Showing papers on "Grain boundary published in 1981"
596 citations
TL;DR: In this paper, the authors conducted deformation experiments on two dunites (Anita Bay, of 100 μm grain size, and Aheim, of 900 mm grain size) at strain rates from 10−3 to 10−6 s−1 and temperatures from 1000°C to 1300°C in a gas-medium deformation apparatus at 300 MPa confining pressure.
387 citations
TL;DR: In this paper, the equilibrium grain-boundary segregation of phosphorus was investigated in Fe-P, Fe-C-P and Fe-Cr-P alloys after annealing at temperatures in the range 400-800°C.
Abstract: The equilibrium grain-boundary segregation of phosphorus was investigated in Fe–P, Fe–C–P, Fe–Cr–P, and Fe–Cr–C–P alloys after annealing at temperatures in the range 400–800°C. The grain-boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. The phosphorus segregation in Fe–P alloys with 0·003–0·3%P can be described by the McLean equation, the free energy of segregation is ∆G° = −34 300–21·5T (J mol−1). The percentage of intergranular fracture is increased by the presence of phosphorus at grain boundaries. In Fe–C–P alloys grain-boundary segregation is affected by site competition. Increasing carbon content of the samples causes an increase of the carbon concentration and a decrease of the phosphorus concentration at the grain boundaries; the intergranular part of fracture is simultaneously lowered. The addition of chromium causes an increase of phosphorus segregation in Fe–C–P alloys but no change in Fe–P alloys. The behaviour in carbon-containi...
316 citations
TL;DR: In this article, the effect of grain size on the sensitivity of polysilicon resistivity versus doping concentration is studied theoretically and experimentally, and it is shown that an increase in grain size from 230 to 1220 A drastically reduces the sensitivity to doping levels by two orders of magnitude.
Abstract: The processing parameters of monolithic polycrystalline silicon resistors are examined, and the effect of grain size on the sensitivity of polysilicon resistivity versus doping concentration is studied theoretically and experimentally. Because existing models for polysilicon do not accurately predict resistivity dependence on doping concentration as grain size increases above 600 A, a modified trapping model for polysilicon with different grain sizes and under various applied biases is introduced. Good agreement between theory and experiments demonstrates that an increase in grain size from 230 to 1220 A drastically reduces the sensitivity of polysilicon resistivity to doping levels by two orders of magnitude. Such an increase is achieved by modifications of the integrated-circuit processes. Design criteria for the optimization of monolithic polysilicon resistors have also been established based on resistivity control, thermal properties, and device geometry.
298 citations
TL;DR: In this paper, the authors proposed a mechanism in which differences in the diffusion coefficients of the diffusing species along the grain boundary cause a self-sustaining climb of grain boundary dislocations and motion of their associated grain boundary steps.
286 citations
TL;DR: In this paper, complex admittance measurements were performed on high-purity ceramics prepared by means of the alkoxide synthesis and on less pure ones obtained from the citrate synthesis.
Abstract: Complex admittance measurements are performed on high-purity ceramics prepared by means of the alkoxide synthesis and on less pure ceramics obtained from the citrate synthesis. The results on ceramic materials with grain sizes ranging from 0.4 to 20 µm are compared with those from a single crystal. The activation enthalpy for grain boundary conductivity Hgb = (118 ± 2) kJ/mol for the samples studied, is independent of composition, grain size, and preparation method. Grain boundary conductivity values and consequently the relevant pre-exponential factors are an order of magnitude smaller for the alkoxide materials than for the citrate materials. The ratio of grain bulk and grain boundary conductivity (Qb/Qgb) for alkoxide materials with grain-sizes 0.4 to 0.8 µm varies from 8.5 to 1.0 in the temperature range 500 to 700 °C.
276 citations
TL;DR: In this paper, the deformation rate of polycrystalline ceramics that contain a residual glass phase is analyzed in terms of material transport through the liquid phase, where molecules are transported and deposited in the direction of the positive normal traction gradient along the interfaces.
275 citations
TL;DR: The product of the grain boundary diffusion coefficient and width, D′δ, has been measured directly for the diffusion of 63Ni along grain boundaries in high-purity NiO as discussed by the authors.
