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Showing papers on "Grain boundary published in 2001"


Journal ArticleDOI
08 Mar 2001-Nature
TL;DR: The combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity, suggesting that this new superconductor class is not compromized by weak-link problems.
Abstract: The discovery of superconductivity at 39 K in magnesium diboride1, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance2 is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field–temperature (H–T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field3,4. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields3. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.

886 citations


Journal ArticleDOI
TL;DR: In this paper, the discovery of superconductivity at 39 K in MgB2 has raised many issues, such as whether this new superconductor resembles a high-temperature cuprate super-conductor or a low-treme metallic super-conductors in terms of its current carrying characteristics in applied magnetic fields.
Abstract: The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One of the central questions is whether this new superconductor resembles a high-temperature-cuprate superconductor or a low-temperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks[2]. Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive[3,4]. These studies of MgB2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains[3]. Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determined by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered.

760 citations


Journal ArticleDOI
TL;DR: In this article, the authors reviewed the creep deformation resistance and rupture life of high Cr ferritic steel with a tempered martensitic lath structure, and focused on the following three subjects: creep mechanism of the ferritic steels, its alloy design for further strengthening, and loss of its creep rupture strength after long-term use.
Abstract: The creep deformation resistance and rupture life of high Cr ferritic steel with a tempered martensitic lath structure are critically reviewed on the basis of experimental data. Special attention is directed to the following three subjects: creep mechanism of the ferritic steel, its alloy design for further strengthening, and loss of its creep rupture strength after long-term use. The high Cr ferritic steel is characterized by its fine subgrain structure with a high density of free dislocations within the subgrains. The dislocation substructure is the most densely distributed obstacle to dislocation motion in the steel. Its recovery controls creep rate and rupture life at elevated temperatures. Improvement of creep strength of the steel requires a fine subgrain structure with a high density of free dislocations. A sufficient number of pinning particles (MX particles in subgrain interior and M 23 C 6 particles on sub-boundaries) are necessary to cancel a large driving force for recovery due to the high dislocation density. Coarsening and agglomeration of the pinning particles have to be delayed by an appropriate alloy design of the steel. Creep rupture strength of the high Cr ferritic steel decreases quickly after long-term use. A significant improvement of creep rupture strength can be achieved if we can prevent the loss of rupture strength. In the steel tempered at high temperature, enhanced recovery of the subgrain structure along grain boundaries is the cause of the premature failure and the consequent loss of rupture strength. However, the scenario is not always applicable. Further studies are needed to solve this important problem of high Cr ferritic steel. MX particles are necessary to retain a fine subgrain structure and to achieve the excellent creep strength of the high Cr ferritic steel. Strengthening mechanism of the MX particles is another important problem left unsolved.

610 citations


Journal ArticleDOI
TL;DR: In this article, a constitutive equation based on these experimental results that includes flow laws for these four creep mechanisms is described. But this equation is in excellent agreement with published laboratory creep data for coarse-grained samples at high temperatures.
Abstract: Creep experiments on fine-grained ice reveal the existence of three creep regimes: (1) a dislocation creep regime; (2) a superplastic flow regime in which grain boundary sliding is an important deformation process; and (3) a basal slip creep regime in which the strain rate is limited by basal slip. Dislocation creep in ice is likely climb-limited, is characterized by a stress exponent of 4.0, and is independent of grain size. Superplastic flow is characterized by a stress exponent of 1.8 and depends inversely on grain size to the 1.4 power. Basal slip limited creep is characterized by a stress exponent of 2.4 and is independent of grain size. A fourth creep mechanism, diffusional flow, which usually occurs at very low stresses, is inaccessible at practical laboratory strain rates even for our finest grain sizes of approximately 3 micrometers. A constitutive equation based on these experimental results that includes flow laws for these four creep mechanisms is described. This equation is in excellent agreement with published laboratory creep data for coarse-grained samples at high temperatures. Superplastic flow of ice is the rate-limiting creep mechanism over a wide range of temperatures and grain sizes at stresses less than or equal to 0.1 MPa, conditions which overlap those occurring in glaciers, ice sheets, and icy planetary interiors.

