Showing papers on "Grain growth published in 1983"

Infrared-transparent mullite ceramic

Abstract: Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al2O3 yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A12O3 formed precipitates of α-Al2O3 at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO2-Al2O3 phase diagram.

Effect of MgO Solute on the Kinetics of Grain Growth in A12O3

Abstract: The kinetics of grain growth in fully dense Al2O3 with and without MgO solute additions were measured. The MgO was found to retard boundary migration and to rkvelop more uniform microstructures. A grain-growth mechanism involving solute partitioning of segregated ions (calcium and magnesium) between different boundary types is proposed. Implications regarding the sintering of Al2O3 are discussed.

Kinetics of the Q-state Potts model in two dimensions

Abstract: Resultats de simulations sur ordinateur de la cinetique d'un modele de Potts a etat Q ferromagnetique

Effect of phosphorus doping on stress in silicon and polycrystalline silicon

Abstract: An investigation of the polysilicon stress properties as a function of film thicknesses and phosphorus doping showed that as‐deposited films are moderately compressive, and become less compressive with increasing film thickness. High temperature PBr3 diffusion in silicon produces wafer bending corresponding to a tensile stress in wafer. Following a PBr3 diffusion, polysilicon films, however, become less compressive. Subsequent oxidation introduces an additional compressive stress component of the order of 2−3×109 dyne/cm2 for oxidation temperatures between ∼900−1000 °C. The thermal expansion coefficients were similar for doped and undoped films (α∼2.9 ppm/°C) and slightly less than for 〈100〉 silicon, while the doped films were found to be less stiff than undoped ones but both were less stiff than 〈100〉 silicon. The observed changes in polysilicon stress due to film thickness and phosphorus doping have been interpreted in terms of a grain growth model wherein those factors which lead to enhanced grain grow...

The Effect of Grain Size on Fatigue Growth of Short Cracks

Abstract: The influence of alloy grain size on growth rates of surface cracks 20 to 500 μm in length was studied in Al 7075-T6 specimens prepared in 12 and 130 μn grain sizes. Grain boundaries temporarily interrupt the propagation of cracks shorter than several grain diameters in length. Linear elastic fracture mechanics is inadequate to describe resulting average growth rates which must instead be characterized as a function of cyclic stress amplitude, σa, and alloy grain size as well as stress intensity range, σK. These observations are rationalized using two models, one that relates crack closure stress to alloy grain size, and a second that relates the development of microplasticity in a new grain in the crack path to grain size. In addition, growth rates were found to be faster in fully reversed loading than in tension-tension loading, especially in the large grained material. Evidence is presented to demonstrate that this is a consequence of the fatigue induced development of a compressive residual surface stress during tension-tension loading. These complex effects, and the role of grain size in determining short crack growth, are discussed.

The effects of segregation on the kinetics of irrtergranular cavity growth under creep conditions

Abstract: Intergranular cavity growth under creep conditions is examined with particular reference to the potential effects of segregation on cavity growth. When cavities are present on all of the boundaries in a polycrystalline solid, they are able to grow in an unconstrained manner. Under these conditions the rate of cavity growth may be controlled by grain boundary diffusion (D GB), surface diffusion (D S), or power law creep, which in turn is controlled by lattice diffusion (D L). When only isolated boundaries are cavitated, cavity growth is constrained and may be completely limited by creep flow of the surrounding grains. The segregation of solute to the grain boundary and the cavity surface can influence the kinetics of cavity growth in several different ways. The reduction in surface energy associated with segregation can increase the rate of cavity growth when the cavities are crack-like. The effects of segregation on grain boundary and surface diffusion can also influence the rate of cavity growth. A phenomenological relation proposed by Borisovet al indicates thatD GB /DL decreases with segregation, thus causing grain boundary diffusion controlled cavity growth to be slowed by segregation. In some cases solute additions increaseD L, thus increasing the rate of creep controlled cavity growth. When these effects are sufficiently large,D GB can increase with solute additions, in spite of the effect of segregation. In some alloysD S increases with segregation by several orders of magnitude. Similar effects on cavity growth are not expected even for surface diffusion controlled cavity growth because the rate of cavity growth is limited by other factors whenD S is very large.

