Nucleation Of Dislocation Loops Research Articles

Understanding the dose-rate effect on loop characteristics in Kr+-irradiated CeO2 by in-situ TEM study

Irradiation of CeO2 with heavy ions is commonly used to accelerate the evaluation of the irradiation performance of UO2 nuclear fuel. However, the differences in damage rates between ions and neutrons have led to doubts about this method, making it especially important to study the dose rate effect. Here, two kinds of dose rates were used to in-situ irradiate CeO2 using 400 keV Kr+ at 550 °C and 700 °C to study the dose rate effect on loop characteristics. High dose rate (HR) led to low-density and small-sized dislocation loops compared to low dose rate (LR), which was attributed to the initial high concentration of oxygen vacancies and enhanced recombination of point defects. HR was found to promote the direct nucleation of perfect dislocation loops (PDLs), while under LR irradiation, the PDLs tended to be formed by the transformation of Frank dislocation loops (FDLs) through a typical unfaulting. At a same dose, the proportion of PDL under the HR condition was much higher than that under the LR condition. Temperature greatly affected the loop characteristics. High temperature caused large-sized and small dense loops, as well as high PDL proportion. Moreover, the newly nucleated PDL tended to be close to the pre-existing PDLs and had the same Burgers vector as the adjacent PDL under HR irradiation at 700 °C. Current results provide underlying insights for assessing the effect of dose rate on irradiation-induced loop evolution and a reference for understanding the dose rate effect.

Effect of hydrogen, helium and dose rate on the evolution of dislocation loop in pure iron during in-situ 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+) were 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.

The effect of hydrogen-helium synergism on dislocation loops in RAFM steel at different temperatures by dual-beam ion irradiation

Simultaneous Fe+H ion irradiation (denoted as Fe+H) and Fe+H ion irradiation with pre-implantation of He (denoted as He/Fe+H) were carried out on Chinese Low-Activation Martensitic (CLAM) steels at 350 ℃-550 ℃ to investigate the synergistic effect between H and He. The peak dose was 15 dpa, the He concentration in the observation area was 11 appm/dpa and the H concentration was 44 appm/dpa. The microstructure and hardening of the irradiated samples were evaluated by TEM and nanoindentation, respectively. Bubbles were not observed in irradiated CLAM steels at all temperatures, which implies that the promotion of bubbles by H may not be as strong as that by He and that H attenuates the promotion of bubbles by He. The hardening induced by He/Fe+H irradiation at 450°C is about 1.6 times that of Fe+H irradiation, the synergistic effect of H and He is most significant at this temperature. Comparison with Fe+He irradiation and single Fe irradiation reveals that Fe+H irradiation at 350°C induces lower interstitial defect damage than Fe+He irradiation, but the level of interstitial defects induced by the synergistic effect of both H and He with displacement damage is comparable at higher temperatures. The average size of dislocation loops induced by Fe+H irradiation is between that of single Fe irradiation and Fe+He irradiation, but the number density of dislocation loops induced by Fe+H irradiation is higher than that of Fe+He irradiation. The synergistic effect of He with displacement damage tends to promote the growth of dislocation loops, while the synergistic effect of H tends to promote the nucleation of dislocation loops.

Variation of first pop-in loads in nanoindentation to detect chemical short-range ordering in the equiatomic Cr-Co-Ni medium-entropy alloy

The variations of the first pop-in load in nanoindentation with annealing temperature and time have been investigated both by experiment and theoretical molecular dynamics (MD) simulations for the equiatomic Cr-Co-Ni medium-entropy alloy (MEA). Regardless of the loading rates, the first pop-in load tends to increase with the increase in electrical resistivity consistently by experiment and MD simulations. The trend in the variation of pop-in load with annealing temperature/time coincides with what is observed in the variation of electrical resistivity in our previous study, indicating that the first pop-in load in nanoindentation can thus be used to qualitatively detect the evolution of short-range ordering (SRO) in the Cr-Co-Ni MEA, and that the peak temperature for SRO for the equiatomic Cr-Co-Ni MEA is 673 K. MD simulations indicate that the first pop-in corresponds to the nucleation of partial dislocation loops beneath the indenter for both annealing conditions and that the increased first pop-in load in nanoindentation is attributed to the higher energy barrier for the nucleation of dislocation loops due to the higher degree of SRO.