Abstract: The product of the grain boundary diffusion coefficient and width, D′δ, has been measured directly for the diffusion of 63Ni along grain boundaries in high-purity NiO. These measurements have been made at an oxygen pressure of 1 atm in the temperature range 500–800°C, and also at an oxygen pressure of 7 × 10−6 atm at 700°C. At po 2 = 1 atm, D′δ is given by D′δ = 3·0× 10−8 exp {−1.78(eV)/kT} cm3 S−l. The width of the boundary, δ, is estimated to be 7 × 10−8 cm and in this region near the boundary the concentration and mobility of Ni vacancies are greater than in the bulk. The measured values of D’δ confirm quantitatively that the rate of oxidation of Ni to NiO at 500–800°C is controlled by the diffusion of Ni along grain boundaries in the NiO scale.
269 citations
TL;DR: In this article, a thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported, which includes severe overaging, deformation, and recrystallization steps.
Abstract: A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.
254 citations
TL;DR: In this article, a study of oxygen chemisorption at grain boundaries confirms the importance of grain boundary effects in ZnO polycrystalline films, which have a conductivity varying from 10−2 to 50 Ω−1 cm−1.
Abstract: Polycrystalline transparent semiconducting zinc oxide films have been deposited by the oxidation of diethyl zinc. The film growth rate is controlled by a complex multistep oxidation process which is dominated by radical reactions. The effect of substrate temperature and gas pressures have been studied. Samples deposited between 280 and 350 °C have a conductivity varying from 10−2 to 50 Ω−1 cm−1. The electrical properties of the films which are typical of polycrystalline material with small crystallites are shown to depend very closely on the film growth conditions. A study of oxygen chemisorption at grain boundaries confirms the importance of grain boundary effects in ZnO polycrystalline films.
245 citations
TL;DR: In this paper, the authors investigated the thermal diffusivity of fine grained hot-pressed compacts of heavily doped n-type silicon germanium alloy and found that the lattice thermal conductivity decreases with a reduction in grain size.
Abstract: Thermocouples made from heavily doped n - and p-type silicon-germanium alloys are used in the nuclear powered thermoelectric generators which provides onboard power to the American Voyager spacecraft. The isotopic heat source is expensive and any improvement in the generators' heat energy to electrical energy conversion efficiency would save both money and weight. The possibility of reducing the thermal conductivity of thermoelectric semiconductor alloys by compacting fine grained material has been considered and various estimates have been made of the reduction in thermal conductivity with grain size. The electrical properties of hot pressed compacts of heavily doped silicon germanium alloys which possess density values dose to that of single crystal are unaffected by grain boundary scattering effects. Consequently, this method of preparation holds out a possibility of improving the ‘figure of merit’ of the thermocouple material and hence the conversion efficiency of the generator. Here the first measurements of the thermal diffusivity of fine grained hot pressed compacts of heavily doped n-type silicon germanium alloy are reported. The compacts investigated possessed grain sizes (L) in the range 10
TL;DR: In this paper, the authors considered the diffusive cavitation of grain facets in circumstances for which the cavitated facets are well separated from one another, and showed that the rupture time tr, is given by the sum of two terms, one proportional to 1 Dσ ∞ (where D is the grain boundary diffusion parameter and σ∞ the stress which would act on a non-cavitated facet) and another proportional to E ∞(here E∞ is the creep rate of a similarly loaded polycrystal with uncavitated boundaries).
TL;DR: In this article, the authors investigated the sliding behavior of grain boundaries and found that an off-coincidence boundary changed into an almost exact coincidence boundary by the absorption of lattice dislocations.
Abstract: Dislocation structures of grain boundaries whose sliding behaviour had already been studied, have been observed by transmission electron microscopy of the slid grain boundaries. The ordered (coincidence) and random boundaries exhibited quite different sliding behaviour and dislocation structures. In the case of ordered boundaries, sliding was difficult and remarkable slide-hardening was shown. Transmission electron microscopy evidenced the presence of a number of EGBDs in these grain boundaries. In contrast, sliding of random boundaries easily took place showing slight slide-hardening. EGBDs were hardly observed in random boundaries. A systematic change in grain boundary misorientation during sliding was observed. An off-coincidence boundary changed into an almost exact coincidence boundary by the absorption of lattice dislocations. These results can be qualitatively explained by a dislocation model of sliding in which the absorption of lattice dislocations into the grain boundary is considered t...
