Showing papers on "Grain boundary published in 1991"

The dependence of grain size on stress during dynamic recrystallisation

Abstract: The steady state grain size during dynamic recrystallisation by grain boundary migration is shown to be simply related to deformation stress for a number of metals and minerals. The data plotted on a scale of grain size (D) divided by Burger's vector (b) against stress (σ) divided by shear modulus (μ) fills in a remarkably narrow range bounded by loci of form σ μ ( D b 2/3 = K with K = 1 and 10. Models of the dynamic recrystallisation process are developed to show that such a relation can be predicted by considering a dynamic balance between the rate of formation of the deformation substructure and the mean velocity of recrystallising grain boundaries. Hence providing a physical basis for the empirical relation derived from the normalised plot of experimental results.

α'-Sialon ceramics : a review

Abstract: a' -Sialon ceramics promise the possibility of a reduction of the amount of glassy grain boundary phases by incorporating the oxides, present in the starting mixture as either sintering additives or impurities, into its rmallattice. It has been shown that techniques such as gas pressure sintering can be used successfully to obtain fully reacted and dense a'- or (a' + s')-sialons. By combination of suitable modifier cations and heat treatment the amount of grain boundary phases can be kept very low. This may lead to improved mechanical properties, especially at elevated temperatures. The equiaxed microstructure of a' -sialons, compared with the needlelike structure of s'sialons, leads to specific differences in mechanical behavior. However, at this moment the complicated interrelationships between the formation sequence, fabrication conditions, properties, and microstructures are still insufficiently understood. Yet, even with the resent-day materials, wear resistance, high-temperature mechanical strength, thermal shock resistance, and oxidation resistance are such that further studies are worth-while.

Bi‐epitaxial grain boundary junctions in YBa2Cu3O7

Abstract: We have developed a new way of making grain boundary junctions in YBa2Cu3O7 thin films by controlling the in‐plane epitaxy of the deposited film using seed and buffer layers. We produce 45° grain boundaries along photolithographically defined lines. The typical value of the critical current density of the junctions is 103–104 A/cm2 at 4.2 K and 102–103 A/cm2 at 77 K, while the rest of the film has a critical current density of 1–3×106 A/cm2 at 77 K. The current‐voltage characteristics of the junctions show resistively shunted junction behavior and we have used them to fabricate dc superconducting quantum interference devices (SQUIDs) which show modulation at temperatures well above 77 K. This is the first planar high Tc Josephson junction technology that appears readily extendable to high Tc integrated circuits.

Intrinsic stress in diamond films prepared by microwave plasma CVD

Abstract: The stress in diamond films prepared by microwave plasma CVD was investigated as a function of methane concentration (0.2%–3.0%) and deposition temperature (600–900 °C). Tensile and compressive total (thermal and intrinsic) stress were observed, depending on the deposition conditions. The thermal stress is compressive and relatively constant (0.215–0.275 GPa) over the temperature range investigated. The intrinsic stress is tensile and its origin is interpreted in terms of the grain boundary relaxation model. Calculations indicate a value of 0.84 GPa, using the grain boundary model, which is in fair agreement with the measured value. For the methane series, the tensile intrinsic stress decreases with increasing the methane fraction. The increasing compressive stress is ascribed to increased impurity (hydrogen and nondiamond phase carbon) incorporation with increasing methane fraction. 15N nuclear reaction analysis shows a linear correlation between hydrogen in the film and methane in the supply gas while spectroscopic ellipsometry shows a direct correlation between optically absorbing nondiamond (sp2) carbon incorporation and methane. For the temperature series, the intrinsic tensile stress increases with deposition temperature. The increase is ascribed to decreasing sp2 C incorporation with temperature, as confirmed by spectroscopic ellipsometry measurements.

