Showing papers on "Grain boundary published in 2000"

Sintering dense nanocrystalline ceramics without final-stage grain growth

Abstract: Sintering is the process whereby interparticle pores in a granular material are eliminated by atomic diffusion driven by capillary forces. It is the preferred manufacturing method for industrial ceramics. The observation of Burke and Coble that certain crystalline granular solids could gain full density and translucency by solid-state sintering was an important milestone for modern technical ceramics. But these final-stage sintering processes are always accompanied by rapid grain growth, because the capillary driving forces for sintering (involving surfaces) and grain growth (involving grain boundaries) are comparable in magnitude, both being proportional to the reciprocal grain size. This has greatly hampered efforts to produce dense materials with nanometre-scale structure (grain size less than 100 nm), leading many researchers to resort to the 'brute force' approach of high-pressure consolidation at elevated temperatures. Here we show that fully dense cubic Y2O3 (melting point, 2,439 degrees C) with a grain size of 60 nm can be prepared by a simple two-step sintering method, at temperatures of about 1,000 degrees C without applied pressure. The suppression of the final-stage grain growth is achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. Such a process should facilitate the cost-effective preparation of other nanocrystalline materials for practical applications.

Superplastic extensibility of nanocrystalline copper at room temperature

Abstract: A bulk nanocrystalline (nc) pure copper with high purity and high density was synthesized by electrodeposition. An extreme extensibility (elongation exceeds 5000%) without a strain hardening effect was observed when the nc copper specimen was rolled at room temperature. Microstructure analysis suggests that the superplastic extensibility of the nc copper originates from a deformation mechanism dominated by grain boundary activities rather than lattice dislocation, which is also supported by tensile creep studies at room temperature. This behavior demonstrates new possibilities for scientific and technological advancements with nc materials.

Temperature and gate voltage dependence of hole mobility in polycrystalline oligothiophene thin film transistors

Abstract: We have performed current–voltage measurement on polycrystalline sexithiophene (6 T) thin film transistors at temperatures ranging from 10 to 300 K. A method is developed to extract the carrier mobility from an analysis of the transfer characteristics. In particular, data are corrected for contact resistance. The carrier mobility is found to increase quasilinearly with gate voltage at room temperature. The dependence becomes superlinear at low temperatures. The temperature dependence shows three domains. For 100 K

An experimental study of the recrystallization mechanism during hot deformation of aluminium

Abstract: Discontinuous dynamic recrystallization (involving nucleation and grain growth) is rarely observed in metals with high stacking fault energies, such as aluminium. In this metal, two other types of recrystallization have been observed: continuous dynamic recrystallization (CDRX, i.e. the transformation of subgrains into grains); and geometric dynamic recrystallization (due to the evolution of the initial grains). The main purpose of this work was to bring clearly into evidence and to better characterize CDRX. Uniaxial compression tests were carried out at 0.7 T m and 10 −2 s −1 on three types of polycrystalline aluminium: a pure aluminium (1199), a commercial purity aluminium (1200) and an Al-2.5wt.%Mg alloy (5052), and also on single crystals of pure aluminium. In addition, 1200 aluminium specimens were strained in torsion. The deformed microstructures were investigated at various strains using X-ray diffraction, optical microscopy, scanning electron microscopy and electron back-scattered diffraction. Observations of the single crystalline samples confirm that subgrain boundaries can effectively transform into grain boundaries, especially when the initial orientation is unstable. In the case of polycrystalline specimens, after separating the effects of the initial and new grain boundaries, it turns out that CDRX operates faster in the 1200 aluminium compared to the two other grades. Moreover, it appears that the strain path does not alter noticeably the CDRX kinetics.

Sintering activation by external electrical field

Abstract: Field assisted sintering technique (FAST) is a non-conventional powder consolidation method in which densification is enhanced by the application of an electrical discharge combined with resistance heating and pressure. Interest in FAST is motivated by its ability to consolidate a large variety of powder materials to high densities in short times. Full densification of metal and ceramic powders has been achieved within minutes, with a reduced number of processing steps, no need for sintering aids and more flexibility in powder handling. Although the electrical discharge effects have not been completely elucidated, distinct surface effects created by micro-discharges have been noticed in FAST consolidated specimens such as atomically clean grain boundaries and new resistivity peaks in superconductors. On-going experimental and theoretical studies to provide more quantitative insight into the relevant FAST mechanisms are presented.

