Evolution Of Dislocation Loops Research Articles

Molecular dynamics simulations on the evolution of irradiation-induced dislocation loops in FeCoNiCrCu high-entropy alloy

High-entropy alloys (HEAs) have received extensive attention due to their excellent irradiation resistance. In this work, the displacement cascade simulations were performed by using the molecular dynamics (MD) method to study the dislocation loop evolution in FeCoNiCrCu HEA. The simulation results showed the dislocation loops evolution in pure Ni were dominated by Frank loops with larger size but lower density, which was caused by the absorption of prismatic dislocation loops by Frank loops. In contrast, prismatic dislocation loops were more prevailing in FeCoNiCrCu HEA with smaller size but higher density, since the interactions between dislocation loops were suppressed in HEA. To figure out the influence of HEA on dislocation loop evolution, the formation energy, interaction energy and mobility were analyzed. It was found that formation energy and interaction energy were basically the same, while the mobility of prismatic dislocation loop in HEA was much lower than that in pure Ni, which was considered as the main reason why the irradiation-induced dislocation loops were more difficult to interact and grow in HEA. The current work provides new insights into understanding the irradiation resistance from micro-mechanism in FeCoNiCrCu HEAs.

Hydrogen modified dislocation loop types and shapes in irradiated iron

Dislocation loops are one type of irradiation defects that severely degrade the mechanical properties of nuclear materials. In this study, we found that hydrogen atoms in irradiation environment significantly modify the loop properties including loop types and shapes during in-situ hydrogen irradiation. <100> loops have been energetically stable from 300 °C in H+ irradiated iron whereas the stability of <100> loops is delayed until 500 °C in Fe+ irradiated iron. Meanwhile, both <100> loops and 1/2<111> loops exhibit polygonal shapes with sharp corners at 400 °C after H+ irradiation, similar to loops predicted at 900 °C without hydrogen. Our results highlight the importance of considering the hydrogen effects in dislocation loop evolution and stability.

Atomistic analysis of the mechanisms underlying irradiation-hardening in Fe–Ni–Cr alloys

This work presents a comprehensive examination of the physical mechanisms driving hardening in irradiated face-centered cubic FeNiCr alloys. The evolution of irradiation-induced defects during shear deformation is modeled by atomistic simulations through overlapping cascade simulations, where the nucleation and evolution of dislocation loops is validated by transmission electron microscopy images obtained from irradiated FeNiCr alloys using tandem accelerator. The effect of different shear rates on the microstructure of irradiated materials with a specific focus on the changes in the density of voids and dislocation loops induced by irradiation was analyzed. Additionally, the fundamental interaction processes between single irradiation-induced defects contributing to irradiation hardening, such as voids and dislocation loops in the alloy are explained. The analysis at atomic level indicates that both the dislocation loops and the voids exhibit strengthening effects. Furthermore, the nanometric voids are much stronger obstacles than dislocation loops of comparable size. The mechanism of cutting the voids leads to an increase of voids density and thus contributes to an increase in irradiation hardening. The mechanism of collapse of small voids into dislocation loops leads to decrease of voids density and at the same time increase of loops density. The coupling effect between the density of voids and dislocation loops is determined. Finally, the novel, physical mechanisms-based model of irradiation hardening and dislocation-radiation defect reaction kinetics are developed, which consider the mechanisms of void cutting, void shrink and void collapse to dislocation loop.

The effect of stress state and He concentration on the dislocation loop evolution in Ni superalloy irradiated by Ni+ & He+ dual-beam ions: In-situ TEM observation and MD simulations

In-situ TEM observation was conducted during Ni+&He+dual-beam irradiation to monitor the evolution of dislocation loops accompanied by He bubbles in the Ni-based alloy GH3535. Two distinct evolutions of dislocation loops, driven by residual stresses, were observed within the monitored grains. Hence, molecular dynamics (MD) simulations were employed to reveal the effects of stress magnitude and direction on loop evolution, including size, number density, type and variation. The simulations revealed that the presence of compressive stress reduced the formation energy of perfect dislocation loops, thus promoting their formation. Stress state was found to influence the preferential orientation of the loops, and compressive stress resulted in a decreased number density of dislocation loops but an increase in their size. This establishes a clear relationship between stress state and magnitude and the evolution of dislocation loops during ion beam irradiation. Additionally, the nature and characteristics of dislocation loops were quantified to explore the effects of He concentrations on their evolution. The higher He concentration not only promotes the nucleation of dislocation loops, leading to their higher number density, but also facilitates the unfaulting evolution by increasing the stacking fault energy (SFE). Moreover, the accumulation of He in the lower-He-concentration sample led to the growth of dislocation loops in multiple stages, explaining their nearly identical average sizes when compared to the higher-He-concentration sample.

