Formation Of Dislocation Loops Research Articles

The effects of post-irradiation isochronous annealing on defects evolution and hardening in Hastelloy N alloy

The nickel-based Hastelloy N alloy samples were implanted with helium ions at room temperature and then annealed at 600°C (sample S2), 800°C (sample S3) and 900°C (sample S4) for 1 hour, respectively. The effects of post-irradiation isochronous annealing on defects evolution and hardening in the alloy were investigated by transmission electron microscopy (TEM) and nanoindenter. TEM characterization shows that the mean diameter and number density of helium bubbles in sample S2 keep unchanged. It is found that the increments of helium bubbles size in samples S3 and S4 are mainly caused by migration and coalescence (MC) and Ostwald ripening (OR) mechanisms, respectively. Additionally, the MC mechanism can promote the formation of dislocation loops due to matrix atoms diffusion. In sample S4, the recovery effects of defects are striking, resulting in a significant reduction in the number density of dislocation loops. The values of Vac. / He. at different regions have been calculated to explain the size variation of helium bubbles in all the samples. Moreover, nanohardness increments calculated by dispersed barrier hardening (DBH) model agree with those measured by nanoindentation.

Model of Formation of Discrete Dislocations of the Thermoelastic Martensitic Transformation

The problem of the transition of a pre-martensite nanostructure of a SME crystal to a structure of discrete dislocation loops of the martensitic transformation is considered in the framework of the Ginzburg–Landau (GL) theory of nonequilibrium first-order phase transitions and a number of reasonable assumptions for the first time. Two asymptotic solutions of the GL equation have been obtained: one solution for a homogeneous source of crystal nanomodulation and another, for a spatially periodic pre-martensite nanostructure. An analysis of these solutions shows that the consolidation of local elastic lines of the nanostructure can be a cause of the formation of discrete dislocation loops of the martensitic transformation.

Striking effect of solute elements (Mn, Ni) on radiation-induced segregation/precipitation in iron-based model alloys

The evolution of radiation damage in steels is a major issue for the safe operation of nuclear power plants. Mn and Ni contribution to the formation and evolution of microstructural features remains a controversial issue. The present study aims at investigating their effects on microstructure by irradiating undersaturated BCC Fe-3.3at.%Ni and Fe-2.8at.%Mn model alloys. Two different ion irradiation conditions have been applied to study the effect of irradiation dose rate in both of these Fe-based alloys at a temperature of 673 K. In all cases, irradiation promotes the formation of dislocation loops. In the FeNi alloy, radiation-induced segregation is observed on loops, leading to precipitation of a FCC phase in the low dose rate irradiation condition only. In the case of FeMn, even if no precipitation was revealed, highly enriched decorated dislocation loops are observed, with a higher Mn segregation at radiation defects than Ni.

Fundamental effects of copper on dislocation loops and mechanical property of tungsten under irradiation

Molecular dynamics simulations have been conducted to investigate the effects of Cu on point defects, dislocation loops, and mechanical properties of W under irradiation. A realistic n-body potential of W-Cu is first corrected by the universal short-range potential of Ziegler, Biersack and Littmark for the irradiation process. Based on this potential, the point defects and mechanical properties of W and W80Cu20 solid solutions under irradiation are derived and compared with each other. It is revealed that the addition of Cu could induce the uniform distribution of point defects of W, impede the formation of dislocation loops in W under irradiation, and fundamentally improve the irradiation resistance of W in terms of mechanical properties. The simulation results are in good agreement with relevant experimental and theoretical observations in the literature, and could provide a deep understanding of points defects and mechanical properties of W and WCu solid solutions under irradiation.