Conductance, work function and catalytic activity of SnO2-based gas sensors

Abstract: We have studied the geometric microstructure as well as the elemental composition and distribution of SnO2-based polycrystalline sensors with X-ray diffraction and surface spectroscopic techniques. Electrical properties and responses against reducing gases are characterized by a.c. and d.c. measurements of resistances and/or conductances. The models explaining the high sensitivity of polycrystalline SnO2 against reducing gases in terms of changes of the intergrain conductivity have been completed. The Schottky-barrier mechanism of the electron transport across the grain boundaries is valid only for SnO2 grains larger than the Debye length of electrons. For grains smaller than the Debye length, the band bending at the surface can be neglected compared with the overall shift of the Fermi level in the grains during gas exposure. From combined measurements of conductances, work function changes and catalytic activities as functions of temperature, CO and H2O partial pressures, we deduce that OH dipoles, which do not influence the oxidation kinetics of carbon monoxide, are formed during interaction with water at 673 K.

Plastic deformation and fracture of binary TiAl-base alloys

Abstract: The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α 2 lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.

Structural origin of reduced critical currents at YBa2Cu3O7–δ grain boundaries

Abstract: THE critical current density across individual grain boundaries in thin films of the high-Tc superconductor YBa2Cu3O7–δ (YBCO) has been found1–4 to be inversely proportional to lattice misorientation for tilts up to ∼10°. Reports of impurity segregation5,6 at grain boundaries, and variations in the chemical stoichiometry7,8, have led to the view that deviations from the ideal composition are responsible for the depressed superconducting order parameter at the boundary. Here we present images of YBCO grain boundaries obtained by a scanning transmission electron microscope in Z-contrast mode9,10, which show that chemical segregation does not necessarily occur at these boundaries. A simple model of the strain associated with the grain-boundary dislocations provides a reasonable physical explanation of the suppressed superconductivity. The surprisingly large effect of strain implied by our model has implications beyond critical currents, for the physics and applications of any thin-film YBCO structures involving strained epitaxial layers.

Transport processes and noise in YBa2Cu3O7−δ grain boundary junctions

Abstract: In the high temperature superconductors the weak link nature of extended defects as grain boundaries is responsible for the deterioration of the superconducting transport properties. From the study of individual grain boundaries in thin-film bicrystals of YBa2Cu3O7−δ general relations describing the superconducting transport and noise characteristics of grain boundaries in the high temperature superconductors were found. Firstly, the critical current density Jgbc decreases as the misorientation angle between the adjoining grains increases. Secondly, the Jgbc ϱN products, where ϱN is the normal resistance times unit area of the grain boundaries, scale proportional to about (1/ϱN)q with q ranging between 1 and 1.5. Thirdly, all grain boundaries show large amounts of low frequency 1/f noise. The transport and noise characteristics can be explained by a junction model which is based on an insulating layer at the grain boundary interface containing a large number of localized defect states.

The effect of liquid‐phase sintering on the properties of Pr6O11‐based ZnO varistors

Abstract: Conventionally batched varistors in the ZnO–Co3O4–Pr6O11 system that were sintered at or above 1280 °C exhibited a sharp increase in the average breakdown voltage per grain boundary. In these samples, barrier heights and donor concentration values, derived from capacitance‐voltage analysis, exhibited sharp changes at the same soaking temperature. At or above 1280 °C, grain growth in this system proceeds with the assistance of an eutectic liquid. The improvement of the distribution of Pr6O11, by precipitation of Pr(OH)3 during batching, resulted in varistors markedly more insensitive to firing temperature than conventionally batched ones. The presence of a liquid phase, at least during grain growth, results in an efficient distribution of grain boundary materials and dopants in general. Suitable chemisorbed gases along the grain boundaries are believed to be responsible for the formation of interface states. Segregated materials appeared to provide for enhanced transport of gaseous species along the grain boundaries. The need for the addition of oxides with large ionic‐radius cations, i.e., varistor‐forming oxides such as bismuth, barium, praseodymium, or lanthanum oxide, to varistor compositions is explained within this context.

