Number Density Of Defects Research Articles

Dislocation density-based constitutive model for the mechanical behaviour of irradiated Cu

Performance degradation of structural steels in nuclear environments results from the formation of a high number density of nanometre-scale defects. The defects observed in copper-based alloys are composed of vacancy clusters in the form of stacking fault tetrahedra and/or prismatic dislocation loops that impede the motion of dislocations. The mechanical behaviour of irradiated copper alloys exhibits increased yield strength, decreased total strain to failure and decreased work hardening as compared to their unirradiated behaviour. Above certain critical defect concentrations (neutron doses), the mechanical behaviour exhibits distinct upper yield points. In this paper, we describe the formulation of an internal state variable model for the mechanical behaviour of such materials subject to these (irradiation) environments. This model has been developed within a multiscale materials-modelling framework, in which molecular dynamics simulations of dislocation–radiation defect interactions inform the final coarse-grained continuum model. The plasticity model includes mechanisms for dislocation density growth and multiplication and for irradiation defect density evolution with dislocation interaction. The general behaviour of the constitutive (homogeneous material point) model shows that as the defect density increases, the initial yield point increases and the initial strain hardening decreases. The final coarse-grained model is implemented into a finite element framework and used to simulate the behaviour of tensile specimens with varying levels of irradiation-induced material damage. The simulation results compare favourably with the experimentally observed mechanical behaviour of irradiated materials.

Addition of nanoparticle dispersions to enhance flux pinning of the YBa2Cu3O7-x superconductor.

Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length ( approximately 2-4 nm for YBCO at temperatures < or =77 K) and when the areal number density of defects is of the order of (H/2) x 10(11) cm(-2), where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of approximately 8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77 K) by a factor of two to three for high magnetic fields.

The pinning property of Bi-2212 single crystals with columnar defects

It is qualitatively understood that the condensation energy density in oxide superconductors,which is one of the essential parameters for determining their pinning strength,becomes large with increasing dimensionality of the superconductor. However, thecondensation energy density has not yet been evaluated quantitatively. Its value canbe estimated from the elementary pinning force of a known defect. Columnardefects created by heavy ion irradiation are candidates for being such defects. Thatis, the size and number density of columnar defects can be given. In addition,it is known that two-dimensional vortices like those in Bi-2212 are forced intothree-dimensional states by these defects in a magnetic field parallel to the defects. Thus,the condensation energy density can be estimated from the pinning propertyof the columnar defects even for two-dimensional superconductors. A similaranalysis was performed also for three-dimensional Y-123. A discussion is given ofthe relationship between the condensation energy density and the anisotropyparameter estimated from measurements of anisotropic resistivity and peak field.

Micro-Macro Combined Simulation of the Damage Progress in Low-Alloy Steel Welds Subject to Type IV Creep Failure

A simulation method for microscopic creep damage at grain boundaries in the fine-grain heat-affected zone of low-alloy steel welds involving high energy piping was proposed on the basis of the combination of elastic-creep FEM analysis and random fracture resistance modeling of the materials. First, the initiation and growth-driving forces of small defects were concretely determined based on microscopic observation of the damage progress at the grain boundaries of the material, taking into account dependence on stress and temperature. Then, a simulation procedure combining the stress distribution from elastic-creep FEM and the random fracture resistance model was proposed, and this procedure was applied to the simulation of the microscopic damage progress in a welded joint model test and in actual power piping. The results in terms of the simulated number density of small defects throughout the wall thickness were in good agreement with the observed results.

Effect of Zr on the Irradiated Microstructure and Hardening in 304 Stainless Steel

ABSTRACTModel alloys of 304 Stainless Steels (SS) (Fe-18Cr-9.5Ni-1.75Mn) and 304 SS+Zr (Fe-18Cr-9.5Ni-1.75Mn+0.04Zr and Fe-18Cr-9.5Ni-1.75Mn+ 0.16Zr) were irradiated with 3.2 MeV protons to a dose of 1.0 dpa at 400°C. Following irradiation, the microstructure was characterized. The number density, defect size, and size distributions for faulted loops and voids were determined. Swelling for each irradiation condition was calculated based on the void measurements. The effect of Zr addition on the irradiated microstructure and hardening is clearly demonstrated. The number density of defects decreased with the Zr addition while the size change of faulted loops and voids is less pronounced. Radiation hardening was reduced by Zr addition.Void swelling is decreased with Zr addition. The reduction in void density and swelling may be caused by the enhanced recombination of defects at oversized Zr solute atoms, suppressing the vacancy super saturation and therefore directly suppressing void nucleation. The reduction in loop density is believed due to the enhanced point defects recombination caused by oversized solute Zr.