608 citations


Journal ArticleDOI
TL;DR: In this article, the microstructures and dislocation configurations in nanostructured Cu processed by a new technique, repetitive corrugation and straightening (RCS), were studied using transmission electron microcopy (TEM) and high resolution TEM.

516 citations


Journal ArticleDOI
Dierk Raabe1, M. Sachtleber1, Z. Zhao1, Franz Roters1, Stefan Zaefferer1 
TL;DR: In this article, a polycrystalline aluminum sample with a quasi-2D single layer of coarse grains is plastically deformed in a channel die plane strain set-up at ambient temperature and low strain rate.

390 citations


Journal ArticleDOI
TL;DR: In this paper, the authors studied the nucleation of extended dislocations from the grain boundaries in nanocrystalline aluminum by molecular-dynamics simulation and found that the length of the stacking fault connecting the two Shockley partials that formed the extended dislocation depends not only on the stacking-fault energy but also on the resolved nucleation stress.

369 citations


Journal ArticleDOI
TL;DR: In this paper, the grain boundary electrical properties of high purity ZrO 2 ceramic materials doped with 2, 3, and 8 mol % Y 2 O 3, and with 0.4 mol % Al 2 O3 were theoretically analyzed in the temperature range of 200 to 500°C.
Abstract: The grain boundary electrical properties of high purity ZrO 2 ceramic materials doped with 2, 3, and 8 mol % Y 2 O 3 , and 8 mol % Y 2 O 3 co-doped with 0.4 mol % Al 2 O 3 were studied in the temperature range of 200 to 500°C by electrochemical techniques and were theoretically analyzed. Although the presence of a siliceous phase is shown to be a major cause for the grain boundary blocking effect, the grain boundary properties appear to he significantly influenced by space charges, particularly in materials of high purity. The oxygen vacancy distribution and the grain boundary resistivity were calculated for 8 mol % Y 2 O 3 doped ZrO 2 by assuming double Schottky barriers, and the results were compared with the experiment. It is shown that reasonable space charge potentials lead to grain boundary effects which are consistent with the experimental features. In contrary to the bulk in which defect associates prevail (at temperatures <560°C), in the boundary regions, association effects can be assumed to be much less pronounced due to the vacancy depletion.

368 citations


Journal ArticleDOI
TL;DR: In this paper, the authors compare the predictions implied by their hypothesis with those of other models for dynamic recrystallization and conclude that a temperature dependence of the relationship between recrystized grain size and flow stress cannot be neglected a priori.
Abstract: It is widely believed that grain size reduction by dynamic recrystallization can lead to major rheological weakening and associated strain localization by bringing about a switch from grain size insensitive dislocation creep to grain size sensitive diffusion creep. Recently, however, we advanced the hypothesis that, rather than a switch, dynamic recrystallization leads to a balance between grain size reduction and grain growth processes set up in the neighborhood of the boundary between the dislocation creep field and the diffusion creep field. In this paper, we compare the predictions implied by our hypothesis with those of other models for dynamic recrystallization. We also evaluate the full range of models against experimental data on a variety of materials. We conclude that a temperature dependence of the relationship between recrystallized grain size and flow stress cannot be neglected a priori. This should be taken into account when estimating natural flow stresses using experimentally calibrated recrystallized grain size piezometers. We also demonstrate experimental support for the field boundary hypothesis. This support implies that significant weakening by grain size reduction in localized shear zones is possible only if caused by a process other than dynamic recrystallization (such as syntectonic reaction or cataclasis) or if grain growth is inhibited.

358 citations


Journal ArticleDOI
TL;DR: In this paper, a fast and non-destructive method for generating three-dimensional maps of the grain boundaries in undeformed polycrystals is presented, which relies on tracking of micro-focused high-energy X-rays.
Abstract: A fast and non-destructive method for generating three-dimensional maps of the grain boundaries in undeformed polycrystals is presented. The method relies on tracking of micro-focused high-energy X-rays. It is verified by comparing an electron microscopy map of the orientations on the 2.5 × 2.5 mm surface of an aluminium polycrystal with tracking data produced at the 3DXRD microscope at the European Synchrotron Radiation Facility. The average difference in grain boundary position between the two techniques is 26 µm, comparable with the spatial resolution of the 3DXRD microscope. As another extension of the tracking concept, algorithms for determining the stress state of the individual grains are derived. As a case study, 3DXRD results are presented for the tensile deformation of a copper specimen. The strain tensor for one embedded grain is determined as a function of load. The accuracy on the strain is Δ∊ ≃ 10−4.

