, rather limited.

Chemically-biased diffusion and segregation impede void growth in irradiated Ni-Fe alloys

Recent irradiations of Ni-Fe concentrated solid solution alloys have demonstrated significant improvement of radiation performance. This improvement is attributed to redistribution of the alloying elements near sinks of point defects (voids, dislocations) due to chemically-biased atomic diffusion, where vacancies have preference to migrate via Fe atoms and interstitials via Ni atoms. In Ni-Fe, all sinks are enriched by Ni atoms, which strongly affects further interactions of radiation-produced mobile defects with voids and dislocations, hence void growth and dislocation climb. Ni-decorated sinks interact stronger with interstitial atoms than vacancies, which enhances dislocation loops growth. At the same time, Ni segregation creates Fe-enriched “channels” for vacancy migration out of the damage region to agglomerate in the outer regions, inaccessible to interstitial atoms. Strong effect of chemically-biased diffusion is supported by transmission electron microscope characterization and calls for special attention in designing alloys with desired properties through tuning defect mobilities.

Molecular dynamics investigation of grain boundaries and surfaces in U3Si2

Uranium-silicide (U-Si) fuels are being pursued as a possible accident tolerant fuel (ATF). This uranium alloy benefits from higher thermal conductivity and higher fissile density compared to uranium dioxide (UO2). In order to perform engineering scale nuclear fuel performance simulations, the material properties of the fuel must be known. Currently, the experimental data available for U-Si fuels is rather limited. Thus, multi-scale modeling efforts are underway to address this gap in knowledge. Interfaces play a critical role in the microstructural evolution of nuclear fuel under irradiation, acting both as sinks for point defects and as preferential nucleation sites for fission gas bubbles. In this study, a semi-empirical modified Embedded-Atom Method (MEAM) potential is utilized to investigate grain boundaries and free surfaces in U3Si2. The interfacial energy as a function of temperature is investigated for ten symmetric tilt grain boundaries, eight unique free surfaces and voids of radius up to 35 Å. The point defect segregation energy for both U and Si interstitials and vacancies is also determined for two grain boundary orientations. Finally, the entropy change and free energy change for grain boundaries is calculated as a function of temperature. This is the first study into grain boundary properties of U-Si nuclear fuel.

Improved high temperature radiation damage tolerance in a three-phase ceramic with heterointerfaces

Radiation damage tolerance for a variety of ceramics at high temperatures depends on the material’s resistance to nucleation and growth of extended defects. Such processes are prevalent in ceramics employed for space, nuclear fission/fusion and nuclear waste environments. This report shows that random heterointerfaces in materials with sub-micron grains can act as highly efficient sinks for point defects compared to grain boundaries in single-phase materials. The concentration of dislocation loops in a radiation damage-prone phase (Al2O3) is significantly reduced when Al2O3 is a component of a composite system as opposed to a single-phase system. These results present a novel method for designing exceptionally radiation damage tolerant ceramics at high temperatures with a stable grain size, without requiring extensive interfacial engineering or production of nanocrystalline materials.

Atomistic Simulation of the Interaction Between Point Defects and Twin Boundary

While nanotwinned metals have been proven to show excellent mechanical properties, they are generally anticipated to be less effective in the alleviation of radiation damage. However, recent in situ studies have indicated that some nanotwinned metals exhibit unprecedented radiation tolerance, and the unexpected self‐healing of twin boundaries in response to radiation was observed. To reveal the underlying atomic mechanisms, we performed long‐time molecular dynamics simulations to study the dynamic interaction between twin boundary and some typical radiation‐induced point defects, including vacancy cluster and self‐interstitial atoms. The defective structures of coherent twin boundary which contains incoherent twin segment or self‐interstitial atoms are considered, and these structure features are found to effectively improve the ability of twin boundary to act as a sink for point defects.

Atomistic modeling of α’ precipitation in Fe-Cr alloys under charged particles and neutron irradiations: Effects of ballistic mixing and sink densities

The α′ precipitation in Fe-Cr alloys under electron, neutron and ion irradiations is modelled by cluster dynamics and atomistic kinetic Monte Carlo simulations. The former method provides the evolution of the density of point defect sinks. This density is then introduced in the Monte Carlo which models the diffusion of point defects and atoms. By comparison with previous studies, this provides a better description of the evolution of point defect concentrations and therefore of the balance between two competing irradiation effects: the acceleration of diffusion and the ballistic mixing. The prediction of the model is in agreement with existing atomic-scale experiments already reported after electron, ion and neutron irradiations or obtained for the present study. Irradiation is found to accelerate the precipitation by orders of magnitude, except for the case of ion irradiation at high doses rates, where the simulations predict a ballistic dissolution of α′ precipitates. According to our model, this dissolution is mainly due to a high sink density, which reduces the concentrations of point defects and limits the acceleration of diffusion. This effect may be reinforced by carbon contamination, which reduces the mobility of point defects.

