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Showing papers on "Grain boundary published in 1995"


Book
01 Jan 1995
TL;DR: Analytical Models of Grain Boundary Diffusion as mentioned in this paper have been proposed for determining grain boundary data, including diffusion along Dislocations and Small Angle Grain Boundaries, in thin films.
Abstract: Analytical Models of Grain Boundary Diffusion. Diffusion Along Dislocations and Small-Angle Grain Boundaries. Grain Boundary Diffusion in Thin Films. Diffusion Along Migrating Grain Boundaries. Structural Effects on and Mechanisms of Grain Boundary Diffusion. Experimental Methods for Determination of Grain Boundary Diffusion Data. Index.

840 citations


Journal ArticleDOI
TL;DR: This paper investigated the effect of melt on the creep behavior of water-free olivine aggregates deformed in the dislocation creep regime and found that the influence of the melt phase is modest at melt fractions less than ∼ 0.04.
Abstract: Experiments have been conducted to investigate the effect of melt on the creep behavior of water-free olivine aggregates deformed in the dislocation creep regime. The influence of the melt phase is modest at melt fractions less than ∼0.04. However, at melt fractions > 0.04, the creep rate of melt-added samples is enhanced by more than an order of magnitude relative to melt-free aggregates. This unexpectedly large influence of melt on strain rate arises because deformation occurs by grain boundary sliding (GBS) accommodated by a dislocation creep process. Four observations support this hypothesis. (1) The strain rate enhancement observed in the dislocation creep regime can be related to the stress concentration caused by the reduction in the solid-solid grain boundary area. (2) Both melt-free and melt-added samples exhibit strain rates indicating that deformation is limited by slip on (010)[100], the easiest slip system in olivine. (3) The GBS mechanism occurs near the transition between diffusion and dislocation creep. (4) Grains in specimens deformed in the GBS regime are not significantly flattened, even after ∼50% shortening. In melt-free aggregates, a transition from the GBS mechanism to dislocation creep limited by slip on (010)[001], the hardest slip system, is observed with an increase in grain size. A transition to (010)[001] limited creep was not observed for partially molten aggregates because grain growth was inhibited by the presence of melt. The results of this study indicate that the viscosity of the upper mantle may decrease by at least an order of magnitude if the retained melt fraction exceeds 0.04 or if the onset of melting results in a reduction in grain size and a concomitant transition from (010)[001] to (010)[100] limited creep.

573 citations


Journal ArticleDOI
TL;DR: In this article, the commercial grain refining practice of aluminium has been experimentally simulated by introducing synthetic TiB2 crystallites into melts by a specially developed technique, and experimental findings indicate that TiB 2 crystallites alone do not nucleate α-Al.
Abstract: Despite the commercial importance of grain refinement and the volume of scientific studies on this topic, its mechanism is still unclear. There are several theories as to how and why commercial grain refiners (Al-Ti and Al-Ti-B) work, but careful analysis shows that no clear consensus has yet emerged. In the present study, the commercial grain refining practice of aluminium has been experimentally simulated by introducing synthetic TiB2 crystallites into melts by a specially developed technique. Experimental findings indicate that TiB2 crystallites alone do not nucleate α-Al. On the other hand, in the presence of dissolved Ti even below the peritectic level, an interfacial TiAl3 layer is formed at the TiB2/melt interface which subsequently nucleates the α-Al. The theoretical and practical implications of grain refinement phenomena are discussed in the light of the present experimental findings. A ‘duplex’ nucleation mechanism is proposed based on solute segregation to the substrate/melt interface.

405 citations


Journal ArticleDOI
TL;DR: In this article, the potential impact of grain boundary design and control on the bulk sensitization and intergranular corrosion resistance of a commercial corrosion resistant nickel-based austenitic alloy: Alloy 600 (UNS N06600) was assessed.
Abstract: The objective of this study was to assess, for the first time, the potential impact of ``Grain Boundary Design and Control`` on the bulk sensitization and intergranular corrosion resistance of a commercial corrosion resistant nickel-based austenitic alloy: Alloy 600 (UNS N06600). Increasing the special grain boundary ({Sigma} {le} 29) frequency in thermomechanically processed Alloy 600 from 37% to 71% has been shown to result in commensurate decreases in bulk intergranular corrosion susceptibility in both the solution annealed and sensitized condition; these findings being attributed to both the intrinsic corrosion resistance, and resistance to solute segregation and precipitation exhibited by structurally-ordered low {Sigma} grain boundaries. These results provide considerable promise for the practical application of grain boundary design and control considerations in the general field of corrosion prevention and control.

