Showing papers on "Grain boundary published in 1977"
TL;DR: In this paper, an approximate theoretical relation is derived which relates stress during steady state creep to both subgrain size and dynamically recrystallized grain size, based on equating the dislocation strain energy in the grain boundary to that in the enclosed volume.
Abstract: An approximate theoretical relation is derived which relates stress during steady state creep to both subgrain size and dynamically recrystallized grain size. The relation results from equating the dislocation strain energy in the grain boundary to that in the enclosed volume. Available data on metals and silicates are in excellent agreement with the theory. For paleopiezometry, the recrystallized grain size must be preserved by quenching, by cooling under stress, or by inhibition of grain growth by intimate mixture of two or more phases. In general, stress may be underestimated using rocks in which grain size has been reduced by dynamic recrystallization, especially if the grain size is very small. Stress may be overestimated using coarse grained rocks in which the grain size has increased toward the steady state value. Quantitative limits remain to be established. The theoretical relation can in principle be applied to any metal or mineral if only the effective isotropic elastic moduli and the Burgers vector are known. When used as a paleopiezometer, the technique indicates that high stresses on the order of 100 MPa are not infrequently associated with mantle diapirism and with large scale thrust faulting. Consideration of the Mt. Albert ultramafic body suggests that texturally inferred stresses from peridotite massifs and from ultramafic xenoliths in alkali olivine basalts might reflect either horizontal variations in stress across a rising diapir or else a vertical variation in stress as defined by the pyroxene geobarometer (Mercier et al. 1977). In either case the stresses are probably characteristic of local diapirism. Stresses characteristic of global upper mantle flow might be inferred from xenoliths originating from above kimberlite-producing diapirs.
515 citations
TL;DR: The dependence of minority carrier lifetime (τ) on the doping concentration N d, grain size d and interface state density N is at the grain boundaries in polycrystalline semiconductors has been calculated analytically as discussed by the authors.
Abstract: The dependence of minority carrier lifetime (τ) on the doping concentration N d , grain size d and interface state density N is at the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential V d adjacent to the boundaries, and ranges from \simeq 10^{2} to 106cm . s-1depending on N is and N d . Under illumination, the population of the interface states is altered considerably from its dark level and as a result, V d decreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing N d . Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities of \simeq 10^{11} cm-2eV-1, τ decreases from 10-6to 10-10s as the grain size is reduced from 1000 to 0.1 µm (for N_{d} = 10^{16} cm-3). For a constant grain size, τ decreases with increasing N d . The open-circuit voltage of p-n junction solar cells decreases for \tau \leq 10^{-7} s, whereas that for Schottky barrier cells remains at its maximum value until \tau \lsim 10^{-8} s.
259 citations
TL;DR: In this article, high-resolution electron microscopy lattice imaging studies of an MgO fluxed hot-pressed silicon nitride were conducted to show that intergranular second phases do indeed exist, but they are heterogeneously distributed.
Abstract: Although high-temperature strength loss in ceramics such as silicon nitride has been attributed to the presence of intergranular amorphous phases, until now no direct proof has been offered. The present paper describes high resolution electron microscopy lattice imaging studies of an MgO fluxed hot-pressed silicon nitride. These studies indicate that intergranular second phases do indeed exist, but they are heterogeneously distributed, appearing especially at multiple grain junctions. These room temperature observations are compatible with the microstructures expected at elevated temperatures.
233 citations
TL;DR: In this article, the surface tension between ice and water has been measured by observations of the equilibrium shape of grain boundary grooves at an interface stabilized by an imposed temperature gradient, which is in agreement with the theory of Nash and Glicksman (1971) which predicts groove shapes for the case of unequal thermal conductivities in two phases.
