Vacancy-type Dislocation Loops Research Articles

Nucleation and Growth of Voids in Silicon

ABSTRACTNucleation of voids and vacancy-type dislocation loops in Si under vacancy supersaturation conditions has been considered. Based upon nucleation barrier calculations, it has been found that voids can be nucleated, but not dislocation loops. The homogeneous nucleation rate of voids has been calculated for different temperatures by assuming different enthalpy values of Si vacancy formation. The process of void growth due to precipitation of vacancies has been numerically simulated. Comparing results of the nucleation and the growth modeling and taking into account the competition between the two processes, the limited time available, and the crystal cooling rate after growth, it has been shown that homogeneous nucleation of voids to experimentally observed densities and void growth to observed sizes is possible if enthalpy of Si vacancy formation is within the range of 2.9 to 3.6 eV with the nucleation temperature in the range of 980–1080 °C.

In situ observation of austenitic stainless steels during dual-beam irradiation

To investigate the effect of helium and hydrogen on the microstructural development in Fe16wt%Cr14wt%Ni xwt%P ywt%Ti ( x = 0, 0.01, y = 0, 0.05, 0.1) model alloys during dual beam irradiation with electrons/He +-ions and electrons/H 2 +-ions, in situ observations were carried out using a high voltage electron microscope (1000 kV) connected with a 300 keV ion accelerator. The simultaneous ion injection rate was 20 at.ppm He (or H)/dpa at 70 keV and the irradiation temperatures were 723 K and 773 K. In the phosphorus containing alloy vacancy type dislocation loops were nucleated in the early irradiation stage with single-electron beam irradiation and with electrons/H 2 +-ions. Voids were nucleated from lower dose during electron /He +-ion dual beam irradiations. In the titanium containing alloy, under electron/He-ion dual irradiation, void nucleation was suppressed compared to phosphorus containing alloy under single-electron beam or electron/H 2 +-ion dual beam irradiation. From these results it was clarified that helium extremely influenced the dislocation structure and void nucleation during irradiation and phosphorus interact strongly with interstitial defect, while the effect of hydrogen was weaker than that of helium.

Effect of additional minor elements on void nucleation in stainless steels during simultaneous irradiation with helium ions and electrons

The synergistic effect of helium and the addition of the minor elements phosphorus and titanium on void formation in austenitic stainless steels has been studied using a HVEM connected with an ion accelerator. The specimens used were pure Fe16Cr14Ni and the same alloy containing either 0.1% P or 0.1% Ti. The electron-induced damage rate was 2 × 10 −3 dpa/s and the helium injection rate was 20 at.ppm/dpa at 70 keV. Dislocation loops of vacancy type were nucleated in the early stage of the electron-only irradiation in the specimen containing phosphorus, but under dual beam irradiation many voids were nucleated and the dislocation loops grew rapidly. When the specimen with titanium was electron-irradiated, void nucleation tended to be reduced but significant void formation occurred due to dual-beam irradiation. In order to explain the result, the free defect concentration has been calculated using a kinetic model based on binding between interstitial and the minor element.

Vacancy-type dislocation loops observed at fairly high temperatures in nearly perfect Al crystals

Vacancy-type dislocation loops appeared at 230°C even during a slow-cooling process from 300°C in nearly perfect aluminum crystals when examined by synchrotron radiation topography with white radiation. The temperature of the specimen was cyclicly varied between 300 and 230°C at a cooling rate of 2000°C/h and a heating rate of 1000°C/h. Almost all of these vacancy loops appeared at the same place at each repetition of the cooling process, and some of the loops were formed along the dislocation lines observed in the preceding stage. Formation of these loops cannot be explained by homogeneous nucleation theory. Therefore, it is concluded that the vacancy loops grown at fairly high temperatures were nucleated heterogeneously, and the nucleation site appears to be inclusions in the specimen. Almost all vacancy loops that remained in a nearly perfect aluminum crystal are formed by heterogeneous nucleation during the cooling process.