Small-Angle Twist Grain Boundaries as Sinks for Point Defects

It is known that grain boundaries (GBs) provide sinks for defects induced into a solid by irradiation. At the same time radiation can change the atomic structure and chemistry of GBs, which in turn impacts the ability of GBs to continue absorbing defects. Although a number of studies have been reported for tilt GBs acting as defect sinks, the questions of how twist GBs evolve to absorb non-equilibrium concentrations of defects and whether GBs saturate as defect sinks for typical irradiation conditions have remained largely unanswered. Here, we used a combination of molecular dynamics and grand canonical Monte Carlo simulations to determine how twist GBs accommodate point defects. We used SiC and {001} and {111} twist GBs as model systems. We found that diffusion of defects along GBs in this material is slow and for most experimentally relevant conditions point defects will accumulate at twist GBs, driving structural and chemical evolution of these interfaces. During irradiation, screw dislocations within GB planes absorb interstitials by developing mixed dislocation segments that climb. Formation of mixed dislocations occurs either by nucleation of interstitial loops or by faulting/unfaulting of stacking faults. Both types of twist GBs can accommodate a high density of interstitials without losing the crystalline structure, irrespectively of the interstitial flux.

Effects of grain boundaries on irradiation-induced defects in tungsten by molecular dynamics simulations

The effects of two different symmetric tilt grain boundaries (GBs), Σ13[001](230) GB and Σ17[001](140) GB, on displacement cascade processes in tungsten were investigated using molecular dynamics simulations. By quantifying the number of interstitials and vacancies surviving after irradiation with the kinetic energy of primary knock-on atom energies of 1, 3 and 5 keV, respectively, in these simulations, it is found that the GBs have dual nature for radiation-induced defects: They absorb interstitials while leaving more vacancies to survive in the grains. The net effect is that the number of total surviving defects in the GB system is not always less than that in the single crystal. These defect behaviors are understood by quantitatively analyzing the recovery fraction of irradiation-induced defects, the time to reach steady state and the mobility of vacancies and interstitials. It is also found that the Σ17 GB is a more effective sink of radiation-induced point defects than the Σ13 GB. One of the main reasons is that the Σ17 GB has a higher GB energy.

Radiation tolerance of nanotwinned metals – An atomistic perspective

Remarkable properties of ultra-high strength, ductility, and stability have been achieved through the introduction of twin boundaries in nanostructured metals. Here, we report molecular dynamics simulations which elucidate the synergistic role of grain boundaries and twin boundaries in enhancing the radiation tolerance of nanotwinned metals. While grain boundaries are known to be excellent sinks for point defects, coherent twin boundaries do not absorb point defects. A beneficial corollary is that the structural integrity of coherent twin boundaries remains intact as radiation-induced defects pass through them and ultimately get absorbed into grain boundaries. Hence, the twin boundaries can continue to play a role in imparting high strength even after being subjected to irradiation. Thus, nanotwinned structures may indeed be optimal motifs for radiation tolerant materials that preserve high strength and ductility.

Solid state amorphization of metastable Al0.5TiZrPdCuNi high entropy alloy investigated by high voltage electron microscopy

The phase stability of high entropy alloy (HEA), Al0.5TiZrPdCuNi, under fast electron irradiation was studied by in-situ high voltage electron microscopy (HVEM). The initial phase of this alloy quenched from the melt was dependent on cooling rate. At high cooling rates an amorphous phase was obtained, whereas a body-centered cubic (b.c.c.) phase were obtained at low cooling rates. By thermal crystallization of the amorphous phase b.c.c. phase nano-crystals were formed. Upon fast electron irradiation solid state amorphization (SSA) was observed in b.c.c. phase regardless of the initial microstructure (i.e., “coarse crystalline structure” or “nano-crystalline structure with grain boundaries as a sink for point defects”). SSA behavior in the Al0.5TiZrPdCuNi HEAs was investigated by in-situ transmission electron microscopy observations. Because the amorphization is very rarely achieved in a solid solution phase under fast electron irradiation in common metallic materials, this result suggests that the Al0.5TiZrPdCuNi HEA from other common alloys and the other HEAs. The differences in phase stability against the irradiation between the Al0.5TiZrPdCuNi HEA and the other HEAs were discussed. This is the first experimental evidence of SSA in HEAs stimulated by fast electron irradiation.

