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Search for interstitial dislocation loops produced in displacement cascades at 20 K in copper
Abstract
A low-temperature in situ ion-irradiation and annealing experiment has been performed in copper. Most clusters which persisted through an anneal to 120 K showed no size changes within the resolution (0.5 nm) of a new weak-beam sizing technique. Of the 55 defects measured under a range of weakly diffracting conditions, seven showed measurable size decreases while three showed size increases. We argue that these clusters are likely to be of vacancy and interstitial nature, respectively. Also on annealing to 120 K a fraction of about 25% of the clusters formed by irradiation with 600 kV Cu+ ions at 20 K disappeared, while a similar number of clusters appeared at different locations. The remaining defects persisted through the anneal, sometimes however with modified morphologies. Video microscopy suggested that the disappearance and appearance of clusters occurred gradually and was unlikely to be due to loop movement. Some arguments on the possible nature of these clusters are presented. On warming specimens to room temperature, a high density of small stacking-fault tetrahedra appeared close to the electron-exit surface of the foil in regions which had been exposed to the electron beam at low temperatures. These are most likely due to the clustering of vacancies produced by sputtering at the back surface.
... GND accumulation is typically interpreted as resulting from lattice curvature changes associated with the emergence of a dislocation substructure with an unbalanced Burgers vector. A problem immediately arises when trying to reconcile the observation of GND with the known fact that the only source of dislocation content in irradiated metals is the production of small, perfect prismatic loops originating within displacement cascades (22)(23)(24). By themselves, these loops contribute no net GND density in a global sense (i.e., when the Nye tensor is calculated along the entire loop contour in a single sweep), and, thus, the puzzle of how they form remains. ...
The development of radiation-tolerant structural materials is an essential element for the success of advanced nuclear energy concepts. A proven strategy to increase radiation resistance is to create microstructures with a high density of internal defect sinks, such as grain boundaries (GBs). However, as GBs absorb defects, they undergo internal transformations that limit their ability to capture defects indefinitely. Here, we show that, as the sink efficiency of GBs becomes exhausted with increasing irradiation dose, networks of irradiation loops form in the vicinity of saturated or near-saturated GB, maintaining and even increasing their capacity to continue absorbing defects. The formation of these networks fundamentally changes the driving force for defect absorption at GB, from “chemical” to “elastic.” Using thermally-activated dislocation dynamics simulations, we show that these networks are consistent with experimental measurements of defect densities near GB. Our results point to these networks as a natural continuation of the GB once they exhaust their internal defect absorption capacity.
... Based on CA assumption, diffraction contrast of defects can be calculated by solving ordinary differential equations (ODE) [6,7] rather than partial differential equations (PDE) [7][8][9].Through the comparison between dynamical two-beam imaging and image contrast simulations, the nature of defects larger than about 5 nm can be decided reliably [1,8]. However, when the size of defects is smaller than 5 nm, their visibility is usually better under weak-beam diffraction conditions [8,10,11]. The use of elasticity theory is likely to be a good approximation for loops of sizes greater than 2 nm [8,12,13], but the deformation across dislocation core is not continuous. ...
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.
... From this result, it appears that there are two types of irradiation-induced loops with different growth rates. The previous observations of irradiated iron have indicated that the irradiation-induced loops have b = a < 100 > [11,12]. While, the a/2 < 111 > loops would be favorable in terms of energy [13]. ...
To investigate the influence of heat load on microstructure change, we conducted in-situ experiments of annealing and irradiation for a model alloy (Fe-0.75Mn-0.47Mo-0.45Ni) using a multi-beam high voltage electron microscope. Electron-irradiation resulted in the formation of black dots in the model alloy. It is noted that some of black dots grew and changed to dislocation loops, but some did not. Furthermore, the 500 °C annealing to the electron-irradiated sample resulted in the shrinkage and/or the disappearance of black dots and small loops (<10 nm). It is suggested that the annealing would cause the release of interstitial atoms from small loops, which could flow into adjacent larger loops or diffuse to the surface.
... To prove the feasibility of applying in-situ ion irradiation technique to the study the microstructural evolution of advanced cladding materials using the facility that was in development at Sandia, Dr. Marcus Kirk at Argonne National Lab was gracious enough to permit the in-situ ion irradiation of a steel sample at their facility. [26][27][28][29] This permitted both discussions on the best design of the facility and the optimal experimental conditions for a variety of conditions, as well as a preliminary study of 1 MeV krypton ions using the intermediate voltage TEM facility for insitu ion irradiation facility at Argonne. The initial results from these experiments can be seen in Figure 57. ...
