Showing papers on "Grain size published in 1974"

Grain-size effects on properties of some ferroelectric ceramics

Abstract: Hot-pressed ceramics of the lead titanate zirconate system with different grain sizes were prepared and some of their electrical properties measured. The values of the dielectric and piezoelectric properties were analysed together with information on barium titanate ceramics already published in the literature. Many similarities were found between both systems. There are two main contributions to the properties; the single-crystal single-domain intrinsic effect and the contribution of the wall motion. The intrinsic effect is analysed considering the grains as monodomains. A model is developed which proposes that each grain has a different transition temperature Tc. The Tc values are taken as following a Gaussian distribution and the behaviour of the dielectric constant against temperature is computed and checked with experimental results. All the results can be explained qualitatively with the model; a connection between findings and the idea of diffuse phase transitions is outlined.

Theoretical considerations of electrical conductivity in a partially molten mantle and implications for geothermometry

Abstract: Relationships between bulk effective electrical conductivity, melt fraction, and liquid path connectivity are derived for a partially melted material. Hashin-Shtrikman bounds are determined for the conductivity on the basis of entropy production and compared with results obtained for exact geometrical models for the limiting cases of isolated melt pockets and complete grain boundary wetting. Models used are (1) a spherical particle assemblage of an infinite number of different-sized composite spheres, each containing an inner core of one phase, surrounded by an outer shell of a second phase, and (2) a three-dimensional periodic array of identical-sized cubes of one phase, surrounded by a second phase that extends continuously throughout the body. Both models yield the same expressions as those of the upper Hashin-Shtrikman bound for the bulk conductivity dependence on melt fraction for small melt fractions with complete liquid bridging. A small dependence on grain size and shape is inferred. Partial melting in the form of isolated melt pockets is found to be ineffective in raising bulk conductivity significantly over that of the solid phase material. Quasi-continuous grain boundary wetting is considered in terms of analog modeling. Melt fraction and liquid path connectivity are found to be main determining factors for electrical conductivity. It is concluded that grain boundary and/or edge wetting is necessary for partial melting to raise bulk conduction in the mantle appreciably over that of its solid phase material. Such wetting is therefore inferred to be a highly probable mechanism responsible for high conductivity anomalies observed in deep geomagnetic soundings. Implications for geothermometry are considered.

Magnetic properties and domain structure in grain-oriented 3% Si-Fe

Abstract: Magnetic domain observations have been used to clarify the relationship between metallurgical structure and the core loss, permeability, and magnetostriction of grain-oriented 3% Si-Fe. Both the prediction of domain structures based on anisotropy, magnetostatics, stress, etc., and the various methods of making domain observations are reviewed. Both static and dynamic domain structures are shown and used to illustrate the relationships among domain structure, metallurgical structure, and magnetic properties. Core losses are shown to depend on the domain wall spacing and mobility, which in turn are controlled by grain size, stress, and defect structure. The high field permeability ( B at H = 10 Oe) is related to the amount of flux closure structure present, which is determined by the misorientation of the grains in the material. Magnetostriction is caused entirely by the motion of 90° domain walls which are introduced during the formation of flux closure structure and by residual or applied stress.

Deformation mechanism maps based on grain size

Abstract: A new form of deformation mechanism map is introduced based on grain size. Maps are developed for pure aluminum at two different homologous temperatures, and the relative contributions of the various deformation processes are estimated as a function of stress at different grain sizes. A simple method of constructing these maps is presented, requiring only a knowledge of the relevant constitutive equations for the various mechanisms and a minimum of calculation.

Theory of the optical properties of snow

Abstract: The optical properties of a snowpack are calculated, giving relations between the asymptotic flux extinction coefficient and albedo under diffuse illumination on the one hand and the density, grain size, and wavelength, on the other. A geometrical optics calculation of the properties of a single grain is used with approximate solutions of the radiative transfer problem. Most of the scattering is the result of change in direction of the light beam upon transmission through the grain, rather than reflection. The asymptotic flux extinction coefficient is directly proportional to the snowpack density and inversely proportional to the square root of the grain size. The albedo under diffuse illumination is independent of density and proportional to the square root of the grain size. The derived relations agree with experimental data to within 20% for the flux extinction coefficient and to better than 2% for the albedo without using adjustable parameters. Higher values of the flux extinction coefficient will be measured if the snow is confined within a finite cylinder with absorbing sides. An analysis of this effect is given.

