Self-interstitial Atom Clusters Research Articles

Variation in formation and migration of self-interstitial atom clusters in electron irradiated copper with material purity and specimen preparation method

ABSTRACT In situ observation with high-voltage electron microscopy was used to survey effects of impurity atoms on the formation and one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in copper under electron irradiation at 300 K. Specimens were prepared from copper materials of five nominal purities (9N, 6N, 5N, 4N, and 3N) using three methods with different annealing conditions. In standard (STD) specimens prepared through cold rolling and annealing in vacuum, SIA cluster formation and 1D migration depended little on the nominal purity. In non-annealed (NA) specimens prepared from high-purity materials (9N and 6N) using mechanical processing and electropolishing, the defect structure was found to be coarser than in STD specimens. In fact, SIA clusters in NA specimens had number density of more than an order of magnitude lower and an average size more than four times greater. Furthermore, 1D migration had the frequency of about an order of magnitude higher and distance extending more than twice longer. Results of bulk annealing (BA) specimens showed that annealing of as-received block material had minor effects. Distributions of 1D migration distance for all specimens were described using an earlier proposed trap model. These results were discussed assuming that SIA clusters are trapped by impurity atoms existing in as-received materials and those induced by annealing.

Vacancy migration in α-iron investigated using in situ high-voltage electron microscopy

ABSTRACT Modeling the microstructural evolution of irradiated materials requires knowledge of the migration energies of point defects, which results from energetic particle radiation. Herein, we measured the growth rate of self-interstitial atom (SIA) clusters in electron-irradiated α-iron at 275–320 K using in situ high-voltage electron microscopy. To improve the statistical accuracy of the measurement, we used photographic films and video data. This enabled analysis of a considerable amount of data by extracting several SIA clusters and tracking their size growth using image processing techniques. By fitting the temperature-dependent cluster growth rate to the Arrhenius relations derived using rate theory analysis, we obtained vacancy migration energy of eV. The validity of the estimation method was confirmed from the consistency of the dependence of the damage rate on the cluster growth rate using the applied rate theory analysis. We also investigated the possible roles of faster motion of tri-vacancies compared to those of mono- and di-vacancies, as demonstrated by first-principles calculations, on the obtained migration energy using a kinetic analysis model. In addition, the effects of impurities leading to decrease in the cluster growth rate were briefly discussed.

A study of primary damage formation in collision cascades in titanium

Molecular dynamics (MD) simulations were applied to study radiation damage formation in collision cascades initiated by primary knock-on atoms (PKA) with energy EРКА = 5, 10, 15 and 20 keV in α-Ti at T = 100, 300, 600 and 900 K ambient temperatures. A series of 24 collision cascades was simulated for each (EРКА, Т) pair. The necessary sampling set size was justified by a simple a posteriori procedure. The number of Frenkel pairs and the fraction of vacancies, εv, and self-interstitial atoms (SIAs), εi in point defect clusters were evaluated as functions of (EРКА, Т). It was established that collision cascades in α-Ti are extended along PKA trajectories and tend to split into subcascades. In contrast to other elemental metals with close-packed crystal structure, εv≥ εi in collision cascades in α-Ti. Moreover, both εv and εi ௜demonstrate weak temperature dependence. This is anindirect indication that both vacancy and SIA clusters created in collision cascades in α-Ti are stable in the considered temperature range

The influence of nitrogen and nitrides on the structure and properties of proton irradiated ferritic/martensitic steel

