Showing papers on "Grain size published in 1987"

Calculations of the Critical Shear Stress for Motion of Uniform and Heterogeneous Sediments

Abstract: An expression for the critical shear stress noncohesive sediment is derived from the balance of forces on individual particles at the surface of a bed. The resulting equation, for a given grain size and density, depends on the near-bed drag force, lift force to drag force ratio, and particle angle of repose. Calculated values of the critical shear stress for uniformly sized sediment correspond closely to those determined from Shields' diagram. The initial motion problem for mixed grain sizes additionally depends on the relative protrusion of the grains into the flow and the particle angle of repose. The latter decreases when the diameter of a moving grain, D, is larger than the length scale of the bed roughness, ks (D/ks > 1), and increases when D/ks < 1, producing a corresponding decrease or increase in critical shear stress. Using the Miller and Byrne experimental relationship between D/ks and particle angle of repose, which is consistent with Shields' definition of initial motion, we obtain results that are in good agreement with the available experimental critical shear stress data for heterogeneous beds.

Collisional charging of interstellar grains

Abstract: Collisional charging of small interstellar particles is reexamined, including effects due to electrostatic polarization of grains by the electric fields of approaching charged particles. The interaction potentials and resulting energy-dependent collisional cross sections are discussed and used to obtain the collisional charging rate for grains of specified charges in a thermal plasma. The probability distribution of the grain charges is obtained, and the application of the results to the interstellar medium is illustrated by considering the power-law grain size distribution of Mathis, Rumpl, and Nordsieck (1977). It is found that dust grains with such a size distribution can contribute appreciably to the recombination of metal ions in dark clouds.

Microstructure of hot‐pressed and die‐upset NdFeB magnets

Abstract: Transmission electron microscopy was used to establish densification, alignment, and magnetization reversal mechanisms in hot‐pressed and die‐upset NdFeB magnets. Microstructures of these materials reveal two principal phases: Nd2Fe14B grains and a grain boundary phase of approximate composition Nd7Fe3. The grains in the hot‐pressed material are polygonal and isotropic, while those in the die‐upset material are flat platelets. The alignment of die‐upset magnets arises exclusively from the fact that hot deformation produces platelets of Nd2Fe14B grains with their c axes parallel to the stress axis. It is argued that yielding plays an important role in the densification of the hot‐pressed samples, and diffusion slip is critical to the alignment. The grain boundary phase appears as a crystalline or noncrystalline phase in the hot‐pressed samples but has an fcc structure in the die‐upset samples. Magnetic domain walls are shown to be pinned at the grain boundary phase in both cases. A Nd‐rich spongy phase is ...

Magnetic properties of hydrothermally recrystallized magnetite crystals.

Abstract: The discrepancy between the magnetic hysteresis properties of magnetite crystals that are precipitated from solution ( 0.3 micrometer) is not an inherent property of magnetite but is caused by the highly stressed state of crushed material and by adhering finer fragments. The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals. Coercive forces of these narrowly sized crystals follow a power law over a wide size range (0.1 micrometer to 1 millimeter) as predicted by theory. Dislocation etch pits show similar dislocation densities for hydrothermally grown (3 x 1010 meter -2) and natural (1 x 1010 meter-2) magnetite crystals. Hysteresis parameters of hydrothermally grown crystals are similar to those of natural crystals but are about one-fifth of those for crushed grains.

Yttria-zirconia: Effect of microstructure on conductivity

Abstract: Complex impedance measurements and detailed analysis of the grain-boundary microstructure have been made on fully stabilized yttria-zirconia sintered bodies as a function of grain size. The prereacted yttria-zirconia powder used in this study was obtained from a commercial source. The powder has very high reactivity and starts sintering around 1200° C. The densification process is complete around 1350° C but the grain growth continues almost linearly with sintering temperature. The grain size variation obtained was between 1 and 30 μm. The grain-boundary resistivity when plotted against grain size showed an inflection in the vicinity of 1500° C sintering temperature. These results have been explained in terms of the grain-boundary microstructure changing with the sintering temperature. The thickness of the grain-boundary layer determined from impedance data and transmission electron micrographs are in reasonably good agreement. The activation energy for the grain-boundary resistivity was only slightly higher than that for the lattice resistivity.