Segregation of Magma from a Mostly Crystalline Mush

Abstract: A model based on the fluid dynamics of intermixed fluids indicated that favorable conditions for segregation of melt from a mostly crystalline mush upwelling from the asthenosphere included a high concentration of melt, a narrow conduit containing the mush, a large grain size, and a large ratio of the viscosity of grains to the viscosity of melt. The greater depth, inferred geochemically, of the source regions of seamount lavas, compared with mid-oceanic ridge lavas, is attributable to the dependence of segregation on conduit width. For thermal reasons, conduit width is smaller at greater depths for small sources of material such as seamounts. Observed systematic enrichment of seamount and island-arc lavas in radiogenic isotopics (which, for chemical reasons, would be most abundant in domains containing high fractions of water) relative to mid-oceanic ridge lavas may be due to disproportionate representation of ubiquitous, small-scale, water-rich domains in the source region of seamount lavas. The fraction of melt is strongly dependent on water content in the deep source regions of seamount lavas but only weakly dependent in the shallow source regions of mid-oceanic ridge lavas. This also complicates identification of subducted sediments in island-arc lavas. The viscosity of partial melt in the asthenosphere, inferred from seismic attenuation studies, is too high for that melt to segregate efficiently.

Grain-Size Effects during Dynamic Recrystallization of Nickel

Abstract: The effect of initial grain size on the hot torsional behaviour of nickel has been examined in the range 800–1000° C over a wide range of strain rates. Significant changes in the torque/twist curves have been observed. Metallographic examination has shown that the kinetics of dynamic recrystallization is accelerated by decrease in grain size. Recrystallization proceeds by nucleation of new grains with limited growth forming colonies of new grains. Grain-boundary nucleation is usually observed but in certain circumstances intragranular nucleation occurs.

Grain size dependence of yield stress in marble

Abstract: The data collected indicate that the yield stress of marble increases linearly with the inverse square root of the mean grain size. Increased confining pressure to 2 kbars causes the grain size effect to become more pronounced, whereas increased temperature to 300°C causes this effect to become less important. The Petch [1953] theory of the yield stress-grain size relationship, although it is in partial agreement with the data, is found to be incomplete.

Grain-size and pressure effects on the dielectric and piezoelectric properties of hot-pressed PZT-5

Abstract: Experimental measurements and model interpretations are given for properties of hot-pressed PZT-5 specimens with grain sizes from ∼2 to 5 μm. The dielectric and piezoelectric parameters are studied as functions of temperature, of hydrostatic pressure up to 2800 kg cm−2, of grain size and of the poling history, using a number of methods in order to disentangle domain wall effects from others. The models presented are a multi-grain model to describe the diffuse transition, and a model of moving 90° domain walls for the piezoelectric variations with grain size and with poling and with pressure. We suggest that some differences from the behaviour of barium titanate ceramics result from thinner domain walls.

Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel and an iron-nickel-carbon alloy

Abstract: The effects of austenitizing conditions on the kinetics at the start of martensite formation in Fe-31Ni and Fe-31 Ni-0.28C alloys have been studied using electrical-resistance measurements during cooling of the specimens to follow the course of the transformation. The primary object of the study was to decide whether or not a change in austenitizing temperature, in the absence of a change in austenite grain size, has any effect on the Ms temperature or the burst characteristics of athermal martensite. It is concluded that it does not, suggesting that the potential nuclei (embryos) of martensite are mechanically stable crystal defects. Another interesting observation is that when the austenite grain size is small, the Mb temperature increases with increasing grain size and the burst is always small. When the austenite grains are coarse, the Mb temperature is independent of the grain size and the burst is large. It is suggested that this phenomenon is a result of the elastic shear stress concentration being related to the size of the first martensite plate and, in turn, to the size of the austenite grain.

Effect of grain size on void formation during high-energy electron irradiation of austenitic stainless steel

Abstract: Thin foils of an ‘ experimental ’ austenitic stainless steel, with and without dispersions of aluminium oxide particles, are irradiated with 1 MeV electrons in a High Voltage Electron Microscope at 600°C. Evidence of grain size dependent void nucleation, void concentration, and void volume swelling are presented for grains in the size range 0°45 to ∼ 50 μ. In both undoped and helium-doped samples the void nucleation is delayed, void concentration is lowered, and void volume swelling is reduced by decreasing the size of the grain under irradiation. The results are discussed in terms of a ‘ defect depletion ’ model based on the property of grain boundaries as neutral and unsaturable sinks for vacancies and self-interstitials. It is suggested that even in the presence of sufficiently large amount of impurity gas atoms, a critical level of vacancy supersaturation is necessary to produce critically sized vacancy clusters which upon stabilization become viable void nuclei. It is concluded that the pres...