The 12Cr1MoWV (wt%) ferritic/martensitic steel HT9 is a candidate material for fuel cladding in advanced nuclear reactors, such as the Versatile Test Reactor currently under development. As such, understanding the relationship between microstructure and mechanical properties in the context of irradiation environments for these steels is critical. N content, and more specifically interstitial N, has been hypothesized to be detrimental to irradiated properties at lower temperatures (less than 0.3Tm) to a total of 6 dpa; however, in this work at a dose of 1 dpa the irradiated microstructure was improved with added N, leading to less irradiation hardening. Three variants of HT9 were irradiated with 1.5 MeV protons to a dose of 1 dpa at 300 ˚C. The HT9 variants included Low (10 ppm), Mid (190 ppm), and High (440 ppm) N alloys that were otherwise nearly identical. Changing the N content had a variety of effects on the irradiated defect structures. As N content increased, the average dislocation loop diameter decreased, while the number density of loops increased. Additionally, extensive Ni clustering was observed on dislocations and interfaces. The Mid and High N specimens exhibited significantly less hardening (ΔHV≅100) relative to the Low N specimen (ΔHV≅160). The decrease in hardening is attributed to vanadium carbonitride acting as a sink for Ni clusters that would otherwise form on dislocations. Under the irradiation conditions used, these results suggest increasing the N content in HT9 may have a desirable effect on the irradiated structure and properties at the dose studied, as well as the swelling resistance at higher doses. In other words, N content appears to be a powerful tool for tailoring the self-interstitial atom cluster mobility in F/M steels for different temperature and dose applications.

<i>In-situ</i> study of one-dimensional motion of interstitial-type dislocation loops in hydrogen-ion-implanted aluminum

The one-dimensional (1D) glide motion of dislocation loops along the direction of Burgers vector in various metallic materials has attracted considerable attention in recent years. During the operation of nuclear fusion reactor, component materials will be bombarded by high energy neutrons, resulting in production of radiation defects such as self-interstitial-atoms (SIAs), vacancies and their clusters. These defects feature large difference in migration energy, which may lead to concentration imbalance between SIAs and vacancies, and eventually irradiation damages such as swelling and embrittlement. Generally speaking, the mobility of a defect cluster is lower than that of a point defect. However, fast 1D motion may also take place among SIA clusters in the form of prismatic dislocation loops. This increases the transport efficiency of SIAs towards grain boundaries, surface and interface sites in the material, in favour of defect concentration imbalance and damage accumulation. To date, most literature works have found that the 1D motion of dislocation loops exhibited short-range (nanometer-scale) character. In addition, such experimental studies were generally conducted in pure metals using high voltage electron microscopes (HVEM) operated at acceleration voltages ≥1000 kV. However, for pure aluminum (Al), the maximum transferable kinetic energy from 200 keV electrons is 19.5 eV, while the displacement threshold energy is only 16 eV. Therefore, the observation and mechanistic investigation of 1D motion of dislocation loops in Al should also be possible with conventional transmission electron microscopes (C-TEM), as it may also exhibit the effects of beam heating and point defect production in HVEM. In view of the shortage of HVEM, this work reports the 1D motion of dislocation loops in pure Al implanted with hydrogen ions using C-TEM. Simultaneous dislocation loop motion in opposite directions of Burgers vector 1/2<inline-formula><tex-math id="Z-20211226170459">\begin{document}$\left\langle {110} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170459.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170459.png"/></alternatives></inline-formula>has been captured, as well as the collective 1D motion of an array of dislocation loops in the direction of Burgers vector 1/3<inline-formula><tex-math id="Z-20211226170340">\begin{document}$\left\langle {111} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170340.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170340.png"/></alternatives></inline-formula> under 200 keV electron irradiation. In addition, 1D motion of dislocation loops up to micron-scale range along the direction of Burgers vector 1/3<inline-formula><tex-math id="Z-20211226170427">\begin{document}$\left\langle {111} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170427.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170427.png"/></alternatives></inline-formula>, and up to a few hundred nanometers range along the direction of Burgers vector 1/2<inline-formula><tex-math id="Z-20211226170442">\begin{document}$\left\langle {110} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170442.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20211229_Z-20211226170442.png"/></alternatives></inline-formula> have been found, which is different from previous literature works. A characteristic migration track would form behind the moving dislocation loop, lasting for about tens of seconds. The more rapid the dislocation loop motion, the longer the migration track length is. The concentration gradient of SIAs by electron irradiation and the redistribution of hydrogen atoms caused by the moving dislocation loops may account for the observed micron-scale 1D motion of dislocation loops and the migration tracks.