Transmission electron microscopy study of extended defect evolution and amorphization in SiC under Si ion irradiation

ABSTRACTThe damage induced in 3C‐SiC epilayers on a silicon wafer by 2.3‐MeV Si ion irradiation for fluences of 1014, 1015, and 1016 cm−2, was studied by conventional and high‐resolution transmission electron microscopy (TEM/HRTEM). The evolution of extended defects and lattice disorder is followed in both the 3C‐SiC film and Si substrate as a function of ion fluence, with reference to previous FTIR spectroscopy data. The likelihood of athermal unfaulting of native stacking faults by point defect migration to the native stacking faults is discussed in relation to damage recovery. Threshold energy densities and irradiation doses for dislocation loop formation and amorphous phase transformation are deduced from the damage depth profile by nuclear collisions. The role of electronic excitations on the damage recovery at high fluence is also addressed for both SiC and Si.

Effect of helium concentration on irradiation damage of Fe-ion irradiated SIMP steel at 300 °C and 450 °C* *Project supported by the National Natural Science Foundation of China (Grant Nos. U1832133 and 12075194), Sichuan Provial Science and Technology Program, China (Grant No. 2020ZYD055), and the Doctor Research Foundation of Southwest University of Science and Technology (Grant No. 18zx714101).

SIMP steel is newly developed fully martensitic steel for lead-cooled fast reactors and accelerator-driven systems. It is important to evaluate its radiation resistance via high flux neutron irradiation, where dense He atoms can be formed via (n, α) transmutation reaction. Co-irradiation with Fe and He ions, instead of neutron, was performed. Specimens were irradiated with 6.4-MeV Fe ions to the damage dose of 5 dpa at a depth of 600 nm. Three different helium injection ratios of 60-appm He/dpa (dpa: displacements per atom), 200-appm He/dpa and 600-appm He/dpa at a depth of 600 nm, were performed. Two different irradiation temperatures of 300 °C and 450 °C were carried out. The effect of helium concentration on the microstructure of Fe-irradiated SIMP steel was investigated. Microstructural damage was observed using transmission electron microscopy. The formed dislocation loops and bubbles depended on the helium injection ratio and irradiation temperature. Lots of dislocation loops and helium bubbles were homogeneously distributed at 300 °C, but not at 450 °C. The causes of observed effects are discussed.

Molecular dynamics simulation of helium bubble ultimate pressure in α-Fe

In order to understand further the micro-mechanism of helium bubble punching out of the dislocation loop in α-Fe, it is necessary to study the ultimate pressure characteristics of helium bubble punching out of the dislocation loop. In this paper, a cubic representative volume element (RVE) model of the metal-helium bubble is established. For eight types of spherical helium bubbles with different initial radii, molecular dynamics simulations are carried out with the initial helium-to-vacancy ratio serving as a variable and the ultimate pressure of helium bubble and the critical helium-to-vacancy ratio at the beginning of dislocation loop formation in each model are obtained. The results show that for helium bubbles with dimensionless radius ranging from 2 to 10, both the ultimate pressure and the critical helium-to-vacancy ratio of helium bubble punching out of the dislocation loop decrease nonlinearly with the increase of initial helium bubble radius. The relationships of the ultimate pressure and the critical helium-to-vacancy ratio with the initial radius of helium bubble are fitted respectively according to the simulation results and the fitted two equations are in good agreement with the results of previous theoretical studies. The critical helium-to-vacancy ratio required for helium bubble punching out of the dislocation loop in α-Fe has an obvious size effect. For the helium bubble in the late nucleation stage (e.g. helium bubble with an initial radius of 0.81 nm) and non-ideal gas stage (e.g. helium bubble with an initial radius ranging from 1.00 nm to 2.50 nm), the critical helium-to-vacancy ratios for punching out of the dislocation loop are the same as the initial helium-to-vacancy ratio corresponding to the peak pressure point of helium bubble. But for early or middle nucleation stage, such as helium bubble with an initial radius of 0.50 nm, the critical helium-to-vacancy ratio for punching out of the dislocation loop is about 13.46% greater than the initial helium-to-vacancy ratios corresponding to the peak pressure points. At the initial moment (0 ps), in the cross section passing through the center of cubic RVE, the shear stress is concentrated, and the maximum shear stress of Fe atom array around the helium bubble is located at the intersection points (i.e. at 45°) of diagonal and helium bubble boundary, and it is distributed symmetrically with respect to the double fold lines of the cross section parallel to the sides. Both the range of shear stress concentrating area and the maximum shear stress increase with the initial helium-to-vacancy ratio increasing. The dislocation loop’s punching direction corresponds to the direction of the maximum shear stress. The research in this paper can deepen the understanding of the physical properties of helium bubbles in metals and lay a useful foundation for the subsequent analyzing of the effects of helium bubbles on the macroscopic physical and mechanical properties of materials.

STEM Characterization of Dislocation Loops in Irradiated FCC Alloys

In this study, we demonstrate the methodology systematically developed for dislocation loop (perfect and faulted loops) imaging and analysis in irradiated face-centered-cubic (FCC) alloys using scanning transmission electron microscopy (STEM). On-zone [001] STEM imaging was identified as the preferred choice for its accuracy and effectiveness based on the comparison with other dislocation loop imaging techniques including: (i) on-zone STEM imaging using other major low-index zone axes, (ii) kinematic two-beam conditions bright field imaging near the [001] zone axis in conventional TEM (CTEM) mode, and (iii) Rel-Rod CTEM dark-field (DF) imaging near the [011] zone axis. The effect of STEM collection angle on the contrast formation of dislocation loops was also investigated. The developed method was confirmed by imaging all populations of perfect and faulted loops of types a/2〈110〉{110} and a/3〈111〉{111} found in an ion irradiated Ni40Fe40Cr20 alloy. The proposed STEM-based technique can easily identify said loops with a size greater than 10 nm without any assumptions such as those commonly made using the conventional Rel-Rod CTEM-DF technique. The recommended methodology in this study is developed as a quick and convenient tool that can be generally applied to irradiated FCC-based materials due to their common crystallography.

Defect and structural evolution under high-energy ion irradiation informs battery materials design for extreme environments

Understanding defect evolution and structural transformations constitutes a prominent research frontier for ultimately controlling the electrochemical properties of advanced battery materials. Herein, for the first time, we utilize in situ high-energy Kr ion irradiation with transmission electron microscopy to monitor how defects and microstructures evolve in Na- and Li-layered cathodes with 3d transition metals. Our experimental and theoretical analyses reveal that Li-layered cathodes are more resistant to radiation-induced structural transformations, such as amorphization than Na-layered cathodes. The underlying mechanism is the facile formation of Li-transition metal antisite defects in Li-layered cathodes. The quantitative mathematical analysis of the dynamic bright-field imaging shows that defect clusters preferentially align along the Na/Li ion diffusion channels (a-b planes), which is likely governed by the formation of dislocation loops. Our study provides critical insights into designing battery materials for extreme irradiation environments and understanding fundamental defect dynamics in layered oxides.

Effect of H on the formation of vacancy dislocation loops in α-Fe

Genetic algorithm was applied to search the energy minimization configuration of vacancy clusters in α-Fe. Molecular statics calculations and dynamics annealing relaxation were employed to calculate the formation and binding energies of vacancies and 3D vacancy or vacancy-hydrogen (H) clusters, as well as 2D vacancy or vacancy-H clusters on (111), (011) and (211) planes. Our calculations show that vacancies prefer to form 3D clusters and vacancy dislocation loops are difficult to form, while vacancy-H clusters prefer to shape into 2D clusters, especially on (211) planes. Since H prefers the directional bonding, a vacancy cluster with H atoms tend to form a vacancy dislocation loop with its slip direction along the 〈100〉 direction and on the (211) habit plane. Our results are consistent with the experimental observations, and provide a possible mechanism for the formation of vacancy dislocation loops in α-Fe. Furthermore, we have also explored how dislocation loops trap self-interstitial atoms, vacancies and H atoms. It is of interest to note that H atoms strongly bound to a 〈100〉 vacancy dislocation loop, and are able to enhance the ability for the dislocation loop to further trap vacancies and reduce its ability to absorb self-interstitial atoms, thus promoting the growth of the 〈100〉 vacancy dislocation loops.

