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Showing papers on "Grain boundary published in 2007"


Journal ArticleDOI
TL;DR: A brief overview of the recent progress made in improving mechanical properties of nanocrystalline materials, and in quantitatively and mechanistically understanding the underlying mechanisms is presented in this paper.

994 citations


Journal ArticleDOI
TL;DR: In this article, the evolution of the microstructure from as-cast to cast-T4 to castT6 involves solid solution+eutectic compound+supersaturated solid solution + cuboid-shaped compound.

610 citations


Journal ArticleDOI
TL;DR: In this article, the authors reported an explanation for the inverse Hall-Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is dependent on strain rate and temperature and deviates from the Hall-petch relationship below a critical grain size.

459 citations


Journal ArticleDOI
TL;DR: In this paper, the microstructures of age hardened Mg-2.0Gd-1.2Y-xZn-0.2Zr (x = 0, 0.3, and 1.0) (at.%) alloys were investigated to understand the remarkable age-hardening and unusual plastic elongation behavior.

427 citations


Journal ArticleDOI
TL;DR: In this article, the Gibbs adsorption isotherm and Wagner's definition of excess solute at surfaces and grain boundaries are both extended to include other crystalline defects, like dislocations and vacancies.

403 citations


Journal ArticleDOI
TL;DR: Transmission electron microscopy and atomistic simulations demonstrate that shear banding instability no longer afflicts the 5- to 10-nm-thick nanolaminate glassy layers during tensile deformation, which also act as high-capacity sinks for dislocations, enabling absorption of free volume and free energy transported by the dislocation.
Abstract: It is known that the room-temperature plastic deformation of bulk metallic glasses is compromised by strain softening and shear localization, resulting in near-zero tensile ductility. The incorporation of metallic glasses into engineering materials, therefore, is often accompanied by complete brittleness or an apparent loss of useful tensile ductility. Here we report the observation of an exceptional tensile ductility in crystalline copper/copper–zirconium glass nanolaminates. These nanocrystalline–amorphous nanolaminates exhibit a high flow stress of 1.09 ± 0.02 GPa, a nearly elastic-perfectly plastic behavior without necking, and a tensile elongation to failure of 13.8 ± 1.7%, which is six to eight times higher than that typically observed in conventional crystalline–crystalline nanolaminates (<2%) and most other nanocrystalline materials. Transmission electron microscopy and atomistic simulations demonstrate that shear banding instability no longer afflicts the 5- to 10-nm-thick nanolaminate glassy layers during tensile deformation, which also act as high-capacity sinks for dislocations, enabling absorption of free volume and free energy transported by the dislocations; the amorphous–crystal interfaces exhibit unique inelastic shear (slip) transfer characteristics, fundamentally different from those of grain boundaries. Nanoscale metallic glass layers therefore may offer great benefits in engineering the plasticity of crystalline materials and opening new avenues for improving their strength and ductility.

402 citations


Journal ArticleDOI
TL;DR: In this article, the evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated, and it was shown that the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar.

401 citations


Journal ArticleDOI
TL;DR: In this paper, a crystal plasticity model for hcp materials is presented which is based on dislocation glide and pinning, and it is shown that the primary effect of elastic anisotropy during subsequent plastic flow is to increase local, grain-level, accumulated slip.

