Measurement of grain-boundary diffusion at low temperature by the surface-accumulation method. II. Results for gold-silver system

International Nuclear Information System (INIS)

Hwang, J.C.M.; Pan, J.D.; Balluffi, R.W.

1979-01-01

Grain-boundary diffusion rates in the gold-silver system were measured at relatively low temperatures by the surface-accumulation method which was analyzed in Paper I. The specimen was a polycrystalline gold film possessing columnar grains on which a silver layer was initially deposited epitaxially on one surface. During subsequent low-temperature annealing lattice diffusion was frozen out, and diffusion then occurred along the grain boundary and free-surface short circuits. The silver, therefore, diffused into the film from the silver layer along the boundaries, eventually reaching the opposite surface where it accumulated and was measured by Auger spectroscopy. The silver layer acted as an effective constant silver source, and grain-boundary diffusivities were calculated from the accumulation data. However, the exact location of the effective constant source in the silver layer could not be determined and this led to an uncertainty in the values of the grain-boundary diffusivities of a factor of 10. Lower- and upper-bound values were therefore described by D/sub b/(lower bound) =7.8 x 10 -6 exp(-0.62eV/kT) and D/sub b/(upper bound) =7.8 x 10 -5 exp(-0.62eV/kT) cm 2 /s in the temperature range 30--269 0 C. An examination of available grain-boundary diffusion data (including the present) suggests a tendency for the observed activation energy to decrease with decreasing temperature, and this was ascribed to a spectrum of activated jumps in the grain boundary and/or a spectrum of grain-boundary types in the specimen employed. The constant source behavior was tentatively ascribed, at least in part, to a grain-boundary ''Kirkendall effect'' resulting from the faster diffusion of silver than gold. The work indicates a need for increased understanding of the details of grain-boundary diffusion in alloys

Diffusion of hydrogen from plasma source by grain boundaries in EFG silicon

International Nuclear Information System (INIS)

Fedotov, A.; Saad, Anis M.H.; Drozdov, N.; Mazanik, A.; Ulyashin, A.; Fahrner, W.R.; Stognii, A.

2001-01-01

Diffusion of atomized hydrogen along grain boundaries (GBs) studied by transformation of their electrical activity in p-type silicon bi crystalline samples cut from EFG silicon crystals was investigated. The changes in electrical activity of GBs was estimated relative to both minority (MiC) and majority (MaC) carriers and demonstrated the correlation between the type, structure and thermal pre-history of GBs. It was shown on the base of this study that diffusion along GBs depends essentially on three factors: type of GBs, state of ribbons (as-grown or annealed) and concurrence of grain boundary dangling bonds and boron passivation effects. The model of the longitudinal hydrogen diffusion that explains these results is proposed

Oxygen Permeability and Grain-Boundary Diffusion Applied to Alumina Scales

Science.gov (United States)

Smialek, James L.; Jacobson, Nathan S.; Gleeson, Brian; Hovis, David B.; Heuer, Arthur H.

2013-01-01

High-temperature oxygen permeability measurements had determined grain-boundary diffusivities (deltaD(sub gb)) in bulk polycrystalline alumina (Wada, Matsudaira, and Kitaoka). They predict that oxygen deltaD(sub gb,O) varies with oxygen pressure as P(O2)(sup -1/6) at low pressure whereas aluminum deltaD(sub gb),Al varies with P(O2)(sup +3/16) at high pressure. These relations were used to evaluate alumina scale growth in terms of diffusivity and grain size. A modified Wagner treatment for dominant inward oxygen growth produces the concise solution: ?(sub i) = k(sub p,i)×G(sub i) = 12 deltaD(sub gb,O,int), where ?(sub i) is a constant and k(sub p,i) and G(sub i) refer to instantaneous values of the scale parabolic growth constant and grain size, respectively. A commercial FeCrAl(Zr) alloy was oxidized at 1100 to 1400 degC to determine k(sub p,i), interfacial grain size, ?, and thus deltaD(sub gb,O,int). The deltaD(sub gb,O,int) values predicted from oxidation at (375 kJ/mole) were about 20 times less than those obtained above (at 298 kJ/mole), but closer than extrapolations from high-temperature bulk measurements. The experimental oxidation results agree with similar FeCrAl(X) studies, especially where both k(sub p,i) and G(sub i) were characterized. This complete approach accounts for temperature-sensitive oxidation effects of grain enlargement, equilibrium interface pressure variation, and grain-boundary diffusivity.

Diffuse-interface polycrystal plasticity: expressing grain boundaries as geometrically necessary dislocations

Science.gov (United States)

Admal, Nikhil Chandra; Po, Giacomo; Marian, Jaime

2017-12-01

The standard way of modeling plasticity in polycrystals is by using the crystal plasticity model for single crystals in each grain, and imposing suitable traction and slip boundary conditions across grain boundaries. In this fashion, the system is modeled as a collection of boundary-value problems with matching boundary conditions. In this paper, we develop a diffuse-interface crystal plasticity model for polycrystalline materials that results in a single boundary-value problem with a single crystal as the reference configuration. Using a multiplicative decomposition of the deformation gradient into lattice and plastic parts, i.e. F( X,t)= F L( X,t) F P( X,t), an initial stress-free polycrystal is constructed by imposing F L to be a piecewise constant rotation field R 0( X), and F P= R 0( X)T, thereby having F( X,0)= I, and zero elastic strain. This model serves as a precursor to higher order crystal plasticity models with grain boundary energy and evolution.

Density functional theory metadynamics of silver, caesium and palladium diffusion at β-SiC grain boundaries

Energy Technology Data Exchange (ETDEWEB)

Rabone, Jeremy, E-mail: jeremy.rabone@ec.europa.eu [European Commission, Joint Research Centre, Institute for Transuranium Elements, D-76125 Karlsruhe (Germany); López-Honorato, Eddie [Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV), Unidad Saltillo, Industria Metalúrgica 1062, Parque Industrial, Ramos Arizpe 25900, Coahuila (Mexico)

2015-03-15

Highlights: • DFT metadynamics of diffusion of Pd, Ag and Cs on grain boundaries in β-SiC. • The calculated diffusion rates for Pd and Ag tally with experimental release rates. • A mechanism of release other than grain boundary diffusion seems likely for Cs. - Abstract: The use of silicon carbide in coated nuclear fuel particles relies on this materials impermeability towards fission products under normal operating conditions. Determining the underlying factors that control the rate at which radionuclides such as Silver-110m and Caesium-137 can cross the silicon carbide barrier layers, and at which fission products such as palladium could compromise or otherwise alter the nature of this layer, are of paramount importance for the safety of this fuel. To this end, DFT-based metadynamics simulations are applied to the atomic diffusion of silver, caesium and palladium along a Σ5 grain boundary and to palladium along a carbon-rich Σ3 grain boundary in cubic silicon carbide at 1500 K. For silver, the calculated diffusion coefficients lie in a similar range (7.04 × 10{sup −19}–3.69 × 10{sup −17} m{sup 2} s{sup −1}) as determined experimentally. For caesium, the calculated diffusion rates are very much slower (3.91 × 10{sup −23}–2.15 × 10{sup −21} m{sup 2} s{sup −1}) than found experimentally, suggesting a different mechanism to the simulation. Conversely, the calculated atomic diffusion of palladium is very much faster (7.96 × 10{sup −11}–7.26 × 10{sup −9} m{sup 2} s{sup −1}) than the observed penetration rate of palladium nodules. This points to the slow dissolution and rapid regrowth of palladium nodules as a possible ingress mechanism in addition to the previously suggested migration of entire nodules along grain boundaries. The diffusion rate of palladium along the Σ3 grain boundary was calculated to be slightly slower (2.38 × 10{sup −11}–8.24 × 10{sup −10} m{sup 2} s{sup −1}) than along the Σ5 grain boundary. Rather

A theoretical model of grain boundary self-diffusion in metals with phase transitions (case study into titanium and zirconium)

Science.gov (United States)

Semenycheva, Alexandra V.; Chuvil'deev, Vladimir N.; Nokhrin, Aleksey V.

2018-05-01

The paper offers a model describing the process of grain boundary self-diffusion in metals with phase transitions in the solid state. The model is based on ideas and approaches found in the theory of non-equilibrium grain boundaries. The range of application of basic relations contained in this theory is shown to expand, as they can be used to calculate the parameters of grain boundary self-diffusion in high-temperature and low-temperature phases of metals with a phase transition. The model constructed is used to calculate grain boundary self-diffusion activation energy in titanium and zirconium and an explanation is provided as to their abnormally low values in the low-temperature phase. The values of grain boundary self-diffusion activation energy are in good agreement with the experiment.

Micromagnetic simulation of anisotropic grain boundary diffusion for sintered Nd-Fe-B magnets

Science.gov (United States)

Li, W.; Zhou, Q.; Zhao, L. Z.; Wang, Q. X.; Zhong, X. C.; Liu, Z. W.

2018-04-01

A systematic investigation on the anisotropic grain boundary diffusion in sintered Nd-Fe-B magnets is carried out by micromagnetic simulation. The results indicate that the critical reason for the anisotropic diffusion effect is not the difference in the amount of Dy diffused along different directions but the macroscopic demagnetizing field. The diffusion parallel to the easy axis from both pole surfaces of the magnet can increase the nucleation fields in the two major regions with large macroscopic demagnetizing fields, where the reverse domains can nucleate easily. As a consequence, the grain boundary diffusion along the directions parallel to the easy axis from two pole surfaces is more effective to improve the coercivity of the magnets than that along other directions. It is also found that, to enhance the coercivity, only a limited diffusion depth is required. The present result is in good agreement with the recent experimental findings.

Grain boundary structure and properties

International Nuclear Information System (INIS)

Balluffi, R.W.

1979-01-01

An attempt is made to distinguish those fundamental aspects of grain boundaries which should be relevant to the problem of the time dependent fracture of high temperature structural materials. These include the basic phenomena which are thought to be associated with cavitation and cracking at grain boundaries during service and with the more general microstructural changes which occur during both processing and service. A very brief discussion of the current state of our knowledge of these fundamentals is given. Included are the following: (1) structure of ideal perfect boundaries; (2) defect structure of grain boundaries; (3) diffusion at grain boundaries; (4) grain boundaries as sources/sinks for point defects; (5) grain boundary migration; (6) dislocation phenomena at grain boundaries; (7) atomic bonding and cohesion at grain boundaries; (8) non-equilibrium properties of grain boundaries; and (9) techniques for studying grain boundaries

Meso-microstructural computational simulation of the hydrogen permeation test to calculate intergranular, grain boundary and effective diffusivities

Energy Technology Data Exchange (ETDEWEB)

Jothi, S., E-mail: s.jothi@swansea.ac.uk [College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP (United Kingdom); Winzer, N. [Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, 79108 Freiburg (Germany); Croft, T.N.; Brown, S.G.R. [College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP (United Kingdom)

2015-10-05

Highlights: • Characterized polycrystalline nickel microstructure using EBSD analysis. • Development meso-microstructural model based on real microstructure. • Calculated effective diffusivity using experimental electrochemical permeation test. • Calculated intergranular diffusivity of hydrogen using computational FE simulation. • Validated the calculated computation simulation results with experimental results. - Abstract: Hydrogen induced intergranular embrittlement has been identified as a cause of failure of aerospace components such as combustion chambers made from electrodeposited polycrystalline nickel. Accurate computational analysis of this process requires knowledge of the differential in hydrogen transport in the intergranular and intragranular regions. The effective diffusion coefficient of hydrogen may be measured experimentally, though experimental measurement of the intergranular grain boundary diffusion coefficient of hydrogen requires significant effort. Therefore an approach to calculate the intergranular GB hydrogen diffusivity using finite element analysis was developed. The effective diffusivity of hydrogen in polycrystalline nickel was measured using electrochemical permeation tests. Data from electron backscatter diffraction measurements were used to construct microstructural representative volume elements including details of grain size and shape and volume fraction of grains and grain boundaries. A Python optimization code has been developed for the ABAQUS environment to calculate the unknown grain boundary diffusivity.

Diffusion-accomodated rigid-body translations along grain boundaries in nanostructured materials

International Nuclear Information System (INIS)

Bachurin, D.V.; Nazarov, A.A.; Shenderova, O.A.; Brenner, D.W.

2003-01-01

A model for the structural relaxation of grain boundaries (GBs) in nanostructured materials (NSMs) by diffusion-accommodated rigid body translations along GBs is proposed. The model is based on the results of recent computer simulations that have demonstrated that the GBs in NSMs retain a high-energy structure with random translational states due to severe geometrical constraints applied from neighboring grains (J. Appl. Phys. 78 (1995) 847; Scripta Metall. Mater. 33 (1995) 1245). The shear stresses within a GB caused by non-optimized rigid-body translations (RBTs) can be accommodated by diffusive flow of atoms along a GB. This mechanism is particularly important for low-angle and vicinal GBs, the energy of which noticeably depends on the rigid body translations. At moderate and high temperatures the model yields relaxation times that are very short and therefore GBs in NSMs can attain an equilibrium structure with optimized rigid body translations. In contrast, at room temperature the model predicts that in some metals non-equilibrium structures can be preserved for a long time, which may result in the observation of grain boundary structures different from those in coarse grained polycrystals

Diffusion-induced grain boundary migration during ion beam mixing of Au/Cu bilayers

International Nuclear Information System (INIS)

Alexander, D.E.; Baldo, P.M.; Rehn, L.E.

1992-09-01

Experiments were performed to evaluate the effect of 1.5 MeV Kr irradiation on diffusion-induced grain boundary migration (DIGM) in Au/Cu bilayers in the temperature range of 300≤T≤050K. The experimental results were consistent with DIGM occurring in bilayers both during irradiation and during annealing treatments. Rutherford backscattering spectrometry showed a nearly uniform distribution of Cu present through the entire thickness of appropriately prepared polycrystalline Au films irradiated or annealed at temperatures ≥400K. No parallel effect was seen in similarly treated single-crystal films. In each polycrystalline sample studied, irradiation resulted in greater amounts of Cu present uniformly in the Au compared to annealing-only. The magnitudes of measured Cu compositions were substantially greater than that expected solely from grain boundary diffusion. A simple analysis of the process indicated that ion irradiation affects DIGM by increasing the composition of Cu present in alloyed zones and/or by increasing the grain boundary velocity in the Au

Grain boundary diffusion of Dy films prepared by magnetron sputtering for sintered Nd–Fe–B magnets

Science.gov (United States)

Chen, W.; Luo, J. M.; Guan, Y. W.; Huang, Y. L.; Chen, M.; Hou, Y. H.

2018-05-01

Dy films, deposited on the surface of sintered Nd–Fe–B magnets by magnetron sputtering, were employed for grain boundary diffusion source. High coercivity sintered Nd–Fe–B magnets were successfully prepared. Effects of sputtering power and grain boundary diffusion processes (GBDP) on the microstructure and magnetic properties were investigated in detail. The dense and uniform Dy films were beneficial to prepare high coercivity magnets by GBDP. The maximum coercivity value of 1189 kA m‑1 could be shown, which was an amplification of 22.3%, compared with that of as-prepared Nd–Fe–B magnet. Furthermore, the improved remanence and maximum energy product were also achieved through tuning grain boundary diffusion processes. Our results demonstrated that the formation of (Nd, Dy)2Fe14B shell surrounding Nd2Fe14B grains and fine, uniform and continuous intergranular RE-rich phases jointly contribute to the improved coercivity.

The potential role of diffusion-induced grain-boundary migration in extended life prediction

International Nuclear Information System (INIS)

Handwerker, C.A.; Blendell, J.E.; Interrante, C.G.; Ahn, T.M.

1993-01-01

The selection of materials that are suitable for various high-level waste-packaging designs must reflect the need to meet requirements for long-term performance in repository environments that change with time. With this in mind, we examine how grain boundaries in materials are induced to migrate as a result of solute diffusion even at low temperatures, how the composition of the matrix material is changed significantly by this diffusion-induced grain boundary migration (DIGM), and how the changing microstructures and compositions during DIGM lead to major changes in materials performance, such as corrosion or embrittlement. Methods are discussed for prediction of the long-term behavior of materials affected by DIGM

Migration of liquid film and grain boundary in Mo-Ni induced by W diffusion

International Nuclear Information System (INIS)

Kang, H.K.; Hackney, S.; Yoon, D.N.

1988-01-01

The liquid films and grain boundaries in liquid phase sintered Mo-Ni alloy are observed to migrate during heat-treatment after adding W to the liquid matrix. Behind the migrating boundaries forms Mo-Ni-W solid solution with the W concentration decreasing with the migration distance because of W depletion in the liquid matrix. The migration rate during the heat-treatment at 1540 0 C after adding W decreases with the decreasing pretreatment sintering temperature. When the sintering temperature is 1420 0 C, the migration rate is almost reduced to O. Under this condition, the coherency strain due to the simultaneous diffusion of W and Ni into the grain surfaces is estimated to be almost O. The results thus show that the coherency strain due to lattice diffusion is the driving force for the liquid film and grain boundary migration

Disorder-induced transition from grain boundary to bulk dominated ionic diffusion in pyrochlores

International Nuclear Information System (INIS)

Perriot, Romain; Dholabhai, Pratik P.; Uberuaga, Blas P.

2017-01-01

In this paper, we use molecular dynamics simulations to investigate the role of grain boundaries (GBs) on ionic diffusion in pyrochlores, as a function of the GB type, chemistry of the compound, and level of cation disorder. We observe that the presence of GBs promotes oxygen transport in ordered and low-disordered systems, as the GBs are found to have a higher concentration of mobile carriers with higher mobilities than in the bulk. Thus, in ordered samples, the ionic diffusion is 2D, localized along the grain boundary. When cation disorder is introduced, bulk carriers begin to contribute to the overall diffusion, while the GB contribution is only slightly enhanced. In highly disordered samples, the diffusive behavior at the GBs is bulk-like, and the two contributions (bulk vs. GB) can no longer be distinguished. There is thus a transition from 2D/GB dominated oxygen diffusivity to 3D/bulk dominated diffusivity versus disorder in pyrochlores. Finally, these results provide new insights into the possibility of using internal interfaces to enhance ionic conductivity in nanostructured complex oxides.

Thermodynamic model for grain boundary effects on hydrogen solubility, diffusivity and permeability in poly-crystalline tungsten

Energy Technology Data Exchange (ETDEWEB)

Oda, Takuji, E-mail: oda@snu.ac.kr

2016-11-15

Highlights: • A thermodynamic model to simulate grain boundary effects on hydrogen behaviors in poly-crystalline W was established. • With this model, the effective solubility, diffusivity and permeability of hydrogen are calculated as a function of grain size. • Grain boundary significantly change the hydrogen behaviors in poly-crystalline W up to around 1000 K. - Abstract: A thermodynamic model to evaluate effects of grain boundary (GB) on hydrogen behaviors in poly-crystalline tungsten is established. With this model, the effective solubility, diffusivity and permeability of hydrogen in tungsten equilibrated with surrounding H{sub 2} gas can be calculated as a function of grain size, temperature and H{sub 2} partial pressure. By setting 1.0 eV to the binding energy of hydrogen to GBs and 0.4 eV to the diffusion barrier of hydrogen along GBs, the model reasonably reproduces some experimental data on the effective diffusivity and permeability. Comparisons between calculation results by the model and available experimental data show that GBs significantly affect the hydrogen behaviors up to around 1000 K or higher in practical materials. Therefore, the effects of GBs need to be considered in analysis of experimental results, for which the present model can be utilized, and in prediction of tritium inventory and leakage in fusion reactors.

Coercivity enhancement and thermal-stability improvement in the melt-spun NdFeB ribbons by grain boundary diffusion

Science.gov (United States)

Xie, Jiajun; Yuan, Chao; Luo, Yang; Yang, Yuanfei; Hu, Bin; Yu, Dunbo; Yan, Wenlong

2018-01-01

Rapidly quenched NdFeB ribbons with high coercivity were obtained by Nd70Cu30 diffusion process. Samples with a high coercivity of 22.02 kOe at room temperature were obtained after grain boundary diffusion with 20 wt% Nd70Cu30 alloys. The NdCu diffusion process promoted grain growth in the ribbons, and grain boundary phases were formed with Cu segregation among NdFeB grains. Coercivity above 10 kOe at 150 °C was achieved in the bonded magnets with NdCu content over 10 wt%. The flux loss of bonded magnets was reduced by ∼32% at 120 °C after diffusion treatment with only a small amount (2 wt%) of NdCu.

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films.

Science.gov (United States)

Ciesielski, Richard; Schäfer, Frank; Hartmann, Nicolai F; Giesbrecht, Nadja; Bein, Thomas; Docampo, Pablo; Hartschuh, Achim

2018-03-07

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects. Whereas long-ranged diffusive charge carrier transport is observed within single crystals, no detectable diffusive transport occurs across grain boundaries. The observed PL transients are found to crucially depend on the microscopic geometry of the crystal and the point of observation. In particular, spatially restricted diffusion of charge carriers leads to slower PL decay near crystal edges as compared to the crystal center. In contrast to many reports in the literature, our experimental results show no quenching or additional loss channels due to grain boundaries for the studied material, which thus do not negatively affect the performance of the derived thin-film devices.

Hydrogen diffusion along grain boundaries in erbium oxide coatings

International Nuclear Information System (INIS)

Mao, Wei; Chikada, Takumi; Suzuki, Akihiro; Terai, Takayuki

2014-01-01

Diffusion of interstitial atomic hydrogen in erbium oxide (Er 2 O 3 ) was investigated using density functional theory (DFT) and molecular dynamics (MD) methods. Hydrogen diffusivity in bulk, on (0 0 1) surface, and along Σ13 (4–3–1)/[1 1 1] symmetric tilt grain boundaries (GBs) were evaluated in a temperature range of 673–1073 K, as well as hydrogen diffusion barriers. It was found that H diffusion shows the faster on (0 0 1) surface than along GBs and in bulk. Also, energy barrier of H diffusion in bulk estimated by DFT and MD methods is somewhat higher than that along GBs evaluated in the experiments. This suggests that H diffusion in Er 2 O 3 coatings depends on GBs rather than bulk. In addition, with a correction of GB density, the simulated diffusivity along GBs in MD simulations is in good agreement with the experimental data within one order of magnitude. The discrepancy of H diffusivity between the experiments and the simulations should be reduced by considering H concentration, H diffusion direction, deviations of the initial configuration, vacancy defects, etc

Auto and hetero-diffusion along grain and interphase boundaries in α-Zr and Zr-2.5wt%Nb

International Nuclear Information System (INIS)

Dyment, F.; Iribarren, M.J.; Vieregge, K.; Herzig, C.

1993-01-01

Grain-boundary diffusion measurements made in α-Zr and interphase-boundary diffusion measurements made in the (α+β) region of Zr-2.5wt%Nb were considered together with the aim of gaining a better understanding of the behaviour of these boundaries in Zr-based materials which are relevant for the nuclear industry. When comparing the total set of data it turns out that, from the diffusion point of view, both types of boundaries provide similar short-circuit diffusion paths. (orig.)

Grain boundary imaging, gallium diffusion and the fracture behavior of Al-Zn Alloy - An in situ study

CERN Document Server

Tsai, W L; Chen, C H; Chang, L W; Je, J H; Lin, H M; Margaritondo, G

2003-01-01

Phase contrast radiology using unmonochromatic synchrotron X-ray successfully imaged the grain boundaries of Al and AlZn alloy without contrast agent. Combining the high penetration of X-ray and the possibility of 3D reconstruction by tomorgraphy or stereography method, this approach can be very used for nondestructive characterization of polycrystalline materials. By examine the images with 3D perspective, we were able locate the observed void-like defects which lies exclusively on the grain boundary and identify their origin from last stage of the rolling process. We studied the Ga Liquid metal diffusion in the AlZn alloy, under different temperature and stress conditions. High resolution images, approx 2 mu m, of Ga liquid metal diffusion in AlZn were obtained in real time and diffusion paths alone grain boundaries and surfaces were clearly identified. Embrittled AlZn responses to the tensile stress and fractures in a drastic different manner than the pure AlZn. These results, although very much expected f...

Study of some properties of point defects in grain boundaries

International Nuclear Information System (INIS)

Martin, Georges

1973-01-01

With the aim of deducing simple informations on the grain boundary core structure, we investigated self diffusion under hydrostatic pressure, impurity diffusion (In and Au), electromigration (Sb) along certain types of grain boundaries in Ag bicrystals, and the Moessbauer effect of 57 Co located in the grain boundaries of polycrystalline Be. Our results lead to the following conclusions: the formation of a vacancy like defects is necessary to grain boundary diffusion; solute atoms may release most of their elastic energy of dissolution as they segregate at the boundary; in an electrical field, the drift of Sb ions parallel to the boundary takes place toward the anode as in the bulk. The force on the grain boundary ions is larger than in the bulk; Moessbauer spectroscopy revealed the formation of Co-rich aggregates, which may proves important in the study of early stages of grain boundary precipitation. (author) [fr

A Study of diffusion and grain boundaries in ionic compounds by the molecular dynamic method

International Nuclear Information System (INIS)

Karakasidis, Theodoros

1995-01-01

In the first part, we present a model of variable cationic charges based on a rigid ion potential. In order to implement the model we performed static and dynamic simulations in calcium fluoride. The structural properties do not depend on the way the model is adjusted but the anion diffusion and the high frequency dielectric constant do. These results allowed to specify the criteria to adjust the variable charge model. As indicated by the behaviour of optical phonons this model introduced a supplementary polarisation mechanism to the rigid ion model. In the second part of this work, we studied the structural and diffusional behaviour of a high angle tilt grain boundary in NiO by molecular dynamics, using a usual rigid ion model. We examined structures with and without point defects between 0 K and 2500 K. The structure without defects presents always the lowest potential energy. In the others structures the defects can cluster and sometimes cause local changes in the boundary. Computer simulated images of high resolution electron microscopy, produced using these structures, present a similarity with the experimental ones. We calculated in the same boundary the diffusion coefficient of a doubly charged nickel vacancy between 2250 K and 2650 K. The atomic trajectories reveal the existence of preferential migration paths for the vacancy. The grain boundary diffusion is slightly anisotropic which is in agreement with an extrapolation of experimental results. A similar study in the volume reveals a migration energy higher than in the grain boundary. The calculated quantities allow for an estimation of the nickel diffusion acceleration due to the boundary. This acceleration is significant, but lower than the one measured by certain authors in polycrystalline, NiO; other authors studying bicrystals have not observed any acceleration. (author) [fr

Diffusion of He interstitial and di-He cluster at grain boundaries in α-Fe

International Nuclear Information System (INIS)

Gao, F.; Heinisch, H.L.; Kurtz, R.J.

2007-01-01

A systematic molecular dynamics study of the diffusion mechanisms of He interstitial and di-He cluster at two representative interfaces has been carried out in α-Fe. The diffusion coefficient of a He interstitial and the effective migration energies were determined. The He atom diffuses along the Σ11 grain boundary one-dimensionally along specific directions, while it migrates two-dimensionally at low temperatures, and three-dimensionally at higher temperatures, in the Σ3 grain boundary. The di-He interstitial cluster can migrate rapidly along the Σ3 interface at low temperatures, but not at the Σ11 interface. It has been observed that a di-He interstitial cluster can kick out a self interstitial atom (SIA) at high temperatures, forming a He 2 V complex. The SIA migrates rapidly near interfaces, whereas the He 2 V complex is immobile at the temperatures considered. This small cluster may serve as the smallest nucleation for the formation of helium bubbles at interfaces

A constitutive model of nanocrystalline metals based on competing grain boundary and grain interior deformation mechanisms

KAUST Repository

Gurses, Ercan

2011-12-01

In this work, a viscoplastic constitutive model for nanocrystalline metals is presented. The model is based on competing grain boundary and grain interior deformation mechanisms. In particular, inelastic deformations caused by grain boundary diffusion, grain boundary sliding and dislocation activities are considered. Effects of pressure on the grain boundary diffusion and sliding mechanisms are taken into account. Furthermore, the influence of grain size distribution on macroscopic response is studied. The model is shown to capture the fundamental mechanical characteristics of nanocrystalline metals. These include grain size dependence of the strength, i.e., both the traditional and the inverse Hall-Petch effects, the tension-compression asymmetry and the enhanced rate sensitivity. © 2011 Elsevier B.V. All rights reserved.

Grain boundary imaging, gallium diffusion and the fracture behavior of Al-Zn Alloy - An in situ study

Science.gov (United States)

Tsai, W. L.; Hwu, Y.; Chen, C. H.; Chang, L. W.; Je, J. H.; Lin, H. M.; Margaritondo, G.

2003-01-01

Phase contrast radiology using unmonochromatic synchrotron X-ray successfully imaged the grain boundaries of Al and AlZn alloy without contrast agent. Combining the high penetration of X-ray and the possibility of 3D reconstruction by tomorgraphy or stereography method, this approach can be very used for nondestructive characterization of polycrystalline materials. By examine the images with 3D perspective, we were able locate the observed void-like defects which lies exclusively on the grain boundary and identify their origin from last stage of the rolling process. We studied the Ga Liquid metal diffusion in the AlZn alloy, under different temperature and stress conditions. High resolution images, ˜2 μm, of Ga liquid metal diffusion in AlZn were obtained in real time and diffusion paths alone grain boundaries and surfaces were clearly identified. Embrittled AlZn responses to the tensile stress and fractures in a drastic different manner than the pure AlZn. These results, although very much expected from the known weakening effect of the liquid metal embrittlement demonstrated, however, that this particular radiology method is fully capable of dynamic study in the micrometer scale.

Separate measurement of local diffusion coefficients in grain boundaries and in adjacent regions

International Nuclear Information System (INIS)

Klotsman, S.M.; Kajgorodov, V.N.

1994-01-01

A new measuring technique is presented that allows one separate determination of grain boundary width and local diffusion coefficients. With the use of the technique presented phenomenological description is accompished for time and temperature dependences of relative and absolute level populations in a zone of preferential intercrystalline diffusion. Local diffusion coefficients obtained for the upper temperature limit of applicability of the technique proposed are in a good agreement with values calculated form coordinate distribution of atoic probes. Local diffusion coefficients determined at lower temperatures essentially differ from those calculated assuming that suction coefficient is equal to a coefficient of volume diffusion. Experimental dta are given for diffusion parameters in Ag, Pd and W polycrystals. 16 refs., 3 figs., 2 tabs

Grain boundary diffusion and segregation of Ni in Cu

International Nuclear Information System (INIS)

Divinski, Sergiy; Ribbe, Jens; Schmitz, Guido; Herzig, Christian

2007-01-01

Grain boundary (GB) diffusion of 63 Ni in polycrystalline Cu was investigated by the radiotracer technique in an extended temperature interval from 476 to 1156K. The independent measurements in Harrison's C and B kinetic regimes resulted in direct data of the GB diffusivity D gb and of the so-called triple product P=s.δ.D gb (s and δ are the segregation factor and the diffusional GB width, respectively). Arrhenius-type temperature dependencies for both the D gb and P values were measured, resulting in the pre-exponential factors D gb 0 =6.93x10 -7 m 2 s -1 and P 0 =1.89x10 -16 m 3 s -1 and the activation enthalpies of 90.4 and 73.8kJmol -1 , respectively. Although Ni is completely soluble in Cu, it reveals a distinct but still moderate ability to segregate copper GBs with a segregation enthalpy of about -17kJmol -1

Mathematical simulation of point defect interaction with grain boundaries

International Nuclear Information System (INIS)

Bojko, V.S.

1987-01-01

Published works, where the interaction of point defects and grain boundaries was studied by mathematical simulation methods, have been analysed. Energetics of the vacancy formation both in nuclei of large-angle special grain boundaries and in lattice regions adjoining them has been considered. The data obtained permit to explain specific features of grain-boundary diffusion processes. Results of mathematical simulation of the interaction of impurity atoms and boundaries have been considered. Specific features of the helium atom interaction with large-angle grain boundaries are analysed as well

A study of diffusion and grain boundaries in ionic compounds by the molecular dynamic method; Etude par dynamique moleculaire de la diffusion et des joints de grains dans les composes ioniques

Energy Technology Data Exchange (ETDEWEB)

Karakasidis, T

1995-11-13

In the first part, we present a model of variable cationic charges based on a rigid ion potential. In order to implement the model we performed static and dynamic simulations in calcium fluoride. The structural properties do not depend on the way the model is adjusted but the anion diffusion and the high frequency dielectric constant do. These results allowed to specify the criteria to adjust the variable charge model. As indicated by the behaviour of optical phonons this model introduced a supplementary polarisation mechanism to the rigid ion model. In the second part of this work, we studied the structural and diffusional behaviour of a high angle tilt grain boundary in NiO by molecular dynamics, using a usual rigid ion model. We examined structures with and without point defects between 0 K and 2500 K. The structure without defects presents always the lowest potential energy. In the others structures the defects can cluster and sometimes cause local changes in the boundary. Computer simulated images of high resolution electron microscopy, produced using these structures, present a similarity with the experimental ones. We calculated in the same boundary the diffusion coefficient of a doubly charged nickel vacancy between 2250 k and 2650 K. The atomic trajectories reveal the existence of preferential migration paths for the vacancy. The grain boundary diffusion is slightly anisotropic which is in agreement with an extrapolation of experimental results. A similar study in the volume reveals a migration energy higher than in the grain boundary. The calculated quantities allow for an estimation of the nickel diffusion acceleration due to the boundary. This acceleration is significant, but lower than the one measured by certain authors in polycrystalline, NiO; other authors studying bicrystals have not observed any certain authors in polycrystalline, NiO; other authors studying bicrystals have not observed any acceleration. (author) 118 refs.

Self-diffusion and heterodiffusion in Zr-2.5%Nb α/β interfaces comparison with grain boundary diffusion in α-Zr

International Nuclear Information System (INIS)

Iribarren, M.J.; Dyment, F.

1991-01-01

Conventional radioactive tracer section techniques were used to make an experimental determination of diffusion parameters for Zr, Nb and Ni along the α/β boundary interfaces of Zr-2.5%Nb and comparing them with those for Zr, Nb and Co in α-Zr grain boundaries. Both determinations were made for a wide range of temperatures, including reactor working temperatures. Different materials were used for this purpose, both specially prepared alloys for diffusion experiments and part of the material from the actual pressure tubes. Different stabilizing thermal treatments were performed and results were analyzed based on the different morphologies obtained. (Author) [es

Interactions of impurities with a moving grain boundary

Energy Technology Data Exchange (ETDEWEB)

Bauer, C L [Carnegie-Mellon Univ., Pittsburgh, Pa. (USA)

1975-01-01

Most theories developed to explain interaction of impurities with a moving grain boundary involve a uniform excess impurity concentration distributed along a planar grain boundary. As boundary velocity increases, the excess impurities exert a net drag force on the boundary until a level is reached whereat the drag force no longer can balance the driving force and breakaway of the boundary from these impurities occurs. In this investigation, assumptions of a uniform lateral impurity profile and a planar grain boundary shape are relaxed by allowing both forward and lateral diffusion of impurities in the vicinity of a grain boundary. It is found that the two usual regions (drag of impurities by, and breakaway of a planar grain boundary) are separated by an extensive region wherein a uniform lateral impurity profile and a planar grain boundary shape are unstable. It is suspected that, in this unstable region, grain boundaries assume a spectrum of more complex morphologies and that elucidation of these morphologies can provide the first definitive description of the breakaway process and insight to more complex phenomena such as solid-solution strengthening, grain growth and secondary recrystallization.

Measurement and modeling of radiation-induced grain boundary grain boundary segregation in stainless steels

International Nuclear Information System (INIS)

Bruemmer, S.M.; Charlot, L.A.; Simonen, E.P.

1995-08-01

Grain boundary radiation-induced segregation (RIS) in Fe-Ni-Cr stainless alloys has been measured and modelled as a function of irradiation temperature and dose. Heavy-ion irradiation was used to produce damage levels from 1 to 20 displacements per atom (dpa) at temperatures from 175 to 550 degrees C. Measured Fe, Ni, and Cr segregation increased sharply with irradiation dose (from 0 to 5 dpa) and temperature (from 175 to about 350 degrees C). However, grain boundary concentrations did not change significantly as dose or temperatures were further increased. Impurity segregation (Si and P) was also measured, but only Si enrichment appeared to be radiation-induced. Grain boundary Si levels peaked at an intermediate temperature of ∼325 degrees C reaching levels of ∼8 at. %. Equilibrium segregation of P was measured in the high-P alloys, but interfacial concentration did not increase with irradiation exposure. Examination of reported RIS in neutron-irradiated stainless steels revealed similar effects of irradiation dose on grain boundary compositional changes for both major alloying and impurity element's. The Inverse Kirkendall model accurately predicted major alloying element RIS in ion- and neutron-irradiated alloys over the wide range of temperature and dose conditions. In addition, preliminary calculations indicate that the Johnson-Lam model can reasonably estimate grain boundary Si enrichment if back diffusion is enhanced

UO2 Grain Growth: Developing Phase Field Models for Pore Dragging, Solute Dragging and Anisotropic Grain Boundary Energies

International Nuclear Information System (INIS)

Ahmed, K.; Tonks, M.; Zhang, Y.; Biner, B.

2016-01-01

A detailed phase field model for the effect of pore drag on grain growth kinetics was implemented in MARMOT. The model takes into consideration both the curvature-driven grain boundary motion and pore migration by surface diffusion. As such, the model accounts for the interaction between pore and grain boundary kinetics, which tends to retard the grain growth process. Our 2D and 3D simulations demonstrate that the model capture all possible pore-grain boundary interactions proposed in theoretical models. For high enough surface mobility, the pores move along with the migrating boundary as a quasi-rigid-body, albeit hindering its migration rate compared to the pore-free case. For less mobile pores, the migrating boundary can separate from the pores. For the pore-controlled grain growth kinetics, the model predicts a strong dependence of the growth rate on the number of pores, pore size, and surface diffusivity in agreement with theroretical models. An evolution equation for the grain size that includes these parameters was derived and showed to agree well with numerical solution. It shows a smooth transition from boundary-controlled kinetics to pore-controlled kinetics as the surface diffusivity decreases or the number of pores or their size increases. This equation can be utilized in BISON to give accurate estimate for the grain size evolution. This will be accomplished in the near future. The effect of solute drag and anisotropy of grain boundary on grain growth will be investigated in future studies.

Advection-diffusion model for normal grain growth and the stagnation of normal grain growth in thin films

International Nuclear Information System (INIS)

Lou, C.

2002-01-01

An advection-diffusion model has been set up to describe normal grain growth. In this model grains are divided into different groups according to their topological classes (number of sides of a grain). Topological transformations are modelled by advective and diffusive flows governed by advective and diffusive coefficients respectively, which are assumed to be proportional to topological classes. The ordinary differential equations governing self-similar time-independent grain size distribution can be derived analytically from continuity equations. It is proved that the time-independent distributions obtained by solving the ordinary differential equations have the same form as the time-dependent distributions obtained by solving the continuity equations. The advection-diffusion model is extended to describe the stagnation of normal grain growth in thin films. Grain boundary grooving prevents grain boundaries from moving, and the correlation between neighbouring grains accelerates the stagnation of normal grain growth. After introducing grain boundary grooving and the correlation between neighbouring grains into the model, the grain size distribution is close to a lognormal distribution, which is usually found in experiments. A vertex computer simulation of normal grain growth has also been carried out to make a cross comparison with the advection-diffusion model. The result from the simulation did not verify the assumption that the advective and diffusive coefficients are proportional to topological classes. Instead, we have observed that topological transformations usually occur on certain topological classes. This suggests that the advection-diffusion model can be improved by making a more realistic assumption on topological transformations. (author)

Study of rapid grain boundary migration in a nanocrystalline Ni thin film

International Nuclear Information System (INIS)

Kacher, Josh; Robertson, I.M.; Nowell, Matt; Knapp, J.; Hattar, Khalid

2011-01-01

Research highlights: → Abnormal growth is distributed randomly in the foil and initiates at different times. → Growth occurs from seemingly uncorrelated regions of the grain boundary. → Growth twins are created during all stages of abnormal grain growth. → Grain growth patterns are qualitatively similar to a vacancy diffusion model. → Grain boundaries and orientations evolve during growth to minimize system energy. - Abstract: Grain boundary migration associated with abnormal grain growth in pulsed-laser deposited Ni was studied in real time by annealing electron transparent films in situ in the transmission electron microscope. The resulting texture evolution and grain boundary types produced were evaluated by ex situ electron backscatter diffraction of interrupted anneals. The combination of these two techniques allowed for the investigation of grain growth rates, grain morphologies, and the evolution of the orientation and grain boundary distributions. Grain boundaries were found to progress in a sporadic, start/stop fashion with no evidence of a characteristic grain growth rate. The orientations of the abnormally growing grains were found to be predominately //ND throughout the annealing process. A high fraction of twin boundaries developed during the annealing process. The intermittent growth from different locations of the grain boundary is discussed in terms of a vacancy diffusion model for grain growth.

Creep of quartz by dislocation and grain boundary processes

Science.gov (United States)

Fukuda, J. I.; Holyoke, C. W., III; Kronenberg, A. K.

2015-12-01

Wet polycrystalline quartz aggregates deformed at temperatures T of 600°-900°C and strain rates of 10-4-10-6 s-1 at a confining pressure Pc of 1.5 GPa exhibit plasticity at low T, governed by dislocation glide and limited recovery, and grain size-sensitive creep at high T, governed by diffusion and sliding at grain boundaries. Quartz aggregates were HIP-synthesized, subjecting natural milky quartz powder to T=900°C and Pc=1.5 GPa, and grain sizes (2 to 25 mm) were varied by annealing at these conditions for up to 10 days. Infrared absorption spectra exhibit a broad OH band at 3400 cm-1 due to molecular water inclusions with a calculated OH content (~4000 ppm, H/106Si) that is unchanged by deformation. Rate-stepping experiments reveal different stress-strain rate functions at different temperatures and grain sizes, which correspond to differing stress-temperature sensitivities. At 600-700°C and grain sizes of 5-10 mm, flow law parameters compare favorably with those for basal plasticity and dislocation creep of wet quartzites (effective stress exponents n of 3 to 6 and activation enthalpy H* ~150 kJ/mol). Deformed samples show undulatory extinction, limited recrystallization, and c-axis maxima parallel to the shortening direction. Similarly fine-grained samples deformed at 800°-900°C exhibit flow parameters n=1.3-2.0 and H*=135-200 kJ/mol corresponding to grain size-sensitive Newtonian creep. Deformed samples show some undulatory extinction and grain sizes change by recrystallization; however, grain boundary deformation processes are indicated by the low value of n. Our experimental results for grain size-sensitive creep can be compared with models of grain boundary diffusion and grain boundary sliding using measured rates of silicon grain boundary diffusion. While many quartz mylonites show microstructural and textural evidence for dislocation creep, results for grain size-sensitive creep may apply to very fine-grained (<10 mm) quartz mylonites.