347 citations


Journal ArticleDOI
TL;DR: In this paper, the microstructural changes of an aged Al-1.7 at% Cu alloy associated with severe plastic deformation have been studied by transmission electron microscopy (TEM) and energy-filtered transmission electron microscope (EF-TEM).

Journal ArticleDOI
TL;DR: In this paper, the grain boundary distributions were analyzed with special emphasis on grain boundary character along intergranular stress corrosion cracks and at crack arrest points, and it was established that only coherent twin Σ3 boundaries could be considered as special boundaries with regard to crack resistance.

Journal ArticleDOI
TL;DR: In this article, a commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size, which was subsequently degassed, hot isostatically pressed, and extruded.
Abstract: A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al2O3 particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.

Journal ArticleDOI
TL;DR: In this paper, the microstructure and fatigue properties of three model AS7G03 cast aluminium alloys containing artificial pore have been studied by using Synchrotron X-ray tomography.
Abstract: The microstructure and fatigue properties of three model AS7G03 cast aluminium alloys containing artificial pores have been studied. Synchrotron X-ray tomography has been used to characterise in three dimensions the pore population in the alloys. The development of fatigue cracks in relation with local crystallography has been studied by means of electron back scattered diffraction (EBSD). Both the average number of cycles to failure and the lifetime scatter depend on the pore content specially at high stress level. The mechanism leading to the initiation of a crack from a pore has been identified. The crack propagation at high stress level appears to be quite insensitive to microstructural barriers and can be reasonably well described by a Paris type law. At low stresses, however, short cracks are often observed to be stopped at grain boundaries and the fatigue life is no longer predicted by a simple propagation law.

Journal ArticleDOI
TL;DR: In this article, the interrelationship between defect chemistry, non-stoichiometry, grain boundary transport and chemical stability of proton conducting perovskites (doped alkaline earth cerates and zirconates) has been investigated.
Abstract: The interrelationship between defect chemistry, non-stoichiometry, grain boundary transport and chemical stability of proton conducting perovskites (doped alkaline earth cerates and zirconates) has been investigated. Non-stoichiometry, defined as the deviation of the A : M molar ratio in AMO3 from 1 : 1, dramatically impacts conductivity, sinterability and chemical stability with respect to reaction with CO2. In particular, alkaline earth deficiency encourages dopant incorporation onto the A-atom site, rather than the intended M-atom site, reducing the concentration of oxygen vacancies. Transport along grain boundaries is, in general, less favorable than transport through the bulk, and thus only in fine-grained materials does microstructure impact the overall electrical properties. The chemical stability of high conductivity cerates is enhanced by the introduction of Zr. The conductivity of BaCe0.9−x Zr x M0.1O3 perovskites monotonically decreases with increasing x (increasing Zr content), with the impact of Zr substitution increasing in the order M = Yb → Gd → Nd. Furthermore, the magnitude of the conductivity follows the same sequence for a given zirconium content. This result is interpreted in terms of dopant ion incorporation onto the divalent ion site.

Journal ArticleDOI
TL;DR: In this paper, a general physical model has been developed for the precipitation of Al3Zr dispersoids in aluminium alloys, which has been used to study the effects of homogenization conditions and alloy composition on dispersoid formation and has been shown to be a powerful tool for optimising the dispersoid distribution in 7xxx series aluminum alloys.

Journal ArticleDOI
D.H Jeong1, Francisco Gonzalez, Gino Palumbo, K.T. Aust1, Uwe Erb1 
TL;DR: In this paper, the effect of grain size reduction on the wear resistance of electrodeposited nanoc-rystalline pure nickel coatings was investigated quantitatively by the Taber abrasive wear test, astandard test often applied in industrial testing.

