Showing papers on "Grain boundary published in 1969"

Average Grain Size in Polycrystalline Ceramics

Abstract: A model is proposed which realistically characterizes the grain structure of polycrystalline ceramics. The average grain size of a log-normal distribution of grain sizes with tetrakaidecahedral (truncated octahedral) shape is related to the average intercept size by a proportionality constant. This result can be used to determine the average grain size of a sintered powder compact composed of nontextured grains which shows no discontinuous grain growth.

Electrical resistivity model for polycrystalline films: the case of specular reflection at external surfaces

Abstract: A model is developed for estimating effects due to electron scattering from grain boundaries, occurring simultaneously with background scattering. Since grain‐boundary effects are negligible in bulk materials, the model is particularly relevant to polycrystalline metal films in which a very fine‐grained structure is often found. It is shown by solution of the appropriate Boltzmann equation, that the total resistivity can be strongly dominated by grain‐boundary scattering. If grain size increases with film thickness, a marked dependence of resistivity on thickness exists, even when scattering from external surfaces is negligible or is completely specular.

New analysis of attenuation in partially melted rock

Abstract: A model of partially melted rock is analyzed in which melt occurs as thin films along grain boundaries. For analysis, the liquid phase is assumed to be a dispersion of randomly oriented ellipsoidal inclusions with minor axes much smaller than major axes. Expressions are derived for complex bulk modulus and rigidity. Rigidity and attenuation in pure shear are found to depend on the number of sites at which melting has occurred rather than to depend only on the concentration of melt, in agreement with published observations. On the other hand, response in pure dilatation depends on concentration alone, with the result that attenuation in pure dilatation is low and bulk modulus is approximately equal to that for unmelted material.

An experimental determination of the surface energies of ice

Abstract: A direct experimental method has been used to determine the magnitudes of the ice-water vapour, ice-water, and ice-ice (grain boundary) surface free energies. The magnitudes were found to be 109, 33 and 65 erg/cm2, respectively, at 0°C. The experimental results indicate that the grain boundary surface free energy varies slightly with the mismatch between the grains but that variations with crystal orientation of the ice-water vapour and the ice-water surface free energies are small.

Aluminum + water reaction

Abstract: During the aluminum + water reaction, aluminum ions and electrons are removed in separate steps at different sites on the surface. Ions are removed nearly uniform over the entire surface, which is covered at all times with a thin amorphous oxide film. The outer surface of this oxide is first hydrolyzed and then dissolves to yield soluble species which either remain in solution or, at intermediate values of pH, precipitate as a porous hydroxide of extremely small particle size. The hydroxide appears to be identical to pseudoboehmite. The overall rate of the corrosion reaction is controlled by this dissolution of the film and by the disposition of the soluble products. The corrosion rate is nearly independent of specimen potential, solution pH below 10, and of the presence in solution of many salts at concentrations as high as 1 mole/1. The rate is strongly dependent on temperature, on the presence of specific inhibitive salts, and increases rapidly at high pH. In environments which preclude oxide dissolution, e.g., water + dioxane mixtures or water vapour, the corrosion rate is drastically reduced. The corrosion rate is constant when no solid hydroxide is formed, and is otherwise a strong function of the amount of reaction. At high temperatures the rate decreases with time as the precipitated hydroxide hinders transport. At lower temperatures the rate may first increase with time as nucleation of hydroxide provides sinks for soluble species close to the interface, and then at long times the rate decreases as the hydroxide layer thickens. Electrons are removed more readily at special sites, and in our specimens these sites were primarily at the grain boundaries. Electron removal results in increased hydroxyl ion concentration, and this in turn results in more rapid attack on the protective oxide. At such cathodes the oxide film is maintained at a small constant thickness at which there is a balance between the rates of its dissolution by the basic solution and growth because of the great affinity of aluminum for oxygen. At cathodic sites there is thus anodic activity as well, but electron removal predominates. The metal is corroded more rapidly at cathodes, producing pronounced grain boundary attack in our specimens. Application of anodic potentials eliminates grain boundary attack.