389 citations


Journal ArticleDOI
TL;DR: In this paper, high energy ball milling has been used as a versatile alternative to other processing routes, including vapor evaporation, liquid quenching and chemical synthesis methods.

374 citations


Journal ArticleDOI
TL;DR: In this paper, superconducting and mechanical properties of YBa2Cu3O7−δ (YBCO) films on Ni-based alloys with a textured yttria-stabilized zirconia (YSZ) buffer layer were reported.
Abstract: We report superconducting and mechanical properties of YBa2Cu3O7−δ (YBCO) thick films on Ni‐based alloys with a textured yttria‐stabilized zirconia (YSZ) buffer layer. The YBCO and YSZ layers were deposited by pulsed laser deposition and ion beam assisted deposition, respectively. It was found that the transport critical current density (Jc) correlates very well with the YBCO mosaic spread. Jc over 1×10 6 A/cm2 at 75 K and ∼1×107 A/cm2 at 4 K were obtained in the 1‐μm thick YBCO films. Zero field critical current of 120 amps at 75 K was obtained in a 2‐μm‐thick and 1‐cm‐wide YBCO film. Angular dependence measurement revealed Jc peaks for both H∥c and H∥a‐b. The peak for H∥c implies additional pinning due to defects such as small angle grain boundaries or twin boundaries. Bending tests at 75 K showed that the YBCO thick films on the metallic substrates could sustain a strain of 0.4% and over 1% for tension and compression, respectively.

342 citations


Journal ArticleDOI
TL;DR: In this paper, the energy associated with the trapping of hydrogen to defects in a nickel lattice was investigated and several dislocations and grain boundaries which occur in nickel are studied.
Abstract: This paper addresses the energy associated with the trapping of hydrogen to defects in a nickel lattice. Several dislocations and grain boundaries which occur in nickel are studied. The dislocations include an edge, a screw, and a Lomer dislocation in the locked configuration, i.e. a Lomer-Cottrell lock (LCL). For both the edge and screw dislocations, the maximum trap site energy is approximately 0.1 eV occurring in the region where the lattice is in tension approximately 3-4 angstroms from the dislocation core. For the Lomer-Cottrell lock, the maximum binding energy is 0.33 eV and is located at the core of the a/6(110) dislocation. Several low-index coincident site lattice grain boundaries are investigated, specifically the Sigma 3(112), Sigma 9(221) and Sigma 11(113) tilt boundaries. The boundaries all show a maximum binding energy of approximately 0.25 eV at the tilt boundary. Relaxation of the boundary structures produces an asymmetric atomic structure for both the Sigma 3 and Sigma 9 boundaries and a symmetric structure for the Sigma 11 tilt boundary. The results of this study can be compared to recent experimental studies showing that the activation energy for hydrogen-initiated failure is approximately 0.3-0.4 eV in the Fe-based superalloy IN903. From the results of this comparison it can be concluded that the embrittlement process is likely associated with the trapping of hydrogen to grain boundaries and Lomer-Cottrell locks.

326 citations


Journal ArticleDOI
TL;DR: In this paper, a brief overview of the present understanding of the mechanical properties of nanophase metals is presented, including grain boundary sliding and other microstructural features, as well as grain size changes are discussed.

321 citations


Journal ArticleDOI
TL;DR: In this article, the possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystine materials.
Abstract: The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

297 citations


Journal ArticleDOI
TL;DR: In this paper, the defects which penetrate the GaN films are predominantly perfect edge dislocations with Burgers vectors of the 1/3-1120-type, lying along the [0001] growth direction.
Abstract: Microstructure of α‐GaN films grown by organometallic vapor phase epitaxy on sapphire substrates using low temperature AlN (or GaN) buffer layers has been studied by transmission electron microscopy. The defects which penetrate the GaN films are predominantly perfect edge dislocations with Burgers vectors of the 1/3〈1120〉 type, lying along the [0001] growth direction. The main sources of threading dislocations are the low angle grain boundaries, formed during coalescence of islands at the initial stages of GaN growth. The grain sizes range from 50 to 500 nm, with in‐plane misorientations of less than 3°. The nature of these threading dislocations suggests that the defect density would not likely decrease appreciably at increasing film thickness, and the suppression of these dislocations could be more difficult.

274 citations


Journal Article
TL;DR: In this article, the authors compare the properties of fine and coarse grained ceramics and compare them with relaxor and thin-firm Ceramics in terms of dielectric and elastic properties.
Abstract: Conventional ferroelectric perovskite type ceramics have dielectric, piezoelectric and elastic properties which depend on grain size and on domain configuration. Very fine grained ceramic is not splitted in domains. This causes strong elastic stress fields in the grains which counteract ferroelectricity. Tetragonal fine grained ceramic has a simple laminar domain structure and high elastic stress fields inside the grain and at the grain boundaries. These stress fields cause very high permittivity. In coarse grained ceramics the stress fields inside the grain are eliminated by a three-dimensional network of domains. In fine and in coarse grained ceramics the domain walls contribute considerably to the dielectric, piezoelectric and elastic constants at frequencies below a relaxation frecuency which is between 200 and 1000 MHz. At low temperatures, however, the domain wall contributions freeze in. Acceptor doping lowers the domain wall contributions and shifts the relaxation frequency to higher values. The properties of the conventional ceramics will be compared wiht properties of thin firms and with properties of relaxor ceramics.

Journal ArticleDOI
TL;DR: In this article, the influence of the processing parameters on the PTCR related properties is discussed, and special emphasis is placed on the phenomenon that the conductivity and grain size decrease with increasing donor concentration above ∼ 0.3 at%.
Abstract: Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

Journal ArticleDOI
TL;DR: In this article, the deformation behavior of nanostructured gold thin films, with grain diameters of 10 nm and film thicknesses of 10-20 nm, was studied by means of in situ high-resolution transmission electron microscopy.

Journal ArticleDOI
TL;DR: In this article, the present understanding of an important and fundamental part of this relationship, namely, the structural aspects of grain boundary segregation, is reviewed and their application to materials is discussed.
Abstract: Physical and chemical properties of solid materials are strongly. influenced by the chemical composition of internal interfaces, One of the crucial parameters affecting interfacial chemistry is the atomic structure of the interface. Due to its importance. a considerable amount of work was done to elucidate the relationship between structure and chemical composition of interfaces. This article reviews the present understanding of an important and fundamental part of this relationship, namely, the structural aspects of grain boundary segregation. After a brief outline of grain boundary structure and geometry. thermodynamic approaches to describe grain boundary segregation are summarized and their application to materials is discussed. covering particular sites at a single grain boundary as well as the role of interfaces in polycrystals. Both the experimental evidence of grain boundary segregation anisotropy and the theoretical results of computer simulations of grain boundary segregation are summar...

Journal ArticleDOI
TL;DR: In this article, the effects of increased diffusivity at base material grain boundaries, of grain boundary motion, and grain boundary grooving, on isothermal solidification during TLP bonding are described.
Abstract: Modelling of transient liquid phase (TLP) bonding is reviewed. The outputs produced during analytical and numerical modelling are discussed in detail and compared with actual experimental results produced when joining simple binary alloy systems. The effects of increased diffusivity at base material grain boundaries, of grain boundary motion, and of grain boundary grooving, on isothermal solidification during TLP bonding are described. There is a critical need for detailed research in which modelling output is closely related to direct microstructural observations during bonding of complex alloy systems.