X-ray diffuse scattering of quenched copper single crystals

X-ray diffuse scattering was measured for quenched copper. The diffuse scattering near the (111) and (222) reflections was observed at room temperature for as quenched and successively annealed samples from 200 to 800°C. A sharp and strong subpeak accompanied by a thermal diffuse scattering was appeared on a smaller angle side of the (111) Bragg angle. The subpeak was disappeared after annealing at 400°C. A characteristic feature of this scattering is from vacancy type dislocation loops, of which the mean radius is estimated to be about 1 nm, associated with a caustic scattering.

Influence of dilute impurities on the evolution of defect cascades in nickel

We have employed transmission electron microscopy to study the formation of vacancy-type dislocation loops from defect cascades produced by a 50 keV Kr + ion irradiation of Ni and its dilute alloys with Si and Al. An unusual and reproducible dependence of the probability of loop formation and loop size distributions on solute content was found. The results are explained by impurity caused changes in both the collisional and thermal spike (molten zone) phase of the defect cascade. Specific mechanisms include disruption of focussons by impurities to change the energy density profile within the collision cascade, interruption of replacement collision sequences, and interstitial trapping to reduce recombination during the thermal spike phase. Applying these mechanisms to a statistical distribution of the spatial extent of cascades and to the molten zones, all the experimental observations can be explained.

Defect formation during zinc diffusion into GaAs

The defect structure induced by zinc diffusion at 1170 K into undoped and indium-doped, semi-insulating GaAs single crystals was characterized for various diffusion times by analytical transmission electron microscopy of cross-sectional specimens. The results were compared with zinc concentration profiles obtained by spreading-resistance measurements on the same samples. Corresponding to zones of different zinc concentrations, three regions of characteristically different defect structures were distinguished. In the diffusion front region, interstitial-type dislocation loops, dislocation networks and gallium precipitates with voids in spatial correlation with the dislocations were found independent of diffusion conditions. The defect structure in the surface region depends on the diffusion conditions and is dominated by zinc-rich precipitates. For longer diffusion times, a transition region is observed where faceted voids and large vacancy-type dislocation loops coexist. A model for the formation of the diffusion-induced defects is presented which is based on the creation of a supersaturation of interstitial gallium atoms by the interstitial-substitutional zinc exchange and a resulting supersaturation of arsenic vacancies during diffusion.

Effects of the primary recoil spectrum on microstructural evolution

For quantitative predictions and comparisons of microstructures that evolve during exposure to different radiation environments at elevated temperature one needs to develop methods that go beyond those based on the number of displacements per atom. The number of freely migrating defects that contribute to the microstructural development is far less than the total number of defects produced, as has been recognized for some time from measurements of radiation-induced segregation and of radiation-enhanced diffusion. One major reason for the small amount of defects available for long range migration is the high concentration and close spatial correlation of vacancies and, to a somewhat lesser degree, of interstitials in cascades produced by high energy knock-ons. As a consequence, many defects either recombine or form immobile defect clusters during the defect formation and cooling phases of the cascades. After doses exceeding a few tenths of a displacement per atom, the residue of small clusters and dislocation loops of vacancy type remaining in the central portions of energetic cascades and subcascades, is the second major reason for the reduction of the mean free path of defects between creation and annihilation. Defect production in various neutron and ion irradiation environments is discussed in light of these facts. A method to calculate the fraction of freely migrating defects from the cluster size distribution of defects produced in cascades is suggested. The results are in good agreement with available data.

Effects of the primary recoil spectrum on microstructural evolution

Abstract For quantitative predictions and comparisons of microstructures that evolve during exposure to different radiation environments at elevated temperature one needs to develop methods that go beyond those based on the number of displacements per atom. The number of freely migrating defects that contribute to the microstructural development is far less than the total number of defects produced, as has been recognized for some time from measurements of radiation-induced segregation and of radiation-enhanced diffusion. One major reason for the small amount of defects available for long range migration is the high concentration and close spatial correlation of vacancies and, to a somewhat lesser degree, of interstitials in cascades produced by high energy knock-ons. As a consequence, many defects either recombine or form immobile defect clusters during the defect formation and cooling phases of the cascades. After doses exceeding a few tenths of a displacement per atom, the residue of small clusters and dislocation loops of vacancy type remaining in the central portions of energetic cascades and subcascades, is the second major reason for the reduction of the mean free path of defects between creation and annihilation. Defect production in various neutron and ion irradiation environments is discussed in light of these facts. A method to calculate the fraction of freely migrating defects from the cluster size distribution of defects produced in cascades is suggested. The results are in good agreement with available data.