The effect of grain boundaries on the electrical resistivity of polycrystalline copper and aluminium

Abstract: The contribution to the electrical resistivity caused by grain boundaries has been measured in polycrystalline specimens of high purity copper and various grades of high purity aluminium. The specific grain boundary resistivity appeared to be independent of the impurity content, depended to a small extent on the temperature in the range 4·2 to 77°K, and at 4·2°K had the values (3·12 ± 0·18) × 10−12 ohm cm2 for copper and (2·45 ± 0·09) × 10−12 ohm cm2 for aluminium.

Mechanical Properties of Polycrystalline β‐Sic

Abstract: The mechanical properties of chemically vapor-deposited β-Sic were measured in bending between room temperature and 1400°C. Material with grain diameters from less than 1 to 15 μm was tested. No grain-size dependence of the bend strength of dense (<99% of theoretical) Sic was observed at any test temperature. The fracture strength of dense Sic remained approximately constant between room temperature and about 900°C and then increased sharply up to the maximum test temperature of 1215° to 1400°C. This increase in fracture stress coincided with the onset of plastic yielding detectable in the stress-strain curves. The fracture mode of this material was transgranular cleavage at all test temperatures. The fracture stress of Sic of lower density, which was characterized by the presence of grain boundary flaws, decreased slightly at high temperature. The fracture mode of the low-density (3.17 g/cm3) β-Sic underwent a transition from predominantly transgranular at room temperature to predominantly intergranular at high temperature.

The Structure of High-Angle Grain Boundaries in Aluminium

Abstract: A transmission electron-microscopy study of high-angle grain boundaries has been made. When thin foils are cut in precisely oriented bicrystals of aluminium, some details of the grain-boundary structure appear. Micrographs of symmetrical tilt boundaries around [001] show linear defects parallel to the tilt axis. Their properties are tightly bound to the geometry of the bicrystal: this provides a good support to the idea of a transitional lattice as a structure of a high-angle boundary. A theoretical interpretation of these observations is developed essentially on geometrical considerations.

An electron microscope investigation of the flux pinning centres in superconducting V3Ga tapes

Abstract: The defect structure of V 3 Ga tapes produced by a solid-state diffusion process, has been studied by electron transmission using a microscope with an acceleration voltage of 500 kV. The only defects found in appreciable numbers are grain boundaries. It is concluded that these are the most important flux pinning centers. The amount of lossless current which can be carried by these tapes in the superconducting state is correlated with the grain size.

Growth of Grain-Boundary Precipitates

Abstract: A model descriptive of the growth of precipitates which nucleate at grain boundaries and grow pre ferentially along them (grain‐boundary allotriomorphs) is studied. The sequential processes during growth are presumed to be volume diffusion of solute to the grain boundary, transport along the grain boundary to the precipitate, and interfacial diffusion and deposition over the surface of the growing allotriomorph. An analysis of this mode of growth is applied to the results of recent observations of the lengthening and the thickening rates of θ‐allotriomorphs in an Al‐4% Cu alloy. On the basis of these measurements the diffusivity within the grain boundary Db is found to be 0.1 exp (−18480/RT) cm2sec−1, while the diffusivity within the allotriomorph‐matrix interface, D8, is D8=1.8 exp (−22020/RT) cm2 sec−1 when the mole fraction of solute in this interface is taken to be the average of that in θ and in α at the boundary, or D8=0.06 exp (−12740/RT) cm2 sec−1 when this surface concentration is assumed to be s...

Mechanisms of stress relief in polycrystalline films

Abstract: The stress required to operate dislocation sources within a grain, at a grain boundary, and at surfaces is found to be larger than the intrinsics tresses observed in polycrystalline films. It is therefore unlikely that a dislocation flow mechanism can relieve stresses in films. Grain boundary sliding and diffusional creep can, however, relieve stresses in films and equations describing the kinetics of stress relaxation are derived. It is suggested that stress relief occurs primarily by a diffusion-creep mechanism. Growth of hillocks during annealing of a film is briefly discussed in terms of the diffusion-creep mechanism.