In-situ TEM study of the effect of hydrogen-helium ratio on defect characteristics in Fe9Cr1.5W0.4Si F/M steel during H2+ & He+ simultaneous irradiation

The effect of hydrogen and helium interaction, especially H–He ratio, on the irradiation behavior of nuclear materials has not yet been resolved. However, this is an important basis for evaluating the irradiation properties of nuclear materials and developing high irradiation resistant materials. Here, 30 keV H2+ and He+ dual beams with four H–He ratios of 0:10, 3:10, 15:10, and 30:10 were used to irradiate the newly developed Fe9Cr1.5W0.4Si F/M steel in TEM to in-situ study the interaction and ratio effect of hydrogen and helium. The addition of H atoms significantly promoted the nucleation of dislocation loops and bubbles. In the early stage of irradiation, the average size and density of dislocation loops increased with the increase of H–He ratio. Meanwhile, the larger the H–He ratio, the easier it was to form a complex dislocation network. Furthermore, the final saturation size of bubbles increased with the increase of H–He ratio. It was first found that the swelling was affected by H concentrations, with high H concentrations slowing down the increase in swelling. For a certain irradiation dose, a specific H–He ratio would lead to a swelling peak of Fe9Cr1.5W0.4Si F/M steel. The super-sized bubbles at grain boundaries (GBs) were found after H addition, resulting in a bigger swelling of GBs than the matrix. Both the swelling of the GBs and the matrix show a dependence on the H–He ratio. The current work is of great significance for understanding the interaction between hydrogen and helium in nuclear materials.

Evidence of dislocation loop preferential nucleation in irradiated aluminum under stress

To explain irradiation creep, several mechanisms have been proposed. Some are based on the effect of stress on either nucleation or growth of dislocation loops. To investigate these mechanisms in aluminum we combine in-situ transmission electron microscope irradiation under stress and two simulation approaches (object kinetic Monte-Carlo, molecular dynamics Frenkel pair accumulation). We observe the selectivity of Frank loop variants under electron irradiation and applied stress. When the stress is turned on after loop formation, there is no loop variant selectivity, suggesting the absence of preferential absorption on already formed loops. Object kinetic Monte-Carlo simulations, including the effect of stress on the diffusion of point defects, show no growth rate difference between loop variants. Frenkel pair accumulation simulations exhibit variant selectivity of nucleated loops. This shows that loop selectivity is due to preferential nucleation of well oriented loops under stress and not to differential growth of loops.

Fertile metastability

ABSTRACTWe deal here with three metastable systems: 1° – the dowser texture, 2° – supertwisted cholesterics and 3° – hypotwisted cholesterics. We outline remarkable properties of the tropisms of the dowser texture stemming from its low symmetry, and we show that, using setups called Dowsons Colliders, nematic monopoles can be nucleated, set into motion and brought into collisions in the dowser texture. Subsequently, we point out that nucleation of dislocation loops occurs in cholesteric layers compressed or dilated between cylindrical mica sheets. Under compression, three modes of nucleation of dislocation loops have been identified: individual, serial and continuous. The serial mode generates dislocation nets made of L circular loops connected by C radial crossing into superloops. Similar superloops can also be generated under dilation. We analyse the topology of superloops in terms of the theory of knots and point out that they can be reduced to the unknot by Reidemeister moves. In other words, superloops are multiply folded loops that can be continuously unfolded.