Faceted shell structure in grain boundary diffusion-processed sintered Nd–Fe–B magnets

International Nuclear Information System (INIS)

Seelam, U.M.R.; Ohkubo, T.; Abe, T.; Hirosawa, S.; Hono, K.

2014-01-01

Graphical abstract: The grain boundary diffusion process (GBDP) using a heavy rare earth elements (HRE) such as Dy and Tb is known as an effective method to enhance the coercivity of Nd–Fe–B sintered magnets without reducing remanence. This process has been industrially implemented to manufacture Nd–Fe–B based sintered magnets with high coercivity and high remanence. In this process, Dy is considered to diffuse through grain boundaries (GBs) to form (Nd 1−x Dy x ) 2 Fe 14 B shells surrounding the Nd 2 Fe 14 B grains and the higher anisotropy field of the Dy-rich shell is considered to suppress the nucleation of reverse domains at low magnetic field. Although there are several investigations on the microstructure of HRE GBDP Nd–Fe–B magnets, no paper addressed the origin of the asymmetric formation of HRE rich shells. Based on detailed analysis of facet planes of core/shell interfaces, we propose a mechanism of the faceted core/shell microstructure formation in the GBDP sintered magnets. We believe that this gives new insights on understanding the coercivity enhancement by the GBDP. - Highlights: • Faceting was observed at the interfaces of cores and shells. • The core/shell interfaces are sharp with an abrupt change in Dy concentration. • Meting occurs at the interfaces of metalic Nd-rich/Nd 2 Fe 14 B phases above 685 °C due to eutectic reaction. • Solidification of Dy-enriched liquid phase from 900 °C can result in the shell formation. - Abstract: Dysprosium enriched shell structure formed by the grain boundary diffusion process (GBDP) of a sintered Nd–Fe–B magnet was characterized by using scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Faceted core–shell interfaces with an abrupt change in Dy concentration suggest the Dy-rich shells are formed by the solidification of the liquid phase during cooling from the GBDP temperature. The Nd-rich phases are almost free from Dy, and

Structure, electronic properties, and oxygen incorporation/diffusion characteristics of the Σ 5 TiN(310)[001] tilt grain boundary

Science.gov (United States)

McKenna, Keith P.

2018-02-01

First principles calculations are employed to investigate the structure, electronic properties, and oxygen incorporation/diffusion characteristics of the Σ 5 TiN(310) tilt grain boundary with relevance to applications of polycrystalline TiN in microelectronics and protective coatings. We show that the grain boundary does not significantly modify electronic states near the Fermi energy but does induce an upward shift of up to 0.6 eV in a number of deeper occupied bands. We also show that oxygen is preferentially incorporated into the TiN grain boundary (GB) but must overcome relatively high activation energies for further diffusion. These predictions are consistent with the "stuffed barrier model" proposed to explain the good barrier characteristics of TiN. We also show that while the oxidizing power of TiN GBs is not sufficient to reduce HfO2 (a prototypical gate dielectric material), they can act as a scavenger for interstitial oxygen. Altogether, these results provide the much needed atomistic insights into the properties of a model GB in TiN and suggest a number of directions for future investigation.

Strong trapping and slow diffusion of helium in a tungsten grain boundary

Energy Technology Data Exchange (ETDEWEB)

Wang, Xin-Xin [Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 (China); School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026 (China); Niu, Liang-Liang, E-mail: nliangli@umich.edu [Department of Physics, Beihang University, Beijing 100191 (China); Department of Nuclear Engineering and Radiological Science, University of Michigan, Ann Arbor, MI 48109 (United States); Wang, Shaoqing [Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 (China)

2017-04-15

We have investigated the segregation, trapping and diffusion of He in a ∑3<110>{111} W grain boundary (GB) using combined techniques of ab initio and classical atomistic simulations. We show that, with an average segregation energy of −3.20 eV, the strong He trapping can be attributed to a GB interstitial trapping or a vacancy trapping mechanism, while an average energy barrier of 1.97 eV leads to a slow diffusion of He in the GB plane. We further reveal by molecular dynamics simulations that the He diffusion will be dictated by GB migration through the motion of GB disconnections. Interestingly, we also observe a He-induced GB structural transition in classical simulations. The present work suggests that the GB does not provide fast transport channel for He, providing useful reference for the possible application of polycrystalline W under He irradiation in advanced nuclear fusion reactors.

Properties of grain boundaries in BCC iron and iron-based alloys

International Nuclear Information System (INIS)

Terentyev, D.; He, Xinfu

2010-01-01

The report contains a summary of work done within the collaboration established between SCK-CEN and CIEA, performed during the internship of Xinfu He (CIAE) in the period of September 2009 to June 2010. In this work, we have carried out an atomistic study addressing the properties of grain boundaries in BCC Fe and Fe-Cr alloys. Throughout this work we report on the structural and cohesive properties of grain boundaries; thermal stability; interaction of grain boundaries with He and diffusivity of He in the core of the grain boundaries; equilibrium segregation of Cr near the grain boundary zone; cleavage fracture of grain boundaries; influence of the Cr precipitates, voids and He bubbles on the structure and strength of grain boundaries.

Properties of grain boundaries in BCC iron and iron-based alloys

Energy Technology Data Exchange (ETDEWEB)

Terentyev, D.; He, Xinfu

2010-08-15

The report contains a summary of work done within the collaboration established between SCK-CEN and CIEA, performed during the internship of Xinfu He (CIAE) in the period of September 2009 to June 2010. In this work, we have carried out an atomistic study addressing the properties of grain boundaries in BCC Fe and Fe-Cr alloys. Throughout this work we report on the structural and cohesive properties of grain boundaries; thermal stability; interaction of grain boundaries with He and diffusivity of He in the core of the grain boundaries; equilibrium segregation of Cr near the grain boundary zone; cleavage fracture of grain boundaries; influence of the Cr precipitates, voids and He bubbles on the structure and strength of grain boundaries.

Direct imaging of thermally-activated grain-boundary diffusion in Cu/Co/IrMn/Pt exchange-bias structures using atom-probe tomography

Energy Technology Data Exchange (ETDEWEB)

Letellier, F.; Lardé, R.; Le Breton, J.-M., E-mail: jean-marie.lebreton@univ-rouen.fr [Groupe de Physique des Matériaux, UMR 6634 CNRS/Université et INSA de Rouen, F-76801 Saint Etienne du Rouvray (France); Lechevallier, L. [Groupe de Physique des Matériaux, UMR 6634 CNRS/Université et INSA de Rouen, F-76801 Saint Etienne du Rouvray (France); Département de GEII, Université de Cergy-Pontoise, F-95031 Cergy-Pontoise (France); Akmaldinov, K. [SPINTEC, Univ. Grenoble-Alpes/CNRS/INAC-CEA, F-38000 Grenoble (France); CROCUS Technology, F-38025 Grenoble (France); Auffret, S.; Dieny, B.; Baltz, V., E-mail: vincent.baltz@cea.fr [SPINTEC, Univ. Grenoble-Alpes/CNRS/INAC-CEA, F-38000 Grenoble (France)

2014-11-28

Magnetic devices are often subject to thermal processing steps, such as field cooling to set exchange bias and annealing to crystallize amorphous magnetic electrodes. These processing steps may result in interdiffusion and the subsequent deterioration of magnetic properties. In this study, we investigated thermally-activated diffusion in Cu/Co/IrMn/Pt exchange biased polycrystalline thin-film structures using atom probe tomography. Images taken after annealing at 400 °C for 60 min revealed Mn diffusion into Co grains at the Co/IrMn interface and along Pt grain boundaries for the IrMn/Pt stack, i.e., a Harrison type C regime. Annealing at 500 °C showed further Mn diffusion into Co grains. At the IrMn/Pt interface, annealing at 500 °C led to a type B behavior since Mn diffusion was detected both along Pt grain boundaries and also into Pt grains. The deterioration of the films' exchange bias properties upon annealing was correlated to the observed diffusion. In particular, the topmost Pt capping layer thickness turned out to be crucial since a faster deterioration of the exchange bias properties for thicker caps was observed. This is consistent with the idea that Pt acts as a getter for Mn, drawing Mn out of the IrMn layer.

Faceted shell structure in grain boundary diffusion-processed sintered Nd–Fe–B magnets

Energy Technology Data Exchange (ETDEWEB)

Seelam, U.M.R.; Ohkubo, T.; Abe, T.; Hirosawa, S.; Hono, K., E-mail: kazuhiro.hono@nims.go.jp

2014-12-25

Graphical abstract: The grain boundary diffusion process (GBDP) using a heavy rare earth elements (HRE) such as Dy and Tb is known as an effective method to enhance the coercivity of Nd–Fe–B sintered magnets without reducing remanence. This process has been industrially implemented to manufacture Nd–Fe–B based sintered magnets with high coercivity and high remanence. In this process, Dy is considered to diffuse through grain boundaries (GBs) to form (Nd{sub 1−x}Dy{sub x}){sub 2}Fe{sub 14}B shells surrounding the Nd{sub 2}Fe{sub 14}B grains and the higher anisotropy field of the Dy-rich shell is considered to suppress the nucleation of reverse domains at low magnetic field. Although there are several investigations on the microstructure of HRE GBDP Nd–Fe–B magnets, no paper addressed the origin of the asymmetric formation of HRE rich shells. Based on detailed analysis of facet planes of core/shell interfaces, we propose a mechanism of the faceted core/shell microstructure formation in the GBDP sintered magnets. We believe that this gives new insights on understanding the coercivity enhancement by the GBDP. - Highlights: • Faceting was observed at the interfaces of cores and shells. • The core/shell interfaces are sharp with an abrupt change in Dy concentration. • Meting occurs at the interfaces of metalic Nd-rich/Nd{sub 2}Fe{sub 14}B phases above 685 °C due to eutectic reaction. • Solidification of Dy-enriched liquid phase from 900 °C can result in the shell formation. - Abstract: Dysprosium enriched shell structure formed by the grain boundary diffusion process (GBDP) of a sintered Nd–Fe–B magnet was characterized by using scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Faceted core–shell interfaces with an abrupt change in Dy concentration suggest the Dy-rich shells are formed by the solidification of the liquid phase during cooling from the GBDP temperature. The Nd-rich phases

Evidence of preferential diffusion and segregation of impurities at grain boundaries in very pure niobium used for radiofrequency cavities

International Nuclear Information System (INIS)

Antoine, C.; Bonin, B.; Safa, H.; Berthier, B.; Tessier, E.; Trocelier, P.; Chevarier, A.; Chevarier, N.; Roux, B.

1996-04-01

In order to overcome dissipation due to impurity segregation at grain boundary, niobium cavities are submitted to a purification annealing (1300 deg C ± 200 deg C under vacuum) during which titanium is evaporated onto the Nb surface. The resulting titanium layer acts as a solid state getter reacting with light impurities (H, C, N, O), thereby removing these impurities from the bulk of the niobium. Evidence of preferential titanium diffusion and segregation at grain boundaries has been studied using PIXE analysis induced by proton microbeam. (author)

Coercivity enhancement of hot-deformed Nd-Fe-B magnets by the eutectic grain boundary diffusion process

International Nuclear Information System (INIS)

Liu, Lihua; Sepehri-Amin, H.; Ohkubo, T.; Yano, M.; Kato, A.; Shoji, T.; Hono, K.

2016-01-01

Nd-M (M = Al, Cu, Ga, Zn, Mn) alloys with compositions close to eutectic points were investigated as diffusion sources for the grain boundary diffusion process to hot-deformed Nd-Fe-B magnets. Coercivity enhancement was observed for most of the alloys. Among them, the sample processed with Nd 90 Al 10 exhibited the highest coercivity of 2.5 T at room temperature. However, the sample processed with Nd 70 Cu 30 exhibited the highest coercivity of 0.7 T at 200 ° C. Microstructural observations using scanning transmission electron microscope (STEM) showed that nonferromagnetic Nd-rich intergranular phase envelops the Nd 2 Fe 14 B grains after the diffusion process. Abnormal grain growth and the dissolution of Al into the Nd 2 Fe 14 B grains were observed in the sample processed with Nd 90 Al 10 , which explains its inferior thermal stability of coercivity compared to the sample processed with Nd 70 Cu 30 . The coercivity enhancement and poor thermal stability of the coercivity of the Nd 90 Al 10 diffusion-processed sample are discussed based on microstructure studies by transmission electron microscopy. - Highlights: • Coercivity of hot-deformed Nd-Fe-B magnets is enhanced by the infiltration of various R-TM eutectic alloys. • The sample infiltrated with Nd 90 Al 10 shows the highest coercivity of 2.5 T at room temperature. • At 200 °C, Nd 70 Cu 30 diffusion-processed sample possesses the highest coercivity of 0.7 T.

Grain-boundary, glassy-phase identification and possible artifacts

International Nuclear Information System (INIS)

Simpson, Y.K.; Carter, C.B.; Sklad, P.; Bentley, J.

1985-01-01

Specimen artifacts such as grain boundary grooving, surface damage of the specimen, and Si contamination are shown experimentally to arise from the ion milling used in the preparation of transmission electron microscopy specimens. These artifacts in polycrystalline, ceramic specimens can cause clean grain boundaries to appear to contain a glassy phase when the dark-field diffuse scattering technique, the Fresnel fringe technique, and analytical electron microscopy (energy dispersive spectroscopy) are used to identify glassy phases at a grain boundary. The ambiguity in interpreting each of these techniques due to the ion milling artifacts will be discussed from a theoretical view point and compared to experimental results obtained for alumina

submitter Thermal stability of interface voids in Cu grain boundaries with molecular dynamic simulations

CERN Document Server

Xydou, A; Aicheler, M; Djurabekova, F

2016-01-01

By means of molecular dynamic simulations, the stability of cylindrical voids is examined with respect to the diffusion bonding procedure. To do this, the effect of grain boundaries between the grains of different crystallographic orientations on the void closing time was studied at high temperatures from 0.7 up to 0.94 of the bulk melting temperature $(T_m)$. The diameter of the voids varied from 3.5 to 6.5 nm. A thermal instability occurring at high temperatures at the surface of the void placed in a grain boundary triggered the eventual closure of the void at all examined temperatures. The closing time has an exponential dependence on the examined temperature values. A model based on the defect diffusion theory is developed to predict the closing time for voids of macroscopic size. The diffusion coefficient within the grain boundaries is found to be overall higher than the diffusion coefficient in the region around the void surface. The activation energy for the diffusion in the grain boundary is calculate...

Molecular dynamics study of grain boundary diffusion of hydrogen in tungsten

International Nuclear Information System (INIS)

Von Toussaint, U; Gori, S; Manhard, A; Höschen, T; Höschen, C

2011-01-01

Understanding the influence of the microstructure of tungsten on hydrogen transport is crucial for the use of tungsten as first-wall material in fusion reactors. Here, we report the results of molecular dynamics and transition state studies on the influence of grain boundaries in tungsten on the transport of hydrogen. An exhaustive mapping of possible minimum activation energy migration trajectories for hydrogen as the trace impurity reveals a strongly modified activation energy distribution in the neighborhood of grain boundaries together with an altered connectivity matrix. The results indicate that grain boundaries in polycrystalline tungsten may provide an important transport channel, especially for neutron-damaged tungsten.

Molecular dynamics simulations of self-diffusion near a symmetrical tilt grain boundary in UO{sub 2}

Energy Technology Data Exchange (ETDEWEB)

Vincent-Aublant, E.; Delaye, J.M. [CEA-Marcoule, DEN/DTCD/SECM, B.P. 17171, 30207 Bagnols sur Ceze cedex (France); Van Brutzel, L. [CEA-Saclay, DEN-DANS/DPC/SCP/LM2T, 91191 Gif-sur-Yvette (France)

2008-07-01

Molecular dynamics (MD) simulations have been used to study the influence of symmetrical tilt grain boundaries (GBs) in stoichiometric UO{sub 2} on uranium and oxygen self-diffusions. The study was performed on a large range of temperature varying from 300 K to 2100 K. First, the effect of the temperature on the structure and the formation energies of 6 relaxed tilt GBs was investigated. The {sigma}5 and {sigma}41 GBs geometries were chosen to study the diffusion. O and U diffusion coefficients have been calculated and compared to those obtained in a perfect stoichiometric UO{sub 2} as well as in over and under-stoichiometric matrices. (authors)

Anomalous diffusion of water molecules at grain boundaries in ice Ih.

Science.gov (United States)

Moreira, Pedro Augusto Franco Pinheiro; Veiga, Roberto Gomes de Aguiar; Ribeiro, Ingrid de Almeida; Freitas, Rodrigo; Helfferich, Julian; de Koning, Maurice

2018-05-23

Using ab initio and classical molecular dynamics simulations, we study pre-melting phenomena in pristine coincident-site-lattice grain boundaries (GBs) in proton-disordered hexagonal ice Ih at temperatures just below the melting point Tm. Concerning pre-melt-layer thicknesses, the results are consistent with the available experimental estimates for low-disorder impurity-free GBs. With regard to molecular mobility, the simulations provide a key new insight: the translational motion of the water molecules is found to be subdiffusive for time scales from ∼10 ns up to at least 0.1 μs. Moreover, the fact that the anomalous diffusion occurs even at temperatures just below Tm where the bulk supercooled liquid still diffuses normally suggests that it is related to the confinement of the GB pre-melt layers by the surrounding crystalline environment. Furthermore, we show that this behavior can be characterized by continuous-time random walk models in which the waiting-time distributions decay according to power-laws that are very similar to those describing dynamics in glass-forming systems.

Coercivity enhancement of hot-deformed Nd-Fe-B magnets by the eutectic grain boundary diffusion process

Energy Technology Data Exchange (ETDEWEB)

Liu, Lihua [Elements Strategy Initiative Center for Magnetic Materials, National Institute of Materials Science, Tsukuba 305-0047 (Japan); Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8577 (Japan); Sepehri-Amin, H.; Ohkubo, T. [Elements Strategy Initiative Center for Magnetic Materials, National Institute of Materials Science, Tsukuba 305-0047 (Japan); Yano, M.; Kato, A.; Shoji, T. [Toyota Motor Corporation, Advanced Material Engineering Div., Susono 410-1193 (Japan); Hono, K., E-mail: kazuhiro.hono@nims.go.jp [Elements Strategy Initiative Center for Magnetic Materials, National Institute of Materials Science, Tsukuba 305-0047 (Japan); Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8577 (Japan)

2016-05-05

Nd-M (M = Al, Cu, Ga, Zn, Mn) alloys with compositions close to eutectic points were investigated as diffusion sources for the grain boundary diffusion process to hot-deformed Nd-Fe-B magnets. Coercivity enhancement was observed for most of the alloys. Among them, the sample processed with Nd{sub 90}Al{sub 10} exhibited the highest coercivity of 2.5 T at room temperature. However, the sample processed with Nd{sub 70}Cu{sub 30} exhibited the highest coercivity of 0.7 T at 200 {sup °}C. Microstructural observations using scanning transmission electron microscope (STEM) showed that nonferromagnetic Nd-rich intergranular phase envelops the Nd{sub 2}Fe{sub 14}B grains after the diffusion process. Abnormal grain growth and the dissolution of Al into the Nd{sub 2}Fe{sub 14}B grains were observed in the sample processed with Nd{sub 90}Al{sub 10}, which explains its inferior thermal stability of coercivity compared to the sample processed with Nd{sub 70}Cu{sub 30}. The coercivity enhancement and poor thermal stability of the coercivity of the Nd{sub 90}Al{sub 10} diffusion-processed sample are discussed based on microstructure studies by transmission electron microscopy. - Highlights: • Coercivity of hot-deformed Nd-Fe-B magnets is enhanced by the infiltration of various R-TM eutectic alloys. • The sample infiltrated with Nd{sub 90}Al{sub 10} shows the highest coercivity of 2.5 T at room temperature. • At 200 °C, Nd{sub 70}Cu{sub 30} diffusion-processed sample possesses the highest coercivity of 0.7 T.

Competing Grain Boundary and Interior Deformation Mechanisms with Varying Sizes

Energy Technology Data Exchange (ETDEWEB)

Zhang, Wei [University of Tennessee (UT); Gao, Yanfei [ORNL; Nieh, T. G. [University of Tennessee, Knoxville (UTK)

2018-01-01

In typical coarse-grained alloys, the dominant plastic deformations are dislocation gliding or climbing, and material strengths can be tuned by dislocation interactions with grain boundaries, precipitates, solid solutions, and other defects. With the reduction of grain size, the increase of material strengths follows the classic Hall-Petch relationship up to nano-grained materials. Even at room temperatures, nano-grained materials exhibit strength softening, or called the inverse Hall-Petch effect, as grain boundary processes take over as the dominant deformation mechanisms. On the other hand, at elevated temperatures, grain boundary processes compete with grain interior deformation mechanisms over a wide range of the applied stress and grain sizes. This book chapter reviews and compares the rate equation model and the microstructure-based finite element simulations. The latter explicitly accounts for the grain boundary sliding, grain boundary diffusion and migration, as well as the grain interior dislocation creep. Therefore the explicit finite element method has clear advantages in problems where microstructural heterogeneities play a critical role, such as in the gradient microstructure in shot peening or weldment. Furthermore, combined with the Hall-Petch effect and its breakdown, the above competing processes help construct deformation mechanism maps by extending from the classic Frost-Ashby type to the ones with the dependence of grain size.

Additive recovery at lateral boundaries of grains under electronic exposure

International Nuclear Information System (INIS)

Plotnikov, S.V.; Postnikov, D.V.

2000-01-01

The experimental investigation of additive re-distribution under electronic beam revealed a recovery of the additive at grain boundaries. Additive accumulation mainly takes place at the boundaries that are perpendicular to material surface, whereas there is no an observed recovery of additive at the boundaries that are parallel to the surface. To construe the processes of additive recovery at grain boundaries, we may use the kinetic diffusion equation describing the mass transfer processes in the presence of temperature gradients and non-equilibrium vacancies. The additive recovery is caused by spot fault gradients near the grain boundary. The grain boundary is an intensive run-off region of vacancies. Therefore, the average vacancy distribution profile near the grain boundary changes its pattern. The above case indicates that there are two additive fluxes. One of them is vectored perpendicular to the surface, and the other one is parallel to it, i.e. it is vectored to the grain boundary. A study of the perpendicular and parallel boundaries shows that there is no additive settling at the boundaries that are parallel to the surface, since the general flux is vectored to the parallel boundaries. There is no such kind of phenomenon at the grain boundaries that are perpendicular to the surface. Besides, the perpendicular boundaries are more effective run-off regions for vacancies, since there is a slower build-up of the region with vacancies due to displacement of the vacancies to the surface

Grain Boundary Segregation in Metals

CERN Document Server

Lejcek, Pavel

2010-01-01

Grain boundaries are important structural components of polycrystalline materials used in the vast majority of technical applications. Because grain boundaries form a continuous network throughout such materials, their properties may limit their practical use. One of the serious phenomena which evoke these limitations is the grain boundary segregation of impurities. It results in the loss of grain boundary cohesion and consequently, in brittle fracture of the materials. The current book deals with fundamentals of grain boundary segregation in metallic materials and its relationship to the grain boundary structure, classification and other materials properties.

Grain boundary migration

International Nuclear Information System (INIS)

Dimitrov, O.

1975-01-01

Well-established aspects of grain-boundary migration are first briefly reviewed (influences of driving force, temperature, orientation and foreign atoms). Recent developments of the experimental methods and results are then examined, by considering the various driving of resistive forces acting on grain boundaries. Finally, the evolution in the theoretical models of grain-boundary motion is described, on the one hand for ideally pure metals and, on the other hand, in the presence of solute impurity atoms [fr

Additive recovery of lateral boundaries of grains under electronic exposure

International Nuclear Information System (INIS)

Postnikov, D.V.; Plotnikov, S.V.

2002-01-01

The experimental investigation of additive re-distribution under electronic beam revealed a recovery of the additive at grain boundaries. Additive accumulation mainly takes place at the boundaries that are perpendicular to material surface, whereas there is no an observed recovery of additive at the boundaries that are parallel to the surface. The additive recovery is caused by spot fault gradients near the grain boundary. The grain boundary is an intensive run-off region of vacancies. Therefore, the average vacancy distribution profile near the grain boundary changes its pattern. The above case indicates that there are two additive fluxes. One of them is vectored perpendicular to the surface, and the other one is parallel to it, i. e. it is vectored to the grain boundary. A study of the perpendicular and parallel boundaries shows that there is no additive settling at the boundaries that are parallel to the surface, since the general flux is vectored to the parallel boundaries. There is no such kind of phenomenon at the grain boundaries that are perpendicular to the surface. Besides, the perpendicular boundaries are more effective run-off regions for vacancies, since there is a slower build-up of the region with vacancies due to displacement of the vacancies to the surface. To compute concentration of vacancies we will consider a grain of the surface as a model. The computations indicate the presence of vacancy gradients vectored to the surface and grain boundaries, which are perpendicular to the surface. Comparison of the experimental and theoretical outcomes shows a good agreement between the theoretical model and actual processes occurring under the exposure. This theory disclose wide potentials for application of diffusion processes in alloys

Atom Probe Tomography of Phase and Grain Boundaries in Experimentally-Deformed and Hot-Pressed Wehrlite

Science.gov (United States)

Cukjati, J.; Parman, S. W.; Cooper, R. F.; Zhao, N.

2017-12-01

Atom probe tomography (APT) was used to characterize the chemistry of three grain boundaries: an olivine-olivine (ol-ol) and olivine-clinopyroxene (ol-cpx) boundary in fine-grained experimentally-deformed wehrlite and an ol-cpx boundary in a fine-grained, hot-pressed wehrlite. Grain boundaries were extracted and formed into APT tips using a focused ion beam (FIB). The tips were analyzed in a reflectron-equipped LEAP4000HR (Harvard University) at 1% or 0.5% detection rate, 5pJ laser energy and 100kHz pulse rate. Total ion counts are between 40 and 100 million per tip. Examination of grain and phase boundaries in wehrlite are of interest since slow-diffusing and olivine-incompatible cations present in cpx (e.g. Ca and Al) may control diffusion-accommodated grain boundary sliding and affect mantle rheology (Sundberg & Cooper, 2008). At steady state, ol-cpx aggregates are weaker than either ol or cpx end member, the results of which are not currently well-explained. We investigate grain boundary widths to understand the transport of olivine-incompatible elements. Widths of grain/phase boundary chemical segregation are between 3nm and 6nm for deformed ol-ol and ol-cpx samples; minimally-deformed (hot-pressed) samples having slightly wider chemical segregation widths. Chemical segregation widths were determined from profiles of Na, Al, P, Cl, K, Ca, or Ni, although not all listed elements can be used for all samples (e.g. Na, K segregation profiles can only be observed for ol-ol sample). These estimates are consistent with prior estimates of grain boundary segregation by atom probe tomography on ol-ol and opx-opx samples (Bachhav et al., 2015) and are less than ol-ol interface widths analyzed by STEM/EDX (Hiraga, Anderson, & Kohlstedt, 2007). STEM/EDX will be performed on deformed wehrlite to investigate chemical profile as a function of applied stress orientation and at length scales between those observable by APT and EPMA. Determination of phase boundary chemistry and

A phase field study of strain energy effects on solute–grain boundary interactions

International Nuclear Information System (INIS)

Heo, Tae Wook; Bhattacharyya, Saswata; Chen Longqing

2011-01-01

We have studied strain-induced solute segregation at a grain boundary and the solute drag effect on boundary migration using a phase field model integrating grain boundary segregation and grain structure evolution. The elastic strain energy of a solid solution due to the atomic size mismatch and the coherency elastic strain energy caused by the inhomogeneity of the composition distribution are obtained using Khachaturyan’s microelasticity theory. Strain-induced grain boundary segregation at a static planar boundary is studied numerically and the equilibrium segregation composition profiles are validated using analytical solutions. We then systematically studied the effect of misfit strain on grain boundary migration with solute drag. Our theoretical analysis based on Cahn’s analytical theory shows that enhancement of the drag force with increasing atomic size mismatch stems from both an increase in grain boundary segregation due to the strain energy reduction and misfit strain relaxation near the grain boundary. The results were analyzed based on a theoretical analysis in terms of elastic and chemical drag forces. The optimum condition for solute diffusivity to maximize the drag force under a given driving force was identified.

How grain boundaries affect the efficiency of poly-CdTe solar-cells: A fundamental atomic-scale study of grain boundary dislocation cores using CdTe bi-crystal thin films.

Energy Technology Data Exchange (ETDEWEB)

Klie, Robert [Univ. of Illinois, Chicago, IL (United States)

2016-10-25

It is now widely accepted that grain boundaries in poly-crystalline CdTe thin film devices have a detrimental effect on the minority carrier lifetimes, the open circuit voltage and therefore the overall solar-cell performance. The goal of this project was to develop a fundamental understanding of the role of grain boundaries in CdTe on the carrier life-time, open-circuit voltage, Voc, and the diffusion of impurities. To achieve this goal, i) CdTe bi-crystals were fabricated with various misorientation angels, ii) the atomic- and electronic structures of the grain boundaries were characterized using scanning transmission electron microscopy (STEM), and iii) first-principles density functional theory modeling was performed on the structures determined by STEM to predict the grain boundary potential. The transport properties and minority carrier lifetimes of the bi-crystal grain boundaries were measured using a variety of approaches, including TRPL, and provided feedback to the characterization and modeling effort about the effectiveness of the proposed models.

Ferroelectric domain continuity over grain boundaries

DEFF Research Database (Denmark)

Mantri, Sukriti; Oddershede, Jette; Damjanovic, Dragan

2017-01-01

Formation and mobility of domain walls in ferroelectric materials is responsible for many of their electrical and mechanical properties. Domain wall continuity across grain boundaries has been observed since the 1950's and is speculated to affect the grain boundary-domain interactions, thereby...... impacting macroscopic ferroelectric properties in polycrystalline systems. However detailed studies of such correlated domain structures across grain boundaries are limited. In this work, we have developed the mathematical requirements for domain wall plane matching at grain boundaries of any given...... orientation. We have also incorporated the effect of grain boundary ferroelectric polarization charge created when any two domains meet at the grain boundary plane. The probability of domain wall continuity for three specific grain misorientations is studied. Use of this knowledge to optimize processing...

Effect of solute concentration on grain boundary migration with segregation in stainless steel and model alloys

Science.gov (United States)

Kanda, H.; Hashimoto, N.; Takahashi, H.

The phenomenon of grain boundary migration due to boundary diffusion via vacancies is a well-known process for recrystallization and grain growth during annealing. This phenomenon is known as diffusion-induced grain boundary migration (DIGM) and has been recognized in various binary systems. On the other hand, grain boundary migration often occurs under irradiation. Furthermore, such radiation-induced grain boundary migration (RIGM) gives rise to solute segregation. In order to investigate the RIGM mechanism and the interaction between solutes and point defects during the migration, stainless steel and Ni-Si model alloys were electron-irradiated using a HVEM. RIGM was often observed in stainless steels during irradiation. The migration rate of boundary varied, and three stages of the migration were recognized. At lower temperatures, incubation periods up to the occurrence of the boundary migration were observed prior to first stage. These behaviors were recognized particularly for lower solute containing alloys. From the relation between the migration rates at stage I and inverse temperatures, activation energies for the boundary migration were estimated. In comparison to the activation energy without irradiation, these values were very low. This suggests that the RIGM is caused by the flow of mixed-dumbbells toward the grain boundary. The interaction between solute and point defects and the effective defect concentration generating segregation will be discussed.

Modelling Cr depletion under a growing Cr2O3 layer on austenitic stainless steel: the influence of grain boundary diffusion

DEFF Research Database (Denmark)

Hansson, Anette Nørgaard; Hattel, Jesper Henri; Dahl, Kristian Vinter

2009-01-01

according to a parabolic rate law as a consequence of rate limiting diffusion of Cr cations through the oxide layer; the retraction of the oxide/alloy interface associated with the removal of Cr atoms from the substrate is included in the calculations. Numerically, the movement of the oxide/alloy interface......The oxidation behaviour of austenitic stainless steels in the temperature range 723–1173K is strongly influenced by the grain size of the oxidizing alloy. In this work the evolution of the concentration profiles of Cr, Ni and Fe in the substrate below a growing Cr2O3 layer is simulated...... with a Fisher-type numerical model, which takes both volume and grain boundary diffusion into consideration. The model is based on a two-dimensional control volume-based solution of Fick’s 2nd law for multicomponent diffusion and includes crossterm diffusion coefficients. The oxide layer is assumed to grow...

Observations of secondary oscillations in thermal grain boundary grooves

International Nuclear Information System (INIS)

Sachenko, P.P.; Schneibel, J.H.; Zhang, W.

2004-01-01

Thermal grain boundary grooving by surface diffusion is accompanied not only by main maxima on either side of a groove, but also by secondary maxima and minima. We measure these oscillations in tungsten and give reasons why the observed secondary maxima and minima are larger than predicted

Structure and properties of fluid-filled grain boundaries under stress in geological materials. Geologica Ultraiectina (290)

NARCIS (Netherlands)

van Noort, R.

2008-01-01

Two of the three processes making up the deformation mechanism of intergranular pressure solution, being dissolution and diffusion, take place in the grain boundary fluid phase. Hence, the structure and physical properties of wet grain boundaries under stress can be expected to influence the

Grain boundary segregation and intergranular failure

International Nuclear Information System (INIS)

White, C.L.

1980-01-01

Trace elements and impurities often segregate strongly to grain boundaries in metals and alloys. Concentrations of these elements at grain boundaries are often 10 3 to 10 5 times as great as their overall concentration in the alloy. Because of such segregation, certain trace elements can exert a disproportionate influence on material properties. One frequently observed consequence of trace element segregation to grain boundaries is the occurrence of grain boundary failure and low ductility. Less well known are incidences of improved ductility and inhibition of grain boundary fracture resulting from trace element segregation to grain boundaries in certain systems. An overview of trace element segregation and intergranular failure in a variety of alloy systems as well as preliminary results from studies on Al 3% Li will be presented

The effect of grain boundary chemistry on the slip transmission process through grain boundaries in Ni3Al

International Nuclear Information System (INIS)

Robertson, I.M.; Lee, T.C.; Subramanian, R.; Birnbaum, H.K.

1992-01-01

This paper reports on the conditions established in disordered FCC systems for predicting the slip system that will be activated by a grain boundary to relieve a local stress concentration that have been applied to the ordered FCC alloy Ni 3 Al. The slip transfer behavior in hypo-stoichiometric Ni 3 Al with (0.2 at. %B) and without boron was directly observed by performing the deformation experiments in situ in the transmission electron microscope. In the boron-free and boron-doped alloys, lattice dislocations were incorporated in the grain boundary, but did not show evidence of dissociation to grain boundary dislocations or of movement in the grain boundary plane. The stress concentration associated with the dislocation pileup at the grain boundary are relieved by the emission of dislocations from the grain boundary in the boron-doped alloy. The slip system initiated in the adjoining grain obeyed the conditions established for disordered FCC systems. In the boron-free alloy, the primary stress relief mechanism was grain-boundary cracking, although dislocation emission from the grain boundary also occurred and accompanied intergranular crack advance

A Numerical Model of Anisotropic Mass Transport Through Grain Boundary Networks

Science.gov (United States)

Wang, Yibo

Tin (Sn) thin films are commonly used in electronic circuit applications as coatings on contacts and solders for joining components. It is widely observed, for some such system, that whiskers---long, thin crystalline structures---emerge and grow from the film. The Sn whisker phenomenon has become a highly active research area since Sn whiskers have caused a large amount of damage and loss in manufacturing, military, medical and power industries. Though lead (Pb) addition to Sn has been used to solve this problem for over five decades, the adverse environmental and health effects of Pb have motivated legislation to severely constrain Pb use in society. People are researching and seeking the reasons which cause whiskers and corresponding methods to solve the problem. The contributing factors to cause a Sn whisker are potentially many and much still remains unknown. Better understanding of fundamental driving forces should point toward strategies to improve (a) the accuracy with which we can predict whisker formation, and (b) our ability to mitigate the phenomenon. This thesis summarizes recent important research achievements in understanding Sn whisker formation and growth, both experimentally and theoretically. Focus is then placed on examining the role that anisotropy in grain boundary diffusivity plays in determining whisker characteristics (specifically, whether they form and, if so, where on a surface). To study this aspect of the problem and to enable future studies on stress driven grain boundary diffusion, this thesis presents a numerical anisotropic mass transport model. In addition to presenting details of the model and implementation, model predictions for a set of increasingly complex grain boundary networks are discussed. Preliminary results from the model provide evidence that anisotropic grain boundary diffusion may be a primary driving mechanism in whisker formation.

Evolution of grain boundary character distributions in alloy 825 tubes during high temperature annealing: Is grain boundary engineering achieved through recrystallization or grain growth?

International Nuclear Information System (INIS)

Bai, Qin; Zhao, Qing; Xia, Shuang; Wang, Baoshun; Zhou, Bangxin; Su, Cheng

2017-01-01

Grain boundary engineering (GBE) of nickel-based alloy 825 tubes was carried out with different cold drawing deformations by using a draw-bench on a factory production line and subsequent annealing at various temperatures. The microstructure evolution of alloy 825 during thermal-mechanical processing (TMP) was characterized by means of the electron backscatter diffraction (EBSD) technique to study the TMP effects on the grain boundary network and the evolution of grain boundary character distributions during high temperature annealing. The results showed that the proportion of ∑ 3 n coincidence site lattice (CSL) boundaries of alloy 825 tubes could be increased to > 75% by the TMP of 5% cold drawing and subsequent annealing at 1050 °C for 10 min. The microstructures of the partially recrystallized samples and the fully recrystallized samples suggested that the proportion of low ∑ CSL grain boundaries depended on the annealing time. The frequency of low ∑ CSL grain boundaries increases rapidly with increasing annealing time associating with the formation of large-size highly-twinned grains-cluster microstructure during recrystallization. However, upon further increasing annealing time, the frequency of low ∑ CSL grain boundaries decreased markedly during grain growth. So it is concluded that grain boundary engineering is achieved through recrystallization rather than grain growth. - Highlights: •The grain boundary engineering (GBE) is applicable to 825 tubes. •GBE is achieved through recrystallization rather than grain growth. •The low ∑ CSL grain boundaries in 825 tubes can be increased to > 75%.

Structure and transport at grain boundaries in polycrystalline olivine: An atomic-scale perspective

Science.gov (United States)

Mantisi, Boris; Sator, Nicolas; Guillot, Bertrand

2017-12-01

Structure and transport properties at grain boundaries in polycrystalline olivine have been investigated at the atomic scale by molecular dynamics simulation (MD) using an empirical ionocovalent interaction potential. On the time scale of the simulation (a few tens of nanoseconds for a system size of ∼650,000 atoms) grain boundaries and grain interior were identified by mapping the atomic displacements along the simulation run. In the investigated temperature range (1300-1700 K) the mean thickness of the grain boundary phase is evaluated between 0.5 and 2 nm, a value which depends on temperature and grain size. The structure of the grain boundary phase is found to be disordered (amorphous-like) and is different from the one exhibited by the supercooled liquid. The self-diffusion coefficients of major elements in the intergranular region range from ∼10-13 to 10-10 m2/s between 1300 and 1700 K (with DSigb Kubo relation expressing the viscosity as function of the stress tensor time correlation function. In spite of a slow convergence of the calculation by MD, the grain boundary viscosity was estimated about ∼105 Pa s at 1500 K, a value in agreement with high-temperature viscoelastic relaxation data. An interesting information gained from MD is that sliding at grain boundaries is essentially controlled by the internal friction between the intergranular phase and the grain edges.

On the elastic stiffness of grain boundaries

International Nuclear Information System (INIS)

Zhang Tongyi; Hack, J.E.

1992-01-01

The elastic softening of grain boundaries is evaluated from the starting point of grain boundary energy. Several examples are given to illustrate the relationship between boundary energy and the extent of softening. In general, a high grain boundary energy is associated with a large excess atomic volume in the boundary region. The consequent reduction in grain boundary stiffness can represent a significant fraction of that observed in bulk crystals. (orig.)

Grain-boundary engineering applied to grain growth in a high temperature material

International Nuclear Information System (INIS)

Huda, Z.

1993-01-01

Crystallography of grain boundaries are determined for a high temperature material, before and after grain growth processes, so as to study the induction of special properties useful for application in components of a gas-turbine engine. The philosophy of grain-boundary engineering is applied to grain growth in APK-6, a powder formed nickel-base superalloy so as to establish the possible structure/property relationships. The alloy in the as received condition is shown to possess a strong texture and contained coincident site lattices (CSL) boundaries with most boundaries having sigma values in the range of 3 > sigma > 25. A normal grain-growth heat treatment result in a good population of low angle grain boundaries, and drastically reduces the proportion of CSL boundaries. A strong [011] annealing texture is observed after an intermediate grain growth; most grain boundaries, here, tend to be high angle indicating a possibility of possessing special properties. (author)

Grain boundary engineering in sintered Nd-Fe-B permanent magnets for efficient utilization of heavy rare earth elements

Energy Technology Data Exchange (ETDEWEB)

Loewe, Konrad

2016-10-18

The first part of the thesis investigates the diffusion of rare-earth (RE) elements in commercial sintered Nd-Fe-B based permanent magnets. A strong temperature dependence of the diffusion distance and resulting change in magnetic properties were found. A maximum increase in coercivity of ∼+350 kA/m using a Dy diffusion source occurred at the optimum annealing temperature of 900 C. After annealing for 6 h at this temperature, a Dy diffusion distance of about 4 mm has been observed with a scanning Hall probe. Consequently, the maximum thickness of grain boundary diffusion processed magnets with homogeneous properties is also only a few mm. The microstructural changes in the magnets after diffusion were investigated by electron microscopy coupled with electron probe microanalysis. It was found that the diffusion of Dy into sintered Nd-Fe-B permanent magnets occurs along the grain boundary phases, which is in accordance with previous studies. A partial melting of the Nd-Fe-B grains during the annealing process lead to the formation of so - called (Nd,Dy)-Fe-B shells at the outer part of the grains. These shells are μm thick at the immediate surface of the magnet and become thinner with increasing diffusion distance towards the center of the bulk. With scanning transmission electron microscopy coupled with electron probe analysis a Dy content of about 1 at.% was found in a shell located about 1.5 mm away from the surface of the magnet. The evaluation of diffusion speeds of Dy and other RE (Tb, Ce, Gd) in Nd-Fe-B magnets showed that Tb diffuses significantly faster than Dy, and Ce slightly slower than Dy, which is attributed to differences in the respective phase diagrams. The addition of Gd to the grain boundaries has an adverse effect on coercivity. Exemplary of the heavy rare earth element Tb, the nano - scale elemental distribution around the grain boundaries after the diffusion process was visualized with high resolution scanning transmission electron microscopy

Grain Boundary Engineering of Electrodeposited Thin Films

DEFF Research Database (Denmark)

Alimadadi, Hossein

is not yet well-understood. This, at least partly, owes to the lack of robust characterization methods for analyzing the nature of grain boundaries including the grain boundary plane characteristics, until recently. In the past decade, significant improvements in the 2-dimensional and 3-dimensional analysis...... of the favorable boundaries that break the network of general grain boundaries. Successful dedicated synthesis of a textured nickel film fulfilling the requirements of grain boundary engineered materials, suggests improved boundary specific properties. However, the textured nickel film shows fairly low...... thermal stability and growth twins annihilate by thermal treatment at 600 degree C. In contrast, for oriented grains, growth nano-twins which are enveloped within columnar grains show a high thermal stability even after thermal treatment at 600 degree C. In order to exploit the high thermal...