On the nature of TEM-observed defect clusters in low-dose neutron- and ion-irradiated Cu, Ag and Au at room temperature

The nature and the depth of TEM-visible defect clusters were precisely analyzed in 40 keV Cu +-ion and 14 MeV neutron-irradiated thin foil Cu, Ag and Au at room temperature. Results of ion irradiation showed that the fraction of vacancy-type defect clusters increased with the dose and decreased with specimen thickness, leading to the conclusion that vacancies produced by displacement collision cascades thermally migrate toward vacancy-type defect clusters created by collision cascades, and that the probability of the loss of interstitial atoms from the surface is small in a thicker section of thin foil specimens. The same thickness dependence was found in a 14 MeV neutron-irradiated Au in which vacancy-type stacking fault tetrahedra and vacancy-type dislocation loops were dominant in a thinner section and considerable number of interstitial-type dislocation loops were formed in a thicker section.

Dislocation loops in single-crystal NiAl

In the conventional viewpoint, dislocation loops of vacancy type in single crystal NiAl can only exist in Al-rich compositions. Their configurations and their changes when deformation occurs, and their effects on the deformation have not been studied thoroughly.In the present study, nickel-rich NiAl single crystals of 48.1∼48.6 at %A1 were annealed at 1315°C for 50 hours, then cooled at 200°C/hr to permit vacancy condensation. Single-crystal samples cut with an [001] axis then underwent high temperature (850°C) tensile and cyclic deformation. TEM specimens were cut from the undeformed, 0.3% tensile strain, ±0.5 cyclic and 30∼40% tensile deformation samples.Two kinds of dislocations, edge and screw, were found in the undeformed NiAl. Edge dislocations consisted of concentric dislocation loops (Fig. 1) and spiral dislocations (dislocation A in Fig. 2). They lie on cubic planes and have cubic line orientations. Invisibility conditions of g.b=0 and g.b x u=0 show that they have <001> type Burgers vectors normal to the loop planes. The shift in image position on changing the sign of g was used to verify that the loops are vacancy in nature.

Defect Formation During Zn Diffusion into GaAs

AbstractThe microstructure induced by the Zn diffusion at 1170 K into doped and undoped semi-insulating GaAs single crystals was characterized for various diffusion times t < 1740 min by analytical electron microscopy. The results were compared with Zn concentration profiles obtained by spreading resistance measurements (SRM) on the same samples. At the diffusion front the formation of prismatic interstitial dislocation loops, dislocation networks, and of cavities partly filled with Ga was observed. Closer to the surface facetted voids and, for the undoped samples, vacancy-type dislocation loops formed. The near surface region of highest Zn-concentration showed a high density of Zn-rich precipitates. A model is presented which accounts .for these observations. It is based on fast interstitial Zn diffusion and the kick-out mechanism for interstitial-substituional exchange.

Formation of defect clusters in nickel and molybdenum by the collisions of heavy-ions with energies of around 100 MeV

Nickel and molybdenum specimens were irradiated with 84–120 MeV C, Cl, Br and I ions at temperatures about 150 K. After the irradiation, the development of defect structures was observed by electron microscopy. In molybdenum, the defect clusters introduced by the irradiation are small dislocation loops of vacancy type. The majority of defect clusters in nickel are stacking fault tetrahedra. The number of these defect clusters increases linearly with the ion fluence, whereas the size of the defect clusters do not increase markedly. This implies that each single collision having energy greater than some threshold causes the formation of a defect cluster. The threshold energies for defect clusters production and the surviving probabilities are determined by the results of the present experiments.

Defect structures induced by inert-gases in rapidly solidified type 304 stainless steel

Defects-are formed in most plastically deformed, quenched, and radiation damaged materials, and their type and distribution depend on the experimental conditions. Extensive research on radiation damage has shown that inert gases accumulate in materials and cause significant alterations of the microstructure and mechanical properties. In the centrifugal atomization process, the exposure of Type 304 stainless steel droplets to inert gas environments presents opportunities for their entrapment. The observation of large number density defects such as vacancy type dislocation loops and stacking faults in as-solidified Type 304 stainless steel powder is attributed to the inert gas/vacancy interaction.The purpose of this work is to examine the defect microstructure of extruded powder metallurgy (P/M) Type 304 stainless steel after preconditioning heat treatments at 900, 1000,1100, and 1200C for 1 hour followed by water quenching. Also, ingot metallurgy (l/M) Type 304 stainless steel (remnants of the feed stock for the powders) was heat treated at 1000 and 1100C for 1 hour followed by water quenching for comparison.

Kinetics of heterogeneous nucleation of aligned vacancy type dislocation loops under radiation and stress

Abstract By the molecular dynamics technique (MDT) it is shown that in fcc metals vacancy type dislocation loops aligned preferentially with an external stress may occur within cascade regions (CRs) during a thermal spike. The influence of this process on radiation-induced creep is discussed.

Direct comparison of electron and self-ion damage in aluminum as a fusion neutron simulation study

The microstructural changes in aluminum both by electron and self-ion irradiations have been observed in-situ within the same grain of the specimen. Electron irradiation is found to produce denser but smaller loops than Al + ion irradiation. Defect accumulation near dislocations and loop denuded zone near grain boundaries are observed in the case of Al + ion irradiation but not in the case of electron irradiation. The relation of microstructural changes with dose (dpa) is remarkably different between the two irradiations. This indicates an inadequacy of the unit of dpa as a correlation parameter. In addition, the effects of irradiation have been investigated on a quenched specimen which contains vacancy type dislocation loops. These loops shrink continuously by electron irradiation, whereas they crumble by Al + ion irradiation.

Simulation irradiation studies on iron

4 MeV Ni ion irradiation was performed for pure Fe up to 50 dpa between 350°C and 500°C with and without simultaneous He ion injection to simulate and understand the fundamental irradiation behaviour of ferritic stainless steels in the fusion environment. Results were compared with those from electron (HVEM), neutron and proton irradiated pure Fe specimens. Voids were observed in Ni ion irradiated Fe at 450°C and 500°C for both single and dual beam irradiation, but no voids were observed at 350°C and 400°C. A number of small vacancy type dislocation loops were observed at 350°C and 400°C especially along dislocation lines. It is considered that these small loops are closely associated with cascade formation and contribute to suppression of void formation at low temperatures.

Formation of vacancy-type dislocation loops in tungsten bombarded by 60 keV Au ions

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Identification of interstitial- and vacancy-type dislocation loops in ion-implanted silicon

A quick method of distinguishing between vacancy-type and interstitial-type dislocation loops is described which is applicable to foils near <111> orientation and loops in the size range which gives rise to double-arc contrast. This method utilizes the Kikuchi line pattern and some general rules concerning electron diffraction contrast from small loops. With this method, small prismatic loops in As+-implanted silicon have been shown to be of interstitial type.

Interstitial Clusters in Electron Irradiated Aluminum

Variation of the image position of the loop dislocation with the change in diffraction condition indicates that the dislocation loops formed in aluminum by electron irradiation in a high voltage electron microscope at room temperature or above may be identified to be of interstitial type. This result is confirmed by comparison of the weak and strong contrast from the stacking fault of irradiation induced loops with that of quench induced vacancy type dislocation loops. Quench induced vacancy type dislocation loops shrink and disappear by electron irradiation, and their absorption of interstitial atoms suppresses the formation of irradiation induced interstitial type loops. The upper limit of temperature for loop formation depends on irradiation intensity, and is about 160°C for 2×10 19 electrons/cm 2 ·sec at 1000 kV. Enhanced formation of interstitial loops is observed on both intrinsic and extrinsic stacking faults which suggests the exsistence of attractive interaction between the fault and interstitial...