Temperature dependent phase field dislocation dynamics model

In this work, we present a three-dimensional phase field-based formulation for simulating the temperature-dependent motion of discrete dislocations in crystals. For demonstration, this thermal phase field dislocation dynamics method is applied to study pyramidal-type dislocations in three exemplar hexagonal close packed materials, Mg, Ti, and Zr. Pyramidal-type dislocations are well known for temperature sensitivity and influence on the yield strength of these metals. Calculations include the predictions of activation stress, dislocation velocity, and dislocation nucleation over temperature ranges from 0 K to up to half the melting temperatures of these metals. We show that pyramidal slip is glissile, but the activation stress and stacking fault widths are asymmetric with respect to forward/backward glide. The stress to initiate glide reduces logarithmically with homologous temperature. The role of temperature in Frank-Read source operation is shown to decrease the size and time of first loop formation. The analysis identifies a master inverse relationship for the dislocation loop nucleation rate with homologous temperature followed by all three metals.

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.

On the theory of dislocation loop nucleation in irradiated crystals

The theory of dislocation loop nucleation in irradiated crystals is critically analysed and revised. The key parameters of the nucleation model are reassessed within the traditional (mean field) rate theory for irradiation defects. In the new approach, it is shown that at relatively low irradiation temperatures all interstitial loops are supercritical (due to the absence of a critical nucleus) and grow until they coalesce with other loops or with the dislocation network. In the transition temperature range, a large critical size of interstitial loops arises and the nucleation rate decreases significantly. At higher temperatures, all interstitial loops become subcritical and shrink, in qualitative agreement with experimental observations in irradiated steels. The vacancy loops are shown to be subcritical at all irradiation temperatures and thus shrink and disappear, in agreement with observations. To further improve the predictions of the nucleation theory, possible modifications of the mean field rate theory for irradiation defects are discussed.

Effect of Li element on shocking behavior of Fe-Li alloys

Understanding the effect of alloying elements on shock behaviors is significant for constitutive model of alloy materials at under dynamic high pressure. However, an in-depth understanding about the microscopic mechanism of alloying elements effect on plasticity and phase transition is still limited. In this work, applying non-equilibrium molecular dynamic (NEMD) simulations, shock-induced plasticity and phase transition of Fe-Li alloys was investigated in terms of the compositions of Li, crystallographic direction and shock velocity. The results show that yield stress (or phase change pressure) of Fe-Li alloy was reduced doping Li element, and the large internal stresses caused by concentration of Li element effectively activated the nucleation and multiplication of dislocation loops. But with the increase of Li composition, phase transition is inhibited for shock along [111] direction because of stress release caused by multiplication of dislocation. There is a strong dependence between these microscopic processes and the law of plastic wave propagation. Moreover, with the increase of Li composition, the diversity of phase transition variants was inhibited for shock along [110] direction due to the destruction of initial stress and potential energy uniformity.

Effect of different pre-existing dislocation densities on the microstructure evolution of W-0.5ZrC alloy during in-situ He+ irradiation

The evolution of dislocation loops and helium bubbles in W-0.5ZrC alloy with different pre-existing dislocation densities was investigated by in-situ transmission electron microscopy during 30 keV He+ irradiation at 800 °C. The nucleation, migration, aggregation, and growth of dislocation loops and helium bubbles occurred with the whole irradiation process. The variation trends of average size and volume number density of dislocation loops and helium bubbles with the increase of irradiation dose in the regions with different pre-existing dislocation densities were obtained. It was found that the growth of dislocation loops was inhibited in the region with a low pre-existing dislocation density (about 5.2 × 1013 /m2), and the nucleation of dislocation loops was inhibited in the region with the dispersed medium density (about 1.6 × 1014 /m2, dislocations were crossed but dispersed), while both the nucleation and growth of dislocation loops were inhibited in the region with the compact medium density (about 1.5 × 1014 /m2, dislocations were crossed and compact). During the irradiation process, the pre-existing dislocations would interact with the irradiation-induced dislocation loops and helium bubbles, but their influence weakened with the increase of irradiation dose, and finally resulted in the disappearance or deformation of pre-existing dislocations. The irradiation hardening caused by dislocation loops and helium bubbles was increased with the increase of irradiation dose, and the corresponding mechanism was analyzed.