微視損傷進展を考慮した低合金鋼溶接継手のクリープ損傷評価法の提案

Creep damage analysis method of welded joint on the basis of the random fracture resistance model was proposed and was applied to the creep damage evaluation of welded joint of low alloy steel. Firstly a random fracture resistance of grain boundaries, an initiation and a growth driving forces of small defects, which were necessary in the above model, were determined considering the stress-dependent damage progress of the fine-grain HAZ material of 21/4Cr-1Mo steel. Secondly a procedure combining both the stress distribution in the welded joint and the random fracture resistance model was concretely proposed and was applied to the creep damage evaluation of the model test and the actual power piping. Experimental results of both the time history and the distribution of the number density of small defects in the welded joint were successfully reproduced by the proposed method.

Localized deformation and hardening in irradiated metals: Three-dimensional discrete dislocation dynamics simulations

When irradiated, metals undergo significant internal damage accumulation and degradation of mechanical properties. Damage takes the form of a high number density of nanosize defect clusters (stacking-fault tetrahedrons (SFTs) or interstitial loops). The alteration of mechanical properties is manifested in a hardening behavior and localized plastic deformation in defect-free channels. This work uses discrete dislocation dynamics (DD) to capture these effects. It sets the framework for the elastic interaction between gliding dislocations and defect clusters and details a scheme for loop unfaulting and absorption into dislocations. Here, it is shown that SFTs represents weaker pinning points for dislocation motion than parent dislocation loops. It is also shown that appreciable yield drop can be attributed to high density of defects decorating the dislocations. Strong obstacles cause dislocations in Cu to continually double cross slip causing the formation of defect-free channels. Finally, the correlation between yield stress increase and defect number density is in excellent agreement with the experiment.

Heat conductivity of La 1− xSr xMnO 3 surface layers

Using the transient thermoelectric effect (TTE), we have measured the thermoelectric power S( T) and the heat conductivity κ( T) of La 1− x Sr x MnO 3 ceramic surface layers with nominal compositions x=0.075, 0.1, 0.15, 0.2 and 0.3 in the temperature range 50–340 K. Except for a shift in the characteristic temperatures, the surface layer S( T) is still similar to the bulk thermopower while the heat conductivity κ( T) is markedly different, even changing the slope at intermediate temperatures. As S is less structure sensitive than κ, we relate these differences to an increased number density of defects near the surface. In particular, a stoichiometry gradient close to the surface and oxygen vacancy related (two level tunneling-) modes are indicated. In addition, new S and κ data are presented for lower doping, i.e. x=0.075 and 0.1.

Phase transition dynamics in the hot Abelian Higgs model

We present a detailed numerical study of the equilibrium and non-equilibrium dynamics of the phase transition in the finite-temperature Abelian Higgs model. Our simulations use classical equations of motion both with and without hard-thermal-loop corrections, which take into account the leading quantum effects. From the equilibrium real-time correlators, we determine the Landau damping rate, the plasmon frequency and the plasmon damping rate. We also find that, close to the phase transition, the static magnetic field correlator shows power-law magnetic screening at long distances. The information about the damping rates allows us to derive a quantitative prediction for the number density of topological defects formed in a phase transition. We test this prediction in a non-equilibrium simulation and show that the relevant time scale for defect formation is given by the Landau damping rate.

Defect formation rates in cosmological first-order phase transitions

In cosmological first-order phase transitions, the progress of true-vacuum bubbles is expected to be significantly retarded by the interaction between the bubble wall and the hot plasma. It has been claimed that this leads to a significant reduction in the number of topological defects formed per bubble, as a result of phase equilibration between bubbles. This claim has been verified for spontaneously-broken global symmetries. We perform a series of simulations of complete phase transitions in the 2+1-dimensional U(1)-Abelian Higgs model, for a range of bubble wall velocities, in order to obtain a quantitative measure of the effect of bubble wall speed on the number density of topological defects. We find that the number of defects formed is i) significantly lower in the local than the global case and ii) decreases exponentially as a function of wall velocity. Slow-moving bubbles also lead, however, to the nucleation of more bubbles before the phase transition is complete. Our simulations show that this is in fact the dominant effect, and so we predict {\em more} defects per unit volume as a result of the sub-luminal bubble wall terminal velocity.

Influence of casting technique and hot isostatic pressing on the fatigue of an Al-7Si-Mg alloy

The influence of the casting filling technique and hot isostatic pressing (hipping) on the fatigue-life distribution of Al-7Si-Mg alloy castings has been studied. To vary the number density and size of oxide-film defects in the castings, test bars were cast using bottom-gated filling systems with and without filtration. Some unfiltered castings were subjected to a hipping treatment of 100 MPa at 500 °C for 6 hours. Test pieces were machined from the castings and were fatigue tested in pull-pull sinusoidal loading, at maximum stresses of 150 and 240 MPa under a stress ratio of R=+0.1. The fatigue lives at any probability of failure and Weibull statistical parameters of the filtered castings were higher than those of the unfiltered and nonhipped castings, illustrating the importance of the casting technique. However, the unfiltered but hipped castings exhibited higher performance. It is proposed that the significant improvement in fatigue life after hipping is due to the deactivation of entrained double oxide-film defects as fatigue-crack initiators.

Theoretical analysis of formation of defects in hafnia

Quasichemical equations describing the dependences of the number density of defects formed in HfO2 by the Schottky, Frenkel, and anti-Frenkel mechanisms as a function of the partial pressure of oxygen and the temperature are derived and solved. Equations describing chemical disordering in the ion and electron subsystems and equations describing the processes of ionization of defects in oxides are derived for ZrO2 and HfO2 as an example. The theoretical data obtained can be used for deriving equations for calculating the number density of defects in HfO2 in any range of temperature and pressure. Results of an experimental test of the derived disordering equations by means of a precision thermomassometric method are presented. An evaluation has shown that the value of the deviation in HfO2 from index 2 amounts to about 0.0002 from the fraction O2. It is assumed that three-dimensional diffusion of oxygen by Ginstling’s equation is the limiting stage of the decrease in the mass.

Theoretical analysis of defect formation processes in silicon nitride

Processes of defect formation in Si3N4 by the Schottky, Frenkel, and anti-Frenkel mechanisms are analyzed theoretically within the method of quasichemical approximation. Equations describing the formation of defects in Si3N4 are derived, analyzed, and solved for different conditions of electroneutrality, and dependences of the number density of point defects in Si3N4 on the partial pressure of nitrogen under equilibrium conditions are determined. It is assumed that nitrogen vacancies are the predominant kind of point defect of nonstoichiometric origin.

Study of Defect Numbers and Distributions in Pecvd S102 Transparent Barrier Coatings on Pet

AbstractTransparent ceramic barrier films are the object of increasing interest in packaging, pharmaceutical, optical, and electronics industries, since they can reduce the permeation rate of oxygen (OTR) and/or water vapor (WVTR) by several orders of magnitude. We prepare silicon dioxide (Si02) or nitride (“SiN”) barrier layers by plasma-enhanced chemical vapor deposition (PECVD). We have shown that OTR (and WVTR) decrease with increasing coating thickness, d, to a certain asymptotic minimum value, when d ≥ 50 nm; the residual permeation is attributed to the presence of microscopic defects in the coating. Since the coatings are transparent and very thin, the detection of defects and the analysis of their origin are very difficult.We have recently developed techniques based on reactive ion etching (RIE) in oxygen plasma to render visible micrometer- or sub-micrometer-sized defects in transparent barrier coatings on transparent polymers. These techniques can be used to better understand the origins of defects in these coatings on a microscopic scale, as well as for mapping and counting defect density on a macroscopic scale (tens of cm2 or more).In this article we present a correlation between measured 02 transmission rate (OTR) values and the number density and size distribution of defects in SiO2 barrier coatings. Excellent agreement between measured and calculated OTR values allows us to confirm that residual OTR is indeed controlled by pinhole defects; our results also compare well with published data for aluminized PET.