Ta concentration effect on nucleation of defects in W-Ta alloy from first-principles model

Tungsten alloys are candidates for materials in DEMO diverter applications due to their excellent thermal and mechanical properties. Using the first-principles method, we researched the effect of Ta concentration on the nucleation of vacancy and self-interstitial atom clusters to better understand the evolution of defect behaviours in W-Ta alloys. Our calculation results indicate that Ta atoms can not only effectively inhibit the formation of mono-vacancy, but also hinder the nucleation of vacancy clusters. The magnitude of this interaction on vacancies would be more obvious with increasing Ta atom concentration. In addition, the study of “nSIA-mTa” structures indicates that the Ta atom can obstruct the nucleation of the SIA cluster, but competition exists between SIAs and the Ta concentration. The diffusion behaviours of <111>dumbbell have been researched using the CI-NEB method, indicating that Ta can slow down the diffusion of W-SIA by compelling it to change the diffusion direction. Therefore, it is conjectured that the Ta atom can effectively promote the recombination rate of the Frenkel pairs, resulting in an enhancement of defect self-repair of W-Ta alloy materials under neutron irradiation.

Interactions of solute atoms with self-interstitial atoms/clusters in vanadium: A first-principles study

The interactions of solute atoms with self-interstitial atoms (SIA)/clusters and their effect on stability of single SIAs and small SIA clusters in vanadium (V) were investigated using first-principles calculations. We calculated the binding energies of 16 substitutional solute elements (Cr, Ti, Al, Fe, Mo, Y, Nb, Cu, Mn, Ni, Ta, Zr, W, Si, S and P) with SIAs and found that the solute Cr/Fe/Mn/P/S prefers to form solute-V<111> mixed interstitial dumbbell. The interaction between solute Ti/Si/Y/Nb/Ta and <111> SIA dumbbell is attractive, while most of other solutes repel with the <111> SIA. The stable Cr- < 111> mixed interstitials and the Ti- < 111> SIA can attract five metal solutes (Ti, Y, Zr, Nb, and Ta), further stabilizing the SIA. The stability of small SIA clusters are determined and the parallel <111> SIA dumbbell clusters are the most stable configurations, followed by the parallel <110> SIA configurations. The small SIA clusters can attract three metallic solutes (Ti, Y and Zr) and two impurities (P and S), but will repel most solute atoms. The presence of solute atoms does not change the relative stability order among SIA clusters. The present calculations provide the basic data for understanding the influence solutes on the stability of SIAs/SIA clusters in V alloys under irradiation.

Migration energy of a self-interstitial atom in α-iron estimated by in situ observation of interstitial clusters at low temperatures using high-voltage electron microscopy

ABSTRACT Modeling cluster dynamics or rate theory to describe the microstructural evolution of irradiated materials requires a precise knowledge of the migration energy of a self-interstitial atom (SIA), a product of energetic particle radiation. We measured the evolution of the number density of SIA clusters in electron-irradiated α-iron at low temperatures (110–320 K) by in situ observation using high-voltage electron microscopy. We identified temperature-dependent physical quantities, including (1) the peak density of SIA clusters and (2) the critical defect-free zone thickness in a thin foil specimen, associated with interstitial mobility. By fitting these quantities to the Arrhenius relations derived by rate theory analysis, we obtained estimated interstitial migration energy values of and eV for (1) and (2), respectively.