The goal of this LDRD project is to develop a rapid first-order experimental procedure for the testing of advanced cladding materials that may be considered for generation IV nuclear reactors. In order to investigate this, a technique was developed to expose the coupons of potential materials to high displacement damage at elevated temperatures to simulate the neutron environment expected in Generation IV reactors. This was completed through a high temperature high-energy heavy-ion implantation. The mechanical properties of the ion irradiated region were tested by either micropillar compression or nanoindentation to determine the local properties, as a function of the implantation dose and exposure temperature. In order to directly compare the microstructural evolution and property degradation from the accelerated testing and classical neutron testing, 316L, 409, and 420 stainless steels were tested. In addition, two sets of diffusion couples from 316L and HT9 stainless steels with various refractory metals. This study has shown that if the ion irradiation size scale is taken into consideration when developing and analyzing the mechanical property data, significant insight into the structural properties of the potential cladding materials can be gained in about a week.
... [42]. It is now known from MD that SIA clusters can form with a size distribution comparable to the vacancy component, and SIA loops visible by TEM have been reported recently [43]. Intracascade clustering has been a common feature of all reported MD simulations of cascades in metals, irrespective of the crystal structure. ...
Atomic- and continuum-scale computer simulation techniques have now become sufficiently powerful that many phenomena associated with radiation damage effects in metals can be modelled with a high degree of realism. Recent studies of several such phenomena are reviewed here. Primary knock-on atoms (PKAs) that recoil under collision from energetic atomic particles such as neutrons or ions are the principal source of damage. At high enough recoil energy, they create cascades of atomic displacements that result in single and clustered self-interstitial and vacancy defects. The time and length scales of the cascade process are ideally suited to atomic-scale computer simulation by molecular dynamics (MD). This method provides data on the number of defects produced and their distribution in clusters. This information was not available from earlier models and so the nature of the primary damage state is now much clearer. MD is also being used to reveal the nature of the motion and interaction of defects. Prediction and understanding of mechanical properties of irradiated metals requires simulation of dislocation–obstacle interactions. Models based on the continuum approximation are now being extended by large-scale MD simulations of the motion of dislocations gliding through irradiation-induced features such as voids and precipitates, and these reveal the strength and weakness of the earlier studies. Dynamic effects at temperatures greater than 0K are also being investigated. The place of modelling in the multiscale problems of radiation damage is emphasised.
... This inspired a low-temperature, in situ ion-irradiation and annealing experiment on copper. 35 The low-dose ion irradiation (600 keV Cu) and all TEM observations were performed at 20 K. Most clusters that persisted through an anneal to 120 K showed no size changes within the resolution (0.5 nm) of a weak-beamsizing technique. 36 An example of image data is shown in Fig. 11. ...
In situ observation is of great value in the study of radiation damage utilizing electron or ion irradiation. We summarize the facilities and give examples of work found around the world. In situ observations of irradiation behavior have fallen into two broad classes. One class consists of long-term irradiation, with observations of microstructural evolution as a function of the radiation dose in which the advantage of in situ observation has been the maintenance of specimen position, orientation, and temperature. A second class has involved the recording of individual damage events in situations in which subsequent evolution would render the correct interpretation of ex situ observations impossible. In this review, examples of the first class of observation include ion-beam amorphization, damage accumulation, plastic flow, implant precipitation, precipitate evolution under irradiation, and damage recovery by thermal annealing. Examples of the second class of observation include single isolated ion impacts that produce defects in the form of dislocation loops, amorphous zones, or surface craters, and single ion impact-sputtering events. Experiments in both classes of observations attempt to reveal the kinetics underlying damage production, accumulation, and evolution.
... The collapse of cascades into vacancy clusters was demonstrated in some metals on the basis of low temperature experiments [45] and used in theoretical treatment of void swelling [46]. The direct formation of clusters of SIAs in cascades was predicted by both MD modelling of displacement cascades [21] and diffusion-based theoretical calculations [47] in the early 1990s and then was confirmed by TEM [48]. Since then the intracascade clustering of vacancies and SIAs has been substantiated in a variety of metals using MD simulations. ...