Concurrent grain boundary motion and grain rotation under an applied stress

International Nuclear Information System (INIS)

Gorkaya, Tatiana; Molodov, Konstantin D.; Molodov, Dmitri A.; Gottstein, Guenter

2011-01-01

Simultaneous shear coupling and grain rotation were observed experimentally during grain boundary migration in high-purity Al bicrystals subjected to an external mechanical stress at elevated temperatures. This behavior is interpreted in terms of the structure of the investigated planar 18.2 o non-tilt grain boundary with a 20 o twist component. For characterization of the grain rotation after annealing under stress the bicrystal surface topography across the boundary was measured by atomic force microscopy. The temperature dependence of the boundary migration rate was measured and the migration activation energy determined.

Micromechanical Modeling of Grain Boundaries Damage in a Copper Alloy Under Creep

International Nuclear Information System (INIS)

Voese, Markus

2015-01-01

In order to include the processes on the scale of the grain structure into the description of the creep behaviour of polycrystalline materials, the damage development of a single grain boundary has been initially investigated in the present work. For this purpose, a special simulationmethod has been used, whose resolution procedure based on holomorphic functions. The mechanisms taken into account for the simulations include nucleation, growth by grain boundary diffusion, coalescence and shrinkage until complete sintering of grain boundary cavities. These studies have then been used to develop a simplified cavitation model, which describes the grain boundary damage by two state variables and the time-dependent development by a mechanism-oriented rate formulation. To include the influence of grain boundaries within continuum mechanical considerations of polycrystals, an interface model has been developed, that incorporates both damage according to the simplified cavitation model and grain boundary sliding in dependence of a phenomenological grain boundary viscosity. Furthermore a micromechanical model of a polycrystal has been developed that allows to include a material's grain structure into the simulation of the creep behaviour by means of finite element simulations. Thereby, the deformations of individual grains are expressed by a viscoplastic single crystal model and the grain boundaries are described by the proposed interface model. The grain structure is represented by a finite element model, in which the grain boundaries are modelled by cohesive elements. From the evaluation of experimental creep data, the micromechanical model of a polycrystal has been calibrated for a copper-antimony alloy at a temperature of 823 K. Thereby, the adjustment of the single crystal model has been carried out on the basis of creep rates of pure copper single crystal specimens. The experimental determination of grain boundary sliding and grain boundary porosity for coarse-grained

A comparison of grain boundary evolution during grain growth in fcc metals

International Nuclear Information System (INIS)

Brons, J.G.; Thompson, G.B.

2013-01-01

Grain growth of Cu and Ni thin films, subjected to in situ annealing within a transmission electron microscope, has been quantified using a precession-enhanced electron diffraction technique. The orientation of each grain and its misorientation with respect to its neighboring grains were calculated. The Cu underwent grain growth that maintained a monomodal grain size distribution, with its low-angle grain boundaries being consumed, and the Ni exhibited grain size distributions in stages, from monomodal to bimodal to monomodal. The onset of Ni’s abnormal grain growth was accompanied by a sharp increase in the Σ3 and Σ9 boundary fractions, which is attributed to simulation predictions of their increased mobility. These Σ3 and Σ9 fractions then dropped to their room temperature values during the third stage of grain growth. In addition to the Σ3 and Σ9 boundaries, the Σ5 and Σ7 boundaries also underwent an increase in total boundary fraction with increasing temperature in both metals

A Cosserat crystal plasticity and phase field theory for grain boundary migration

Science.gov (United States)

Ask, Anna; Forest, Samuel; Appolaire, Benoit; Ammar, Kais; Salman, Oguz Umut

2018-06-01

The microstructure evolution due to thermomechanical treatment of metals can largely be described by viscoplastic deformation, nucleation and grain growth. These processes take place over different length and time scales which present significant challenges when formulating simulation models. In particular, no overall unified field framework exists to model concurrent viscoplastic deformation and recrystallization and grain growth in metal polycrystals. In this work a thermodynamically consistent diffuse interface framework incorporating crystal viscoplasticity and grain boundary migration is elaborated. The Kobayashi-Warren-Carter (KWC) phase field model is extended to incorporate the full mechanical coupling with material and lattice rotations and evolution of dislocation densities. The Cosserat crystal plasticity theory is shown to be the appropriate framework to formulate the coupling between phase field and mechanics with proper distinction between bulk and grain boundary behaviour.

Grain boundary motion and grain rotation in aluminum bicrystals: recent experiments and simulations

International Nuclear Information System (INIS)

Molodov, D A; Barrales-Mora, L A; Brandenburg, J-E

2015-01-01

The results of experimental and computational efforts over recent years to study the motion of geometrically different grain boundaries and grain rotation under various driving forces are briefly reviewed. Novel in-situ measuring techniques based on orientation contrast imaging and applied simulation techniques are described. The experimental results obtained on specially grown aluminum bicrystals are presented and discussed. Particularly, the faceting and migration behavior of low angle grain boundaries under the curvature force is addressed. In contrast to the pure tilt boundaries, which remained flat/faceted and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry. The shape evolution and shrinkage kinetics of cylindrical grains with different tilt and mixed boundaries were studied by molecular dynamics simulations. The mobility of low angle <100> boundaries with misorientation angles higher than 10°, obtained by both the experiments and simulations, was found not to differ from that of the high angle boundaries, but decreases essentially with further decrease of misorientation. The shape evolution of the embedded grains in simulations was found to relate directly to results of the energy computations. Further simulation results revealed that the shrinkage of grains with pure tilt boundaries is accompanied by grain rotation. In contrast, grains with the tilt-twist boundaries composed of dislocations with the mixed edge-screw character do not rotate during their shrinkage. Stress driven boundary migration in aluminium bicrystals was observed to be coupled to a tangential translation of the grains. The activation enthalpy of high angle boundary migration was found to vary non-monotonically with

Swelling and gas release of grain-boundary pores in uranium dioxide

International Nuclear Information System (INIS)

Schrire, D.I.

1983-12-01

The swelling and gas release of overpressured grain boundary pores is sintered unirradiated uranium dioxide were investigated under isothermal conditions. The pores became overpressured when the ambient pressure was reduced, and the excess pressure driving force caused growth and interconnection of the pores, leading to eventual gas release. Swelling was measured continuously by a linear variable differential transformer, and open and closed porosity fractions were determined after the tests by immersion density and quantitative microscopy measurements. The sinter porosity consisted of pores situated on grain faces, grain edges, and grain corners. Isolated pores maintained their equilibrium shape while growing, without any measurable change in dihedral angle. Interconnection occurred predominantly along grain edges, without any evidence of pore sharpening or crack propagation at low driving forces. Extensive open porosity occurred at a threshold density of about 85% TD. There was an almost linear dependence of the initial swelling rate on the driving force, with an activation energy of 200+- 8 kJ/mole, in good agreement with published values of the activation energy for grain boundary diffusion

Creep crack extension by grain-boundary cavitation

International Nuclear Information System (INIS)

Bassani, J.L.

1981-01-01

Recent work by Riedel and coworkers has led to various descriptions of stationary and moving crack tip fields under creep conditions. For stationary and growing cracks, several flow mechanisms (e.g., elastic, time-independent plastic, primary creep, and secondary creep) can dictate the analytical form of the crack tip field. In this paper, relationship between overall loading and crack velocities are modelled based upon grain-boundary cavity growth and coalescence within the zone of concentrated strain in the crack tip field. Coupled diffusion and creep growth of the cavities is considered. Overall crack extension is taken to be intermittent on a size scale equivalent to the size of a grain. Numerical results are presented for a center-cracked panel of 304 stainless steel. (author)

Deformation by grain boundary sliding and slip creep versus diffusional creep

International Nuclear Information System (INIS)

Ruano, O A; Sherby, O D; Wadsworth, J.

1998-01-01

A review is presented of the debates between the present authors and other investigators regarding the possible role of diffusional creep in the plastic flow of polycrystalline metals at low stresses. These debates are recorded in eleven papers over the past seventeen years. ln these papers it has been shown that the creep rates of materials in the so-called diffusional creep region are almost always higher than those predicted by the diffusional creep theory. Additionally, the predictions of grain size effects and stress exponents from diffusional creep theory are often not found in the experimental data. Finally, denuded zones have been universally considered to be direct evidence for diffusional creep; but, those reported in the literature are shown to be found only under conditions where a high stress exponent is observed. Also, the locations of the denuded zones do not match those predicted. Alternative mechanisms are described in which diffusion-controlled dislocation creep and/or grain boundary sliding are the dominant deformation processes in low-stress creep. It is proposed that denuded zones are formed by stress-directed grain boundary migration with the precipitates dissolving in the moving grain boundaries. The above observations have led us to the conclusion that grain boundary sliding and slip creep are in fact the principal mechanisms for observations of plastic flow in the so-called diffusional creep regions

Three dimensional grain boundary modeling in polycrystalline plasticity

Science.gov (United States)

Yalçinkaya, Tuncay; Özdemir, Izzet; Fırat, Ali Osman

2018-05-01

At grain scale, polycrystalline materials develop heterogeneous plastic deformation fields, localizations and stress concentrations due to variation of grain orientations, geometries and defects. Development of inter-granular stresses due to misorientation are crucial for a range of grain boundary (GB) related failure mechanisms, such as stress corrosion cracking (SCC) and fatigue cracking. Local crystal plasticity finite element modelling of polycrystalline metals at micron scale results in stress jumps at the grain boundaries. Moreover, the concepts such as the transmission of dislocations between grains and strength of the grain boundaries are not included in the modelling. The higher order strain gradient crystal plasticity modelling approaches offer the possibility of defining grain boundary conditions. However, these conditions are mostly not dependent on misorientation of grains and can define only extreme cases. For a proper definition of grain boundary behavior in plasticity, a model for grain boundary behavior should be incorporated into the plasticity framework. In this context, a particular grain boundary model ([l]) is incorporated into a strain gradient crystal plasticity framework ([2]). In a 3-D setting, both bulk and grain boundary models are implemented as user-defined elements in Abaqus. The strain gradient crystal plasticity model works in the bulk elements and considers displacements and plastic slips as degree of freedoms. Interface elements model the plastic slip behavior, yet they do not possess any kind of mechanical cohesive behavior. The physical aspects of grain boundaries and the performance of the model are addressed through numerical examples.

GRAIN-BOUNDARY PRECIPITATION UNDER IRRADIATION IN DILUTE BINARY ALLOYS

Institute of Scientific and Technical Information of China (English)

S.H. Song; Z.X. Yuan; J. Liu; R.G.Faulkner

2003-01-01

Irradiation-induced grain boundary segregation of solute atoms frequently bring about grain boundary precipitation of a second phase because of its making the solubility limit of the solute surpassed at grain boundaries. Until now the kinetic models for irradiation-induced grain boundary precipitation have been sparse. For this reason, we have theoretically treated grain boundary precipitation under irradiation in dilute binary alloys. Predictions ofγ'-Ni3Si precipitation at grain boundaries ave made for a dilute Ni-Si alloy subjected to irradiation. It is demonstrated that grain boundary silicon segregation under irradiation may lead to grain boundaryγ'-Ni3 Si precipitation over a certain temperature range.

Evidence of preferential diffusion and segregation of impurities at grain boundaries in very pure niobium used for radiofrequency cavities

International Nuclear Information System (INIS)

Antoine, C.; Bonin, B.; Safa, H.; Chevarier, A.; Chevarier, N.; Roux, B.

1996-01-01

Grain boundaries (GB) of titaniferous, heat treated and then etched niobium have been observed by nuclear microprobe analysis. The very small area of the probe allows to measure by PIXE quantities of titanium as low as one monolayer at the GB. Concentrations of titanium as high as some atomic percent were found on 6 μm etched samples, giving indication of a preferential diffusion and/or segregation at GB. Titanium was detectable also on 15 μm etched samples but was bellow the sensitivity of the microprobe for 35 μm etched samples. (author)

Grain boundary corrosion of copper canister material

International Nuclear Information System (INIS)

Fennell, P.A.H.; Graham, A.J.; Smart, N.R.; Sofield, C.J.

2001-03-01

The proposed design for a final repository for spent fuel and other long-lived residues in Sweden is based on the multi-barrier principle. The waste will be encapsulated in sealed cylindrical canisters, which will then be placed in granite bedrock and surrounded by compacted bentonite clay. The canister design is based on a thick cast inner container fitted inside a corrosion-resistant copper canister. During fabrication of the outer copper canisters there will be some unavoidable grain growth in the welded areas. As grains grow they will tend to concentrate impurities within the copper at the new grain boundaries. The work described in this report was undertaken to determine whether there is any possibility of enhanced corrosion at grain boundaries within the copper canister. The potential for grain boundary corrosion was investigated by exposing copper specimens, which had undergone different heat treatments and hence had different grain sizes, to aerated artificial bentonite-equilibrated groundwater with two concentrations of chloride, for increasing periods of time. The degree of grain boundary corrosion was determined by atomic force microscopy (AFM) and optical microscopy. AFM showed no increase in grain boundary 'ditching' for low chloride groundwater. In high chloride groundwater the surface was covered uniformly with a fine-grained oxide. No increases in oxide thickness were observed. No significant grain boundary attack was observed using optical microscopy either. The work suggests that in aerated artificial groundwaters containing chloride ions, grain boundary corrosion of copper is unlikely to adversely affect SKB's copper canisters

The influence of vortex pinning and grain boundary structure on critical currents across grain boundaries in YBa2Cu3Ox

International Nuclear Information System (INIS)

Miller, D. J.

1998-01-01

We have used studies of single grain boundaries in YBCO thin films and bulk bicrystals to study the influence of vortex pinning along a grain boundary on dissipation. The critical current density for transport across grain boundaries in thin films is typically more than an order of magnitude larger than that measured for transport across grain boundaries in bulk samples. For low disorientation angles, the difference in critical current density within the grains that form the boundary can contribute to the substantial differences in current density measured across the boundary. However, substantial differences exist in the critical current density across boundaries in thin film compared to bulk bicrystals even in the higher angle regime in which grain boundary dissipation dominates. The differences in critical current density in this regime can be understood on the basis of vortex pinning along the boundary

Multiscale model of metal alloy oxidation at grain boundaries

International Nuclear Information System (INIS)

Sushko, Maria L.; Alexandrov, Vitaly; Schreiber, Daniel K.; Rosso, Kevin M.; Bruemmer, Stephen M.

2015-01-01

High temperature intergranular oxidation and corrosion of metal alloys is one of the primary causes of materials degradation in nuclear systems. In order to gain insights into grain boundary oxidation processes, a mesoscale metal alloy oxidation model is established by combining quantum Density Functional Theory (DFT) and mesoscopic Poisson-Nernst-Planck/classical DFT with predictions focused on Ni alloyed with either Cr or Al. Analysis of species and fluxes at steady-state conditions indicates that the oxidation process involves vacancy-mediated transport of Ni and the minor alloying element to the oxidation front and the formation of stable metal oxides. The simulations further demonstrate that the mechanism of oxidation for Ni-5Cr and Ni-4Al is qualitatively different. Intergranular oxidation of Ni-5Cr involves the selective oxidation of the minor element and not matrix Ni, due to slower diffusion of Ni relative to Cr in the alloy and due to the significantly smaller energy gain upon the formation of nickel oxide compared to that of Cr 2 O 3 . This essentially one-component oxidation process results in continuous oxide formation and a monotonic Cr vacancy distribution ahead of the oxidation front, peaking at alloy/oxide interface. In contrast, Ni and Al are both oxidized in Ni-4Al forming a mixed spinel NiAl 2 O 4 . Different diffusivities of Ni and Al give rise to a complex elemental distribution in the vicinity of the oxidation front. Slower diffusing Ni accumulates in the oxide and metal within 3 nm of the interface, while Al penetrates deeper into the oxide phase. Ni and Al are both depleted from the region 3–10 nm ahead of the oxidation front creating voids. The oxide microstructure is also different. Cr 2 O 3 has a plate-like structure with 1.2–1.7 nm wide pores running along the grain boundary, while NiAl 2 O 4 has 1.5 nm wide pores in the direction parallel to the grain boundary and 0.6 nm pores in the perpendicular direction providing an additional

The interactions of radiation damage with grain boundaries

International Nuclear Information System (INIS)

King, A.H.

1979-01-01

This thesis reports a theoretical and experimental study of the fundamental effects giving rise to zones adjacent to grain boundaries which are denuded of irradiation-induced damage. The results, however, have significance in the wider field of point-defect absorption (and emission) by grain boundaries. Particular emphasis has been laid upon correlating the point-defect sink behaviour of grain boundaries with their structures and to this end, grain boundaries with periodically repeating structures have been chosen for study. The hypotheses that point-defect absorption is achieved by the climb of grain boundary dislocation spirals, loops and structural arrays have been investigated and firm evidence has been found to support the two latter mechanisms in specific cases. Loops, in particular, have been found to grow only on coherent twin boundary planes. Chapter two of the thesis investigates the crystallographic nature of the possible reactions of point-defects with periodic boundaries and demonstrates that effects such as grain boundary migration and grain translations may be associated with point-defect absorption. Chapter three presents a theoretical study of the effects of elastic interactions between point-defects and grain boundary dislocations and gives predictions of sink strength and bias of a grain boundary as a function of its structure. Chapter four consists of experimental examples of grain boundaries observed during and after irradiation. Chapter five discusses the results of chapters two, three and four considering their implications for the various hypotheses and presents the conclusions of the thesis and some suggestions for further work. (author)

Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries

Science.gov (United States)

Vasco, Enrique; Polop, Celia

2017-12-01

The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.

Stress-assisted grain growth in nanocrystalline metals: Grain boundary mediated mechanisms and stabilization through alloying

International Nuclear Information System (INIS)

Zhang, Yang; Tucker, Garritt J.; Trelewicz, Jason R.

2017-01-01

The mechanisms of stress-assisted grain growth are explored using molecular dynamics simulations of nanoindentation in nanocrystalline Ni and Ni-1 at.% P as a function of grain size and deformation temperature. Grain coalescence is primarily confined to the high stress region beneath the simulated indentation zone in nanocrystalline Ni with a grain size of 3 nm. Grain orientation and atomic displacement vector mapping demonstrates that coalescence transpires through grain rotation and grain boundary migration, which are manifested in the grain interior and grain boundary components of the average microrotation. A doubling of the grain size to 6 nm and addition of 1 at.% P eliminates stress-assisted grain growth in Ni. In the absence of grain coalescence, deformation is accommodated by grain boundary-mediated dislocation plasticity and thermally activated in pure nanocrystalline Ni. By adding solute to the grain boundaries, the temperature-dependent deformation behavior observed in both the lattice and grain boundaries inverts, indicating that the individual processes of dislocation and grain boundary plasticity will exhibit different activity based on boundary chemistry and deformation temperature.

Subgroup report on grain boundary and interphase boundary structure and properties

International Nuclear Information System (INIS)

Balluffi, R.W.; Cannon, R.M.; Clarke, D.R.; Heuer, A.H.; Ho, P.S.; Kear, B.H.; Vitek, V.; Weertman, J.R.; White, C.L.

1979-01-01

In many high temperature structural applications, the performance characteristics of a materials system are largely controlled by the properties of its grain and interphase boundaries. Failure in creep and fatigue frequently occurs by cavitation, or cracking along grain boundaries. In a few special cases, this failure problem has been overcome by directional alignment of grain and interphase boundaries by various types of metallurgical processing such as directional solidification and directional recrystallization. A good example is to be found in the application of directionally aligned structures in high performance gas-turbine airfoils. However, where fine, equiaxed grain structures are desirable, other methods of controlling grain boundary properties have been developed. Important among these has been the introduction of improvements in primary melting practices, designed to control important impurities. This is of decisive importance because even traces of certain impurity elements present in grain boundaries in high temperature materials can seriously affect properties. Impurities are deleterious and need to be removed. However, in certain cases, (e.g., creep fracture) controlled impurity additions can be beneficial and result in improved properties

Grain Boundaries From Theory to Engineering

CERN Document Server

Priester, Louisette

2013-01-01

Grain boundaries are a main feature of crystalline materials. They play a key role in determining the properties of materials, especially when grain size decreases and even more so with the current improvements of  processing tools and methods that allow us to control various elements in a polycrystal. This book presents the theoretical basis of the study of  grain boundaries and aims to open up new lines of research in this area. The treatment is light on mathematical approaches while emphasizing practical examples; the issues they raise are discussed with reference to theories. The general approach of the book has two main goals: to lead the reader from the concept of ‘ideal’ to ‘real’ grain boundaries; to depart from established knowledge and address the opportunities emerging through "grain boundary engineering",  the control of morphological and crystallographic features that affect material properties. The book is divided in three parts:  I ‘From interganular order to disorder’ deals wit...

Distribution of the grain limit character in SiC and its effect on the diffusion of fission products in the TRISO fuel particles

International Nuclear Information System (INIS)

Cancino T, F.; Lopez H, E.

2017-09-01

At present is accepted that silver diffuses through silicium carbide (SiC) by diffusion in grain boundaries, although little is known about the characteristics of grain boundaries in SiC, and how these change depending on the type of sample. In this work, was observed that there are small but important differences between the SiC in the tri-structural isotropic (TRISO) particles and that of the monoliths, which could explain some of the differences observed in experiments on diffusion in the literature. Five different types (coatings and monoliths) of SiC produced by chemical vapor deposition (CVD) were characterized by electron backscatter diffraction (EBSD). In all the samples the SiC was mainly composed of high-angle grain boundaries (∼ 65%), with a small fraction of grain boundaries of low-angle (about 15%) and 20% of the coincidence site lattice (CSL). The morphology of the monoliths is constituted by large grains, surrounded by smaller grains; in the particles of the TRISO fuel, both columnar and equi axial grains were observed, with a more uniform distribution over the surface of the coating. (Author)

Study of twist boundaries in aluminium. Structure and intergranular diffusion

International Nuclear Information System (INIS)

Lemuet, Daniel

1981-01-01

This research thesis addresses the study of grain boundaries in oriented crystals, and more particularly the systematic calculation of intergranular structures and energies of twist boundaries of <001> axis in aluminium, the determination of intergranular diffusion coefficients of zinc in a set of twist bi-crystals of same axis encompassing a whole range of disorientations, and the search for a correlation between these experimental results and calculated structures

Surface concentration of defects at grain boundaries in sintered alumina determined by positron annihilation lifetime spectroscopy

International Nuclear Information System (INIS)

Kansy, J.; Ahmad, A.Si.; Moya, G.; Liebault, J.

2001-01-01

Sintered alumina samples of grain diameters spanning from 1.2 to 4.5 μm have been investigated by positron annihilation lifetime spectroscopy. One series of samples was produced from material containing about 150 ppm impurities (mainly SiO 2 ). The second one was made from material having about 2700 ppm of various elements (SiO 2 , MgO, CaO). Two models of positron trapping at grain boundaries are compared: The first one relates to the diffusion-limited regime; and the other one - to the transmission-limited regime of trapping. As a results of relative change of surface concentration of defects at grain boundaries is determined. Additionally, positron diffusion constant in bulk alumina at room temperature, D + = 0.36 ± 10 cm 2 /s, is estimated. (author)

High-coercivity ultrafine-grained anisotropic Nd–Fe–B magnets processed by hot deformation and the Nd–Cu grain boundary diffusion process

International Nuclear Information System (INIS)

Sepehri-Amin, H.; Ohkubo, T.; Nagashima, S.; Yano, M.; Shoji, T.; Kato, A.; Schrefl, T.; Hono, K.

2013-01-01

The grain boundary diffusion process using an Nd 70 Cu 30 eutectic alloy has been applied to hot-deformed anisotropic Nd–Fe–B magnets, resulting in a substantial enhancement of coercivity, from 1.5 T to 2.3 T, at the expense of remanence. Scanning electron microscopy showed that the areal fraction of an Nd-rich intergranular phase increased from 10% to 37%. The intergranular phase of the hot-deformed magnet initially contained ∼55 at.% ferromagnetic element, while it diminished to an undetectable level after the process. Microscale eutectic solidification of Nd/NdCu as well as a fine lamellae structure of Nd 70 (Co,Cu) 30 /Nd were observed in the intergranular phase. Micromagnetic simulations indicated that the reduction of the magnetization in the intergranular phases leads to the enhancement of coercivity in agreement with the experimental observation

Grain boundaries in high temperature superconductors

NARCIS (Netherlands)

Hilgenkamp, Johannes W.M.; Mannhart, J.

2002-01-01

Since the first days of high-Tc superconductivity, the materials science and the physics of grain boundaries in superconducting compounds have developed into fascinating fields of research. Unique electronic properties, different from those of the grain boundaries in conventional metallic

Crystalline orientation dependent photoresponse and heterogeneous behaviors of grain boundaries in perovskite solar cells

Science.gov (United States)

Jiang, Chuanpeng; Zhang, Pengpeng

2018-02-01

Using photoconductive atomic force microscopy and Kelvin probe force microscopy, we characterize the local electrical properties of grains and grain boundaries of organic-inorganic hybrid perovskite (CH3NH3PbI3) thin films on top of a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/ITO substrate. Three discrete photoconductivity levels are identified among perovskite grains, likely corresponding to the crystal orientation of each grain. Local J-V curves recorded on these grains further suggest an anti-correlation behavior between the short circuit current (JSC) and open circuit voltage (VOC). This phenomenon can be attributed to diffusion-limited surface recombination at the non-selective perovskite-tip contact, where a higher carrier mobility established in the perovskite grain results in an enhanced surface recombination and thus a lower VOC. In addition, the photoresponse of perovskite films displays a pronounced heterogeneity across the grain boundaries, with the boundaries formed between grains of the same photoconductivity level displaying even enhanced photocurrent and open circuit voltage compared to those of the adjacent grain interiors. These observations highlight the significance of controlling the microstructure of perovskite thin films, which will be a necessary route for further improving the efficiency of perovskite solar cells.

Towards realistic molecular dynamics simulations of grain boundary mobility

International Nuclear Information System (INIS)

Zhou, J.; Mohles, V.

2011-01-01

In order to investigate grain boundary migration by molecular dynamics (MD) simulations a new approach involving a crystal orientation-dependent driving force has been developed by imposing an appropriate driving force on grain boundary atoms and enlarging the effective range of driving force. The new approach has been validated by the work of the driving force associated with the motion of grain boundaries. With the new approach the relation between boundary migration velocity and driving force is found to be nonlinear, as was expected from rate theory for large driving forces applied in MD simulations. By evaluating grain boundary mobility nonlinearly for a set of symmetrical tilt boundaries in aluminum at high temperature, high-angle grain boundaries were shown to move much faster than low-angle grain boundaries. This agrees well with experimental findings for recrystallization and grain growth. In comparison with the available data the simulated mobility of a 38.21 o Σ7 boundary was found to be significantly lower than other MD simulation results and comparable with the experimental values. Furthermore, the average volume involved during atomic jumps for boundary migration is determined in MD simulations for the first time. The large magnitude of the volume indicates that grain boundary migration is accomplished by the correlated motion of atom groups.

Special grain boundaries in the nugget zone of friction stir welded AA6061-T6 under various welding parameters

Energy Technology Data Exchange (ETDEWEB)

Tao, Wang [Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061 (China); Yong, Zou, E-mail: yzou@sdu.edu.cn [Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061 (China); Xuemei, Liu [Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061 (China); Matsuda, Kenji [Department of Materials Science and Technology, Faculty of Engineering, University of Toyama, Toyama 930-8555 (Japan)

2016-08-01

The age hardenable AA6061-T6 plate was butt welded by friction stir welding. The total heat input, generated by friction between the tool and work piece and plastic deformation, results in a consumption of meta-stable phases in the nugget zone. Precipitation phenomena were closely related to the diffusion of the solute atoms. The existence of special grain boundaries like Σ1a and Σ3 will increase the difficulty in diffusion, which will improve the hardness in the nugget zone. Furthermore, the formation of Σ3 grain boundaries can result from an impingement of re-crystallized grains coming from texture components in twin relationship already. An appropriate strain level may benefit the development of the twin components with a similar intensity. The welding parameters have an effect on heat source mode and the strain level. Then, the type of dynamic re-crystallization and distribution of the special grain boundaries was altered by changing the parameters.

Grain boundary structure and properties

International Nuclear Information System (INIS)

Balluffi, R.W.

1979-05-01

An attempt is made to distinguish those fundamental aspects of grain boundaries which should be relevant to the problem of the time dependent fracture of high temperature structural materials. These include the basic phenomena which are thought to be associated with cavitation and cracking at grain boundaries during service and with the more general microstructural changes which occur during both processing and service. A very brief discussion of the current state of knowledge of these fundamentals is given

Grain boundary and grain interior conduction in γ'-Bi2MoO6

International Nuclear Information System (INIS)

Vera, C.M.C.; Aragon, R.

2005-01-01

Impedance spectroscopy of fine grained ( 2 MoO 6 samples, in the frequency range of 0.1 Hz-250 kHz, relevant to sensor applications, up to 800 deg. C, has been used to characterize grain boundary and grain interior contributions to conduction. Above 500 deg. C, the grain boundary contribution is no longer rate limiting and conduction is dominated by the grain interior component. The corresponding activation energies are 0.98 eV for grain boundary and 0.73 eV for grain interior components. The weak dependence of conductivity on oxygen partial pressure below 500 deg. C can be attributed to electrode-electrolyte interface phenomena, whereas the robust response to ethanol is commensurate with changes in intrinsic ionic conductivity

A simple method to evaluate the fission gas release at fuel grain boundary including the grain growth both at constant and at transient power histories

International Nuclear Information System (INIS)

Paraschiv, M.; Paraschiv, A.

1991-01-01

A method to rewrite Fick's second law for a region with a moving boundary when the moving law in time of this boundary is known, has been proposed. This method was applied to Booth's sphere model for radioactive and stable fission product diffusion from the oxide fuel grain in order to take into account the grain growth. The solution of this new equation was presented in the mathematical formulation for power histories from ANS 5.4 model for the stable species. It is very simple to apply and very accurate. The results obtained with this solution for constant and transient temperatures show that the fission gas release (FGR) at grain boundary is strongly dependent on kinetics of grain growth. The utilization of two semiempirical grain growth laws, from published information, shows that the fuel microstructural properties need to be multicitly considered in the fission gas release for every manufacturer of fuel. (orig.)

Grooving of grain boundaries in multicrystalline silicon: Effect on solar cell performance

International Nuclear Information System (INIS)

Dimassi, W.; Bouaicha, M.; Nouri, H.; Boujmil, M.F.; Ben Nasrallah, S.; Bessais, B.

2006-01-01

In this work, we investigate the effect of grooving of grain boundaries (GB) in multicrystalline silicon using chemical etching in HF/HNO 3 solutions. The grain boundaries were grooved in order to reduce the area of these highly recombining regions. Using optimized conditions, grooved GBs enable deep phosphorus diffusion and deep metallic contacts. As a result, the internal quantum efficiency (IQE), and the I-V characteristics under the dark and AM1.5 illumination were improved. It was also observed a reduction of the GB recombination velocity, which was deduced from light-beam-induced-current (LBIC) measurements. Such grooving in multicrystalline silicon enables passivation of GB-related defects. These results are discussed and compared to solar cells based on untreated multicrystalline silicon wafers

Viscoelastic sliding and diffusive relaxation along grain boundaries in polycrystalline boron nitride

International Nuclear Information System (INIS)

Pezzotti, G.; Nishida, Toshihiko; Kleebe, H.J.; Ota, Kenichi

1997-01-01

Dense hexagonal boron nitride (BN) materials were prepared via two different processing routes: (1) hot-pressing with the addition of a Ca/B-containing glass and (2) chemical vapor deposition (CVD). The resulting microstructure of both materials was studied by scanning and transmission electron microscopy. While the hot-pressed BN material shows, apart from large BN matrix grains, an inhomogeneous distribution of residual glass at room temperature, the CVD deposition yields a homogeneous fine grained microstructure with no amorphous residue detectable. Internal-friction experiments were performed to study the micromechanical response of the materials when exposed to high temperatures. The CVD material revealed no relaxation peak during testing up to 2,300 C, while the glass-doped sample showed a pronounced relaxation peak at a peak-top temperature of about 600 C. This temperature corresponds to the softening temperature known for bulk Ca/B-glasses and it is, therefore, concluded that the glass homogeneously wets the BN grains at elevated temperatures. The results presented are seen as the first clear evidence that the internal friction peak monitored for various glass-containing ceramics is indeed related to a viscous sliding process along grain boundaries

The effect of surface contact conditions on grain boundary interdiffusion in a semi-infinite bicrystal

Czech Academy of Sciences Publication Activity Database

Svoboda, Jiří; Fischer, F. D.; Klinger, L.; Rabkin, E.

2014-01-01

Roč. 94, č. 30 (2014), s. 3398-3412 ISSN 1478-6435 Institutional support: RVO:68081723 Keywords : grain boundary diffusion * liquid metals * stress analysis * interfacial thermodynamics Subject RIV: BJ - Thermodynamics Impact factor: 1.825, year: 2014

Grain boundary cavitation under reversed constant stress

International Nuclear Information System (INIS)

Hales, R.

1978-06-01

The growth of grain boundary cavities by diffusion processes has been examined for cyclic stresses. It is found that the time required to grow a void by a predetermined amount (tsub(t)) is always longer than the time required to shrink the same defect to its original size (tsub(c)) under reversed stress. The ratio tsub(c)/tsub(t) is a function of the magnitude of the applied stress and tensile hold time. Similar calculations have been performed for gas filled bubbles. Similar results to those for voids are found at long hold times, but a significantly different ratio of tsub(c)/tsub(t) is obtained at short times. (author)

Neodymium ion diffusion during sintering of Nd : YAG transparent ceramics

Energy Technology Data Exchange (ETDEWEB)

Hollingsworth, Joel P; Kuntz, Joshua D; Soules, Thomas F [Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA, 94550 (United States)

2009-03-07

Using an electron microprobe, we measured and characterized the Nd{sup 3+} ion diffusion across a boundary between Nd doped and undoped ceramic yttrium aluminium garnet (YAG) for different temperature ramps and hold times and temperatures. The results show significant Nd ion diffusion on the order of micrometres to tens of micrometres depending on the time and temperature of sintering. The data fit well a model including bulk diffusion, grain boundary diffusion and grain growth. Grain boundary diffusion dominates and grain growth limits grain boundary diffusion by reducing the total cross-sectional area of grain boundaries. (fast track communication)

Role of Grain Boundaries under Long-Time Radiation

Science.gov (United States)

Zhu, Yichao; Luo, Jing; Guo, Xu; Xiang, Yang; Chapman, Stephen Jonathan

2018-06-01

Materials containing a high proportion of grain boundaries offer significant potential for the development of radiation-resistant structural materials. However, a proper understanding of the connection between the radiation-induced microstructural behavior of a grain boundary and its impact at long natural time scales is still missing. In this Letter, point defect absorption at interfaces is summarized by a jump Robin-type condition at a coarse-grained level, wherein the role of interface microstructure is effectively taken into account. Then a concise formula linking the sink strength of a polycrystalline aggregate with its grain size is introduced and is well compared with experimental observation. Based on the derived model, a coarse-grained formulation incorporating the coupled evolution of grain boundaries and point defects is proposed, so as to underpin the study of long-time morphological evolution of grains induced by irradiation. Our simulation results suggest that the presence of point defect sources within a grain further accelerates its shrinking process, and radiation tends to trigger the extension of twin boundary sections.

A grain boundary phase transition in Si–Au

International Nuclear Information System (INIS)

Ma, Shuailei; Meshinchi Asl, Kaveh; Tansarawiput, Chookiat; Cantwell, Patrick R.; Qi, Minghao; Harmer, Martin P.; Luo, Jian

2012-01-01

A grain boundary transition from a bilayer to an intrinsic (nominally clean) boundary is observed in Si–Au. An atomically abrupt transition between the two complexions (grain boundary stabilized phases) implies the occurrence of a first-order interfacial phase transition associated with a discontinuity in the interfacial excess. This observation supports a grain-boundary complexion theory with broad applications. This transition is atypical in that the monolayer complexion is absent. A model is proposed to explain the bilayer stabilization and the origin of this complexion transition.

Electronic and atomic structures of KFe2Se2 grain boundaries

International Nuclear Information System (INIS)

Fan, Wei; Liu, Da-Yong; Zeng, Zhi

2014-01-01

Highlights: •Twist grain boundary has lower grain-boundary energy. •Twist grain-boundary has similar electronic structure to that in crystal. •Charge and magnetic-moment fluctuations are large within tilt grain boundary. •Bi-collinear AFM is most stable even with existence of grain boundary. •Insulating Fe-vacancy phase is stable with existence of twist grain boundary. -- Abstract: The electronic and atomic structures of the twist and tilt grain boundaries (GB) of the iron-based superconductor KFe 2 Se 2 are studied based on the simulations of the first principles density functional theory. Our results have clarified that the Σ5[0 0 1] twist grain boundary of KFe 2 Se 2 with layered structure has the lower grain-boundary energy. The local structure and the main features of the basic electronic structure within the [0 0 1] twist grain-boundary region have small differences compared with those in KFe 2 Se 2 crystal. The large fluctuations of the charges and magnetic moments are found in the [0 0 1] tilt grain-boundary regions, especially the former are more prominent. The bi-collinear anti-ferromagnetic order is the most stable magnetic order even with grain boundaries in the bulk. The √(5)a×√(5)a superstructure of Fe-vacancies in K 2 Fe 4 Se 5 phase is intrinsically related to the coincident-site lattice of Σ5[0 0 1] twist grain boundary

Interdiffusion and grain-boundary migration in Au-Cu bilayers during ion-irradiation

International Nuclear Information System (INIS)

Alexander, D.E.; Rehn, L.E.; Baldo, P.M.

1991-11-01

Ion irradiation and annealing experiments have been conducted on Au/Cu bilayer films to evaluate the effect of irradiation on diffusion-induced grain boundary migration (DIGM). The Au films were prepared with a large-grained microstructure with grain boundaries perpendicular to the film surface and extending through the film thickness. Irradiations were conducted with 1.5 MeV Kr at 228 degree C. Rutherford backscattering spectrometry of the samples revealed that interdiffusion was substantially enhanced in the irradiated area relative to the unirradiated area. Both irradiated and annealed-only areas were characterized by a nearly uniform composition of 14 at.% and 7 at.% Cu respectively through the entire thickness of the underlying Au film. Small probe X-ray energy dispersive spectroscopy showed significant lateral compositional homogeneities in both irradiated and annealed areas. These two results are consistent with previous observations of DIGM in the Au/Cu system, suggesting that this previously unexamined mechanism contributes to ion beam mixing

Basic aspects of high-Tc grain boundary devices

International Nuclear Information System (INIS)

Mannhart, J.; Moler, K.A.; Sigrist, M.

1996-01-01

Grain boundaries are extensively used as high-quality Josephson junctions in high-T c superconductors. Their superconducting characteristics can generally be well described by conventional models of strongly coupled Josephson junctions. Here, we report on highly anomalous critical current vs. magnetic field dependencies of grain boundaries in YBa 2 Cu 3 O 7-x . Direct imaging with scanning SQUID microscopy provides evidence of magnetic flux generated by single grain boundaries. Conventional Josephson junction models cannot explain these effects if a superconducting order parameter with a pure s-wave symmetry is assumed. The results have significant implications for our understanding of the properties of grain boundaries in high-T c superconductors and for their applications. (orig.)

Effects of grain size and grain boundaries on defect production in nanocrystalline 3C-SiC

International Nuclear Information System (INIS)

Swaminathan, N.; Kamenski, Paul J.; Morgan, Dane; Szlufarska, Izabela

2010-01-01

Cascade simulations in single crystal and nanocrystalline SiC have been conducted in order to determine the role of grain boundaries and grain size on defect production during primary radiation damage. Cascades are performed with 4 and 10 keV silicon as the primary knock-on atom (PKA). Total defect production is found to increase with decreasing grain size, and this effect is shown to be due to increased production in grain boundaries and changing grain boundary volume fraction. In order to consider in-grain defect production, a new mapping methodology is developed to properly normalize in-grain defect production rates for nanocrystalline materials. It is shown that the presence of grain boundaries does not affect the total normalized in-grain defect production significantly (the changes are lower than ∼20%) for the PKA energies considered. Defect production in the single grain containing the PKA is also studied and found to increase for smaller grain sizes. In particular, for smaller grain sizes the defect production decreases with increasing distance from the grain boundary while for larger grain sizes the presence of the grain boundaries has negligible effect on defect production. The results suggest that experimentally observed changes in radiation resistance of nanocrystalline materials may be due to long-term damage evolution rather than changes in defect production rates from primary damage.

Dynamical simulation of structural multiplicity in grain boundaries

International Nuclear Information System (INIS)

Majid, I.; Bristowe, P.D.

