Showing papers on "Grain boundary published in 1988"

High critical currents in Y‐Ba‐Cu‐O superconductors

Abstract: Melt‐textured growth of polycrystalline YBa2Cu3O7−δ superconductor using directional solidification created an essentially 100% dense structure consisting of long, needle‐ or plate‐shaped crystals preferentially aligned parallel to the a‐b conduction plane. The new microstructure, which completely replaces the previous granular and random structure in the sintered precursor, exhibits dramatically improved transport Jc values at 77 K of ∼17 000 A/cm2 in zero field and ∼4000 A/cm2 at H=1 T (as compared to ∼500 and ∼1 A/cm2, respectively, for the as‐sintered structure), with the severe field dependence of Jc (‘‘weak‐link’’ problem) no longer evident in the new melt‐textured material. The improvement in Jc is attributed to the combined effects of densification, alignment of crystals, and formation of cleaner grain boundaries. Microstructure and distribution of various phases present in the melt‐textured material are discussed in relation to the superconducting properties.

Microcracking in Ceramics Induced by Thermal Expansion or Elastic Anisotropy

Abstract: The effect of crystal anisotropy on the formation of grain-boundary microcracks is analyzed, by considering a planar array of hexagonal grains as a model of a polycrystalline ceramic. The stress singularities at triple-grain junctions are analyzed by an asymptotic method as well as by a numerical solution, and the critical size of a grain-boundary defect is investigated by a crack analysis. It is found that elastic anisot-ropies can significantly increase the stress levels near triple points, which results in a smaller critical grain size for microcracking.

Creep in metallic materials

Abstract: 1. Deformation and Creep. Deformation. Definition of creep. Time dependence of creep strain. Creep Curve. Mechanisms of plastic deformation: A general survey. Mechanical equation of state. Creep test compared with tensile test at constant strain rate and constant loading rate. Creep tests at constant load and constant stress. 2. Motion of Dislocations. Dynamic Recovery. Motion of dislocations. Free, mobile and moving dislocations. Dynamic recovery. 3. Temperature Dependence of Creep Rate. Activation energy of creep. Methods of determination of activation energy of creep. Correction of experimentally determined activation energy of creep for temperature dependence of elastic modulus. Activation energy or creep and activation enthalpy of diffusion. 4. Applied Stress Dependence of Creep Rate. Initial creep rate. Steady-state creep. Transient creep. 5. Influence of Grain Size and Stacking Fault Energy. Grain size. Stacking fault energy. 6. Orowan and Bailey-Orowan Equations. Orowan equation. Bailey-Orowan equation. Relation between Orowan equation and Bailey-Orowan equations. A consequence of the equivalence of Orowan and Bailey-Orowan equations. Experimental verification of Bailey-Orowan equation. Experimental determination of quantities r and h. Incubation period and ``Frictional'' stress. 7. Back Stress. Internal, threshold and frictional stress. Internal and effective stress. Concept of internal and effective stress and the mechanical equation of state. Definitions of experimental parameters. Interpretation of experimental parameters. 8. Dislocation Structure. Development of dislocation structure during creep. Basic quantitative characteristics of dislocation structure. Subgrain structure. Subgrain structure and long-range internal stress. Behaviour of sub-boundaries. Interaction of dislocations with sub-boundaries. Generation of dislocations. Structural steady state. Concept of hard and soft regions and measured internal stress. 9. Dislocation Creep in Pure Metals. Creep controlled by recovery. Creep controlled by dislocation glide. Models based on thermally activated glide and diffusion controlled recovery. Relation between constants A and n in the dorn creep equation and the natural third power law. Harper-Dorn creep. 10. Creep in Solid Solution Alloys. Introduction. Mechanisms of creep strengthening in solid solutions. Creep controlled by viscous dislocation glide. 11. Creep in Precipitation and Dispersion Strengthened Alloys. Models of Ansell and Weertman. Back stress concept. 12. Diffusional Creep. Nabarro-Herring and Coble creep. Subgrain boundaries as sources and sinks for vacancies. Diffusional creep and grain boundary sliding. Reactions on grain boundaries. Diffusional caritational creep. 13. Deformation Mechanism Maps. Equations used for construction of deformation mechanism maps. Examples of deformation mechanism maps. ``Generalized'' deformation mechanism map for pure metals. 14. Grain Boundary Sliding.