Differences in coupling between nuclear and electronic energy losses in UO2 with irradiation temperature: An in situ TEM study

To investigate the coupling between nuclear and electronic energy losses in UO2, we irradiated thin foils with 0.39 MeV Xe and/or 6 MeV Si ions at 93 K using single or simultaneous dual beam ion irradiations. The evolution of perfect dislocation loops was characterized by in situ transmission electron microscopy (TEM). Additional ex situ TEM characterizations at room temperature revealed for the first time in UO2 the presence of faulted Frank loops too small to be measured during in situ experiments and conventional bright field kinematical imaging conditions.For the single Xe irradiation, which favor dominant ballistic energy losses, we observed a continuous nucleation of small perfect dislocation loops, which increase in size for our last fluences by growing through mainly coalescence effect. Both the single Si and dual Xe & Si irradiations showed a coupling between nuclear and electronic energy losses, resulting in a significant loop density increase and a tangled line network formation, respectively. These phenomena occur at lower dpa levels, compared to the single Xe irradiation, likely resulting from the thermal spike effect of Si ions. The present results were compared to our previous work at 293 K to investigate the role of irradiation temperature on the energy losses coupling. For the Xe irradiation, the density increases and the loops are smaller at 93 K compared to 293 K, resulting from the uranium interstitials mobility being prevented or allowed. For the Si irradiation, the dislocation evolution kinetics are similar at both temperatures. The electronic excitations effect seems greater than the irradiation temperature effect in this temperature range. For the Xe & Si irradiation, the loop kinetics change resulting in a tangled line network formation is faster and thus the loop transformation into lines occurs at lower dpa levels at 93 K compared to 293 K. It appears that the irradiation temperature affecting the mobility of some small point defects reduces the electronic excitation effect in this case.

Fluctuations in misfit volume by interstitial carbon atoms contribute to the unusual dislocation loop evolution in high-entropy alloys under irradiation

Dislocation loops play a pivotal role for the irradiation behavior and associated mechanical properties of materials. This study employs advanced transmission electron microscopy, nanoindentation and first-principles calculations to elucidate the impact of interstitial carbon atoms on the evolution of dislocation loops in an Fe49.5Mn30Co10Cr10C0.5 high-entropy alloy under ion irradiation at temperatures ranging from 420 °C to 540 °C with a peak dose of 50 DPA. Interstitial carbon atoms have been revealed to broaden the size distribution of dislocation loops, rendering their morphologies more jagged, and consequently inducing a more substantial pinning effect. These outcomes can be attributed to the enhancement of misfit volume fluctuations of the alloying elements, which drives more severe local lattice distortions and larger variation of local chemical environments. As a result, these fluctuations promote the local pinning of dislocation loops, lead to their rugged movement and contribute to the rise in hardening. Our findings bring previously unrecognized insights into the role of interstitial carbon atoms in influencing the nature of dislocation loops in high-entropy alloys, and provide important clues to the design of alloys for nuclear applications.

Effect of hydrogen, helium and dose rate on the evolution of dislocation loop in pure iron during in-situ ion irradiation

Simulating neutron irradiation with ion irradiation requires considering the comprehensive effects of hydrogen, helium and dose rate to fully understand the microstructure evolution of material in an irradiation environment. Here, the characterization and evolution of dislocation loops in pure iron were investigated under heavy ion irradiation with four different injected gas ion types (Fe+, Fe++H2+, Fe++He+and Fe++H2++He+) and under dual-beam (Fe++He+) irradiation with four different dose rates using in-situ transmission electron microscopy at 723 K. Comparing to the single-beam (Fe+) irradiation, the additional injection of H or He, or co-injected H and He, mainly promoted the nucleation of dislocation loops at the early stage of irradiation and enhanced loop growth after that. Among them, the simultaneous irradiation of triple beams (Fe++H2++He+) was most pronounced. Moreover, triple-beam (Fe++H2++He+) irradiation resulted in the highest ratio of the rotation of loop habit-plane. Under dual-beam (Fe++He+) irradiation, the results demonstrated that as the dose rate increased, the loop density increased and the loop size decreased. Moreover, the fraction of 〈100〉 loops at low dose rate was noticeably greater than that at high dose rate. The results of the current work provide important references for a better understanding of the effectiveness of ion irradiation in simulating neutron irradiation.