Recent theoretical calculations and atomistic computer simulations have shown that glissile clusters of self-interstitial atoms (SIAs) play an important role in the evolution of microstructure in metals and alloys under cascade damage conditions. Over the past decade or so, the properties of SIA clusters in fcc, bcc and hcp lattices have been widely studied. In this paper we review key properties of these defects and also those of vacancy clusters formed directly in cascades, and present an atomic-level picture based on computer modelling of how these properties may change in the presence of other defects, impurities, stress fields, etc. We then examine the role of cluster properties and the consequences of their interactions in the process of damage accumulation and changes in mechanical and physical properties. We focus on the formation of defect clusters (e.g. dislocation loops and stacking fault tetrahedra (SFT)) and their segregation in the form of rafts of dislocation loops and atmospheres of loops decorating dislocations. Finally, we address the problem of radiation hardening by considering interactions between mobile dislocations and defect clusters (e.g. SIA dislocation loops, SFT and microvoids) produced during irradiation.
... The collapse in cascades of vacancy clusters into dislocation loops was demonstrated in some metals on the basis of low temperature experiments [25] and used in theoretical treatment of void swelling [26]. The direct formation of SIA clusters in cascades was predicted by both MD modelling of HEDCs [11] and diffusion-based theoretical calculations [27] and has been confirmed recently by TEM [28]. Now, the intracascade clustering of vacancies and SIAs has been substantiated in a variety of bcc, fcc and hcp metals using MD simulations. ...
Considerable success has been achieved in recent years in the understanding of radiation damage production in high-energy displacement cascades, the properties of the defects and evolution of radiation damage in metals. Two main reasons form the basis of this success. First, the significant increase in computing power has allowed simulation of realistic cascade energies with good statistics and relatively long-time evolution of defects to be carried out. Second, new experimental findings and corresponding theoretical calculations have allowed interpretation of a number of mechanisms and phenomena crucial for understanding and prediction of practically important radiation effects, such as void swelling, radiation growth, matrix hardening and plastic flow localisation. In this paper we review the most significant results in atomic-scale computer modelling related to these issues, mainly focusing on new achievements such as the formation of extended defect clusters, the dynamic properties of defect clusters, interaction between radiation defects and strengthening of material due to radiation defects.
Based on column approximation (CA) assumption, many‐beam Schaeublin‐Stadelmanndiffraction equations are employed for simulating thetransmission electron microscopy (TEM) diffraction image contrast ofdislocation loops withinthin TEM foilof finite thickness, and two beam and many beam diffraction conditions are compared.Moreover, the effects of materials anisotropy and free surface relaxation induced elastic fields distortion of dislocation loops on the black‐white image contrast are specially focused.It is found that anisotropy has a remarkable impact on the TEM imagecontrast of dislocation loop, and free surface relaxation induced image forces can changethe black‐white contrastfeatures when dislocation loops are near TEM foil free surfaces. Thus, in order to make reliable judgment on the nature of defects, effects of free surface and anisotropy should be included when analyzing irradiation induced dislocation loops and other type of defects in in‐situ electron, proton, heavy‐ion irradiation experiments under TEM environments. This article is protected by copyright. All rights reserved
In situ straining in the transmission electron microscope has been combined with molecular dynamics computer simulations to investigate the nature of the interaction of glissile dislocations with radiation-produced defects (loops, stacking-fault tetrahedra, and He bubbles), and to determine the mechanisms by which the dislocation loops and stacking-fault tetrahedra are annihilated and defect-free channels are created. The defect pinning strength depends on the defect and on the interaction geometry. The experiments and simulations show that a single interaction is not always sufficient to annihilate a dislocation loop or a stacking-fault tetrahedra and that the nature of the defect may be changed because of the interaction. The edge/screw character of the dislocation is also important as they have different efficiencies for annihilating a defect. The dislocations responsible for creating the defect-free channels are not the preexisting dislocations but originate from grain boundaries and other stress concentrators. Cross-slip of dislocations within the channels is important for clearing and widening the channel and can create new channels. Based on these observations a dispersed-barrier hardening model in which the influence of the radiation defects and dislocation density are combined. The resulting model predicts the observed behavior, including the apparent yield drop at high detect densities.