Frequency dependent electrical properties of polycrystalline olivine compacts

Abstract: The complex electrical properties of poly crystalline San Carlos olivine compacts were measured over the range of frequency l0−4–104Hz from 800° to 1400°C under controlled oxygen fugacity. The impedance data display a strong frequency dependence that is evidenced most clearly when the results are displayed in the complex impedance plane. A parameterized model of the frequency dependent electrical response using equivalent electrical circuits is presented. Two distinct conduction mechanisms of the sample are observed: grain interior and grain boundary conduction. Each occurs over a different range of frequency. The resistance of each mechanism adds in series resulting in a lower total DC conductivity for polycrystalline olivine than for either mechanism separately. The total DC conductivity is dominated by the grain interior conductivity above 1200°C, whereas the grain boundary conductivity has the strongest influence below 1000° C. Impedance spectra of natural dunite samples exhibit a similar type of frequency dependence. The grain interior conductivity displays a change in slope at 1344°C and has activation energies of 1.45 eV (800°–1344°C) and 4.87 eV (>1344°C). The grain boundary conductivity has an activation energy of 2.47 eV. In these cases, the ƒO2 for each experimental run was controlled at that of the wustite-magnetite oxygen buffer. Experiments on samples with different grain sizes reveal no dependence of DC conductivity on grain size for either mechanism, although the relaxation time and real relative permittivity of the grain boundary mechanism are dependent on grain size. Because of the electrical response observed at low frequencies, care must be taken in the inversion of electromagnetic field observations using laboratory measurements made in the kilohertz range since they may not be the same as DC measurements. Impedance measurements must be performed over a range of relatively low frequencies to assess the role of grain boundaries on the overall electrical response of polycrystalline materials.

Carbon Additions to Molybdenum Disilicide: Improved High‐Temperature Mechanical Properties

Abstract: C addition (2 wt%) to MoSi2 acted as a deoxidant, removing the otherwise ubiquitous siliceous grain boundary phase in hot-pressed samples, and causing formation of SiC and Mo5Si3C1 (a variable-composition Nowotny phase). Both hardness and fracture toughness of the C-containing alloy were higher than those of the C-free (and oxygen-rich) material; more significantly, the fracture toughness of the MoSi2+ 2% C alloy increased from 5.5 MPa·m1/2 at 800°C to ∼11.5 MPa·m1/2 at 1400°C.

Microstructure of epitaxial YBa2Cu3O7 films on step-edge SrTiO3 substrates

Abstract: The microstructure of YBa2Cu3O7 films epitaxially grown on step-edge (100) SrTiO3 substrates has been characterized by means of high resolution transmission electron microscopy. The results indicate a relationship between the microstructure of the film acreoss a step and the angle step makes with the substrate plane. On a steep, high-angle step, the film grows with its c-axis perpendicular to that of the film on the substrate surface so that two grain boundaries are formed. On a low-angle step, the film grows without any change in c-axis orientation across the step and without grain boundaries. Epitaxial second phases intergrowths across the steps have been found in some cases which may act as barrier layers when they cut through th YBa2Cu3O7 film.

Compositional changes of minerals associated with dynamic recrystallizatin

Abstract: The rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously. The enhanced exchange rate may result from a reduction in grain size (shorter distance for volume diffusion), dissolution and growth of grains by diffusion creep (pressure solution), or the movement of high-angle grain boundaries through strained grains during recrystallization in the dislocation creep regime. The migration of high-angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization. The operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases (An1 and An79) at 900°C, 1 GPa confining pressure, and a strain rate of ∼2x10-6s-1. The polygonal, recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition. At the conditions of these experiments, the volume interdiffusion rate of NaSi/CaAl is too slow to produce any observable chemical change, and microstructural-chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine-grained (2–10 μm) aggregates. These results indicate that strain-induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars, pyroxenes, and amphiboles.

