Frank 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.

Molecular dynamics study of tribological properties of Ni–Cr alloy coatings with different pore shapes

The presence of nanopores with different shapes cannot be avoided during the coating preparation process, and the pore shape is an important factor affecting the coating friction mechanism. This paper applies molecular dynamics to study the tribological properties of Ni–Cr alloy coatings containing spherical, rectangular and ellipsoid-shaped pores. It is found that the coating with spherical pores has the smallest fluctuation of friction coefficient and the shallowest depth of wear scar. The atoms near the rectangular pores are more likely to move towards the corners of the pores due to shear strain, and the stress concentration in the spherical pores is less than that in the rectangular and ellipsoidal pores. The spherical pore has the least effect on the potential energy inside the coating, and the interatomic bond strength is optimal. The ellipsoidal pore is also more effective in insulating the heat transfer to the bottom of the coating, protecting the substrate underneath the coating and prolonging the coating life. Coatings with rectangular pores generate more interstitial atoms during friction, producing Frank dislocation loop and prismatic dislocation loops.

Diffusion of small anti-Schottky clusters in UO2

The aggregation of irradiation-induced defect clusters in UO2 leads to the formation of dislocation loops and cavities which contribute swelling and ultimately produce deleterious effects on the fuel. Despite their fundamental role in fuel evolution, the kinetics of these defect clusters are not currently well understood. In this work, we investigate the diffusive behavior of interstitial clusters via Molecular Dynamics (MD) simulations. Our investigation considers a range of defect cluster sizes, each composed of N UO2 units or anti-Schottky defects. We report a complex cluster size - mobility relation; increasing cluster size corresponds to an increase in mobility up until a critical size (N=4) where diffusivity reaches a maximum, after which the trend reverts and further increasing size corresponds to a decrease in mobility. The rapid migration observed correlates well with the very low barriers reported in a few historic experimental studies. Further analysis of the cluster shape and orientation reveals a strong structural preference for near-planar configurations oriented normal to the 〈100〉 direction. Rotational energy barriers are found to be similar in both magnitude and trend to the observed diffusion barriers. Lastly, connections are drawn to the behavior of large defect clusters including the formation of (1/3)〈111〉 Frank dislocation loops.

In Ukrainian

Investigating the formation of inversion layers (IL) at the Si-SiO2 interface in the manufacturing technology of silicon photodetectors, some dynamics of dislocations after isothermal annealing were revealed, which were absent in samples without inversion. After selective etching of samples with inversion layers, localization of dislocations on the periphery of responsive elements (RE) with accumulation of guard rings (GR) or other elements of n+-type topology outside the RE was observed. This testified to the movement of dislocations on the surface of the Si-SiO2 structures with IL in the direction of the periphery of the crystal during isothermal annealing, which contributed to a significant decrease in the density of structural defects in RE. The described phenomenon can be used to obtain highly doped defect-free silicon structures. Since the presence of dislocations or other violations of the crystal lattice negatively affect the parameters of the products. In the case of using the described phenomenon as a technological method of “cleaning” the surface of silicon structures, there is a need for controlled formation of IL. One of the methods of forming inversion layers can be thermal oxidation in hydrochloric acid vapors according to the principle of dry-wet-dry oxidation (for p-type silicon). Another method that does not require additional materials is the annealing of Si-SiO2 structures at a temperature of 900–950 Celsium degrees in a nitrogen atmosphere for ≥ 240 minutes. Inversion channels, in this case, will be formed due to the redistribution and diffusion of metal impurities in the oxide (which were introduced during previous thermal operations) to the Si-SiO2 interface. In the described case, these structural defects after annealing were localized in the GR, which is also an active element of the phododiodes, as it limits the dark current of the RE, accordingly, the dark current of the GR should also be low. To be able to implement this method, it is necessary to create passive n+-regions on the periphery of the crystals, limited by oxide, which will be the locations of defects after annealing. It can be both separate areas of arbitrary shape and a concentric ring outside the GR. Elements that will be the locations of defects on the periphery can be cut off at the stage of separating the substrates into crystals. After annealing, it is necessary to remove the IL and form an anti-reflective coating by any known method, since the presence of inversion channels contributes to the growth of dark currents. When examining the morphology of defect localization areas after annealing under high-magnification microscopes and with the help of an atomic force microscope, the formation of hexagonal and round defects, which are partial marginal Frank dislocation loops, was observed. The mechanism of dislocation movement described in this article has not been thoroughly studied by us and requires additional research, but it may be related to Cottrell atmospheres and their interaction with IL