Molecular dynamics study of defect evolutions and mechanical properties of W/Fe semi-coherent interfaces under irradiation

Molecular dynamic simulation has been performed to systematically investigate defect evolutions and mechanical properties of W/Fe semi-coherent interfaces under irradiation. For the simulation of damage cascades, a recently published EAM potential by our group has been first corrected using the universal potential of ZBL. On this basis, the dislocation loop evolutions and stress-strain curves of the irradiated W(110)/Fe(110) and W(100)/Fe(100) semi-coherent interfaces are derived. It has been found that interstitial atoms of pure W and Fe seriously accumulate with the increasing of irradiation times. Surprisingly, the formation of semi-coherent interfaces can trap the interstitials, and consequently inhibit the dislocation loop nucleation in W and Fe bulks to improve the irradiation resistance. The fundamental effects of interface orientation are also disscussed, and give a deep understanding of mechanical properties of W/Fe interfaces under irradiation.

Chemical short-range ordering regulated dislocation cross slip in high-entropy alloys

High-entropy alloys (HEAs) have received extensive interest owing to their unusual mechanical properties as a result of the extreme compositional complexity. Understanding the dislocation behavior under the influence of chemical disorder, especially chemical short-range ordering (SRO), which is commonly found in HEAs, is fundamental to the rational design of high-performance alloy systems. Here, we systematically investigate the effect of disorder and SRO on the regulation mechanism of dislocation nucleation and cross-slip in HEAs through a combination of Monte Carlo methods, molecular dynamic (MD) simulations, and theoretical analysis. Our results demonstrate that the incipient plasticity of HEAs under nanoindentation is mainly controlled by the nucleation and cross-slip of Shockley partial dislocation loops, which are found to be highly dependent on SRO. In contrast to traditional strengthening, SRO leads to a significant increase in the stable nucleation radius of the dislocation loop and cross-slip resistance. Strengthening is then achieved due to the extension of the dislocation nucleation period and the high resistance for dislocation cross-slip. Aided by the transition path analysis and theoretical model, we show that the local compositional changes originating from SRO, specifically for strong Co-Cr pairs, are the dominant factors governing the dislocation properties and accordingly, the strengthening effects. Finally, the SRO strengthen effect is further validated against different alloy systems with different SRO types. These findings suggest that the elasticity and strength of HEAs can be improved through the prolonging of the nucleation period of dislocations resulting from SRO, dictating an avenue for enhancing the mechanical properties of HEAs. • The incipient plasticity of HEAs under nanoindentation is controlled by the nucleation and cross-slip of partial dislocation loops. • Strong SRO increases the nucleation energy barriers and the stable nucleation length is longer simultaneously. • Dislocation cross-slip is inhibited by SRO. • SRO increase the critical transition radius from dislocation slip to cross slip. • Strength originating from the interaction between solute and dislocation is enhanced by strong SRO.

Assessment of meta-atom scheme for nucleation of dislocation loops in TWIP steel

We use atomistic calculations to explore the process of homogeneous nucleation of dislocation loops in TWIP steel with the nudged-elastic-band method and meta-atom interatomic potential. Energy barriers to the nucleation process have been estimated at different applied shear stresses, and an atomistically informed nucleation model is fitted to the results for both Shockley and twinning dislocation loops. Besides exhibiting excellent agreement with the direct atomistic results, the nucleation model also provides the critical size of the dislocation loop. Comparison of the critical loop diameters with the intrinsic spatial scale of the meta-atom potential reveals that the loop nucleation is sensitive to the local compositional heterogeneity. As a result, the simulation results obtained with a meta-atom potential are shown to require careful interpretation in terms of their statistical ramifications.