Abstract: The surface tension between ice and water has been measured by observations of the equilibrium shape of grain boundary grooves at an interface stabilized by an imposed temperature gradient The experiments have been designed to minimize the perturbing effects of heat flow in the cell walls which contain the samples The observed grooves are in agreement with the theory of Nash and Glicksman (1971) which predicts groove shapes for the case of unequal thermal conductivities in the two phases The measurements give a value of 29·1±0·8 mJ m−2 for the ice-water surface tension
215 citations
TL;DR: In this article, the absorption process is extended to random boundaries with the help of a new model for their structure in which the existence of an irregular arrangement of certain low energy groups of atoms is postulated.
Abstract: A lattice dislocation may lower its elastic energy by dissociation in a high angle grain boundary. The absorption process involves the separation of the product grain boundary dislocations at a rate limited in general by climb and their interaction with any pre-existing network. This description of the absorption process is extended to random boundaries with the help of a new model for their structure in which the existence of an irregular arrangement of certain low energy groups of atoms is postulated. Experimental results are analysed in support of the model for the absorption process.
180 citations
TL;DR: In this article, a modification to the transmission Kossel diffraction technique was developed that enabled useful orientation information to be obtained from deformed aluminium, and the misorientation produced by moderate deformation by compression of coarse-grained aluminium was studied.
177 citations
TL;DR: In this paper, the authors compared their measurements with the equivalent predictions for the grain boundary segregation of phosphorus in their steel and showed that these experimental measurements are fully compatible with an equilibrium segregation theory without reference to the effects of grain boundary carbides on segregation.
171 citations
148 citations
TL;DR: A high-angle tilt grain boundary viewed end-on in a (011) thin crystal of Ge has been imaged with atomic resolution in the Kyoto 500 kV electron microscope.
Abstract: A high-angle tilt grain boundary viewed end-on in a (011) thin crystal of Ge has been imaged with atomic resolution in the Kyoto 500 kV electron microscope. The boundary is shown to consist of alte...
146 citations
TL;DR: The results of a computer simulation of the structure of periodic grain boundaries between twin related crystals of aluminium are described in this article, where an interatomic potential derived on the basis of pseudo-potential theory is used.
Abstract: The results of a computer simulation of the structure of periodic grain boundaries between twin related crystals of aluminium are described. An interatomic potential derived on the basis of pseudo-potential theory was used. The algorithm employed allows simultaneous local atomic relaxation and rigid body translation of the adjacent grains. It was found that rigid body translation is a dominant contribution to relaxation, and that the energy of a boundary, $\gamma $, is not simply related to boundary periodicity. In addition, annealing twins in aluminium were observed by using transmission electron microscopy and detailed correspondence with theoretical predictions was found in two areas; the calculated $\gamma $'s of experimentally observed boundary planes were lower than those of geometrically possible alternatives, and excellent agreement between predicted and experimentally measured rigid body translations was obtained for two types of tilt boundary.
141 citations
TL;DR: In this article, the authors measured the length of the longest and width of the widest allotriomorph in each sample and calculated the parabolic rate constants for lengthening and thickening by means of the Atkinson analysis for oblate ellipsoids.
Abstract: The kinetics of lengthening and thickening of grain boundary allotriomorphs of proeutectoid ferrite were measured at several temperatures in high purity Fe−C alloys containing 0.11 pct, 0.23 pct and 0.42 pct C. These measurements were conducted by measuring the length of the longest and the width of the widest allotriomorph in each specimen. All specimens were austenitized so as to make the grain boundaries perpendicular to the plane of polish. This measurement technique appreciably reduced the scatter in the parabolic rate constant data previously encountered in thermionic emission microscopy measurements. Parabolic rate constants for lengthening and thickening were calculated, using the experimental aspect ratio, by means of the Atkinson analysis for oblate ellipsoids. The ratio of the measured to the calculated constants was in all cases less than unity. The previously made suggestion that these slow growth kinetics are due to faceting was supported through the observation of facets on allotriomorphs by means of scanning and transmission electron microscopy. The aspect ratio of ferrite allotriomorphs was shown to beca 1/3, independent of reaction time temperature and carbon content. The dihedral angle of the allotriomorphs was found to be 100±5 deg, as compared with a published angle for recrystallized and equilibrated specimens ofca 115 deg. Several possible explanations for the aspect ratio and dihedral angle findings are considered.
TL;DR: The melting behavior of drawn poly(ethylene terephthalate) bristles has been studied by means of differential scanning calorimetry as discussed by the authors, and the wide-angle X-ray diffraction pattern was analyzed.
TL;DR: Work hardening was found nearly independent of grain size for sizes below 1 μm, consistent with an absence of cell formation in sub-micron grain sizes as discussed by the authors. But the slip lengths for small grain sizes were found to be almost independent of strain.
TL;DR: In this article, the atomic structure and energy of high angle grain boundaries in high purity silver and silver alloys with or without solute segregation were investigated by identifying the orientation relationships corresponding boundaries of low energy.
TL;DR: In this paper, the compressive strength of polycrystalline alumina at 23°C is found to be strain-rate sensitive, but insensitive to environment, and the origin of the strength-strain-rate dependence is localized plasticity in the form of twinning and, possibly, slip.
Abstract: The compressive strength of polycrystalline alumina at 23° C is found to be strain-rate sensitive, but insensitive to environment. Scanning electron microscopy of specimens loaded to near failure indicates the origin of the strength-strain-rate dependence to be localized plasticity in the form of twinning and, possibly, slip. The interaction of these deformation bands with grain boundaries causes the initiation of microcracks. Higher stresses produce still more twin/slip-nucleated microcracks, which finally coalesce at failure. It is suggested that twinning also may be related to the tensile failure of alumina.
TL;DR: Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities as mentioned in this paper.
Abstract: Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities (Cr, Fe). Divalent iron in solid solution was found to enhance both aluminium lattice and oxygen grain-boundary diffusion. Large concentrations of divalent iron led to viscous Coble creep which was rate-limited entirely by oxygen grain-boundary diffusion. Nabarro-Herring creep which was rate-limited by aluminium lattice diffusion was observed in pure and chromium-doped material. Chromium additions had no effect on diffusional creep rates but significantly depressed non-viscous creep modes of deformation. Creep deformation maps were constructed at various iron dopant concentrations to illustrate the relative contributions of aluminium grain boundary, aluminium lattice, and oxygen grain-boundary diffusion to the diffusional creep of polycrystalline alumina.
TL;DR: In this paper, both conventional smooth and machine notched stress-rupture samples and dead weight loaded fatigue precracked fracture toughness specimens were evaluated in comparison with powder metallurgy techniques.
Abstract: During creep loading of IN-792, grain boundary morphology in conjunction with grain size strongly affected crack propagation. Compositional variations and fabrication techniques showed no significant effect. A primary requirement for materials to be used in gas turbine engine discs is satisfactory resistance to crack growth resistance in the 650 to 760°C range. Both conventional smooth and machine notched stress-rupture samples and dead weight loaded fatigue precracked fracture toughness specimens were evaluated in this study. Creep fractures took place by grain boundary cracking followed by rapid transgranular fractures. Composition variations had only very slight effects on crack propagation. Materials hot worked from castings had the same properties as those made by powder metallurgy techniques. The primary factors influencing the crack growth behavior were the grain size and grain shape. Increasing grain size markedly improved the toughness. By slow cooling through the gamma prime solvus a serrated grain boundary structure was developed that also improved the cracking resistance. Earlier creep fracture toughness studies have shown that the slow crack growth behavior can be described by a critical strain model in which the crack propagation is controlled by the yield strength, grain size, and a critical strain parameter. The present results are consistent with this model, with serrated grain boundaries introducing a four-fold increase in the critical strain parameter over that of smooth grained material.
TL;DR: The most important factor responsible for superplastic behavior was found to be the very fine grain size as mentioned in this paper, which is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.