Thermal Shrinkage Behavior of CH3O-Multielement-Molecular-Ion-Implantation-Induced Dislocation Loops Studied by Real-Time Transmission Electron Microscopy Observation

We investigated the thermal behavior of dislocation loops formed in a CH3O-multielement-molecular-ion-implanted epitaxial silicon (Si) wafer by real-time cross-sectional TEM observation with in situ heating. We found that the CH3O-ion-implantation-induced faulted Frank dislocation loops (FDLs) shrink at a low rate at the beginning of heat treatment (1st stage), and then the shrinkage rate rapidly increased (2nd stage), resulting in the dissolution of the defects. The activation energies for the shrinkage of FDLs in the 1st and 2nd stages (E D-1 and E D-2) were found to be 2.94 ± 0.31 and 4.95 ± 0.25 eV, respectively. The shrinkage behavior in the 1st stage is the desorption of C and O atoms that segregated along the edge of an FDL because of the interaction between the CH3O-ion-implantation-induced FDL and the segregated impurities. On the other hand, the 2nd stage corresponds to the desorption of Si atoms from FDLs and its migration. Compared to our previous study on the shrinkage behavior of CH4N-ion-implantation-induced FDLs (J. Electrochem. Soc. 169, 047521 (2022)), E D-2 of the CH3O-ion-implantation-induced FDLs is almost the same as that of the CH4N-ion-implantation-induced FDLs, while the values of E D-1 are quite different. The difference between the E D-1 values of CH3O- and CH4N-ion-implantation-induced FDLs is suggested to be the difference of the kind of segregated impurities. Our experimental results suggest that thermal stability of the dislocation loop is determined by the kind of segregated impurities around the dislocation loop.

The evolution and interaction of irradiation defects in CeO2 during H2+ & He+ dual-beam synergistic irradiation investigated by in-situ TEM

As a surrogate material of UO2, non-radioactive CeO2 was investigated by in-situ transmission electron microscopy during 30 keV H2+ & He+ dual-beam synergistic irradiation at 1073 K. The relationships between the size/density of dislocation loops and irradiation fluence were established, as well as for gas bubbles. Based on the analysis of the Burgers vectors, the formation of perfect dislocation loop (PDL) through the reaction between PDLs and the direct transformation from Frank dislocation loop (FDL) to PDL were obtained. The shrinkage of FDLs at high dose was observed. It was not only found that the number density and size of bubbles near the grain boundary were lower than those in the crystal, but also found that no loop-bubbles complex was formed. The mechanism of irradiation-induced excess oxygen vacancies should be the reason for the shrinkage of dislocation loops at high dose and the depletion of bubbles in the dislocation loops.

In-situ TEM investigation of unfaulting behavior of Frank loops in FCC Pd during H2+ & He+dual-beam irradiation

A faulted Frank dislocation loop (FDL) can transform into an unfaulted perfect dislocation loop (PDL) under plastic deformation, quenching or particle irradiation. Here, we report our direct observation of the unfaulting processes of FDLs in palladium (Pd) by in-situ ion irradiation in transmission electron microscopy. Results indicate that the FDL unfaulting is a combination of three processes involving the interaction between adjacent FDLs, the interaction between FDL and PDL, and the nucleation and growth of Shockley partial dislocations within FDL. Meanwhile, bubbles tend to nucleate within dislocation loops, and the PDLs capture more He/H2 over FDLs, which confirms for the first time that the irradiation-induced loops play an important role in the aging of Pd and the unfaulting strengthens this process.

Dislocation loop evolution and radiation hardening in nickel-based concentrated solid solution alloys

Effects of chemical composition, ion irradiation dose and temperature on unfaulting of irradiation induced Frank dislocation loops to perfect loops in two nickel based single-phase solid solution alloys, Ni–20Fe and NiFe–20Cr, have been studied. The fraction of Frank loops decreases with irradiation dose from 7.2 to 38.4 dpa at 500°C, but with more Frank loops remaining in the ternary alloy. However, perfect loops and dislocation networks become the dominant features of defects at 580°C in both alloys. The results indicate a thermally assisted loop unfaulting process that may be hindered by more sluggish defect motion in the alloy with more chemical components. Nano-indentation with both continuous stiffness method and single indentation method are used to measure radiation hardening. Loop unfaulting in both alloys irradiated at 580°C reduced radiation hardening while significant hardening is observed after irradiation at 500°C. The quasi-static single indentation method exhibits lower hardness results compared to continuous stiffness method, because dislocations induced from the cyclic loading in the latter method get relaxed and stabilized, resulting in higher resistance to the indenter.