Short and long fatigue crack growth: A unified model

Abstract: In this model the growth of a crack is analysed in terms of the successive blocking of the plastic zone by slip barriers (e.g. grain boundaries) and the subsequent initiation of the slip in the next grain. The discontinuous character of the slip process (slip jumps) plays a fundamental role in the model. The factor governing the transfer of slip across a grain boundary is considered to be the stress concentration ahead of the plastic zone which, for a constant applied stress τ, is found to be dependent only on a parameter n = a/c defining the position of the crack tip relative to the grain boundary. The discrete behaviour of the slip has a strong influence in the short-crack period and hence cannot be neglected in the analysis of the crack growth rate. This period is characterized by large variations in the parameter n. In the long-crack period the slip jumps do not influence the overall description of the growth and the parameter n is almost constant. By making the crack extension per cycle prop...

Weakly coupled grain model of high‐frequency losses in high Tc superconducting thin films

Abstract: We propose a model of Josephson coupling between grains to explain the millimeter‐wave surface impedance of oriented, polycrystalline thin films of high Tc superconductors. An effective junction IcR product and effective grain size are calculated based on recent measurements of the surface impedance. We suggest a criterion on film quality for the observation of losses intrinsic in the superconductor. The effects of crystalline orientation on surface impedance are considered.

Sulphuric acid at grain boundaries in Antarctic ice

Abstract: It has been suggested1,2 that acids in the cold polar ice sheets may exist as aqueous mixtures at grain boundaries. This assumption can correctly predict the d.c. conductivity of polar ice2, but this does not prove the existence of acids or liquid veins at grain boundaries, and this remains controversial3–5. In this study we used a scanning electron microscope (SEM), equipped with a cold stage and an energy-dispersive X-ray microanalysis facility, to determine the location of sulphur in ice from the Antarctic Peninsula. As expected, sulphur was undetectable in the bulk of the ice. However, at the junctions where three grains met (triple-junctions), sulphur was found in concentrations greater than 1 M in areas of < 1 μm2. Calculations show that between 40 and 100% of the sulphuric acid present in this ice was found at the triple-junctions, and would have been liquid at ice-sheet temperatures. This finding, if general, has considerable implications for many of the physical properties of polar ice.

The kinetics of ferrite nucleation at austenite grain boundaries in Fe-C alloys

Abstract: The nucleation kinetics of grain boundary allotriomorphs of proeutectoid ferrite at austenite grain faces have been measured in three high purity Fe-C alloys as a function of isothermal reaction time and temperature. Several correction techniques, including discrimination between different nucleation sites and the effect of carbon diffusion fields on further nucleation of ferrite, were incorporated into a stereological procedure utilizing the SchwartzJSaltykov size distribution analysis. This analysis enabled the number of ferrite particles per unit unreacted grain boundary area to be obtained as a function of isothermal reaction time, and thus the time-dependent nucleation kinetics to be obtained as a function of temperature and carbon concentration. These rates were then compared with those predicted by classical heterogeneous nucleation theory using various models for the critical nucleus. It was concluded that viable critical nuclei must have predominately low energy interphase boundaries. Only a very small fraction of the austenite grain face area appears to be capable of supporting nucleation.

Development and Microstructural Characterization of High‐Thermal‐Conductivity Aluminum Nitride Ceramics

Abstract: The effect of several additives, such as CaC2, CaO, Y2O3, and C, on thermal conductivity of hot-pressed AlN ceramics was investigated. The addition of CaC2 reductant was found to be useful for achieving high thermal conductivity of 180 W/(m·K) at room temperature. The characterization of AlN ceramics with CaC2 additive was performed by chemical analysis of Ca, C, and O and microstructural analysis using transmission electron microscopes equipped with an energy-dispersive X-ray analyzer and an electron energy loss spectrometer. The major influence on high thermal conductivity is the disappearance of a thermal barrier caused by oxygen impurities at the grain boundary.

Observation of tilt boundaries in graphite by scanning tunneling microscopy and associated multiple tip effects

Abstract: Tilt boundaries have been observed on the (0001) surface of graphite by scanning tunneling microscopy (STM). Rotation angles about the c axis of 6.5°, 8°, and 19° were found, indicating no preferential orientation of grains in the basal plane of graphite. The grain boundary region between crystallites appears disordered with a width varying between 10 and 100 A. Moire patterns are observed near grain boundaries when multiple tips scanning over different grains contribute to the image simultaneously. Such images support the theory that multiple isolated tips, occasionally hundreds of angstroms apart, can contribute to STM images.