Formation of prismatic dislocation loops during unloading in nanoindentation

The formation of prismatic dislocation loops during nanoindentation of bcc iron is simulated with discrete dislocation plasticity. Interestingly, it is not the loading phase, but the unloading phase of the test which is found to be crucial for prismatic loop formation when indenting a (001) crystal surface. In this case, prismatic loops do not form during loading, but develop as the indenter is removed, in contrast to (111) indentation where prismatic loops are nucleated almost immediately upon loading.

In situ microstructural evolution in face-centered and body-centered cubic complex concentrated solid-solution alloys under heavy ion irradiation

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr18Fe27Mn27Ni28, Cr15Fe35Mn15Ni35, and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the CrFeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr++ ions to various levels of displacements per atom (dpa) using in-situ transmission electron microscopy. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in face-centered cubic (FCC) CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect production is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr18Fe27Mn27Ni28 and 10 nm in Cr15Fe35Mn15Ni35. This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy.

Formation of prismatic dislocation loops in a spherical particle embedded in a semi-infinite matrix

The introduction of prismatic dislocation loops in the interface between a misfitting spherical particle and its semi-infinite matrix has been theoretically investigated. The equilibrium position of one isolated loop has been first determined from an energy variation calculation and a shifting effect on the dislocation position relative to the particle equatorial plane has been identified due to the matrix free-surface. The case of two dislocation loops lying in the particle–matrix interface has been then discussed.

Reduction of dislocation, mean free path, and migration barriers using high entropy alloy: insights from the atomistic study of irradiation damage of CoNiCrFeMn

High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradiation resistance, partially due to sluggish diffusion. The mechanism from a defect-generation perspective, however, has received much less attention. In this paper, the formation of dislocation loops, and migration of interstitials and vacancies in CoNiCrFeMn high entropy alloy under consecutive bombardments were studied by molecular dynamics simulations. Compared to pure Ni, less defects were produced in the CoNiCrFeMn. Only a few small dislocation loops were observed, and the length of dislocation was small. The dislocation loops in Ni matrix were obviously longer and so was the length of dislocation. The interstitial clusters had much smaller mean free path during migration in the CoNiCrFeMn. The mean free path of 10-interstitial clusters in CoNiCrFeMn was reduced over 40 times compared to that in pure Ni. In addition, CoNiCrFeMn had a smaller difference of migration energy between interstitial and vacancy, which increased the opportunity of recombination of defects, therefore, led to less defects and much fewer dislocation loops. Our results provide insights into the mechanism of irradiation resistance in the high entropy alloy and could be useful in material design for irradiation tolerance and accident tolerance materials in nuclear energy.

Size effect of He clusters on the interactions with self-interstitial tungsten atoms at different temperatures**Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11705157), the Henan Provincial Key Research Projects, China (Grant No. 17A140027), and the Ninth Group of Key Disciplines in Henan Province of China (Grant No. 2018119).

The behaviors of helium clusters and self-interstitial tungsten atoms at different temperatures are investigated with the molecular dynamics method. The self-interstitial tungsten atoms prefer to form crowdions which can tightly bind the helium cluster at low temperature. The crowdion can change its position around the helium cluster by rotating and slipping at medium temperatures, which leads to formation of combined crowdions or dislocation loop locating at one side of a helium cluster. The combined crowdions or dislocation loop even separates from the helium cluster at high temperature. It is found that a big helium cluster is more stable and its interaction with crowdions or dislocation loop is stronger.

In Situ Microstructural Evolution in Fcc and Bcc Complex Concentrated Solid-Solution Alloys Under Heavy Ion Irradiation

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr18Fe27Mn27Ni28, Cr15Fe35Mn15Ni35, and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the CrFeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr++ ions to various levels of dpa using in-situ TEM. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in FCC CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect accumulation is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr18Fe27Mn27Ni28 and 10 nm in Cr15Fe35Mn15Ni35. This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy.