Cascades Damage in γ-Iron from Molecular Dynamics Simulations

The degradation of austenitic stainless steels under irradiation environment is a known problem for nuclear reactors, which starts from atoms displacement cascade. Here, molecular dynamics (MD) simulations have been used to investigate the formation of atomic displacement cascade in γ-iron for energies of the primary knock-on atom (PKA) up to 40 keV at 300 K. The number of Frenkel pairs increased sharply until a peak value was reached, which occurred at a time in the range of 0.1-2 ps. After that, a number of defects gradually decreased and became stabilized. Compared with α-iron, there was less defects in the stable stage, and more clustered defects were produced in γ-iron. Within the range of PKA energies, two regimes of power-law energy-dependence of the defect production were observed, which converge on 16.8 keV. The transition energy also marks the onset of the formation of large self-interstitial atom (SIA) clusters and vacancy clusters. Interstitial and vacancy clusters were in the form of Shockley, Frank dislocation loops and Stir-Rod dislocation loops.

Mechanical response and plastic deformation of coherent twin boundary with perfect and defective structures

Coherent twin boundary (CTB) is an important structural defect, which is generally considered to have a perfect structure in the previous theoretical and computational studies. However, the CTBs observed in the recent experimental research in the submicrometer range were found to have inherent imperfections. In this work, we investigated the difference between perfect and defective CTB structures in mechanical responses and deformation mechanisms using molecular dynamics simulations. Two typical defective CTB structures with either incoherent twin boundary (ITB) step or self-interstitial atoms (SIA) cluster were considered. The simulation samples were deformed under uniaxial strain in different directions and were deformed under shear strain. Compare to the CTB with perfect structure, the stability of the defective CTB decreased significantly, and the simulated samples softened due to the existence of the ITB step and SIA cluster. When the critical stress was reached, the first dislocations were nucleated in the crystal lattice of the bicrystal sample with a perfect CTB structure. However, the CTB with structural defects played a role as the dislocation source to release dislocations, thus reducing the yield stress. Under uniaxial tension and compression, the yield stress of CTB with the SIA cluster was lower than that of CTB with ITB step in all simulated conditions. Dislocation analysis indicated that the difference was ascribed to the dissociation of an existed Frank dislocation loop on the twin plane rather than the nucleation of new dislocations from the CTB/ITB junction. Under shear deformation, the shear strain separated the structural defects from the CTB plane and transformed the defective CTB structure into a single perfect CTB. The simulation results provide a better understanding of the mechanical properties and deformation mechanisms of CTB-strengthened materials.

Simulations of weak-beam diffraction contrast images of dislocation loops by the many-beam Schaeublin–Stadelmann equations

Based on many-beam Schaeublin–Stadelmann diffraction equations and elasticity theories of defects, the transmission electron microscopy (TEM) diffraction contrast of dislocation loops within thin TEM foil is investigated systematically. Firstly, TEM diffraction contrast images of Frank dislocation loops in ion-irradiated copper are simulated with two-beam Howie–Whelan and many-beam Schaeublin–Stadelmann diffraction equations for verification; Then, the classic L–g–b relation of dislocation loop diffraction contrast is revisited, and effects of two-beam and many-beam diffraction conditions on the image contrast features are compared. Afterward, effects of anisotropic ratio on the black–white lobe features of dislocation loops are studied. Additionally, effects of loop depth within thin foil on the black–white lobes feature of dislocation loops are studied, and anisotropy will not change the flipping feature of the diffraction contrast lobes.

Microstructural evolution of NF709 austenitic stainless steel under in-situ ion irradiations at room temperature, 300, 400, 500 and 600 °C

Irradiation induced microstructural changes in the NF709 austenitic stainless steel were investigated under 1 MeV Kr ion irradiations to a dose of 3 dpa at room temperature (RT), 300, 400, 500 and 600 °C, and a dose of 20 dpa at RT. The irradiation-induced defects and the stability of precipitates were characterized with transmission electron microscopy (TEM). Frank dislocation loops were observed in all the irradiated samples, and the loop sizes were much larger at 600 °C than those at lower temperatures. “Raft” defect structures, formed through self-alignment of small dislocation loops, were also observed in all irradiated samples. M23C6 precipitates were amorphorized under irradiations at RT and 300 °C, but remained to be crystalline at 400 °C and above. MX precipitates were stable under irradiations at RT up to 20 dpa, and at temperatures below 600 °C to 3 dpa. At 600 °C, some MX precipitates were observed to dissolve during in-situ irradiation, suggesting possible precipitate instability at this irradiation temperature.

