Showing papers on "Grain growth published in 1978"

Grain Growth Mechanism of Heavily Phosphorus‐Implanted Polycrystalline Silicon

Abstract: Grain growth phenomena of heavily phosphorus‐implanted polycrystalline silicon films owing to high temperature annealing are investigated by transmission electron microscope. Phosphorus doping in excess of is found to enhance grain growth. This growth is broken down into primary and secondary recrystallization. Isochronal annealing reveals the activation energies for these as 2.4 and 1.0 eV, respectively. The driving force of the primary recrystallization is found to be the interface energy. Therefore, the elementary process behind the primary recrystallization is attributed to silicon diffusion across the grain boundary region.

Effects of nitrogen on crop development and grain growth of winter wheat in relation to assimilation and utilization of assimilates and nutrients

Abstract: Grain growth and yield components of winter wheat cv. Lely were studied in a field experiment in 1976 with 4 rates of N (50, 100, 100 + 50 or 100 + 100 kg N/ha). Growing conditions were characterized by a high level of solar radiation, warmth, ample nutrient supply and no damage by diseases. N raised grain number/m2 from 16 700 to 20 600 and grain yield from 640 to 821 g dry wt./m2. Grain growth duration was short, due to warmth, but the rate of the grain filling was very high (from 24.0 to 29.2 g/m2 day during the effective grain-filling period). A high grain yield was associated with a high grain N content which resulted in a grain protein yield ranging from 63.8 to 107.1 g/m2 with increased N rate from 50 to 200 kg/ha. The carbohydrate demand of the grains was provided by current photosynthesis and relocation of stem reserves. The latter was reflected in a decline of the stem wt. after the mid-kernel filling stage. N and P demands of the grains were supplied by withdrawal from the vegetative organs (leaves, stem, chaff) and to a large extent by post-floral uptake and assimilation. Under the prevailing growing conditions the grains turned out to be very strong sinks for carbohydrate, N and P as shown by the harvest indices. Additional N dressings increased the harvest indices of DN, N and P from 0.40 to 0.48, from 0.75 to 0.81 and from 0.91 to 0.93 resp. It was suggested that a more restricted vegetative crop development at high N levels and a longer duration of root activity, photosynthesis and grain growth after anthesis would considerably favour grain yield. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Grain rearrangements during superplastic deformation

Abstract: Current models for obtaining large superplastic flow without change of grain size are two-dimensional; they therefore involve rearrangement of grains without increasing the surface area of the specimen as it deforms. A new model is proposed in which grainboundary sliding (GBS) in a group of grains is accommodated by a grain emerging from the next layer of grains, giving the correct increase in surface area. This also produces curved grain boundaries and there is some rotation of grains involving plastic flow in a zone along grain boundaries (the “mantle”) of predictable width. Grains do not have to be uniform and regular for the process. Characteristic configurations of marker lines are produced by the deformation. All these features are shown to have been observed in the literature. The model does not predict a threshold stress. It can be linked with a previous constitutive equation based on the climb and glide of dislocations in the grain mantles.