Journal ArticleDOI
TL;DR: In this paper, the thermal stability of electroplated nanocrystalline Ni of 10 and 20 nm grain size was investigated by differential scanning calorimetry (DSC).
Abstract: Thermal stability of electroplated nanocrystalline Ni of 10 and 20 nm grain size was investigated by differential scanning calorimetry (DSC). The temperature dependence and heat release ΔH during grain growth have been determined by linear anisothermal measurements (linear heating at 10 K min −1 ). The corresponding change in microstructure has been monitored in the temperature range between 373 K and 693 K using transmission electron microscopy (TEM). The TEM and DSC studies identified three exothermic reactions: “nucleation” and abnormal grain growth (353–562 K), normal grain growth (562–593 K) and growth towards equilibrium (643–773 K). The grain growth behaviour, and the similar heat releases ΔH = 18 J g −1 , and 16 J g −1 measured for the 10 nm and 20 nm Ni nanocrystals respectively in the DSC experiments may be related to the observed sulphur segregation at grain boundaries and triple junctions.

Journal ArticleDOI
TL;DR: In this paper, the structural transformation of polycrystalline Si induced by high energy ball milling has been studied and two phase amorphous and nanocrystalline si has been produced.
Abstract: The structural transformation of polycrystalline Si induced by high energy ball milling has been studied. The structure and property characteristics of the milled powder have been investigated by x-ray diffraction, scanning electron microscopy, high-resolution electron microscopy, differential scanning calorimetry, Raman scattering, and infrared absorption spectroscopy. Two phase amorphous and nanocrystalline Si has been produced by ball milling of polycrystalline elemental Si. The nanocrystalline components contain some defects such as dislocations, twins, and stacking faults which are typical of defects existing in conventional coarse-grained polycrystalline materials. The volume fraction of amorphous Si is about 15% while the average size of nanocrystalline grains is about 8 nm. Amorphous elemental Si without combined oxygen can be obtained by ball milling. The distribution of amorphous Si and the size of nanocrystalline Si crystallites is not homogeneous in the milled powder. The amorphous Si formed is concentrated near the surface of milled particles while the grain size of nanocrystalline Si ranges from 3 to 20 nm. Structurally, the amorphous silicon component prepared by ball milling is similar to that obtained by ion implantation or chemical vapor deposition. The amorphous Si formed exhibits a crystallization temperature of about 660 °C at a heating rate of 40 K/min and crystallization activation energy of about 268 kJ/mol. Two possible amorphization mechanisms, i.e., pressure-induced amorphization and crystallite-refinement-induced amorphization, are proposed for the amorphization of Si induced by ball milling.

Journal ArticleDOI
TL;DR: A detailed analysis of superplasticity in powder metallurgy aluminum alloys and composites has been reviewed through a detailed analysis as discussed by the authors, where the role of increasing misorientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined.
Abstract: Superplasticity in powder metallurgy aluminum alloys and composites has been reviewed through a detailed analysis. The stress-strain curves can be put into four categories: a classical well-behaved type, continuous strain hardening type, continuous strain softening type and a complex type. The origin of these different types of stress-strain curves is discussed. The microstructural features of the processed material and the role of strain have been reviewed. The role of increasing misorientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined. Threshold stresses have been determined and analyzed. The parametric dependencies for superplastic flow in modified conventional aluminum alloys, mechanically alloyed alloys and aluminum alloy matrix composites is determined to elucidate the superplastic mechanism at high strain rates. The role of incipient melting has been analyzed. A stress exponent of 2, an activation energy equal to that for grain boundary diffusion and a grain size dependence of 2 generally describes superplastic flow in modified conventional aluminum alloys and mechanically alloyed alloys. The present results agree well with the predictions of grain boundary sliding models. This suggests that the mechanism of high strain rate superplasticity in the above-mentioned alloys is similar to conventional superplasticity. The shift of optimum superplastic strain rates to higher values is a consequence of microstructural refinement. The parametric dependencies for superplasticity in aluminum alloy matrix composites, however, is different. A true activation energy of 313 kJ mol−1 best describes the composites having SiC reinforcements. The role of shape of the reinforcement (particle or whisker) and processing history is addressed. The analysis suggests that the mechanism for superplasticity in composites is interface diffusion controlled grain boundary sliding.

Journal ArticleDOI
Z. Yang1, Kevin P. Homewood1, M. S. Finney1, M.A. Harry1, Karen J. Reeson 
TL;DR: In this paper, the existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three-parameter thermodynamic model and the Einstein model.
Abstract: Ion beam synthesized polycrystalline semiconducting FeSi2 on Si(001) has been investigated by transmission measurements at temperatures between 10 and 300 K. The existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three‐parameter thermodynamic model and the Einstein model. Band tail states and states on a shallow impurity level were found to give rise to the absorption below the fundamental edge. The presence of an Urbach exponential edge was shown and the temperature dependence of the Urbach tail width was also studied based on the Einstein model. A strong structural disorder associated with grain boundaries between and within the FeSi2 grains and their related defects was found to be the dominant contribution at room temperature.