Molecular dynamic simulations evaluating the effect of the stacking fault energy on defect formations in face-centered cubic metals subjected to high-energy particle irradiation

Austenitic stainless steels, which are used as incore structural materials in light water reactors, are characterized by an extremely low stacking fault energy (SFE) among face-centered cubic (FCC) metals. To evaluate the effects of SFE on defect formation under high-energy particle irradiation, molecular dynamics simulations were performed using the interatomic potential sets for FCC metals with different SFEs and a primary knock-on atom energy (EPKA) of 100 keV at 600 K. The results show that the number of residual defects is independent of the SFE. However, the characteristics of self-interstitial atom (SIA) clusters do depend on the SFE. For clusters smaller than a certain size, the ratio of glissile SIA clusters decreases as the SFE increases, which is similar to the trend observed at the low EPKA. However, for larger clusters, which can be detected only at a high EPKA, the ratio of glissile clusters increases. These results correspond to static energy calculations, in which the difference in the formation energy between a Frank loop and perfect loop (ΔEF-P) for the small clusters decreases as the SFE increases. In contrast, for the larger clusters, the SFE dependence of ΔEF-P changes due to the shape restrictions of stable perfect loops. At a high temperature of 600 K, large vacancy clusters with stacking faults can be detected at EPKA = 100 keV, resulting in the enhanced formation of these clusters at lower SFEs. Furthermore, several of these clusters were similar to perfect loops, with the edges split into two partial dislocations with stacking faults, although the largest clusters detected at low EPKAs were similar to stacking fault tetrahedrons.

Effect of confinement of SIA cluster diffusion by impurities on radiation defect accumulation due to 14 MeV neutrons in tungsten

Impurities acting as traps reduce the self-interstitial atom (SIA) cluster mobility, thereby increasing recombination and, in turn, enhancing radiation-resistance of polycrystalline materials. However, at elevated temperatures, depending on SIA-trap (impurity) binding energy, SIA clusters may trap and detrap (detach) numerous times. In tungsten, SIA clusters of all sizes glide one-dimensionally (1D) along their Burgers vector direction. This can result in detrapped SIA clusters to retrace their original 1D path, confining their 1D-glide between traps. However, SIA clusters can change the direction of their 1D-glide by overcoming a rotation barrier and impurities or solutes are known to reduce this barrier. A lower rotation barrier can result in detrapped SIA clusters to diffuse in a random 1D direction, effectively leading to 3D (net-3D) diffusion. Here we investigate the effect of two cases of SIA diffusion, namely the confined-1D diffusion and the net-3D diffusion, on the damage accumulation in polycrystalline tungsten under 14 MeV neutron irradiation using the object kinetic Monte Carlo method. Simulations were performed using cascade debris obtained from molecular dynamics simulations with primary knock-on atom (PKA) energies corresponding to 14 MeV neutrons. In the simulations with net-3D diffusion, SIA clusters of all sizes diffuse in a random 1D direction after the clusters detrap. While for the confined 1D diffusion case, clusters larger than size 5 retain their original direction of 1D glide. In both types of simulations, trapped vacancies are assumed to be permanently immobilized. Unexpectedly, the damage accumulation was lower with confined 1D diffusion than that with net-3D. We present a systematic comparison of the influence of the two cases of SIA diffusion on the radiation damage accumulation as a function of dose rate, detrapping barrier, and impurity concentration at 1025 K.

Effects of self-interstitial atom on behaviors of hydrogen and helium in tungsten

Neutron irradiation induces a large number of vacancies and self-interstitial atoms (SIAs) that interact with hydrogen atoms (H) and degrade the mechanical properties of tungsten (W) in a fusion environment. Vacancies increase free space and trap H atoms, however, the effects of SIAs on trapping H atoms are still unclear since SIAs decrease free space. We therefore perform systematical ab initio calculations to study the interactions of SIAs and SIAs clusters with H atoms in W. Our results suggest that SIA dumbbell makes interstitial H atoms easy to accumulate in W. With accumulation of H atoms near a SIA dumbbell, the binding energy decreases firstly and then levels off at about 0.3 eV. For comparison, the interactions of SIAs and SIAs clusters with helium (He) atoms are considered. It is found that SIAs and SIAs clusters can also act as trapping centers for interstitial He atoms. The combinations of SIAs with H and He hinder the fast movement of SIAs in W to annihilate with vacancies, increasing the concentrations of SIAs and vacancies in grain. SIAs and vacancies provide more sites for trapping H and He atoms and thus increase irradiation damage in W. We therefore suggest reducing retention of H isotopes and He in W to suppress irradiation damage.