Grain growth and grain size distributions in thin germanium films

Abstract: We have observed and characterized normal and secondary grain growth in thin films of germanium on silicon dioxide. Films were deposited on thermally oxidized silicon wafers, encapsulated with 1000‐A‐thick sputtered SiO2 films and annealed in evacuated ampoules at 900 and 915 °C. After 5 min, the films had developed a columnar grain structure as a result of normal grain growth. The grain size distributions were lognormal with mean grain diameters of about 2.5 times the film thickness. The standard deviation of the normal grain size distribution did not change significantly with annealing time and temperature. Secondary grain growth, which can lead to grains that are much larger than the film thickness, occurred in films that were annealed for longer periods of time. The normal grain size distribution remained stationary, i.e., the peak height and width did not increase with annealing time. Secondary grains were few in number compared to normal grains, and were manifest as a small tail on the normal grain size distribution. The rate of secondary grain growth was constant and largest in the thinnest films.

Stochastic modeling of the independent roles of particle size and grain size in transgranular cleavage fracture

Abstract: The independent roles of grain size and particle size on sharp crack and rounded notch toughness are investigated over a range of temperatures from the lower shelf into the early ductile/brittle transition region. The results are interpreted in terms of a weakest link statistical model wherein the onset of failure coincides with the critical propagation of a particle microcrack into the matrix. It is shown that, for a fixed particle size distribution, both sharp-crack and rounded-notch toughness decrease with increasing grain size. However, at fixed grain size, the sharp-crack toughness increases, while the rounded-notch toughness decreases with increasing particle size. Such effects result primarily from the difference in the number of activated particles in the plastic zone.

Magnetite in soils; I, The synthesis of single-domain and superparamagnetic magnetite

Abstract: A series of experiments has been carried out to investigate the possible formation of magnetite, Fe3O4, under ambient soil-forming conditions. Rapid and easy synthesis of magnetite was achieved through controlled oxidation of Fe2+ solutions at room temperatures and near neutral pH values. The synthetic products were found to range in size between 0·01–0·07 µm (mean diameter) and hence span the theoretical superparamagnetic-single-domain grain-size boundary.

Effect of composition and grain size on slow plastic flow properties of NiAl between 1200 and 1400 K

Abstract: A series of about 15-micron diameter polycrystalline B2 crystal structure NiAl alloys ranging in composition from 43.9 to 52.7 Al (at. pct) have been compression tested at constant velocities in air between 1200 and 1400 K. All materials were fabricated via powder metallurgy techniques with hot extrusion as the densification process. Seven intermediate compositions were produced by blending various amounts of two master heats of prealloyed powder; in addition, a tenth alloy of identical composition, 48.25 Al, as one of the blended materials, was produced from a third master heat. Comparison of the flow stress-strain rate behavior for the two 48.25 Al alloys revealed that their properties were identical. The creep strength of materials for Al/Ni not above 1.03 was essentially equal, and deformation could be described by a single stress exponent and activation energy. Creep at low temperatures and faster strain rates is independent of grain sizes and appears to be controlled by a subgrain mechanism. However, at higher temperatures and slower strain rates, diffusional creep seems to contribute to the overall deformation rate.

Grain‐Boundary Microcracking Due to Thermal Expansion Anisotropy in Aluminum Titanate Ceramics

Abstract: The relation between grain size and grain-boundary microcracking during cooling in aluminum titanate ceramics was studied. Microcracking temperature was determined by the measurement of thermal contraction and expansion, which was accompanied by acoustic emission. When the ceramics were cooled at a rate of 6°C/min, stress relaxation did not occur below the sintering temperature of 1500°C. The relation between the temperature difference from the sintering temperature to the microcracking temperature and the grain size showed good agreement with the prediction based on the energy criterion of grain-boundary microcracking.

Magnetic properties of decarburized steels: An investigation of the effects of grain size and carbon content

Abstract: The effects of carbon on the magnetic properties of steel have been of great interest over the years both because of the great commercial importance of steel and because it is known that carbon is the most important single factor affecting the magnetic properties of steel. Slight changes in the amount and distribution of the carbon content can completely mask any changes due to grain size, stress, and the effects of other impurities. The magnetic properties of several specimens of steel in which the carbon content has been deliberately reduced are investigated so that the more subtle effects on magnetic properties caused by these other factors can be determined. The results presented include magnetic Barkhausen measurements, magnetoacoustic emission measurements, and magnetization data. The grain structure of each specimen was closely monitored and the resulting magnetization data correlated with the microstructure.

Means for stabilizing polycrystalline semiconductor layers

Abstract: Implantation of oxygen or nitrogen in polysilicon layers to a dose above about 10 15 ions/cm 2 retards rapid grain boundary migration of conventional dopants such as B, P, As, Sb, and the like during dopant activation. Pre-annealing of the poly films to increase the grain size also decreases rapid grain boundary migration. The effects can be combined by first pre-annealing and then implanting oxygen or nitrogen before introducing the dopant. It is desirable to anneal the oxygen implant before introducing the dopant to allow for oxygen diffusion to the grain surfaces where it precipitates and blocks the grain boundaries. Vertical and lateral migration of the dopants can be inhibited by placing the implanted oxygen or nitrogen between the dopant and the location desired to be kept comparatively free of dopants. When very high dopant activation temperatures are used the blocking effect of the oxygen on the grain boundaries is overwhelmed by dopant diffusion through the grains.