Grain-Boundary Diffusion Kinetics in Silicate and Oxide Minerals

Abstract: Geologists have long held the belief that diffusion along grain boundaries in rocks would provide the means for mass transport over distances in the range of centimetres to metres needed to explain various metasomatic phenomena. Indeed, measured grain-boundary diffusion coefficients support this belief, as the ratio D i GB /D i VOL is commonly greater than 104 at metamorphic temperatures. To assess the role of grain-boundary diffusion in metamorphic processes this contribution: (1) reviews the current understanding of the nature of grain boundaries in oxides and silicates; (2) reviews the mathematical description of grain-boundary diffusion; (3) summarizes available grain-boundary diffusion data for oxides and silicates; (4) uses these results to explore the scale of mass transport by grain-boundary diffusion in metamorphic systems at geologic time scales.

Microstructure and strength of nanocrystalline copper alloy prepared by mechanical alloying

Abstract: Polycrystalline materials having an ultrafine grain size may be prepared by mechanical alloying. Such a material has been prepared here with a copper matrix and a uniform dispersion of particles which stabilises the fine microstructure. It is shown that the grain size of the copper matrix may be explained in terms of the conventional models of boundary pinning by particles, even for grain sizes below 40 nm. For grain sizes larger than about 100 nm, material strength may be explained by dislocation-particle interactions as illustrated by TEM observations. For grain sizes below this limit, however, strengthening is not as great as dislocation theory would predict based on the distribution of particles in the material; in addition TEM observations show no indication of the presence of dislocations. A different deformation mechanism seems to control strengthening for these materials of nano-scale grain size.

Analysis of texture evolution in channel die compression. I, Effects of grain interaction

Abstract: The effect of grain interactions on deformation patterns and texture evolution is examined by the finite element method for a particular set of grains deformed in channel die compression. The material behavior within each element is determined from a slip based constitutive formulation that fully accounts for finite deformations and lattice rotations with deformation. Compatibility and equilibrium are enforced across the grain boundaries, and grain boundary sliding is not permitted. In order to make the analyses numerically tractable, an idealized two-dimensional geometric model is used to model the deformation in the longitudinal-transverse plane; and the strains are assumed to be uniform through the compression direction. Boundary conditions simulating homogeneous plane-strain compression are applied to the model region. The results reveal complex deformation patterns arising from grain interactions. The analyses also show that the crystallographic texture and the spread of orientations within a grain depend not only on the orientations of the neighboring grains, but also on the constraints provided by grains located several grains away.

Atomistic and computer modeling of metallization failure of integrated circuits by electromigration

Abstract: Nonstationary and stationary concentration profiles of vacancies in a grain boundary are calculated for the case of electromigration and various conditions of vacancy formation and annihilation. If supersaturation of vacancies is assumed for void formation, current densities necessary for voiding have to be larger than a critical value jcrit, which is related to the length l of the grain boundary by the relation ljcrit=const. An estimation of the value of the constant in the last equation is in agreement with experimental values. An approximate equation for the median time to failure (MTF) is derived from the model for grain boundaries conducting the flux of matter parallel or in series. The results explain many features of experimental studies. First values of MTF for a network of grain boundaries obtained by computer simulations are presented as well, and the effect of resistance heating and nonstationarity on the current exponent is discussed in some detail.

Polysilicon thin-film transistors with channel length and width comparable to or smaller than the grain size of the thin film

Abstract: Poly-Si thin-film transistors (TFTs) with channel dimensions (width W, and length L) comparable to or smaller than the grain size of the poly-Si film were fabricated and characterized. The grain size of the poly-Si film was enhanced by Si ion implantation followed by a low-temperature anneal and was typically 1 to 3 mu m in diameter. A remarkable improvement was observed in the device characteristics as the channel dimensions decreased to W=L=2 mu m. On the other hand, TFTs with submicrometer channel dimensions were characterized by an extremely abrupt switching in their I/sub D/ versus V/sub GS/ characteristics. The improvement was attributed to a reduction in the effect of the grain boundaries and to the effect of the device's floating body. >