Journal ArticleDOI
TL;DR: In this article, density-functional-based tight-binding molecular-dynamics calculations of high-energy high-angle twist (100) grain boundaries in diamond were used as a model for the UNCD grain boundaries.
Abstract: Ultrananocrystalline diamond (UNCD) films grown from hydrogen-poor plasmas have grain sizes of 3-10 nm, resulting in a large number of grain boundaries. We repon on density-functional-based tight-binding molecular-dynamics calculations of high-energy high-angle twist (100) grain boundaries in diamond as a model for the UNCD grain boundaries. About one-half of the carbons in the grain boundary are threefold coordinated and are responsible for states introduced into the band gap. Simulations were also performed for N, Si, and H impurities in (100) twist grain boundaries where substitution energies, optimized geometries, and electronic structures were calculated. Substitution energies were found to be substantially lower for the grain boundaries compared to the bulk diamond crystal. Nitrogen increases the number of threefold-coordinated carbons while hydrogen saturates dangling bonds. The electronic structure of UNCD is characterized by a large number of states in the band gap attributed to the bonding disorder and impurities in the grain boundaries.

Journal ArticleDOI
TL;DR: In this article, a phenomenological constitutive equation predicting the effect of grain size on the yield stress of metals is discussed and extended to the nanocrystalline regime, which is shown to predispose the material to shear localization.

Journal ArticleDOI
TL;DR: In this paper, the effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles and the weld of Al alloy 5083 had a slightly greater hardness in the stir zone than the base material.
Abstract: Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld of Al alloy 1080 had a slightly greater hardness in the stir zone than the base material. The maximum hardness was located in the thermomechanically affected zone (TMAZ). The stir zone consisted of recrystallized fine grains, while the TMAZ had a recovered grain structure. The increase in hardness in the stir zone can be explained by the Hall-Petch relationship. On the other hand, the hardness profiles in the weld of Al alloy 5083 were roughly homogeneous. Friction-stir welding created the fine recrystallized grains in the stir zone and recovered grains in the TMAZ in the weld of this alloy. The stir zone and the TMAZ had slightly higher dislocation densities than the base material. Many small Al6(Mn,Fe) particles were detected in all the grains of the weld. The hardness profiles could not be explained by the Hall-Petch relationship, but rather by Orowan hardening. The results of the present study suggest that the hardness profile is mainly affected by the distribution of small particles in friction-stir welds of Al alloys containing many such particles.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the grain boundary mobilities during grain growth are affected by solute and impurity chemistry, chemical fugacity of trace and major elements, pore size and number, polygonal pore fluid chemistry, the presence of melts, and presence of solid second phases, as well as temperature and pressure.
Abstract: Static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature. Given the relative simplicity of the protocol for grain growth experiments, measurements of grain boundary mobility show surprising variations. Boundary mobilities during grain growth are affected by solute and impurity chemistry, chemical fugacity of trace and major elements, pore size and number, pore fluid chemistry, the presence of melts, and the presence of solid second phases, as well as temperature and pressure. All of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments. Provided that the necessary kinetics parameters are known, bounds may be placed on the interface mobility when pores, partial melts, or solutes are present. To predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature.

Journal ArticleDOI
TL;DR: In this article, the effects of solution temperatures and cooling rate on Widmanstatten morphologies and on mechanical properties have been determined, showing that the Widman-statten plates size increases when the cooling rate decreases and a certain decrease of α-allotromorphous phase size at the grain boundaries can be observed when increased.

Journal ArticleDOI
TL;DR: In this paper, the influence of grain sizes on the processing of thin metal sheets was investigated in two different ways: first, by reduction of the sheet thickness at a constant grain size (investigated in tension), and secondly, by changing the grain size at constant sheet thickness, and the results showed that in both the experiments, the yield strength as well as the maximum load decrease with a decreasing number of grains over the thickness.