Microstructural evolution of ion-irradiated commercially pure titanium

Radiation-induced dislocation loop evolution in ion-irradiated commercially pure (CP) Ti was investigated through both ex situ 4 MeV Ar beams and by in situ TEM irradiation with 1 MeV Kr ion beams at different temperatures. The dominant mechanism for dislocation loop growth was loop coalescence. Both <a>- and c-component loops exhibited an increase in size and density with increased irradiation dose and temperature. C-component loops were observed only after reaching a threshold incubation dose (TID). In CP Ti, these loops initiated at lower doses compared to previous observations in Zr, and the TID decreased with increasing temperature.

Extending damage accumulation of commercial reactor irradiated 316 stainless steel with ion irradiation

Austenitic 316 stainless steel flux thimble tubes (FTTs) removed from a commercial pressurized water reactor (PWR) were re-irradiated using nickel ions to evolve the irradiated microstructure to higher damage levels than those achieved in reactor. The microstructures of the neutron-plus-ion irradiated samples were compared with those of neutron irradiated only samples at the same doses to evaluate the effectiveness of ion irradiation to extend the damage range and match that created in reactor. Ion irradiations effectively captured, both qualitatively and semi-quantitatively, the evolution of dislocation loops, nanocavities, nanocluster size and density, and radiation-induced segregation (RIS) with dose. Only Ni-Si clusters were observed in ion irradiated FTT samples while Ni-Si-Mn clusters that were frequently observed in reactor irradiated samples above ∼41 dpa were absent in ion irradiated samples probably due to ballistic mixing at the high ion irradiation damage rate. Two samples were ion irradiated to ∼160 dpa to predict the irradiated microstructure that may develop due to an increase of the damage accumulation by extending a PWR life from 40 years to 60 years. The extended ion irradiation results indicated that significant microstructure changes were unlikely to occur even to very high doses near common PWR relevant temperature conditions. Overall, the microstructure resulting from ion irradiation following neutron irradiation matched that of neutron to the same dose reasonably well in most cases.

A spatially resolved analysis of dislocation loop and nanohardness evolution in proton irradiated Zircaloys

Proton irradiation is increasingly used as a surrogate for neutron irradiation, providing similar damage structures at significantly lower costs and less time compared to neutron irradiation. However, in contrast to neutrons, protons produce a depth dependent damage profile that incorporates a plateau region and a Bragg peak in the tens of microns region depending on the material and proton energy. Here we demonstrate that this depth dependent damage can be utilised to obtain new understanding about irradiation induced damage at different dose levels from a single sample by combining spatially resolved micro-beam synchrotron X-ray diffraction-based dislocation analysis and nanoindentation. For this purpose, and to study the damage evolution starting from very early stages, we have investigated microstructure evolution and hardening behaviour of Zircaloy-2 and Zircaloy-4 proton irradiated samples between 0.01 dpa and 17 dpa at 350 °C. The results highlight good agreements of SRIM-based damage depth predictions with irradiation-induced dislocation appearance and nanohardness. However, at even relatively low damage levels within the plateau region, dislocation line density and nanohardness profiles appear saturated across the entire irradiated region, even when the dpa levels differed significantly. When relating dislocation loop line densities to nanohardness, it was found that from a certain point hardness increased only very slightly with increasing line densities. This apparent discrepancy can be explained by a decreasing loop size for the highest line densities identified by from the diffraction line profile analysis and the application of a Dispersed Barrier Hardening model that relates obstacle strength to dislocation loop size.