Abstract: The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium. Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight line when plotted as log (ekTd*
2/DgbGb3) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.
TL;DR: In this article, the authors carried out a systematic experimental study of fracture in materials which contain hard second phase particles and found that the growth of cavities in the grain boundaries was the rate limiting step in the fracture process.
Abstract: We have carried out a systematic experimental study of fracture in materials which contain hard second phase particles. The principal variables in this study were the average size and spacing of the second phase particles, grain size, temperature, and the strain rate. Polycrystalline copper containing a dispersion of silica particles was the material used in these experiments. Three modes of fracture were observed: transgranular necking fracture, fracture by the propagation of intergranular cracks initiated at the surface, and intergranular fracture by grain boundary cavitation throughout the entire specimen cross-section. The transition between the fracture modes was shown to shift systematically with temperature, strain rate, and the microstructure. The intergranular fracture mode was studied in detail. The growth of cavities in the grain boundaries was determined to be the rate limiting step in the fracture process. It was determined that in the range of 10-4 to 10-7 s-1 in strain rate, the dominant growth mechanism of the cavities was power-law creep rather than diffusional transport. The ductility of the material in the intergranular mode of fracture was found to be strongly dependent on the area fraction of the second phase in the grain boundary and on the strain rate sensitivity of the material; it was weakly dependent on the grain size. A theoretical lower bound and a practical upper bound of the ductility in the intergranular fracture mode were established. The results are in qualitative agreement with the data on nickel-base alloys and other materials published in the literature.
Abstract: The electrical resistivities, current‐voltage characteristics, and thermopowers of a series of n‐doped BaTiO3 ceramics having systematically varying resistivity anomalies (posistor or PTC effect) are analyzed in light of the theoretical description of the grain‐boundary resistivity by a Schottky‐type potential barrier model (Heywang’s barrier). The observed current‐voltage dependences agree well with this model over a wide voltage range. However, the agreement between the theoretical and experimental temperature curves of the low‐voltage resistivities is weak if the surface acceptor states, which result as recently suggested from an excess of barium vacancies in the immediate vicinity of the grain surface, are assumed to be at the same energy. Satisfactory agreement is obtained when the acceptor states are distributed over a certain energy interval. An estimate of the surface charge then reveals that the resistivity below the Curie point is governed by a residual potential barrier originating from an exce...
TL;DR: In this paper, the surface and grain boundary energies of polycrystalline stoichiometric UO 2 at 1500, 1750 and 1900°C were determined using the multiphase equilibrium method for determination of wetting angles.
TL;DR: In this paper, it was found that the number of splits increased with decreasing finishing temperature and impact test temperature, and the amount of energy consumed in impact testing was inversely proportional to number of splitting.
Abstract: Splitting,i.e., delamination behavior in Charpy specimens of low-carbon steel was found to be caused by cracks propagating between pancake grains parallel to the rolling plane of the as-rolled plate. The results suggest the mechanism is a decohesion of grain boundaries that is independent of the texture of the material. The number of splits increased with decreasing finishing temperature and impact test temperature. The energy absorbed in impact testing was inversely proportional to the number of splits. In specimens annealed after rolling splitting was evident until the material was almost completely recrystallized.
TL;DR: In this paper, the lattice and grain boundary diffusion of 51Cr in Inconel-600 (nickel base alloy) has been studied using Gruzin's residual activity technique in the temperature range of 673-1523K.
TL;DR: In this article, a crystallographic treatment is developed which clarifies the relation between the structure of a grain boundary and its location between relatively translated crystals, and characterisation of line defects which can exist in grain boundaries is also facilitated by using this treatment, and the following topics are considered: (1) the computed structures in part I of this work; (2) steps at the cores of perfect grain boundary dislocations; (3) boundary structures related by c.l.