The properties of tungsten processed by chemically activated sintering

Abstract: Two tungsten powders have been treated with small concentrations of sintering activators to provide for enhanced low temperature sintering. The experimental study focused on the determination of the processing effects on properties such as sintered density, grain size, hardness, and strength. Variables in the plan included tungsten particle size, type of activator, amount of activator, compaction pressure, and sintering temperature. The sintered density is found to have a dominant effect on strength and hardness. The various processing variables are analyzed in terms of their effects on density. At high sintered densities, grain growth acts to degrade the strength. Additionally, the nature of the sintering activator influences the fracture strength. In this study optimal strength occurred with a 0.7 μm tungsten powder treated with 0.29 wt pct Ni, sintered at 1200 °C for one hour. The resulting density was 18.21 g/cm3, with aR A hardness of 69 and a transverse rupture strength of 460 MPa.

Enhancement of Gas Sensitivity by Controlling Microstructure of α–Fe2O3Ceramics

Abstract: We have found that the gas sensitivity of α–Fe2O3 ceramics can be greatly enhanced by controlling its microstructure by adding quadrivalent Ti, Zr or Sn These metals have the effect on suppressing the grain growth and crystallization, and consequently increasing the specific surface area of the sensing element With Sn in particular, the sensing element consists of ultra-fine grains as small as about 100 A and has an extremely large specific surface area of 125 m2/g However, the additives are effective only in the presence of the sulfate ion, which is considered to localize mainly on the surface of the α–Fe2O3 grains Thus the surface of the grains, which is fairly amorphous, is thought to play an essential role in the gas sensitive mechanism

The inhibition of grain-boundary motion by a dispersion of particles

Abstract: The resistance to grain-boundary motion provided by a dispersion of spherical particles has been calculated for primary and secondary recrystallization (normal grain growth). Specific account has been taken of grain-boundary flexibility, and of those forces, which aid and those which hinder motion. When neither the intrinsic surface energy of particles nor that of grain boundaries is changed by the presence of the other, the resistance decreases markedly with increasing volume fraction of the particulate phase. Where there is interaction this resistance can be markedly larger, particularly at the highest volume fractions.

Microstructure of Sintered Mullite-Zirconia Composites

Abstract: Fused mullite, pure and with 10 to 25 vol% ZrO2 added, was milled in an attritor and sintered in air at temperatures near 1600°C to a dense fine-grained ceramic The ZrO2 promoted densification and retarded grain growth of the mullite phase Transmission electron microscopy showed that compositions containing ZrO2 were substantially free of glass, whereas pure mullite bodies contained a glassy phase The ZrO2 formed inter granular, isolated particles

Investigation of Twin Lamellae in BaTiO3 Ceramics

Abstract: The microstructure of many electroceramic components based on barium titanate is dominated by anomalous grain growth. At firing temperatures below the eutectic temperature nearly all large crystallites were found to contain lamellae. The crys-tallographic orientation of these lamellae was studied by TEM. By combining image and diffraction information it could be directly shown that the lamellae lie on (111) planes and represent twins, which are ∼0.5 μm thick. The relevance of twin formation for the mechanism of anomalous grain growth is briefly discussed.

Grain growth control in ZnO varistors using seed grains

Abstract: A method for making low‐voltage ZnO varistors having extraordinarily large grain size without necessity of sintering at much higher temperatures or for much longer times is reported. The device is made by sintering a mixture of ZnO fine powder, additives, and ZnO seed grains having grain sizes of 63–105 μm obtained by washing a ZnO sintered body containing BaO in boiling water. The device has a grain size around 500 μm and low threshold voltage of 6 V/mm. Such anomalous grain growth is caused by the difference between the radii of curvature of ZnO fine powder and ZnO seed grains.

