Vacancy Loops Research Articles

Simulations of reactions between irradiation induced [formula omitted]-loops and mixed dislocation lines in zirconium

The interaction between mixed dislocations gliding in the basal plane with elliptical prism plane vacancy loops characteristic of irradiation induce loops is studied using Molecular Dynamics simulations. A methodology to create elliptical loops in the prism plane with major axes in the [0001] direction and with Burgers vectors in the [1¯21¯0] direction is described. Dislocation reactions involving mixed dislocations, parallel to [1¯21¯0] with a Burgers vector of a/3[2¯110] and gliding in the basal plane in the [101¯0] direction are investigated. The reaction of the mixed dislocation and vacancy loops in prism planes (1¯100) or (1¯010) and with Burgers vector a/3[1¯21¯0] leads to the creation of two jogs in the vacancy loop. These loops present a significant barrier to the motion of the mixed dislocation gliding in the basal plane. The reaction of the mixed dislocation and a vacancy loop in the (101¯0) prism plane with Burgers vector a/3[2¯110] leads to the cleaving of the vacancy loop and the absorption of part of the vacancy loop into the gliding dislocation. This loop presents a relatively weaker barrier to the motion of the mixed dislocation gliding in the basal plane. Strain rate is observed to have a large impact on the shear stress-displacement behavior of the reactions, but not on the reaction mechanisms.

How do energetic ions damage metallic surfaces?

Surface modification of structural and functional materials under bombardment by energetic ions is observed under different conditions and can be either an unavoidable effect of the irradiation or an intentional modification to enhance materials properties. Understanding the basic mechanisms is necessary for predicting property changes. The mechanisms activated during ion irradiation are of atomic scale and atomic scale modeling is the most suitable tool to study these processes. In this paper, we present results of an extensive simulation program aimed at developing an understanding of primary surface damage in iron induced by energetic particles. We simulated 25keV self-ion bombardment of Fe thin films with (100) and (110) surfaces at room temperature. A large number of simulations, ∼400, were carried out allow a statistically significant treatment of the results. The particular mechanism of surface damage depends on how the destructive supersonic shock wave generated by the displacement cascade interacts with the free surface. Three scenarios were primarily observed, with the limiting cases being damage created far below the surface with little or no impact on the surface itself, and extensive direct surface damage on the timescale of a few picoseconds. In some cases, formation of large 〈100〉 vacancy loops beneath the free surface was observed, which may explain some earlier experimental observations.

Theoretical investigation of microstructure evolution and deformation of zirconium under neutron irradiation

The radiation growth of zirconium is studied using a reaction–diffusion model which takes into account intra-cascade clustering of self-interstitial atoms and one-dimensional diffusion of interstitial clusters. The observed dose dependence of strain rates is accounted for by accumulation of sessile dislocation loops during irradiation. The computational model developed and fitted to available experimental data is applied to study deformation of Zr single crystals under irradiation up to hundred dpa. The effect of cold work and the reasons for negative prismatic strains and co-existence of vacancy and interstitial loops are elucidated.

Preferential Cu precipitation at extended defects in bcc Fe: An atomistic study

As a starting point to understand Cu precipitation in RPV alloys, molecular dynamics and Metropolis Monte-Carlo simulations are carried out to study the effect of lattice defects on Cu precipitation by taking Fe–Cu system as a model alloy. Molecular dynamics simulations show that owing to the high heat of mixing and positive size mismatch, Cu is attracted by vacancy type defects such as vacancies and voids, and tensile stress fields. In accordance, preferential precipitation of Cu is observed in Metropolis Monte-Carlo simulations at dislocations, prismatic loops and voids. The interaction of Cu with a stress field, e.g., that associated with a dislocation or a prismatic loop, is dominated by elastic effect and can be well described by the linear-elasticity theory. For prismatic loops, the attraction to Cu is found to be size-dependent with opposite trends displayed by vacancy and interstitial loops. The size-dependences can be explained by considering the stress fields produced by these loops. The current results will be useful for understanding the effect of neutron irradiation on Cu precipitation in reactor-pressure-vessel steels.