Effects of cold work on the oxidation behavior and carburization resistance of Alloy 800

Abstract: The structure of the oxide layer formed on Alloy 800 at 600 °C in superheated steam markedly indicates the role of the grain boundaries as easy diffusion paths of Cr and Mn to the alloy/oxide interface. Increasing the number of grain boundaries by 10-90% cold work leads to increasing Cr- and Mn-content in the scale and to decreasing oxide growth rates. Variation of the grain size by different annealing treatments leads – since the Cr-content in the scale is decreasing with the grain size – to a linear relation of growth rate and grain size. The effect of cold work was also demonstrated on the protectiveness of the oxide scale towards carbon uptake and carburization of Alloy 800. After preoxidation of differently deformed specimens at 900 °C, these were exposed to a CO-CO2H2O-H2 mixture at 700 °C for long time. The gas mixture was tagged with 14C so that the C-ingress into the oxide scale and into the alloy could be sensitively monitored by autoradiography and (upon stepwise polishing) radioactivity measurements of the carbon penetration. The carbon uptake is effectively reduced with cold working; in contrast a non-deformed, electropolished and preoxidized specimen shows relatively high C-content after exposure. The investigations prove the highly favorable effect of mechanical pretreatment on the formation of the oxide scale on an austenitic Fe-Ni-Cr alloy. Cold work and other methods of surface deformation (grinding, polishing, sand blasting, shot peening) generate easy diffusion paths for fast Cr-diffusion to the surface and sufficient supply of Cr to form a protective oxide layer. Einflus der Kaltverformung auf Oxidationsverhalten und Aufkohlungsbestandigkeit von Alloy 800 Der Aufbau der Oxidschicht, die sich auf Alloy 800 bei 600 °C in uberhitztem Dampf bildet, zeigt deutlich die wichtige Rolle der Korngrenzen als schnelle Diffusionswege des Cr and Mn zur Grenzflache Legierung/Oxid. Eine 10-90% Kaltverformung fuhrt aufgrund der zunehmenden Zahl an Korngrenzen zu erhohtem Cr- und Mn-Gehalt der Oxidschichten und zu abnehmenden Oxidationsgeschwindigkeiten. Bei Veranderungen der Korngrose durch verschiedene Gluhbehandlungen erhalt man – bei mit der Korngrose abnehmendem Cr-Gehalt – eine lineare Beziehung zwischen Oxidationsgeschwindigkeit und Korngrose. Die Auswirkung der Kaltverformung wurde ebenfalls fur die Schutzwirkung der Oxidschicht gegen das Eindringen von Kohlenstoff und die Aufkohlung von Alloy 800 gezeigt. Nach Voroxidation verschieden verformter Proben bei 900 °C wurden diese langere Zeit einer CO-CO2-H2-H2O-Atomosphare bei 700 °C ausgesetzt. Das Gasgemisch war mit 14C indiziert, so das die C-Aufnahme in die Oxidschicht und in die Legierung durch Autoradiographie und Radioaktivitats-Messungen (unter schrittweisem Abpolieren) sehr empfindlich nachgewiesen werden konnte. Die C-Aufnahme wird mit zunehmendem Verformungsgrad wirksam vermindert – im Gegensatz hierzu zeigt eine unverformte, elektropolierte und dann voroxidierte Probe relativ hohe C-Gehalte nach der Auslagerung. Diese Untersuchungen belegen die gunstige Auswirkung einer mechanischen Vorbehandlung auf die Bildung einer schutzenden Oxidschicht auf einer austenitischen Fe-Ni-Cr Legierung. Kaltverformung und andere gleichartig wirksame Methoden der Oberflachenbearbeitung (wie Schmirgeln, Polieren, Sandstrahlen) erzeugen schnelle Diffusionswege, so das Cr schnell zur Oberflache diffundieren kann und in hinreichender Menge fur die Bildung einer schutzenden Oxidschicht zur Verfugung steht.