Grain growth in samples of aluminum containing alumina particles

Abstract: A study of the two-dimensional and three-dimensional grain size distributions before and after grain growth treatments has been made in samples having a range of oxide contents. In order to collect statistically useful amounts of data, an automatic image analyzer was used and the resulting data were subjected to a series of statistical tests which evaluate the difference between related distributions. Normal grain growth was observed in samples of the lowest oxide content; as the oxide level was increased, a significant narrowing of the grain size distribution was observed. In the latter samples the maximum or limiting grain size measured after prolonged heat treatment is approximately one half that predicted by Zener and is in broad agreement with the values calculated from the models due to Hillert and Gladman. Anomalous grain growth has been observed in samples where normal grain growth is restricted and where the initial grain size distribution is very wide.

The role of Coble creep and interface control in superplastic Sn-Pb alloys

Abstract: The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260μm. In Sn-2 wt% Pb with grain sizes larger than ∼ 50 μm, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses (∼ 0.1 MPa) and strain rates (∼ 10−10 sec−1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.

Effect of grain size on creep rate in type 316 stainless steel at 873 and 973 K

Abstract: The influence of grain size on creep rate in AISI type 316 stainless steel has been investigated at 873 and 973 K over a wide range of applied stresses. Grain boundaries contribute to strengthening at high stresses (180–260 MN m−2) at 873 K but this strengthening does not correlate with the available models which attempt to incorporate the Hall–Petch strengthening effect into high temperature strain rate equations. The importance of loading strain in determining creep rate has been indicated. The creep rate was generally constant at 973 K but increased at small grain sizes and at lower stresses and this has been attributed to the increased contribution of grain boundary sliding at fine grain sizes. The difference in the grain size effects on creep at the two temperatures is attributed to the difference in the substructures developed during creep.

Enhanced Sintering of Tin Dioxide with Additives under Isothermal Condition

Abstract: The enhanced sintering of tin dioxide with additives, viz. MnO2, CuO, ZnO, Li2CO3 and Sb2O3 has been studied under isothermal condition. The efficiency of different additives in attaining higher density is discussed in terms of grain growth and pore morphology studied by SEM fractograph.The time exponents of the kinetic equation and activation energies are calculated from isothermal sintering data for various systems and these parameters are used to interpret the sintering mechanism for different additives. It has been found that different mechanisms are predominant in different systems and in some cases it depends on sintering temperature also. From SEM fractograph it appears that due to pore trapping within the grains it is not possible to attain higher density.

Observations of plane-matching grain boundaries by electron channelling patterns

Abstract: Observations of {110} plane-matching grain boundaries have been made by electron channelling patterns (ECPs) for the recrystallization structure of an Fe-3% Si alloy. It has been found that the {110} plane-matching grain boundary plays an important role in the grain growth during recrystallization of the alloy. The grains whose surfaces were near {100} preferentially grew by the migration of a {110} plane-matching grain boundary at the expense of the matrix grain whose surface was near {111} in the direction parallel to {110} plane. The anisotropy of the migration of the {110} plane-matching grain boundaries has been found. A step mechanism is suggested to explain the observed anisotropy of grain boundary migration.

Sintering of glass bonded ceramic barium hexaferrite magnetic powders

Abstract: Mixtures of low-melting glasses (glazes) and barium hexaferrite magnetic powders sinter at relatively low temperatures (800° C) and produce dense items in which excessive grain growth has been inhibited.

Effect of annealing temperature on structure and properties of fine-grained aluminium

Abstract: Ultrafine-grained Al samples of purity grades 2N7 and 4N were prepared by hydrostatic extrusion. The changes in structure and properties in the grain growth regime after annealing were studied in terms of the role of grain boundary structure. Important differences in grain growth kinetics were observed at annealing temperatures higher than 620 K and grain growth is shown to be dependent on the predominant type of grain boundary. This interpretation is supported by the results of mechanical testing and Hall–Petch analysis, leading to the conclusion that the parameters σ0 and k are more strongly dependent on grain boundary structure than on material purity.