Deformation behavior of copper under conditions of loading by spherically converging shock waves: High-intensity regime of loading

Methods of X-ray diffraction analysis, optical metallography, transmission electron microscopy, and microhardness measurements have been used to perform a layer-by-layer study of the structure of a 64-mm copper ball after loading by spherically converging shock waves. It has been revealed that the high-velocity plastic deformation of copper under these loading conditions is mainly realized via slip and, in the middle and deep layers, by the formation of localized-deformation bands at grain boundaries. On the macroscopic level, shear bands are observed and, on the microlevel, a homogeneous dislocation structure, microbands, microtwins, a banded structure, and dislocation vacancy loops arise.

Displacement cascades and defects annealing in tungsten, Part I: Defect database from molecular dynamics simulations

Molecular dynamics simulations have been used to generate a comprehensive database of surviving defects due to displacement cascades in bulk tungsten. Twenty-one data points of primary knock-on atom (PKA) energies ranging from 100eV (sub-threshold energy) to 100keV (∼780×Ed, where Ed=128eV is the average displacement threshold energy) have been completed at 300K, 1025K and 2050K. Within this range of PKA energies, two regimes of power-law energy-dependence of the defect production are observed. A distinct power-law exponent characterizes the number of Frenkel pairs produced within each regime. The two regimes intersect at a transition energy which occurs at approximately 250×Ed. The transition energy also marks the onset of the formation of large self-interstitial atom (SIA) clusters (size 14 or more). The observed defect clustering behavior is asymmetric, with SIA clustering increasing with temperature, while the vacancy clustering decreases. This asymmetry increases with temperature such that at 2050K (∼0.5Tm) practically no large vacancy clusters are formed, meanwhile large SIA clusters appear in all simulations. The implication of such asymmetry on the long-term defect survival and damage accumulation is discussed. In addition, 〈100〉{110} SIA loops are observed to form directly in the highest energy cascades, while vacancy 〈100〉 loops are observed to form at the lowest temperature and highest PKA energies, although the appearance of both the vacancy and SIA loops with Burgers vector of 〈100〉 type is relatively rare.

Redistribution of alloying elements in Zircaloy-2 after in-reactor exposure

An atom probe tomography study of the microstructure of a Zircaloy-2 material subjected to 9 annual cycles of BWR exposure has been conducted. Upon dissolution of secondary phase particles, Fe and Cr are seen to reprecipitate in large numbers of clusters and particles of 1–5nm sizes throughout the Zr metal matrix. Fe and Sn were observed to segregate to ring-shaped features in the metal that are interpreted to be <c>-component vacancy loops. This implies that these two elements play a major role in the irradiation growth phenomenon in Zr alloys, which is believed to be caused by the formation of <c>-loops. Similarly to autoclave-corroded Zr alloys, the formation of a sub-oxide layer of approximate composition ZrO was observed. On the other hand, no oxygen saturated metal phase was detected underneath the oxide scale.

The behavior of vacancy-type dislocation loops under electron irradiation in iron

Two different types of dislocation loops were found in hydrogen ion implanted pure iron after annealing. The natures of these dislocation loops have been characterized by the inside–outside method of transmission electron microscope (TEM). It is found that the dislocation loops in specimens annealed at 670K are interstitial type with b=1/2〈111〉, n≈〈112〉, and the dislocation loops in the specimens annealed at 770K are vacancy type with b=1/2〈111〉, n≈〈011〉 or b=〈100〉 and, n≈〈112〉. Adding nickle element in iron could decrease the formation temperature of the vacancy loops.

Irradiation-enhanced twin boundary migration in BCC Fe

The effects of irradiation on twin boundary migration in BCC Fe are studied by atomistic simulations. It is found that under the applied shear strain–stress, thermal spikes may create twinning dislocation loops (TDLs) at twin boundaries, so triggering twinning. Irradiation-generated clusters of point defect at twin boundaries may act as sources to nucleate TDLs. When a vacancy loop intersects with a twin boundary, the critical stress to activate a TDL is less than half of that required for a defect-free twin boundary.