Journal ArticleDOI
TL;DR: In this article, pressureless sintered alumina compacts with a submicrometer microstructure exhibit a hardness that approaches or even exceeds the level of advanced hot-pressed composites of Al{sub 2 O{sub 3} + 35 vol% TiC, whereas the strength of both ceramics is approximately the same.
Abstract: Pressureless sintered alumina compacts with a submicrometer microstructure exhibit a hardness that approaches or even exceeds the level of advanced hot-pressed composites of Al{sub 2}O{sub 3} + 35 vol% TiC, whereas the strength of both ceramics is approximately the same. The combination of reduced dislocation mobility (due to the small grain size), high density, and density homogeneity are the prerequisites for the surprisingly high hardness. Quasi-conventional powder processing is used to produce these outstanding alumina bodies.

Journal ArticleDOI
TL;DR: In this paper, the size of recrystallized marble marble was measured by optical microscopy on ultrathin sections, and automatic grain size measurement techniques were used to study the microstructural and mechanical property changes associated with dynamic recrycstallization.
Abstract: Carrara marble has been deformed experimentally at temperatures ranging between 500° and 1000°C, at confining pressures of 200 and 300 MPa, and up to very large strains in extension and compression, in order to study the microstructural and mechanical property changes associated with dynamic recrystallization. Microstructural studies were made by optical microscopy on ultrathin sections, and automatic grain size measurement techniques were used. When the temperature is sufficiently high (≈ 600°C) and the stresses are high enough for deformation twinning to occur, twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size. At stress levels too low to activate twinning, recrystallization occurs in two stages: the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions, followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume. The recrystallization process requires larger prestrains at the lower temperatures. The size to which migration-recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms. Within the range of experimental observations, dynamically recrystallized grain size does not depend on strain rate or temperature. Separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry. No unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found. However, extrapolation of the experimental data to low, natural strain rates suggests that weakening following recrystallization may occur in nature.

Book
30 Sep 1995
TL;DR: In this article, a method for estimating the density of volume elements by a method of size distribution reconstruction is presented. But the method is limited to the case of 2D lines on a plane.
Abstract: Introduction Elements of the Microstructures of Materials Size and Shape The Influence of Microstructure on the Properties of Materials Stochastic Character of the Geometry of Microstructures Observations of the Microstructure of Materials About the Book Rerferences Basic Concepts, Definitions, Techniques, and Relationships Populations, Individual Microstructural Elements, and Distribution Functions Distribution Functions Parameters of the Populations Normal and Log-Normal Distribution Sampling Size of the Samples Microstructural Homogeneity General Characterization of Material Microstructures Geometrical Probing Random and Oriented Sections Random Oriented Lines Point Testing Number and Weighted Distributions Notation Volume Fraction Determination Density of Two-Dimensional Features in Space Density of Lines on a Plane Density of Lines in Space Density of Elements and Average Properties Modification for Anisotropic Lines and Surfaces Anisotropic Distribution of Lines Estimation of the Density of Volume Elements, NV, by a Method of Size Distribution Reconstruction Description of Particle Agglomeration Curvature of Lines on a Plane References Modern Stereology Disector-New Approach to Particle Density Selector-Particle Volume Distribution Point Sampled Intercepts Second Order Statistics-Methods for Estimating the Variance of Particle Volume Fractionator Nucleator Vertical Sectioning-Area of the Surfaces Orientator Trisector References Computer Aided Characterization of the Microstructures of Materials Image Processing Computer Aided Procedures of Data Analysis Elements of a System for Image Analysis References Characterization of Linear and Planar Elements of Microstructures Dislocations Internal Surfaces External Surfaces Quantitative Characterization of Thin Film Structures References The Geometry of Grains and Its Effect on the Properties of Polycrystals Introduction Grains as Space Filling Volumetric Elements Stochastic Character of Grains Geometry The Relationship Between 3-D Grains and Their 2-D Sections Grain Size Grain Area Distribution Function and Its Relationship to Grain Volume Distribution Mean Size of Grains Relationship Between Linear Parameters of Grain Size Degree of Grain Size Uniformity Data for Metallic Polycrystals Grain Size Effect Geometry of Grains and Physical Properties of Grain Boundaries References Changes in the Geometry of Grains and Grain Boundaries Under the Action of External and Internal Factors Tensile Test Changes in the Geometry of Grains in Strained Polycrystals-Uniform Deformation Approximation Spherical Grains Approach Stochastic Geometry Modelling Statistically Compatible Deformation Localized Deformation An Application to the Studies of Plastic Deformation Mechanisms Grain Geometry Evolution Under the Action of Internal Factors Annealing of Deformed Materials References Particles, Pores, and Other Isolated Volumetric Elements of the Microstructure Particle Sampling and Choice of Parameters General Characteristics of Particle Populations: Volume, Surface Area, Curvature, and Number Individual Properties of Particles: Mean Volume and Coefficient of Variation Particle Arrangements and Orientations Description of Particle Shapes Shape of Particle Sections Fourier Analysis of Sections Shape References