A computer program has been developed to solve numerically the Howie-Basinski equations of electron diffraction theory, which avoid the so-called column approximation. In this paper we describe the basis of the numerical approach, and apply it to simulate images of small loops in copper under a variety of weak-beam imaging conditions. Simulations were carried out for faulted Frank loops of size 2-10 nm with systematic variations in imaging parameters (the loop orientation, the diffraction vector, the deviation parameter, the loop depth, the foil thickness and beam convergence). Comparisons are made with experiments in ion-irradiated copper. The simulated images were found to be in good qualitative agreement with experimental TEM micrographs. We are able to reach conclusions on the likely visibility of very small clusters, and we discuss the implications of the simulations for experimental measurements of loop number densities and sizes.
Single crystals of Ni were irradiated with 137MeV xenon ions (Xe10+) of fluences up to 4.9×1013ions/cm2 below 13K. After the irradiation, the X-ray cryostat with the specimens was transferred from an irradiation chamber to the X-ray diffraction equipment below 20K. The results of X-ray diffuse scattering near Bragg reflections at low temperature indicate that the defects are located in cascades and suggest that a small amorphous zone exists within the cascade region. By the thermal annealing at 40K corresponding to the recovery stage IA, the defects in the cascades rearrange.
We developed a reliable method of transferring specimens at low temperature after ion irradiation with a cryostat for X-ray diffraction measurements. Single crystal Ni specimens have been irradiated with heavy ions at 13K. Transferring the irradiated specimens at low temperature without any warming up, defect structures were studied using X-ray diffuse scattering on annealing between 18K and room temperature. We observed a q−4-dependence of the diffuse scattering intensity after the irradiation and the results indicated that there were both vacancies and interstitial clusters, which were associated with a displacement cascade. With increasing the annealing temperature of the irradiated Ni, the interstitial dislocation loops grew at 200K and the radius and number of those loops began to decrease below 300K.
The effect of grain size on void swelling has its origin in the
intrinsic property of grain boundaries as neutral and unsaturable sinks
for both vacancies and selfinterstitial atoms. The phenomenon had
already been investigated in the 1970s and it was demonstrated that the
grain-size-dependent void swelling measured under irradiation producing
only Frenkel pairs could be satisfactorily explained in terms of the
standard rate theory (SRT) and dislocation bias. Experimental results
reported in the 1980s demonstrated, on the other hand, that the effect
of grain boundaries on void swelling under cascade damage conditions was
radically different and could not be explained in terms of the SRT. In
an effort to understand the source of this significant difference, the
effect of grain size on void swelling under cascade damage conditions
has been investigated both experimentally and theoretically in pure
copper irradiated with fission neutrons at 623 K to a dose level of
about 0.3 displacement per atom. The post-irradiation defect
microstructure including voids was investigated using transmission
electron microscopy and positron annihilation spectroscopy. The
evolution of void swelling was calculated within the framework of the
production bias model (PBM) and the SRT. The grain-size-dependent void
swelling measured experimentally is in good accord with the theoretical
results obtained using the PBM. The implications of these results on the
modelling of void swelling under cascade damage conditions are
discussed.
A weak-beam transmission electron microscopy study was carried out for matrix damage in A533B reactor pressure vessel (RPV) steel produced by 3MeV Ni2+ ion irradiation to a dose of 1dpa at 290°C. The matrix damage was found to consist of small dislocation loops. The observed and analyzed dislocation loops have Burgers vectors b=a〈100〉. The dislocation loops have a mean image size d=2.5nm and the number density is about 1×1022m−3. Most of the loops are stable after thermal annealing at 400°C for 30min. This indirect evidence suggests that their nature is interstitial.
We present results of a weak-beam transmission electron microscopy study of “matrix damage” in two nearly-pure irons (designated alloys 1A and 2A) produced by neutron irradiation to a fluence of 0.06 dpa at 280°C. The matrix damage in both materials was found to consist of small (2-6 nm) dislocation loops. About 80 % have Burgers vectors b = a<100>, and the remainder have b = a/2<111>. The loops in alloy 1A have a mean image size dmean = 2.8± 0.1 nm and a mean maximum image size dmax = 4.2± 0.3 nm, while those in 2A have d mean = 3.4± 0.1 nm and d max = 4.5± 0.3 nm. The number densities are about 8.5 × 1021 m−3 in alloy 1A, and 6.6 × 1021 m−3 in 2A. It can be shown that the loops can account for the observed irradiation hardening. At least some loops are stable under thermal annealing to temperatures of at least 430°C. This and other indirect evidence suggests that their nature is interstitial.
The recent molecular dynamics (MD) simulations have provided insights into the nature of displacement cascades, revealing defect cluster formation and their stability. In order to get direct experimental insights into the defect accumulation processes, in situ TEM observations in copper under irradiations with 100 keV C+ and 240 keV Cu+ ions at temperatures from 573 to 823 K were performed. Defect clusters produced by cascades were observed to be unstable with lifetimes of seconds, which depend on temperature, ion species and fluence. Multiple (2 or 3) defect clusters showing up their contrast in the same video frames, having a time resolution of 1/30 s, were concluded to be features related to subcascades and fast diffusion of defect clusters when located within 30 nm and from 30 to 140 nm, respectively. The detailed analysis of the defect cluster distribution shows that the direction of the fast diffusion is strongly related to crowdion directions, suggesting that the mechanism is based on motion of crowdion bundles. Instability and diffusion of defect clusters detected under ion irradiation are interpreted in terms of transformation into crowdion bundles, which is well described by MD simulations of dislocation loop stability under a compressive stress.
The application of the two-and-a-half-dimensional (2 1/2D) technique to the analysis of the nature of small point-defect clusters in ion-irradiated silver and copper has been explored. A modification of the technique which allows the identification of reciprocal-lattice spike effects has been made. However, even with this modification a comparison of analyses of the same clusters by 2 1/2D and the Black - White contrast method showed that 2 1/2D analyses of small faulted point-defect clusters are unreliable.
The black - white (B - W) stereo technique has been critically assessed and applied to the determination of the nature of small point-defect clusters in copper produced by Kr+ heavy-ion irradiation at room temperature. Several precautions were introduced in an attempt to improve the reliability of the analysis. It was found that most defects lying in the first approximately 10 nm of the foil are vacancy in nature, together with some of the deeper clusters. There was no unequivocal evidence for interstitial clusters. Some were more likely to be interstitial than vacancy on the criteria chosen, although the evidence is weak. Defects sometimes showed inconsistent contrast from one imaging condition to another, demonstrating the difficulty of placing them in the layer structure with complete confidence. The near impossibility of establishing that interstitial clusters are not present is shown by the fact that over half (53%) the total number of clusters could not be analysed because they did not show clear B - W contrast in a minimum number of imaging conditions. Various correlations are made between parameters such as defect sizes with the depths of the defects in the foil.
The effects of heavy-ion irradiation of copper at 30 and 300 K and the effect of successive irradiations on the fate of individual defects can be understood within the framework of the local melting model that stemmed from molecular dynamic computer simulations
MD simulations of displacement cascades in a variety of pure metals and alloys of different crystal structure are reviewed. For low recoil energies, these simulations have provided extensive results on the orientation-dependence and mean value of the displacement threshold energy in different crystal systems, and this information is tabulated. Large numbers of recoils have been simulated at true cascade energies, and the results show that Frenkel-pair production at the end of the cascade process is well below the NRT theoretical value in all metals and alloys. A new empirical relationship between Frenkel-pair number and damage energy is proposed. In contrast with this, antisite production efficiency in ordered alloys increases with increasing energy. Clustering of interstitials is a feature of cascade processes for all metals, but the degree of clustering is material-dependent. Atomic mixing in cascades is strongly dependent on cascade energy and is shown to be independent of crystal structure. The mechanisms underlying these results are discussed, particularly in relation to the highly disordered zone formed at the end of the thermal spike.
The black-white (B-W) stereo technique has been critically assessed and applied to the determination of the nature of small point-defect clusters in copper produced by Kr+ heavy-ion irradiation at room temperature. Several precautions were introduced in an attempt to improve the reliability of the analysis. It was found that most defects lying in the first approximately 10 nm of the foil are vacancy in nature, together with some of the deeper clusters. There was no unequivocal evidence for interstitial clusters. Some were more likely to be interstitial than vacancy on the criteria chosen, although the evidence is weak. Defects sometimes showed inconsistent contrast from one imaging condition to another, demonstrating the difficulty of placing them in the layer structure with complete confidence. The near impossibility of establishing that interstitial clusters are not present is shown by the fact that over half (53%) the total number of clusters could not be analysed because they did not show clear B-W contrast in a minimum number of imaging conditions. Various correlations are made between parameters such as defect sizes with the depths of the defects in the foil.