Columnar growth in thin films.

Abstract: A theory for the growth of columnar microstructures in thin films is presented. The zone-I to zone-II transition temperature is predicted. The surface morphology and the columnar grain structure are obtained both analytically and numerically, and the scaling behavior of the columnar grain size with film thickness is derived. Monte Carlo simulations are used to follow the evolution of the three-dimensional zone-II microsctuctures and to account for the formation of film texture. These results agree with experimental observations.

Domain wall pinning versus nucleation of reversed domains in R‐Fe‐B magnets (invited)

Abstract: The validity of nucleation and domain wall pinning models has been examined in R‐Fe‐B magnets by correlating the hard magnetic properties with the microstructure and magnetic domain structure The nucleation model can be used to explain the magnetic properties of R‐Fe‐B magnets at low temperatures, but it fails at high temperatures On the other hand, domain wall pinning can explain the magnetic properties over a much wider temperature range (up to the Curie temperature) The magnetic and structural data suggest that the coercivities (Hc) of powders and stoichiometric melt‐spun ribbons are due to ‘‘strong pinning’’ by a random distribution of pins while the Hc of sintered magnets and ribbons with off‐stoichiometric composition are due to ‘‘localized pinning’’ at grain boundaries

Ceramic Microstructures '86: Role of Interfaces

Abstract: Progress in the Understanding of Ceramic Microstructures and Interfaces since 1976.- Ceramic Microstructures: The Art of the Possible.- Designing Material Structures.- Purposive Design of Nanocomposites: Entire Class of New Materials.- Design of High Thermal Expansion Glass-Ceramics Through Micros truetura1 Control.- Microdesigning of Ceramic-Metal Composites.- Characterization of Microstructures.- Grain Boundaries and Interfaces: Some Applications of Electron Microscopy and Microanalysis.- Quantitative Characterization of Microstructural Geometry of Interfaces.- Gas Transport as a Tool for Structural Characterization of Inter facial Phases.- Unit Cell Distortion of Pb(Zr,Ti)O3 by Pb(Co,Nb)O3.- Electron Microscopy Studies of BaTiO3~NaNbO3 Ceramics.- Analytical Electron Microscopy of the Glassy Phase in Mullite/Zirconia Composites.- Microstructure Characterization of Basalt Glass-Ceramics.- Interfacial Reaction Between Bioactive Glass and Synthetic Physiological Solution.- Oxidation and Microstructure of Sintered Silicon Nitride.- Origin of Cracking Phenomena Observed in Decomposition Reactions.- Characterization of Interfaces.- Oxide Interfaces: Theory of Oxide-Oxide and Oxide-Metal Interfaces.- Green Function Method for Calculation of Atomistic Structure of Grain Boundary Interfaces in Ionic Crystals.- Comparison of the Energies of Symmetrical and Asymmetrical Grain Boundaries.- The Structure of a Natural Grain Boundary in a Magnetite Bicrystal Studied by XRD Technique.- Interface Morphology in Ceramics.- HREM Study of a Tetragonal ? Monoclinic Martensitic Interface in a Y2O3-Stabilized ZrO2 Alloy.- Control of the Tetragonal to Monoclinic Phase Transformation of Yttria-doped Tetragonal ZrO2 Polycrystals by Annealing in Water.- Grain Boundary Energy of Twisted MgO Bicrystals on (100).- T.E.M. Statistical Characterization of Grain Boundaries in Polycrystalline NiO Scales Obtained by High Temperature Oxidation of Nickel.- Structure of High-Angle Grain Boundaries in NiO.- High-Angle Grain Boundaries in Sheet Silicates (Biotite/Chlorite): A TEM Study.- HRTEM Analysis of Ordered Grain Boundaries in High Purity Alpha-SiC.- Atomic Structure of Interfaces in Sialon Ceramics.- Characterization of Impurity Segregations.- Atomistic Lattice Simulations of the Structure, Energetics and Impurity Segregation Behaviour of Ceramic Boundaries.- Distribution and Influence of Minor Constituents on Ceramic Formulations.- Characterization of Interphase Interfaces.- Diffusion Bonding of Metal/Ceramic Interfaces-A Model Study at the Nb/Al2O3Interfaces.- Ceramic-Metal Reactions and Their Effect on the Interface Microstructure.- The Influence of Surface Roughness on Interface Formation in Metal/Ceramic Diffusion Bonds.- A Reappraisal of Wetting in the System AI-AI2O3 from 750-1000 C.- Microstructural Characterization of Ni/Al203 Diffusion Bonds.