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.

Interatomic potentials for irradiation-induced defects in iron

Empirical potentials using embedded atom method are developed for Fe, mainly to study irradiation-induced defects such as self-interstitial atom clusters or dislocation loops. The potentials are fitted using experimental values of solid-state properties, ab initio formation energies of basic point defects and ab initio forces acting on the atoms in the liquid or random state configurations. Various bulk and defect properties are compared to validate the transferability of the new potential. In this paper, we also investigate the energetic landscape of C15 self-interstitial atom clusters. In order to simplify and to facilitate the construction of lowest energy configurations in the complex energy landscape of C15 clusters, we test and propose three selection rules.

The effect of rhenium on the diffusion of small interstitial clusters in tungsten

In this work, we use atomistic simulations to investigate the mobility and stability of self-interstitial atom (SIA) clusters of size 1–5 in W-Re alloys. We apply molecular statics and molecular dynamics (MD) simulations to determine the dimensionality of diffusion of the clusters, the activation energy of translation and rotation, and the energy of dissociation. The results show a strong effect of Re on the diffusion properties of SIA clusters, but not on its stability. The diffusion mechanism of the single SIA changes from 1-D migration with on-site rotations to full 3-D diffusion on the MD time and length scale due to the addition of Re. Further, the mobility of the SIA clusters is greatly reduced by the addition of Re. The obtained results can be readily used to parameterize coarse grain models such as object kinetic Monte Carlo and rate theory models.

Quantum de-trapping and transport of heavy defects in tungsten.

The diffusion of defects in crystalline materials1 controls macroscopic behaviour of a wide range of processes, including alloying, precipitation, phase transformation and creep2. In real materials, intrinsic defects are unavoidably bound to static trapping centres such as impurity atoms, meaning that their diffusion is dominated by de-trapping processes. It is generally believed that de-trapping occurs only by thermal activation. Here, we report the direct observation of the quantum de-trapping of defects below around one-third of the Debye temperature. We successfully monitored the de-trapping and migration of self-interstitial atom clusters, strongly trapped by impurity atoms in tungsten, by triggering de-trapping out of equilibrium at cryogenic temperatures, using high-energy electron irradiation and in situ transmission electron microscopy. The quantum-assisted de-trapping leads to low-temperature diffusion rates orders of magnitude higher than a naive classical estimate suggests. Our analysis shows that this phenomenon is generic to any crystalline material.

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The interactions between the energetic particles and atoms in materials would result in the atomic displacements and the associated radiation defects. The interstitial dislocation loop, as one of the primary radiation defects, is formed by the clustering of the supersaturated self-interstitial atoms from the displacement damages in body centered cubic (bcc) iron based materials. The radiation hardening, embrittlement, swelling, creep, etc. are generally related to these loops and their interactions with other defects. In addition, the irradiation would also result in the formation of the micro-cracks from the surface of the materials and also from the interface of grains, precipitates, and gas-bubbles inside the materials, which would result in the irradiation assisted stress corrosion crack (IASCC). Therefore, to understand the interaction between interstitial dislocation loop and micro-crack under the irradiation, is one of key steps to understand the underlying mechanism of IASCC. In this work, the interaction between interstitial dislocation loop and micro-crack is simulated by molecular dynamics method on an atomic scale. The distance, relative position between them and radius of dislocation loop, as the main factors affecting their interactions, are studied to explore the underlying reason for inducing the micro-crack to expand on the slip plane. The simulation results indicate that when the interaction between them dominates the whole process with the distance between them within the critical value, the dislocation network containing the &lt;inline-formula&gt;&lt;tex-math id="Z-20200522122407-1"&gt;\begin{document}$ \langle 100 \rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20200317_Z-20200522122407-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20200317_Z-20200522122407-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; and 1/2&lt;inline-formula&gt;&lt;tex-math id="Z-20200522122407-2"&gt;\begin{document}$ \langle 111 \rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20200317_Z-20200522122407-2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20200317_Z-20200522122407-2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; segments, would interact with the crack tip to inhibit the crack from expanding through the pinning effect. When the size of loop is different, the pining effect would be available only when the interaction between loop core and crack tip dominates with the distance between them within the critical value. All these results provide new understanding for further exploring the IASCC under irradiation.