Journal ArticleDOI
TL;DR: In this article, tensile specimens of Type 316L stainless steel having grain sizes in the range 3.1-86.7 μm were deformed to 34% strain at temperatures 24, 400 and 700°C and strain rate 1 × 10−4s−1 to investigate the Hall-Petch (H-P) relationship, the nature of stress-strain curves and the substructure development.
Abstract: Tensile specimens of Type 316L stainless steel having grain sizes in the range 3.1–86.7 μm were deformed to 34% strain at temperatures 24, 400 and 700°C and strain rate 1 × 10−4s−1 to investigate the Hall-Petch (H-P) relationship, the nature of stress-strain curves and the substructure development. Upto ∼5% strain the H-P relationship exhibits bi-linearity whereas the single Hall-Petch relation is exhibited at larger strains. The presence of bi-linearity is explained by the back stress associated with the difference in the dislocation densities in the vicinity of grain boundary and in the grain interior. The log stress (σ)-log strain (e) plots depict three regimes and follow the relationship σ = Ken in each regime, but with varying magnitudes of the strength coefficient (K) and strain-hardening exponent (n).

Journal ArticleDOI
TL;DR: In this article, the characteristics of high temperature flow curves and evolution of new grain structures under conditions of dynamic recrystallization (DRX) are reviewed, together with the associated development of dislocation substructures.

Journal ArticleDOI
TL;DR: In this article, the role of grain boundary phases and their influence on grain boundary resistivity in ZrO 2 -based electrolyte systems have been investigated by impedance spectroscopy and microstructural analysis.

Journal ArticleDOI
TL;DR: In this article, the Si microstructure of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix.
Abstract: Heterogeneous nucleation of solidification in melt spun Al-Si and Al-Si-P has been studied using differential scanning calorimetry, and transmission, scanning transmission and high resolution electron microscopies. The microstructures of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix. The Si microstructure depends on the level of P: coarse faceted Si particles are nucleated by AlP particles in Al-Si containing 2 ppm P and Al-Si-P containing 35 ppm P whereas eutectic droplets of fine Si particles are nucleated by the surrounding Al matrix at a high undercooling in Al-Si containing 0.25 ppm P. The Si nucleation onset temperature remains approximately constant while the peak and end temperatures both decrease with increasing cooling rate, in agreement with classical nucleation theory. Kinetic analysis, using the spherical cap model gives contact angles of 10°, 43° and 10° for Si nucleation in low and high purity Al-Si and Al-Si-P respectively.

Journal ArticleDOI
TL;DR: In this article, the electrical conductivity of monocrystalline (3.0, 9.5, 12.0 and 17.9 mol% yttria)-doped zirconia samples was studied using impedance spectroscopy in the temperature range of 250-1200 °C.

Journal ArticleDOI
TL;DR: In this paper, a coupling equation of diffusion and reaction on a spherical quartz grain with diameter d and grain boundary width w was derived, and the flow law of pressure solution creep was derived.