Effects of neutron irradiation and post-irradiation annealing on the microstructure of HT-UPS stainless steel

Microstructural changes resulted from neutron irradiation and post-irradiation annealing in a high-temperature ultra-fine precipitate strengthened (HT-UPS) stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Three HT-UPS samples were neutron-irradiated to 3 dpa at 500 °C, and after irradiation, two of them were annealed for 1 h at 600 °C and 700 °C, respectively. Frank dislocation loops were the dominant defect structure in both the as-irradiated and 600 °C post-irradiation-annealed (PIAed) samples, and the loop sizes and densities were similar in these two samples. Unfaulted dislocation loops were observed in the 700 °C PIAed sample, and the loop density was greatly reduced in comparison with that in the as-irradiated sample. Nano-sized MX precipitates were observed under TEM in the 700 °C PIAed sample, but not in the 600 °C PIAed or the as-irradiated samples. The titanium-rich clusters were identified in all three samples using APT. The post-irradiation annealing (PIA) caused the growth of the Ti-rich clusters with a stronger effect at 700 °C than at 600 °C. The irradiation caused elemental segregations at the grain boundary and the grain interior, and the grain boundary segregation behavior is consistent with observations in other irradiated austenitic steels. APT results showed that PIA reduced the magnitude of irradiation induced segregations.

Irradiation-induced \u03b2 to \u03b1 SiC transformation at low temperature

We observed that β-SiC, neutron irradiated to 9 dpa (displacements per atom) at ≈1440 °C, began transforming to α-SiC, with radiation-induced Frank dislocation loops serving as the apparent nucleation sites. 1440 °C is a far lower temperature than usual β → α phase transformations in SiC. SiC is considered for applications in advanced nuclear systems, as well as for electronic or spintronic applications requiring ion irradiation processing. β-SiC, preferred for nuclear applications, is metastable and undergoes a phase transformation at high temperatures (typically 2000 °C and above). Nuclear reactor concepts are not expected to reach the very high temperatures for thermal transformation. However, our results indicate incipient β → α phase transformation, in the form of small (~5–10 nm) pockets of α-SiC forming in the β matrix. In service transformation could degrade structural stability and fuel integrity for SiC-based materials operated in this regime. However, engineering this transformation deliberately using ion irradiation could enable new electronic applications.

Thermodynamic and morphological analysis of large silicon self-interstitial clusters using atomistic simulations

We study computationally the formation of thermodynamics and morphology of silicon self-interstitial clusters using a suite of methods driven by a recent parameterization of the Tersoff empirical potential. Formation free energies and cluster capture zones are computed across a wide range of cluster sizes (2 < Ni < 150) and temperatures (0.65 < T/Tm < 1). Self-interstitial clusters above a critical size (Ni ∼ 25) are found to exhibit complex morphological behavior in which clusters can assume either a variety of disordered, three-dimensional configurations, or one of two macroscopically distinct planar configurations. The latter correspond to the well-known Frank and perfect dislocation loops observed experimentally in ion-implanted silicon. The relative importance of the different cluster morphologies is a function of cluster size and temperature and is dictated by a balance between energetic and entropic forces. The competition between these thermodynamic forces produces a sharp transition between the three-dimensional and planar configurations, and represents a type of order-disorder transition. By contrast, the smaller state space available to smaller clusters restricts the diversity of possible structures and inhibits this morphological transition.

Atomic structure of extended defects in boron-implanted silicon layers

The structure of extended defects introduced into Si by means of boron implantation followed by thermal annealing at T = 900 °C is studied by the method of high-resolution transmission electron microscopy and computer modeling for different values of the implantation dosage (D) and concentration of boron atoms in substitutional positions B0\((C_{B_0 } )\) injected into the Si lattice before implantation. It is shown that the Frank dislocation loops of both interstitial (I) and vacancy (V) type at a ratio of 4: 1 are observed in Si samples with D = 1016 cm−2 up to \(C_{B_0 } \) = 0.8·1020 cm−3. The atomic structure of the I-type Frank dislocation loops is heavily deformed, which suggests segregation of finely dispersed boron in the defect plane. At the same time, the structure of the V-type Frank dislocation loops tends to be reconstructed due to interaction with self-interstitials. At \(C_{B_0 } \) = 2.5·1020 cm−3, the I-type Frank dislocation loops are found to transform to perfect dislocation loops, and boron precipitates with a high density appear in Si. Based on the results obtained, probable reasons for vacancy deficit formation in boron-implanted Si are discussed.