Investigations and applications of severe plastic deformation

Abstract: Preface Introduction I: Innovations in Severe Plastic Deformation Processing and Process Modeling Severe Plastic Deformation of Materials by Equal Channel Angular Extrusion (ECAE) RE Goforth, et al Severe Plastic Deformation of Steels: Structure, Properties and Techniques SV Dobatkin Application of ECAP - Technology for Producing Nano- and Microcrystalline Materials VI Kopylov Severe Deformation Based Process for Grain Subdivision and Resulting Microstructures AK Ghosh, W Huang Modeling of Continual Flows in Angular Domains BV Koutcheryaev Synthesis and Characterization of Nanocrystalline Tial Based Alloys ON Senkov, FH Froes Formation of Submicrocrystalline Structure in TiAl and Ti3Al Intermetallics via Hot Working G Salishchev, et al Severe Plastic Deformation Processes Modeling and Workability SL Semiatin, et al The Effect of Strain Path on the Rate of Formation of High Angle Grain Boundaries During ECAE PB Prangnell, et al Thermomechanical Conditions for Submicrocrystalline Structure Formation by Severe Plastic Deformation FZ Utyashev, et al II: Microstructural Characterization and Modeling of Severe Plastic Deformation Materials Strengthening Processes of Metals by Severe Plastic Deformation Analyses with Electron and Synchrotron Radiation MJ Zehetbauer Size Distribution of Grains or Subgrains, Dislocation Density and Dislocation Character by Using the Dislocation Model of Strain Anisotropy in X-Ray Line Profile Analysis T Ungar X Ray-Studies and Computer Simulation of Nanostructured SPD Metals IV Alexandrov An Analysis of Heterophase Structures of Ti3Al, TiAl, Ni3Al Intermetallics Synthesized by the Method of the SphericalShock Wave Action BA Greenberg, et al Structural Changes Induced by Severe Plastic Deformation of Fe- and Co-Based Amorphous Alloys N Noskova, et al Structure of Grains and Internal Stress Fields in Ultrafine Grained NI Produced by Severe Plastic Deformation NA Koneva, et al Crystal Lattice Distorsions in Ultrafine-Grained Metals Produced by Severe Plastic Deformation AN Tyumentsev, et al Grain and Subgrain Size-Distribution and Dislocation Densities in Severely Deformed Copper Determined by a New Procedure of X-Ray Line Profile Analysis T Ungar, et al Calculation of Energy Intensity and Temperature of Mechanoactivation Process in Planetary Ball Mill by Computer Simulation EV Shelekhov, et al III: Microstructure Evolution During Severe Plastic Deformation Processing Microstructural Evolution During Processing by Severe Plastic Deformation TG Langdon, et al Characterization of Ultrafine-Grained Structures Produced by Severe Plastic Deformation Z Horita, et al Fragmentation in Large Strain Cold Rolled Aluminium as Observed by Synchrotron X-Ray Bragg Peak Profile Analysis (SXPA), Electron Back Scatter Patterning (EBSP) and Transmission Electron Microscopy (TEM) E Schafler, et al Influence of Thermal Treatment and Cyclic Plastic Deformation on the Defect Structure in Ultrafine-Grained Nickel E Thiele, et al Nanostructure State as Nonequilibrium Transition in Grain Boundary Defects in SPD Condition OB Naimark Texture, Structural Evolution and Mechanical Properties in AA5083 Processed by ECAE L Dupuy, et al A TEM-Based Disclination Model for the Substructure Evolution under Severe Plastic Deformation M Seefeldt, et al Physical Mesomechanics of Ultrafine-Grained Metals VE Panin Microstructure Evo

Hot working of commercial Ti–6Al–4V with an equiaxed α–β microstructure: materials modeling considerations

Abstract: The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s−1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol−1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol−1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s−1 and these are manifested as adiabatic shear bands in the α–β regime.

Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si3N4/a- and nc-TiSi2 nanocomposites with HV= 80 to ≥ 105 GPa

Abstract: Multiphase nanocomposite coatings (3–20 μm thick) consisting of nanocrystalline TiN, amorphous Si 3 N 4 , and amorphous and nanocrystalline TiSi 2 , nc-TiN/a-SiN x /a- and nc-TiSi 2 were deposited on steel substrates by means of plasma CVD. The load-independent Vickers microhardness from 80 to >105 GPa was measured by the load–depth sensing technique for applied loads between 30 and 200 mN and verified by measuring the size of the remaining plastic indentation using SEM. The results of a complex analysis provide a consistent picture of the nature of the grain boundaries which determines the hardness in the whole range of silicon content between approximately 3 and 22 at.%. At a high discharge current density of ≥2.5 mA/cm 2 the a-Si 3 N 4 forms the grain boundaries and the nanocomposites are superhard (40–50 GPa) as we reported earlier. At a lower current density of ≤1 mA/cm 2 a mixture of TiSi 2 and Si 3 N 4 is formed. With increasing Si-content the amount of a-TiSi 2 in the grain boundaries of the TiN nanocrystals increases, and above 10 at.% of Si approximately 3 nm small TiSi 2 nanocrystals precipitate. The hardness depends critically and in a complex way on the Si 3 N 4 content and the TiSi 2 /Si 3 N 4 ratio. The ultrahardness 1 of ≥80 GPa is achieved when the surface of the TiN nanocrystals is covered with approximately one monolayer of Si 3 N 4 . Under these conditions the ultrahardness of 80–100 GPa depends on the amount of a- and nc-TiSi 2 .

Electroplasticity in metals and ceramics

Abstract: The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75TM and on the superplasticity of fine-grained oxides (MgO, Al2O3 and ZrO2) at T>0.5TM are considered. Regarding NaCl, at T≤0.5TM an electric field E≥10 kV cm−1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm−1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5TM, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5TM, an internal electric field E≤0.3 kV cm−1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.

Magnetic properties of nanostructured ferrimagnetic zinc ferrite

Abstract: Nanostructured ZnFe2O4 ferrites with different grain sizes were prepared by high energy ball milling for various milling times. Both the average grain size and the root mean square strain were estimated from the x-ray diffraction line broadening. The lattice parameter initially decreases slightly with milling and it increases with further milling. The magnetization is found to increase as the grain size decreases and its large value is attributed to the cation inversion associated with grain size reduction. The Fe-57 Mossbauer spectra were recorded at 300 K and 77 K for the samples with grain sizes of 22 and 11 nm. There is no evidence for the presence of the Fe2+ charge state. At 77 K the Mossbauer spectra consist of a magnetically ordered component along with a doublet due to the superparamagnetic behaviour of small crystalline grains with the superparamagnetic component decreasing with grain size reduction. At 4.2 K the sample with 11 nm grain size displays a magnetically blocked state as revealed by the Mossbauer spectrum. The Mossbauer spectrum of this sample recorded at 10 K in an external magnetic field of 6 T applied parallel to the direction of gamma rays clearly shows ferrimagnetic ordering of the sample. Also, the sample exhibits spin canting with a large canting angle, maybe due to a spin-glass-like surface layer or grain boundary anisotropies in the material.

Microstructural evolution, microhardness and thermal stability of HPT-processed Cu

Abstract: Coarse-grained copper was subject to high-pressure torsion (HPT) and thermal treatment to study the effects of increasing amounts of deformation and subsequent annealing on the evolution of microstructure and microhardness. Cellular subgrains with low-angle grain boundaries were first formed at low strain. Some of the low-angle subgrain boundaries transformed to high-angle grain boundaries at higher strains, refining the average grain size from 200 μm to 150 nm. X-ray diffraction patterns showed the formation of crystallographic texture. Microhardness increased monotonically with increasing torsional strain. High internal stress and nonequilibrium grain boundaries were observed in unannealed samples. Annealing as-deformed samples at temperatures as low as 50°C decreased the microhardness, indicating a very low thermal stability of the deformation induced microstructures. Differential scanning calorimetry (DSC) revealed an exothermal peak between 180 and 280°C, caused by recrystallization. Annealing twins were also formed during recrystallization.