1987-06-01

Work on a computer simulation study of a low-energy high-angle boundary structure which is not periodic have been recently reported. This result is of interest since grain boundary structures are usually assumed to have a periodicity corresponding to the appropriate coincidence site lattice (CSL) and many experimental observations of the structure of grain boundaries performed using conventional and high-resolution electron microscopy, electron diffraction and x-ray diffraction appear to support this work. However, this work, using empirical interatomic pair potentials and the relaxation method of molecular statics, have simulated a Σ = 5 36.87 0 (001) twist boundary and found a low energy structure having a larger repeat cell than the CSL and is composed of two different types of structural unit that are randomly distributed in the boundary plane. This result, which has been termed the multiplicity of grain boundary structures, has also been found in the simulation of tilt boundaries. The multiplicity phenomenon is of special interest in twist boundaries since it is used as a structural model to explain the x-ray scattering from a Σ = 5 boundary in gold. These scattering patterns had previously remained unexplained using stable structures that had simple CSL periodicity. Also, the effect of having a multiple number of low energy structural units coexisting in the grain boundary is of more general interest since it implies that the boundary structures may be quasi-periodic and, in some circumstances, may even result in a roughening of the boundary plane. This paper extends this work by showing, using molecular dynamics, that a multiplicity of structural units can actually nucleate spontaneously in a high-angle grain boundary at finite temperatures

Advantageous grain boundaries in iron pnictide superconductors

Science.gov (United States)

Katase, Takayoshi; Ishimaru, Yoshihiro; Tsukamoto, Akira; Hiramatsu, Hidenori; Kamiya, Toshio; Tanabe, Keiichi; Hosono, Hideo

2011-01-01

High critical temperature superconductors have zero power consumption and could be used to produce ideal electric power lines. The principal obstacle in fabricating superconducting wires and tapes is grain boundaries—the misalignment of crystalline orientations at grain boundaries, which is unavoidable for polycrystals, largely deteriorates critical current density. Here we report that high critical temperature iron pnictide superconductors have advantages over cuprates with respect to these grain boundary issues. The transport properties through well-defined bicrystal grain boundary junctions with various misorientation angles (θGB) were systematically investigated for cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films fabricated on bicrystal substrates. The critical current density through bicrystal grain boundary (JcBGB) remained high (>1 MA cm−2) and nearly constant up to a critical angle θc of ∼9°, which is substantially larger than the θc of ∼5° for YBa2Cu3O7–δ. Even at θGB>θc, the decay of JcBGB was much slower than that of YBa2Cu3O7–δ. PMID:21811238

Grain boundary corrosion of copper canister weld material

International Nuclear Information System (INIS)

Gubner, Rolf; Andersson, Urban; Linder, Mats; Nazarov, Andrej; Taxen, Claes

2006-01-01

The proposed design for a final repository for spent fuel and other long-lived residues in Sweden is based on the multi-barrier principle. The waste will be encapsulated in sealed cylindrical canisters, which will then be placed in granite bedrock and surrounded by compacted bentonite clay. The canister design is based on a thick cast inner container fitted inside a corrosion-resistant copper canister. During fabrication of the outer copper canisters there will be some unavoidable grain growth in the welded areas. As grains grow, they will tend to concentrate impurities within the copper at the new grain boundaries. The work described in this report was undertaken to determine whether there is any possibility of enhanced corrosion at grain boundaries within the copper canister, based on the recommendations of the report SKB-TR--01-09 (INIS ref. 32025363). Grain boundary corrosion of copper is not expected to be a problem for the copper canisters in a repository. However, as one step in the experimental verification it is necessary to study grain boundary corrosion of copper in an environment where it may occur. A literature study aimed to find one or several solutions that are aggressive with respect to grain boundary corrosion of copper. Copper specimens cut from welds of real copper canisters where exposed to aerated ammonium hydroxide solution for a period of 14 days at 80 degrees C and 10 bar pressure. The samples were investigated prior to exposure using the scanning Kelvin probe technique to characterize anodic and cathodic areas on the samples. The degree of corrosion was determined by optical microscopy. No grain boundary corrosion could be observed in the autoclave experiments, however, a higher rate of corrosion was observed for the weld material compared to the base material. The work suggests that grain boundary corrosion of copper weld material is most unlikely to adversely affect SKB's copper canisters under the conditions in the repository

Grain boundary corrosion of copper canister weld material

Energy Technology Data Exchange (ETDEWEB)

Gubner, Rolf; Andersson, Urban; Linder, Mats; Nazarov, Andrej; Taxen, Claes [Corrosion and Metals Research Inst. (KIMAB), Stockholm (Sweden)

2006-01-15

The proposed design for a final repository for spent fuel and other long-lived residues in Sweden is based on the multi-barrier principle. The waste will be encapsulated in sealed cylindrical canisters, which will then be placed in granite bedrock and surrounded by compacted bentonite clay. The canister design is based on a thick cast inner container fitted inside a corrosion-resistant copper canister. During fabrication of the outer copper canisters there will be some unavoidable grain growth in the welded areas. As grains grow, they will tend to concentrate impurities within the copper at the new grain boundaries. The work described in this report was undertaken to determine whether there is any possibility of enhanced corrosion at grain boundaries within the copper canister, based on the recommendations of the report SKB-TR--01-09 (INIS ref. 32025363). Grain boundary corrosion of copper is not expected to be a problem for the copper canisters in a repository. However, as one step in the experimental verification it is necessary to study grain boundary corrosion of copper in an environment where it may occur. A literature study aimed to find one or several solutions that are aggressive with respect to grain boundary corrosion of copper. Copper specimens cut from welds of real copper canisters where exposed to aerated ammonium hydroxide solution for a period of 14 days at 80 degrees C and 10 bar pressure. The samples were investigated prior to exposure using the scanning Kelvin probe technique to characterize anodic and cathodic areas on the samples. The degree of corrosion was determined by optical microscopy. No grain boundary corrosion could be observed in the autoclave experiments, however, a higher rate of corrosion was observed for the weld material compared to the base material. The work suggests that grain boundary corrosion of copper weld material is most unlikely to adversely affect SKB's copper canisters under the conditions in the repository.

The influence of grain boundary structure on diffusional creep

DEFF Research Database (Denmark)

Thorsen, Peter Anker; Bilde-Sørensen, Jørgen

1999-01-01

the deformation caused by deposition of material at (or removal of material from) grain boundaries. The misorientation across the grain boundaries, and hence the character of the boundaries, was measured with the use of electron back-scattering patterns. The deformation behavior of the individual boundaries......A Cu-2wt%Ni-alloy was deformed in tension in the diffusional creep regime (Nabarro-Herring creep). A periodic grid consisting of alumina was deposited on the surface of the creep specimen prior to creep. This makes it possible to separate the deformation caused by grain boundary sliding from...

Grain boundaries of nanocrystalline materials - their widths, compositions, and internal structures

International Nuclear Information System (INIS)

Fultz, B.; Frase, H.N.

2000-01-01

Nanocrystalline materials contain many atoms at and near grain boundaries. Sufficient numbers of Moessbauer probe atoms can be situated in grain boundary environments to make a clear contribution to the measured Moessbauer spectrum. Three types of measurements on nanocrystalline materials are reported here, all using Moessbauer spectrometry in conjunction with X-ray diffractometry, transmission electron microscopy, or small angle neutron scattering. By measuring the fraction of atoms contributing to the grain boundary component in a Moessbauer spectrum, and by knowing the grain size of the material, it is possible to deduce the average width of grain boundaries in metallic alloys. It is found that these widths are approximately 0.5 nm for fcc alloys and slightly larger than 1.0 nm for bcc alloys.Chemical segregation to grain boundaries can be measured by Moessbauer spectrometry, especially in conjunction with small angle neutron scattering. Such measurements on Fe-Cu and Fe 3 Si-Nb were used to study how nanocrystalline materials could be stabilized against grain growth by the segregation of Cu and Nb to grain boundaries. The segregation of Cu to grain boundaries did not stabilize the Fe-Cu alloys against grain growth, since the grain boundaries were found to widen and accept more Cu atoms during annealing. The Nb additions to Fe 3 Si did suppress grain growth, perhaps because of the low mobility of Nb atoms, but also perhaps because Nb atoms altered the chemical ordering in the alloy.The internal structure of grain boundaries in nanocrystalline materials prepared by high-energy ball milling is found to be unstable against internal relaxations at low temperatures. The Moessbauer spectra of the nanocrystalline samples showed changes in the hyperfine fields attributable to movements of grain boundary atoms. In conjunction with SANS measurements, the changes in grain boundary structure induced by cryogenic exposure and annealing at low temperature were found to be

Migration mechanisms of a faceted grain boundary

Science.gov (United States)

Hadian, R.; Grabowski, B.; Finnis, M. W.; Neugebauer, J.

2018-04-01

We report molecular dynamics simulations and their analysis for a mixed tilt and twist grain boundary vicinal to the Σ 7 symmetric tilt boundary of the type {1 2 3 } in aluminum. When minimized in energy at 0 K , a grain boundary of this type exhibits nanofacets that contain kinks. We observe that at higher temperatures of migration simulations, given extended annealing times, it is energetically favorable for these nanofacets to coalesce into a large terrace-facet structure. Therefore, we initiate the simulations from such a structure and study as a function of applied driving force and temperature how the boundary migrates. We find the migration of a faceted boundary can be described in terms of the flow of steps. The migration is dominated at lower driving force by the collective motion of the steps incorporated in the facet, and at higher driving forces by the step detachment from the terrace-facet junction and propagation of steps across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in the facet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics. Since the mechanisms seem likely to be very general on kinked grain-boundary planes, the step-flow description is a promising approach to more quantitative modeling of general grain boundaries.

Structure of grain boundaries in hexagonal materials

International Nuclear Information System (INIS)

Sarrazit, F.

1998-05-01

The work presented in this thesis describes experimental and theoretical aspects associated with the structure of grain boundaries in hexagonal materials. It has been found useful to classify grain boundaries as low-angle, special or general on the basis of their structure. High-angle grain boundaries were investigated in tungsten carbide (WC) using conventional electron microscopy techniques, and three examples characteristic of the interfaces observed in this material were studied extensively. Three-dimensionally periodic patterns are proposed as plausible reference configurations, and the Burgers vectors of observed interfacial dislocations were predicted using a theory developed recently. The comparison of experimental observations with theoretical predictions proved to be difficult as contrast simulation techniques require further development for analysis to be completed confidently. Another part of this work involves the characterisation of high-angle grain boundaries in zinc oxide (ZnO) using circuit mapping. Two boundaries displayed structural features characteristic of the 'special' category, however, one boundary presented features which did not conform to this model. It is proposed that the latter observation shows a structural transition from the special to a more general type. Material fluxes involved in defect interactions were considered using the topological framework described in this work. A genera) expression was derived for the total flux arising which allows the behaviour of line-defects to be studied in complex interfacial processes. (author)

HREM investigation of the structure of the Σ5(310)/[001] symmetric tilt grain boundaries in Nb

International Nuclear Information System (INIS)

King, W.E.; Compbell, G.H.; Coombs, A.; Ruehle, M.

1991-01-01

This paper reports on atomistic simulations using interatomic potentials for Nb developed employing the embedded atom method (EAM) and the model generalized pseudopotential theory (MGPT) that have indicated a possible cusp at the Σ5 (310) orientation in the energy vs tilt angle curves for left-angle 001 right-angle symmetric tilt grain boundaries. In addition, the most stable structure predicted using EAM exhibits shifts of one crystal relative to the other along the tilt axis and along the direction perpendicular to the tilt axis lying in the boundary plane. The structure predicted using the MGPT was mirror symmetric across the plane of the grain boundary. This boundary has been prepared for experimental study using the ultra high vacuum diffusion bonding method. A segment of this boundary has been studied using high resolution electron microscopy

Grain boundary sinks in neutron-irradiated Zr and Zr-alloys

International Nuclear Information System (INIS)

Griffiths, M.; Gilbert, R.W.; Coleman, C.E.

1988-01-01

Samples of annealed sponge and crystal-bar Zr and Zircaloy-2 have been examined following irradiation in EBR-II at temperatures ≅ 700 K. Loop analysis shows that there is selective denuding of interstitial loops near to some grain boundaries indicating that such boundaries are net sinks for interstitial point defects. Furthermore, in sponge Zr and Zircaloy-2, vacancy c-component loops are observed running into the grain boundaries showing that the grain boundaries are not preferred sinks for vacancies. Cavities are observed in all samples. In crystal-bar Zr and sponge Zr they are mostly observed adjacent to grain boundaries. They are also sometimes found within grains associated with precipitates. The cavities are more common in the crystal-bar Zr and this is probably because both the sponge Zr and Zircaloy-2 contain vacancy c-component loops which compete for vacancies (assuming that the cavities are vacancy sinks). Only some of the grain boundaries have cavities adjacent to them and this may be related to the orientation of the boundary. (orig.)

Segregation to grain boundaries in nimonic PE16 superalloy

International Nuclear Information System (INIS)

Nettleship, D.J.; Wild, R.K.

1990-01-01

Nimonic PE16 alloy is a nickel-based superalloy containing 34 wt.% iron and 16wt.% chromium with additions of molybdenum, titanium and aluminium. It is used in the fuel assembly of the UK advanced gas-cooled reactors (AGR). This component supports significant loads in service and its mechanical integrity is therefore of paramount importance. Mechanical properties may be influenced by the grain size and grain boundary composition, both of which can themselves alter during service. Scanning Auger microscopy is a well-established method for investigating grain boundaries, and has now been applied to the study of PE16. In order to expose PE16 grain boundary surfaces it is necessary to hydrogen charge samples and fracture by pulling in tension at a slow strain rate within the ultra-high vacuum chamber of the Auger microprobe. A series of casts of nimonic PE16 alloy that have received a range of thermal ageing treatments have been fractured in an intergranular manner and the grain boundary composition determined. Segregation of trace and minority elements, particularly Mo and P, has been detected at grain boundaries. Significant variations between different as-manufactured casts were observed, whilst ageing brought about the growth of chromium-rich particles on the grain boundaries. Ductile fracture in PE16 followed a path through Ti(C, N) particles. Many of these particles incorporated large amounts of sulphur. (author)

A constitutive model of nanocrystalline metals based on competing grain boundary and grain interior deformation mechanisms

KAUST Repository

Gurses, Ercan; El Sayed, Tamer S.

2011-01-01

In this work, a viscoplastic constitutive model for nanocrystalline metals is presented. The model is based on competing grain boundary and grain interior deformation mechanisms. In particular, inelastic deformations caused by grain boundary

Grain-size distributions and grain boundaries of chalcopyrite-type thin films

International Nuclear Information System (INIS)

Abou-Ras, D.; Schorr, S.; Schock, H.W.

2007-01-01

CuInSe 2 , CuGaSe 2 , Cu(In,Ga)Se 2 and CuInS 2 thin-film solar absorbers in completed solar cells were studied in cross section by means of electronbackscatter diffraction. From the data acquired, grain-size distributions were extracted, and also the most frequent grain boundaries were determined. The grain-size distributions of all chalcopyrite-type thin films studied can be described well by lognormal distribution functions. The most frequent grainboundary types in these thin films are 60 - left angle 221 right angle tet and 71 - left angle 110 right angle tet (near) Σ3 twin boundaries. These results can be related directly to the importance of {112} tet planes during the topotactical growth of chalcopyrite-type thin films. Based on energetic considerations, it is assumed that the most frequent twin boundaries exhibit a 180 - left angle 221 right angle tet constellation. (orig.)

Evidence of preferential diffusion and segregation of impurities at grain boundaries in very pure niobium used for radiofrequency cavities

International Nuclear Information System (INIS)

Antoine, C.; Bonin, B.; Safa, H.; Berthier, B.; Tessier, E.; Trocelier, P.; Chevarier, A.; Chevarier, N.; Roux, B.

1996-01-01

Complete text of publication follows. Grain boundaries (GB) of titanified, heat treated and then etched niobium have been observed by the mean of the nuclear microprobe from the Laboratoire Pierre Sue at Saclay. The very small area of the probe allows to measure by PIXE 1 quantities of titanium as low as one monolayer at the GB. Indeed concentrations of titanium as high as some atomic percent were found on 6 μm etched samples, giving indication of a preferential diffusion and/or segregation at GB. Titanium was detectable also on 15 μm etched samples but was bellow the sensitivity of the microprobe for 35 μm etched samples. Moreover it was shown that not all boundaries were polluted with titanium, and that their behaviour was correlated with orientation. A discussion of the literature shows that all these facts are consistent with the behaviour of very pure metals. Segregation at GB is also known to influence dramatically the GB resistivity in metals and superconductors. For the latter, it has been shown that the GB resistivity can be responsible of occurrence of granular superconductivity phenomena. The presence of Ti deep into the Nb GB explains why a strong etching is needed after a purification heat treatment. Moreover, it has been shown that a heat treatment at lower temperature, although much longer in time, allows less deep diffusion of Ti and then needs a lighter etch. (author)

Effect of crystal orientation on grain boundary migration and radiation-induced segregation

International Nuclear Information System (INIS)

Hashimoto, N.; Eda, Y.; Takahashi, H.

1996-01-01

Fe-Cr-Ni, Ni-Al and Ni-Si alloys were electron-irradiated using a high voltage electron microscope (1 MeV), and in situ observations of the structural evolution and micro-chemical analysis were carried out. During the irradiation, the grain boundaries in the irradiated region migrated, while no grain boundary migration occurred in the unirradiated area. The occurrence of boundary migration depended on the orientation relationship of the boundary interfaces. Grain boundary migration took place in Fe-Cr-Ni and Ni-Si alloys with large crystal orientation difference between the two grains across a grain boundary. In Ni-Al, however, the grain boundary migration did not occur. The solute segregation was caused at grain boundary under irradiation and this segregation behavior was closely related to solute size, namely the concentrations of undersized Ni and oversized Cr elements in Fe-Cr-Ni alloy increased and reduced at grain boundary, respectively. The same dependence of segregation on the solute size was derived in Ni-Si and Ni-Al alloys, in which Si and Al solutes are undersized and oversized elements, respectively. Therefore, Si solute enriched and Al solute depleted at grain boundary. From the present segregation behavior, it is suggested that the flow of point defects into the boundary is the cause of grain boundary migration. (orig.)

Segregation of solute elements at grain boundaries in an ultrafine grained Al-Zn-Mg-Cu alloy

International Nuclear Information System (INIS)

Sha, Gang; Yao, Lan; Liao, Xiaozhou; Ringer, Simon P.; Chao Duan, Zhi; Langdon, Terence G.

2011-01-01

The solute segregation at grain boundaries (GBs) of an ultrafine grained (UFG) Al-Zn-Mg-Cu alloy processed by equal-channel angular pressing (ECAP) at 200 o C was characterised using three-dimensional atom probe. Mg and Cu segregate strongly to the grain boundaries. In contrast, Zn does not always show clear segregation and may even show depletion near the grain boundaries. Trace element Si selectively segregates at some GBs. An increase in the number of ECAP passes leads to a decrease in the grain size but an increase in solute segregation at the boundaries. The significant segregation of alloying elements at the boundaries of ultrafine-grained alloys implies that less solutes will be available in the matrix for precipitation with a decrease in the average grain size. -- Research Highlights: → Atom probe tomography has been employed successfully to reveal unique segregation of solutes at ultrafine grained material. → Mg and Cu elements segregated strongly at the grain boundary of an ultrafine grained Al-Zn-Mg-Cu alloy processed by 4-pass and 8-pass ECAP at 200 o C. Zn frequently depleted at GBs with a Zn depletion region of 7-15 nm in width on one or both sides of the GBs. Only a small fraction (3/13) of GBs were observed with a low level of Zn segregation where the combined Mg and Cu excess is over 3.1 atom/nm 2 . Si appeared selectively segregated at some of the GBs. → The increase in number of ECAP passes from 4 to 8 correlated with the increase in mean level segregation of Mg and Cu for both solute excess and peak concentration. → The change of plane normal of a grain boundary within 30 o only leads to a slight change in the solute segregation level.

Grain boundary sliding mechanism during high temperature deformation of AZ31 Magnesium alloy

Energy Technology Data Exchange (ETDEWEB)

Roodposhti, Peiman Shahbeigi, E-mail: pshahbe@ncsu.edu [North Carolina State University (United States); University of Connecticut (United States); Sarkar, Apu; Murty, Korukonda Linga [North Carolina State University (United States); Brody, Harold [University of Connecticut (United States); Scattergood, Ronald [North Carolina State University (United States)

2016-07-04

High temperature tensile creep tests were conducted on AZ31 Magnesium alloy at low stress range of 1–13 MPa to clarify the existence of grain boundary sliding (GBS) mechanism during creep deformation. Experimental data within the GBS regime shows the stress exponent is ~2 and the activation energy value is close to that for grain boundary diffusion. Analyses of the fracture surface of the sample revealed that the GBS provides many stress concentrated sites for diffusional cavities formation and leads to premature failure. Scanning electron microscopy images show the appearances of both ductile and brittle type fracture mechanism. X-ray diffraction line profile analysis (based on Williamson-Hall technique) shows a reduction in dislocation density due to dynamic recovery (DRV). A correlation between experimental data and Langdon's model for GBS was also demonstrated.

The Pinning by Particles of Low and High Angle Grain Boundaries during Grain Growth

DEFF Research Database (Denmark)

Tweed, C.J.; Ralph, B.; Hansen, Niels

1984-01-01

A study has been made using transmission electron microscopy of the pinning of grain boundaries in aluminium during grain growth by fine dispersions of alumina particles. The boundary parameters have been determined with precision and the pinning effects measured using an approach due to Ashby...

In-situ transmission electron microscopy study of glissile grain boundary dislocation relaxation in a near Σ = 3 {1 1 1} grain boundary in copper

International Nuclear Information System (INIS)

Couzinie, J.P.; Decamps, B.; Boulanger, L.; Priester, L.

2005-01-01

An in-situ annealing experiment has been performed on an intergranular dislocation configuration composed only of glissile grain boundary dislocations observed in a near Σ = 3 {1 1 1} grain boundary in copper. Relaxation phenomena are not obvious than those predicted by theoretical models. Upon annealing, glissile intergranular dislocations are shown to overcome dislocation obstacles by node movement leading to a decrease of the total grain boundary energy

Science at the interface : grain boundaries in nanocrystalline metals.

Energy Technology Data Exchange (ETDEWEB)

Rodriguez, Mark Andrew; Follstaedt, David Martin; Knapp, James Arthur; Brewer, Luke N.; Holm, Elizabeth Ann; Foiles, Stephen Martin; Hattar, Khalid M.; Clark, Blythe B.; Olmsted, David L.; Medlin, Douglas L.

2009-09-01

Interfaces are a critical determinant of the full range of materials properties, especially at the nanoscale. Computational and experimental methods developed a comprehensive understanding of nanograin evolution based on a fundamental understanding of internal interfaces in nanocrystalline nickel. It has recently been shown that nanocrystals with a bi-modal grain-size distribution possess a unique combination of high-strength, ductility and wear-resistance. We performed a combined experimental and theoretical investigation of the structure and motion of internal interfaces in nanograined metal and the resulting grain evolution. The properties of grain boundaries are computed for an unprecedented range of boundaries. The presence of roughening transitions in grain boundaries is explored and related to dramatic changes in boundary mobility. Experimental observations show that abnormal grain growth in nanograined materials is unlike conventional scale material in both the level of defects and the formation of unfavored phases. Molecular dynamics simulations address the origins of some of these phenomena.

Molecular dynamics study on microstructure of near grain boundary distortion region in small grain size nano- NiAl alloy

International Nuclear Information System (INIS)

Wang, J.Y.; Wang, X.W.; Rifkin, J.; Li, D.X.

2001-12-01

Using the molecular dynamics simulation method, the microstructure of distortion region near curved amorphous-like grain boundary in nano-NiAl alloy is studied. The results showed that due to the internal elastic force of high energy grain boundary, distortion layer exists between grain and grain boundary. The lattice expansion and structure factor decreasing are observed in this region. Stacking fault in sample with grain size 3.8nm is clearly observed across the distortion region at the site very close to grain. The influences of different grain sizes on average distortion degree and volume fractions of distortion region, grain and grain boundary are also discussed. (author)

A first-principles analysis of ballistic conductance, grain boundary scattering and vertical resistance in aluminum interconnects

Science.gov (United States)

Zhou, Tianji; Lanzillo, Nicholas A.; Bhosale, Prasad; Gall, Daniel; Quon, Roger

2018-05-01

We present an ab initio evaluation of electron scattering mechanisms in Al interconnects from a back-end-of-line (BEOL) perspective. We consider the ballistic conductance as a function of nanowire size, as well as the impact of surface oxidation on electron transport. We also consider several representative twin grain boundaries and calculate the specific resistivity and reflection coefficients for each case. Lastly, we calculate the vertical resistance across the Al/Ta(N)/Al and Cu/Ta(N)/Cu interfaces, which are representative of typical vertical interconnect structures with diffusion barriers. Despite a high ballistic conductance, the calculated specific resistivities at grain boundaries are 70-100% higher in Al than in Cu, and the vertical resistance across Ta(N) diffusion barriers are 60-100% larger for Al than for Cu. These results suggest that in addition to the well-known electromigration limitations in Al interconnects, electron scattering represents a major problem in achieving low interconnect line resistance at fine dimensions.

Accelerated diffusion controlled creep of polycrystalline materials. Communication 1. Model of diffusion controlled creep acceleration

International Nuclear Information System (INIS)

Smirnova, E.S.; Chuvil'deev, V.N.

1998-01-01

The model is suggested which describes the influence of large-angle grain boundary migration on a diffusion controlled creep rate in polycrystalline materials (Coble creep). The model is based on the concept about changing the value of migrating boundary free volume when introducing dislocations distributed over the grain bulk into this boundary. Expressions are obtained to calculate the grain boundary diffusion coefficient under conditions of boundary migration and the parameter, which characterized the value of Coble creep acceleration. A comparison is made between calculated and experimental data for Cd, Co and Fe

Binding energetics of substitutional and interstitial helium and di-helium defects with grain boundary structure in α-Fe

International Nuclear Information System (INIS)

Tschopp, M. A.; Gao, F.; Yang, L.; Solanki, K. N.

2014-01-01

The formation/binding energetics and length scales associated with the interaction between He atoms and grain boundaries in BCC α-Fe were explored. Ten different low Σ grain boundaries from the 〈100〉 and 〈110〉 symmetric tilt grain boundary systems were used. In this work, we then calculated formation/binding energies for 1–2 He atoms in the substitutional and interstitial sites (HeV, He 2 V, HeInt, He 2 Int) at all potential grain boundary sites within 15 Å of the boundary (52 826 simulations total). The present results provide detailed information about the interaction energies and length scales of 1–2 He atoms with grain boundaries for the structures examined. A number of interesting new findings emerge from the present study. For instance, the Σ3(112) twin boundary in BCC Fe possesses a much smaller binding energy than other boundaries, which corresponds in long time dynamics simulations to the ability of an interstitial He defect to break away from the boundary in simulations on the order of nanoseconds. Additionally, positive correlations between the calculated formation/binding energies of the He defects (R > 0.9) asserts that the local environment surrounding each site strongly influences the He defect energies and that highly accurate quantum mechanics calculations of lower order defects may be an adequate predictor of higher order defects. Various metrics to quantify or classify the local environment were compared with the He defect binding energies. The present work shows that the binding and formation energies for He defects are important for understanding the physics of He diffusion and trapping by grain boundaries, which can be important for modeling He interactions in polycrystalline steels

Binding energetics of substitutional and interstitial helium and di-helium defects with grain boundary structure in α-Fe

Energy Technology Data Exchange (ETDEWEB)

Tschopp, M. A., E-mail: mark.tschopp@gatech.edu [Dynamic Research Corporation, (on site at) U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (United States); Center for Advanced Vehicular Systems, Mississippi State University, Starkville, Mississippi 39762 (United States); Gao, F.; Yang, L. [Pacific Northwest National Laboratory, Richland, Washington 99352 (United States); Solanki, K. N. [Arizona State University, School for Engineering of Matter, Transport and Energy, Tempe, Arizona 85287 (United States)

2014-01-21

The formation/binding energetics and length scales associated with the interaction between He atoms and grain boundaries in BCC α-Fe were explored. Ten different low Σ grain boundaries from the 〈100〉 and 〈110〉 symmetric tilt grain boundary systems were used. In this work, we then calculated formation/binding energies for 1–2 He atoms in the substitutional and interstitial sites (HeV, He{sub 2}V, HeInt, He{sub 2}Int) at all potential grain boundary sites within 15 Å of the boundary (52 826 simulations total). The present results provide detailed information about the interaction energies and length scales of 1–2 He atoms with grain boundaries for the structures examined. A number of interesting new findings emerge from the present study. For instance, the Σ3(112) twin boundary in BCC Fe possesses a much smaller binding energy than other boundaries, which corresponds in long time dynamics simulations to the ability of an interstitial He defect to break away from the boundary in simulations on the order of nanoseconds. Additionally, positive correlations between the calculated formation/binding energies of the He defects (R > 0.9) asserts that the local environment surrounding each site strongly influences the He defect energies and that highly accurate quantum mechanics calculations of lower order defects may be an adequate predictor of higher order defects. Various metrics to quantify or classify the local environment were compared with the He defect binding energies. The present work shows that the binding and formation energies for He defects are important for understanding the physics of He diffusion and trapping by grain boundaries, which can be important for modeling He interactions in polycrystalline steels.

Grain boundary and grain interior conduction in {gamma}'-Bi{sub 2}MoO{sub 6}

Energy Technology Data Exchange (ETDEWEB)

Vera, C.M.C. [Laboratorio de Peliculas Delgadas, Facultad de Ingenieria, Universidad de Buenos Aires, Paseo Colon 850, 1063 Buenos Aires (Argentina)]. E-mail: cvera@fi.uba.ar; Aragon, R. [Laboratorio de Peliculas Delgadas, Facultad de Ingenieria, Universidad de Buenos Aires, Paseo Colon 850, 1063 Buenos Aires (Argentina); CINSO, CONICET, CITEFA, Lasalle 4397, Villa Martelli, Buenos Aires (Argentina)

2005-07-25

Impedance spectroscopy of fine grained (<10 {mu}m) {gamma}'-Bi{sub 2}MoO{sub 6} samples, in the frequency range of 0.1 Hz-250 kHz, relevant to sensor applications, up to 800 deg. C, has been used to characterize grain boundary and grain interior contributions to conduction. Above 500 deg. C, the grain boundary contribution is no longer rate limiting and conduction is dominated by the grain interior component. The corresponding activation energies are 0.98 eV for grain boundary and 0.73 eV for grain interior components. The weak dependence of conductivity on oxygen partial pressure below 500 deg. C can be attributed to electrode-electrolyte interface phenomena, whereas the robust response to ethanol is commensurate with changes in intrinsic ionic conductivity.

Observing grain boundaries in CVD-grown monolayer transition metal dichalcogenides

KAUST Repository

Ly, Thuchue

2014-11-25

Two-dimensional monolayer transition metal dichalcogenides (TMdCs), driven by graphene science, revisit optical and electronic properties, which are markedly different from bulk characteristics. These properties are easily modified due to accessibility of all the atoms viable to ambient gases, and therefore, there is no guarantee that impurities and defects such as vacancies, grain boundaries, and wrinkles behave as those of ideal bulk. On the other hand, this could be advantageous in engineering such defects. Here, we report a method of observing grain boundary distribution of monolayer TMdCs by a selective oxidation. This was implemented by exposing directly the TMdC layer grown on sapphire without transfer to ultraviolet light irradiation under moisture-rich conditions. The generated oxygen and hydroxyl radicals selectively functionalized defective grain boundaries in TMdCs to provoke morphological changes at the boundary, where the grain boundary distribution was observed by atomic force microscopy and scanning electron microscopy. This paves the way toward the investigation of transport properties engineered by defects and grain boundaries. (Figure Presented).

Cadmium Telluride and Grain Boundaries: A Preliminary Study

Science.gov (United States)

Liao, Michael Evan

The efficacy of the CdCl2 treatment on polycrystalline CdTe-based solar cells was discovered over a quarter of a century ago; and yet, the exact mechanism of this treatment is still not fully understood to this day. In fact, the lack of understanding stems from a debate on the exact role of grain boundaries in CdCl2-treated CdTe solar cells. Some hypothesize that the CdCl2-treatment causes grain boundaries to become beneficial to solar cell performance while others disagree and claim that the treatment simply mitigates the harmful effects of grain boundaries via passivation. A future goal of this project is to determine which, if either, hypothesis is correct by direct wafer bonding single crystalline CdTe. Direct wafer bonding of single crystalline materials would create only one grain boundary at the bonded interface. This approach allows the orientation and surface chemistry of interfaces to be controlled in order to study the chemistry of grain boundaries methodically. However, before any direct wafer bonding can be done, a preliminary study of single crystalline CdTe is necessary. High-quality direct wafer bonding can only be achieved if the surfaces of each wafer satisfy certain requirements. Additionally, analyzing single crystalline CdTe materials prior to bonding is crucial in order to make any insightful connections between results found from direct bonding of single crystalline CdTe and what is observed in polycrystalline CdTe. First, the surface of an (001) CdTe layer epitaxially grown on an (001) InSb substrate is studied using atomic force microscopy. Stacking faults on the CdTe surface are observed and the thickness of the grown CdTe epilayer is calculated by considering the interplanar angles between the (001) and (111) crystallographic planes as well as the dimensions of the stacking faults. While the stacking faults will inhibit successful wafer bonding, the roughness of the regions outside the stacking faults is 0.9 nm, which is an acceptable

Grain-boundary unzipping by oxidation in polycrystalline graphene

Science.gov (United States)

Alexandre, Simone; Lucio, Aline; Nunes, Ricardo

2011-03-01

The need for large-scale production of graphene will inevitably lead to synthesis of the polycrystalline material [1,2]. Understanding the chemical, mechanical, and electronic properties of grain boundaries in graphene polycrystals will be crucial for the development of graphene-based electronics. Oxidation of this material has been suggested to lead to graphene ribbons, by the oxygen-driven unzipping mechanism. A cooperative-strain mechanism, based on the formation of epoxy groups along lines of parallel bonds in the hexagons of graphene's honeycomb lattice, was proposed to explain the unzipping effect in bulk graphene In this work we employ ab initio calculations to study the oxidation of polycrystalline graphene by chemisorption of oxygen at the grain boundaries. Our results indicate that oxygen tends to segregate at the boundaries, and that the unzipping mechanism is also operative along the grain boundaries, despite the lack of the parallel bonds due to the presence of fivefold and sevenfold carbon rings along the boundary core. We acknowledge support from the Brazilian agencies: CNPq, Fapemig, and INCT-Materiais de Carbono.

Cross-sectional measurement of grain boundary segregation using WDS

Energy Technology Data Exchange (ETDEWEB)

Christien, F., E-mail: frederic.christien@emse.fr [Laboratoire Georges Friedel, CNRS, Ecole des Mines de Saint-Etienne, 158 Cours Fauriel, 42023 Saint-Etienne (France); Risch, P. [Institut des Matériaux Jean Rouxel (IMN), CNRS, Université de Nantes, Rue Christian Pauc, 44306 Nantes (France)

2016-11-15

A new method is proposed for the quantification of grain boundary segregation using Wavelength Dispersive Spectroscopy (WDS) in a Scanning Electron Microscope (SEM). Analyses are undertaken on a simple metallographically polished section of material. The method is demonstrated for the model system of sulphur segregation to nickel grain boundaries. Quantification was carried out from sulphur concentration profiles acquired across 11 grain boundaries of a nickel specimen containing 5.4 wt ppm of sulphur in the bulk and equilibrated at 550 °C. The average sulphur grain boundary concentration determined is µ=35.2 ng cm{sup −2}=6.6×10{sup 14} atoms cm{sup −2}≈0.5 monolayer, which is in good agreement with a previous quantification obtained from SIMS (Secondary Ion Mass Spectrometry) on the same material. However this is lower by a factor of two than the quantification obtained using “surface” techniques on fractured specimens of the same material. With the conditions of analysis used in this study, the limit of detection of the method developed is found to be better than 10% of a sulphur monolayer. - Highlights: • Impurity grain boundary segregation can be measured using WDS in a SEM. • The method proposed is quantitative. • The specimen preparation is simple: metallographical section.

Radiation-induced grain boundary segregation in austenitic stainless steels

International Nuclear Information System (INIS)

Bruemmer, S.M.; Charlot, L.A.; Vetrano, J.S.; Simonen, E.P.

1994-11-01

Radiation-induced segregation (RIS) to grain boundaries in Fe-Ni-Cr-Si stainless alloys has been measured as a function of irradiation temperature and dose. Heavy-ion irradiation was used to produce damage levels from 1 to 20 displacements per atom (dpa) at temperatures from 175 to 550 degrees C. Measured Fe, Ni, and Cr segregation increased sharply with irradiation dose (from G to 5 dpa) and temperature (from 175 to about 350 degrees C). However, grain boundary concentrations did not change significantly as dose or temperatures were further increased. Although interfacial compositions were similar, the width of radiation-induced enrichment or depletion profiles increased consistently with increasing dose or temperature. Impurity segregation (Si and P) was also measured, but only Si enrichment appeared to be radiation-induced. Grain boundary Si peaked at levels approaching 10 at% after irradiation doses to 10 dpa at an intermediate temperature of 325 degrees C. No evidence of grain boundary silicide precipitation was detected after irradiation at any temperature. Equilibrium segregation of P was measured in the high-P alloys, but interfacial concentration did not increase with irradiation exposure. Comparisons to reported RIS in neutron-irradiated stainless steels revealed similar grain boundary compositional changes for both major alloying and impurity elements

Impurity effects on the grain boundary cohesion in copper

Science.gov (United States)

Li, Yunguo; Korzhavyi, Pavel A.; Sandström, Rolf; Lilja, Christina

2017-12-01

Segregated impurities at grain boundaries can dramatically change the mechanical behavior of metals, while the mechanism is still obscure in some cases. Here, we suggest a unified approach to investigate segregation and its effects on the mechanical properties of polycrystalline alloys using the example of 3 s p impurities (Mg, Al, Si, P, or S) at a special type Σ 5 (310 )[001 ] tilt grain boundary in Cu. We show that for these impurities segregating to the grain boundary, the strain contribution to the work of grain boundary decohesion is small and that the chemical contribution correlates with the electronegativity difference between Cu and the impurity. The strain contribution to the work of dislocation emission is calculated to be negative, while the chemical contribution is calculated to be always positive. Both the strain and chemical contributions to the work of dislocation emission generally become weaker with the increasing electronegativity from Mg to S. By combining these contributions together, we find, in agreement with experimental observations, that a strong segregation of S can reduce the work of grain boundary separation below the work of dislocation emission, thus embrittling Cu, while such an embrittlement cannot be produced by a P segregation because it lowers the energy barrier for dislocation emission relatively more than for work separation.

Study of grain boundary tunneling in barium-titanate ceramic films

CERN Document Server

Wong, H; Poon, M C

1999-01-01

The temperature and the electric-field dependences of the current-voltage characteristics and the low-frequency noise of barium-titanate ceramic films are studied. An abnormal field dependence is observed in the resistivity of BaTiO sub 3 materials with a small average grain size. In addition, experiments show that the low-frequency noise behaviors are governed by grain-boundary tunneling at room temperature and by trapping-detrapping of grain-boundary states at temperatures above the Curie point. Physical models for the new observations are developed. Results suggest that grain-boundary tunneling of carriers is as important as the double Schottky barrier in the current conduction in BaTiO sub 3 materials with small grain sizes.

Measurement of gap and grain-boundary inventories of 129I in used CANDU fuels

International Nuclear Information System (INIS)

Stroes-Gascoyne, S.; Moir, D.L.; Kolar, M.; Porth, R.J.; McConnell, J.L.; Kerr, A.H.

1995-01-01

Combined gap and grain-boundary inventories of 129 I in 14 used CANDU fuel elements were measured by crushing and simultaneously leaching fuel segments for 4 h in a solution containing KI carrier. From analogy with previous work a near one-to-one correlation was anticipated between the amount of stable Xe and the amount of 128 I in the combined gap and grain-boundary regions of the fuel. However, the results showed that such a correlation was only apparent for low linear power rating (LLPR) fuels with an average linear power rating of 44 kW/m), the 129 I values were considerably smaller than expected. The combined gap and grain-boundary inventories of 129 I in the 14 fuels tested varied from 1.8 to 11.0%, with an average value of 3.6 ± 2.4% which suggests that the average value of 8.1 ± 1% used in safety assessment calculations overestimates the instant release fraction for 129 I. Segments of used CANDU fuels were leached for 92 d (samples taken at 5, 28 and 92 d) to determine the kinetics of 129 I release. Results could be fitted tentatively to half-order reaction kinetics, implying that 129 I release is a diffusion-controlled process for LLPR fuels, and also for HLPR fuels, once the gap inventory has been leached. However, more data are needed over longer leaching periods to gain more understanding of the processes that control grain-boundary release of 129 I from used CANDU fuel

Improvement of creep-rupture properties by serrated grain boundaries in high-tungsten cobalt-base superalloys

International Nuclear Information System (INIS)

Tanaka, Manabu

1993-01-01

The improvement of creep-rupture properties by serrated grain boundaries was investigated using cobalt-base superalloys containing about 14 to 20 wt.% tungsten at 1089 and 1311 K. Serrated grain boundaries improved both the rupture life and the ductility, especially under lower stresses at 1089 K. The increase in rupture life was larger in the alloys containing a larger amount of W. Ductile grain boundary fracture surfaces, which involved dimple patterns and grain boundary ledges, were observed in the specimens with serrated grain boundaries whereas brittle grain boundary facets were observed in the specimens with normal straight grain boundaries ruptured at 1089 K. The strengthening by serrated grain boundaries was also effective at 1311 K, but there was little difference in rupture life between the specimens with serrated grain boundaries and those with straight grain boundaries under lower stresses, since serrated grain boundaries developed also in the specimens with straight grain boundaries according to grain boundary precipitates forming during creep at 1311 K. The increase in W content of the alloys led to the increase in rupture life of the specimens with serrated grain boundaries at 1089 and 1311 K. (orig.) [de

Grain-boundary melting: A Monte Carlo study

DEFF Research Database (Denmark)

Besold, Gerhard; Mouritsen, Ole G.

1994-01-01

Grain-boundary melting in a lattice-gas model of a bicrystal is studied by Monte Carlo simulation using the grand canonical ensemble. Well below the bulk melting temperature T(m), a disordered liquidlike layer gradually emerges at the grain boundary. Complete interfacial wetting can be observed...... when the temperature approaches T(m) from below. Monte Carlo data over an extended temperature range indicate a logarithmic divergence w(T) approximately - ln(T(m)-T) of the width of the disordered layer w, in agreement with mean-field theory....

Grain Boundary Complexions

Science.gov (United States)

2014-05-01

Cantwell et al. / Acta Materialia 62 (2014) 1–48 challenging from a scientific perspective, but it can also be very technologically rewarding , given the...energy) is a competing explanation that remains to be explored. Strategies to drive the grain boundary energy toward zero have produced some success...Thompson AM, Soni KK, Chan HM, Harmer MP, Williams DB, Chabala JM, et al. J Am Ceram Soc 1997;80:373. [172] Behera SK. PhD dissertation, Materials Science

Abnormal grain growth: a non-equilibrium thermodynamic model for multi-grain binary systems

International Nuclear Information System (INIS)

Svoboda, J; Fischer, F D

2014-01-01

Abnormal grain growth as the abrupt growth of a group of the largest grains in a multi-grain system is treated within the context of unequal retardation of grain growth due to the segregation of solute atoms from the bulk of the grains into the grain boundaries. During grain boundary migration, the segregated solute atoms are dragged under a small driving force or left behind the migrating grain boundary under a large driving force. Thus, the solute atoms in the grain boundaries of large grains, exhibiting a large driving force, can be released from the grain boundary. The mobility of these grain boundaries becomes significantly higher and abnormal grain growth is spontaneously provoked. The mean-field model presented here assumes that each grain is described by its grain radius and by its individual segregation parameter. The thermodynamic extremal principle is engaged to obtain explicit evolution equations for the radius and segregation parameter of each grain. Simulations of grain growth kinetics for various conditions of segregation with the same initial setting (100 000 grains with a given radius distribution) are presented. Depending on the diffusion coefficients of the solute in the grain boundaries, abnormal grain growth may be strongly or marginally pronounced. Solute segregation and drag can also significantly contribute to the stabilization of the grain structure. Qualitative agreement with several experimental results is reported. (paper)

Metallographic screening of grain boundary engineered type 304 austenitic stainless steel

Energy Technology Data Exchange (ETDEWEB)

Hanning, F., E-mail: Fabian.Hanning@googlemail.com; Engelberg, D.L., E-mail: Dirk.engelberg@manchester.ac.uk

2014-08-15

An electrochemical etching method for the identification of grain boundary engineered type 304 austenitic stainless steel microstructures is described. The method can be applied for rapid microstructure screening to complement electron backscatter diffraction analysis. A threshold parameter to identify grain boundary engineered microstructure is proposed, and the application of metallographic etching for characterising the degree of grain boundary engineering discussed. - Highlights: • As-received (annealed) and grain boundary engineered microstructures were compared. • Electro-chemical polarisation in nitric acid solutions was carried out. • A metallographic screening method has been developed. • The screening method complements EBSD analysis for microstructure identification.

Influence of S, P, C on grain boundary diffusion and creep properties of Alloy 800; Einfluss von S, P, C auf die Korngrenzendiffusion und Kriecheigenschaften von Alloy 800

Energy Technology Data Exchange (ETDEWEB)

Lindemann, J.; Hannesen, K.; Mast, R.; Viefhaus, H. [Max-Planck-Institut fuer Eisenforschung GmbH, Duesseldorf (Germany); Grabke, H.J.

1998-12-31

The paper reports examinations of Alloy 800 and specific commercially available variants known under the names of 800H, 800HT, and 800LC, differing in their concentrations of C, Al, and Ti. In addition, melts also containing phosphorus (0.09 wt-%) or sulfur (0.04 wt-%) as additional alloying materials have been prepared for the experiments. The volume diffusion and the grain boundary diffusion of {sup 59}Fe in those alloys was measured at temperatures between 800 and 1000 C by means of a radioactive tracer method combined with residual activity measurements. It was found that accompanying elements like phosphorus and sulfur increase the activation energy of the grain boundary diffusion of the iron and thus delay the grain boundary self-diffusion in Alloy 800. Creep curves were measured of the same materials after age-hardening treatment for 100 hours at 800 C, measurements performed at constant temperature but at three different, constant creep stress loads. The results showed that addition of phosphorus markedly increases the lifetime of Alloy 800, and reduces the creep rupture strain. The minimum strain rate in Alloy 800 containing 0.09 wt-% of phosphorus was found to be lower by a factor of 100, as compared to the other Alloy 800 materials used. (orig./CB) [Deutsch] Alloy 800 ist ein austenitischer Fe-Ni-Cr Stahl, der relativ geringe, aber wichtige Konzentrationen von Kohlenstoff, Aluminium und Titan enthaelt. Besondere Varianten von Alloy 800, bekannt als 800H, 800HT und 800LC, unterscheiden sich in den Konzentrationen dieser Elemente. Diese kommerziellen Legierungen wurden untersucht, und zusaetzlich wurden Schmelzen mit zulegiertem Phosphor (0,09 Gew-%) bzw. Schwefel (0,04 Gew-%) hergestellt. Mittels einer radioaktiven Tracermethode in Verbindung mit Restaktivitaetsmessungen wurde die Volumen- und Korngrenzendiffusion von {sup 59}Fe in diesen Legierungen im Temperaturbereich 800-1000 C gemessen. Es wurde gefunden, dass Begleitelemente wie Phosphor und

Grain boundary engineering of highly deformable ceramics

International Nuclear Information System (INIS)

Mecartney, M.L.

2000-01-01

Highly deformable ceramics can be created with the addition of intergranular silicate phases. These amorphous intergranular phases can assist in superplastic deformation by relieving stress concentrations and minimizing grain growth if the appropriate intergranular compositions are selected. Examples from 3Y-TZP and 8Y-CSZ ceramics are discussed. The grain boundary chemistry is analyzed by high resolution analytical TEM is found to have a strong influence on the cohesion of the grains both at high temperature and at room temperature. Intergranular phases with a high ionic character and containing large ions with a relatively weak bond strength appear to cause premature failure. In contrast, intergranular phases with a high degree of covalent character and similar or smaller ions than the ceramic and a high ionic bond strength are the best for grain boundary adhesion and prevention of both cavitation at high temperatures and intergranular fracture at room temperature

Diffusion in moving grain-boundaries; Diffusion aux joints de grains en mouvement; Diffuziya na peremeshchayushchikhsya granichnykh poverkhnostyakh zeren; Difusion de los limites intergranulares en movimiento

Energy Technology Data Exchange (ETDEWEB)

Blackburn, D A; Brown, A F [Solid State Laboratory, University of Edinburgh, Edinburgh (United Kingdom)

1962-01-15

Bicrystals of copper have been sheared along the common boundary at rates of 0.06-0.3 {mu}m/h at temperatures around 725{sup o} C. Penetration of Ag{sup 110} into static boundaries and boundaries moving in this way was studied by autoradiography. For both moving and static boundaries good agreement is obtained with Fisher's formula. The ratio of the diffusivities in the grain boundary and the bulk crystal (D'/D) is about 10{sup 6}. For the boundaries studied D'/D appears to increase slightly with total shear and with shear rate, but only by a factor of not more than two over the ranges investigated. (author) [French] Des bicristaux de cuivre ont ete sectionnes le long du joint commun a raison de 00,6 a 0,3 {mu} par heure, a des temperatures d'environ 725{sup o} C. La penetration de l'argent-110 aux joints fixes et aux joints en mouvement a ete etudiee par autoradiographie. Pour les joints en mouvement et r o m les joints fixes, les auteurs ont obtenu des resultats qui concordent avec la formule de Fisher. Le coefficient d'autodiffusion intergranulaire et massique (D'/D) est d'environ 10{sup 6}. Pour les joints etudies, le rapport D'/D semble augmenter legerement avec le sectionnement total et la vitesse de sectionnement, mais sans doubler pour les valeurs considerees. (author) [Spanish] Los autores han cizallado bicristales de cobre a lo largo de su limite comun a velocidades de 0,06-0,3 {mu}/h y a temperaturas del orden de los 725{sup o} C, a fin de estudiar por autorradiografia la penetracion del {sup 110}Ag wen los limites estacionarios y en los limites en movimiento. Los resultados obtenidos para ambos tipos de limites concuerdan satisfactoriamente con la formula de Fisher. La razon D'/D entre los coeficientes de difusion en los limites intergranulares y en la masa del cristal es del orden de 10{sup 6}. En el caso de los limites estudiados, este cociente parece crecer ligeramente con el esfuerzo total y con la velocidad de cizallamiento, pero el aumento no

The atomic-scale origins of grain boundary superconducting properties

International Nuclear Information System (INIS)

Pennycook, S.J.; Chisholm, M.F.; Buban, J.; Browning, N.D.; Prouteau, C.; Univ. of Illinois, Chicago, IL; Nellist, P.D.

1998-02-01

Due to the extremely short coherence lengths of the high-T c superconductors, defects such as grain boundaries are obvious barriers to the flow of supercurrent. Within a few months of the discovery of these materials, it was shown how the critical current dropped four orders of magnitude as the grain boundary misorientation increased from zero to 45 degree. Even today, there is no quantitative understanding of this behavior. A qualitative understanding is however possible through atomic resolution Z-contrast imaging on YBa 2 Cu 3 O 7-δ and SrTiO 3 bicrystal grain boundaries, combined with bond-valence-sum analysis. The Z-contrast image of a YBa 2 Cu 3 O 7-δ low angle grain boundary shows the same kind of reconstructed dislocation cores as seen in SrTiO 3 , containing reconstructions on both the Cu and Y/Ba sublattices. An image of an asymmetric 30 degree boundary in YBa 2 Cu 3 O 7-δ shows the same units and unit sequence as expected for SrTiO 3 . YBa 2 Cu 3 O 7-δ boundaries are wavy because of their non-equilibrium growth process, and therefore mostly asymmetric in nature, although small segments have the symmetric structure. It seems reasonable to assume that boundaries of other angles will also have similar structures in these two materials

Grain-boundary microchemistry and intergranular cracking of irradiated austenitic stainless steels

International Nuclear Information System (INIS)

Chung, H.M.; Ruther, W.E.; Sanecki, J.E.; Kassner, T.F.

1993-01-01

Constant-extension-rate tensile tests and grain-boundary analysis by Auger electron spectroscopy were conducted on high and commercial-purity (HP and CP) Type 304 stainless steel (SS) specimens from irradiated boiling-water reactor (BWR) components to identify the mechanisms of irradiation-assisted stress corrosion cracking (IASCC). Contrary to previous beliefs, susceptibility to intergranular fracture could not be correlated with radiation-induced segregation of impurities such as Si, P, C, or S, but a correlation was obtained with grain-boundary Cr concentration, indicating a role for Cr depletion. Detailed analysis of grain-boundary chemistry was conducted on BWR neutron absorber tubes that were fabricated from two similar heats of HP Type 304 SS of virtually identical bulk chemical composition but exhibiting a significant difference in susceptibility to IASCC after irradiation to ∼2 x 10 21 n/cm 2 (E > 1 MeV). Grain-boundary concentrations of Cr Ni, Si, P, S, and C of the cracking-resistant and -susceptible HP heats were virtually identical. However, grain boundaries of the cracking-resistant material contained less N and more B and Li than those of the cracking-susceptible material. This observation indicates that, besides the deleterious effect of grain-boundary Cr depletion, a synergism between grain-boundary segregation of N and B and transmutation to H and Li plays an important role in IASCC

Analysis of grain boundaries, twin boundaries, and Te precipitates in CdZnTe grown by high-pressure Bridgeman method

International Nuclear Information System (INIS)

Heffelfinger, J.R.; Medlin, D.L.; James, R.B.

1998-03-01

Grain boundaries and twin boundaries in commercial Cd 1-x Zn x Te, which is prepared by a high pressure Bridgeman technique, have been investigated with transmission electron microscopy, scanning electron microscopy, infrared light microscopy and visible light microscopy. Boundaries inside these materials were found to be decorated with Te precipitates. The shape and local density of the precipitates were found to depend on the particular boundary. For precipitates that decorate grain boundaries, their microstructure was found to consist of a single, saucer shaped grain of hexagonal Te (space group P3 1 21). Analysis of a Te precipitate precipitates by selected area diffraction revealed the Te to be aligned with the surrounding Cd 1-x Zn x Te grains. This alignment was found to match the (111) Cd 1-x Z x Te planes with the (1 bar 101) planes of hexagonal Te. Crystallographic alignments between the Cd 1-x Zn x Te grains were also observed for a high angle grain boundary. The structure of the grain boundaries and the Te/Cd 1-x Zn x Te interface are discussed

The Role of Grain Orientation and Grain Boundary Characteristics in the Mechanical Twinning Formation in a High Manganese Twinning-Induced Plasticity Steel

Science.gov (United States)

Shterner, Vadim; Timokhina, Ilana B.; Rollett, Anthony D.; Beladi, Hossein

2018-04-01

In the current study, the dependence of mechanical twinning on grain orientation and grain boundary characteristics was investigated using quasi in-situ tensile testing. The grains of three main orientations (i.e., , , and parallel to the tensile axis (TA)) and certain characteristics of grain boundaries (i.e., the misorientation angle and the inclination angle between the grain boundary plane normal and the TA) were examined. Among the different orientations, and were the most and the least favored orientations for the formation of mechanical twins, respectively. The orientation was intermediate for twinning. The annealing twin boundaries appeared to be the most favorable grain boundaries for the nucleation of mechanical twinning. No dependence was found for the inclination angle of annealing twin boundaries, but the orientation of grains on either side of the annealing twin boundary exhibited a pronounced effect on the propensity for mechanical twinning. Annealing twin boundaries adjacent to high Taylor factor grains exhibited a pronounced tendency for twinning regardless of their inclination angle. In general, grain orientation has a significant influence on twinning on a specific grain boundary.

Simulation of grain boundary effects on electronic transport in metals, and detailed causes of scattering

Energy Technology Data Exchange (ETDEWEB)

Feldman, Baruch [Process Technology Modeling, Design and Technology Solutions, Technology and Manufacturing Group, Intel Corporation, Santa Clara, CA 95052 (United States); Department of Physics, University of Washington, Seattle, WA 98195 (United States); Park, Seongjun; Haverty, Michael; Shankar, Sadasivan [Process Technology Modeling, Design and Technology Solutions, Technology and Manufacturing Group, Intel Corporation, Santa Clara, CA 95052 (United States); Dunham, Scott T. [Department of Physics, University of Washington, Seattle, WA 98195 (United States); Department of Electrical Engineering, University of Washington, Seattle, WA 98195 (United States)

2010-07-15

We present first-principles simulations of single grain boundary reflectivity of electrons in noble metals, Cu and Ag. We examine twin and non-twin grain boundaries using non-equilibrium Green's function and first principles methods. We also investigate the determinants of reflectivity in grain boundaries by modeling atomic vacancies, disorder, and orientation and find that both the change in grain orientation and disorder in the boundary itself contribute significantly to reflectivity. We find that grain boundary reflectivity may vary widely depending on the grain boundary structure, consistent with published experimental results. Finally, we examine the reflectivity from multiple grain boundaries and find that grain boundary reflectivity may depend on neighboring grain boundaries. This study raises some potential limitations in the independent grain boundary assumptions of the Mayadas-Shatzkes (MS) model. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

Molecular dynamics study of the role of symmetric tilt grain boundaries on the helium distribution in nickel

Science.gov (United States)

Torres, E.; Pencer, J.

2018-04-01

Helium impurities, from either direct implantation or transmutation reactions, have been associated with embrittlement in nickel-based alloys. Helium has very low solubility in nickel, and has been found to aggregate at lattice defects such as vacancies, dislocations, and grain boundaries. The retention and precipitation of helium in nickel-based alloys have deleterious effects on the material mechanical properties. However, the underlying mechanisms that lead to helium effects in the host metal are not fully understood. In the present work, we investigate the role of symmetric tilt grain boundary (STGB) structures on the distribution of helium in nickel using molecular dynamics simulations. We investigate the family of STGBs specific to the 〈 110 〉 tilt axis. The present results indicate that accumulation of helium at the grain boundary may be modulated by details of grain boundary geometry. A plausible correlation between the grain boundary energy and misorientation with the accumulation and mobility of helium is proposed. Small clusters with up to 6 helium atoms show significant interstitial mobility in the nickel bulk, but also become sites for nucleation and grow of more stable helium clusters. High-energy GBs are found mainly populated with small helium clusters. The high mobility of small clusters along the GBs indicates the role of these GBs as fast two-dimensional channels for diffusion. In contrast, the accumulation of helium in large helium clusters at low-energy STGB creates a favorable environment for the formation of large helium bubbles, indicating a potential role for low-energy STGB in promoting helium-induced GB embrittlement.

AES/STEM grain boundary analysis of stabilized zirconia ceramics

NARCIS (Netherlands)

Winnubst, Aloysius J.A.; Kroot, P.J.M.; Burggraaf, A.J.

1983-01-01

Semiquantitative Auger Electron Spectroscopy (AES) on pure monophasic (ZrO2)0.83(YO1.5)0.17 was used to determine the chemical composition of the grain boundaries. Grain boundary enrichment with Y was observed with an enrichment factor of about 1.5. The difference in activation energy of the ionic

Grain-boundary free energy in an assembly of elastic disks.

Science.gov (United States)

Lusk, Mark T; Beale, Paul D

2004-02-01

Grain-boundary free energy is estimated as a function of misoriention for symmetric tilt boundaries in an assembly of nearly hard disks. Fluctuating cell theory is used to accomplish this since the most common techniques for calculating interfacial free energy cannot be applied to such assemblies. The results are analogous to those obtained using a Leonard-Jones potential, but in this case the interfacial energy is dominated by an entropic contribution. Disk assemblies colorized with free and specific volume elucidate differences between these two characteristics of boundary structure. Profiles are also provided of the Helmholtz and Gibbs free energies as a function of distance from the grain boundaries. Low angle grain boundaries are shown to follow the classical relationship between dislocation orientation/spacing and misorientation angle.

Atomic scale study of grain boundary segregation before carbide nucleation in Ni-Cr-Fe Alloys

Science.gov (United States)

Li, Hui; Xia, Shuang; Liu, Wenqing; Liu, Tingguang; Zhou, Bangxin

2013-08-01

Three dimensional chemical information concerning grain boundary segregation before carbide nucleation was characterized by atom probe tomography in two Ni-Cr-Fe alloys which were aged at 500 °C for 0.5 h after homogenizing treatment. B, C and Si atoms segregation at grain boundary in Alloy 690 was observed. B, C, N and P atoms segregation at grain boundary in 304 austenitic stainless steel was observed. C atoms co-segregation with Cr atoms at the grain boundaries both in Alloy 690 and 304 austenitic stainless steel was found, and its effect on the carbide nucleation was discussed. The amount of each segregated element at grain boundaries in the two Ni-Cr-Fe alloys were analyzed quantitatively. Comparison of the grain boundary segregation features of the two Ni-Cr-Fe alloys were carried out based on the experimental results. The impurity and solute atoms segregate inhomogeneously in the same grain boundary both in 304 SS and Alloy 690. The grain boundary segregation tendencies (Sav) are B (11.8 ± 1.4) > P (5.4 ± 1.4) > N (4.7 ± 0.3) > C (3.7 ± 0.4) in 304 SS, and B (6.9 ± 0.9) > C (6.7 ± 0.4) > Si (1.5 ± 0.2) in Alloy 690. Cr atoms may co-segregate with C atoms at grain boundaries before carbide nucleation at the grain boundaries both in 304 SS and Alloy 690. Ni atoms generally deplete at grain boundary both in 304 SS and Alloy 690. The literature shows that the Ni atoms may co-segregate with P atoms at grain boundaries [28], but the P atoms segregation do not leads to Ni segregation in the current study. In the current study, Fe atoms may segregate or deplete at grain boundary in Alloy 690. But Fe atoms generally deplete at grain boundary in 304 SS. B atoms have the strongest grain boundary segregation tendency both in 304 SS and Alloy 690. The grain boundary segregation tendency and Gibbs free energy of B in 304 SS is higher than in Alloy 690. C atoms are easy to segregate at grain boundaries both in 304 SS and Alloy 690. The grain boundary segregation

Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries

Science.gov (United States)

Sheikhzada, Ahmad K.

As a major component of linear particle accelerators, superconducting radio-frequency (SRF) resonator cavities are required to operate with lowest energy dissipation and highest accelerating gradient. SRF cavities are made of polycrystalline materials in which grain boundaries can limit maximum RF currents and produce additional power dissipation sources due to local penetration of Josephson vortices. The essential physics of vortex penetration and mechanisms of dissipation of vortices driven by strong RF currents along networks of grain boundaries and their contribution to the residual surface resistance have not been well understood. To evaluate how GBs can limit the performance of SRF materials, particularly Nb and Nb3Sn, we performed extensive numerical simulations of nonlinear dynamics of Josephson vortices in grain boundaries under strong dc and RF fields. The RF power due to penetration of vortices both in weakly-coupled and strongly-coupled grain boundaries was calculated as functions of the RF field and frequency. The result of this calculation manifested a quadratic dependence of power to field amplitude at strong RF currents, an illustration of resistive behavior of grain boundaries. Our calculations also showed that the surface resistance is a complicated function of field controlled by penetration and annihilation of vortices and antivortices in strong RF fields which ultimately saturates to normal resistivity of grain boundary. We found that Cherenkov radiation of rapidly moving vortices in grain boundaries can produce a new instability causing generation of expanding vortex-antivortex pair which ultimately drives the entire GB in a resistive state. This effect is more pronounced in polycrystalline thin film and multilayer coating structures in which it can cause significant increase in power dissipation and results in hysteresis effects in I-V characteristics, particularly at low temperatures.

Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries

Energy Technology Data Exchange (ETDEWEB)

Sheikhzada, Ahmad [Old Dominion Univ., Norfolk, VA (United States)

2017-05-01

As a major component of linear particle accelerators, superconducting radio-frequency (SRF) resonator cavities are required to operate with lowest energy dissipation and highest accelerating gradient. SRF cavities are made of polycrystalline materials in which grain boundaries can limit maximum RF currents and produce additional power dissipation sources due to local penetration of Josephson vortices. The essential physics of vortex penetration and mechanisms of dissipation of vortices driven by strong RF currents along networks of grain boundaries and their contribution to the residual surface resistance have not been well understood. To evaluate how GBs can limit the performance of SRF materials, particularly Nb and Nb3Sn, we performed extensive numerical simulations of nonlinear dynamics of Josephson vortices in grain boundaries under strong dc and RF fields. The RF power due to penetration of vortices both in weakly-coupled and strongly-coupled grain boundaries was calculated as functions of the RF field and frequency. The result of this calculation manifested a quadratic dependence of power to field amplitude at strong RF currents, an illustration of resistive behavior of grain boundaries. Our calculations also showed that the surface resistance is a complicated function of field controlled by penetration and annihilation of vortices and antivortices in strong RF fields which ultimately saturates to normal resistivity of grain boundary. We found that Cherenkov radiation of rapidly moving vortices in grain boundaries can produce a new instability causing generation of expanding vortex-antivortex pair which ultimately drives the entire GB in a resistive state. This effect is more pronounced in polycrystalline thin film and multilayer coating structures in which it can cause significant increase in power dissipation and results in hysteresis effects in I-V characteristics, particularly at low temperatures.

Transport properties of olivine grain boundaries from electrical conductivity experiments

Science.gov (United States)

Pommier, Anne; Kohlstedt, David L.; Hansen, Lars N.; Mackwell, Stephen; Tasaka, Miki; Heidelbach, Florian; Leinenweber, Kurt

2018-05-01

Grain boundary processes contribute significantly to electronic and ionic transports in materials within Earth's interior. We report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties. We performed electrical conductivity measurements of melt-free polycrystalline olivine (Fo90) samples that had been previously deformed at 1200 °C and 0.3 GPa to shear strains up to γ = 7.3. The electrical conductivity and anisotropy were measured at 2.8 GPa over the temperature range 700-1400 °C. We observed that (1) the electrical conductivity of samples with a small grain size (3-6 µm) and strong crystallographic preferred orientation produced by dynamic recrystallization during large-strain shear deformation is a factor of 10 or more larger than that measured on coarse-grained samples, (2) the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size, and (3) conductivity is up to a factor of 4 larger in the direction of shear than normal to the shear plane. Based on these results combined with electrical conductivity data for coarse-grained, polycrystalline olivine and for single crystals, we propose that the electrical conductivity of our fine-grained samples is dominated by grain boundary paths. In addition, the electrical anisotropy results from preferential alignment of higher-conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains.

Engineering Mixed Ionic Electronic Conduction in La 0.8 Sr 0.2 MnO 3+ δ Nanostructures through Fast Grain Boundary Oxygen Diffusivity

KAUST Repository

Saranya, Aruppukottai M.

2015-04-09

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Nanoionics has become an increasingly promising field for the future development of advanced energy conversion and storage devices, such as batteries, fuel cells, and supercapacitors. Particularly, nanostructured materials offer unique properties or combinations of properties as electrodes and electrolytes in a range of energy devices. However, the enhancement of the mass transport properties at the nanoscale has often been found to be difficult to implement in nanostructures. Here, an artificial mixed ionic electronic conducting oxide is fabricated by grain boundary (GB) engineering thin films of La0.8Sr0.2MnO3+δ. This electronic conductor is converted into a good mixed ionic electronic conductor by synthesizing a nanostructure with high density of vertically aligned GBs with high concentration of strain-induced defects. Since this type of GBs present a remarkable enhancement of their oxide-ion mass transport properties (of up to six orders of magnitude at 773 K), it is possible to tailor the electrical nature of the whole material by nanoengineering, especially at low temperatures. The presented results lead to fundamental insights into oxygen diffusion along GBs and to the application of these engineered nanomaterials in new advanced solid state ionics devices such are micro-solid oxide fuel cells or resistive switching memories. An electronic conductor such as La0.8Sr0.2MnO3+δ is converted into a good mixed ionic electronic conductor by synthesizing a nanostructure with excellent electronic and oxygen mass transport properties. Oxygen diffusion highways are created by promoting a high concentration of strain-induced defects in the grain boundary region. This novel strategy opens the way for synthesizing new families of artificial mixed ionic-electronic conductors by design.

Characterization and Modeling of Grain Boundary Chemistry Evolution in Ferritic Steels under Irradiation

Energy Technology Data Exchange (ETDEWEB)

Marquis, Emmanuelle [Univ. of Michigan, Ann Arbor, MI (United States); Wirth, Brian [Univ. of Tennessee, Knoxville, TN (United States); Was, Gary [Univ. of Michigan, Ann Arbor, MI (United States)

2016-03-28

grain boundaries in the ion-irradiated alloys. More significant segregation was observed in the neutron irradiated alloys. For the more concentrated alloys, irradiation did not affect existing Cr segregation to grain boundaries and segregation to dislocation loops was not observed perhaps due to a change in the dislocation loop structure with increasing Cr concentration. Precipitation of α’ was observed in the neutron irradiated alloys containing over 9 at.% Cr. However ion irradiation appears to suppress the precipitation process. Initial low dose ion irradiation experiments strongly suggest a cascade recoil effect. The systematic analysis of grain boundary orientation on Cr segregation was significantly challenged by carbon contamination during ion irradiation or by existing carbon and therefore carbide formation at grain boundaries (sensitization). The combination of the proposed systematic experimental approach with atomistic modeling of diffusion processes directly addresses the programmatic need for developing and benchmarking predictive models for material degradation taking into account atomistic kinetics parameters

Migration energy barriers of symmetric tilt grain boundaries in body-centered cubic metal Fe

International Nuclear Information System (INIS)

Wu, Minghui; Gu, Jianfeng; Jin, Zhaohui

2015-01-01

Graphical abstract: DFT calculated migration energy barrier (left) for symmetric grain boundary in metals is an essential physical property to measure the trend of grain boundary migration, in particular, in terms of the classical homogeneous nucleation model of GB dislocation/disconnection loops (right). - Migration energy barriers of two symmetric tilt grain boundaries in body-centered cubic metal Fe are obtained via first-principles calculations in combination with the nudged elastic band methods. Although the two grain boundaries show similar grain boundary energies, the migration energy barriers are different. Based on a homogeneous nucleation theory of grain-boundary dislocation loops, the calculated energy barrier provides a measure of intrinsic grain-boundary mobility and helps to evaluate effects due to vacancy and interstitial atoms such as carbon

Applied Thermodynamics: Grain Boundary Segregation

Directory of Open Access Journals (Sweden)

Pavel Lejček

2014-03-01

Full Text Available Chemical composition of interfaces—free surfaces and grain boundaries—is generally described by the Langmuir–McLean segregation isotherm controlled by Gibbs energy of segregation. Various components of the Gibbs energy of segregation, the standard and the excess ones as well as other thermodynamic state functions—enthalpy, entropy and volume—of interfacial segregation are derived and their physical meaning is elucidated. The importance of the thermodynamic state functions of grain boundary segregation, their dependence on volume solid solubility, mutual solute–solute interaction and pressure effect in ferrous alloys is demonstrated.

Electrical properties of grain boundaries in polycrystalline materials under intrinsic or low doping

International Nuclear Information System (INIS)

Chowdhury, M H; Kabir, M Z

2011-01-01

An analytical model is developed to study the electrical properties (electric field and potential distributions, potential energy barrier height and polarization phenomenon) of polycrystalline materials at intrinsic or low doping for detector and solar cell applications by considering an arbitrary amount of grain boundary charge and a finite width of grain boundary region. The general grain boundary model is also applicable to highly doped polycrystalline materials. The electric field and potential distributions are obtained by solving Poisson's equation in both depleted grains and grain boundary regions. The electric field and potential distributions across the detector are analysed under various doping, trapping and applied biases. The electric field collapses, i.e. a nearly zero-average electric field region exists in some part of the biased detector at high trapped charge densities at the grain boundaries. The model explains the conditions of existence of a zero-average field region, i.e. the polarization mechanisms in polycrystalline materials. The potential energy barrier at the grain boundary exists if the electric field changes its sign at the opposite side of the grain boundary. The energy barrier does not exist in all grain boundaries in the low-doped polycrystalline detector and it never exists in intrinsic polycrystalline detectors under applied bias condition provided that there is no charge trapping in the grain.

On the fission gas release from oxide fuels during normal grain growth

International Nuclear Information System (INIS)

Paraschiv, M.C.; Paraschiv, A.; Glodeanu, F.

1997-01-01

A mathematical formalism for calculating the fission gas release from oxide fuels considering an arbitrary distribution of fuel grain size with only zero boundary condition for gas diffusion at the grain boundary is proposed. It has also been proved that it becomes unnecessary to consider the grain volume distribution function for fission products diffusion when the grain boundary gas resolution is considered, if thermodynamic forces on grain boundaries are only time dependent. In order to highlight the effect of the normal grain growth on fission gas release from oxide fuels Hillert's and Lifshitz and Slyozov's theories have been selected. The last one was used to give an adequate treatment of normal grain growth for the diffusion-controlled grain boundary movement in oxide fuels. It has been shown that during the fuel irradiation, the asymptotic form of the grain volume distribution functions given by Hillert and Lifshitz and Slyozov models can be maintained but the grain growth rate constant becomes time dependent itself. Experimental results have been used to correlate the two theoretical models of normal grain growth to the fission gas release from oxide fuels. (orig.)

On Techniques to Characterize and Correlate Grain Size, Grain Boundary Orientation and the Strength of the SiC Layer of TRISO Coated Particles: A Preliminary Study

Energy Technology Data Exchange (ETDEWEB)

I.J.van Rooyen; J.L. Dunzik Gougar; T. Trowbridge; Philip M van Rooyen

2012-10-01

The mechanical properties of the silicon carbide (SiC) layer of the TRi-ISOtropic (TRISO) coated particle (CP) for high temperature gas reactors (HTGR) are performance parameters that have not yet been standardized by the international HTR community. Presented in this paper are the results of characterizing coated particles to reveal the effect of annealing temperature (1000 to 2100°C) on the strength and grain size of unirradiated coated particles. This work was further expanded to include possible relationships between the grain size and strength values. The comparative results of two strength measurement techniques and grain size measured by the Lineal intercept method are included. Preliminary grain boundary characterization results determined by electron backscatter diffraction (EBSD) are included. These results are also important for future fission product transport studies, as grain boundary diffusion is identified as a possible mechanism by which 110mAg, one of the fission activation products, might be released through intact SiC layers. Temperature is a parameter known to influence the grain size of SiC and therefore it is important to investigate the effect of high temperature annealing on the SiC grain size. Recommendations and future work will also be briefly discussed.

Transformation of slip dislocation in ä3 grain boundary

Czech Academy of Sciences Publication Activity Database

Gemperlová, Juliana; Jacques, A.; Gemperle, Antonín; Zárubová, Niva

2002-01-01

Roč. 10, - (2002), s. 51-57 ISSN 0927-7056 R&D Projects: GA ČR GA202/98/1281; GA ČR GA202/01/0670 Institutional research plan: CEZ:AV0Z1010914 Keywords : in situ TEM * slip dislocations * grain boundary * grain boundary dislocations * plasticity Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.767, year: 2002

Influence of grain boundaries on elasticity and thermal conductivity of nanocrystalline diamond films

International Nuclear Information System (INIS)

Mohr, Markus; Daccache, Layal; Horvat, Sebastian; Brühne, Kai; Jacob, Timo; Fecht, Hans-Jörg

2017-01-01

Diamond combines several outstanding material properties such as the highest thermal conductivity and highest elastic moduli of all materials. This makes diamond an interesting candidate for a multitude of applications. Nonetheless, nanocrystalline diamond films, layers and coatings, usually show properties different to those of single crystalline diamond. This is usually attributed to the larger volume fraction of the grain boundaries with atomic structure different from the single crystal. In this work we measured Young's modulus and thermal conductivity of nanocrystalline diamond films with average grain sizes ranging from 6 to 15 nm. The measured thermal conductivities are modeled considering the thermal boundary conductance between grains as well as a grain size effect on the phonon mean free path. We make a comparison between elastic modulus and thermal boundary conductance of the grain boundaries G_k for different nanocrystalline diamond films. We conclude that the grain boundaries thermal boundary conductance G_k is a measure of the cohesive energy of the grain boundaries and therefore also of the elastic modulus of the nanocrystalline diamond films.

Carbon diffusion paths and segregation at high-angle tilt grain boundaries in α-Fe studied by using a kinetic activation-relation technique

Science.gov (United States)

Restrepo, Oscar A.; Mousseau, Normand; Trochet, Mickaël; El-Mellouhi, Fedwa; Bouhali, Othmane; Becquart, Charlotte S.

2018-02-01

Carbon diffusion and segregation in iron is fundamental to steel production but is also associated with corrosion. Using the kinetic activation-relaxation technique (k-ART), a kinetic Monte Carlo (KMC) algorithm with an on-the-fly catalog that allows to obtain diffusion properties over large time scales taking into account long-range elastic effects coupled with an EAM force field, we study the motion of a carbon impurity in four Fe systems with high-angle grain boundaries (GB), focusing on the impact of these extended defects on the long-time diffusion of C. Short and long-time stability of the various GBs is first analyzed, which allows us to conclude that the Σ 3 (1 1 1 ) θ =109 .53∘ GB is unstable, with Fe migration barriers of ˜0.1 eV or less, and C acts as a pinning center. Focusing on three stable GBs, in all cases, these extended defects trap C in energy states lower than found in the crystal. Yet, contrary to general understanding, we show, through simulations extending to 0.1 s, that even tough C diffusion takes place predominantly in the GB, it is not necessarily faster than in the bulk and can even be slower by one to two orders of magnitude depending on the GB type. Analysis of the energy landscape provided by k-ART also shows that the free cavity volume around the impurity is not a strong predictor of diffusion barrier height. Overall, results show rather complex diffusion kinetics intimately dependent on the local environment.

Progress in sintered NdFeB magnets with Dy/Tb grain boundary diffusion%晶界扩散Dy/Tb烧结NdFeB研究进展

Institute of Scientific and Technical Information of China (English)

刘涛; 周磊; 张昕; 程星华; 林德; 喻晓军; 李波

2011-01-01

由于汽车和电子应用领域对节能电动机的需求,在VCM、电动机、信号发生器、手机和MRI等领域中得到广泛应用的烧结钕铁硼在电动机市场应用会进一步扩展.剩磁和矫顽力等磁性能的提高有利于烧结磁体在电动机市场快速增长.通常,烧结磁体一些性能的改善总是以牺牲其他性能为代价.而晶界扩散Dy/Tb烧结磁体与传统的同类磁体相比较,其矫顽力和剩磁可同时得以改善,本文对晶界扩散Dy/Tb钕铁硼的一些研究成果进行了综述和总结.晶界扩散Dy/Tb过程中烧结磁体晶界的成份和微观组织的变化对烧结磁体磁性能改善至为重要.%Many new applications of Nd-Fe-B magnets, such as voice coil motor (VCM) in hard disc drive, motors, generators, mobile phone, MRI, etc, are expected to expand in future because of strong energy saving requirements from the automobile and electric appliances markets. Magnetic property improvement in residual (Br) and coercive force (Hcj) helps the rapid growth of the motor market. In many instances, an improvement of some characteristics of sintered magnet is achieved but only at the expense of other characteristics. It has been found that Dy/Tb grain boundary diffusion processed Nd-Fe-B sintered magnets have improvement in both the highest remanence and cocercivity among conventional rare-earth permanent mgnets, which have been introduced in this paper. The influence of Dy/Tb grain boundary diffusion on composition and microstructure of grain boundaries phase is a key point for magnetic property improvement of sintered magnet.

Diffusive boundary layers over varying topography

KAUST Repository

Dell, R.  W.; Pratt, L.  J.

2015-01-01

Diffusive bottom boundary layers can produce upslope flows in a stratified fluid. Accumulating observations suggest that these boundary layers may drive upwelling and mixing in mid-ocean ridge flank canyons. However, most studies of diffusive bottom

Spatially resolved mapping of electrical conductivity across individual domain (grain) boundaries in graphene.

Science.gov (United States)

Clark, Kendal W; Zhang, X-G; Vlassiouk, Ivan V; He, Guowei; Feenstra, Randall M; Li, An-Ping

2013-09-24

All large-scale graphene films contain extended topological defects dividing graphene into domains or grains. Here, we spatially map electronic transport near specific domain and grain boundaries in both epitaxial graphene grown on SiC and CVD graphene on Cu subsequently transferred to a SiO2 substrate, with one-to-one correspondence to boundary structures. Boundaries coinciding with the substrate step on SiC exhibit a significant potential barrier for electron transport of epitaxial graphene due to the reduced charge transfer from the substrate near the step edge. Moreover, monolayer-bilayer boundaries exhibit a high resistance that can change depending on the height of substrate step coinciding at the boundary. In CVD graphene, the resistance of a grain boundary changes with the width of the disordered transition region between adjacent grains. A quantitative modeling of boundary resistance reveals the increased electron Fermi wave vector within the boundary region, possibly due to boundary induced charge density variation. Understanding how resistance change with domain (grain) boundary structure in graphene is a crucial first step for controlled engineering of defects in large-scale graphene films.

High angle grain boundaries as sources or sinks for point defects

Energy Technology Data Exchange (ETDEWEB)

Balluffi, R.W.

1979-09-01

A secondary grain boundary dislocation climb model for high angle grain boundaries as sources/sinks for point defects is described in the light of recent advances in our knowledge of grain boundary structure. Experimental results are reviewed and are then compared with the expected behavior of the proposed model. Reasonably good consistency is found at the level of our present understanding of the subject. However, several gaps in our present knowledge still exist, and these are identified and discussed briefly.

Influences of triple junctions on stress-assisted grain boundary motion in nanocrystalline materials

International Nuclear Information System (INIS)

Aramfard, Mohammad; Deng, Chuang

2014-01-01

Stress-assisted grain boundary motion is among the most studied modes of microstructural evolution in crystalline materials. In this study, molecular dynamics simulations were used to systematically investigate the influences of triple junctions on the stress-assisted motion of symmetric tilt grain boundaries in Cu by considering a honeycomb nanocrystalline model. It was found that the grain boundary motion in nanocrystalline models was highly sensitive to the loading mode, and a strong coupling effect which was prevalent in bicrystal models was only observed when simple shear was applied. In addition, the coupling factor extracted from the honeycomb model was found to be larger and more sensitive to temperature change than that from bicrystal models for the same type of grain boundary under the same loading conditions. Furthermore, the triple junctions seemed to exhibit unusual asymmetric pinning effects to the migrating grain boundary and the constraints by the triple junctions and neighboring grains led to remarkable non-linear grain boundary motion in directions both parallel and normal to the applied shear, which was in stark contrast to that observed in bicrystal models. In addition, dislocation nucleation and propagation, which were absent in the bicrystal model, were found to play an important role on shear-induced grain boundary motion when triple junctions were present. In the end, a generalized model for shear-assisted grain boundary motion was proposed based on the findings from this research. (paper)

Grain boundary precipitation strengthening mechanism in W containing advanced creep resistant ferritic steels

Energy Technology Data Exchange (ETDEWEB)

Shibata, T.; Hasegawa, Y. [Tohoku Univ., Sendai (Japan)

2010-07-01

Grain boundary precipitation strengthening is expected to be a decisive factor in developing ferritic creep resistant steels. This study examined the grain boundary precipitation strengthening mechanism extracting the effect of the tempered martensitic microstructure and precipitates on the high angle grain boundary in M{sub 23}C4{sub 6} type carbide and the Fe{sub 2}W type Laves phase effect of the creep deformation fixing the grain boundary according to transmission electron microscope (TEM) observation. A creep test was carried out at high temperature in order to evaluate the high angle boundary strengthening effect simulating the long-term creep deformation microstructure by the lath structure disappearance. The correlation of the creep rupture time and the grain boundary shielding ratio were found to be independent of precipitate type. The creep deformation model represents block boundary shielding by precipitates as the decisive factor for W containing ferritic creep resistant steels. (orig.)

Grain boundary engineering for structure materials of nuclear reactors

Science.gov (United States)

Tan, L.; Allen, T. R.; Busby, J. T.

2013-10-01

Grain boundary engineering (GBE), primarily implemented by thermomechanical processing, is an effective and economical method of enhancing the properties of polycrystalline materials. Among the factors affecting grain boundary character distribution, literature data showed definitive effect of grain size and texture. GBE is more effective for austenitic stainless steels and Ni-base alloys compared to other structural materials of nuclear reactors, such as refractory metals, ferritic and ferritic-martensitic steels, and Zr alloys. GBE has shown beneficial effects on improving the strength, creep strength, and resistance to stress corrosion cracking and oxidation of austenitic stainless steels and Ni-base alloys.

Grain boundary engineering for structure materials of nuclear reactors

Energy Technology Data Exchange (ETDEWEB)

Tan, L., E-mail: tanl@ornl.gov [Materials Science and Technology Division, Oak Ridge National Laboratory (United States); Allen, T.R. [Department of Engineering Physics, University of Wisconsin–Madison (United States); Busby, J.T. [Materials Science and Technology Division, Oak Ridge National Laboratory (United States)

2013-10-15

Grain boundary engineering (GBE), primarily implemented by thermomechanical processing, is an effective and economical method of enhancing the properties of polycrystalline materials. Among the factors affecting grain boundary character distribution, literature data showed definitive effect of grain size and texture. GBE is more effective for austenitic stainless steels and Ni-base alloys compared to other structural materials of nuclear reactors, such as refractory metals, ferritic and ferritic–martensitic steels, and Zr alloys. GBE has shown beneficial effects on improving the strength, creep strength, and resistance to stress corrosion cracking and oxidation of austenitic stainless steels and Ni-base alloys.

Interaction between particles and grain boundaries under conditions of cooperative migration

International Nuclear Information System (INIS)

Marvina, L.A.; Marvin, V.B.

1996-01-01

The analysis of particle grain boundary interaction is performed for dispersion hardened alloys when cooperative migration takes place. It is shown that in a general case the particle experiences a Zener force and a force of grain boundary surface tension due to boundary bending between particles. Approximate numerical estimates are made for the force acting on a particle in dispersion hardened alloy Ni-HfO 2 . It is noted that during cooperative migration of particle and grain boundary the velocity of the particle is directed along the resulting force. The latter equals the sum of surface tension and Zener forces. 6 refs., 2 figs

Changes in grain boundary composition induced by neutron irradiation of austenitic stainless steels

International Nuclear Information System (INIS)

Asano, K.; Nakata, K.; Fukuya, K.; Kodama, M.

1992-01-01

The radiation induced segregation of solutes to the grain boundary in austenitic stainless steels were studied. Type 304 and type 316 steel samples neutron irradiated at 561K up to 9.2x10 25 n/m 2 were obtained and minute compositional profiles across grain boundaries were examined using an analytical scanning transmission electron microscope equipped with a field emission electron gun. Chromium was slightly enriched at grain boundaries at the lowest irradiation dose but decreased with increasing fluence. Higher fluence irradiation resulted in depletion in chromium and molybdenum, and enrichment in nickel, silicon and phosphorus. These changes in grain boundary chemistry were limited within about 5nm of the boundary. Significant depletion of chromium and enrichment of impurities on the grain boundary occurred at fluences roughly coincidental with that of SCC susceptibility change obtained from another project

Evaluation of deformation behavior of in grains and grain boundaries of L-grade austenitic stainless steel 316L

International Nuclear Information System (INIS)

Nagashima, Nobuo; Hayakawa, Masao; Tsukada, Takashi; Kaji, Yoshiyuki; Miwa, Yukio; Ando, Masami; Nakata, Kiyotomo

2009-01-01

In this study, micro-hardness tests and AFM observations were performed on SUS 316L low-carbon austenitic stainless steel pre-strained by cold rolling to investigate its deformation behavior. The following results were obtained. Despite the fact that the same plastic strain was applied, post-tensile test AFM showed narrower slip-band spacing in a reduction in area of 30% cold-rolled specimen than the unrolled specimen. Concentrated slip bands were observed near grain boundaries. These were presumably due to slip blocking at grain boundaries. SCC sensitivity increased at a hardness of 300 or higher, the frequency occurrence of a hardness of 300 or higher in the micro-hardness measurements was compared. The micro-hardness did not exceed 300 both within grains and at grain boundaries in the unrolled and up to a reduction in area of 20% cold-rolled specimens of before and after the tensile tests. Micro-hardness exceeding 300 was found to occur frequently in after tensile test specimens with a reduction in area of 30% or more, particularly at grain boundaries. It is suggested that the nonuniformity of deformation at grain boundaries plays an important role of IGSCC crack propagation mechanism of low-carbon austenitic stainless steel. (author)

Wetting and crystallization at grain boundaries: Origin of aluminum-induced crystallization of amorphous silicon

International Nuclear Information System (INIS)

Wang, J.Y.; He, D.; Zhao, Y.H.; Mittemeijer, E.J.

2006-01-01

It has been shown experimentally that the grain boundaries in aluminium in contact with amorphous silicon are the necessary agents for initiation of the crystallization of silicon upon annealing temperatures as low as 438 K. Thermodynamic analysis has shown (i) that Si can 'wet' the Al grain boundaries due to the favorable Si/Al interface energy as compared to the Al grain-boundary energy and (ii) that Si at the Al grain boundaries can maintain its amorphous state up to a thickness of about 1.0 nm. Beyond that thickness crystalline Si develops at the Al grain boundaries

Temperature dependence of grain boundary free energy and elastic constants

International Nuclear Information System (INIS)

Foiles, Stephen M.

2010-01-01

This work explores the suggestion that the temperature dependence of the grain boundary free energy can be estimated from the temperature dependence of the elastic constants. The temperature-dependent elastic constants and free energy of a symmetric Σ79 tilt boundary are computed for an embedded atom method model of Ni. The grain boundary free energy scales with the product of the shear modulus times the lattice constant for temperatures up to about 0.75 the melting temperature.

Diffusion of 51Cr along high-diffusivity paths in Ni-Fe alloys

International Nuclear Information System (INIS)

Cermak, J.

1990-01-01

Penetration profiles of 51 Cr in polycrystalline alloys Ni-xFe (x = 0, 20, 40, and 60 wt.% Fe) after diffusion anneals at temperatures between 693 and 1473 K are studied. Sectioning of diffusion zones of samples annealed above 858 K is carried out by grinding, at lower temperatures by DC glow discharge sputtering. The concentration of 51 Cr in depth x is assumed to be proportional to relative radioactivity of individual sections. With help of volume and pipe self-diffusion data taken from literature, the temperature dependence of product P = δD g (δ and D g are grain boundary width and grain boundary diffusion coefficient, respectively) is obtained: P = (2.68 - 0.88 +1.3 ) x 10 -11 exp [-(221.3 ± 3.0) kJ/mol/RT]m 3 /s. This result agrees well with the previous measurements of 51 Cr diffusivity in Fe-18 Cr-12 Ni and Fe-21 Cr-31 Ni. It indicates that the mean chemical composition of Fe-Cr-Ni ternary alloys is not a dominant factor affecting the grain boundary diffusivity of Cr in these alloys. (author)

Investigation of slip transfer across HCP grain boundaries with application to cold dwell facet fatigue

International Nuclear Information System (INIS)

Zheng, Zebang; Balint, Daniel S.; Dunne, Fionn P.E.

2017-01-01

This paper addresses the role of grain boundary slip transfer and thermally-activated discrete dislocation plasticity in the redistribution of grain boundary stresses during cold dwell fatigue in titanium alloys. Atomistic simulations have been utilised to calculate the grain boundary energies for titanium with respect to the misorientation angles. The grain boundary energies are utilised within a thermally-activated discrete dislocation plasticity model incorporating slip transfer controlled by energetic and grain boundary geometrical criteria. The model predicts the grain size effect on the flow strength in Ti alloys. Cold dwell fatigue behaviour in Ti-6242 alloy is investigated and it is shown that significant stress redistribution from soft to hard grains occurs during the stress dwell, which is observed both for grain boundaries for which slip transfer is permitted and inhibited. However, the grain boundary slip penetration is shown to lead to significantly higher hard-grain basal stresses near the grain boundary after dwell, thus exacerbating the load shedding stress compared to an impenetrable grain boundary. The key property controlling the dwell fatigue response is argued to remain the time constant associated with the thermal activation process for dislocation escape, but the slip penetrability is also important and exacerbates the load shedding. The inclusion of a macrozone does not significantly change the conclusions but does potentially lead to the possibility of a larger initial facet.

Thermally activated phase slippage in high-Tc grain-boundary Josephson junctions

International Nuclear Information System (INIS)

Gross, R.; Chaudhari, P.; Dimos, D.; Gupta, A.; Koren, G.

1990-01-01

The effect of thermally activated phase slippage (TAPS) in YBa 2 Cu 3 O 7 grain-boundary Josephson junctions has been studied. TAPS has been found to be responsible for the dc noise voltage superimposed on the dc Josephson current near the transition temperature. Because of the reduced Josephson coupling energy of the grain-boundary junctions, which is caused by a reduced superconducting order parameter at the grain-boundary interface, TAPS is present over a considerable temperature range. The implications of TAPS on the applicability of high-T c Josephson junctions are outlined

Interactions between Dislocations and Grain Boundaries

NARCIS (Netherlands)

Soer, Wouter Anthon

2006-01-01

Dislocations (line defects) and grain boundaries (planar defects) are two types of lattice defects that are crucial to the deformation behavior of metals. Permanent deformation of a crystalline material is microscopically associated with the nucleation and propagation of dislocations, and extensive

Chevron defect at the intersection of grain boundaries with free surfaces in Au

International Nuclear Information System (INIS)

Radetic, T.; Lancon, F.; Dahmen, U.

2002-01-01

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90 deg. tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary

Studies of Grain Boundaries in Materials Subjected to Diffusional Creep

DEFF Research Database (Denmark)

Nørbygaard, Thomas

Grain boundaries in crystalline Cu(2%Ni) creep specimens have been studied by use of scanning and transmission electron microscopy in order to establish the mechanism of deformation. Creep rate measurements and dependencies were found to fit reasonably well with the model for diffusional creep......) with the activity displayed during diffusional creep testing. It was found that boundaries with low deviation from perfect Σ did not contribute macroscopically to the creep strain. A resist deposition procedure was examined to improve the reference surface grid so as to allow determination of the grain boundary...... plane by use of simple stereomicroscopy directly on the surface. The etched pattern deteriorated heav-ily during creep testing, supposedly because of dislocation creep, due to exces-sive creep stress. Grain boundaries have been studied and characterised by TEM providing an insight into the diversity...

Temperature jump boundary conditions in radiation diffusion

International Nuclear Information System (INIS)

Alonso, C.T.

1976-12-01

The radiation diffusion approximation greatly simplifies radiation transport problems. Yet the application of this method has often been unnecessarily restricted to optically thick regions, or has been extended through the use of such ad hoc devices as flux limiters. The purpose of this paper is to review and draw attention to the use of the more physically appropriate temperature jump boundary conditions for extending the range of validity of the diffusion approximation. Pioneering work has shown that temperature jump boundary conditions remove the singularity in flux that occurs in ordinary diffusion at small optical thicknesses. In this review paper Deissler's equations for frequency-dependent jump boundary conditions are presented and specific geometric examples are calculated analytically for steady state radiation transfer. When jump boundary conditions are applied to radiation diffusion, they yield exact solutions which are naturally flux- limited and geometry-corrected. We believe that the presence of temperature jumps on source boundaries is probably responsible in some cases for the past need for imposing ad hoc flux-limiting constraints on pure diffusion solutions. The solution for transfer between plane slabs, which is exact to all orders of optical thickness, also provides a useful tool for studying the accuracy of computer codes

Effective search for stable segregation configurations at grain boundaries with data-mining techniques

Science.gov (United States)

Kiyohara, Shin; Mizoguchi, Teruyasu

2018-03-01

Grain boundary segregation of dopants plays a crucial role in materials properties. To investigate the dopant segregation behavior at the grain boundary, an enormous number of combinations have to be considered in the segregation of multiple dopants at the complex grain boundary structures. Here, two data mining techniques, the random-forests regression and the genetic algorithm, were applied to determine stable segregation sites at grain boundaries efficiently. Using the random-forests method, a predictive model was constructed from 2% of the segregation configurations and it has been shown that this model could determine the stable segregation configurations. Furthermore, the genetic algorithm also successfully determined the most stable segregation configuration with great efficiency. We demonstrate that these approaches are quite effective to investigate the dopant segregation behaviors at grain boundaries.

Development of helium porosity near-by grain boundaries in nickel-carbon alloys

International Nuclear Information System (INIS)

Reutov, I.V.; Reutov, V.F.

1995-01-01

The peculiarities of development of helium porosity near grain boundaries in nickel with 0.002-0.065 at.% carbon uniformly doped with helium up to 2·10 -2 at.% in the process of post-irradiation isothermal annealing at 800 deg C for 1-50 hours are studied. It is stated that at this annealing temperature intensive nucleation and growth of bubbles are observed in near-boundary region whose width grows both with increase of annealing time and carbon content. The TEM studies have shown that in near-boundary zone itself the process of bubble growth is non-uniform: bubble size increases and their density decreases as the distance from grain boundary is increased. The effect observed is discussed from the point of view of formation of two zones with different level of swelling in a grain (near-by boundary and matrix) and consequently, hydrostatic stress as well conditioning the flux of vacancies and helium-vacancy complexes from matrix to grain boundary. 8 refs., 5 figs

Diffusive boundary layers over varying topography

KAUST Repository

Dell, R. W.

2015-03-25

Diffusive bottom boundary layers can produce upslope flows in a stratified fluid. Accumulating observations suggest that these boundary layers may drive upwelling and mixing in mid-ocean ridge flank canyons. However, most studies of diffusive bottom boundary layers to date have concentrated on constant bottom slopes. We present a study of how diffusive boundary layers interact with various idealized topography, such as changes in bottom slope, slopes with corrugations and isolated sills. We use linear theory and numerical simulations in the regional ocean modeling system (ROMS) model to show changes in bottom slope can cause convergences and divergences within the boundary layer, in turn causing fluid exchanges that reach far into the overlying fluid and alter stratification far from the bottom. We also identify several different regimes of boundary-layer behaviour for topography with oceanographically relevant size and shape, including reversing flows and overflows, and we develop a simple theory that predicts the regime boundaries, including what topographies will generate overflows. As observations also suggest there may be overflows in deep canyons where the flow passes over isolated bumps and sills, this parameter range may be particularly significant for understanding the role of boundary layers in the deep ocean.

New deformation model of grain boundary strengthening in polycrystalline metals

International Nuclear Information System (INIS)

Trefilov, V.I.; Moiseev, V.F.; Pechkovskij, Eh.P.

1988-01-01

A new model explaining grain boundary strengthening in polycrystalline metals and alloys by strain hardening due to localization of plastic deformation in narrow bands near grain boundaries is suggested. Occurrence of localized deformation is caused by different flow stresses in grains of different orientation. A new model takes into account the active role of stress concentrator, independence of the strengthening coefficient on deformation, influence of segregations. Successful use of the model suggested for explanation of rhenium effect in molybdenum and tungsten is alloys pointed out

Spatially resolved analytical electron microscopy at grain boundaries of {alpha}-Al{sub 2}O{sub 3}; Ortsaufgeloeste analytische Elektronenmikroskopie an Korngrenzen in {alpha}Al{sub 2}O{sub 3}

Energy Technology Data Exchange (ETDEWEB)

Nufer, S.

2001-10-01

Aluminum oxide, {alpha}-Al{sub 2}O{sub 3}, is a common structural ceramic material. The most technologically important properties are either determined or strongly influenced by the polycrystalline microstructure. For instance, the grain boundaries control the mechanical behavior (e.g. plasticity, creep, and fracture) or various transport phenomena (e.g. ion diffusion, segregation, and electrical resistivity). In order to understand the structure-properties relationships, it is therefore important to characterize the structure and chemistry of grain boundaries, both experimentally and theoretically. In this work the electronic structure of the basal and rhombohedral twin grain boundaries and the impurity excess at different tilt grain boundaries in bicrystals were investigated, using electron energy-loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS). The electronic structure of the rhombohedral twin grain boundary was determined by comparing spatially resolved EELS measurements of the O-K ionisation edge with the theoretical density of states (DOS), obtained from local density functional theory (LDFT) calculations. The interface excess of impurities was quantitatively analysed at grain boundaries with and without Y-doping. (orig.)

Atomic scale study of grain boundary segregation before carbide nucleation in Ni–Cr–Fe Alloys

Energy Technology Data Exchange (ETDEWEB)

Li, Hui, E-mail: huili@shu.edu.cn [Key Laboratory for Microstructures, Shanghai University, Shanghai 200444 (China); Institute of Materials, Shanghai University, Shanghai 200072 (China); Xia, Shuang [Institute of Materials, Shanghai University, Shanghai 200072 (China); Liu, Wenqing [Key Laboratory for Microstructures, Shanghai University, Shanghai 200444 (China); Liu, Tingguang; Zhou, Bangxin [Institute of Materials, Shanghai University, Shanghai 200072 (China)

2013-08-15

Highlights: • Impurities segregated at grain boundaries were observed by atom probe tomography. • The comparison of segregation features in two Ni–Cr–Fe alloys was studied by APT. • C and Cr atoms co-segregated at grain boundaries before carbide precipitation. -- Abstract: Three dimensional chemical information concerning grain boundary segregation before carbide nucleation was characterized by atom probe tomography in two Ni–Cr–Fe alloys which were aged at 500 °C for 0.5 h after homogenizing treatment. B, C and Si atoms segregation at grain boundary in Alloy 690 was observed. B, C, N and P atoms segregation at grain boundary in 304 austenitic stainless steel was observed. C atoms co-segregation with Cr atoms at the grain boundaries both in Alloy 690 and 304 austenitic stainless steel was found, and its effect on the carbide nucleation was discussed. The amount of each segregated element at grain boundaries in the two Ni–Cr–Fe alloys were analyzed quantitatively. Comparison of the grain boundary segregation features of the two Ni–Cr–Fe alloys were carried out based on the experimental results.

Grain boundaries and defects in superconducting Bi-Sr-Ca-Cu-O ceramics

International Nuclear Information System (INIS)

Ramesh, R.; Bagley, B.G.; Tarascon, J.M.; Green, S.M.; Rudee, M.L.; Luo, H.L.

1990-01-01

Defects and structural interfaces in superconducting Bi-Sr-Ca-Cu-O have been characterized by transmission electron microscopy. The superconducting phase exhibits frequent variations in the stacking sequence (polytypoids). Dislocations, observed inside the grains, either introduce or accommodate the shear in the a-b plane and the local composition fluctuations. In general, the grains exhibit a platelike morphology with the a-b plane as the grain boundary plane. Grain boundaries along the short edge are generally disordered, whereas those near the long edge generally have a thin layer of the lower T c polytypoid. Coherent intragranular boundaries are also observed

Complementary AES and AEM of grain boundary regions in irradiated γ'-strengthened alloys

International Nuclear Information System (INIS)

Farrell, K.; Kishimoto, N.; Clausing, R.E.; Heatherly, L.; Lehman, G.L.

1986-01-01

Two microchemical analysis techniques are used to measure solute segregation at grain boundaries in two γ'-strengthened, fcc Fe-Ni-Cr alloys that display radiation-induced intergranular fracture. Scanning Auger electron spectroscopy (AES) of grain boundary fracture surfaces and analytical electron microscopy (AEM) of intact grain boundaries using energy-dispersive x-ray spectroscopy show good agreement on the nature and extent of segregation. The elements Ni, Si, Ti, and Mo are found to accumulate in G, Laves and γ' phases on the grain boundaries. Segregation of P is detected by AES. The complementary features of the two analytical techniques are discussed briefly

Algorithm based on regional separation for automatic grain boundary extraction using improved mean shift method

Science.gov (United States)

Zhenying, Xu; Jiandong, Zhu; Qi, Zhang; Yamba, Philip

2018-06-01

Metallographic microscopy shows that the vast majority of metal materials are composed of many small grains; the grain size of a metal is important for determining the tensile strength, toughness, plasticity, and other mechanical properties. In order to quantitatively evaluate grain size in metals, grain boundaries must be identified in metallographic images. Based on the phenomenon of grain boundary blurring or disconnection in metallographic images, this study develops an algorithm based on regional separation for automatically extracting grain boundaries by an improved mean shift method. Experimental observation shows that the grain boundaries obtained by the proposed algorithm are highly complete and accurate. This research has practical value because the proposed algorithm is suitable for grain boundary extraction from most metallographic images.

Effect of grain boundary structures on the behavior of He defects in Ni: An atomistic study

Institute of Scientific and Technical Information of China (English)

H F Gong; Y Yan; X S Zhang; W Lv; T Liu; Q S Ren

2017-01-01

We investigated the effect of grain boundary structures on the trapping strength of HeN (N is the number of helium atoms) defects in the grain boundaries of nickel.The results suggest that the binding energy of an interstitial helium atom to the grain boundary plane is the strongest among all sites around the plane.The HeN defect is much more stable in nickel bulk than in the grain boundary plane.Besides,the binding energy of an interstitial helium atom to a vacancy is stronger than that to a grain boundary plane.The binding strength between the grain boundary and the HeN defect increases with the defect size.Moreover,the binding strength of the HeN defect to the Σ3 (1 12)[110] grain boundary becomes much weaker than that to other grain boundaries as the defect size increases.

Misorientation related microstructure at the grain boundary in a nickel-based single crystal superalloy

International Nuclear Information System (INIS)

Huang, Ming; Zhuo, Longchao; Liu, Zhanli; Lu, Xiaogang; Shi, Zhenxue; Li, Jiarong; Zhu, Jing

2015-01-01

The mechanical properties of nickel-based single crystal superalloys deteriorate with increasing misorientation, thus the finished product rate of the casting of single crystal turbine airfoils may be reduced due to the formation of grain boundaries especially when the misorientation angle exceeds to some extent. To this day, evolution of the microstructures at the grain boundaries with misorientation and the relationship between the microstructures and the mechanical properties are still unclear. In this work a detailed characterization of the misorientation related microstructure at the grain boundary in DD6 single crystal superalloy has been carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques; the elemental distribution at the grain boundaries has been analyzed by energy dispersive (EDS) X-ray mapping; and the effect of precipitation of μ phases at the grain boundary on the mechanical property has been evaluated by finite element calculation. It is shown that the proportion of γ phase at the grain boundaries decreases, while the proportion of γ′ phase at the grain boundaries increases with increasing misorientation; the μ phase is precipitated at the grain boundaries when the misorientation angle exceeds about 10° and thus it could lead to a dramatic deterioration of the mechanical properties, as well as that the enrichment of Re and W gradually disappears as the misorientation angle increases. All these factors may result in the degradation of the mechanical properties at the grain boundaries as the misorientation increases. Furthermore, the finite element calculation confirms that precipitation of μ phases at the grain boundary is responsible for the significant deterioration of the mechanical properties when the misorientation exceeds about 10°. This work provides a physical imaging of the microstructure for understanding the relationship between the mechanical properties and the misorientation

Influence of annealing temperature on the Dy diffusion process in NdFeB magnets

Science.gov (United States)

Hu, Sheng-qing; Peng, Kun; Chen, Hong

2017-03-01

Sintered NdFeB magnets were coated with a layer of Dy metal using electron beam evaporation method and then annealed at various temperatures to investigate the temperature dependence of Dy diffusion process in NdFeB magnets. A Dy-rich phase was observed along the grain boundaries after the grain boundary diffusion process, the diffusion coefficients of various temperatures were obtained, the diffusion coefficients of Dy along the grain boundaries at 800 °C and 900 °C were determined to be 9.8×10-8 cm2 s-1 and 2.4×10-7 cm2 s-1, respectively. The diffusion length depended on the annealing temperature and the maximum diffusion length of approximately 1.8 mm and 3.0 mm can be obtained after annealing at 800 °C and 900 °C for 8 h. Higher diffusion temperature results in the diffusion not only along the grain boundaries but also into grains and then decrease in magnetic properties. The optimum annealing conditions can be determined as 900 °C for 8 h. The coercivity was improved from 1040 kA/m to 1450 kA/m and its magnetization has no significant reduction after the grain boundary diffusion process at the optimum annealing conditions.

A creep rupture model accounting for cavitation at sliding grain boundaries

NARCIS (Netherlands)

Giessen, Erik van der; Tvergaard, Viggo

1991-01-01

An axisymmetric cell model analysis is used to study creep failure by grain boundary cavitation at facets normal to the maximum principal tensile stress, taking into account the influence of cavitation and sliding at adjacent inclined grain boundaries. It is found that the interaction between the

Ultra-low-angle boundary networks within recrystallizing grains

DEFF Research Database (Denmark)

Ahl, Sonja Rosenlund; Simons, Hugh; Zhang, Yubin

2017-01-01

We present direct evidence of a network of well-defined ultra-low-angle boundaries in bulk recrystallizing grains of 99.5% pure aluminium (AA1050) by means of a new, three-dimensional X-ray mapping technique; dark-field X-ray microscopy. These boundaries separate lattice orientation differences o...

Determination of grain boundary mobility during recrystallization by statistical evaluation of electron backscatter diffraction measurements

International Nuclear Information System (INIS)

Basu, I.; Chen, M.; Loeck, M.; Al-Samman, T.; Molodov, D.A.

2016-01-01

One of the key aspects influencing microstructural design pathways in metallic systems is grain boundary motion. The present work introduces a method by means of which direct measurement of grain boundary mobility vs. misorientation dependence is made possible. The technique utilizes datasets acquired by means of serial electron backscatter diffraction (EBSD) measurements. The experimental EBSD measurements are collectively analyzed, whereby datasets were used to obtain grain boundary mobility and grain aspect ratio with respect to grain boundary misorientation. The proposed method is further validated using cellular automata (CA) simulations. Single crystal aluminium was cold rolled and scratched in order to nucleate random orientations. Subsequent annealing at 300 °C resulted in grains growing, in the direction normal to the scratch, into a single deformed orientation. Growth selection was observed, wherein the boundaries with misorientations close to Σ7 CSL orientation relationship (38° 〈111〉) migrated considerably faster. The obtained boundary mobility distribution exhibited a non-monotonic behavior with a maximum corresponding to misorientation of 38° ± 2° about 〈111〉 axes ± 4°, which was 10–100 times higher than the mobility values of random high angle boundaries. Correlation with the grain aspect ratio values indicated a strong growth anisotropy displayed by the fast growing grains. The observations have been discussed in terms of the influence of grain boundary character on grain boundary motion during recrystallization. - Highlights: • Statistical microstructure method to measure grain boundary mobility during recrystallization • Method implementation independent of material or crystal structure • Mobility of the Σ7 boundaries in 5N Al was calculated as 4.7 × 10"–"8 m"4/J ⋅ s. • Pronounced growth selection in the recrystallizing nuclei in Al • Boundary mobility values during recrystallization 2–3 orders of magnitude

The influence of the grain boundary strength on the macroscopic properties of a polycrystalline aggregate

International Nuclear Information System (INIS)

Simonovski, Igor; Cizelj, Leon; Garrido, Oriol Costa

2013-01-01

Highlights: ► Grain boundary stiffness should be at least 1.5× higher that the stiffness of bulk grains. ► The ratio δ n pl /δ n el should be at least 400. ► Simultaneous increase of δ n el and δ n pl at constant grain boundary strength increases numerical stability but results in high percentage of damage grain boundary area. ► Shear contributes significantly to damage initialization. -- Abstract: In this work a model, based on a X-ray diffraction contrast tomography data of a stainless steel wire with a diameter of 0.4 mm is presented. As measured 3D grain geometry and crystallographic orientation of individual grains are directly transferred into a finite element model. Anisotropic elasticity and crystal plasticity constitutive laws are used for the bulk grain material while the grain boundaries are explicitly modeled using the cohesive zone approach. A parametric study on the effects of the grain boundary strength and other cohesive zone parameters on the macroscopic response and damaged grain boundary area of a polycrystalline aggregate is presented. Recommendations for the cohesive zone parameters values aimed at achieving low damaged grain boundary area during numerical tensile tests are given while at the same time taking into account the numerical stability of the simulations

Boundary fluxes for nonlocal diffusion

Science.gov (United States)

Cortazar, Carmen; Elgueta, Manuel; Rossi, Julio D.; Wolanski, Noemi

We study a nonlocal diffusion operator in a bounded smooth domain prescribing the flux through the boundary. This problem may be seen as a generalization of the usual Neumann problem for the heat equation. First, we prove existence, uniqueness and a comparison principle. Next, we study the behavior of solutions for some prescribed boundary data including blowing up ones. Finally, we look at a nonlinear flux boundary condition.

Electron scattering at surfaces and grain boundaries in thin Au films

International Nuclear Information System (INIS)

Henriquez, Ricardo; Flores, Marcos; Moraga, Luis; Kremer, German; González-Fuentes, Claudio; Munoz, Raul C.

2013-01-01

The electron scattering at surfaces and grain boundaries is investigated using polycrystalline Au films deposited onto mica substrates. We vary the three length scales associated with: (i) electron scattering in the bulk, that at temperature T is characterized by the electronic mean free path in the bulk ℓ 0 (T); (ii) electron-surface scattering, that is characterized by the film thickness t; (iii) electron-grain boundary scattering, that is characterized by the mean grain diameter D. We varied independently the film thickness from approximately 50 nm to about 100 nm, and the typical grain size making up the samples from 12 nm to 160 nm. We also varied the scale of length associated with electron scattering in the bulk by measuring the resistivity of each specimen at temperatures T, 4 K 0 (T) by approximately 2 orders of magnitude. Detailed measurements of the grain size distribution as well as surface roughness of each sample were performed with a Scanning Tunnelling Microscope (STM). We compare, for the first time, theoretical predictions with resistivity data employing the two theories available that incorporate the effect of both electron-surface as well as electron-grain boundary scattering acting simultaneously: the theory of A.F. Mayadas and M. Shatzkes, Phys. Rev. 1 1382 (1970) (MS), and that of G. Palasantzas, Phys. Rev. B 58 9685 (1998). We eliminate adjustable parameters from the resistivity data analysis, by using as input the grain size distribution as well as the surface roughness measured with the STM on each sample. The outcome is that both theories provide a fair representation of both the temperature as well as the thickness dependence of the resistivity data, but yet there are marked differences between the resistivity predicted by these theories. In the case of the MS theory, when the average grain diameter D is significantly smaller than ℓ 0 (300) = 37 nm, the electron mean free path in the bulk at 300 K, the effect of electron-grain

Grain-Boundary Resistance in Copper Interconnects: From an Atomistic Model to a Neural Network

Science.gov (United States)

Valencia, Daniel; Wilson, Evan; Jiang, Zhengping; Valencia-Zapata, Gustavo A.; Wang, Kuang-Chung; Klimeck, Gerhard; Povolotskyi, Michael

2018-04-01

Orientation effects on the specific resistance of copper grain boundaries are studied systematically with two different atomistic tight-binding methods. A methodology is developed to model the specific resistance of grain boundaries in the ballistic limit using the embedded atom model, tight- binding methods, and nonequilibrium Green's functions. The methodology is validated against first-principles calculations for thin films with a single coincident grain boundary, with 6.4% deviation in the specific resistance. A statistical ensemble of 600 large, random structures with grains is studied. For structures with three grains, it is found that the distribution of specific resistances is close to normal. Finally, a compact model for grain-boundary-specific resistance is constructed based on a neural network.

Grain Boundary Engineering the Mechanical Properties of Allvac 718Plus(Trademark) Superalloy

Science.gov (United States)

Gabb, Timothy P.; Telesman, Jack; Garg, Anita; Lin, Peter; Provenzano, virgil; Heard, Robert; Miller, Herbert M.

2010-01-01

Grain Boundary Engineering can enhance the population of structurally-ordered "low S" Coincidence Site Lattice (CSL) grain boundaries in the microstructure. In some alloys, these "special" grain boundaries have been reported to improve overall resistance to corrosion, oxidation, and creep resistance. Such improvements could be quite beneficial for superalloys, especially in conditions which encourage damage and cracking at grain boundaries. Therefore, the effects of GBE processing on high-temperature mechanical properties of the cast and wrought superalloy Allvac 718Plus (Allvac ATI) were screened. Bar sections were subjected to varied GBE processing, and then consistently heat treated, machined, and tested at 650 C. Creep, tensile stress relaxation, and dwell fatigue crack growth tests were performed. The influences of GBE processing on microstructure, mechanical properties, and associated failure modes are discussed.

Electrical characterization of grain boundaries of CZTS thin films using conductive atomic force microscopy techniques

Energy Technology Data Exchange (ETDEWEB)

Muhunthan, N.; Singh, Om Pal [Compound Semiconductor Solar Cell, Physics of Energy Harvesting Division, New Delhi 110012 (India); Toutam, Vijaykumar, E-mail: toutamvk@nplindia.org [Quantum Phenomena and Applications Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012 (India); Singh, V.N., E-mail: singhvn@nplindia.org [Compound Semiconductor Solar Cell, Physics of Energy Harvesting Division, New Delhi 110012 (India)

2015-10-15

Graphical abstract: Experimental setup for conducting AFM (C-AFM). - Highlights: • Cu{sub 2}ZnSnS{sub 4} (CZTS) thin film was grown by reactive co-sputtering. • The electronic properties were probed using conducting atomic force microscope, scanning Kelvin probe microscopy and scanning capacitance microscopy. • C-AFM current flow mainly through grain boundaries rather than grain interiors. • SKPM indicated higher potential along the GBs compared to grain interiors. • The SCM explains that charge separation takes place at the interface of grain and grain boundary. - Abstract: Electrical characterization of grain boundaries (GB) of Cu-deficient CZTS (Copper Zinc Tin Sulfide) thin films was done using atomic force microscopic (AFM) techniques like Conductive atomic force microscopy (CAFM), Kelvin probe force microscopy (KPFM) and scanning capacitance microscopy (SCM). Absorbance spectroscopy was done for optical band gap calculations and Raman, XRD and EDS for structural and compositional characterization. Hall measurements were done for estimation of carrier mobility. CAFM and KPFM measurements showed that the currents flow mainly through grain boundaries (GB) rather than grain interiors. SCM results showed that charge separation mainly occurs at the interface of grain and grain boundaries and not all along the grain boundaries.

An Analysis of Hole Trapping at Grain Boundary or Poly-Si Floating-Body MOSFET.

Science.gov (United States)

Jang, Taejin; Baek, Myung-Hyun; Kim, Hyungjin; Park, Byung-Gook

2018-09-01

In this paper, we demonstrate the characteristics of the floating body effect of poly-silicon with grain boundary by SENTAURUS™ TCAD simulation. As drain voltage increases, impact ionization occurs at the drain-channel junction. And these holes created by impact ionization are deposited on the bottom of the body to change the threshold voltage. This feature, the kink effect, is also observed in fully depleted silicon on insulator because grain boundary of the poly-silicon serve as a storage to trap the holes. We simulate the transfer curve depending on the density and position of the grain boundary. The trap density of the grain boundary affects the device characteristics significantly. However similar properties appear except where the grain boundary is located on the drain side.

Diffusion processes in the magnetopause boundary layer

International Nuclear Information System (INIS)

Tsurutani, B.T.; Thorne, R.M.

1982-01-01

Anomalous cross-field diffusion of magnetosheath ions and electrons is a direct consequence of cyclotron-resonant scattering by electrostatic and electromagnetic emissions which are continuously present within the magnetopause boundary layer. Expressions for the rate of cross-field diffusion involving either type of wave are developed and expressed in terms of the absolute upper limit referred to as Bohm diffusion. For the typical average intensity of waves observed in the boundary layer, resonant electron cross-field diffusion is always insignificant. However, magnetosheath ions, resonant with low frequency electrostatic waves, may be transported inward at a rate approaching one tenth the Bohm rate (D/sub perpendiculartsperpendicular/roughly-equal10 3 km 2 /s). While this is not the only mechanism capable of explaining the presence of the low latitude boundary layer it is adequate to account for the typical boundary layer thickness and it should occur at all local times and under all interplanetary conditions. It consequently provides a continuous mechanism for significant mass and momentum transfer across the magnetopause under conditions when field merging is inoperative

Grain boundary cavity growth under applied stress and internal pressure

International Nuclear Information System (INIS)

Mancuso, J.F.

1977-08-01

The growth of grain boundary cavities under applied stress and internal gas pressure was investigated. Methane gas filled cavities were produced by the C + 4H reversible CH4 reaction in the grain boundaries of type 270 nickel by hydrogen charging in an autoclave at 500 0 C with a hydrogen pressure of either 3.4 or 14.5 MPa. Intergranular fracture of nickel was achieved at a charging temperature of 300 0 C and 10.3 MPa hydrogen pressure. Cavities on the grain boundaries were observed in the scanning electron microscope after fracture. Photomicrographs of the cavities were produced in stereo pairs which were analyzed so as to correct for perspective distortion and also to determine the orientational dependence of cavity growth under an applied tensile stress

Reconstruction of 3d grain boundaries from rock thin sections, using polarised light

Science.gov (United States)

Markus Hammes, Daniel; Peternell, Mark

2016-04-01

Grain boundaries affect the physical and chemical properties of polycrystalline materials significantly by initiating reactions and collecting impurities (Birchenall, 1959), and play an essential role in recrystallization (Doherty et al. 1997). In particular, the shape and crystallographic orientation of grain boundaries reveal the deformation and annealing history of rocks (Kruhl and Peternell 2002, Kuntcheva et al. 2006). However, there is a lack of non-destructive and easy-to-use computer supported methods to determine grain boundary geometries in 3D. The only available instrument using optical light to measure grain boundary angles is still the polarising microscope with attached universal stage; operated manually and time-consuming in use. Here we present a new approach to determine 3d grain boundary orientations from 2D rock thin sections. The data is recorded by using an automatic fabric analyser microscope (Peternell et al., 2010). Due to its unique arrangement of 9 light directions the highest birefringence colour due to each light direction and crystal orientation (retardation) can be determined at each pixel in the field of view. Retardation profiles across grain boundaries enable the calculation of grain boundary angle and direction. The data for all positions separating the grains are combined and further processed. In combination with the lateral position of the grain boundary, acquired using the FAME software (Hammes and Peternell, in review), the data is used to reconstruct a 3d grain boundary model. The processing of data is almost fully automatic by using MATLAB®. Only minor manual input is required. The applicability was demonstrated on quartzite samples, but the method is not solely restricted on quartz grains and other birefringent polycrystalline materials could be used instead. References: Birchenall, C.E., 1959: Physical Metallurgy. McGraw-Hill, New York. Doherty, R.D., Hughes, D.A., Humphreys, F.J., Jonas, J.J., Juul Jensen, D., Kassner, M

Modeling of stresses at grain boundaries with respect to occurrence of stress corrosion cracking

Energy Technology Data Exchange (ETDEWEB)

Kozaczek, K.J. [Oak Ridge National Lab., TN (United States); Sinharoy, A.; Ruud, C.O. [Pennsylvania State Univ., University Park, PA (United States); McIlree, A.R. [Electric Power Research Institute, Palo Alto, CA (United States)

1995-12-31

The distributions of elastic stresses/strains in the grain boundary regions were studied by the analytical and the finite element models. The grain boundaries represent the sites where stress concentration occurs as a result of discontinuity of elastic properties across the grain boundary and the presence of second phase particles elastically different from the surrounding matrix grains. A quantitative analysis of those stresses for steels and nickel based alloys showed that the stress concentrations in the grain boundary regions are high enough to cause a local microplastic deformation even when the material is in the macroscopic elastic regime. The stress redistribution as a result of such a plastic deformation was discussed.

Creep crack growth by grain boundary cavitation under monotonic and cyclic loading

Science.gov (United States)

Wen, Jian-Feng; Srivastava, Ankit; Benzerga, Amine; Tu, Shan-Tung; Needleman, Alan

2017-11-01

Plane strain finite deformation finite element calculations of mode I crack growth under small scale creep conditions are carried out. Attention is confined to isothermal conditions and two time histories of the applied stress intensity factor: (i) a monononic increase to a plateau value subsequently held fixed; and (ii) a cyclic time variation. The crack growth calculations are based on a micromechanics constitutive relation that couples creep deformation and damage due to grain boundary cavitation. Grain boundary cavitation, with cavity growth due to both creep and diffusion, is taken as the sole failure mechanism contributing to crack growth. The influence on the crack growth rate of loading history parameters, such as the magnitude of the applied stress intensity factor, the ratio of the applied minimum to maximum stress intensity factors, the loading rate, the hold time and the cyclic loading frequency, are explored. The crack growth rate under cyclic loading conditions is found to be greater than under monotonic creep loading with the plateau applied stress intensity factor equal to its maximum value under cyclic loading conditions. Several features of the crack growth behavior observed in creep-fatigue tests naturally emerge, for example, a Paris law type relation is obtained for cyclic loading.

SILICON CARBIDE GRAIN BOUNDARY DISTRIBUTIONS, IRRADIATION CONDITIONS, AND SILVER RETENTION IN IRRADIATED AGR-1 TRISO FUEL PARTICLES

Energy Technology Data Exchange (ETDEWEB)

Lillo, T. M.; Rooyen, I. J.; Aguiar, J. A.

2016-11-01

Precession electron diffraction in the transmission electron microscope was used to map grain orientation and ultimately determine grain boundary misorientation angle distributions, relative fractions of grain boundary types (random high angle, low angle or coincident site lattice (CSL)-related boundaries) and the distributions of CSL-related grain boundaries in the SiC layer of irradiated TRISO-coated fuel particles. Two particles from the AGR-1 experiment exhibiting high Ag-110m retention (>80%) were compared to a particle exhibiting low Ag-110m retention (<19%). Irradiated particles with high Ag-110m retention exhibited a lower fraction of random, high angle grain boundaries compared to the low Ag-110m retention particle. An inverse relationship between the random, high angle grain boundary fraction and Ag-110m retention is found and is consistent with grain boundary percolation theory. Also, comparison of the grain boundary distributions with previously reported unirradiated grain boundary distributions, based on SEM-based EBSD for similarly fabricated particles, showed only small differences, i.e. a greater low angle grain boundary fraction in unirradiated SiC. It was, thus, concluded that SiC layers with grain boundary distributions susceptible to Ag-110m release were present prior to irradiation. Finally, irradiation parameters were found to have little effect on the association of fission product precipitates with specific grain boundary types.

The influence of the grain boundary strength on the macroscopic properties of a polycrystalline aggregate

Energy Technology Data Exchange (ETDEWEB)

Simonovski, Igor, E-mail: Igor.Simonovski@ec.europa.eu [European Commission, DG-JRC, Institute for Energy and Transport, P.O. Box 2, NL-1755 ZG Petten (Netherlands); Cizelj, Leon, E-mail: Leon.Cizelj@ijs.si [Jožef Stefan Institute, Reactor Engineering Division, Jamova Cesta 39, SI-1000 Ljubljana (Slovenia); Garrido, Oriol Costa, E-mail: Oriol.Costa@ijs.si [Jožef Stefan Institute, Reactor Engineering Division, Jamova Cesta 39, SI-1000 Ljubljana (Slovenia)

2013-08-15

Highlights: ► Grain boundary stiffness should be at least 1.5× higher that the stiffness of bulk grains. ► The ratio δ{sub n}{sup pl}/δ{sub n}{sup el} should be at least 400. ► Simultaneous increase of δ{sub n}{sup el} and δ{sub n}{sup pl} at constant grain boundary strength increases numerical stability but results in high percentage of damage grain boundary area. ► Shear contributes significantly to damage initialization. -- Abstract: In this work a model, based on a X-ray diffraction contrast tomography data of a stainless steel wire with a diameter of 0.4 mm is presented. As measured 3D grain geometry and crystallographic orientation of individual grains are directly transferred into a finite element model. Anisotropic elasticity and crystal plasticity constitutive laws are used for the bulk grain material while the grain boundaries are explicitly modeled using the cohesive zone approach. A parametric study on the effects of the grain boundary strength and other cohesive zone parameters on the macroscopic response and damaged grain boundary area of a polycrystalline aggregate is presented. Recommendations for the cohesive zone parameters values aimed at achieving low damaged grain boundary area during numerical tensile tests are given while at the same time taking into account the numerical stability of the simulations.

On the grain boundary hardening in a B-bearing 304 austenitic stainless steel

International Nuclear Information System (INIS)

Yao, X.X.

1999-01-01

The precipitates, (Cr,Fe) 23 (C,B) 6 carbides and (Cr,Fe) 2 B borides, formed along the grain boundaries in a 304 austenitic stainless steel containing boron of 33 ppm after solution treatment at 1100 C for 1 h followed by isothermal ageing for 0.5 h at temperatures ranging from 750 to 1050 C have been identified. The influence of these precipitates on the grain boundary hardening has been investigated by means of micro-Vickers hardness measurements. It is found that the degree of grain boundary hardening below 900 C decreases, while it increases above 900 C with increasing ageing temperature. The dissolution of (Cr,Fe) 23 (C,B) 6 carbides and the precipitation of (Cr,Fe) 2 B borides are associated with the changes of grain boundary hardening in this B-bearing 304 austenitic stainless steel between 750 and 1100 C. The non-equilibrium boron segregation enhances the grain boundary hardening when the ageing temperature is above 900 C. (orig.)

Study and simulations of quick diffusion in Zr-based alloys

Energy Technology Data Exchange (ETDEWEB)

Corvalán, C., E-mail: corvalan@cnea.gov.ar [UNTREF (Universidad Nacional de Tres de Febrero), General Enrique Mosconi 2736, B1674AHF, Buenos Aires (Argentina); Gerencia de Materiales, CAC, Comisión de Energía Atómica, Av. del Libertador 8250, C1429BNP Buenos Aires (Argentina); Consejo Nacional de Investigaciones Científicas y Técnicas (Argentina); Lucía, A. [UNTREF (Universidad Nacional de Tres de Febrero), General Enrique Mosconi 2736, B1674AHF, Buenos Aires (Argentina); Instituto Sabato, Comisión Nacional de Energía Atómica, Universidad Nacional de San Martín, Buenos Aires (Argentina); Iribarren, M. [Gerencia de Materiales, CAC, Comisión de Energía Atómica, Av. del Libertador 8250, C1429BNP Buenos Aires (Argentina); Instituto Sabato, Comisión Nacional de Energía Atómica, Universidad Nacional de San Martín, Buenos Aires (Argentina); Servant, C. [Laboratoire de Physicochimie de l' Etat Solide, UMR 8182, ICMMO, Université de Paris-Sud XI, 91405 Orsay Cedex (France); Costa e Silva, A. [Universidade Federal Fluminense, Volta Redonda, RJ (Brazil)

2015-11-15

Zirconium and its alloys are widely used in the nuclear industry. Under normal conditions, Zr-alloys are polycrystalline and contain a high density of grain and interphase boundaries. These boundaries function as paths for accelerated matter movement. The movement of fast diffusing elements (Co, Fe, Cr, Ni) in Zr alloys along boundaries produces technologically important changes in the materials in nuclear reactors at normal temperatures (∼550 K) e.g.: segregation, phase precipitation, hydrogen absorption, etc. In this work, diffusion parameters for fast diffusion in Zr at low temperature were assessed for Co and Cr. An improved database for DICTRA (DIffusion-Controlled-TRAnsformation) software for fast diffusion was obtained. The diffusion parameters in grain boundaries of α-Zr for Cr and Co were used from a particular kinetic diffusion model [1]. Simulated profiles were compared with previous experimental work [2]. The results of the comparison and the adequacy of the improved database are discussed. Diffusion profiles on grain boundaries in α-Zr for Cr and Co are presented in the temperature range of 380–460 K. - Highlights: • An improved database for fast diffusion paths was obtained. • The diffusion parameters in type C kinetic on fast paths in α-Zr for Cr and Co were used. • Simulated diffusion profiles were compared with previous experimental results. This comparison and the adequacy of the improved database are discussed. • Diffusion parameters for fast diffusion paths in Zr at low temperature were assessed for Co and Cr. • Diffusion profiles on grain boundaries in α-Zr for Cr and Co are presented in the temperature range of 380–460 K.

Study and simulations of quick diffusion in Zr-based alloys

International Nuclear Information System (INIS)

Corvalán, C.; Lucía, A.; Iribarren, M.; Servant, C.; Costa e Silva, A.

2015-01-01

Zirconium and its alloys are widely used in the nuclear industry. Under normal conditions, Zr-alloys are polycrystalline and contain a high density of grain and interphase boundaries. These boundaries function as paths for accelerated matter movement. The movement of fast diffusing elements (Co, Fe, Cr, Ni) in Zr alloys along boundaries produces technologically important changes in the materials in nuclear reactors at normal temperatures (∼550 K) e.g.: segregation, phase precipitation, hydrogen absorption, etc. In this work, diffusion parameters for fast diffusion in Zr at low temperature were assessed for Co and Cr. An improved database for DICTRA (DIffusion-Controlled-TRAnsformation) software for fast diffusion was obtained. The diffusion parameters in grain boundaries of α-Zr for Cr and Co were used from a particular kinetic diffusion model [1]. Simulated profiles were compared with previous experimental work [2]. The results of the comparison and the adequacy of the improved database are discussed. Diffusion profiles on grain boundaries in α-Zr for Cr and Co are presented in the temperature range of 380–460 K. - Highlights: • An improved database for fast diffusion paths was obtained. • The diffusion parameters in type C kinetic on fast paths in α-Zr for Cr and Co were used. • Simulated diffusion profiles were compared with previous experimental results. This comparison and the adequacy of the improved database are discussed. • Diffusion parameters for fast diffusion paths in Zr at low temperature were assessed for Co and Cr. • Diffusion profiles on grain boundaries in α-Zr for Cr and Co are presented in the temperature range of 380–460 K.

Electron scattering at surfaces and grain boundaries in thin Au films

Energy Technology Data Exchange (ETDEWEB)

Henriquez, Ricardo [Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Casilla 110-V, Valparaíso (Chile); Flores, Marcos; Moraga, Luis [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile); Kremer, German [Bachillerato, Universidad de Chile, Las Palmeras 3425, Santiago 7800024 (Chile); González-Fuentes, Claudio [Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Casilla 110-V, Valparaíso (Chile); Munoz, Raul C., E-mail: ramunoz@ing.uchile.cl [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile)

2013-05-15

The electron scattering at surfaces and grain boundaries is investigated using polycrystalline Au films deposited onto mica substrates. We vary the three length scales associated with: (i) electron scattering in the bulk, that at temperature T is characterized by the electronic mean free path in the bulk ℓ{sub 0}(T); (ii) electron-surface scattering, that is characterized by the film thickness t; (iii) electron-grain boundary scattering, that is characterized by the mean grain diameter D. We varied independently the film thickness from approximately 50 nm to about 100 nm, and the typical grain size making up the samples from 12 nm to 160 nm. We also varied the scale of length associated with electron scattering in the bulk by measuring the resistivity of each specimen at temperatures T, 4 K < T < 300 K. Cooling the samples to 4 K increases ℓ{sub 0}(T) by approximately 2 orders of magnitude. Detailed measurements of the grain size distribution as well as surface roughness of each sample were performed with a Scanning Tunnelling Microscope (STM). We compare, for the first time, theoretical predictions with resistivity data employing the two theories available that incorporate the effect of both electron-surface as well as electron-grain boundary scattering acting simultaneously: the theory of A.F. Mayadas and M. Shatzkes, Phys. Rev. 1 1382 (1970) (MS), and that of G. Palasantzas, Phys. Rev. B 58 9685 (1998). We eliminate adjustable parameters from the resistivity data analysis, by using as input the grain size distribution as well as the surface roughness measured with the STM on each sample. The outcome is that both theories provide a fair representation of both the temperature as well as the thickness dependence of the resistivity data, but yet there are marked differences between the resistivity predicted by these theories. In the case of the MS theory, when the average grain diameter D is significantly smaller than ℓ{sub 0}(300) = 37 nm, the electron mean

Grain boundaries in Ni3Al. 2

International Nuclear Information System (INIS)

Kung, H.; Sass, S.L.

1992-01-01

This paper discusses the dislocation structure of small angle tilt and twist boundaries in ordered Ni 3 Al, with and without boron, investigated using transmission electron microscopy. Dislocation with Burgers vectors that correspond to anti-phase boundary (APB)-coupled superpartials were found in small angle twist boundaries in both boron-free and boron-doped Ni 3 Al, and a small angle tilt boundary in boron-doped Ni 3 Al. The boundary structures are in agreement with theoretical models proposed by Marcinkowski and co-workers. The APB energy determined from the dissociation of the grain boundary dislocations was lower than values reported for isolated APBs in Ni 3 Al. For small angle twist boundaries the presence of boron reduced the APB energy at the interface until it approached zero. This is consistent with the structure of these boundaries containing small regions of increased compositional disorder in the first atomic plane next to the interface

Precession electron diffraction for SiC grain boundary characterization in unirradiated TRISO fuel

International Nuclear Information System (INIS)

Lillo, T.M.; Rooyen, I.J. van; Wu, Y.Q.

2016-01-01

Highlights: • SiC grain orientation determined by TEM-based precession electron diffraction. • Orientation data improved with increasing TEM sample thickness. • Fraction of low angle grain boundaries lower from PED data than EBSD data. • Fractions of high angle and CSL-related boundaries similar to EBSD data. - Abstract: Precession electron diffraction (PED), a transmission electron microscopy-based technique, has been evaluated for the suitability for evaluating grain boundary character in the SiC layer of tristructural isotropic (TRISO) fuel. This work reports the effect of transmission electron microscope (TEM) lamella thickness on the quality of data and establishes a baseline comparison to SiC grain boundary characteristics, in an unirradiated TRISO particle, determined previously using a conventional electron backscatter diffraction (EBSD) scanning electron microscope (SEM)-based technique. In general, it was determined that the lamella thickness produced using the standard focused ion beam (FIB) fabrication process (∼80 nm), is sufficient to provide reliable PED measurements, although thicker lamellae (∼120 nm) were found to produce higher quality orientation data. Also, analysis of SiC grain boundary character from the TEM-based PED data showed a much lower fraction of low-angle grain boundaries compared to SEM-based EBSD data from the SiC layer of a TRISO-coated particle made using the same fabrication parameters and a SiC layer deposited at a slightly lower temperature from a surrogate TRISO particle. However, the fractions of high-angle and coincident site lattice (CSL)-related grain boundaries determined by PED are similar to those found using SEM-based EBSD. Since the grain size of the SiC layer of TRSIO fuel can be as small as 250 nm (Kirchhofer et al., 2013), depending on the fabrication parameters, and since grain boundary fission product precipitates in irradiated TRISO fuel can be nano-sized, the TEM-based PED orientation data

Grain boundary effects in nanocrystalline diamond

Czech Academy of Sciences Publication Activity Database

Mareš, Jiří J.; Hubík, Pavel; Krištofik, Jozef; Nesládek, Miloš

2008-01-01

Roč. 205, č. 9 (2008), 2163-2168 ISSN 1862-6300 R&D Projects: GA ČR(CZ) GA202/06/0040 Institutional research plan: CEZ:AV0Z10100521 Keywords : diamond film * grain boundary * superconductivity * noise * ballistic transport Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.205, year: 2008

Solid-state diffusion-controlled growth of the phases in the Au-Sn system

Science.gov (United States)

Baheti, Varun A.; Kashyap, Sanjay; Kumar, Praveen; Chattopadhyay, Kamanio; Paul, Aloke

2018-01-01

The solid state diffusion-controlled growth of the phases is studied for the Au-Sn system in the range of room temperature to 200 °C using bulk and electroplated diffusion couples. The number of product phases in the interdiffusion zone decreases with the decrease in annealing temperature. These phases grow with significantly high rates even at the room temperature. The growth rate of the AuSn4 phase is observed to be higher in the case of electroplated diffusion couple because of the relatively small grains and hence high contribution of the grain boundary diffusion when compared to the bulk diffusion couple. The diffraction pattern analysis indicates the same equilibrium crystal structure of the phases in these two types of diffusion couples. The analysis in the AuSn4 phase relating the estimated tracer diffusion coefficients with grain size, crystal structure, the homologous temperature of experiments and the concept of the sublattice diffusion mechanism in the intermetallic compounds indicate that Au diffuses mainly via the grain boundaries, whereas Sn diffuses via both the grain boundaries and the lattice.

Formation of intra-island grain boundaries in pentacene monolayers.

Science.gov (United States)

Zhang, Jian; Wu, Yu; Duhm, Steffen; Rabe, Jürgen P; Rudolf, Petra; Koch, Norbert

2011-12-21

To assess the formation of intra-island grain boundaries during the early stages of pentacene film growth, we studied sub-monolayers of pentacene on pristine silicon oxide and silicon oxide with high pinning centre density (induced by UV/O(3) treatment). We investigated the influence of the kinetic energy of the impinging molecules on the sub-monolayer growth by comparing organic molecular beam deposition (OMBD) and supersonic molecular beam deposition (SuMBD). For pentacene films fabricated by OMBD, higher pentacene island-density and higher polycrystalline island density were observed on UV/O(3)-treated silicon oxide as compared to pristine silicon oxide. Pentacene films deposited by SuMBD exhibited about one order of magnitude lower island- and polycrystalline island densities compared to OMBD, on both types of substrates. Our results suggest that polycrystalline growth of single islands on amorphous silicon oxide is facilitated by structural/chemical surface pinning centres, which act as nucleation centres for multiple grain formation in a single island. Furthermore, the overall lower intra-island grain boundary density in pentacene films fabricated by SuMBD reduces the number of charge carrier trapping sites specific to grain boundaries and should thus help achieving higher charge carrier mobilities, which are advantageous for their use in organic thin-film transistors.

Thermal stability of grain boundaries in nanocrystalline Zn studied by positron lifetime spectroscopy

International Nuclear Information System (INIS)

Zhou Kai; Li Hui; Pang Jinbiao; Wang Zhu

2012-01-01

Nanocrystalline Zn prepared by compacting nanoparticles with mean grain size about 55 nm at 15 MPa has been studied by positron lifetime spectroscopy. For the bulk Zn sample, the vacancy defect is annealed out at about 350 °C, but for the nanocrystalline Zn sample, the vacancy cluster in grain boundaries is quite difficult to be annealed out even at very high temperature (410 °C). In the grain boundaries of nanocrystalline Zn, the small free volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ 1 ). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τ av ), which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. This stabilization is very important for the nanocrystalline materials using as radiation resistant materials.

Hot-isostatic pressing of U-10Zr by grain boundary diffusion and creep cavitation. Part 2: Theory and data analysis

International Nuclear Information System (INIS)

McDeavitt, S.M.; Solomon, A.A.

1997-01-01

Uranium-10 wt % zirconium (U-10Zr) is a fuel alloy that has been used in the Experimental Breeder Reactor-II (EBR-II). The high burnup that was desired in this fuel system made high demands on the mechanical compatibility between fuel and cladding both during normal operation and during safety-related transients when rapid differential expansion may cause high stresses. In general, this mechanical stress can be reduced by cladding deformation if the cladding is sufficiently ductile at high burnup, and/or by fuel hot-pressing. Fortunately, the fuel is very porous when it contacts the cladding, but this porosity gradually fills with solid fission products (primarily lanthanides) that may limit the fuel's compressibility. If the porosity remains open, gaseous fission products are released and the porous fuel creeps rather than hot-presses under contact stresses. If the pores are closed by sintering or by solid fission products, the porous fuel will hot-isostatic press (HIP), as represented by the models to be discussed. HIP experiments performed at 700 C on U-10Zr samples with different impurity phase contents (Part 1) are analyzed in terms of several creep cavitation models. The coupled diffusion/creep cavitation model of Chen and Argon shows good quantitative agreement with measured HIP rates for hydride- and metal-derived U-10Zr materials, assuming that pores are uniformly distributed on grain boundaries and are of modal size, and that far-field strain rates are negligible. The analysis predicts, for the first time, an asymmetry between HIP and swelling at identical pressure-induced driving forces due to differences in grain boundary stresses. The differences in compressibility of hydride- and metal-derived U-10Zr can be partially explained by differences in pore size and spacing. The relevance of the experiments to description of in-reactor densification under external pressure or contact stress due to fuel/cladding mechanical interaction is discussed

Testing thermal gradient driving force for grain boundary migration using molecular dynamics simulations

International Nuclear Information System (INIS)

Bai, Xian-Ming; Zhang, Yongfeng; Tonks, Michael R.

2015-01-01

Strong thermal gradients in low-thermal-conductivity ceramics may drive extended defects, such as grain boundaries and voids, to migrate in preferential directions. In this work, molecular dynamics simulations are conducted to study thermal gradient driven grain boundary migration and to verify a previously proposed thermal gradient driving force equation, using uranium dioxide as a model system. It is found that a thermal gradient drives grain boundaries to migrate up the gradient and the migration velocity increases under a constant gradient owing to the increase in mobility with temperature. Different grain boundaries migrate at very different rates due to their different intrinsic mobilities. The extracted mobilities from the thermal gradient driven simulations are compared with those calculated from two other well-established methods and good agreement between the three different methods is found, demonstrating that the theoretical equation of the thermal gradient driving force is valid, although a correction of one input parameter should be made. The discrepancy in the grain boundary mobilities between modeling and experiments is also discussed.

Applied thermodynamics: Grain boundary segregation

Czech Academy of Sciences Publication Activity Database

Lejček, Pavel; Zheng, L.; Hofmann, S.; Šob, Mojmír

2014-01-01

Roč. 16, č. 3 (2014), s. 1462-1484 ISSN 1099-4300 R&D Projects: GA ČR(CZ) GAP108/12/0311; GA ČR GAP108/12/0144; GA MŠk(CZ) ED1.1.00/02.0068 Institutional support: RVO:68378271 ; RVO:68081723 Keywords : interfacial segregation * Gibbs energy of segregation * enthalpy * entropy * volume * grain boundaries * iron Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.502, year: 2014

Progress of HDDR NdFeB powders modulated by the diffusion of low melting point elements and their alloys

Directory of Open Access Journals (Sweden)

Lyu Meng

2017-12-01

Full Text Available The hydrogenation-disproportionation-desorption-recombination (HDDR process is the main technique for the fabrication of anisotropic NdFeB magnetic powder.But the intrinsic coercivity (HC of HDDR magnetic powder is low.The addition of heavy rare earth element Dy could improve its HC.It was found that the added Dy is mainly distributed in the grain boundary of HDDR magnets,which regulates grain boundary phase and increases the thickness of grain boundary to improve the anisotropy field (HA and HC of the magnets.However,Dy becomes scarcer and more expensive,which limits the practical application of HDDR magnets.To reduce the dependence on heavy rare earth elements and cost,researchers replaced the heavy rare earth element Dy by low melting point elements and their alloys through grain boundary diffusion technique.During diffusion process low melting point metal exists as liquid phase that increases the diffusion coefficient of diffusion medium as well as its contact area with grain boundary phases of HDDR magnets,and benefits its diffusion along grain boundaries and regulation of grain boundary phase.The modified grain boundary in magnets improve HC.This review paper focuses on the research progress in improving HC of HDDR NdFeB magnets by low melting point elements and their alloys.

Present state of the controversy about the grain boundary relaxation

International Nuclear Information System (INIS)

Povolo, F.; Molinas, B.J.

1990-04-01

An analysis of the internal friction produced by grain boundary relaxation in metals, alloys and ceramics is presented. The different interpretations given in the literature to relaxation phenomena occurring at temperatures above about half the melting point which include the influence of grain boundaries and their interaction with solutes and precipitates are discussed in detail. A complete set of the experimental data disposable in this field since 1972 until today is reviewed. Finally, some recent experiments are discussed and new ones are suggested. They might solve the actual controversy about the real origin of the relaxation phenomena observed. If this is the case, a considerable amount of information already published can be taken into account with a good degree of confidence. This information contributes to the description of the structure and behaviour of grain boundaries, both being important topics for materials science. (author). 119 refs, 21 figs, 1 tab

Coercivity degradation caused by inhomogeneous grain boundaries in sintered Nd-Fe-B permanent magnets

Science.gov (United States)

Chen, Hansheng; Yun, Fan; Qu, Jiangtao; Li, Yingfei; Cheng, Zhenxiang; Fang, Ruhao; Ye, Zhixiao; Ringer, Simon P.; Zheng, Rongkun

2018-05-01

Quantitative correlation between intrinsic coercivity and grain boundaries in three dimensions is critical to further improve the performance of sintered Nd-Fe-B permanent magnets. Here, we quantitatively reveal the local composition variation across and especially along grain boundaries using the powerful atomic-scale analysis technique known as atom probe tomography. We also estimate the saturation magnetization, magnetocrystalline anisotropy constant, and exchange stiffness of the grain boundaries on the basis of the experimentally determined structure and composition. Finally, using micromagnetic simulations, we quantify the intrinsic coercivity degradation caused by inhomogeneous grain boundaries. This approach can be applied to other magnetic materials for the analysis and optimization of magnetic properties.

Non-equilibrium grain boundary segregation of boron in austenitic stainless steel - IV. Precipitation behaviour and distribution of elements at grain boundaries

International Nuclear Information System (INIS)

Karlsson, L.; Norden, H.

1988-01-01

The distribution of elements and the precipitation behaviour at grain boundaries have been studied in boron containing AISI 316L and ''Mo-free AISI 316L'' type austenitic stainless steels. A combination of microanalytical techniques was used to study the boundary regions after cooling at 0.29-530 0 C/s from 800, 1075 or 1250 0 C. Tetragonal M/sub 2/B, M/sub 5/B/sub 3/ and M/sub 3/B/sub 2/, all rich in Fe, Cr and Mo, precipitated in the ''high B'' (40 ppm) AISI 316L steel whereas orthorhombic M/sub 2/B, rich in Cr and Fe was found in the ''Mo-free steel'' with 23 ppm B. In the ''high B steel'' a thin (<2nm), continuous layer, containing B, Cr, Mo and Fe and having a stoichiometry of typically M/sub 9/B, formed at boundaries after cooling at intermediate cooling rates. For both types of steels a boundary zone was found, after all heat treatments, with a composition differing significantly from the bulk composition. The differences were most marked after cooling at intermediate cooling rates. In both types of steel boundary depletion of Cr and enrichment of B and C occurred. It was found that non-equilibrium grain boundary segregation of boron can affect the precipitation behaviour by making the boundary composition enter a new phase field ''Non-equilibrium phases'' might also form. The synergistic effect of B and Mo on the boundary composition and precipitation behaviour, and the observed indications of C non-equilibrium segregation are discussed

Study of nanophase TiO2 grain boundaries by Raman spectroscopy

International Nuclear Information System (INIS)

Melendres, C.A.; Narayanasamy, A.; Maroni, V.A.; Siegel, R.W.

1989-01-01

Raman spectra have been recorded for as-consolidated nanophase TiO 2 samples with differing grain sizes and on samples annealed in air at a variety of temperatures up to 1273 K. The nanophase samples with the smallest grain size, about 12 nm average diameter, could have 15-30% of their atoms in grain boundaries; nevertheless, the strong Raman-active lines representative of the rutile structure were found to dominate all of the observed spectra, independent of grain size and annealing treatment. These lines were quite broad in the as-consolidated nanophase samples, equally in 12 nm and 100 nm grain-size compacts, but sharpened considerably upon annealing at elevated temperatures. The Raman data give no indication of grain-boundary structures in nanophase TiO 2 that are significantly different from those in conventional polycrystals. However, defect structures within the grains, which anneal out at elevated temperatures, are evidenced by changes in the Raman spectra. 15 refs., 2 figs

Defect and grain boundary scattering in tungsten: A combined theoretical and experimental study

Science.gov (United States)

Lanzillo, Nicholas A.; Dixit, Hemant; Milosevic, Erik; Niu, Chengyu; Carr, Adra V.; Oldiges, Phil; Raymond, Mark V.; Cho, Jin; Standaert, Theodorus E.; Kamineni, Vimal K.

2018-04-01

Several major electron scattering mechanisms in tungsten (W) are evaluated using a combination of first-principles density functional theory, a Non-Equilibrium Green's Function formalism, and thin film Kelvin 4-point sheet resistance measurements. The impact of grain boundary scattering is found to be roughly an order of magnitude larger than the impact of defect scattering. Ab initio simulations predict average grain boundary reflection coefficients for a number of twin grain boundaries to lie in the range r = 0.47 to r = 0.62, while experimental data can be fit to the empirical Mayadas-Schatzkes model with a comparable but slightly larger value of r = 0.69. The experimental and simulation data for grain boundary resistivity as a function of grain size show excellent agreement. These results provide crucial insights for understanding the impact of scaling of W-based contacts between active devices and back-end-of-line interconnects in next-generation semiconductor technology.

The diffusional growth of a grain boundary crack

International Nuclear Information System (INIS)

Puls, M.P.; Dutton, R.

1977-10-01

This report considers the possibility of high temperature rupture occurring by a grain boundary diffusional mechanism. It is assumed that a pre-existing, intergranular crack grows by loss of atoms from the crack tip to the grain boundary. Rupture occurs when the crack has grown to a critical length. A theoretical treatment of the kinetics of crack growth is presented and equations are derived for the crack velocity and time to rupture. A comparison is made with a previous theoretical model developed by Charles, together with rupture data obtained experimentally for the nickel-based alloy, Nimonic 80A. We conclude that experimental verification of the theoretical models requires a comparison with crack velocity data rather than time to rupture data. (author)

Adsorption of metal atoms at a buckled graphene grain boundary using model potentials

International Nuclear Information System (INIS)

Helgee, Edit E.; Isacsson, Andreas

2016-01-01

Two model potentials have been evaluated with regard to their ability to model adsorption of single metal atoms on a buckled graphene grain boundary. One of the potentials is a Lennard-Jones potential parametrized for gold and carbon, while the other is a bond-order potential parametrized for the interaction between carbon and platinum. Metals are expected to adsorb more strongly to grain boundaries than to pristine graphene due to their enhanced adsorption at point defects resembling those that constitute the grain boundary. Of the two potentials considered here, only the bond-order potential reproduces this behavior and predicts the energy of the adsorbate to be about 0.8 eV lower at the grain boundary than on pristine graphene. The Lennard-Jones potential predicts no significant difference in energy between adsorbates at the boundary and on pristine graphene. These results indicate that the Lennard-Jones potential is not suitable for studies of metal adsorption on defects in graphene, and that bond-order potentials are preferable

Grain boundary structures in La2/3Ca1/3MnO3 thin films

International Nuclear Information System (INIS)

Miller, D. J.; Lin, Y.-K.; Vlasko-Vlasov, V.; Welp, U.

1999-01-01

As with many other oxide-based compounds that exhibit electronic behavior, structural defects have a strong influence on the electronic properties of the CMR manganites. In this work, the authors have studied the effect of grain boundaries on the transport properties and on the local orientation of magnetization. Thin films of the perovskite-related La 2/3 Ca 1/3 MnO 3 compound were deposited onto bicrystal substrates using pulsed laser deposition. Transport measurements showed some enhancement of magnetoresistance across the grain boundary. The structure of the boundary was evaluated by electron microscopy. In contrast with the highly meandering boundaries typically observed in bicrystals of high temperature superconductors, the boundaries in these films are relatively straight and well defined. However, magneto-optical imaging showed that the local magnetization was oriented out of the plane at the grain boundary while it was oriented within the plane in the grains on either side. This coordinated reorientation of local magnetization near the grain boundary leads to enhanced magnetoresistance across the boundary in low fields

Boundary fluxes for non-local diffusion

OpenAIRE

Cortazar, C.; Elgueta, M.; Rossi, J. D.; Wolanski, N.

2006-01-01

We study a nonlocal diffusion operator in a bounded smooth domain prescribing the flux through the boundary. This problem may be seen as a generalization of the usual Neumann problem for the heat equation. First, we prove existence, uniqueness and a comparison principle. Next, we study the behavior of solutions for some prescribed boundary data including blowing up ones. Finally, we look at a nonlinear flux boundary condition.

Grain boundary precipitation in an austenitic stainless steel

International Nuclear Information System (INIS)

Jones, A.R.; Howell, P.R.; Ralph, B.

The precipitation of second phase particles of niobium carbide in an austenitic stainless steel is shown to be considerably influenced by the degree of deformation introduced prior to the ageing treatment. Sites for the nucleation of second phase particles are identified and the importance of one type of nucleation site, extrinsic dislocations, to the evolution of the final boundary precipitate distributions is emphasized. Further, it is shown that the presence of a grain boundary can effect precipitation processes for some considerable distance into the matrix on either side of the boundary. (author)

Grain boundary layer behavior in ZnO/Si heterostructure

International Nuclear Information System (INIS)

Liu Bingce; Liu Cihui; Yi Bo

2010-01-01

The grain boundary layer behavior in ZnO/Si heterostucture is investigated. The current-voltage (I-V) curves, deep level transient spectra (DLTS) and capacitance-voltage (C-V) curves are measured. The transport currents of ZnO/Si heterojunction are dominated by grain boundary layer as high densities of interfacial states existed. The interesting phenomenon that the crossing of In I-V curves of ZnO/Si heterojunction at various measurement temperatures and the decrease of its effective barrier height with the decrement of temperature are in contradiction with the ideal heterojunction thermal emission model is observed. The details will be discussed in the following. (semiconductor physics)

On the search for experimentally observed grain boundary phase transitions

International Nuclear Information System (INIS)

Balluffi, R.W.; Hsieh, T.E.

1987-07-01

The phase space for a heterogeneous system containing a grain boundary involves a relatively large number of variables (i.e., at least six plus the number of components), and it is therefore conceptually possible to induce a large variety of grain boundary phase transitions by selectively varying these parameters. Despite this, a review of the literature reveals that there have been virtually no clear-cut experimental observations of transitions reported in which the boundary structure has been observed as a function of time under well defined conditions. In current work, we are searching for roughening/faceting transitions and melting transitions for boundaries in Al by hot stage transmission electron microscopy. A clear example of a reversible roughening/faceting transition has been found. No evidence for melting has been found for temperatures as high as 0.96 T/sub m/ (by monitoring GBD core delocalization in several special boundaries with Σ ≤ 13) or 0.999 T/sub m/ (by observing the local diffraction contrast at general boundaries in polycrystalline specimens)

Influence of annealing temperature on the Dy diffusion process in NdFeB magnets

Energy Technology Data Exchange (ETDEWEB)

Hu, Sheng-qing, E-mail: joy_hsq@126.com [State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083 (China); Peng, Kun, E-mail: kpeng@hnu.edu.cn [State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083 (China); College of Materials Science and Engineering, Hunan University, Changsha 410082 (China); Chen, Hong [State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083 (China)

2017-03-15

Sintered NdFeB magnets were coated with a layer of Dy metal using electron beam evaporation method and then annealed at various temperatures to investigate the temperature dependence of Dy diffusion process in NdFeB magnets. A Dy-rich phase was observed along the grain boundaries after the grain boundary diffusion process, the diffusion coefficients of various temperatures were obtained, the diffusion coefficients of Dy along the grain boundaries at 800 °C and 900 °C were determined to be 9.8×10{sup −8} cm{sup 2} s{sup −1} and 2.4×10{sup −7} cm{sup 2} s{sup −1}, respectively. The diffusion length depended on the annealing temperature and the maximum diffusion length of approximately 1.8 mm and 3.0 mm can be obtained after annealing at 800 °C and 900 °C for 8 h. Higher diffusion temperature results in the diffusion not only along the grain boundaries but also into grains and then decrease in magnetic properties. The optimum annealing conditions can be determined as 900 °C for 8 h. The coercivity was improved from 1040 kA/m to 1450 kA/m and its magnetization has no significant reduction after the grain boundary diffusion process at the optimum annealing conditions. - Highlights: • The optimum annealing conditions can be determined as 900 °C for 8 h. • The diffusion coefficient of Dy at 900 °Care determined to be 2.4×10{sup −7} cm{sup 2} s{sup −1}. • A maximum diffusion length of about 3 mm can be obtained.

Investigation and optimization of YBa2Cu3O7-δ grain boundaries and coated conductors

International Nuclear Information System (INIS)

Held, Rainer Robert Martin

2010-01-01

With increasing misorientation angle grain boundaries strongly reduce the critical current density of high temperature superconductors. For this reason costly techniques are used in production of modern Coated Conductors to induce sharp textures in the polycrystalline superconductor layers. In this dissertation measurements of the critical current density of different grain boundary types are presented showing that out-of-plane grain boundaries exhibit, also in applied magnetic fields, much higher critical current densities than expected. In further analysis of the grain boundaries indications for a microstructural reason of the high critical current densities were found. The high critical current densities of the out-of-plane grain boundaries should in fabrication of Coated Conductors allow for a relaxation of the out-of-plane grain alignment requirements and a concomitant cost reduction. In this work also results of a industrial cooperation with Nexans are presented demonstrating that the critical current density of metal-organic deposited grain boundaries and Coated Conductor layers can be increased by selective Calcium-doping. In the experiments selective Calcium-doping most effectively increased the critical current density of weak spots. (orig.)

Reaction diffusion equations with boundary degeneracy

Directory of Open Access Journals (Sweden)

Huashui Zhan

2016-03-01

Full Text Available In this article, we consider the reaction diffusion equation $$ \\frac{\\partial u}{\\partial t} = \\Delta A(u,\\quad (x,t\\in \\Omega \\times (0,T, $$ with the homogeneous boundary condition. Inspired by the Fichera-Oleinik theory, if the equation is not only strongly degenerate in the interior of $\\Omega$, but also degenerate on the boundary, we show that the solution of the equation is free from any limitation of the boundary condition.

On the orientation dependent grain boundary migration in an Fe-6at.%Si alloy

International Nuclear Information System (INIS)

Lejcek, P.; Adamek, J.

1995-01-01

The [100]symmetrical tilt grain boundaries in an Fe-6at.%Si alloy were found to exhibit as pronounced anisotropy of activation enthalpy of migration characterized by its high values for special boundaries as compared to general ones. This rather surprising posing three main contributions to the migration enthalpy: intrinsic migration enthalpy, migration enthalpy resulting from grain boundary segregation, and migration enthalpy resulting from alloy mixing. It is shown that the differences in migration enthalpy of special and general grain boundaries in a concentrated alloy reflect the prevailing character of the intrinsic migration enthalpy over the weakened segregation effects. (orig.)

Partitioning of water between point defects, dislocations, and grain boundaries in olivine

Science.gov (United States)

Tielke, J. A.; Mecklenburgh, J.; Mariani, E.; Wheeler, J.

2017-12-01

Estimates of the storage capacity of water in the interior of the Earth and other terrestrial planets vary significantly. One interpretation is that water in planetary interiors exists primarily as hydrogen ions, dissociated from liquid water, that are associated with point defects in the crystal structure of nominally anhydrous minerals. However, dislocations and grain boundaries may contribute significantly to the storage capacity of water in planetary interiors, but hydrogen concentrations in dislocations and grain boundaries are difficult to quantify. To measure the water storage capacity of dislocations and grain boundaries, we are analyzing results from high-temperature and high-pressure experiments where deuterium, a stable isotope of hydrogen, was incorporated into olivine, the dominate phase in the upper mantle. Compared to hydrogen, deuterium concentrations can be determined at much higher spatial resolution using secondary-ion mass spectroscopy. The concentration of deuterium in the samples will also be quantified using Fourier transform infrared spectroscopy for comparison to results for hydrogen-bearing olivine. The spatial distribution of regions with different densities of geometrically-necessary dislocations and the locations of grain boundaries will be determined using electron-backscatter diffraction (EBSD) analyses. Correlation of the concentration of deuterium with dislocation densities and grain boundaries will be used to examine the partitioning of water-derived species between the different types of defects. Ultimately, these data will be used to place more realistic bounds on the storage capacity of water in the interior of Earth and of other terrestrial planets.

Thermally activated phase slippage in high- T sub c grain-boundary Josephson junctions

Energy Technology Data Exchange (ETDEWEB)

Gross, R.; Chaudhari, P.; Dimos, D.; Gupta, A.; Koren, G. (IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 (USA))

1990-01-08

The effect of thermally activated phase slippage (TAPS) in YBa{sub 2}Cu{sub 3}O{sub 7} grain-boundary Josephson junctions has been studied. TAPS has been found to be responsible for the dc noise voltage superimposed on the dc Josephson current near the transition temperature. Because of the reduced Josephson coupling energy of the grain-boundary junctions, which is caused by a reduced superconducting order parameter at the grain-boundary interface, TAPS is present over a considerable temperature range. The implications of TAPS on the applicability of high-{ital T}{sub {ital c}} Josephson junctions are outlined.

Micromechanical modelling of nanocrystalline and ultrafine grained metals: A short overview

DEFF Research Database (Denmark)

Mishnaevsky, Leon; Levashov, Evgeny

2015-01-01

An overview of micromechanical models of strength and deformation behaviour of nanostructured and ultrafine grained metallic materials is presented. Composite models of nanomaterials, polycrystal plasticity based models, grain boundary sliding, the effect of non-equilibrium grain boundaries...... and nanoscale properties are discussed and compared. The examples of incorporation of peculiar nanocrystalline effects (like large content of amorphous or semi-amorphous grain boundary phase, partial dislocation GB emission/glide/GB absorption based deformation mechanism, diffusion deformation, etc.......) into the continuum mechanical approach are given. The possibilities of using micromechanical models to explore the ways of the improving the properties of nanocrystalline materials by modifying their structures (e.g., dispersion strengthening, creating non-equilibrium grain boundaries, varying the grain size...

External boundary effects on simultaneous diffusion and reaction processes

International Nuclear Information System (INIS)

Le Roux, M.N.; Wilhelmsson, H.

1989-01-01

External boundaries influence the spatial and temporal structure of evolution of dynamic systems governed by reaction-diffusion equations. Critical limits, i.e. thresholds for explosive growth or onset of diffusion dominated decay, are found to be caused by the presence of the boundary and to depend on: the position of the boundary, where the density is assumed to be zero at any instant of time: the mutual weights (coefficients) and powers of the nonlinear reaction and diffusion processes; and the initial spatial distribution. However, for particular relations between the nonlinear powers of the reaction and diffusion terms the critical limits do not depend on the initial conditions. The results are obtained by simulation experiment for one, two and three dimensions. Trends in the dynamic evolution of the system with an external boundary imposed are compared with the corresponding analytic results obtained for free boundary. Interesting applications are found in various areas, e.g. in the field of high temperature fusion plasma where the evolution of the temperature profile for the so-called H-mode (constant plasma density) is described

Distribution of the grain limit character in SiC and its effect on the diffusion of fission products in the TRISO fuel particles; Distribucion del caracter de limite de grano en SiC y su efecto sobre la difusion de los productos de fision en las particulas de combustible TRISO

Energy Technology Data Exchange (ETDEWEB)

Cancino T, F.; Lopez H, E., E-mail: Eddie.lopez@cinvestav.edu.mx [IPN, Centro de Investigacion y de Estudios Avanzados, Unidad Saltillo, Av. Industria Metalurgica 1062, Zona Industrial, 25900 Ramos Arizpe, Coahuila (Mexico)

2017-09-15

At present is accepted that silver diffuses through silicium carbide (SiC) by diffusion in grain boundaries, although little is known about the characteristics of grain boundaries in SiC, and how these change depending on the type of sample. In this work, was observed that there are small but important differences between the SiC in the tri-structural isotropic (TRISO) particles and that of the monoliths, which could explain some of the differences observed in experiments on diffusion in the literature. Five different types (coatings and monoliths) of SiC produced by chemical vapor deposition (CVD) were characterized by electron backscatter diffraction (EBSD). In all the samples the SiC was mainly composed of high-angle grain boundaries (∼ 65%), with a small fraction of grain boundaries of low-angle (about 15%) and 20% of the coincidence site lattice (CSL). The morphology of the monoliths is constituted by large grains, surrounded by smaller grains; in the particles of the TRISO fuel, both columnar and equi axial grains were observed, with a more uniform distribution over the surface of the coating. (Author)

Grain-boundary effects on the magnetoresistance properties of perovskite manganite films

International Nuclear Information System (INIS)

Gupta, A.; Gong, G.Q.; Xiao, G.; Duncombe, P.R.; Lecoeur, P.; Trouilloud, P.; Wang, Y.Y.; Dravid, V.P.; Sun, J.Z.

1996-01-01

The role of grain boundaries in the magnetoresistance (MR) properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of La 0.67 D 0.33 MnO 3-δ (D=Ca,Sr, or vacancies). While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K. The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region. copyright 1996 The American Physical Society

High temperature microplasticity of fine-grained ceramics

International Nuclear Information System (INIS)

Lakki, A.; Schaller, R.

1996-01-01

Several fine-grained ceramics exhibit enhanced ductility or even structural superplasticity at high temperature. Grain boundaries play a dominant role in the deformation process of these materials which usually involves diffusion-accommodated grain boundary sliding. Sliding is either lubricated by an amorphous intergranular phase or takes place by glide and climb of grain boundary dislocations. At high temperature, anelastic deformation precedes plastic deformation and stems from the short range motion of lattice defects, such as dislocations and grain boundaries. The energy loss (''mechanical loss'') associated with such motion can be measured by using the technique of mechanical spectroscopy. Moreover, at the onset of plasticity (''microplasticity''), long range irrecoverable motion of defects contributes to additional mechanical loss. Mechanical loss spectra may then give an insight into mechanisms operating at the transition between anelastic and plastic deformation. As an illustration, the spectra of three fine-grained ceramics (Si 3 N 4 , ZrO 2 , Al 2 O 3 ) are presented. In all cases, anelastic relaxation phenomena (peak and background) have been observed at high temperature (> 1200 K), bearing a close relation with creep behaviour. Their analysis permits to distinguish between different types of microstructrual elements: bulk regions of amorphous intergranular phase at triple points, grain boundaries separated by a thin glassy film and ''clean'' grain boundaries. (orig.)

Numerical study of the atomic and electronic structure of some silicon grain boundaries

International Nuclear Information System (INIS)

Torrent, M.

