Showing papers on "Grain boundary published in 2003"

Textures of liquid crystals

Abstract: Preface.Foreword. 1. Introduction.2. Surface Anchoring and Elasticity.3. Polarizing Microscopy.4. The Blue Phases.5. The Nematic and Cholesteric Phases.6. Twist Grain Boundary Phases.7. The Fluid Smectic Phases.8. The SmC Subphases.9. The Hexatic Phases.10. Soft Crystal Phases and Crystallization.11. Other Liquid Crystal Phases.Appendix A.Appendix B.Color Plates.Index.

Grain Boundary Scars and Spherical Crystallography

Abstract: We describe experimental investigations of the structure of two-dimensional spherical crystals. The crystals, formed by beads self-assembled on water droplets in oil, serve as model systems for exploring very general theories about the minimum-energy configurations of particles with arbitrary repulsive interactions on curved surfaces. Above a critical system size we find that crystals develop distinctive high-angle grain boundaries, or scars, not found in planar crystals. The number of excess defects in a scar is shown to grow linearly with the dimensionless system size. The observed slope is expected to be universal, independent of the microscopic potential.

Dynamic continuous recrystallization characteristics in two stage deformation of Mg-3Al-1Zn alloy sheet

Abstract: Dynamic recrystallization (DRX) characteristics of a Mg/3Al/1Zn (AZ31) alloy sheet were investigated at temperatures ranging from 200� /450 8C and constant strain rates of 1/10 4 � /2/10 4 s 1 . The average grain size of the as-received alloy was 12 mm and can be refined to 6 mm via deformation at 250 8C, 1/10 4 s 1 to a strain level of 60%. Grain refinement was less effectiv ea t higher temperatures due to rapid grain growth. The grain refinement was attributed to dynamic continuous recrystallization which involves progressive increase in grain boundary misorientation and conversion of low angle boundaries into high angle boundaries. During DRX, subgrains were developed through the conversion of dislocation cell walls into subgrain boundaries. The presence of precipitates was not essential for dynamic recrystallization in the magnesium alloy being investigated because of its limited slip systems, low stacking fault energy and high grain boundary diffusion rate. # 2003 Elsevier Science B.V. All rights reserved.

Deformation mechanism in nanocrystalline Al: Partial dislocation slip

Abstract: We report experimental observation of a deformation mechanism in nanocrystalline face-centered-cubic Al, partial dislocation emission from grain boundaries, which consequently resulted in deformation stacking faults (SFs) and twinning. These results are surprising because (1) partial dislocation emission from grain boundaries has not been experimentally observed although it has been predicted by simulations and (2) deformation stacking faults and twinning have not been reported in Al due to its high SF energy.

Modeling and simulation of polycrystalline ZnO thin-film transistors

Abstract: Thin-film transistors (TFTs) made of transparent channel semiconductors such as ZnO are of great technological importance because their insensitivity to visible light makes device structures simple. In fact, there have been several demonstrations of ZnO TFTs achieving reasonably good field effect mobilities of 1–10 cm2/V s, but the overall performance of ZnO TFTs has not been satisfactory, probably due to the presence of dense grain boundaries. We modeled grain boundaries in ZnO TFTs and performed simulation of a ZnO TFT by using a two-dimensional device simulator in order to determine the grain boundary effects on device performance. Polycrystalline ZnO TFT modeling was started by considering a single grain boundary in the middle of the TFT channel, formulated with a Gaussian defect distribution localized in the grain boundary. A double Schottky barrier was formed in the grain boundary, and its barrier height was analyzed as a function of defect density and gate bias. The simulation was extended to TFTs with many grain boundaries to quantitatively analyze the potential profiles that developed along the channel. One of the main differences between a polycrystalline ZnO TFT and a polycrystalline Si TFT is that the much smaller nanoscaled grains in a polycrystalline ZnO TFT induces a strong overlap of the double Schottky barriers with a higher activation energy in the crystallite and a lower barrier potential in the grain boundary at subthreshold or off-state region of its transfer characteristics. Through the simulation, we were able to estimate the density of total trap states localized in the grain boundaries for polycrystalline ZnO TFT by determining the apparent mobility and grain size in the device.

Anomalous Grain Boundary Physics in Polycrystalline CuInSe2: The Existence of a Hole Barrier

Abstract: First-principles modeling of grain boundaries (GB) in CuInSe2 semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB Consequently, the absence of holes inside the GB prevents GB electrons from recombining At the same time, the GI is purer in polymaterials than in single crystals, since impurities segregated to the GBs This explains the puzzle of the superiority of polycrystalline CuInSe2 solar cells over their crystalline counterpart We identify a simple and universal mechanism for the barrier, arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction This discovery opens up possibilities for the future design of superior polycrystalline devices

Grain size dependence of the plastic deformation kinetics in Cu

Abstract: Data on the effect of grain size d in the range of nm to mm on the plastic deformation kinetics of Cu at 77–373 K are analyzed to determine the influence of grain size on the strain rate-controlling mechanism. Three grain size regimes were identified: Regimes I (d≈10−6–10−3 m), II (d≈10−8–10−6 m) and III (d<∼10−8 m). A dislocation cell structure characterizes Regime II, which no longer occurs in Regime II. The absence of all intragranular dislocation activity characterizes Regime III. The following mechanisms were concluded to be rate-controlling for : (a) Regime I, intersection of dislocations; (b) Regime I, grain boundary shear promoted by dislocation pile-ups; and (c) Regime III, grain boundary shear. The major effect of grain size on the intersection mechanism in Regime I is on the mobile and forest dislocation densities; the effect in Regime II is on the number of dislocations and on the number of grain boundary atom sites; the effect in Regime III is on the number of grain boundary atom sites. The transition grain size from one regime to another depends on the strain rate and temperature. Crystallographic texture is also important.

Blocking Grain Boundaries in Yttria-Doped and Undoped Ceria Ceramics of High Purity

Abstract: CeO 2 samples doped with 10, 1.0, and 0.1 mol% Y 2 O 3 and undoped CeO 2 samples of high purity were studied by impedance spectroscopy at temperatures <800°C and under various oxygen partial pressures. According to microstructural investigations by SEM and analytical STEM (equipped with EDXS), the grain boundaries were free of any second phase, providing direct grain-to-grain contacts. An amorphous siliceous phase was detected at only a few triple junctions, if at all; as a result, its contribution to the grain-boundary resistance was negligible. Nevertheless, the specific grain-boundary conductivities were still 2-7 orders of magnitude lower than the bulk conductivities, depending on dopant concentration, temperature, and oxygen partial pressure. The charge carrier transport across the grain boundaries occurred only through the grain-to-grain contacts, whose properties were then determined by the space-charge layer. The space-charge potential in acceptor-doped CeO 2 was positive, causing the simultaneous depletion of oxygen vacancies and accumulation of electrons in the space-charge layer. The very low grain-boundary conductivities can be accounted for by the oxygen-vacancy depletion; the accumulation of electrons became evident in weakly doped and undoped CeO 2 at high temperatures and under low oxygen partial pressures.

Grain-Boundary Sliding in AZ31 Magnesium Alloys at Room Temperature to 523 K

Abstract: Rolled sheets of AZ31 Mg alloys were subjected to tensile testing at temperatures ranging from room temperature to 523K The occurrence of grain-boundary sliding (GBS) at room temperature was demonstrated by the displacement of scribed lines across grain boundaries of deformed samples Surface relief of deformed samples was measured by use of a scanning laser microscope GBS strain was calculated from the measured surface step height, and its temperature dependence was analyzed by a Dorn-type constitutive equation GBS above 423K was found to be pure GBS that was activated by resolved applied shear stress acting on grain boundaries The activation energy for GBS was found to be 80 kJ/mol, which is in agreement with the activation energy for grain boundary diffusion Meanwhile, GBS below 373K was found to be slipinduced GBS, and its extent was found to be significantly greater than that expected from extrapolation of high-temperature values The slipinduced GBS is considered to occur by plastic compatibility conditions in the presence of plastic strain anisotropy and by absorption and dissociation of lattice dislocations at grain boundaries

Atomistic simulations of elastic deformation and dislocation nucleation during nanoindentation

Abstract: Nanoindentation experiments have shown that microstructural inhomogeneities across the surface of gold thin films lead to position-dependent nanoindentation behavior [Phys. Rev. B (2002), to be submitted]. The rationale for such behavior was based on the availability of dislocation sources at the grain boundary for initiating plasticity. In order to verify or refute this theory, a computational approach has been pursued. Here, a simulation study of the initial stages of indentation using the embedded atom method (EAM) is presented. First, the principles of the EAM are given, and a comparison is made between atomistic simulations and continuum models for elastic deformation. Then, the mechanism of dislocation nucleation in single crystalline gold is analyzed, and the effects of elastic anisotropy are considered. Finally, a systematic study of the indentation response in the proximity of a high angle, high sigma (low symmetry) grain boundary is presented; indentation behavior is simulated for varying indenter positions relative to the boundary. The results indicate that high angle grain boundaries are a ready source of dislocations in indentation-induced deformation.

On the relationship between microstructure, strength and toughness in AA7050 aluminum alloy

Abstract: The effect of process parameters such as quench rate and precipitation heat treatment on the compromise between the toughness and the yield strength of AA7050 aluminum alloy (AlZnMgCu) are investigated, as well as the anisotropy of this compromise in the rolling plane. Fracture toughness is experimentally approached by the Kahn tear test. The microstructure is studied quantitatively in detail by a combination of scanning electron microscopy, transmission electron microscopy and small-angle X-ray scattering, and the relative fractions of the various fracture modes as a function of microstructural state are quantitatively determined on scanning electron microscopy images. Toughness is confirmed to be minimum at peak strength, and lower for an overaged material than for an underaged material of the same yield strength. A lower quench rate is shown to result in an overall reduction of toughness, and in a reduced evolution of this toughness during the aging heat treatment. The overall toughness is also lowered when the main crack propagation direction is parallel to the preferential elongation direction of the coarse constituent particles (rolling direction). The competition between intergranular and transgranular fracture is explained in terms of the modifications of the work hardening rate, and of grain boundary precipitation. The evolution of fracture toughness is qualitatively explained in terms of evolution of yield stress, strain hardening rate, grain boundary precipitation and intragranular quench-induced precipitates.

Diffusion and ionic conduction in nanocrystalline ceramics

Abstract: We review case studies of diffusion in nanocrystalline ceramics, i.e. polycrystalline non-metallic materials with average grain sizes typically in the range from 5 to 50 nm. The experimental methods applied are, on the one hand, tracer diffusion or conductivity methods which are sensitive to macroscopic transport, and, on the other hand, NMR techniques which, complementarily to the previous ones, give access to microscopic diffusion parameters like atomic hopping rates and jump barrier heights. In all cases the diffusion properties of the samples, whether single-phase systems or composites, are dominated by their grain boundaries and interfacial regions, respectively. In principle, all experimental techniques allow one to discriminate between contributions to the diffusion from the crystalline grains and those from the interfacial regions. Corresponding examples are presented for SIMS and impedance measurements on oxygen conductors. NMR studies for various nanocrystalline lithium ion conductors reveal that two lithium species with different diffusivities are present. Comparison with the coarse grained counterparts shows that the slower ions are located inside the crystallites and the faster ones in the structurally disordered interfacial regions. Investigations on composite materials exhibit phenomena which can be explained by the percolation of fast diffusion pathways being formed by the interfaces between the two components.

Last-stage solidification of alloys: Theoretical model of dendrite-arm and grain coalescence

Abstract: Hot tearing in castings is closely related to the difficulty of bridging or coalescence of dendrite arms during the last stage of solidification. The details of the process determine the temperature at which a coherent solid forms; i.e., a solid that can sustain tensile stresses. Based on the disjoining-pressure concept used in fluid dynamics, a theoretical framework is established for the coalescence of primary-phase dendritic arms within a single grain or at grain boundaries. For pure substances, approaching planar liquid/solid interfaces coalesce to a grain boundary at an undercooling (ΔTb), given by $$\Delta T_b = \frac{{\Delta \Gamma _b }}{\delta } = \frac{{\gamma _{gb} - 2\gamma _{sl} }}{{\Delta s_f }}\frac{1}{\delta }$$ where δ is the thickness of an isolated solid-liquid interface, and ΔГb is the difference between the grain-boundary energy, γgb, and twice the solid/liquid interfacial energy, 2γsl, divided by the entropy of fusion. If γgb 2γsl, the two liquid/solid interfaces are “repulsive” and ΔTb>0. In this case, a stable liquid film between adjacent dendrite arms located across such grain boundaries can remain until the undercooling exceeds ΔTb. For alloys, coalescence is also influenced by the concentration of the liquid film. The temperature and concentration of the liquid film must reach a coalescence line parallel to, but ΔTb below, the liquidus line before coalescence can occur. Using one-dimensional (1-D) interface tracking calculations, diffusion in the solid phase perpendicular to the interface (backdiffusion) is shown to aid the coalescence process. To study the interaction of interface curvature and diffusion in the liquid film parallel to the interface, a multiphase-field approach has been used. After validating the method with the 1-D interface tracking results for pure substances and alloys, it is then applied to two-dimensional (2-D) situations for binary alloys. The coalescence process is shown to originate in small necks and involve rapidly changing liquid/solid interface curvatures.

Metal content of multicrystalline silicon for solar cells and its impact on minority carrier diffusion length

Abstract: Instrumental neutron activation analysis was performed to determine the transition metal content in three types of silicon material for cost-efficient solar cells: Astropower silicon-film sheet material, Baysix cast material, and edge-defined film-fed growth (EFG) multicrystalline silicon ribbon. The dominant metal impurities were found to be Fe (6×1014 cm−3 to 1.5×1016 cm−3, depending on the material), Ni (up to 1.8×1015 cm−3), Co (1.7×1012 cm−3 to 9.7×1013 cm−3), Mo (6.4×1012 cm−3 to 4.6×1013 cm−3), and Cr (1.7×1012 cm−3 to 1.8×1015 cm−3). Copper was also detected (less than 2.4×1014 cm−3), but its concentration could not be accurately determined because of a very short decay time of the corresponding radioactive isotope. In all samples, the metal contamination level would be sufficient to degrade the minority carrier diffusion length to less than a micron, if all metals were in an interstitial or substitutional state. This is a much lower value than the actual measured diffusion length of these samples. Therefore, most likely, the metals either formed clusters or precipitates with relatively low recombination activity or are very inhomogeneously distributed within the samples. No significant difference was observed between the metal content of the high and low lifetime areas of each material. X-ray microprobe fluorescence spectrometry mapping of Astropower mc-Si samples confirmed that transition metals formed agglomerates both at grain boundaries and within the grains. It is concluded that the impact of metals on solar cell efficiency is determined not only by the total metal concentration, but also by the distribution of metals within the grains and the chemical composition of the clusters formed by the metals.

Nanostructures in Ti processed by severe plastic deformation

Abstract: Metals and alloys processed by severe plastic deformation (SPD) can demonstrate superior mechanical properties, which are rendered by their unique defect structures. In this investigation, transmission electron microscopy and x-ray analysis were used to systematically study the defect structures, including grain and subgrain structures, dislocation cells, dislocation distributions, grain boundaries, and the hierarchy of these structural features, in nanostructured Ti produced by a two-step SPD procedure—warm equal channel angular pressing followed by cold rolling. The effects of these defect structures on the mechanical behaviors of nanostructured Ti are discussed.