The damage structures produced in Ni by heavy-ion irradiations with 50 keV Kr+ and 50 and 100keV Ni+ ions have been studied as a function of irradiation temperature (30 and 300K) and ion dose (1015–1017 ions m-2). Both irradiations and electron microscopy were performed in the HVEM-Accelerator facility at Argonne National Laboratory. The effects of the irradiations were characterized in terms of the loop densities, loop sizes and the Burgers vector distribution. Dislocation loops were formed from isolated displacement cascades both at room temperature and 30 K, although the loop formation probability was significantly lower at 30 K. Additional loops appeared when specimens irradiated at 30 K were warmed to room temperature. At both irradiation temperatures the defect production rate decreased with increasing ion dose. During continued irradiation it was observed that new loops formed from isolated displacement cascades, and some pre-existing loops disappeared, shifted position of coalesced with a neighbouring loop. These effects can be explained in terms of the molecular dynamic computer simulation model that displays ‘local melting’ within the displacement cascade. The decreasing defect production rate with increasing ion dose is attributed to the coalescence of existing loops and the annihilation/reformation of loops that occurs when a fresh cascade spatially overlaps with the existing loops.
YBa2Cu3O7-δ single crystals were irradiated with 50 and 85 keV Kr and Xe ions to low doses, producing spatially isolated defect cascades. One irradiation was performed at low temperature. The resulting defects were investigated by conventional and high-resolution transmission electron microscopy. They were found to have primarily an amorphous-like structure with an inwardly directed (vacancy) strain field. Evidence for lattice melting, with coherent or incoherent recrystallization at some defect cascade sites, is presented for the first time.
Nine different atomic species, from K to Yb, have been implanted into gold at energies ranging from 20 to 150 keV. The nature and depth distribution of the resultant defect clusters were studied by transmission electron microscopy techniques as well as by a modification of the 21/2-dimensions stereo technique developed by Mitchell and Bell (1976). The effects of implanted ion dose and sample purity were determined. The cluster depth distributions are in overall agreement with the damage distributions deduced from the energy-deposition calculations of Winterbon, Sigmund and Sanders (1970). The nature of the defect clusters is found to depend on the mass and energy of the incoming ion, in agreement with our previously reported work. It is suggested that these provide evidence for the decisive influence of the deposited energy density on the nature of visible damage. We conclude that it is possible to distinguish between cascade and ‘spike’ effects, the latter setting in when the average energy per atom in the cascade is approximately 2 eV/atom. All results known to us (obtained at low doses on pure samples for a variety of ion species in Au, Au, Cu, W, Mo and Ni) may be related to each other in this way.
Cryo-transfer transmission electron microscopy and isochronal annealing experiments have been made on low temperature 14-MeV neutron-irradiated Au, Cu, Cu-0.1 at% Ge and Cu-0.2 at% Ni specimens. A two and a half dimensional stereo analysis has revealed the effects of specimen thickness and neutron dose on the formation of cascade defects. Interstitial-type defects disappear and some of the submicroscopic vacancy-type defects grow and become visible at the stage-III temperature because of migrating vacancies. Both disappearance and appearance of interstitial-type defects are observed below stage-III, which may be due to beam effects in the electron microscope. Above stage-III, residual interstitial-type defects disappear by absorbing vacancies at the temperature at which vacancy-type defects thermally emit these vacancies and disappear. Stable vacancy-type stacking fault tetrahedra seem to be transformed from vacancy-type clusters during isochronal annealing.
A simple and accurate method of determining foil thickness is described. The method makes use of measurements of the spacing of intensity oscillations in convergent beam diffraction patterns obtained with commercial scanning transmission electron microscopes. Extension of the technique to determination of extinction distances and anomalous absorption parameters required for the two-beam dynamical theory is outlined briefly.Une méthode simple et exacte pour déterminer l′épaisseur des échantillons minces est décrite. Cette méthode se sert des mesures d′espacement aux oscillations d′intensité qui se trouvent sur les figures de diffraction obtenues au moyen d′un microscope électronique industriel à balayage par transmission. L′extension de cette méthode pour déterminer les distances d′extinction et des paramètres d′absorption anomalaux nécessaires pour la théorie dynamique à deux rayons est briévement discutée.
We present molecular-dynamics computer-simulation studies of 25-keV displacement cascades in Cu at low temperature. We observe the splitting of a cascade into subcascades and for the first time show by molecular dynamics that cascades in metals may lead to the formation of both vacancy and interstitial dislocation loops. We propose a new mechanism of defect production based on interstitial prismatic dislocation-loop formation and discuss its consequences regarding the primary state of damage in irradiated metals.
- Foreman