Journal ArticleDOI
TL;DR: Wagner's classical treatment of internal oxidation (generic name allowing for reaction with oxygen, nitrogen, carbon or sulfur) assumed ideal conditions such as uninhibited dissolution of the gas, formation of spherical particles, diffusion of the oxidant in the solvent as the rate-controlling step, equilibrium conditions, etc.
Abstract: Wagner's classical treatment of internal oxidation (generic name allowing for reaction with oxygen, nitrogen, carbon or sulfur) assumed ideal conditions such as uninhibited dissolution of the gas, formation of spherical particles, diffusion of the oxidant in the solvent as the rate-controlling step, equilibrium conditions, etc. However, during the 45 years since his treatment, many observations have been made to complicate the idealized situation suggested by Wagner. This paper examines the most important modifications with respect to Wagner's original analysis. The following items are discussed. (a) The role of solute concentration: The parabolic kinetics are much higher than expected for Ni−Al alloys due to rapid interfacial diffusion of oxygen along the interfaces between cylindrical rods of Al2O3 (perpendicular to the surface) and the matrix. (b) Precipitate morphology: Spherical precipitates seem almost to be the exception. A wide variety of forms have been observed, including Widmanstatten platelets, cylindrical rods, hexagonal plates, dendritic or “fishbone” products, etc. The competition between nucleation and growth is useful to explain the observed structures. (c) Intergranular internal oxidation: Rapid oxygen diffusion in grain boundaries may lead to a wide variety of intergranular-precipitate structures. (d) Internal-oxide bands: Wavy, approximately parallel bands form at a finite distance beneath the surface in certain alloys having very reactive solutes, e.g., Ag−Mg. It is postulated that high stresses generated by precipitation play a major role. (e) Surface nodules of pure solvent metal: High stresses generated during precipitation cause extrusion of solute through dislocation pipes, leading to extensive nodule formation on either grain boundaries or on the grains (or both), depending on the alloy and oxidizing conditions. (f) Nonstoichiometric precipitates: Either hypo- or hyperstoichiometric particles can form as very small clusters in certain alloys (Ag−Al). The nature of precursors and changes in stoichiometry during reaction are discussed. (g) Trapping of oxidant: Diffusion of the oxidant may be slowed appreciably by trapping with the solute, although no precipitates need to form. Lower-than-expected kinetics (based on normal diffusivities of the oxidant) result. (h) High-solubility-product precipitates: Concentration profiles of solute, oxidant and precipitate are quite different than those expected for low-solubility-product precipitates as considered by Wagner. In particular, a variable mole fraction of precipitate exists, and further precipitation occurs in the reaction zone after the front has passed by. Linear kinetics have been observed for some Nb-base alloys at very high temperatures and low oxygen pressures. The rate-controlling step is the arrival of oxygen at the surface and not oxygen diffusion in the metal. (i) Dual oxidants: Two gases may diffuse·simultaneously and each forms its own product with the solute. The thermodynamically most-stable compound forms near the surface, and the less-stable compound deeper in the alloy. The less-stable compound is subsequently converted to the more-stable compound with a concomitant release of the second oxidant. Although numerous examples have been reported of systems which do not behave as predicted by Wagner, his theory still remains as the cornerstone of our understanding and is still the starting point for virtually every study in internal oxidation.

Journal ArticleDOI
TL;DR: In this article, the phase identification and surface morphology characterization of sputtering deposited copper thin films were performed by x-ray diffraction, Raman spectroscopy, and atomic force microscopy (AFM).
Abstract: The oxidation of sputtering deposited copper thin films has been investigated. Phase identification and surface morphology characterization were performed by x‐ray diffraction, Raman spectroscopy, and atomic force microscopy (AFM). The copper film is first oxidized to Cu2O below 300 °C and then to CuO at higher temperatures (≥300 °C) near the surface of the copper. AFM results show that the as‐deposited Cu film contains grains of similar sizes. After heat treatment at 200 °C, small grains are observed due to nucleation of the Cu2O phase. Further annealing in air leads to aggregation and growth of the grains whereas the grain boundary of the grains before coalescence is still evident. The Cu2O and CuO phases form alternating layers when the annealing temperature is 400 °C.