Calculated grain size-dependent vacancy supersaturation and its effect on void formation

Abstract: In order to study the effect of grain size on void formation during high-energy electron irradiations, the steady-state point defect concentration and vacancy supersaturation profiles have been calculated for three-dimensional spherical grains up to three microns in size. In the calculations of vacancy supersaturation as a function of grain size, the effects of internal sink density and the dislocation preference for interstitial attraction have been included. The computations show that the level of vacancy supersaturation achieved in a grain decreases with decreasing grain size. The grain size dependence of the maximum vacancy supersaturation in the centre of the grains is found to be very similar to the grain size dependence of the maximum void number density and void volume swelling measured in the central regions of austenitic stainless steel grains. This agreement reinforces the interpretation that the grain size effect is due primarily to the depletion of point defects from the grain interi...

The effect of ingot processing treatments on the grain size and properties of Al alloy 7075

Abstract: An investigation was carried out to determine the effect of various ingot thermal mechanical processing treatments on the grain size and mechanical properties of high purity homogeneous 7075 aluminum alloy sheet and plate. The results indicate that the recrystallization of 7075 alloy into a fine grained structure can be controlled by the distribution of the Cr in the microstructure, as well as by the distribution of the major alloying elements, Zn, Mg and Cu. A new ingot processing technique, FA-ITMT, was developed for producing fine grained 7075 sheet and plate. Data are presented which show that fine grained 7075 sheet and plate have equivalent strength and significantly better ductility than conventionally processed material.

Improvements in the fatigue strength of Ti-6Al-4V through microstructure control

Abstract: Fatigue deformation and stage I (shear mode) crack initiation in Ti-6Al-4V alloy test pieces have been studied using optical microscopy. Two types of stage I fatigue crack initiation were observed, (a) alongα/β interfaces and (b) transcrystalline initiation acrossα grains in partly transformed microstructures and acrossαβ interfaces in fully transformed microstructures. The α/β interface cracking occurred predominantly in the low stress regions of the test pieces. These observations suggested that a microstructure with a smallα grain size, to minimize the mean free slip path, and with minimum lengths ofα/β interface, would have a high fatigue strength. Such a microstructure, with anα grain size of < 10 μm, and spheroidal or near spheroidalβ particles, was produced by thermo-mechanical processing. The rotating cantilever fatigue strength of this microstructure, ± 670 MN m−2 at 10−7 cycles, compares with fatigue strengths in the range ± 480 to ± 590 MN m−2 for commercial Ti-6Al-4V bars.

The influence of grain size and impurities on the magnetic properties of the soft magnetic alloy 47.5% NiFe

Abstract: An attempt has been made to equate quantitatively the coercive force as a function of the grain size and the concentration of nonmagnetic inclusions. It was found that the coercive force H c of recrystallized 47.5% nickel-iron can be completely described, independent of the final anneal temperature, by the grain size d K and the inclusion content N F . Only nonmagnetic inclusions of sub-micron size (0.02- 0.5 μm) were found to be of significance. A definite correlation between the coercive force of 47.5% nickel-iron and those inclusions visible using an optical microscope (> 1μm) was not observed. Furthermore, there was no clear dependence of the coercive force on the sulfur and oxygen content. The grain size term H cK was found to be in good agreement with the theory of A. Mager [1]. Surprisingly, there was no difference between the low-angle grain boundaries of strip with cube texture and the large-angle grain boundaries of un-textured strip with respect to H cK .

Dielectric properties of fine-grained PbTiO3 crystals precipitated in a glass

Abstract: The spontaneous deformation of PbTiO3 crystals precipitated in a PbOTiO2Al2O3SiO2 glass was found to decrease with decreasing grain size. This trend remains even after the glass matrix surrounding PbTiO3 crystals is removed completely by HNO3 acid immersion. Consequently, a linear electro-optic effect cannot be expected from transparent glass-ceramics which necessarily comprise very-fine-grained crystals. The dielectric constant of PbTiO3 crystals surrounded by a glassy matrix shows two maxima at grain sizes 0.15 μm and 250 A, but is still fairly high even at a grain size as small as 160 A. This suggests that a material with a large quadratic electro-optic effect may be produced from a transparent glass-ceramic. The maxima of the dielectric constant at grain sizes 0.15 μm and 250 A were interpreted in terms of the internal stress and internal electric field, respectively.