Evolution of dislocation loops in irradiated α-Uranium: An atomistically-informed cluster dynamics investigation

An atomistically-informed mean field cluster dynamics model has been developed to investigate the nucleation and growth of prismatic loops in irradiated α-Uranium. TEM analysis of neutron irradiated α-Uranium shows the evolution of self-interstitial atom and vacancy loops on (010) and (100) crystallographic planes, respectively, resulting in an anisotropic lattice swelling of its face-centered orthorhombic crystal. To provide model parameters, the crystallography of loops and the binding energy of point defects to these loops were studied using an angular dependent EAM potential and classical molecular dynamics (MD) simulations. Furthermore, using the bond-boost hyperdynamics method, the anisotropic diffusion of interstitials and vacancies in α-Uranium was investigated. The mechanisms of point defect diffusion and the associated migration energies were reported and compared with previous DFT studies. The energetics and kinetic quantities mentioned above were used in the cluster dynamics model to predict the cluster density as a function of dose rate, dose and temperature and the results were compared to the reported neutron irradiation experiments. The model predictions reveal an accumulation of small sized vacancy loops along with a population of large and growing self-interstitial loops, which closely corresponds to the TEM observations.

The design of Pd-containing high-entropy alloys and their hardening behavior under He ion irradiation

Imparting additional chemical heterogeneities of high-entropy alloys (HEAs) is considered as a novel strategy to improve their irradiation resistance on dislocation loop growth; however, there is still a lack of knowledge about the effects of this strategy on bubble evolution and irradiation-induced hardening. In this study, a series of Pd-containing FeCrNiCo HEAs with proven potential for chemical short-range order were designed and irradiated by 400 keV He ions to the fluences of 5 × 1016 cm−2, 1 × 1017 cm−2, and 5 × 1017 cm−2 at 723 K. Microscopic characterization demonstrates a remarkably inhibited growth of dislocation loops and promoted bubble coarsening in the Pd-containing HEAs compared to the HEA without Pd alloying. The underlying mechanism is that although the alloying of Pd can suppress the evolution of dislocation loops through severe local lattice distortion, it also significantly reduces the vacancy formation energy of the entire HEA system, leading to the encouraged bubble evolution. Pd-containing HEAs exhibit improved resistance to irradiation-induced hardening at low doses, but this advantage is unexpectedly absent with increasing dose. The hardening mechanism is explained by identifying the loop-dominated hardening at low dose and the bubble-induced hardening which makes an increased contribution at high dose. In addition, the Ni segregation around bubbles and loop punching mechanism are also discussed to provide essential evidence. These insights can help to shed light on the effects of Pd alloying on bubble evolution and irradiation-induced hardening behavior in HEAs, and thus contribute to the design of irradiation-resistant HEAs.

Evolution of dislocation loops and voids in post-irradiation annealed ThO2: A combined in-situ TEM and cluster dynamics investigation

The effect of isochronal annealing on the evolution of dislocation loop and void population in proton irradiated ThO2 has been investigated. Post-irradiation annealing in other actinide oxides like UO2 shows significant loop coarsening. ThO2 samples were irradiated with 2 MeV protons up to a dose of 0.1 dpa at 600 °C. Post-irradiation isochronal annealing was performed at 600, 800, 1000 and 1100 °C for 1 h at each temperature using in-situ TEM. Only faulted 1/3<111> type dislocation loops were observed, and their sizes and distribution were characterized. The population of self-interstitial atom (SIA) dislocation loops did not show any significant growth and coarsening. Additionally, nanometric voids were observed at annealing temperatures of 1000 and 1100 °C. Using cluster dynamics (CD), we have studied the nucleation and growth of point defects and defect clusters, i.e., SIA prismatic dislocation loops and nanometric and sub-nanometric voids in proton irradiated ThO2. The CD model was further utilized to predict the growth and coarsening of loops and voids during isochronal annealing at the experimental and higher temperatures. The model did not predict significant SIA loop growth which closely corresponds to the TEM observations. CD predicted SIA loop coarsening is insignificant even at high annealing temperature of 1500 °C because the model only considers the growth of defect clusters by absorption of like point defects, i.e., SIA loops absorb interstitials and voids absorb vacancies, and cannot account for their migration and coalescence due to elastic interaction. The CD model also predicts the evolution of nanometric voids having mean size within the error bounds of TEM observations.