Abstract: A crystallographic treatment is developed which clarifies the relation between the structure of a grain boundary and its location between relatively translated crystals. Characterization of line defects which can exist in grain boundaries is also facilitated by using this treatment, and the following topics are considered: (1) the computed structures in part I of this work; (2) steps at the cores of perfect grain boundary dislocations; (3) boundary structures related by c.s.l. symmetry; (4) partial grain boundary dislocations. Transmission electron microscope observations of topic 4 are presented.
TL;DR: In this paper, the authors considered the elastic interaction between solute atoms and tilt boundaries in order to estimate the spectrum of binding energies for such boundaries, and calculated the extent of solute segregation to these boundaries for several sets of parameters that determine the nature of the solute and the grain boundary.
Abstract: Experimental data on the temperature dependence of solute segregation to grain boundaries in polycrystalline metals does not appear to be adequately explained by a single binding energy between solute atoms and grain boundary sites. The extent of segregation in a given alloy appears to decrease more slowly with increasing temperature than is consistent with a theory based on such a single binding energy. This behavior can be explained however, if a spectrum of binding energies is assumed to exist. Such a binding energy spectrum presumably results from the site-to-site variation in the solute environment at the grain boundary. The present paper considers the elastic interaction between solute atoms and tilt boundaries in order to estimate the spectrum of binding energies for such boundaries. The extent of solute segregation to these boundaries is then calculated for several sets of parameters that determine the nature of the solute and the grain boundary.
TL;DR: In this paper, an approximate treatment of cavity growth by plastic flow at the tip of a grain boundary crack is given, where the authors derive a creep rupture equation in the form of the Monkman-Grant relation as follows: ϵ t r = (n 2 − 4) 3 In λ 2a 0 2 n+ 2 2πn 2 9 3 ξ n n+2 λ L n − 2 n + 2 σ E n n +2 where ϵge and tr are the creep rate and rupture time, respectively, λ and a
TL;DR: In this paper, a mechanism for the generation of the fan-shaped precipitate configurations has been proposed, and it has been demonstrated that in the initial stage the dominating driving force is due to balancing of the interfacial energies at the three-phase junctions in the reaction front, i.e., the growing β-particles are pulling the grain boundary.
TL;DR: In this paper, a grain boundary doping scheme was proposed to increase the conversion efficiency of polycrystalline silicon solar cells in which the grain structure is columnar, and the resulting junction around each grain surface collects electrons which might otherwise recombine at the grain boundaries.
Abstract: The possibility of increasing the carrier collection efficiency in polycrystalline silicon by means of a heavily doped region near the grain boundaries is investigated. Phosphorous dopant is preferentially introduced into the grain boundaries of p‐type material by a low‐temperature diffusion process. A subsequent high‐temperature diffusion forms a highly n‐doped skin covering each grain. The resulting junction around each grain surface collects electrons which might otherwise recombine at the grain boundaries. This grain boundary doping scheme makes possible an increase in the conversion efficiency of polycrystalline silicon solar cells in which the grain structure is columnar.
TL;DR: In this paper, a defect-induced mechanism of cone formation was proposed, whereby sputter elaboration of bulk defects (either pre-existing or bombardment-induced) leads to the formation and development of surfa...
Abstract: Observations of ion-bombardment-induced surface modifications on crystalline copper substrates have been made using scanning electron microscopy. The delineation and development of grain boundary edges, faceted and terraced etch pits and small-scale ripple structure, together with the formation of faceted conical features, have all been observed on low and high purity polycrystalline substrates. In general, the density of such surface morphological features, although variable from grain to grain, is higher in the proximity of grain boundaries. In particular, cones are only found within regions where other surface erosional features are present and it would appear that the development of these other surface features is a pre-requisite to cone generation in high-purity crystalline substrates. We suggest the operation of a defect-induced mechanism of cone formation whereby sputter elaboration of bulk defects (either pre-existing or bombardment-induced) leads to the formation and development of surfa...