Cracks and extrusions caused by persistent slip bands

Cottrell’s account of persistent slip poses a puzzle which has challenged all subsequent research. Persistent slip bands (PSB) endure repeated plastic shear which constantly produces narrower and narrower dipoles, as observed by Veyssière. At the same time, because narrow interstitial dipoles have a larger elastic energy than otherwise identical vacancy ones, shear will eject larger interstitial dipoles to the surface, whilst retaining smaller vacancy ones in the interior, thereby producing excess vacancy loops. This causes a longitudinal tensile stress (the ‘fibre stress’) inside the band. The ejection process lowers the energy by a term linear in the fibre stress, but increases it by a term quadratic in the fibre stress, giving rise to an equilibrium value of the fibre stress, tensile, in order-of-magnitude agreement with observations of extrusions at low temperatures, where only plasticity can occur. The fibre stress produces logarithmic infinities in the surface stress at the edges of the PSB, and thus can be responsible for sharp stage I fatigue cracks. At higher temperatures which allow pipe diffusion and/or volume diffusion, interstitial loops are drawn by the tensile fibre stress into the PSB. Then, as cyclic plasticity attempts to maintain equilibrium, the loops are ejected where the band meets the surface, producing growing extrusions. Such extrusions can grow almost without limit. At low temperatures, the rate of extrusion formation is maximal at one Burgers vector per cycle, but it will be slower than this if it is diffusion limited.

Microstructural investigation of Si-ion-irradiated single crystal 3C-SiC and SA-Tyrannohex SiC fiber-bonded composite at high temperatures

Silicon carbides (SiCs) are considered as one of the promising candidates for structural and core materials used in fusion reactor and high temperature gas-cooled reactor (HTGR) due to its high thermal stability, and good resistance to irradiation and chemical attack. Single crystal 3C-SiC with less intrinsic defects was used to precisely characterize the radiation-induced defects in 3C-SiC. In addition, there are limited discussions related to radiation effect of SA-Tyrannohex fiber-bonded composite at high temperatures. Therefore, in this study, single crystal 3C-SiC thin film and SA-Tyrannohex SiC fiber-bonded composite were irradiated at 1000–1350°C with 7MeV Si3+ ion to simulate the neutron irradiation in reactors. The microstructure of the irradiated SiC was examined by using high resolution transmission electron microscope (HRTEM). In irradiated single crystal 3C-SiC, high resolution images showed that the planar defects were extrinsic stacking faulted loop with changing atomic sequences and intrinsic stacking faulted loop, i.e. vacancy loop. In addition, dislocation loops, voids, and edge dislocations in SA-Tyrannohex SiC fiber-bonded composite after irradiation were investigated. Besides, larger voids (with diameter 10–40nm) formed in alumina with preferred orientation after irradiation perhaps resulting in degradation of strength of the SA-Tyrannohex SiC fiber-bonded composite.

In situ study of self-ion irradiation damage in W and W–5Re at 500 °C

In situ self-ion irradiations (150 keV W+) have been carried out on W and W–5Re at 500 °C, with doses ranging from 1016 to 1018 W+m−2 (∼1.0 dpa). Early damage formation (1016W+m−2) was observed in both materials. Black–white contrast experiments and image simulations using the TEMACI software suggested that vacancy loops were formed within individual cascades, and thus, the loop nucleation mechanism is likely to be ‘cascade collapse’. Dynamic observations showed the nucleation and growth of interstitial loops at higher doses, and that elastic loop interactions may involve changes in loop Burgers vector. Elastic interactions may also promote loop reactions such as absorption or coalescence or loop string formation. Loops in both W and W–5Re remained stable after annealing at 500 °C. One-dimensional hopping of loops (b = 1/2 ⟨111>) was only seen in W. At the final dose (1018W+m−2), a slightly denser damage microstructure was seen in W–5Re. Both materials had about 3–4 × 1015 loops m−2. Detailed post-irradiation analyses were carried out for loops of size ⩾ 4 nm. Both b = 1/2 ⟨111⟩ (∼75%) and b = ⟨100> (∼25%) loops were present. Inside–outside contrast experiments were performed under safe orientations to determine the nature of loops. The interstitial-to-vacancy loop ratio turned out close to unity for 1/2 ⟨111⟩ loops in W, and for both 1/2 ⟨111⟩ and ⟨100⟩ loops in W–5Re. However, interstitial loops were dominant for ⟨100⟩ loops in W. Re seemed to restrict loop mobility, leading to a smaller average loop size and a higher number density in the W-Re alloy.