Helium role in Fe9Cr1.5W0.4Si F/M steel during Fe++He+ dual-beam irradiation

In materials exposed to neutron irradiation, helium atoms produced by transmutation reactions are prevalent and play a significant role in the evolution of dislocation loops, causing mechanical property degradation. However, systematic investigation on the helium role is lack and possible origin of helium effects is unclear. Here, by performing 400 keV Fe ion and 30 keV He in-situ irradiation, this study found that about 9.2×1012 cm−3 loops nucleated for every increase of 1 appm He at 0.9 dpa in Fe++He+ dual beam irradiation and 87% loop density was produced with the help of helium at 2262 appm/dpa. The different loop evolution in single He irradiation and Fe++He+ irradiation indicated the presence of high-density TEM-invisible (≤ 2 nm) small loops after Fe+ irradiation at high temperatures. The large number of loops produced by helium irradiation were possibly related with the small bubbles.

Effect of ion flux on one-dimensional migration of dislocation loops in Fe9Cr1.5W0.4Si F/M steel during in-situ Fe+ irradiation

The study of one-dimensional (1D) migration of dislocation loops is essential for understanding the evolution of material microstructure in the irradiation environment. The 1D migration of dislocation loops under thermal activation and electron irradiation has been widely studied, but the effect of ion flux on the 1D migration of dislocation loops has not been concerned. In this study, the 1D migration and evolution of dislocation loops in the Fe9Cr1.5W0.4Si ferrite/martensite steel during 400 keV Fe+ irradiation was investigated by in-situ transmission electron microscopy at room temperature. Compared with 1/2<111> dislocation loops, 〈100〉 dislocation loops had weak migration ability and low migration frequency. The effect of ion flux on the 1D migration of dislocation loops were explored, which showed that increasing the ion flux was not only beneficial to the migration of large dislocation loops, but also increased the migration frequency. In addition, the higher the ion flux, the higher the density of mobile dislocation loops per unit irradiation dose. The knowledge gained here provides an important reference for understanding and predicting the degradation process of irradiated materials.

Pressure Effect on Diffusion of Native Defects and Mg Impurity in Mg‐Ion‐Implanted GaN during Ultrahigh‐Pressure Annealing

Herein, transmission electron microscopy direct observations are used to examine the time evolution of dislocation loops in Mg‐ion‐implanted GaN during annealing in N2 atmospheres of 2.0 and 0.3 GPa. It is indicated in the results that the diffusion of the native defects, for both interstitials and vacancies, is retarded during annealing at the higher pressure. Furthermore, secondary ion mass spectrometry shows that annealing at the higher pressure retards the migration of Mg and increases Mg‐acceptor concentration in the ion‐implanted region. These findings provide a design principle of the postimplantation annealing process to activate ion‐implanted Mg in GaN.

Molecular dynamics simulations of displacement cascades in vanadium: Generation and types of dislocation loops

Vanadium(V)-based alloys have received extensive attention as promising first-wall and blanket candidates for fusion reactors. In the present study, molecular dynamics (MD) simulations have been performed to study the displacement cascades in V bulk with the recoil energy up to 80 keV at 300 K. The results indicate that the clustering fraction and cluster size of vacancies are higher than those of interstitials. The nucleation, evolution, and energy of dislocation loops are studied. It’s found that 1/2 〈1 1 1〉 interstitial and vacancy dislocation loops are the primary ones, while low concentration 〈1 0 0〉 vacancy loops are also formed. In addition, the nucleation of 1/2 〈1 1 1〉 interstitial dislocation loops is earlier than that of 1/2 〈1 1 1〉 vacancy ones. The reason for the appearance of 1/2 〈1 1 1〉 vacancy dislocation loops is that the formation energy of the 1/2 〈1 1 1〉 vacancy dislocation loops is close to that of voids containing the same number of vacancies. These results provide a new understanding of applying V-based alloys in fusion reactors.