1996-01-01

This work contributes to the theoretical study of extended defects in covalent materials. The study is especially devoted to the tilt grain boundaries in silicon as a model material. The theoretical model is based on the self-consistent tight-binding approximation and is applied within two numerical techniques: the fast 'order N' density-matrix method and the diagonalization technique which allows the sampling of the reciprocal space. Total energy parameters of the model have been fitted in order to reproduce the silicon band structure (with a correct gap value) and the transferability of crystalline and mechanical properties of this material. A new type of boundary conditions is proposed and tested. These conditions, named 'ante-periodic' or 'Moebius', allow only one grain boundary per box instead of two and decrease the CPU time by a factor of two. The model is then applied to the study of the Σ=25 [001] (710) grain boundary. The results show the possible presence in this boundary of low energy non-reconstructed atomic structures which are electrically active. This confirms what had been suggested by some experimental observations. The same study is also performed for the Σ=13 [001] (510) grain boundary. In order to compare the intrinsic electrical activity in the previous grain boundaries with the one induced by impurities, a total energy parametrization for the silicon-nickel bond is achieved and used in preliminary calculations. Finally the two variants of the Σ=11 [011] (2-33) interface are studied, especially their respective interfacial energies. The result disagrees with previous calculations using phenomenological potentials. (author)

Importance of low-angle grain boundaries in YBa2Cu3O7-δ coated conductors

International Nuclear Information System (INIS)

Durrell, J H; Rutter, N A

2009-01-01

Over the past ten years the perception of grain boundaries in YBa 2 Cu 3 O 7-δ conductors has changed greatly. They are now not a problem to be eliminated, but an inevitable and potentially favourable part of the material. This change has arisen as a consequence of new manufacturing techniques which result in excellent grain alignment, reducing the spread of grain boundary misorientation angles. At the same time there is considerable recent evidence which indicates that the variation of properties of grain boundaries with mismatch angle is more complex than a simple exponential decrease in critical current. This is due to the fact that low-angle grain boundaries represent a qualitatively different system to high-angle boundaries. The time is therefore right for a targeted review of research into low-angle YBa 2 Cu 3 O 7-δ grain boundaries. This article does not purport to be a comprehensive review of the physics of grain boundaries as found in YBa 2 Cu 3 O 7-δ in general; for a broader overview we would recommend that the reader consult the comprehensive review of Hilgenkamp and Mannhart (2002 Rev. Mod. Phys. 74 485). The purpose of this article is to review the origin and properties of the low-angle grain boundaries found in YBa 2 Cu 3 O 7-δ coated conductors both individually and as a collective system. (topical review)

Stochastic diffusion of dust grains by the interplanetary magnetic field

International Nuclear Information System (INIS)

Hassan, M.H.A.; Wallis, M.K.

1983-10-01

The effects of the sectored Interplanetary Magnetic Field on charged dust grains orbiting around the sun under radiation pressure and Poynting-Robertson drag forces are examined for initially circular and non-inclined orbits. The distribution function of the charged grains satisfies a Fokker-Planck equation in which the sectored field is taken as a source of stochastic impulses. By adopting the integrals of the impulse-free motion as variable parameters, the Fokker-Planck equation can be properly treated as a diffusion equation. Analytic solutions of the resulting diffusion equation show that dust grains injected near the ecliptic plane are scattered strongly to high helio-latitudes. The scattering is more pronounced for small grains injected at large distances from the Sun. (author)

Effect of grain boundary microcracks on crack resistance of annealed tungsten

International Nuclear Information System (INIS)

Babak, A.V.; Uskov, E.I.

1984-01-01

Effect of grain boundary microcracks in tungsten, produced by the method of powder sintering, on its crack resistance after annealing at T=2200 deg C, has been considered. On the basis of complex physncomechanical study of tungsten crack resistance it is shown, that the value of ultimate tensile stress does not depend on temperature. The presence of grain boundary cracks in such material (in the limits from 2 to 8%) does not produce effect on its crack resistance

Statistical model for grain boundary and grain volume oxidation kinetics in UO2 spent fuel

International Nuclear Information System (INIS)

Stout, R.B.; Shaw, H.F.; Einziger, R.E.

1989-09-01

This paper addresses statistical characteristics for the simplest case of grain boundary/grain volume oxidation kinetics of UO 2 to U 3 O 7 for a fragment of a spent fuel pellet. It also presents a limited discussion of future extensions to this simple case to represent the more complex cases of oxidation kinetics in spent fuels. 17 refs., 1 fig

Nanocompositional Electron Microscopic Analysis and Role of Grain Boundary Phase of Isotropically Oriented Nd-Fe-B Magnets

Directory of Open Access Journals (Sweden)

Gregor A. Zickler

2017-01-01

Full Text Available Nanoanalytical TEM characterization in combination with finite element micromagnetic modelling clarifies the impact of the grain misalignment and grain boundary nanocomposition on the coercive field and gives guidelines how to improve coercivity in Nd-Fe-B based magnets. The nanoprobe electron energy loss spectroscopy measurements obtained an asymmetric composition profile of the Fe-content across the grain boundary phase in isotropically oriented melt-spun magnets and showed an enrichment of iron up to 60 at% in the Nd-containing grain boundaries close to Nd2Fe14B grain surfaces parallel to the c-axis and a reduced iron content up to 35% close to grain surfaces perpendicular to the c-axis. The numerical micromagnetic simulations on isotropically oriented magnets using realistic model structures from the TEM results reveal a complex magnetization reversal starting at the grain boundary phase and show that the coercive field increases compared to directly coupled grains with no grain boundary phase independently of the grain boundary thickness. This behaviour is contrary to the one in aligned anisotropic magnets, where the coercive field decreases compared to directly coupled grains with an increasing grain boundary thickness, if Js value is > 0.2 T, and the magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries at the prismatic planes parallel to the c-axis and secondly as Néel domain wall at the basal planes perpendicular to the c-axis. In summary our study shows an increase of coercive field in isotropically oriented Nd-Fe-B magnets for GB layer thickness > 5 nm and an average Js value of the GB layer < 0.8 T compared to the magnet with perfectly aligned grains.

Mapping grain boundary heterogeneity at the nanoscale in a positive temperature coefficient of resistivity ceramic

Science.gov (United States)

Holsgrove, Kristina M.; Kepaptsoglou, Demie M.; Douglas, Alan M.; Ramasse, Quentin M.; Prestat, Eric; Haigh, Sarah J.; Ward, Michael B.; Kumar, Amit; Gregg, J. Marty; Arredondo, Miryam

2017-06-01

Despite being of wide commercial use in devices, the orders of magnitude increase in resistance that can be seen in some semiconducting BaTiO3-based ceramics, on heating through the Curie temperature (TC), is far from well understood. Current understanding of the behavior hinges on the role of grain boundary resistance that can be modified by polarization discontinuities which develop in the ferroelectric state. However, direct nanoscale resistance mapping to verify this model has rarely been attempted, and the potential approach to engineer polarization states at the grain boundaries, that could lead to optimized positive temperature coefficient (PTC) behavior, is strongly underdeveloped. Here we present direct visualization and nanoscale mapping in a commercially optimized BaTiO3-PbTiO3-CaTiO3 PTC ceramic using Kelvin probe force microscopy, which shows that, even in the low resistance ferroelectric state, the potential drop at grain boundaries is significantly greater than in grain interiors. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy reveal new evidence of Pb-rich grain boundaries symptomatic of a higher net polarization normal to the grain boundaries compared to the purer grain interiors. These results validate the critical link between optimized PTC performance and higher local polarization at grain boundaries in this specific ceramic system and suggest a novel route towards engineering devices where an interface layer of higher spontaneous polarization could lead to enhanced PTC functionality.

Mapping grain boundary heterogeneity at the nanoscale in a positive temperature coefficient of resistivity ceramic

Directory of Open Access Journals (Sweden)

Kristina M. Holsgrove

2017-06-01

Full Text Available Despite being of wide commercial use in devices, the orders of magnitude increase in resistance that can be seen in some semiconducting BaTiO3-based ceramics, on heating through the Curie temperature (TC, is far from well understood. Current understanding of the behavior hinges on the role of grain boundary resistance that can be modified by polarization discontinuities which develop in the ferroelectric state. However, direct nanoscale resistance mapping to verify this model has rarely been attempted, and the potential approach to engineer polarization states at the grain boundaries, that could lead to optimized positive temperature coefficient (PTC behavior, is strongly underdeveloped. Here we present direct visualization and nanoscale mapping in a commercially optimized BaTiO3–PbTiO3–CaTiO3 PTC ceramic using Kelvin probe force microscopy, which shows that, even in the low resistance ferroelectric state, the potential drop at grain boundaries is significantly greater than in grain interiors. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy reveal new evidence of Pb-rich grain boundaries symptomatic of a higher net polarization normal to the grain boundaries compared to the purer grain interiors. These results validate the critical link between optimized PTC performance and higher local polarization at grain boundaries in this specific ceramic system and suggest a novel route towards engineering devices where an interface layer of higher spontaneous polarization could lead to enhanced PTC functionality.

Analysis of defect structure in silicon. Effect of grain boundary density on carrier mobility in UCP material

Science.gov (United States)

Dunn, J.; Stringfellow, G. B.; Natesh, R.

1982-01-01

The relationships between hole mobility and grain boundary density were studied. Mobility was measured using the van der Pauw technique, and grain boundary density was measured using a quantitative microscopy technique. Mobility was found to decrease with increasing grain boundary density.

A boundary integral equation for boundary element applications in multigroup neutron diffusion theory

International Nuclear Information System (INIS)

Ozgener, B.

1998-01-01

A boundary integral equation (BIE) is developed for the application of the boundary element method to the multigroup neutron diffusion equations. The developed BIE contains no explicit scattering term; the scattering effects are taken into account by redefining the unknowns. Boundary elements of the linear and constant variety are utilised for validation of the developed boundary integral formulation

Effect of grain boundary on electrical characteristics in B- and P-doped polycrystalline Si1-x-yGe xC y film deposited by ultraclean LPCVD

International Nuclear Information System (INIS)

Shim, Hyunyoung; Sakuraba, Masao; Murota, Junichi

2006-01-01

The effect of grain boundary on electrical characteristics in B- and P-doped polycrystalline (poly) Si 1-x -y Ge x C y films was investigated. Poly-Si 1-x -y Ge x C y films were deposited on thermally oxidized Si(100) at 500-650 deg. C in a SiH 4 -GeH 4 -SiH 3 CH 3 -H 2 gas mixture by an ultraclean hot-wall low-pressure chemical vapor deposition. B and P were doped into the films by ion implantation and diffusion by heat-treatment. The electrical properties are characterized by grain size, width of disordered region near grain boundaries, carrier trap density and the amount of impurity segregation at grain boundaries. In the B-doped poly-Si 1-x -y Ge x C y films heat-treated at 900 deg. C, the increase of carrier concentration n poly and the decrease of resistivity ρ poly with Ge addition are caused by the narrowing of the width of disordered regions, i.e., crystallization of disordered regions induced by Ge atoms. The decrease of n poly and the increase of ρ poly with C addition are explained by the suppression of crystallization of disordered region due to C atom segregation at grain boundaries. In the P-doped poly-Si 1-x -y Ge x C y films, it is found that n poly and ρ poly are influenced by P atom segregation at grain boundaries due to lowering solid solubility of P in grain by the existence of Ge

Investigations of the electrical neutralization and bonding mechanisms of shallow impurities in silicon grain boundaries

International Nuclear Information System (INIS)

Kazmerski, L.L.; Nelson, A.J.; Dhere, R.G.; Abou-Elfotouh, F.

1987-01-01

Interactions between shallow acceptors (B, Al, Ga and In) and hydrogen in polycrystalline Si are investigated. The bonding mechanisms involved in the acceptor neutralization process at grain boundaries are examined using microanalytical techniques. Differences in the incorporation of molecular and atomic hydrogen, and corresponding variations in electrical passivation at grain boundaries, are observed. Low-temperature Auger difference spectroscopy confirms Si-H bonding to dominate B, Ga and In-doped cases, with no direct acceptor-hydrogen bonding. Al-rich grain boundaries show H-complex and hydroxyl bonding. The data confirm chemical bond strength trends with B< Ga< In. Volume-indexed AES is utilized to compare bonding and H-distributions in B- and Al-rich grain boundary regions

Detection of grain-boundary resistance to slip transfer using nanoindentation

NARCIS (Netherlands)

Soer, WA; De Hosson, JTM

2005-01-01

Nanoindentation measurements near a high-angle grain boundary in a Fe-14%Si bicrystal showed dislocation pile-up and transmission across the boundary. The latter is observed as a characteristic displacement jump, from which the Hall-Petch slope can be calculated as a measure for the slip

Effects of microstructures and creep conditions on the fractal dimension of grain boundary fracture in high-temperature creep of heat-resistant alloys

International Nuclear Information System (INIS)

Tanaka, Manabu

1993-01-01

The effects of microstructural aspects, such as grain size and grain boundary configuration, and creep conditions on the fractal dimension of the grain boundary fracture were examined using several heat-resistant alloys, principally in an analysis scale range between one grain boundary length and specimen size. Grain boundary fracture surface profiles in the heat-resistant alloys exhibited a fractal nature in the scale range between one grain boundary length and specimen size as well as in the scale range below one grain boundary length. The fractal dimension of the grain boundary fracture slightly increased with decreasing grain size and was generally a little larger in the specimens with serrated grain boundaries than in those with straight grain boundaries. The fractal dimension of the grain boundary and the number of grain boundary microcracks which affected the grain boundary fracture patterns were a little larger in the specimen with the smaller grain size, and were also larger in the specimen with serrated grain boundaries. The fractal dimension of the grain boundary fracture increased with decreasing creep stress in the temperature range from 973 to 1422 K in these alloys, since more grain boundary microcracks existed in the specimens ruptured under the lower stresses at the higher temperatures. (orig.) [de

Determination of compositional ordering at grain boundaries in boron-doped Ni3Al

International Nuclear Information System (INIS)

Mills, M.J.

1989-01-01

The effects of crystal thickness and defocus on the superlattice contrast from HRTEM images have been demonstrated. The results indicate that fine, FCC fringe spacings in the vicinity of these grain boundaries can be produced if the boundary is slightly inclined to the electron beam, creating the false impression that the region is compositionally disordered. For properly chosen defocus conditions and boundary orientation, contrast typical of the ordered structure extends up to the estimated position of the boundary plane. The lack of a distinct disordered region suggests that microplasticity near grain boundaries is not significantly affected by the presence of B, and that its influence must be highly localized to the boundaries

Anomalous diffusion in niobium. Study of solute diffusion mechanism of iron in niobium

International Nuclear Information System (INIS)

Ablitzer, D.

1977-01-01

In order to explain anomalously high diffusion velocities observed for iron diffusion in niobium, the following parameters were measured: isotope effect, b factor (which expresses the effect of iron on niobium self-diffusion), self-diffusion coefficient of niobium, solute diffusion coefficient of iron in niobium. The results obtained show that neither pure vacancy models, nor diffusion in the lattice defects (dislocations, sub-boundaries, grain boundaries), nor pure interstitialy mechanisms, nor simple or cyclic exchange mechanisms agree with experiments. A mechanism is proposed which considers an equilibrium between substitution iron atoms and interstitial iron atoms. The diffusion of iron then occurs through interstitial vancancy pairs [fr

Effect of grain boundary phase on the magnetization reversal process of nanocrystalline magnet using large-scale micromagnetic simulation

Directory of Open Access Journals (Sweden)

Hiroshi Tsukahara

2018-05-01

Full Text Available We investigated the effects of grain boundary phases on magnetization reversal in permanent magnets by performing large-scale micromagnetic simulations based on Landau–Lifshitz–Gilbert equation under a periodic boundary. We considered planar grain boundary phases parallel and perpendicular to an easy axis of the permanent magnet and assumed the saturation magnetization and exchange stiffness constant of the grain boundary phase to be 10% and 1%, respectively, for Nd2Fe14B grains. The grain boundary phase parallel to the easy axis effectively inhibits propagation of magnetization reversal. In contrast, the domain wall moves across the grain boundary perpendicular to the easy axis. These properties of the domain wall motion are explained by dipole interaction, which stabilizes the antiparallel magnetic configuration in the direction perpendicular to the magnetization orientation. On the other hand, the magnetization is aligned in the same direction by the dipole interaction parallel to the magnetization orientation. This anisotropy of the effect of the grain boundary phase shows that improvement of the grain boundary phase perpendicular to the easy axis effectively enhances the coercivity of permanent magnets.

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

Energy Technology Data Exchange (ETDEWEB)

Devaraj, Arun; Kovarik, Libor; Kautz, Elizabeth; Arey, Bruce; Jana, Saumyadeep; Lavender, Curt; Joshi, Vineet

2018-06-01

Grain boundaries in metallic alloys often play a crucial role, not only in determining the mechanical properties or thermal stability of alloys, but also in dictating the phase transformation kinetics during thermomechanical processing. We demonstrate that locally stabilized structure and compositional segregation at grain boundaries—“grain boundary complexions”—in a complex multicomponent alloy can be modified to influence the kinetics of cellular transformation during subsequent thermomechanical processing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography analysis of a metallic nuclear fuel highly relevant to worldwide nuclear non-proliferation efforts —uranium-10 wt% molybdenum (U-10Mo) alloy, new evidence for the existence of grain boundary complexion is provided. We then modified the concentration of impurities dissolved in Υ-UMo grain interiors and/or segregated to Υ-UMo grain boundaries by changing the homogenization treatment, and these effects were used used to retard the kinetics of cellular transformation during subsequent sub-eutectoid annealing in this U-10-Mo alloy during sub-eutectoid annealing. Thus, this work provided insights on tailoring the final microstructure of the U-10Mo alloy, which can potentially improve the irradiation performance of this important class of alloy fuels.

Evolution of grain structure in nickel oxide scales

International Nuclear Information System (INIS)

Atkinson, H.V.

1987-01-01

In systems such as the oxidation of nickel, in which grain-boundary diffusion in the oxide can control the rate of oxidation, understanding of the factors governing the grain structure is of importance. High-purity mechanically polished polycrystalline nickel was oxidized at 700 0 C, 800 0 C, and 1000 0 C for times up to 20 hr in 1 atm O 2 . The scale microstructures were examined by parallel and transverse cross section transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Texture coefficients were found by x-ray diffraction (XRD). Each grain in the transverse section grain boundary networks was systematically analyzed for width parallel to the Ni-NiO interface and perpendicular length, for boundary radius of curvature and for number of sides. The variation of these parameters with depth in the scale was examined. In particular, grains were increasingly columnar (i.e., with ratio of grain length to width > 1) at higher temperatures and longer times. Columnar grain boundaries tended to be fairly static; the columnar grain width was less than the rate controlling grain size predicted from the oxidation rate. The mean boundary curvature per grain provided a guide to the tendency for grain growth, except in the region of the Ni-NiO interface, where the boundaries were thought to be pinned

Superplasticity and grain boundary character distribution in overaged Al-Li-Cu-Mg-Zr alloy

International Nuclear Information System (INIS)

Avramovic-Cingara, G.; Aust, K.T.; Perovic, D.D.; McQueen, H.J.

1995-01-01

Samples of 8091 alloy were subjected to a thermomechanical processing (TMP) treatment that included the following stages: overaging before deformation, multistage deformation at 300 deg C and strain rate change tests for superplasticity. Torsional deformation was utilized both to develop the refined microstructure and to test for superplasticity. The strain rate sensitivity, m, of the material ranged between 0.30 and 0.45 at 450 deg C for strain rates between 8 x 10 -2 and 10 -3 s -1 . The grain boundary character distribution (GBCD) of thermomechanically processed Al-Li-Cu-Mg-Zr (8091) alloy, which develops good superplastic response, has been determined by an electron backscattering diffraction technique (EBSD). All grain boundaries have been classified into one of three categories in terms of Σ values : low angle, coincidence site lattice and random high angle boundaries. Quantitative studies of grain boundary character were done after various processing stages to obtain evidence about structure evolution and indicate an increase in Σ boundary frequency following TMP. Selected area electron diffraction examination (SAD) gave evidence about the refined structure, in which the grain boundary misorientation increased EBSD how the grain boundary character was changed to high Σ values. TEM analyses indicate that the T 2 phase is responsible for substructure stabilization. There is no evidence of cavity formation during superplastic deformation by torsion, which suggests that cavity nucleation is strongly influenced by the nature of stress. (author). 32 refs., 3 tabs., 9 figs

Grain boundary embrittlement and cohesion enhancement in copper

Energy Technology Data Exchange (ETDEWEB)

Paxton, Anthony; Lozovoi, Alexander [Atomistic Simulation Centre, Queen' s University Belfast, BT7 1NN (United Kingdom); Schweinfest, Rainer [Science+Computing ag, Hagellocher Weg 71-5, 720270 T ubingen (Germany); Finnis, Michael [Imperial College London, Exhibition Road, London SW7 2AZ (United Kingdom)

2008-07-01

There has been a long standing debate surrounding the mechanism of grain boundary embrittlement and cohesion enhancement in metals. Embrittlement can lead to catastrophic failure such as happened in the Hinkley Point disaster, or indeed in the case of the Titanic. This kind of embrittlement is caused by segregation of low solubility impurities to grain boundaries. While the accepted wisdom is that this is a phenomenon driven by electronic or chemical factors, using language such as charge transfer and electronegativity difference; we believe that in copper, at least, both cohesion enhancement and reduction are caused by a simple size effect. We have developed a theory that allows us to separate unambiguously, if not uniquely, chemical and structural factors. We have studied a large number of solutes in copper using first principles atomistic simulation to support this argument, and the results of these calculations are presented here.

Branching-induced grain boundary evolution during directional solidification of columnar dendritic grains

International Nuclear Information System (INIS)

Guo, Chunwen; Li, Junjie; Yu, Honglei; Wang, Zhijun; Lin, Xin; Wang, Jincheng

2017-01-01

We present an investigation of secondary and tertiary branching behavior in diverging grain boundaries (GBs) between two columnar dendritic grains with different crystallographic orientations, both by two-dimensional phase-field simulations and thin-sample experiments. The stochasticity of the GB trajectories and the statistically averaged GB orientations were analyzed in detail. The side-branching dynamics and subsequent branch competition behaviors found in the simulations agreed well with the experimental results. When the orientations of two grains are given, the experimental results indicated that the average GB orientation was independent of the pulling velocity in the dendritic growth regime. The simulation and experimental results, as well as the results reported in the literature exhibit a uniform relation between the percentage of the whole gap region occupied by the favorably oriented grain and the difference in the absolute values of the secondary arm growth directions of the two competitive grains. By describing such a uniform relation with a simple fitting equation, we proposed a simple analytical model for the GB orientation at diverging GBs, which gives a more accurate description of GB orientation selection than the existing models.

Superconductivity of individual grains and inter-grain boundaries for polycrystalline FeSr2YCu2O6+y

International Nuclear Information System (INIS)

Yamaguchi, K.; Hata, Y.; Mochiku, T.; Yasuoka, H.

2013-01-01

Polycrystalline FeSr 2 YCu 2 O 6+y was synthesized and its transport and magnetic properties were studied. Diamagnetism was observed below 60 K. Zero resistivity was observed below 38 K under zero magnetic field and below 10 K under 160 kOe. A two-step transition was observed in resistivity measurement due to the superconductivity in individual grains and across inter-grain boundaries. The critical current density in individual grains, J c intra , at 2 K under 1 kOe was deduced 3.4 × 10 5 A/cm 2 from the Bean model. In contrast, the critical current density in inter-grain boundaries, J c inter , at 2 K was 1.7 A/cm 2 in voltage–current measurement. The two-step transition seems to result from the large difference between J c intra and J c inter

Influence of microstructure on grain boundary sliding of alloys 600 and 690

International Nuclear Information System (INIS)

Kergaravat, J.F.; Guetaz, L.; Baillin, X.; Robert, G.

1995-01-01

The influence of deformation and damage mechanisms, and more especially of the grain boundary sliding effect, on the stress corrosion of nickel base alloys used in nuclear industry (exchanger tubes), has been experimentally examined. The grain boundary sliding effect has been measured at 500 C and 320 C on several samples of alloy 690 and 600 (in the mill annealed and mill annealed heat treated conditions). (author). 4 figs., 1 tab

Large modulation of carrier transport by grain-boundary molecular packing and microstructure in organic thin films

KAUST Repository

Rivnay, Jonathan

2009-11-08

Solution-processable organic semiconductors are central to developing viable printed electronics, and performance comparable to that of amorphous silicon has been reported for films grown from soluble semiconductors. However, the seemingly desirable formation of large crystalline domains introduces grain boundaries, resulting in substantial device-to-device performance variations. Indeed, for films where the grain-boundary structure is random, a few unfavourable grain boundaries may dominate device performance. Here we isolate the effects of molecular-level structure at grain boundaries by engineering the microstructure of the high-performance n-type perylenediimide semiconductor PDI8-CN 2 and analyse their consequences for charge transport. A combination of advanced X-ray scattering, first-principles computation and transistor characterization applied to PDI8-CN 2 films reveals that grain-boundary orientation modulates carrier mobility by approximately two orders of magnitude. For PDI8-CN 2 we show that the molecular packing motif (that is, herringbone versus slip-stacked) plays a decisive part in grain-boundary-induced transport anisotropy. The results of this study provide important guidelines for designing device-optimized molecular semiconductors. © 2009 Macmillan Publishers Limited. All rights reserved.

Grain boundary sweeping and dissolution effects on fission product behaviour under severe fuel damage accident conditions

International Nuclear Information System (INIS)

Rest, J.

1986-01-01

The theoretical FASTGRASS-VFP model has been used in the interpretation of fission gas, iodine, tellurium, and cesium release from severe-fuel-damage (SFD) tests performed in the PBF reactor in Idaho. A theory of grain boundary sweeping of gas bubbles, gas bubble behavior during fuel liquefaction (destruction of grain boundaries due to formation of a U-rich melt phase), and during U-Zr eutectic melting has been included within the FASTGRASS-VFP formalism. The grain-boundary-sweeping theory considers the interaction between the moving grain boundary and two distinct size classes of bubbles, those on grain faces and on grain edges. The theory of the effects of fuel liquefaction and U-Zr eutectic melting on fission product behaviour considers the migration and coalescence of fission gas bubbles in either molten uranium, or a Zircaloy-Uranium eutectic melt. Results of the analyses demonstrate that intragranular fission product behavior during the tests can be interpreted in terms of a grain-growth/grain-boundary-sweeping mechanism that enhances the flow of fission products from within the grains to the grain boundaries. Whereas fuel liquefaction leads to an enhanced release of fission products in trace-irradiated fuel, the occurrence of fuel liquefaction in normally-irradiated fuel can degrade fission product release. This phenomenon is due in part to reduced gas-bubble mobilities in a viscous medium as compared to vapor transport, and in part to a degradation of grain growth rates and the subsequent decrease in grain-boundary sweeping of intragranular fission products into the liquified lamina. The analysis shows that total UO 2 dissolution due to eutectic melting leads to increased release for both trace-irradiated and normally-irradiated fuel. The FASTGRASS-VFP predictions, measured release rates from the above tests, and previously published release rates are compared and differences between fission product behavior in trace-irradiated and in normally

The effect of electron scattering from disordered grain boundaries on the resistivity of metallic nanostructures

International Nuclear Information System (INIS)

Arenas, Claudio; Henriquez, Ricardo; Moraga, Luis; Muñoz, Enrique; Munoz, Raul C.

2015-01-01

Highlights: • Quantum theory of the resistivity arising from electron-grain boundary scattering in nanometric metallic structures. • The resistivity is controlled by the collective properties of the grain assembly, by the allowed Kronig-Penney (KP) bands and by the electron transmission probability across successive grains. • When the grain diameter d is larger than the electron mean free path l, the increase in resistivity arises mainly from a decrease of the number of states at the Fermi surface that are allowed KP bands. • When the grain diameter d is smaller than the electron mean free path l, the increase in resistivity arises primarily from Anderson localization caused by electron transmission across successive grains. - Abstract: We calculate the electrical resistivity of a metallic specimen, under the combined effects of electron scattering by impurities, grain boundaries, and rough surfaces limiting the film, using a quantum theory based upon the Kubo formalism. Grain boundaries are represented by a one-dimensional periodic array of Dirac delta functions separated by a distance “d” giving rise to a Kronig–Penney (KP) potential. We use the Green's function built from the wave functions that are solutions of this KP potential; disorder is included by incorporating into the theory the probability that an electron is transmitted through several successive grain boundaries. We apply this new theory to analyze the resistivity of samples S1, S2, S7 and S8 measured between 4 and 300 K reported in Appl. Surf. Science273, 315 (2013). Although both the classical and the quantum theories predict a resistivity that agrees with experimental data to within a few percent or better, the phenomena giving rise to the increase of resistivity over the bulk are remarkably different. Classically, each grain boundary contributes to the electrical resistance by reflecting a certain fraction of the incoming electrons. In the quantum description, there are states

The effect of electron scattering from disordered grain boundaries on the resistivity of metallic nanostructures

Energy Technology Data Exchange (ETDEWEB)

Arenas, Claudio [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile); Synopsys Inc., Avenida Vitacura 5250, Oficina 708, Vitacura, Santiago (Chile); Henriquez, Ricardo [Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Casilla 110-V, Valparaíso (Chile); Moraga, Luis [Universidad Central de Chile, Toesca 1783, Santiago (Chile); Muñoz, Enrique [Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 7820436 (Chile); Munoz, Raul C., E-mail: ramunoz@ing.uchile.cl [Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2008, Casilla 487-3, Santiago 8370449 (Chile)

2015-02-28

Highlights: • Quantum theory of the resistivity arising from electron-grain boundary scattering in nanometric metallic structures. • The resistivity is controlled by the collective properties of the grain assembly, by the allowed Kronig-Penney (KP) bands and by the electron transmission probability across successive grains. • When the grain diameter d is larger than the electron mean free path l, the increase in resistivity arises mainly from a decrease of the number of states at the Fermi surface that are allowed KP bands. • When the grain diameter d is smaller than the electron mean free path l, the increase in resistivity arises primarily from Anderson localization caused by electron transmission across successive grains. - Abstract: We calculate the electrical resistivity of a metallic specimen, under the combined effects of electron scattering by impurities, grain boundaries, and rough surfaces limiting the film, using a quantum theory based upon the Kubo formalism. Grain boundaries are represented by a one-dimensional periodic array of Dirac delta functions separated by a distance “d” giving rise to a Kronig–Penney (KP) potential. We use the Green's function built from the wave functions that are solutions of this KP potential; disorder is included by incorporating into the theory the probability that an electron is transmitted through several successive grain boundaries. We apply this new theory to analyze the resistivity of samples S1, S2, S7 and S8 measured between 4 and 300 K reported in Appl. Surf. Science273, 315 (2013). Although both the classical and the quantum theories predict a resistivity that agrees with experimental data to within a few percent or better, the phenomena giving rise to the increase of resistivity over the bulk are remarkably different. Classically, each grain boundary contributes to the electrical resistance by reflecting a certain fraction of the incoming electrons. In the quantum description, there are states

Substitutional Boron in Nanodiamond, Bucky-Diamond, and Nanocrystalline Diamond Grain Boundaries

Energy Technology Data Exchange (ETDEWEB)

Barnard, Amanda S.; Sternberg, Michael G.

2006-10-05

Although boron has been known for many years to be a successful dopant in bulk diamond, efficient doping of nanocrystalline diamond with boron is still being developed. In general, the location, configuration, and bonding structure of boron in nanodiamond is still unknown, including the fundamental question of whether it is located within grains or grain boundaries of thin films and whether it is within the core or at the surface of nanoparticles. Presented here are density functional tight-binding simulations examining the configuration, potential energy surface, and electronic charge of substitutional boron in various types of nanocrystalline diamond. The results predict that boron is likely to be positioned at the surface of isolated particles and at the grain boundary of thin-film samples.

Bandgap tunability at single-layer molybdenum disulphide grain boundaries

KAUST Repository

Huang, Yu Li

2015-02-17

Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40±0.05 eV for single-layer, 2.10±0.05 eV for bilayer and 1.75±0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85±0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.

Diffusion of the 65Zn radiotracer in ZnO polycrystalline ceramics

Directory of Open Access Journals (Sweden)

Maria Auxiliadora das Neves Nogueira

2003-06-01

Full Text Available Zinc self-diffusion coefficients were measured in polycrystalline ZnO of high density (>99% of the theoretical density and of high purity (> 99.999%. The diffusion experiments were performed from 1006 to 1377 °C, in oxygen atmosphere, for times between 16 and 574 h. The diffusion profiles were established by means of Residual Activity Method using the 65Zn radioactive isotope as zinc tracer. In our experimental conditions, the zinc volume diffusion coefficients can be described by the following Arrhenius relationship: D(cm²/s = 1.57×10-3 exp[(-2.66 ± 0.26 eV/kT]. In the same experimental conditions, the grain-boundary diffusion coefficients are approximately 4 orders of magnitude greater than the volume diffusion coefficients, and can be described by the Arrhenius relation: D'delta (cm³/s = 1.59×10-6 exp[(-2.44 ± 0.45 eV/kT], where D' is the grain-boundary diffusion coefficient and delta is the grain boundary width.

Effect of texture on grain boundary misorientation distributions in polycrystalline high temperature superconductors

International Nuclear Information System (INIS)

Goyal, A.; Specht, E.D.; Kroeger, D.M.; Mason, T.A.

1996-01-01

Computer simulations were performed to determine the most probable grain boundary misorientation distribution (GBMD) in model polycrystalline superconductors. GBMDs in polycrystalline superconductors can be expected to dictate the macroscopic transport critical current density, J c . Calculations were performed by simulating model polycrystals and then determining the GBMD. Such distributions were calculated for random materials having cubic, tetragonal, and orthorhombic crystal symmetry. In addition, since most high temperature superconductors are tetragonal or pseudotetragonal, the effect of macroscopic uniaxial and biaxial grain orientation texture on the GBMD was determined for tetragonal materials. It is found that macroscopic texture drastically alters the grain boundary misorientation distribution. The fraction of low angle boundaries increases significantly with uniaxial and biaxial texture. The results of this study are important in correlating the macroscopic transport J c with the measured grain orientation texture as determined by x-ray diffraction copyright 1996 American Institute of Physics

Effect of Ion Streaming on Diffusion of Dust Grains in Dissipative System

Science.gov (United States)

Begum, M.; Das, N.

2018-01-01

The presence of strong electric fields in the sheath region of laboratory complex plasma induces an ion drift and perturbs the field around dust grains. The downstream focusing of ions leads to the formation of oscillatory kind of attractive wake potential which superimpose with the normal Debye-Hückel (DH) potential. The structural properties of complex plasma and diffusion coefficient of dust grains in the presence of such a wake potential have been investigated using Langevin dynamics simulation in the subsonic regime of ion flow. The study reveals that the diffusion of dust grains is strongly affected by the ion flow, so that the diffusion changes its character in the wake potential to the DH potential dominant regimes. The dependence of the diffusion coefficient on the parameters, such as the neutral pressure, dust grain size, ion flow velocity, and Coulomb coupling parameter, have been calculated for the subsonic regime by using the Green-Kubo expression, which is based on the integrated velocity autocorrelation function. It is found that the diffusion and the structural property of the system is intimately connected with the interaction potential and significantly get affected in the presence of ion flow in the subsonic regime.

First-principles study of the effects of segregated Ga on an Al grain boundary

International Nuclear Information System (INIS)

Zhang Ying; Lu Guanghong; Wang Tianmin; Deng Shenghua; Shu Xiaolin; Kohyama, Masanori; Yamamoto, Ryoichi

2006-01-01

The effects of different amounts of segregated Ga (substitutional) on an Al grain boundary have been investigated by using a first-principles pseudopotential method. The segregated Ga is found to draw charge from the surrounding Al due to the electronegativity difference between Ga and Al, leading to a charge density reduction between Ga and Al as well as along the Al grain boundary. Such an effect can be enhanced by increasing the Ga segregation amount. With further Ga segregated, in addition to the charge-drawing effect that occurs in the Al-Ga interface, a heterogeneous α-Ga-like phase can form in the grain boundary, which greatly alters the boundary structure. These effects are suggested to be responsible for Ga-induced Al intergranular embrittlement

Physics of grain boundaries in polycrystalline photovoltaic semiconductors

Energy Technology Data Exchange (ETDEWEB)

Yan, Yanfa, E-mail: yanfa.yan@utoledo.edu; Yin, Wan-Jian; Wu, Yelong; Shi, Tingting; Paudel, Naba R. [Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Ohio 43606 (United States); Li, Chen [Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States); Poplawsky, Jonathan [The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States); Wang, Zhiwei [Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Ohio 43606 (United States); National Renewable Energy Laboratory, Golden, Colorado 80401 (United States); Moseley, John; Guthrey, Harvey; Moutinho, Helio; Al-Jassim, Mowafak M. [National Renewable Energy Laboratory, Golden, Colorado 80401 (United States); Pennycook, Stephen J. [Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996 (United States)

2015-03-21

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se{sub 2} (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.

Development of micro tensile testing method in an FIB system for evaluating grain boundary strength

International Nuclear Information System (INIS)

Fujii, Katsuhiko; Fukuya, Koji

2010-01-01

A micro tensile testing method for evaluating grain boundary strength was developed. Specimens of 2 x 2 x 10μm having one grain boundary were made by focused ion beam (FIB) micro-processing and tensioned in an FIB system in situ. The load was measured from the deflection of the silicon cantilever. The method was applied to aged and unaged Fe-Mn-P alloy specimens with different level of grain boundary phosphorus segregation. The load at intergranular fracture decreased with increasing phosphorus segregation. (author)

Microstructure and composition of electromagnetically-characterized YBa2Cu3O7-δ grain boundaries

International Nuclear Information System (INIS)

Babcock, S.E.; Zhang, Na; Cai, Xue Yu; Larbalestier, D.C.; Gao, Yufei; Merkle, K.L.; Kaiser, D.L.

1991-01-01

The electrical character (flux-pinning, Josephson junction, or resistive) of the grain boundaries in approximately twenty flux-grown YBa 2 Cu 3 O 7-δ bicrystals was determined in previous studies. A selection of these same bicrystals now have been thinned for study by transmission and scanning transmission electron microscopy. High-spatial resolution imaging and analytical techniques reveal microstructural differences among these boundaries that are consistent with their diverse electrical characteristics. The observations offer preliminary insight into some of the feature that control the grain boundary superconducting properties and re-emphasize the very fine scale on which the grain boundary electrical character is determined. 11 refs., 6 figs

Enthalpy - entropy compensation effect in grain boundary phenomena

Czech Academy of Sciences Publication Activity Database

Lejček, Pavel

2005-01-01

Roč. 96, č. 10 (2005), s. 1129-1133 ISSN 0044-3093 R&D Projects: GA MPO(CZ) FF-P2/053 Institutional research plan: CEZ:AV0Z10100520 Keywords : compensation effect * enthalpy * entropy * thermodynamics * grain boundary Subject RIV: BJ - Thermodynamics Impact factor: 0.842, year: 2005

Determination of uranium self-diffusion coefficients in the U O2 nuclear fuel by secondary ion mass spectrometry

International Nuclear Information System (INIS)

Ferraz, Wilmar Barbosa

1998-01-01

This study of uranium self-diffusion in UO 2 presents a great technological interest because its knowledge is necessary to interpret the mechanism of many important processes like, for example, sintering, creep, grain growth, in-reactor densification and others. The present work deals with new measurements of uranium diffusion in UO 2 single crystals and polycrystals through an original mythology based on the utilization of 235 U as tracer and depth profiling by secondary ions mass spectrometry (SIMS). The diffusion experiments were performed between 1498 and 1697 deg C, in H 2 atmosphere. In our experimental conditions, the uranium volume diffusion coefficients measured in UO 2 single crystals can be described by the following Arrhenius relation: D(cm 2 /s) = 8.54x10 -7 exp[-4.4(eV)/K T]. The uranium grain-boundary diffusion experiments performed in UO 2 polycrystals corresponded to the type-B diffusion. In this case, it was possible to determine the product D'δ, where D is the grain-boundary diffusion and is the width of the grain-boundary. In our experimental conditions, the product D'δ can be described by the following relation: D'δ (cm 3 /s) = 1.62x10 -5 exp[-5.6(eV)/K T]. These results that the uranium volume diffusion coefficients, measured in UO 2 single crystals, are 5 orders of magnitude lower than the uranium grain boundary diffusion coefficients measured in UO 2 polycrystalline pellets, in the same experimental conditions. This large difference between these two types of diffusivities indicates that the grain boundary is a preferential via for uranium diffusion in UO 2 polycrystalline pellet. (author)

Transport properties through graphene grain boundaries: strain effects versus lattice symmetry

Science.gov (United States)

Hung Nguyen, V.; Hoang, Trinh X.; Dollfus, P.; Charlier, J.-C.

2016-06-01

As most materials available at the macroscopic scale, graphene samples usually appear in a polycrystalline form and thus contain grain boundaries. In the present work, the effect of uniaxial strain on the electronic transport properties through graphene grain boundaries is investigated using atomistic simulations. A systematic picture of transport properties with respect to the strain and lattice symmetry of graphene domains on both sides of the boundary is provided. In particular, it is shown that strain engineering can be used to open a finite transport gap in all graphene systems where the two domains are arranged in different orientations. This gap value is found to depend on the strain magnitude, on the strain direction and on the lattice symmetry of graphene domains. By choosing appropriately the strain direction, a large transport gap of a few hundred meV can be achieved when applying a small strain of only a few percents. For a specific class of graphene grain boundary systems, strain engineering can also be used to reduce the scattering on defects and thus to significantly enhance the conductance. With a large strain-induced gap, these graphene heterostructures are proposed to be promising candidates for highly sensitive strain sensors, flexible electronic devices and p-n junctions with non-linear I-V characteristics.

The grain boundary segregation of phosphorus in thermally aged and irradiated C-Mn submerged-are weld metal

International Nuclear Information System (INIS)

Mendes, C.M.

1999-01-01

The segregation of free phosphorus atoms to grain boundaries in C-Mn steels has been identified as an embrittlement mechanism. A change in the brittle fracture mechanism from transgranular to intergranular has been observed for materials with higher phosphorus grain boundary coverage. The grain boundary segregation of phosphorus in various steels used in the nuclear power industry has been thermodynamically and kinetically modelled mostly with the Langmuir-McLean model. Recent publications have also suggested that neutron irradiation can affect segregation and various attempts at modelling this are currently under way. The present paper describes a data base assembled on phosphorus grain boundary coverage measured by Auger electron spectroscopy on thermally aged and irradiated C-Mn submerged-arc weld specimens. Software tools were developed to evaluate the changes in phosphorus grain boundary coverage associated with instantaneous temperature changes and temperature gradients. The phosphorus free energy change associated with grain boundary segregation was modelled from the thermally aged data and used with the software to determine the phosphorus segregation in submerged-arc weld metals following the post weld stress relief heat treatments received prior to plant operation. The phosphorus grain boundary coverage changes arising from the thermal history of submerged-arc weld materials during irradiation were also modelled and found to compare well with data obtained on irradiated materials. It was concluded that under the irradiation conditions sampled, phosphorus grain boundary segregation in submerged-arc weld materials can be modelled successfully using only the thermal term without appealing to an irradiation induced segregation process. (author)