Processing of crystalline ceramics

Abstract: Keynote Address.- Some Considerations in the State of the Art in Processing Crystalline Ceramics.- I: Fine Particle Science, Technology and Characterization.- Physical and Chemical Parameters Controlling the Homogeneity of Fine Grained Powders and Sintering Materials.- Ultrafine Powders of Oxide and Non-Oxide Ceramic Materials and Their Sinterability.- Handling and Green Forming of Fine Powders.- The Potential of Fine Particle Technology Applied to Ceramic Raw Materials.- Particle Size and Permeability in Slip Casting.- Chemical Processing for Ceramics (and Polymers).- Ceramics Sintered Directly from Sol-Gels.- Characterization of Ceramic Microprocessing.- Packing and Sintering Relations for Binary Powders.- Stress and Density Distributions in the Compaction of Powders.- Pore Morphography in Ceramic Processing.- II: Solid State Sintering and Grain Growth.- Fundamentals of the Sintering of Ceramics.- Current Paradigms in Powder Processing.- Some Effects of Aggregates and Agglomerates in the Fabrication of Fine Grained Ceramics.- The Sintering of Conductive Rutile: A Model System for Sintering Electronic Ceramics.- Sintering of Mullite.- Rate Controlled Sintering as a Processing Method.- A Multiple-Lognormal Model of a Normal Grain Growth.- III: Liquid Phase Sintering And Post-Firing Technology.- Rearrangement During Liquid Phase Sintering of Ceramics.- Reactivity of Alumina Substrates with High Lead Glasses.- Influence of the Processing Parameters on the Properties of Rapid Fired Porcelain.- Strengthening of Lime-Stabilized Zirconia by Post Sintering Heat Treatments.- Strain and Surface Energy Effects in Ceramic Processes.- Dynamic and Material Parameters in Brittle Fracture in Ceramics.- Processing Induced Sources of Mechanical Failure in Ceramics.- IV: Dielectric and Magnetic Ceramics.- Processing of High Density Piezoelectric Ceramic Compositions.- The Role of ZrO2 Powders in Microstructural Development of PZT Ceramics.- Novel Uses of Gravimetry in the Processing of Crystalline Ceramics.- Deformation Processing of Magnetic Hexaferrites for Hc Maximization Through Grain Growth Control.- Processing and Magnetic Properties of Low-Loss and High-Stability Mn-Zn Ferrites.- Relationship Between Processing Conditions, Oxygen Stoichiometry and Strength of MnZn Ferrites.- Sintering of High Density Ferrites.- V: Energy Related Ceramics I: Fast Ion Conductors MHD, Nuclear and Refractory Ceramics.- Processing and Characterization of Polycrystalline ?"-Alumina Ceramic Electrolytes.- Microstructural Control During Sintering of ?"-Alumina Compositions Through Ceramic Processing Modification.- Transient Eutectics in Sintering of Sodium Beta Alumina.- Microstructural Evolution During the Processing of Sodium ?-Alumina.- Fabrication and Performance of MHD Electrodes.- Fabrication and Property Control of LaCrO3 Based Oxides.- Strontium Containing Perovskites and Related Conductive Electronic Ceramics.- Hot Pressed Composite Ceramic MHD Electrode Development.- Processing Variables Affecting the Thermomechanical Degradation of Monolithic Refractory Concretes.- UO2-Gd2O3 Sintering Behavior.- Processing Requirements for Property Optimization of EU2O3-W Cermets for Fast Reactor Neutron Absorber Applications.- Ceramic Processing of Boron Nitride Insulators.- VI: Energy Related Ceramics II: Non-Oxide Ceramics.- Grain Boundary Engineering in Non-Oxide Ceramics.- The Fabrication of Dense Nitrogen Ceramics.- Ceramics in the Si-Al-O-N System Fabricated by Conventional Powder Processing and Sintering Techniques.- Polytypism in Magnesium Sialons.- Dense Silicon Nitride Ceramics: Fabrication and Interrelations with Properties.- High-Pressure Hot-Pressing of Silicon Nitride Powders.- The Structure of Grain Boundaries in Silicon Nitride Based Alloys.- Evolution of Microstructure in Polycrystalline Silicon Carbide.- Thermal Gradient Deposition of SiC Diffusion Tracers.- Contributors.

A Test of the Second‐Phase and Impurity‐Segregation Models for MgO‐Enhanced Densification of Sintered Alumina

Abstract: To determine if MgO added within the solubility limit in Al2O3 is sufficient to suppress discontinuous grain growth, an undoped alumina pellet was sintered next to a preequilibrated, two-phase mixture of spinel (MgAl2O1) and alumina. The MgO-doped outer surface of the undoped pellet did sinter to full density; metallography confirmed that the sample was free of second phase. Grain boundaries were analyzed with a scanning Auger microprobe, which showed that the grain boundaries of the inner porous region had approximately the same Ca segregation as those in the dense outer shell region. Therefore, it was concluded that the beneficial effect of magnesium is due neither to second-phase pinning nor to Ca impurity segregation.

Impurity-Induced Exaggerated Grain Growth in Mn-Zn Ferrites

Abstract: The effects of 20 oxide dopants on the microstructure development of Mn-Zn ferrite during sintering were investigated by diffusion-couple-like experiments. Nine oxides enhanced grain growth in ferrite; the effects of TiO2 dopant were studied in detail. The TiO2-induced exaggerated grain growth shows a parabolic time dependence, as was also true for the diffusion of Ti into ferrite as shown by X-ray spectrometry. For samples sintered at a given temperature but for different times, the Ti concentrations at the growth fronts of the exaggerated grains are identical. Exaggerated grain growth kinetics and Ti diffusivities have similar activation energies. Furthermore, the mobility of the exaggerated grains is reduced to that of the matrix grains after the TiO2 layer is removed, showing that the exaggerated grain growth is sustained by Ti diffusion into ferrite. Probable mechanisms for TiO2-promoted exaggerated grain growth are proposed. Some self-consistent arguments are presented for the observed effects of other dopants. Grain boundary mobilities of undoped Mn-Zn ferrite were measured and are in good agreement with those reported in the literature.