In-Situ Irradiation Studies on the Effects of Helium on the Microstructural Evolution of V-3.8Cr-3.9Ti

AbstractThe role of helium on the microstructural evolution of V-3.8Cr-3.9Ti has been investigated by in-situ transmission electron microscopy observations of as-prepared and He implanted (&lt;10 appm) samples subjected to 200keV He irradiation at room temperature. Quantitative analysis of the defects showed an increase in the defect density and size with irradiation in both types of samples. The unimplanted sample showed a defect density consistent with electron irradiation experiments. In comparison, the He preimplanted sample had slightly larger defects and a substantially greater increase in the number density of defects. This result is consistent with a mechanism of He trapping by the formation of He-vacancy-X (X= C, N, O) complexes.

Defect Cluster Structure and Tensile Properties of Copper Single Crystals Irradiated With 600 MeV Protons

AbstractThe defect microstructures and tensile properties of high purity copper single crystals have been studied after irradiation at room temperature with 600 MeV protons in the dose range from 10-3to 10-1dpa. The defect number density N of clusters shows a linear dependence with dose up to a saturation value of 4×1023m-3, with a broad transition region between doses of 3×10-3and 10-1dpa. The increase in critical resolved shear stress (CRSS) induced by the irradiation is proportional to the square root of the dose, up to a dose of 7×10-3dpa. These results are compared to those of both fission and fusion neutron irradiations, showing that in room temperature irradiations at low fluences, no recoil spectra effects can be detected.

Characterization Of Grown-In Defects In CZ-SI Crystals By Bright Field IR Laser Interferometer

AbstractWe have developed a quantitative measurement method for the number density, size and morphology of grown-in defects in czochralski-grown silicon (CZ-Si) crystals with a bright-field infrared-laser interferometer (known as Oxygen Precipitate Profiler; OPP). Using this method we investigated the effect of crystal cooling condition during crystal growth on the formation of grown-in defects by growth holding experiments. The relation between gate oxide integrity (GOI) and grown-in defects was studied. It was revealed that the grown-in defects have octahedral shape and degrade the GOI performance. We estimate the density as well as cumulative volume of the grown-in defects and discuss the formation mechanism of them.

Separate assessment of porosity and uncombined carbon in cemented carbides

Porosity and uncombined carbon in cemented carbides are traditionally assessed by comparison to standardized microstructure charts, such as those in ISO 4505-1978. To improve the accuracy in the characterization a fully automatic image analysis procedure has been developed. The analysis is performed using light optical microscopy on unetched polished specimens. The implemented algorithm separates uncombined carbon, C-defects, from pores, A- and B-defects. Pores are approximately distributed in a random way in the structure. Uncombined carbon is on the other hand precipitated into clusters of carbon particles. This difference is together with defect size used as a base for the separation. Small defects situated close together are classified as C-defects, small isolated defects as A-pores and large defects as B-pores. Pores are found to be more round than the somewhat elongated C-defects. The difference in shape is significant and can also be used as a separation criterion. Image analysis is used to quantitatively characterise the ISO 4505-1978 standard charts with respect to volume fraction, number density and size distribution of defects. The results are compared to measured distributions for true microstructures.

Charged defects in vitreous silica

Abstract It is shown that special treatments of the conduction band mobility edge position and especially the luminescence spectra indicate that valence alternating pair (VAP) defets must exist in v-SiO2. The number density of such defects is found to be ∼ 1019 cm−3.

A small angle neutron scattering study of the lamellar phase of caesium pentadecafluorooctanoate (CsPFO)/1H-1H- perfluoroheptan-1-ol/2H2O

The ionic surfactant system CsPFO/ 2 H 2 O has been extensively studied in recent years. It exhibits a defected lamellar phase (L α H ) at low temperature or high surfactant concentration followed by nematic (N D ) and isotropic (L 1 ) phases as temperature increases or surfactant concentration decreases. Previous work on the lamellar phase, mainly by X-ray and neutron scattering techniques, has revealed an intra-lamella structure of water filled defects although the precise nature of these defects remains controversial. The addition of the cosurfactant 1H-1H-perfluoroheptan-1-ol to the binary system has been studied by optical microscopy and SANS. As the cosurfactant is added the phase transition temperatures increase and the nematic phase is replaced by a broad L 1 +L α H two phase coexistence region. SANS shows that the reflection from the intra-lamella structure weakens and shifts to lower Q. The number density of defects decreases with increasing cosurfactant although their shape remains roughly constant being elongated holes. Using these results and those for the binary system a more precise picture of the defect structure, which is consistent with theoretical models of the defected lamellar phase, has been obtained