Radiation effects in alpha-doped UO2

UO2 samples doped with the strong α-emitter 238Pu were periodically characterised to study the potential effects of α-self-irradiation on spent nuclear fuel during storage. The degradation of the thermal properties and microstructure of the samples were monitored using the Laser Flash technique (LAF), X-Ray Diffraction (XRD), and Scanning and Transmission Electron Microscopy (SEM, TEM). The thermal conductivity decreased with a non-linear behaviour as a function of the α-dose. Three clearly different stages in the thermal conductivity degradation could be detected, and saturation was reached at relatively low doses (0.035 dpa). In combination with analyses of the microstrain measured by XRD, and TEM images, these three phases could be attributed to three stages of radiation-induced defect generation and clustering. SEM inspection showed the thermal conductivity degradation cannot be due to loss of integrity or grain boundary opening. The radiation damage produced resulted in the formation of dislocation loops as observed by TEM.

Molecular dynamics simulations of radiation damage generation and dislocation loop evolution in Ni and binary Ni-based alloys

As a great potential structural-material candidate for nuclear power applications, single-phase concentrated solid-solution alloys containing different elements have outstanding mechanical properties. In order to explore the effects of their elemental compositions on their radiation tolerance properties, in this work, radiation-induced defect production, as well as defect cluster and dislocation loop evolution, are investigated for equiatomic NiCo and NiFe alloys, and pure Ni, using molecular dynamics simulations. We find that the binary alloys are more radiation-resistant than pure Ni, while the NiFe binary alloy has the best radiation tolerance due to a higher recombination of defects and a lower number of surviving Frenkel pairs after cascade, compared to NiCo and pure Ni. Small defect clusters appearing in the three materials exhibit different migration behaviors, which migrate in a simple one-dimensional mode in pure Ni, but randomly change directions in the binary alloys; this behavior in the latter is thought to enhance the recombination of defects. Vacancy-type stacking fault tetrahedra and mixed interstitial dislocation loops with different types of dislocation segments are also observed. Interstitial dislocation loops generated in the NiCo and NiFe alloys show delayed and sluggish nucleation and evolution processes, which may lead to a higher probability of defect recombination during their evolution. Moreover, the formation of mixed dislocation loops is also discussed.

Effect of 165-keV Ar-ion irradiation on microstructural and mechanical properties of zircaloy-4

Cold-worked zircaloy-4 was irradiated by 165 keV Ar ion at room temperature. The aim of the study is to understand the correlation between the dislocation loops induced by irradiation and material hardening. The results revealed ziracloy-4 swelling even at low dose and a strong lattice disorder along and directions. With increasing dose, the microstrain increases while the domain size decreases attributed to the dislocation loops formation. Nanohardness and dislocation density show a similar evolution with the dose, which demonstrate a good correlation between the two parameters. Consequently zircaloy-4 hardening observed at low dose is due to the dislocation loops formation.

Microstructures and mechanical properties of AlCrN/TiSiN nanomultilayer coatings consisting of fcc single-phase solid solution

AlCrN/TiSiN nanomultilayer coatings, basically consisting of single-phase fcc-AlCrTiSiN solid solution, were prepared by co-deposition method with Al30Cr70 and Ti72Si18 targets. With substrate negative bias voltage increase from 50 V to 110 V, the hardness of nanomulitlayer coatings is increased from 32.1 GPa to 36.5 GPa, and the growth behavior changes from (2 0 0)fcc preferred orientation to coexistence of both (2 0 0)fcc and (1 1 1)fcc preferred orientation. Low interfacial energy supports the epitaxial growth, resulting in an increase in strain energy of coating systems due to the lattice misfit between neighboring sublayers. Increased strain energy is overcompensated by the formation of interfacial dislocations and dislocation loops as well as fcc-AlCrTiSiN nano-twins and hcp-AlN clusters at twin boundaries. Additionally, the Cr-ion etching induces to form a thin amorphous loose layer (LL) between coating and steel substrate during the pretreatment process. High substrate bias voltage promotes the transformation of amorphous phase to the (Cr,Fe)N solid solution during the coating deposition process, resulting in a notable increase in adhesion strength.