Coarsening of tungsten grains in liquid nickel-tungsten matrix

Abstract: In sintered W-Ni alloys with 1,7, and 30 wt pct Ni the tungsten grain growth in liquid matrix at 1540°C was investigated. The observed grain size distributions and growth rate are compared with the theoretical predictions of Wagner, Lifshitz and Slyozov, Lay, and Ardell. In the 70 pct W-30 pct Ni alloy the tungsten particles settled to the bottom of the specimens immediately upon melting of the matrix, but the spherical grain shape is maintained during the initial stage of annealing. In these specimens the linear intercept distribution of the grains agrees with the prediction of the LSW (Lifshitz, Slyozov, and Wagner) theory for the reaction controlled growth mechanism. On the other hand the growth rate appears to follow the t1/3 law predicted for the diffusion controlled mechanism. These results are consistent with Lay and Ardell's theory in which the concentration gradient around grains is inversely proportional to the average grain size in the limit of small matrix fraction. In the alloys with 1 and 7 pct Ni a meaningful comparison of the observed linear intercept distribution of the grains with theoretical predictions is difficult because of grain contact flattening due to densification. The grain growth is larger with less matrix fraction in the specimens and this result provides an evidence for the diffusion controlled grain growth during the liquid phase sintering of this alloy.

High coercivity, isotropic plasma sprayed samarium‐cobalt magnets

Abstract: A value of 675 kOe has been measured for the room temperature coercivity of an isotropic, heat treated, 1‐5 samarium‐cobalt plasma sprayed deposit This value is believed to be the largest published, so far, for a permanent magnet Sprayed magnets are generally found to be better than 93 percent dense with coercivities of 35 to 60 kOe Deposits comprised of fine microstructures yield broad, unresolved x‐ray diffraction peaks Low temperature crystallization of these deposits yields diffraction patterns representative of the high temperature equilibrium structure The emergence of peaks, ascribed to SmCo5, from a low temperature 700 °C heat treatment implies both formation and stability of the SmCo5 phase at this temperature These results dispute the existence of the generally accepted eutectoid decomposition reaction for SmCo5 at 750–800 °C If such a reaction were present the products of crystallization should have included Sm2Co7 and not SmCo5 High temperature investigations on sprayed magnets indicate that temperatures of 1100 °C result in low values of coercivity, possibly due to grain growth

Predicted color excess ratios versus interstellar grain size

Abstract: Various color excess ratios, the ratio (R) of total to selective extinction, and values of the wavelength of maximum interstellar linear polarization (lambda max) are computed and displayed as functions of mean interstellar grain size and absorption coefficients. E(V-K)/E(B-V)-E(u-b)/E(b-y) is shown to be highly sensitive to changes of mean grain size. The shape of the R versus lambda max curve is shown to depend on the amount of absorption initially present in the grains, and on the absorptivity of the material added to the grains. While no simple model of grain growth has been found to fit all the observations, the two stars with the largest values of lambda max (HD 147889 and W67) support the concept of dielectric mantles growing on dielectric grains. It is further shown that more accurate uby and JKL photometry of heavily reddened stars would permit better discrimination between grain growth theories.

Surface Area Reduction Kinetics During Intermediate Stage Sintering

Abstract: The surface-area reduction kinetics for the intermediate stage of sintering was analyzed. The high sintering rate in the initial stage results from the initially large curvature gradients. In the intermediate stage these gradients are reduced and the surface-area reduction kinetics shift to a dependence on the mass transport rate associated with grain growth. This is a much slower process. (FS)

A model of densification with simultaneous grain growth

Abstract: A densification model in which grain growth and densification progress simultaneously in a powdered compact produced the relation between the relative density ρ and average particle/grain size R, logρ (1−ρ0)/ρ0(1−ρ) =k4KG log(R/R0), where R0 is the initial particle size, ρ0 is the green density, KG is the dimensionless parameter dependent on the spread of the distribution of particle/grain size in the compact, and k4=SS/(1−ρ) S (SS is the pore surface area of the compact and S is the total area of particles/grains). Sintering data of BaCl2‐ and halogen‐doped MgO compacts deviated slightly from the theoretically calculated relation. This discrepancy is interpreted in terms of the heterogeneously packed structure of particles/grains.

Method of making cast metal turbine wheel with integral radial columnar grain blades and equiaxed grain disc

Abstract: A cast metal turbine wheel is made comprising a cylindrical disc with equiaxed grains and integral blades with columnar grains oriented substantially parallel to the leading and trailing blade edges. The blades are solidified unidirectionally by withdrawing heat through chill means located adjacent the mold at the blade tips. Cooling is inhibited from other blade surfaces by retaining cast metal in mold portions located above and below the blades. The disc portion of the mold is insulated so that it cools slowly to promote grain nucleation throughout the metal therein and equiaxed grain growth.