Defect clusters formed from large collision cascades in fcc metals irradiated with spallation neutrons

Fcc pure metals were irradiated with spallation neutrons (energies up to 500MeV) at room temperature to a neutron fluence of 1×1018n m−2 at KENS, High Energy Accelerator Research Organization (KEK). Defect clusters induced by large collision cascades were examined using transmission electron microscopy (TEM). In Au, large groups of defects included more than 10 clusters, and the damage zone extended over 50nm, which was larger than that induced by fusion neutron irradiation (<20nm). Although small stacking fault tetrahedra (SFT) are formed in subcascades by fission and fusion neutron irradiation, dislocation loops were also observed in the present experiments. Large dislocation loops (>10nm) were identified as vacancy type by the conventional inside–outside contrast method. Because of the low neutron fluence, spatial overlapping of collision cascades was ignored. Large vacancy loops are formed through cooperative reactions among subcascades in a single collision cascade with large recoil energy.

Self-interstitial configurations in hcp Zr: a first principles analysis

The alignment of vacancy loops and voids along basal planes observed in irradiated Zr and Zr alloys requires anisotropic point-defect transport with a dominant contribution along the basal plane. For neutron irradiation, this can be explained by one-dimensional mobility of self-interstitial atom (SIA) clusters, but experiments with electron irradiation indicate unambiguously that even single SIA should exhibit anisotropic diffusion. No experimental information is available on SIA properties in Zr and the previous ab initio calculations did not provide any evidence of anisotropic diffusion mechanisms. An extensive investigation of SIAs in Zr has been performed from first principles using two different codes. It was demonstrated that the simulation cell size, type of pseudopotential, exchange-correlation functional and the c/a ratio are crucially important for determining the properties of interstitials in hcp Zr. The most stable SIA configurations lie in the basal plane, which should lead to SIA diffusion mainly along basal planes.

Effects of helium and deuterium on irradiation damage in pure iron

Productions of transmute elements (hydrogen and helium) have great influences on the resistance to irradiation damage in structural materials for fusion reactor. The evolution of irradiation damage in bcc iron is investigated with ion implantation and electron irradiation. Pure iron implanted by He+ or D+ ions at room temperature are aged at 500℃ for 1 h, then irradiated by electrons under high voltage electron microscope. The results show that interstitial loops (i-loop) and vacancy loops (v-loop) are formed in He+-implanted iron and D+-implanted iron respectively. Under electron irradiation, due to the absorption of interstitials atom, i-loop grows up while v-loop shrinks. According to the rate of variation of dislocation loop, v-loop absorbs more interstitial atoms, i.e., the dislocation bias of D+-implanted iron is larger than that of He+-implanted iron, which means that the v-loop has the more contributions to irradiation swelling than i-loop. The causes of the different natures of dislocation loops formed in D+-implanted iron and He+-implanted iron are analyzed by the structures of He-V and D-V complexes.

Influence of impurities on the evolution of vacancy-type defects in neutron-irradiated nickel

In order to investigate the effect of impurities on vacancy defect evolution in nickel, specimens with high (5N) and technical (3N) purity were neutron-irradiated at ∼330K in the IVV-2M reactor (Russia) to fluencies in the range of 1×1021–1×1023n/m2 (E>0.1MeV) corresponding to displacement dose levels in the range of about 0.0001–0.01dpa and subsequently stepwise annealed to about 900K. The specimens of Ni with different purities were characterized both in as-irradiated state as well as after post-irradiation annealing by positron annihilation spectroscopy. The formation of three-dimensional vacancy clusters (3D-VCs) in cascades was observed under neutron irradiation. The density and size of 3D-VCs depended not only on dose level, but also on purity. The population of 3D-VCs in the technical Ni is lower than that in the high-purity Ni. 3D-VCs collapse into secondary-type clusters (stacking fault tetrahedra (SFTs) and vacancy loops) during stepwise annealing at 350–450K (stage III in Ni). The suppression of secondary cluster formation in 3N Ni is attributed to an effective vacancy interaction with impurity carbon atoms, which based on a relatively large vacancy–carbon atom binding energy (0.32–0.35eV). The trapping of vacancies released at the collapse of 3D-VCs by the interstitial impurity atoms dominates at low irradiation dose level (10−4dpa). Thus, we found that carbon impurity atoms have strong effects both on the primary vacancy-type defect aggregation and on the secondary vacancy clusters and complexes formation.

The effect of neutron irradiation dose on vacancy defect accumulation and annealing in pure nickel

In order to investigate the dose dependence of vacancy defect evolution in nickel, specimens of high-purity Ni were neutron-irradiated at ∼330 K in the IVV-2M reactor (Russia) to fluencies in the range of 1 × 10 21–1 × 10 23 n/m 2 ( E > 0.1 MeV) corresponding to displacement dose levels in the range of about 0.0001–0.01 dpa and subsequently stepwise annealed to about 900 K. Ni was characterized both in as-irradiated state as well as after post-irradiation annealing by positron annihilation spectroscopy. The formation of three-dimensional vacancy clusters (3D-VCs) in cascades was observed under neutron irradiation, the concentration of 3D-VCs increases with increasing dose level. 3D-VCs collapse into secondary-type clusters (stacking fault tetrahedra (SFTs), and vacancy loops) during stepwise annealing at 350–450 K. It is shown that the thermal stability of SFTs grow with increasing dose level, probably, it is due to growth of the average SFT size during annealing. The results of annealing experiments on electron-irradiated Ni at 300 K are indicated in the paper, for comparison. We also have briefly discussed the positron response to the SFT-like structures.

Radiation-induced damage and evolution of defects in Mo

The formation of defects in bcc Mo lattice as a result of 50-keV Xe bombardment is studied via atomistic simulation with an interatomic potential developed using the force-matching ab initio based approach. The defect evolution in the cascade is described. Diffusion and interaction of interstitials and vacancies are analyzed. Only small interstitial atom clusters form directly in the cascade. Larger clusters grow only via aggregation at temperatures up to 2000 K. Stable forms of clusters demonstrate one-dimensional diffusion with a very high diffusion coefficient and escape quickly to the open surface. Point vacancies have much lower diffusivity and do not aggregate. The possibility of a large prismatic vacancy loop formation near the impact surface as a result of fast recrystallization is revealed. The mobility of the vacancy dislocation loop segments is high, however, the motion of the entire loops is strongly hindered by neighbor point defects. This paper explains the existence of the large prismatic vacancy loops and the absence of the interstitial loops in the recent experiments with ion irradiation of Mo foils.

In situ transmission electron microscopy of electron-beam induced damage process in nuclear grade graphite

Atomic level processes involved in the swelling and crack-closing in nuclear grade graphite under electron irradiation have been observed in real-time using transmission electron microscopy. Noise-filtered lattice images show the formation of vacancy loops, interstitial loops and resulting dislocations with unprecedented clarity. The dislocation dipoles formed via vacancy loops were found to undergo climb resulting in extra basal planes. Concurrent EELS studies showed a reduction in the atomic density because of the breakage of hexagonal carbon rings. The formation of new basal planes via dislocation climb in addition to the bending/breaking of basal planes leads to swelling and closing of micro-cracks.

Microstructure formation in neutron-irradiated V-1.6Y alloy at 800 °C

Microstructure formation in V-1.6Y during neutron irradiation at 800°C was examined with transmission electron microscopy. Faulted vacancy loops were observed in the coarse grains several micrometers in diameter without dispersed Y2O3 particles. The habit plane of the faulted vacancy loop is close to {116}. Formation of faulted vacancy loops in the vanadium matrix could be induced by oxygen picked up from the environment during irradiation. Denuded zone of faulted vacancy loops was observed around both helical dislocations and grain boundaries.