Grain-boundary structures in polycrystalline metals at the nanoscale

Abstract: We present a detailed analysis of grain-boundary structures in computer-generated Cu and Ni three-dimensional nanocrystalline samples. The study includes both totally random and textured microstructures with grain sizes in the range of 5\char21{}12 nm. A detailed direct visualization technique is used at the atomic scale for studying the grain-boundary structural features. The study focuses on determining the presence of regions in the boundary exhibiting order and structural units normally expected for high-angle boundaries. For low-angle boundaries we investigate the presence of dislocation networks accommodating the misfit between the grains. A significant degree of crystalline order is found for all the boundaries studied. The highest degree of structural order was identified for boundaries with misfits within about 10\ifmmode^\circ\else\textdegree\fi{} deviation from the perfect twin. These grain boundaries contain a repeated building structure consisting of structural units typical of a $\ensuremath{\Sigma}=3$ symmetrical tilt twin boundary and highly disordered steps between those structural units. For all other types of misfit, we also observe some degree of structural coherence, and misfit accommodation occurs in a regular pattern. The cases studied include grain boundaries with a high-index common axis and show structural coherency that is independent of the grain size. Similar results are obtained for textured samples containing only low-angle grain boundaries, where regular dislocation arrays that are typical of larger grain materials are observed. These results provide evidence against the view of grain boundaries in nanocrystals as highly disordered, amorphous, or liquidlike interfaces. The results suggest that the grain-boundary structure in nanocrystalline materials is actually similar to that found in larger grain polycrystals.

Size effects in the electrical resistivity of polycrystalline nanowires

Abstract: Grain-boundary and surface scattering are known to increase the electrical resistivity of thin metallic films and wires. The length scale at which these produce appreciable effects is of the order of the electronic mean free path. For the well-studied case of thin films, both mechanisms can, in principle, be used to explain the observed thickness dependence on resistivity. In order to evaluate which of these mechanisms is more relevant, we have carried out an experimental study of the width dependence of the resistivity of narrow thin-film polycrystalline gold wires (nanowires), and computed the expected behavior on the basis of both surface and grain-boundary scattering mechanisms independently. We find that the resistivity increases as wire width decreases in a manner which is dependent on the mean grain size and cannot be explained adequately by either model alone. We propose a modification to the well-known model of Mayadas and Shatzkes, incorporating the variation of mean grain size on wire dimensions.

Dislocation and diffusion creep of synthetic anorthite aggregates

Abstract: Synthetic fine-grained anorthite aggregates were deformed at 300 MPa confining pressure in a Paterson-type gas deformation apparatus. Creep tests were performed at temperatures ranging from 1140 to 1480 K, stresses from 30 to 600 MPa, and strain rates between 2×10−6 and 1×10−3 s−1. We prepared samples with water total contents of 0.004 wt % (dry) and 0.07 wt % (wet), respectively. The wet (dry) material contained 120 MPa we found a stress exponent of n = 3 irrespective of the water content, indicating dislocation creep. However, the activation energy of wet samples is 356±9 kJ mol−1, substantially lower than for dry specimens with 648±20 kJ mol−1. The preexponential factor is log A = 2.6 (12.7) MPa−n s−1 for wet (dry) samples. Microstructural observations suggest that grain boundary migration recrystallization is important in accommodating dislocation creep. In the low-stress regime we observed a stress exponent of n = 1, suggesting diffusion creep. The activation energies for dry and wet samples are 467 ±16 and 170 ± 6 kJ mol−1, respectively. Log A is 12.1 MPa−n μmm s−1 for the dry material and 1.7 MPa−n μmm s−1 for wet anorthite. The data show that the strengths of anorthite aggregates decrease with increasing water content in both the dislocation and diffusion creep regimes. A comparison of the creep data of synthetic plagioclase from this study with published data for feldspar, olivine, and quartz indicates a linear relationship between activation energy and log A similar to the suggested compensation law for diffusion in silicates.

Enhanced supercurrent density in polycrystalline YBa2Cu3O7-δ at 77 K from calcium doping of grain boundaries

Abstract: With the discovery of high-temperature superconductivity1, it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization But it was soon found that the critical current density Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small1,2 In many materials, Jc is suppressed at grain boundaries2,3,4, by phenomena such as interface charging and bending of the electronic band structure5,6,7,8,9 Partial replacement (‘doping’) of the yttrium in YBa2Cu3O7-δ with calcium has been used to increase grain-boundary Jc values substantially, but only at temperatures much lower than 77 K (ref 9) Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of Jc at 77 K that far exceed previously published values Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures