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Showing papers on "Nucleation published in 1972"


Journal ArticleDOI
TL;DR: In this article, the authors distinguish between strain-induced nucleation and stress-assisted nucleation, the latter involving the same sites and embryos as does the regular spontaneous transformation, while the former depends on the creation of new sites by plastic deformation; this phenomenon may also contribute in a major way to autocatalytic nucleation during the course of martensitic transformation.
Abstract: The previous work of Professor W. G. Burgers and Dr. A. J. Bogers is used to develop a mechanism of strain-induced martensitic nucleation, involving two intersecting shear systems. We distinguish between strain-induced nucleation and stress-assisted nucleation, the latter involving the same sites and embryos as does the regular spontaneous transformation. The strain-induced nucleation, on the other hand, depends on the creation of new sites and embryos by plastic deformation; this phenomenon may also contribute in a major way to autocatalytic nucleation during the course of martensitic transformation. For the case of strain-induced nucleation, it is possible to focus on specific intersecting-shear systems when the austenitic stacking-fault energy is low and e (h.c.p.) martensite can form as a part of the shear displacements. It then becomes feasible to extend the intersecting-shear mechanism from this special case to alloys of higher stacking-fault energy, where e is no longer stable relative to the austenite. It should be noted, however, that these events are very early stages in the formation of martensitic plates and relate primarily to the genesis of embryos; the actual growth start-ups which determine the operational (measured) nucleation rates may be controlled by subsequent processes.

789 citations


Journal ArticleDOI
TL;DR: In this paper, time-temperature-precipitation diagrams were determined between 400° and 900°C for up to 3000 hr as a function of carbon content, solution treatment temperature, and cold work.
Abstract: Although Type 316 austenitic stainless steel is widely used in steam generating plants and nuclear reactors the knowledge about aging reactions, nature of precipitates, and precipitation kinetics during high temperature exposure is limited. Time-temperature-precipitation (TTP) diagrams were determined between 400° and 900°C for up to 3000 hr as a function of carbon content, solution treatment temperature, and cold work. The nucleation and growth phenomena, morphology, and composition of the various carbide (M23C6, M6C) and intermetallic phases (σ, χ, η were determined. The complex sequence of phase instabilities can be explained on the basis of the carbon content, effect of molybdenum and chromium on the carbon solubility, thermodynamic stability of the phases, and the kinetics of the various precipitation reactions.

658 citations


Journal ArticleDOI
TL;DR: In this paper, a computer simulation model is described which applies to the molecular processes involved in crystal growth under very general growth conditions, and the transition probabilities used for adding and subtracting molecules from the crystal and for surface diffusion are shown to satisfy microscopic reversibility in equilibrium.
Abstract: A computer simulation model is described which applies to the molecular processes involved in crystal growth under very general growth conditions. The transition probabilities used for adding and subtracting molecules from the crystal and for surface diffusion are shown to satisfy microscopic reversibility in equilibrium. The computer program uses several internal self‐consistency checks, the results of which support the validity of the simulation method. Growth measurements are obtained in a range above and below the surface roughening temperature. The growth rates of the (100) face of a Kossel crystal show a two‐dimensional nucleation barrier at low supersaturations and below the roughening temperature. An almost linear dependence on supersaturation is observed above this temperature. This result is independent of the amount of surface diffusion. A simple step model is described and the resulting growth rates are compared with the simulated rates of faces containing a fixed number of steps. Agreement is...

438 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the scale-forming reaction of Ni-20 wt. % Cr2O3 scales on dispersion-free systems must involve shortcircuit diffusion of chromium through the scale, and that it seems probable that an effect of the dispersion must be to retard or eliminate this short-circuit process.
Abstract: Alloys of Ni-20 wt. % Cr containing 3 vol. % of a dispersed oxide phase have been prepared by a mechanical alloying method and oxidized in oxygen at 100 Torr in the temperature range of 900 to 1200°C. It appears that the dispersed oxide has four distinct effects on the oxidation: (1) the selective oxidation of chromium to form a continuous protective Cr2O3scale is promoted; (2) the rate of growth of Cr2O3is reduced compared with particle-free alloys; (3) the adhesion of the Cr2O3is greatly improved; and (4)the scale-forming reaction appears to be at the scale-metal interface in alloys containing a dispersion, but at the scale-oxygen interface in alloys without a dispersion. It appears that the nature of the dispersed oxide is not important, since very similar effects can be obtained with ThO2,Y2O3,and CeO2dispersions. It is demonstrated that a logical deduction from this evidence is that the growth of Cr2O3scales on dispersion-free systems must involve short-circuit diffusion of chromium through the scale, and that it seems probable that an effect of the dispersion must be to retard or eliminate this short-circuit process. It is suggested that the oxide particles act as nucleation centers for the oxide, thus reducing the oxide grain size; and it is shown that this simple hypothesis is sufficient to explain a number of the experimental observations.

266 citations


Journal ArticleDOI
TL;DR: In this article, the decomposition of Zr in Al-0.18 wt.% alloys has been examined by transmission electron microscopy and the precipitating phase was the metastable cubic Al3Zr, which, during annealing at 460°C, was found to be stable within the investigated time ranges.