TL;DR: In this paper, the theory of non-equilibrium grain-boundary segregation is discussed with particular reference to recent ideas and data relating to boron grain boundary segregation in Type 316 austenitic steel and a model is developed which, it is hoped, will more realistically describe the magnitude and extent of the process.
Abstract: The theory of non-equilibrium grain-boundary segregation is discussed with particular reference to recent ideas and data relating to boron grain-boundary segregation in Type 316 austenitic steel. The kinetics of the non-equilibrium grain-boundary segregation process are considered in depth and a model is developed which, it is hoped, will more realistically describe the magnitude and extent of the process. Reasonable agreement is found between the predictions of the model and experimental evidence for non-equilibrium boron, aluminium and titanium segregation to grain boundaries in austenitic steels. The model predicts, generally, that elements with large misfits with the matrix atoms will segregate most. Larger grain sizes lead to greater grain-boundary segregation. Also, the two critical heat-treatment parameters in non-equilibrium segregation are the solution-treatment temperature and the cooling rate from the solution-treatment temperature. Predictions of the worst combinations of these parameters for maximum non-equilibrium segregation to grain boundaries in austenitic steels are presented.
TL;DR: In this paper, the preparation of fine-grained high-density compacts of heavily doped n-type Si63.5Ge36.5 alloy and the measurement of their thermal diffusivity over the temperature range 300 to 1150 K are reported.
Abstract: The preparation of fine‐grained high‐density compacts of heavily doped n‐type Si63.5Ge36.5 alloy and the measurement of their thermal diffusivity over the temperature range 300 to 1150 K are reported. The compacts investigated possessed grain sizes (L) in the range 10
TL;DR: In this article, two specially developed techniques were used to study the nucleation and growth of grain-boundary cavities in 304 stainless steel at 0.5 Tm and found that cavities nucleated heterogeneously throughout the creep history and those observed were well in their growth stage.
TL;DR: In this article, a simple theoretical approach to the general problem of diffusive growth of grain boundary cavities of both equilibrium and non-equilibrium shape in the presence of power law creep is developed.
TL;DR: In this paper, the influence of powder characteristics, dopant addition, sintering atmosphere, firing schedules and the intrinsic grain boundary properties on microstructural development are discussed, and it is concluded that particle coarsening and rapid grain growth during densification must be minimized, possibly by dopant additions and a control of the sinting atmosphere.
TL;DR: In this article, La2O3 was sintered to full density by a process which uses a controlled transient solid phase to retard grain growth sufficiently that pores remain on grain boundaries and are eliminated by solid-state diffusion.
Abstract: Compositions of La2O3-Y2O3 were sintered to full density by a process which uses a controlled transient solid phase to retard grain growth sufficiently that pores remain on grain boundaries and are eliminated by solid-state diffusion. The temperature is then decreased into a single-phase region and the structural transition yields a pore-free, single-phase body. Optimum compositions are 8.1 to 12.1 mol% La2O3 for a sintering temperature of 2170°C and annealing temperatures of 1920° to 2020°C, resulting in materials with near-theoretical total transmittances and specular transmittance within 6% of theoretical.
TL;DR: In this article, it was shown that the degree of coupling between intragranular deformation and particulate flow may change during the course of natural deformation, leading to a transition from controlled to dependent flow.
TL;DR: In this article, a detailed balance approach has been coupled with a solution of the current continuity equation to yield a full solution to the problem subject to the approximation that majority carrier currents can be properly described by thermionic emission expressions.
Abstract: Calculations have been made of the barrier heights and recombination velocities at semiconductor grain boundaries subject to uniform illumination with above‐band‐gap light. A detailed balance approach has been coupled with a solution of the current continuity equation to yield a full solution of the problem subject to the approximation that majority carrier currents can be properly described by thermionic emission expressions. Silicon bicrystal data are presented and shown to be in good agreement with the predictions of the theory.
TL;DR: In this article, a review summarizes existing theories and attempts to reconcile them with the latest experimental data for reactions which take place from supersaturated solid solutions (eutectoid decomposition is specifically excluded), and it is considered that fuller understanding requires increased application of the latest electron-optical techniques such as high-resolution microanalysis and in situ experiments in the highvoltage microscope.
Abstract: Discontinuous precipitation results in the formation of a two-phase lamellar structure behind moving grain-boundaries. The reaction is of interest because it generally has a deleterious effect on the mechanical, physical, and chemical properties of many commercial alloys. The combination of heterogeneous boundary precipitation and concurrent migration is complex and has resulted in a diverse range of proposed reaction mechanisms, several models of the growth kinetics, and many empirical observations of the effects of ternary additions and lattice strain. This review summarizes existing theories and attempts to reconcile them with the latest experimental data for reactions which take place from supersaturated solid solutions (eutectoid decomposition is specifically excluded). It is considered that fuller understanding requires increased application of the latest electron -optical techniques such as high-resolution microanalysis and in situ experiments in the high-voltage microscope. Furthermore, to...
TL;DR: In this paper, the grain boundary diffusivity was evaluated as a function of temperature and the concentration behind the migrating boundaries was studied as function of the distance from the surface by microprobe measurements.
TL;DR: In this paper, a phenomenological model for the electrical conduction in polycrystalline silicon is developed, which combines dopant segregation, carrier trapping, and carrier reflection at grain boundaries.
Abstract: A new phenomenological model for the electrical conduction in polycrystalline silicon is developed. The combined mechanisms of dopant segregation, carrier trapping, and carrier reflection at grain boundaries are proposed to explain the electrical conduction in polycrystalline silicon. The grain boundaries are assumed to behave as an intrinsic wide-band-gap semiconductor forming a heterojunction with the grains. Thermionic emission over the potential barriers created within the grains due to carrier trapping at the grain boundaries and then tunneling through the grain boundaries is proposed as the carrier transport mechanism. A generalized current-voltage relationship is developed which shows that the electrical properties of polycrystalline silicon depend on the properties of the grain boundaries.
TL;DR: In this article, it was found that boron carbide can be sintered pressureless to ultrafine-grained bodies with densities of more than 95% of the theoretical density of B 4 C, if small additions of carbon are made to bors carbide powders of submicron particle size and if the compacts are heated in inert atmospheres to temperatures near 2150 °C.
Abstract: It was found that boron carbide can be sintered pressureless to ultrafine-grained bodies with densities of more than 95% of the theoretical density of B 4 C, if small additions of carbon are made to boron carbide powders of submicron particle size and if the compacts are heated in inert atmospheres to temperatures near 2150 °C. In particular the carbon acts as an inhibitor of surface-to-surface matter transport so that densification via grain boundary and/or lattice diffusion is enhanced at lower sintering temperatures. The mechanical properties of this new sintered material are summarized and compared with those of the commercial hot-pressed product.
TL;DR: In this paper, a crack initiation criterion based on the maximum stress and oxide depth at the time of crack initiation was found to represent the data very well, and an expression for the initiation fatigue life was developed.
Abstract: Low cycle fatigue of Rene 80 was studied at 871 and 982 °C. It was found that when the data were represented on the basis of plastic strain, the life increased with decreasing frequency and imposition of a 90 s hold at maximum strain. Transmission electron microscopy studies showed that the ′ coarsened and an interfacial array of edge dislocations developed. The density of dislocations in the matrix was very low. Light optical microscopy revealed that cracks generally inititiated at oxide spikes in surface connected grain boundaries. A crack initiation criterion based on the maximum stress and oxide depth at the time of crack initiation was found to represent the data very well. Based on that representation, an expression for the initiation fatigue life was developed. That expression includes temperature, frequency and cyclic stress strain parameters as variables.