On the origin of core losses in a manganese zinc ferrite with appreciable silica content

Abstract: The effect of silica on the magnetic properties and microstructure of a manganese zinc ferrite has been studied. It is found that as silica content increases, the magnetic properties at first improve, then deteriorate around 0.04% silica due to discontinuous grain growth. At still higher silica contents, magnetic properties deteriorate further due to an increase in the total effective magnetic anisotropy.

Reflectance and structure of evaporated chromium and molybdenum films

Abstract: Chromium and molybdenum films deposited in a high vacuum at room temperature are shown to have anomalously low optical reflectance throughout the 0.2–2.5‐μm wavelength range. Deposition at about 700°C for chromium and 1000°C for molybdenum, or postdeposition annealing, gives films with bulk reflectance. X‐ray diffraction and transmission electron microscopy show the films deposited at room temperature to be very fine grained (80–100‐A grain size). Electrical resistivity of the fine grained films is high (3–5 times bulk). High‐temperature deposition or annealing gives larger grains (≳400 A) and lower resistivity. A simple model accounts for the resistivity behavior.

Grain growth in polycrystalline silicon

Abstract: A grain‐growth study of polycrystalline silicon having small dendritic crystallites obtained by cracking SiHCl3 on a hot filament has been carried out in the temperature range 1275–1375 °C. It has been observed that (1) the grain growth in the polycrystalline silicon is due to a two‐dimensional motion of grain boundaries in a plane perpendicular to the grain axis, (2) the size‐limited grain‐growth law is obeyed, and (3) the activation energy of grain growth in polycrystalline silicon is 4.62±0.23 eV, which is nearly the same as the activation energy of the diffusion of Si31 in silicon.

A theoretical model for creep crack growth

Abstract: A theory of creep crack growth has been developed with the presumption that the crack growth occurs by the diffusion of vacancies along the grain boundaries. This is consistent with many experimental results that show that creep fracture is generally of intergranular type and the activation energies for crack growth rates fall within the range of grain boundary diffusion energies. The theory is based on the concept that creep crack growth results from a balance of two competing processes-the diffusion of point defects that contributes to the growth and the creep deformation process that retards the growth and causes even its arrest. The present analysis shows that crack growth via grain boundary diffusion occurs within some temperature range. The upper limiting temperature is determined by the bulk diffusion process which disperses the vacancies, that are diffusing to the crack tip, to the plastic zone ahead of the crack front. The lower temperature limit is set by the fact that the grain boundary diffusion rates decrease with the decrease in temperature and thus large stress intensities approaching the fracture toughness value are required to accomplish crack growth by the grain boundary diffusion. Outside these limits creep crack growth occurs via deformation which is significantly slower than growth by the grain boundary diffusion process. The importance of the present analysis rests on the fact that service conditions for many high temperature structural materials fall within the regime wherein creep crack growth occurs via grain boundary diffusion.

Models for growth kinetics of A-15 compounds by solid state diffusion

Abstract: In the formation of A-15 superconducting compounds by solid state diffusion, the time exponent in the growth law under different experimental conditions varies widely from about 0·25 to 1·0. Specific models of growth for different operative rate-controlling conditions are proposed. When the diffusion of B atoms in the matrix is rate-controlling, the thickness of the reacted compound layer increases as t1/2 or t2/3 . When the diffusion of B atoms through the compound layer is rate controlling, a t1/2 dependence both for bulk diffusion and grain-boundary diffusion is predicted. When substantial grain growth occurs in the reacted layer during the diffusion anneal, the time exponent observed could be as low as 1/4. Experimental data in support of the predictions of the proposed models are presented.

Application of X‐ray synchrotron topography to in situ studies of recrystallization

Abstract: It is demonstrated that X-ray topography with synchrotron radiation provides an excellent means of simultaneously measuring the size and orientation of grains in polycrystalline materials. The technique, which also provides data on the lattice strains, is particularly suited to studies of grain growth at high temperatures and preliminary studies of recrystallization in iron-silicon alloy sheet are reported. The initial radial-growth rate is constant and approximately equal for all grains studied in one recrystallization.

High temperature creep of ultrafine-grained Fe-doped MgO polycrystals

Abstract: Creep deformation in ultrafine-grained (0.1 to 1μm) Fe-doped magnesia polycrystals is studied in compression, at temperatures of 700 to 1050° C, and constant loads of 50 to 140 MPa. The stress exponent observed to be nearly unity and the strong grain size sensitivity (ė∼d−2.85) suggest that diffusional creep mechanisms dominate the deformation. In the grain size range of the present study the grain boundary diffusion contribution is significantly more important than lattice diffusion. Magnesium is tentatively identified as the rate-controlling species along grain boundaries from an analysis of the diffusivities inferred from the present work and from other authors for Fe-doped magnesia.