400 citations


Book
06 Jul 2007
TL;DR: Sintering of Ceramics: Fundamentals The Sintering process driving force for sintering Defects in Crystalline Solids Diffusion in Ionic Crystals: Ambipolar Diffusion Solid-State and Viscous Sinterings Mechanisms of SinterING Effects of Grain Boundaries Theoretical Analysis of SIntering Herring's Scaling Law Analytical Models Numerical Simulation of SINTERing Phenomenological Sinter-ing Equations Sintered Diagrams Sinterouring Diagram as discussed by the authors.
Abstract: Sintering of Ceramics: Fundamentals The Sintering Process Driving Force for Sintering Defects in Crystalline Solids Diffusion in Crystalline Solids The Chemical Potential Diffusional Flux Equations Diffusion in Ionic Crystals: Ambipolar Diffusion Solid-State and Viscous Sintering Mechanisms of Sintering Effects of Grain Boundaries Theoretical Analysis of Sintering Herring's Scaling Law Analytical Models Numerical Simulation of Sintering Phenomenological Sintering Equations Sintering Diagrams Sintering with an Externally Applied Pressure Stress Intensification Factor and Sintering Stress Alternative Derivation of the Sintering Equations Grain Growth and Microstructure Control General Features of Grain Growth Ostwald Ripening Topological and Interfacial Tension Requirements Normal Grain Growth in Dense Solids Abnormal Grain Growth in Dense Solids Grain Growth in Thin Films Mechanisms Controlling the Boundary Mobility Grain Growth and Pore Evolution in Porous Solids Simultaneous Densification and Grain Growth Fabrication Principles for Ceramics with Controlled Microstructure Liquid-Phase Sintering Elementary Features of Liquid-Phase Sintering Stages of Liquid-Phase Sintering Grain Boundary Films The Basic Mechanisms of Liquid-Phase Sintering Numerical Modeling of Liquid-Phase Sintering Hot Pressing with a Liquid Phase Use of Phase Diagrams in Liquid-Phase Sintering Activated Sintering Vitrification Special Topics in Sintering Inhomogeneities and their Effects on Sintering Constrained Sintering I: Rigid Inclusions Constrained Sintering II: Adherent Thin Films Constrained Sintering III: Multilayers Constitutive Models for Porous Sintering Materials Morphological Stability of Continuous Phases Solid Solution Additives and the Sintering of Ceramics Sintering with Chemical Reaction: Reaction Sintering Viscous Sintering with Crystallization Sintering Process Variables and Sintering Practice Sintering Measurement Techniques Conventional Sintering Microwave Sintering Pressure-Assisted Sintering Appendix A Physical Constants Appendix B SI Units - Names and Symbols Appendix C Conversion of Units Appendix D Ionic Crystal Radii (in units of 10-10m) Appendix E Density and Melting Point of Some Elements and Ceramics

400 citations


Journal ArticleDOI
TL;DR: In this paper, the transport properties of yttrium-doped barium zirconate (BYZ) have been explored, with the aim of attaining reproducible proton conductivity in well-densified samples.
Abstract: The factors governing the transport properties of yttrium-doped barium zirconate (BYZ) have been explored, with the aim of attaining reproducible proton conductivity in well-densified samples. It was found that a small initial particle size (50–100 nm) and high-temperature sintering (1600 °C) in the presence of excess barium were essential. By this procedure, BaZr0.8Y0.2O3−δ with 93% to 99% theoretical density and total (bulk plus grain boundary) conductivity of 7.9 × 10−3 S/cm at 600 °C [as measured by alternating current (ac) impedance spectroscopy under humidified nitrogen] could be reliably prepared. Samples sintered in the absence of excess barium displayed yttria-like precipitates and a bulk conductivity that was reduced by more than 2 orders of magnitude.

353 citations


Journal ArticleDOI
TL;DR: In this article, the extended version of the Gibbs adsorption isotherm including dislocations and vacancies is used to analyse existing experimental data, and the analysis of experimental results addresses the question whether zero or negative defect energies are feasible and how this will affect materials behaviour.

Journal ArticleDOI
TL;DR: In this paper, the texture and grain boundary structure of recrystallized materials are dependent upon the character of the deformed matrix and the selective nucleation and growth of crystallites from the deformation structure.

Journal ArticleDOI
TL;DR: In this article, the grain boundary segregation of Ni-W alloys has been investigated and it has been shown that the W content controls the grain size over a very broad range: ∼2-140nm.

Journal ArticleDOI
TL;DR: In this paper, a review of the superconducting properties of MgB2 that are relevant for power applications is presented, focusing on the reversible mixed state parameters, which define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current.
Abstract: This review focuses on the superconducting properties of MgB2 that are relevant for power applications. The reversible mixed state parameters are the most important, since they define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current. They also determine the flux pinning energy, the pinning force and the elastic properties of the flux line lattice and, therefore, strongly influence the critical current densities. The magnetic properties of magnesium diboride are anisotropic and influenced by the two different energy gaps of the σ- and π-bands. Whereas the transition temperature could not be enhanced significantly during the past five years, the upper critical field was considerably increased by impurity scattering or doping. Flux pinning is very weak in MgB2 single crystals and was only improved by irradiation techniques so far. In polycrystalline samples, grain boundary pinning seems to play the dominant role. High critical currents close to the theoretical limit were found in c-axis oriented thin films. The anisotropy of the upper critical field strongly reduces the critical currents in untextured MgB2 at high magnetic fields, where the supercurrents become highly percolative, since not all grains are superconducting anymore. The performance of polycrystalline wires and tapes was significantly improved during the past few years by increasing the upper critical field and by reducing its anisotropy. Pinning seems to be nearly optimized in many forms of this material, but the connectivity between the grains might be further improved.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the influence of the grain size on the energy barrier of the martensitic transformation in nanocrystalline NiTi by transmission electron microscopy (TEM).
Abstract: The analysis of nanocrystalline NiTi by transmission electron microscopy (TEM) shows that the martensitic transformation proceeds by the formation of atomic-scale twins. Grains of a size less than about 50 nm do not transform to martensite even upon large undercooling. A systematic investigation of these phenomena was carried out elucidating the influence of the grain size on the energy barrier of the transformation. Based on the experiment, nanograins were modeled as spherical inclusions containing (0 0 1) compound twinned martensite. Decomposition of the transformation strains of the inclusions into a shear eigenstrain and a normal eigenstrain facilitates the analytical calculation of shear and normal strain energies in dependence of grain size, twin layer width and elastic properties. Stresses were computed analytically for special cases, otherwise numerically. The shear stresses that alternate from twin layer to twin layer are concentrated at the grain boundaries causing a contribution to the strain energy scaling with the surface area of the inclusion, whereas the strain energy induced by the normal components of the transformation strain and the temperature dependent chemical free energy scale with the volume of the inclusion. In the nanograins these different energy contributions were calculated which allow to predict a critical grain size below which the martensitic transformation becomes unlikely. Finally, the experimental result of the atomic-scale twinning can be explained by analytical calculations that account for the transformation-opposing contributions of the shear strain and the twin boundary energy of the twin-banded morphology of martensitic nanograins.

Journal ArticleDOI
TL;DR: In this article, a proton-conducting BaZr 0.9 was successfully sintered at 1325°C with a relative density of 96% via addition of 1.5% ZnO. The formula of the added sample is Ba 0.77 Y 0.19 Zn 0.04 O 3− δ which exhibits a tetragonal structure with space group P 4/mbm (127); a =5.9787(1)

Journal ArticleDOI
TL;DR: In this paper, the deformation behavior of nanocrystalline Ni-W alloys is evaluated by nanoindentation techniques for grain sizes of 3-150nm, spanning both the range of classical Hall-Petch behavior as well as the regime where deviations from the Hall-petch trend are observed.

Journal ArticleDOI
26 Apr 2007-Nature
TL;DR: The present results may lead to the development of predictive models for capillarity-driven microstructure evolution in a wide range of industrial and commercial processing scenarios—such as the heat treatment of metals, or even controlling the ‘head’ on a pint of beer.
Abstract: Cellular structures or tessellations are ubiquitous in nature. Metals and ceramics commonly consist of space-filling arrays of single-crystal grains separated by a network of grain boundaries, and foams (froths) are networks of gas-filled bubbles separated by liquid walls. Cellular structures also occur in biological tissue, and in magnetic, ferroelectric and complex fluid contexts. In many situations, the cell/grain/bubble walls move under the influence of their surface tension (capillarity), with a velocity proportional to their mean curvature. As a result, the cells evolve and the structure coarsens. Over 50 years ago, von Neumann derived an exact formula for the growth rate of a cell in a two-dimensional cellular structure (using the relation between wall velocity and mean curvature, the fact that three domain walls meet at 120 degrees and basic topology). This forms the basis of modern grain growth theory. Here we present an exact and much-sought extension of this result into three (and higher) dimensions. The present results may lead to the development of predictive models for capillarity-driven microstructure evolution in a wide range of industrial and commercial processing scenarios--such as the heat treatment of metals, or even controlling the 'head' on a pint of beer.

Journal ArticleDOI
TL;DR: The polycrystalline sample of NaBa2V5O15 (NBV), a member of tungsten bronze family, is prepared by a mixed oxide-technique and X-ray diffraction analysis shows the formation of single phase compound with an orthorhombic structure at room temperature.

Journal ArticleDOI
TL;DR: In this paper, a comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented, where optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements.
Abstract: A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3×1019to2×1020cm−3.

Journal ArticleDOI
TL;DR: In this paper, the energy of asymmetric tilt grain boundaries in Cu and Al was investigated using atomistic simulations, and the authors found that asymmetric boundaries with low index normals do not necessarily have lower energies than boundaries with similar inclination angles.
Abstract: Atomistic simulations were employed to investigate the structure and energy of asymmetric tilt grain boundaries in Cu and Al. In this work, we examine the Σ5 and Σ13 systems with a boundary plane rotated about the ⟨ 100 ⟩ misorientation axis, and the Σ9 and Σ11 systems rotated about the ⟨ 110 ⟩ misorientation axis. Asymmetric tilt grain boundary energies are calculated as a function of inclination angle and compared with an energy relationship based on faceting into the two symmetric tilt grain boundaries in each system. We find that asymmetric tilt boundaries with low index normals do not necessarily have lower energies than boundaries with similar inclination angles, contrary to previous studies. Further analysis of grain boundary structures provides insight into the asymmetric tilt grain boundary energy. The Σ5 and Σ13 systems in the ⟨ 100 ⟩ system agree with the aforementioned energy relationship; structures confirm that these asymmetric boundaries facet into the symmetric tilt boundaries. The Σ9 and ...

Journal ArticleDOI
TL;DR: In this paper, a nonlinear conjugate gradient algorithm was employed along with an embedded atom method potential for Cu and Al to generate the equilibrium 0'K grain boundary structures.
Abstract: The objective of this research is to use atomistic simulations to investigate the energy and structure of symmetric and asymmetric Σ3 ⟨110⟩ tilt grain boundaries. A nonlinear conjugate gradient algorithm was employed along with an embedded atom method potential for Cu and Al to generate the equilibrium 0 K grain boundary structures. A total of 25 ⟨110⟩ grain boundary structures were explored to identify the various equilibrium and metastable structures. Simulation results show that the Σ3 asymmetric tilt grain boundaries in the ⟨110⟩ system are composed of only structural units of the two Σ3 symmetric tilt grain boundaries. The energies for the Σ3 grain boundaries are similar to previous experimental and calculated grain boundary energies. A structural unit and faceting model for Σ3 asymmetric tilt grain boundaries fits all of the calculated asymmetric grain boundary structures. The significance of these results is that the structural unit and facet description of all Σ3 asymmetric tilt grain boundaries m...

Journal ArticleDOI
TL;DR: Atom probe tomography has played a key role in the understanding of the embrittlement of neutron irradiated reactor pressure vessel steels through the atomic level characterization of the microstructure as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this paper, the authors have developed hydrogen-doped In2O3 films on glass with high mobility and high near-infrared transparency by using sputtering process performed at room temperature, followed by post-annealing treatment at 200 °C.
Abstract: We have developed hydrogen (H)-doped In2O3 films on glass with high mobility and high near-infrared transparency by using sputtering process performed at room temperature, followed by post-annealing treatment at 200 °C. To incorporate H-donor into In2O3 matrix, H2O vapor has been introduced into a chamber during the deposition. In the post-annealing of the films, phase transition from amorphous to polycrytalline was confirmed to occur. The resulting In2O3 films containing 1.9–6.3 at. % H show quite large mobility as high as 98–130 cm2/(V s) at carrier density of (1.4–1.8)×1020 cm-3. We attributed the high mobility in the film to suppression of grain boundary defects as well as multicharged and neutral impurities.

Journal ArticleDOI
TL;DR: In this paper, atomistic computer simulations are used to investigate the equilibrium solute distribution and alloying energy in nanocrystalline Ni-W alloys, and the authors conclude that these alloying are synthesized in a metastable state.

Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the results on stress in polycrystalline thin films on substrates and showed that the tensile and compressive stresses in these films are independent and additive.

Journal ArticleDOI
TL;DR: In this paper, a self-consistent scheme making use of the translated fields technique for elastic-viscoplastic materials is used as micro-macro scale transition, where the representative volume element is composed of grains supposed to be spherical and randomly distributed with a grain size distribution following a log-normal statistical function.

Journal ArticleDOI
TL;DR: In this paper, a physically based grain size dependent strain hardening model has been developed for the ferrite matrix, involving specific laws for the accumulation and saturation of dislocations along grain boundaries and for their net back stress contribution.

Journal ArticleDOI
TL;DR: In this paper, the authors present atomistic simulations that illuminate the stabilizing effect of interfacially segregated, oversized dopants in face-centered cubic cubic (fcc) copper and determine the critical dopant concentration required to eliminate grain growth in bulk and thin film structures.

Journal ArticleDOI
TL;DR: In this paper, low cycle fatigue tests were carried out on a two-dimensional polycrystalline nickel-base alloy, where grain morphology and orientation were determined using electron back scatter diffraction (EBSD), and polycrystal plasticity analyses carried out for the characterised microstructure with identical conditions to the experiment tests.