Environmental embrittlement and grain-boundary fracture in Ni3Al

Abstract: Recent studies have demonstrated that many ordered intermetallics exhibit environmental embrittlement when tested in air at ambient temperatures. Brittle grain-boundary fracture and poor ductility have been observed in a number of binary ordered intermetallics with an L1{sub 2} ordered crystal structure such as Ni{sub 3}Al, Ni{sub 3}Si, Ni{sub 3}Ge, and Ni{sub 3}Ga. The grain boundaries in the alloys have been considered to be intrinsically brittle, as evidenced by detailed Auger analyses and as suggested by atomic-simulation calculations. In this paper the study of environmental embrittlement at room temperature is extended to include binary Ni{sub 3}Al. The present results also show that moisture-induced hydrogen embrittlement contributes substantially to brittle grain-boundary fracture and low tensile ductility in Ni{sub 3}Al containing 24 and 23.5at.% Al, prepared by repeated cold forging and annealing of cast-alloy ingots.

Rare-earth-based alloy electrodes for a nickel-metal hydride battery

Abstract: Extensive work has been carried out on utilizing MmNi 5 -based alloys (Mm, mischmetal) as a low cost negative battery electrode. The replacement of nickel by cobalt was effective in improving cycle lifetime but caused a decrease in capacity and high rate capability. The replacement of lanthanum by large amounts of cerium of neodymium gave the alloy a satisfactory cycle lifetime with small amounts of cobalt present and without impairing the high rate capability. The alloy MmNi 3.5 Co 0.7 Al 0.8 was selected consistent with the above requirements. Induction-melting tests showed that the surface region with a columnar structure had a longer cycle lifetime than the inner region with an equiaxed structure. Annealing treatment also caused a decrese in the cycle lifetime. It was considered that alloys with smaller crystal grains had a longer cycle lifetime because the protective surface layer on the grain would remain effective after pulverization. The deviation from stoichiometric composition to the nickel-rich side or the Mm-rich side also caused a decrease in cycle lifetime, accompanied by the precipitation of nickel or Mm on the grain boundaries. It was concluded that the surface layer of the crystal grain played a very important role in preventing capacity decay.

Effect of Alumina Addition on the Tetragonal‐to‐Monoclinic Phase Transformation in Zirconia–3 mol% Yttria

Abstract: The tetragonal-to-monoclinic martensitic phase transformation in ZrO–3 mol% Y2O3 (PSZ) containing 0 to 12 wt% Al2O3 was investigated by dilatometry, XRD, and SEM-EDS methods. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al2O3. The grain size was independent of the addition of Al2O3. Both Y2O3 and Al2O3 segregated at grain boundaries. From this segregation behavior, it was suggested that a certain compound or phase of Y2O3–Al2O3 could be formed at grain boundaries, which would presumably prevent the propagation of the transformation into interiors of PSZ-containing Al2O3.

Structural characterization of nanometer-sized crystalline Pd by x-ray-diffraction techniques.

Abstract: Quantitative x-ray-diffraction measurements of ultrafine-grained (nanocrystalline) Pd samples and a coarse-grained polycrystalline reference foil were obtained using synchrotron radiation. The intensity profiles of the Bragg reflections from the nanocrystalline samples were considerably better represented by Lorentzian functions than by Gaussian functions, indicating that a large fraction of intensity from the Bragg peaks was found in the tails of the reflections. The remaining intensity differed only slightly for different grain-sized materials, therefore, atomic relaxations in the vicinity of grain boundaries in nanocrystalline Pd must be small in magnitude and/or extremely localized. The results of the present work do not support the previously proposed existence of either a ``gaslike'' grain boundary phase, or large quantities of vacancies or voids within the grains of nanocrystalline Pd, which produce broadly distributed diffuse scattering. The broadening of the Bragg reflections was related to the small particle size of nanocrystalline Pd, and strain located in the grains and/or interfacial regions. Evidence was seen for anisotropic grain shapes preferentially elongated along the [111] direction. The Debye-Waller parameter of nanocrystalline Pd was observed to be larger than the literature value for coarse-grained Pd, which suggests larger displacements of the atoms from their ideal lattice locations in the nanocrystalline material than in the coarse-grained material.