- Bonding Ceramic-Metal Interfaces and Joints.- Copper-Glass-Ceramic Interfaces and Composites: Reactions, Microchemistry, and Electrical Properties.- The Study of Microstructures for Pt-Silicide Interfaces after Nd:YAG Laser Irradiation by XRD, SEM, XPS, and RBS.- High Resolution Electron Microscopy of Silicon Nitride-Metal Bonded Interfaces.- Interfacial Chemistry and Bonding in Fiber Reinforced Glass and Glass-Ceramic Matrix Composites.- Effects of Interfacial Diffusion Barriers on Thermal Stability of Ceramic Fibers.- Solid State Bonding of Alumina and Steel by HIPING.- Thermochemical Analyses of Interface Reactions in Carbon-Fiber Reinforced Glass Matrix Composites.- Interfaces in Heterogeneous Dissolution of Oxides in Molten Ca-Al-Silicates.- Microstructure Development.- Powders, Interfaces, and Processing: Alumina as a Case Study.- The Role of Powder Packing in Sintering.- Effect of Pores on Microstructure Development.- Microstructure Development of Hydrothermal Powders and Ceramics.- The Colloidal Chemistry of Growing Silica Spheres.- Effects of Vapor Transport on Microstructure Development.- Controlling Grain Growth.- The Effects of Grain Growth on the Intergranular Porosity Distributions in Hot-Pressed and Swelled UO2.- Effects of Solutes on the Grain Boundary Mobility of TiO2.- Microstructural Development in Hot Pressed CaO: Density Decrease and Pore Growth During Post Sintering.- Diffusion Induced Interface Migration in Ceramics.- Grain Boundary Diffusion Artefacts in Polycrystalline Nickel Oxide Grown by High Temperature Oxidation.- Variations in Grain Growth of Donor-Doped SrTiO3 with Cation Nonstoichiometry.- The Wetting and Dewetting of Grain Boundaries.- Anion Controlled Microstructures in the Al2O3-AIN System.- Sintering.- Sintering Theory for Crystalline Solids.- Selected Sintering Conditions for SiC and Si3N4 Ceramics.- Colloidal Consolidation and Sintering Behavior of CVD-Processed Mullite Powders.- Sintering of Acicular NiZn-Ferrite Powder.- Continuous SiC Fiber/Glass Composites.- Electrical Properties.- Electrical Conductivity in Ceramics: A Review.- The Crystallographical and Chemical Relationship between Intergranular and Bulk Resistivity in Semi-Conductor Oxides.- Interface Effects in Zinc Oxide Varistors.- The Role of Inter facial Microstructure in ZnO Varistor Materials.- Electrical Properties and Microstructure of ZnO-Nb2O5~MnO Ceramics Sintered in the Liquid Phase.- New Composite PTC Materials Based on PbTiO3-TiO2.- Effects of Microstructure Control on Ferroelectric Ceramics.- Effect of Deviations from Stoichiometry on Microstructure and Magnetic Properties of Ferrites.- Optical Effects of Grain Boundaries in PLZT Ceramics.- Microstructures of High Dielectric Constant Materials.- Interfaces Generated by Electronic Device Development in Beta SiC Thin Films.- Electrical Barriers at Grain Boundaries in Silicon Carbide Materials with BeO Addition.- Microstructural Analysis of Silicon Carbide Materials with BeO Addition.- Mechanical Properties and Behavior.- The New Generation of High Toughness Ceramics.- Influence of Microstructure on Creep Rupture.- Mechanisms of Dynamic Failure in Debased Alumina.- Some Interfacial Related Properties of Transformation Toughened Ceramics.- Precipitation in a (Mg) Partially Stabilized Zirconia during Aging at 1000 C.- High Temperature Mechanical Properties of ZrO2-Based Ceramics.- Microstructure and Mechanical Properties of a Al2TiO5-Mullite-ZrO2 Composite Obtained by Reaction Sintering.- Interfaces in Ceramic Fiber Composites.- Delamination Toughening from Interfacial Cracking in Ceramics and Ceramic Composites.- Development of Structure in Silicon Nitride Grain Boundaries.- The Impact of Compositional Variations and Processing Conditions on Secondary Phase Characteristics in Sintered Silicon Nitride Materials.- Structure and Chemistry of Interfaces in Silicon Carbide-Containing Materials.- Interfaces in Alumina-SiC Whisker Composites.- Creep Performance of Silicon Carbide Whisker-Reinforced Alumina.- Temperature-Dependent Toughening in Whisker-Reinforced Ceramics.- Effect of High Temperature Oxidation on the Microstructure and Mechanical Properties of Whisker-Reinforced Ceramics.- The Role of Interfacial Reactions on the Mechanical Properties of Ceramic Brazements.- Fracture Energies of Ceramic-Metal Interfaces.- On the Fracture Behavior and Microstructure of Metal-to-Ceramic Joints.