The role of boron in ductilizing Ni3Al

Abstract: Ductilization of Ni3Al at room temperature by microalloying with boron has been primarily attributed to the increased grain boundary cohesion in the presence of boron.1 However, another aspect of the role played by boron in ductilizing Ni3Al is revealed when the Hall-Petch relationships for Ni3Al and B-doped Ni3Al are compared. A shallower slope for the B-doped Ni3Al compared to that for Ni3Al indicates a reduced resistance to slip propagation across grain boundaries, and therefore reduced stress concentration at boundaries, in the presence of boron. This comparison of Hall-Petch relationships was carried out by generating data for powder processed B-doped Ni3Al at various grain sizes and by compiling data for Ni3Al from the literature. In addition, the room temperature fracture of B-doped Ni3Al has been shown to initiate along certain grain boundaries. The fracture eventually occurs by transgranular ductile tearing.

Grain Growth in Two‐Phase Ceramics: Al2O3 Inclusions in ZrO

Abstract: The effect of Al2O3 inclusions with a greater average size (0.6 μm) than the average particle size of the major phase powder (<0.1 μm) on grain gowth was examined by sintering ZrO2/Al2O3 composites (0,3,5,10, and 20 vol%) at 1400°C and then heat-treating at temperatures up to 1700°C. Normal grain growth was observed for all conditions. The inclusions appeared to have no effect on grain growth until the ZrO2 grain size was ∼1.5 times the average inclusion size. Grain growth inhibition increased with volume fraction of the Al2O3 inclusion phase. At temperatures 1600°C, the inclusions were relatively immobile and most were located within the ZrO2 grains for volume fractions <0.20; at higher temperatures, the inclusions could move with the grain boundary to coalesce. Grain growth was less inhilited when the inclusions could move with the boundaries, resulting in a larger increase in grain size than observed at lower temperatures. Analogies between mobile voids, entrapped within grain at lower temperature due to abnormal grain growth during the last state of sintering, and the observations concerning the mobile inclusions are made suggesting that grain-boundary movement can “sweep” voids to grain boundaries and eventually of four-grain junctions, where they are more likely to disappear by mass transport.

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract: Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains dα formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size dγ. In the cooling rate range studied (q= 0·05–5 K s−1), dα was found to be proportional to q−0·26dγ 0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q−0·17dγ 0·33...

Oxygen Partial Pressure and Grain Growth in Donor-Doped BaTiO3

Abstract: The grain growth of donor-doped BaTiO3 at different oxygen partial pressures was studied. Results showed that the oxygen pressure had a pronounced influence on the grain growth and related effects. A model for the grain size anomaly during sintering of donor-doped BaTiO3 in the presence of a TiO2-rich liquid phase is proposed.

Function of Substrate Bias Potential for Formation of Cubic Boron Nitride Films in Plasma CVD Technique

Abstract: Cubic BN thin films are formed in RF discharge in B2H6 and N2 at low pressures under a magnetic field to confine the plasma, for negatively self-biased substrate electrodes. Ion bombardment on the growing surface is suggested to play an important role in the formation of cubic BN. The deposited films are characterized by infrared absorption spectroscopy and transmission electron microscopy which show that they are composed of microcrystals of cubic BN with 100-200 A grain size.

Modelling of diffusion bonding of metals

Abstract: Modelling of diffusion bonding has been carried out to quantify the kinetics of the bonding processes and to predict the time for achieving a sound bond. An alternative geometric assumption for the shape of the interfacial cavities to those considered previously was employed. Three subprocesses of bonding were introduced to simplify the modelling. These involved volume and interfacial diffusion coupled with creep, rigid collapse, and surface diffusion. The effects of grain size and phase ratio on diffusion bonding have also been considered. The predictions are compared with existing experimental data for copper and Ti–6Al–4V alloy and in general show good agreement.MST/588

Effects of grain size and precipitate size on the fatigue crack growth behavior of alloy 718 at 427 °C

Abstract: The fatigue crack growth rates of four Alloy 718 microstructures comprising a two-by-two matrix of grain size and γ" precipitate size were determined in air at 427 °C and 033 Hz For a stress ratio of 005, slower Region II rates were obtained for coarse-grained microstructures, independent of γ" size, and microstructures with large γ", independent of grain size In the near-threshold regime, the coarse-grained microstructures again showed slower growth rates (higher ΔKth), whereas the effect of γ" size was mixed Deformation modes were studied using scanning and transmission electron microscopy Fatigue deformation resulted in the formation of slip bands which were longer for coarse-grained microstructures and typically spaced farther apart and more planar for large γ micro-structures The concepts of dislocation reversibility and slip band strain localization were used to explain the microstructural effects Fatigue morphologies and cyclic constitutive behavior were con-sistent with the observed deformation modes For a stress ratio of 075, the effects of grain size and γ" size were essentially identical to those observed for a stress ratio of 005 This indicated that roughness-induced closure had a minimal influence on the differences that were observed in fatigue crack growth behavior