Grain-Size Control in Steels

Abstract: An account has been given of the methods of grain-size control in steels. The importance of particle pinning in inhibiting grain coarsening has been described and some consideration given to the solubities and coarsening characteristics of the particles themselves. Grain-size control by hot working has also been considered, where the recrystallization characteristics of the austenite are important in connection. with the grain size of the ferrite-transformation product. The effects of second-phase particles on the recrystallization of cold-worked carbon steels have also been considered. The importance of the integrated nucleation rate on the final grain size has been demonstrated, independent of whether the ferrite grains are formed by transformation or by recrystallization.The mechanism of secondary recrystallization has been described, and it is suggested that this process can be caused by the presence of critically dispersed second-phase particles.

Structure of electroplated and vapor‐deposited copper films. III. Recrystallization and grain growth

Abstract: Changes in the structure of electroplated and vapor‐deposited copper thin films, as a result of isothermal and isochronal annealing in the temperature range 100–500 °C, have been investigated by x‐ray diffraction and metallographic techniques. In the electroplated films, recrystallization occurred with an activation energy of about 0.65 eV until the recrystallized grains attained sizes comparable to the film thickness. In vapor‐deposited films, such recrystallization is believed to take place immediately after deposition on heated substrates. Grain growth in both sets of copper films proceeded with an activation energy of about 1.55 eV, until a limiting grain size was reached. This limiting grain size is shown to be dependent on the film thickness and the extent of texture present.

Effects of grain size on the electrical properties of PLZT ceramics

Abstract: A group of transparent hot-pressed ferroelectric (Pb0.92La0.08)(Zr0.65Ti0.35)098O3 ceramics with controlled grain size from 1 to 14 μm and with few pores, less than 0.2% at most, were prepared from the oxide powders by applying a hot-pressing procedure including atmosphere control. The electrical and optical constants of these ceramics were determined as a function of grain size. The effect of grain size on each measured constant, χ, was expressed by a simple equation, χ = A + B/√GS . Polarization-reversal properties were also measured and the switching time related with the space charge effects were discussed.

The Evaluation of the Interparticle Spacing in Dispersion Alloys

Abstract: It is well known that dispersed particles (whether of intermetallic or non-metallic phases) can have a considerable influence on the properties of metals, e.g. strengths are increased, recrystallization can be either accelerated or retarded, and the matrix grain size is often much reduced compared with that of a single-phase alloy having the same composition as the matrix of the dispersion alloy. The magnitude of these effects depends on the size and spacing of the dispersed particles. However, these parameters seldom have an absolute value, even in a single specimen, since the particles are rarely, uniform in size, shape, or distribution. In addition, the interparticle spacing can be calculated on the basis of any one of several different definitions and/or formulae, each of which leads to a different numerical value for a given alloy structure.

Grain size effects on the ferroelectric-paraelectric transition, the dielectric constant, and the lattice parameters in lanthana-substituted lead titanate

Abstract: The effect of the grain size on unit-cell dimensions and dielectric properties has been studied in ceramic lanthana-substituted lead titanate of the general composition Pb1-αxLaxTiO3+x (15-α) with α = 120 and x = 007 (PLT7) and x = 018 (PLT18) The grain size ranges from 06 to 11 µm and the effects are most pronounced on the smaller sizes The tetragonal distortion, characterized by the ratio c/a of the unit-cell dimensions c and a, decreases by decreasing grain size This effect becomes relatively smaller by decreasing temperature A decrease of the grain size leads to a decrease of the maximum value of the dielectric constant e' at the Curie temperature T'c and to an increase of e' at lower temperatures The Curie temperature shifts to a lower temperature while the phase transition is less sharp with decreasing grain size The results can be discussed in terms of an average value of internal compression stresses in the direction of the c-axis of the crystal

Chemical Diffusion in Polycrystalline Al2O3 as Determined from Electrical Conductivity Measurements

Abstract: By analyzing the changes with time of the electrical conductivity of polycrystalline Al2O3 after the O2 pressure was changed, a defect diffusion coefficient was obtained which was assigned to the Al or O ion, whichever is the faster-diffusing species. Both decreased grain size and MgO addition increase the defect diffusion coefficient. The grain-boundary defect diffusion coefficient for the undoped material was estimated to be: and that for the MgO-doped material was over the range 1100° to 1350°C (δ is the effective thickness of the boundary and s the coefficient of segregation of defects to the boundary region). The mechanism of grain-boundary diffusion is discussed in terms of defect mobility.