Grain Boundary Segregation of Yttrium in Chromia Scales

Abstract: Influence de l'implantation d'une forte dose d'yttrium sur la vitesse d'oxydation et le mecanisme de croissance de Cr 2 O 3 sur l'alliage Co-45 Cr oxyde dans l'oxygene pur a 1000°C. Observation de la segregation des ions yttrium aux joints de grain de la couche d'oxyde

Ion‐bombardment‐enhanced grain growth in germanium, silicon, and gold thin films

Abstract: Grain growth has been studied in polycrystalline thin films of Ge, Si, and Au during ion bombardment. The phenomenon has been characterized by varying the ion dose, ion energy, ion flux, ion species, substrate temperature, and thin‐film deposition conditions. Films bombarded with Si+, Ar+, Ge+, Kr+, and Xe+ exhibited enhanced grain growth which was weakly temperature dependent and proportional to the energy deposited in elastic collisions at or very near grain boundaries. The effect of these parameters on grain size and microstructure was analyzed both qualitatively and quantitatively using transmission electron microscopy. A transition state model describing the motion of grain boundaries during ion bombardment has been applied to the present experimental data. The results suggest that bombardment‐enhanced grain growth may be due to thermal migration of bombardment‐generated defects across the boundary. The calculated defect yield per incident ion was found to be directly related to enhanced grain growth...

Grain growth in synthetic marbles with added mica and water

Abstract: Evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder, through primary recrystallization and normal grain growth, to a size stabilized by second phases. To form the marbles, reagent grade CaCO3 was mixed with 0, 1 and 5 volume% mica and heat-treated under pressure with added water. Densification with negligible recrystallization occurred within one hour at 500° C and 500 MPa confining pressure. Primary recrystallization occurred at 500–550° C, causing increases of grain size of factors of 2–5. Resulting samples had uniform grain size, gently curved grain boundaries, and near-equilibrium triple junctions; they were used subsequently for normal grain growth studies. Normal grain growth occurred above 550° C; at 800° C, grain size (D) increased from 7 μm (D 0) to 65 μm in 24 hours. Growth rates fit the equation, D n -D 0 =Kt, where K is a constant and n≃2.6. Minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size, D max, which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0.3 and 1. Once D max is reached, normal growth continues only if second phases are mobile or coarsen, or if new driving forces are introduced that cause unpinning of boundaries. Normal grain growth in Solnhofen limestone was significantly slower than in pure synthetic marble, suggesting that migration is also inhibited by second phases in the limestone.

Influence of retrogression and reaging treatments on the strength and stress corrosion resistance of aluminium alloy 7075-T6

Abstract: The influence of retrogression and reaging (RRA) treatments on high strength aluminium alloy 7075 was studied using hardness and tensile testing and by measuring the stress corrosion crack growth rate using double-cantilever-beam specimens. Transmission electron microscopy was used to study the microstructures at both the matrix and the grain boundary. The RRA treatments result in a significant improvement of the stress corrosion resistance of aluminium alloy 7075-T651 without sacrificing its maximum strength. The crack velocity decreases to a level comparable with that of the T73 structure as a result of the optimum RRA treatments. The initial strength decrease during the retrogression treatment is mainly due to a dissolution of small η′ particles. The coarsening of grain boundary precipitates occurs during the retrogression treatment. The steady state crack velocity decreases logarithmically with increase in the volume fraction of grain boundary precipitates.