TL;DR: In this paper, the authors show that fracture surface morphology, mechanical data, and the controlled crack growth analogues suggest that thermal fracture in solidifying basalt is an incremental and cyclic process, involving three steps: (1) the accumulation of elastic strain energy in cooling rock at temperatures below that required for stress relaxation due to viscous flow in the intercrystalline liquid phase, (2) fracture at a ΔT determined primarily by the aggregate thermal expansion coefficient αυ and Young's modulus E, (3) the penetration by the advancing crack tip, of
Abstract: The glass transition has been experimentally detected in basalt as (1) an increase in the aggregate linear thermal expansion coefficient αL, (2) an abrupt change in the temperature dependence of Young's modulus dE/dT, and (3) a change in stress relaxation behavior that effectively separates the T> TG and T TG. Collectively, the mechanical results suggest that for Hawaiian olivine tholeiite at 1-atm pressure, the principal material responses are (1) elastic (T ≤ 600°C), (2) reduced creep (600 980°C). Fracture surface morphologies developed during solidification suggest that the presence of the supercooled melt grain boundary phase may participate in the regulation of the thermal cracking process. Well-preserved fracture surfaces formed by incremental crack growth are found to be covered with striations that correspond to the inferred sequential stopping positions of the advancing fracture front. These striae may be rationalized in terms of experimental analogues that have been produced in viscoelastic polymers by cyclic tension-tension loading. The fracture surface morphology, mechanical data, and the controlled crack growth analogues suggest that thermal fracture in solidifying basalt is an incremental and cyclic process, involving three steps: (1) the accumulation of elastic strain energy in cooling rock at temperatures below that required for stress relaxation due to viscous flow in the intercrystalline liquid phase, (2) fracture at a ΔT determined primarily by the aggregate thermal expansion coefficient αυ and Young's modulus E, (3) the penetration by the advancing crack tip, of the thermal horizon capable of relaxing stress due to the creep of intercrystalline supercooled melt, producing the rough surface texture associated with the termination of a striation. Further crack growth must now await the migration of the solidus. The cycle then repeats. Striations measured in deep Hawaiian lava lakes have been compared with the crack advance increments expected in the vicinity of the glass transition, based on two tests: (1) thermal gradients measured in Kilauea Iki, combined with the mechanical properties of olivine tholeiite evaluated near TG, and (2) the crack advance required to match the recorded seismic stopping phases for prexisting cracks of the dimensions expected for Kilauea Iki. The observed versus predicted comparisons are (1) 31 versus 36 cm; and (2) 31 versus 30 cm. We envision this incremental crack growth process as contributing to the control on the downward movement of the thermal cracking front—and its associated hydrothermal circulation zone—in the upper portions of solidifying subaerial and submarine ponded basalt.
TL;DR: In this article, the assignment of boundary values for the chemical potential and the calculation of energy release rates for the growth of creep cavities along grain boundaries by self-diffusion are discussed.
Abstract: The assignment of boundary values for the chemical potential and the calculation of energy release rates for the growth of creep cavities along grain boundaries by self-diffusion are discussed It is assumed that the boundaries are flat and that surface and gran-boundary diffusion are the dominant transport mechanisms As matter diffuses from the void surface into and along the grain boundary, misfit residual stresses are induced to alleviate the high stress concentration ahead of the cavity apex As a result, it is shown that the contribution of strain energy terms to the chemical potential can be neglected in typical cases Also, contrary to the Griffith crack extension model, the energy dissipation incurred by diffusive removal of material from the cavity surface and deposition in the grain boundary is a major term in the energy transfers associated with cavity growth The primary energy sink in diffusive cavity growth arises from the work done by the grain-boundary normal stress when matter is inserted in the near-tip region by diffusion, and not from the loss of strain energy of matter that is removed from the cavity at its tip or from a work of bond separation Thermodynamic restrictions on the angle formed bymore » the void surfaces at their apex, where they join the grain boundary is described The derivation of boundary values for the chemical potential is carried out in a manner appropriate for arbitrarily large but elastic distortions of material near the cavity tip and, by contrast to most previous work in the area Rigorously the effects of surface tension, as distinct from surface energy, is included« less