Vacancy loops in Breakaway Irradiation Growth of zirconium: Insight from atomistic simulations

Irradiation of zirconium alloys by neutrons causes dimensional changes associated with the formation of dislocation loops. In undeformed single crystals of this hexagonal close-packed material, an expansion in the crystallographic <a>-direction and a contraction in the <c>-direction are observed as a function of neutron fluence consisting of three stages, namely a rapid initial change, followed by a plateau, and then an accelerated “breakaway” growth whilst the volume remains constant. Molecular dynamics simulations suggest an atomic-level explanation: the initial dimensional changes are related to the formation of <a>-type nano-clusters of self-interstitial atoms (SIAs) while vacancies remain mostly isolated. In the plateau-region, formation of <a>-type vacancy loops compensates for the effect of growing SIA clusters and SIA <a>-loops. Upon further irradiation, vacancy <c> loops grow while the anisotropically diffusing SIAs anneal preferentially vacancy <a>-loops. By having a preferential vacancy sink on the basal plane and a net flow of interstitials to <a>-loops, the dimensional changes accelerate, thus leading to breakaway growth. The present simulations yield values for the thermodynamic stability, strain fields, and their effect on dimensional changes for vacancy <a> and <c>-loops as a function of their size, thus providing critical input for continuum models of radiation-induced growth of zirconium.

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.

Effects of one-dimensional migration of self-interstitial atom clusters on the decreasing behaviour of their number density in electron-irradiated α-iron

ABSTRACTAn in- situ observation using high-voltage electron microscopy (HVEM) showed that the number density of stationary self-interstitial atom (SIA) clusters in electron-irradiated α-iron at room temperature seems to reach its peak value at an early stage of irradiation and continuously decreases at the prolonged irradiation. A thin foil specimen is used in the in situ HVEM experiments; hence, the SIA clusters can annihilate because of their escape to the specimen surfaces and their mutual coalescence through one-dimensional (1D) migration. In this study, we derive analytical models associated with the experimentally revealed 1D migration mechanisms to examine the decreasing behaviour of the cluster number density: (1) trapping of one-dimensionally migrating SIA clusters by impurity atoms within the specimen surfaces and (2) detrapping of stationary SIA clusters from a bounded impurity atom caused by impact with incident electrons. By introducing a simple cluster evolution model that accounts for only the trapping and detrapping processes, the model calculation indicates that the detrapping of the stationary SIA clusters causes the surface annihilation of the liberated SIA clusters, leading to the decrease in their number density. The decreasing behaviour is in closer accordance with the experimental data when setting the impurity concentration in the same order as the estimation from the previous in situ HVEM experiment. This result suggests that the trapping and detrapping of the SIA clusters are the possible underlying processes for the decreasing behaviour.

Rearrangement of interstitial defects in alpha-Fe under extreme condition

In this study, by theoretical means, we reveal the main mechanisms that underpin the microstructure evolution driven by the formation of self-interstitial atoms (SIAs) clusters in body centered cubic iron under extreme conditions. Using Frenkel pairs accumulation simulations we point the complex interplay between the two families of interstitial defects, the dislocation loops with Burgers vectors <100> and ½<111> and the tridimensional C15 clusters. We reconcile the previous sparse understanding of microstructure evolution that put in opposition various mechanisms of defects formation by showing that both ½ <111> loops self-interactions and C15 clusters transformations produce <100> loops. Moreover, we exhibit the fact that these tri-dimensional clusters can form under irradiations with only the Frenkel pair accumulation that mimics electron irradiation and not only in high-energy cascades as it was previously stated. Finally, we show that the tridimensional C15 clusters even precede production of loops under irradiation.