Quantification of the number of Si interstitials formed by hydrogen implantation in silicon using boron marker layers

H implantation results in the appearance of tensile out-of-plane strain in the implanted region which evolves during further annealing. VnHm complexes and/or larger platelets, both co-precipitates of vacancies and H atoms, are believed to be responsible for strain generation. However, during H+ implantation, Frenkel pairs i.e., both vacancies and interstitials are generated. Silicon self-interstitials have been rarely detected and thus their possible role in strain generation has been ignored so far. In this work, we demonstrate that Si interstitials are actually present in large measurable quantities in such implanted layers. For this, we have studied by Secondary Ions Mass Spectrometry the diffusion of boron delta layers during annealing at 350°C, 550°C and 850°C after H implantation at 12keV with a fluence of 1×1016H+/cm2. The Si self-interstitial supersaturations were extracted by comparison with simulations. Frank dislocation loops, i.e., precipitates of Si atoms, were observed by Transmission Electron Microscopy growing by Ostwald ripening during 850°C annealing. The supersaturation of Si self-interstitials in dynamical equilibrium with these loops was extracted showing consistency with the values found from the diffusion experiments. These results and more generally the role of interstitials in the strain build up are discussed.

Dislocation Generated by Electron Irradiation

Dislocation loop was generated by electron irradiation in nickel aluminum alloy. It is important to know dislocation characteristics obtained from a high energetic electron irradiation. If b is the Burger vector of a dislocation loop and g is the diffraction vector, dislocation loop will appear larger, smaller or disappear for g.b>0, g.b<0 or g.b=0, respectively. Dislocation loop was determined as follows – first, the appearance of dislocation loops is arranged in observation table. Second, based on type of dislocation loop, Burger vector and diffraction vector, appearance of dislocation loop is arranged in calculation table. Third, based on observation and calculation table, Burger vector and type of dislocation loop is determined. The results show that dislocation loops consist of perfect dislocation loops and Frank dislocation loops. The perfect dislocation loops have Burger vectors of ½[0 ] and ½[ 0] while Frank dislocation loops have Burger vectors of ⅓[1 1], ⅓[11 ], ⅓[ 11], ⅓[111], ⅓[1 1], ⅓[11 ] and ⅓[ 11]. All dislocation loops are interstitial types.

TEM and DSC Investigation of the Recovery of a Recycled Aluminum Processed by Equal Channel Angular Extrusion

TEM investigation was performed on a recycled aluminum processed by Equal Channel Angular Extrusion. The aluminum contains many inclusions which were identified to be rich in iron and silicon.This has a substantial effect in dragging recovery. ECAE was performed via routes B and C using a die formed by two channels characterized by the well defined angles =90° and  = 90°. The evolution of the microstructure seems to be similar whether using route B or C. a high dislocation density was introduced since the first passage together with a high density of Frank dislocation loops. The latter are due to a by-passing of inclusions by cross-slip mechanism. Weak beam experiments enable us to investigate the early stages of recovery consisting in the formation of dislocation walls and few dislocation cells. A partial arrangement of dislocations was observed since the first passage through the die. The formation of dislocation cells began already before annealing and progresses at more elevated temperatures. The substantive recovery seems to push recrystallization process to high temperatures. By contrast, DSC investigations show a significant difference: The amount of energy deduced from the area of the peak relative to recrystallization is higher when using route C.

Frank dislocation loops in HgTe∕CdTe superlattices on CdTe∕Si(211)B substrates

The defect structures in HgTe∕CdTe superlattices (SLs) on CdTe∕Si(211)B substrates grown by molecular-beam epitaxy have been investigated using (scanning) transmission electron microscopy and electron energy loss spectroscopy. Straight Hg-rich defects perpendicular to the SLs have been observed near the substrate while Frank dislocation loops (FDLs) are seen far from the substrate. The Hg-rich defects exhibit only a compositional variation with no significant atomic shift, and can be considered to be a remnant of a FDL which has climbed by thermal diffusion during the growth.