Deformation mechanism of WC single crystals under nanoindentation: Effects of surface defects and orientation on pop-in and hysteresis

Nanoindentation was carried out on pure tungsten carbide (WC) on the basal (0001) and prismatic (1010) planes, using Berkovich and spherical indenters, in both single load and multi-load testing. The work focuses on correlating the load-displacement curves, including elastic to plastic deformation, size effect and hysteresis with the deformation behaviour of WC. With different specimen preparation processes, the elastic to plastic deformation started at different threshold loads: This observation was found to be due to the variation in surface dislocation density. Staircase deformation was observed thought to be caused by dislocation motion and the formation of slip bands; sudden displacement discontinuities in the load-displacement response - associated with dislocation loop nucleation - occurred at, or near the theoretical shear strength. Furthermore, discontinuities in load-displacement curves were also used to confirm that hysteresis loops were a result of plastic deformation, as they were absent when the loading was purely elastic.

Homogeneous Dislocation Nucleation in Molecular Crystal Cyclotetramethylene‐Tetranitramine (β‐HMX)

AbstractThe molecular crystal cyclotetramethylene‐tetranitramine (β‐HMX) is the active ingredient in some broadly used plastic bonded explosives. Plasticity is thought to be essential for reaction initiation and detonation. In this work we use molecular simulations to investigate the conditions under which dislocations nucleate homogeneously in β‐HMX. We evaluate the resolved shear stress leading to homogeneous nucleation of dislocation loops in the planes currently considered to be most important for plasticity in this crystal, (011) and (101), and at pressures up to 20 GPa. The importance of homogeneous nucleation for plastic deformation is discussed in relation to the other plasticity mechanisms operating in HMX. It is concluded that homogeneous nucleation is unlikely to happen at pressures above 5 GPa, while at pressures below this threshold homogeneous nucleation competes with shear localization. The article also presents γ‐surfaces for the (011) and (101) planes, which are used here to interpret the nucleation process and critical stresses, and have broader importance in other contexts. A discussion of the procedure required to compute γ‐surfaces in molecular crystals is included.

Transfer of dislocation slip through grain boundaries in metal-graphene nanocomposites

A model is suggested that describes the onset of plastic deformation in metal-graphene nanocomposites. Within the model, the plastic deformation of a deformed metal-graphene nanocomposite is realized through the generation of perfect or partial dislocation loops. These loops nucleate at grain boundaries (GBs) of the metallic matrix and expand across grains. The expanded dislocation loops promote the nucleation and expansion of secondary dislocation loops in adjacent grains, thereby realizing dislocation transmission across GBs. Within the model, we have calculated the critical stresses, τc1 and τc2, for the generation of a secondary perfect or partial dislocation loop and for the bypass of the graphene inclusion by the secondary loop. The critical stress τc2 increases with an increase in the graphene concentration and with a decrease in the thickness of the inclusion. This implies that the addition of graphene can raise the critical stress for the expansion of the secondary dislocation loops and thus increase the yield strength of metals.

Unveiling the local deformation behavior of typical microstructures of nickel-based single crystals under nanoindentation

In the present study, consider the typical microstructures (γ, γ’ and interface phases) in nickel-based single crystal, three ideal simplified atomic models of Ni, Ni3Al and Ni/Ni3Al interface were established. And nanoindentation molecular dynamics (MD) simulations were performed on the above models. Based on the dynamic deformation behavior and mechanism of nickel-based single crystal microstructure under the local loading, the nanoindentation plastic deformation is divided into two stages Pi and Pii. In the initial Pi stage, dislocations only move and react on the contact surface between the indenter and the substrate. Subsequently, in stage Pii, edge dislocations climb away from the contact surface, screw dislocations interact with each other and finally form dislocation shear loops. The local plastic deformation is dominated by the nucleation and emission of dislocation loops, affected by its resistance and dislocation velocity. It is noted that in the interface model, dislocation shear loops inevitably interact with interface dislocations, which will lead to further plastic deformation. Combined with the dislocation theory and the above research, a model based on the movement of dislocations was established better describe the local plastic behavior of the typical microstructure in nickel-based single crystal under indentation.