In-situ TEM investigation on the evolution of He bubbles and dislocation loops in Ni-based alloy irradiated by H2+ &amp; He+ dual-beam ions

• Synergistic evolution mechanism of He bubbles and loops in Ni-based alloy irradiated by dual-beam ions were investigated via in-situ TEM. • Several evolution stages of the loop-punching mechanism and its effect on evolution of neighboring bubbles and loops were revealed. • The details of the unfaulting processes of the Frank loops and the formation mechanism of rhombic loops were discussed. • Four Frank loop variations were identified, and their different evolution behaviors were studied. The synergistic evolution mechanisms of He bubbles and dislocation loops under 30 keV H 2 + & H e + dual-beam ions irradiation at 650°C in the Ni-based alloy GH3535, which is the most promising candidate structure material for molten salt reactors (MSRs), were revealed via in-situ TEM. The nucleation, merging, and change in the size of the dislocation loops and He bubbles were characterized in detail to study the influences of irradiation fluence and pre-existing dislocation loops on their evolutions. The number density of both the He bubbles and dislocation loops increases rapidly and subsequently saturates, whereas their size continuously increases with the increasing ion fluence. Pre-existing dislocation loops with strong absorption characteristics grow preferentially and suppress the nucleation of dislocation loops during the dual-beam ions irradiation. Moreover, the bubbles tend to nucleate within the dislocation loops to form bubble-loop complexes, and then decrease in their number density. The details of the unfaulting processes of the Frank loops were discussed, where the energy difference between the two types of loops as well as the evolution of the inside Shockley dislocation loops dominates the unfaulting behavior. The several evolution stages of the loop-punching mechanism are revealed, and the emitted loops can directly form perfect loops as well as unfault the neighboring Frank loops. The He bubbles inside the loops provide corresponding stress for the formation of rhombic loops, which can achieve rapid growth and sweep ability by merging with the neighboring loops. Additionally, its dissociation to Shockley dislocation can unfault the Frank loops along their slip direction. Among the four Frank loop variations, the edge-on Frank loop variations have the highest growth rate, followed by the perfect loop. The related mechanisms based on in-situ experimental observation are discussed in depth. Synergistic evolution mechanism of He bubbles and dislocation loops, including loop-punching mechanism and bubble-loop complexes, in Ni-based alloy irradiated by dual-beam ions were investigated via in-situ TEM.

In-situ TEM investigation of dislocation loop evolution in HR3 steel during Fe+ ion irradiation

The evolution and characteristics of dislocation loops, which are affected by the value of temperature play a crucial role in the mechanical properties of structural materials. This study explores the evolution processes of the dislocation loops with the increasing values of the temperature, and the effect of precipitate on the dislocation loop. In-situ TEM experimental observations are performed using HR3 steel, which is commonly used as a structural material in nuclear reactors, during 400 keV Fe+ irradiation at room temperature (RT), 300 °C, 400 °C and 500 °C. The evolution of the dislocation loop at different values of the temperature is characterized in detail. The average size and density of dislocation loops as a function of displacement per atom (dpa) and values of temperature were constructed and analyzed comparatively. Irradiated samples at higher temperatures have a larger average size and a lower density of dislocation loops. The pre-existing dislocation lines have a profound effect on the density and Burgers vector of dislocation loops. Our present results provide a fundamental insight into the evolution and characteristics of dislocation loops in HR3 steel in a gradually changing of temperature environment.

Effect of dose rate on the characteristics of dislocation loops in palladium: In-situ TEM analysis during 30 keV H2+ irradiation

Ion irradiation is usually used to simulate neutron irradiation to accelerate the evaluation of the irradiation behavior of reactor materials. However, the validity of using a high damage rate of ion irradiation to simulate a low damage rate of neutron irradiation has always been a controversial topic. Here, the effect of two dose rates (2.94 × 10–6 and 7.35 × 10–5 dpa s–1) on the characteristics and evolution of dislocation loops in palladium was studied in situ during 30 keV H2+ irradiation using transmission electron microscopy. The dose rate obviously affected the nucleation rate and growth rate of dislocation loops, the types (Frank loops or perfect loops) of dislocation loops, and the irradiation hardening and total damage obtained from the product of average loop size and loop density. At the same irradiation dose, a high dose rate would lead to high loop density, small average loop size, low loop growth rate, and low irradiation hardening and damage induced by loops in pure Pd. Meanwhile, it was found for the first time that a high dose rate was beneficial to the generation of perfect dislocation loops. The effect of dose rate was attributed to the different dynamic equilibrium results between the effective generation rate of point defects and their absorption rate by existing sinks. The present results show that the effect of dose rate should be considered when using ion irradiation to simulate neutron irradiation to evaluate the irradiation damage to materials.