Journal ArticleDOI
TL;DR: In this paper, a three-step manufacturing process to fabricate complex shaped components can be envisaged: cast sheet or hot-pressed powder metallurgy sheet + friction stir processing + superplastic forging or forming.
Abstract: Friction stir processing is a new thermo-mechanical processing technique that leads to a microstructure amenable for high strain rate superplasticity in commercial aluminum alloys. Friction stirring produces a combination of very fine grain size and high grain boundary misorientation angles. Preliminary results on a 7075 Al demonstrate high strain rate superplasticity in the temperature range of 430-510 °C. For example, an elongation of >1000 % was observed at 490 °C and 1 × 10 -2 s -1 . This demonstrates a new possibility to economically obtain a superplastic microstructure in commercial aluminum alloys. Based on these results, a three-step manufacturing process to fabricate complex shaped components can be envisaged: cast sheet or hot-pressed powder metallurgy sheet + friction stir processing + superplastic forging or forming.

Journal ArticleDOI
TL;DR: It is shown that modest levels of atomic disorder induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strengths, and it is anticipated that either chemical doping or mechanical processing should generate similar levels of disorder, and so achieve performance that is technologically attractive in an economically viable way.
Abstract: A relatively high critical temperature, Tc, approaching 40 K, places the recently-discovered superconductor magnesium diboride (MgB2) intermediate between the families of low- and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB2 is that in bulk material the critical current density, Jc, appears to drop more rapidly with increasing magnetic field than it does in the HTS phases. The magnitude and field dependence of Jc are related to the presence of structural defects that can "pin" the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the paucity of suitable defects in MgB2 that causes the rapid decay of Jc with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase Jc significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB2 by economically-viable routes.

Journal ArticleDOI
TL;DR: Equal channel angular pressing (ECAP) was attempted at room temperature to refine grain sizes of six different commercial Al alloys, 1100, 2024, 3004, 5083, 6061 and 7075.

Journal ArticleDOI
TL;DR: In this paper, thermal conductivity measurements for low-pressure chemical-vapor deposition (LPCVD) polysilicon layers of thickness near 1 /spl mu/m doped with boron and phosphorus at concentrations between 2.0/spl times/10/sup 18/ cm/sup -3/ and 4.1/spl µ/m for temperatures from 20 K to 320 K.
Abstract: The thermal conductivities of doped polysilicon layers depend on grain size and on the concentration and type of dopant atoms. Previous studies showed that layer processing conditions strongly influence the thermal conductivity, but the effects of grain size and dopant concentration were not investigated in detail. The current study provides thermal conductivity measurements for low-pressure chemical-vapor deposition (LPCVD) polysilicon layers of thickness near 1 /spl mu/m doped with boron and phosphorus at concentrations between 2.0/spl times/10/sup 18/ cm/sup -3/ and 4.1/spl times/10/sup 19/ cm/sup -3/ for temperatures from 20 K to 320 K. The data show strongly reduced thermal conductivity values at all temperatures compared to similarly doped single-crystal silicon layers, which indicates that grain boundary scattering dominates the thermal resistance. A thermal conductivity model based on the Boltzmann transport equation reveals that phonon transmission through the grains is high, which accounts for the large phonon mean free paths at low temperatures. Algebraic expressions relating thermal conductivity to grain size and dopant concentration are provided for room temperature. The present results are important for the design of MEMS devices in which heat transfer in polysilicon is important.

Journal ArticleDOI
TL;DR: In this article, the effects of 60 MeV 12 C ion irradiation on nanocrystalline gold (nano-Au) were studied and the experimental results showed that the irradiation-produced defects in nano-AU are thermally unstable because of the existence of a large volume fraction of grain boundaries.

Journal ArticleDOI
TL;DR: In this article, the motion of planar symmetrical and asymmetrical tilt boundaries in high-purity aluminium with -and -tilt axes under the influence of an external mechanical stress field was investigated.

Journal ArticleDOI
TL;DR: In this article, a study on the accuracy of cohesive models for capturing dynamic fragmentation of ceramic microstructures is presented, which consists of a combined experimental/numerical approach in which microcracking and damage kinetics are examined by means of plate impact recovery experiments.