Structural Changes during Superplastic Deformation of the Al–Cu Eutectic Alloy

Abstract: The microstructural changes that occur during superplastic deformation of the Al–Cu eutectic alloy are considered. It is shown that considerable modification occurs to the shapes and distribution of the inter-metallic CuAl2 particles during deformation. This effect may be understood in terms of sliding and diffusion-controlled shape changes associated with grain boundaries within the intermetallic. The roles of interface sliding and of diffusion creep in superplasticity are discussed.

Surface energy anisotropy by an improved thermal grooving technique

Abstract: A new technique for determining anisotropics of surface energy is described. Measurements of thermally equilibrated grain boundary grooves in wires exhibiting bamboo structure are numerically analysed and an analytical expression describing the complete y-plot is obtained by a least squares fit of a double Fourier series. The method is applied to pure copper wires which have been annealed at 1030°c in dry hydrogen. The principal features of the y-plot are:

Grain Boundary Self‐Diffusion in Nickel

Abstract: High‐purity nickel bicrystals with controlled grain boundary misorientations have been grown from the melt using an electron beam floating‐zone technique. These bicrystals have symmetrical tilt grain boundaries with 10° misorientation between {111} planes tilted about a 〈112〉 axis, and 10° twist about a {111} axis. The diffusion of nickel‐63 in these boundaries and similar sintered grain boundaries has been studied over the temperature range 600°–970°C using high‐resolution contact autoradiography. Using the dislocation pipe model for low‐angle grain boundaries, edge and screw dislocations were found to have similar activation energies of 40.7 and 44.9 kcal/mole, respectively. The value for the tilt boundaries is shown to agree well with recent work on isolated edge dislocations in nickel. It is concluded that the low‐angle tilt boundaries are composed of 〈112〉 {111} edge dislocation arrays which exhibit the same diffusion behavior as isolated edge dislocations formed by plastic deformation. Also, the hig...

Thermal Stability of Hastelloy Alloy C-276

Abstract: Hastelloy Alloy C has long been of major importance to the chemical process industry, but in many applications, vessels fabricated from Alloy C had to be solution heat treated to remove harmful weld heat affected zone precipitates which reduced corrosion resistance. Modification of the chemical composition produced Hastelloy Alloy C-276, which is more resistant to the precipitation of grain boundary particles than Alloy C. In order to fully utilize the improved alloy, and to understand its responses to fabrication techniques, it is helpful to understand the time-temperature-transformation characteristics. Grain boundary precipitates can form in Alloy C-276 when it is exposed to temperatures from 1200 to 2000 F (649 to 1093 C). The precipitation characteristics of Alloy C-276 compared to those of Alloy C can be shown through the use of a time-temperature-transformation curve (T-T-T). The precipitates have been identified as either “P” or Ni7Mo6-type phases. No M6C precipitate was found in Alloy C-...

Grain‐Boundary Fine Structure in an Iron Alloy

Abstract: Regular substructures were observed in grain boundaries of an annealed Fe‐0.75% Mn alloy by transmission electron microscopy. A detailed analysis was made on one of the grain boundaries. The result indicates that the analyzed grain boundary may be considered as a coincidence site grain boundary.

Ceramic CdS Solar Cell

Abstract: This paper describes the properties and the model of ceramic CdS photovoltaic cell made by treating CdS ceramic plate electrochemically in copper ion solution. The compositions and structures were analysed by means of X-ray Ginuie Camera and X-ray microanalyser. It was found that Cu2-xS (0x0.2) is formed along grain boundaries over a range of a several ten microns of the surface layer, and that the photovoltaic junctions exist at grain boundaries in this surface layer. From examinations of the voltage dependence of the barrier capacitance, the junctions could be classified into step junction, graded junction and Mott barrier, according to the prepared method. The spectral response measurement indicates that the origin of the photo-emf of the Mott barrier is mostly impurity photovoltaic effect.