Vacancies at the Cu–Nb semicoherent interface

We present the 0 K structures and formation energies for vacancy clusters of up to four vacancies and migration barriers for a single vacancy at a semicoherent Kurdjumov–Sachs Cu–Nb interface using ab initio calculations. Two main results emerge from this study, first that the predicted vacancy structure is compact, differing notoriously with predictions based on available empirical potentials, and second that vacancy clusters containing up to four vacancies have a smaller formation energy than monovacancy in bulk. Additionally, the binding energies show that the vacancy clusters are energetically stable for clusters having up to four vacancies. Nudged elastic band calculations of migration barriers show that the migration of a vacancy from one misfit dislocation intersection to another is highly improbable due to the high barriers. These findings suggest that at nonzero temperatures the interface will be preloaded with vacancy clusters with a relatively large capture radius for interstitials in the interface plane, implying that the semicoherent Cu–Nb interface could be a highly effective sink for point defects that form due to irradiation.

Extended defects in ZnO: Efficient sinks for point defects

Dopant-defect reactions dominate the defect formation in mono-crystalline ZnO samples implanted with Ag and B ions. This is in contrast to most other ion species studied and results in an enhanced concentration of extended defects, such as stacking faults and defect clusters. Using a combination of B and Ag implants and diffusion of residual Li atoms as a tracer, we demonstrate that extended defects in ZnO act as efficient traps for highly mobile Zn interstitials. The results imply that dynamic annealing involving interaction of point defects with extended ones can play a key role in the disorder saturation observed for ZnO and other radiation-hard semiconductors implanted with high doses.

Point defect sink efficiency of low-angle tilt grain boundaries

We examine the common assumption that grain boundaries (GBs) are ideal (or perfect) sinks for point defects by comparing and contrasting its implications with an explicit model of a low-angle tilt GB described by an array of edge dislocations which annihilate point defects by climbing. We solve the resultant diffusion equation in the absence and presence of irradiation-induced point defects. The GB sink efficiency depends on the physical parameters describing the boundary geometry (i.e., misorientation), material properties, and/or irradiation conditions (point defect generation and annihilation within the interior of grains). When the constituent dislocation spacing is small (large misorientation), the GB sink efficiency approaches that of the ideal sink. However, for small misorientations, the GB sink efficiency drops rapidly to zero and the ideal sink assumption for the GB fails dramatically. We derive a reduced dimension description of GBs where the influence of GB structure is captured in a single parameter in a Robin boundary condition for the diffusion equation. For the case of a low-angle tilt GB, we explicitly relate this parameter to the GB structure. We discuss the generality of this approach for cases where the low-angle GB model applies and parameterize the model so that it accurately reproduces the results of the two-dimensional dislocation model. The applicability of the approach to more general GBs is discussed as well as the implication of these results for predicting grain size effects under irradiation conditions.

Energetics of point defect interacting with grain boundaries undergone plastic deformations

Grain boundaries (GBs) in the polycrystalline and nanocrystalline materials are usually at their non-equilibrated states due to the plastic deformations. Thus, the point defect sink efficiencies of non-equilibrated GBs may be different from those of equilibrated counterparts, which may influence the irradiation tolerance of materials. In this paper, we firstly performed the shear responses of four copper symmetric tilt grain boundaries (GBs). The plastic deformation modes of four GBs include GB sliding, shear-coupling and complex mechanism due to atom-shuffling, partial dislocation nucleation and local GB dissociations. We then study the energetics of point defects interacting with a series of GB configurations undergone plastic deformations. It is found that the plastic deformation dominated by the sliding and shear-coupling has no effect on the point defect sink efficiency of GB in comparison with initial GB states. However, sink efficiencies of GB configurations produced from the complex deformation mode are generally intensified, for both vacancy and self-interstitial atom. In addition, the residual stress in the crystals due to the dislocation nucleating from GB affects the point defect concentration in the crystals. On the other hand, complex deformation mechanism drives GBs to higher energy states with highly disordered structures. As a result, the distribution of lower point defect formation energies extends a larger distance from GB, which may therefore favor GB absorbing the point defects nearby.