192 citations


Journal ArticleDOI
TL;DR: In this paper, a model for simulation of a crystal-fluid interface of unlimited thickness is described, where the growth rate of a perfect close-packed surface at low temperature shows an exponential dependence on supersaturation indicating a heterogeneous nucleation barrier.

177 citations


Journal ArticleDOI
TL;DR: In this article, high-temperature diffusion kinetics and phase relations between couples of fused SiO2 or cristobalite and sapphire or mullite were investigated in air and in helium.
Abstract: High-temperature diffusion kinetics and phase relations between couples of fused SiO2 or cristobalite and sapphire or mullite were investigated in air and in helium. Subsolidus liquid formation between sapphire and cristobalite indicates the existence of a metastable system without mullite. A liquid phase is considered to be essential for the nucleation of mullite. The growth rate of mullite exceeded its dissolution rate in semi-infinite fused-SiO2-sapphire couples above 1634°C. The inter-facial liquid compositions provided data for a minor revision of the mullite liquidus curve. Diffusion coefficients calculated from the Al profiles vary greatly with concentration and temperature, resulting in a large range of values for apparent activation energy, which decreases with increasing Al2O3 content (∼310 to ∼60 kcal/mol for ∼4 to ∼22 wt% Al2O3). The diffusion process in the liquid is considered to be cooperative movement of oxygen-containing Al and Si complexes whose nature changes with composition and temperature; this change in the diffusing species contributes to the range in the values of experimental apparent activation energies.

171 citations


Journal ArticleDOI
TL;DR: In this article, a model for the mechanism by which nucleation of recrystallization occurs in transition bands during annealing is proposed, and the dependence of these processes on crystallographic orientation is discussed and related to the formation of recrstallization textures in b.c.
Abstract: Transition bands are regions of high lattice distortion which result from inhomogeneous deformation in metals. During annealing of cold-worked metals, nucleation of recrystallization is frequently observed to occur within these bands. An analysis has been made of some of the conditions under which transition bands may develop during deformation, and of the factors which influence their microstructure. A model is proposed for the mechanism by which nucleation of recrystallization occurs in transition bands during annealing. The dependence of these processes on crystallographic orientation is then discussed and related to the formation of recrystallization textures in b.c.c. metals.

152 citations


Patent
04 Dec 1972
TL;DR: In this paper, alluminosilicate zeolites are rapidly and efficiently prepared by seeding an aqueous mixture containing alumina, silica, and alkali metal hydroxide with zeolite seeds, i.e. nucleation centers having an average size below about 0.1 micron.
Abstract: Crystalline alluminosilicate zeolites are rapidly and efficiently prepared by seeding an aqueous mixture containing alumina, silica, and alkali metal hydroxide with zeolite seeds, i.e. nucleation centers having an average size below about 0.1 micron.

139 citations


Journal ArticleDOI
TL;DR: In this paper, the elemental composition of a surface can be derived from the energy spectrum of singly reflected noble gas ions, and it is shown that it is also possible to obtain information about certain structural features on the surface from energy spectra of multiply reflected ions.

109 citations


Journal ArticleDOI
R. A. Logan1, C. D. Thurmond1
TL;DR: In this article, the growth of singlecrystal epitaxial layers onto (0001) oriented sapphire substrates from the liquid phase was shown to require three conditions: (i) the partial pressure of in the H2 ambient, greater than the equilibrium pressure, (ii) the growth melt positioned in a thermal gradient to transport the dissolved to the growth interface, and (iii) the control of nucleation by the addition of Bi to the growing melt.
Abstract: The growth of single‐crystal epitaxial layers of onto (0001) oriented sapphire substrates from the liquid phase is shown to require three conditions:(i) the partial pressure of in the H2 ambient, greater than the equilibrium pressure, (ii) the growth melt positioned in a thermal gradient to transport the dissolved to the growth interface, and (iii) the control of nucleation by the addition of Bi to the growth melt. Studies of growth at temperatures from 850° to 1050°C show that the layer thickness, , is approximately linear in time and varies with growth temperature and atom fraction of Ga, in the growth melt. When , internal cracks in the basal plane hexagonal directions develop at the layer‐substrate interface and for they propagate to the surface and are thermally etched. A description is given of the interaction of liquid Ga with as a function of and . The low solubility of in Ga ( fraction at 1150°C) prevents useful growth by slow cooling of the melt. The inability to change the electron concentration, , with Zn doping and the decrease of with in the growth melt support but do not unambiguously confirm the model that the native donor is a nitrogen vacancy

Journal ArticleDOI
TL;DR: In this paper, a semianalytical procedure is developed to calculate stationary nucleation rates and induction times from the experimental results, which is very useful in situations in which the time-dependence of the mean number of the nuclei formed at constant supersaturation is not available experimentally.

Journal ArticleDOI
TL;DR: In this paper, it was shown that secondary nucleation does not involve a singular mechanism but that different kinds of secondary nucleations can take place under different conditions of supersaturation, liquid velocity, and impurity concentration.

Journal ArticleDOI
TL;DR: In this article, the effect of oxygen activity on the sintering of high-purity Cr2O3 is shown, and the theoretical density is approached at the equilibrium O2 partial pressure needed to maintain the Cr2 O3 phase (Po2=2×10−12 atm).
Abstract: The effect of oxygen activity on the sintering of high-purity Cr2O3 is shown. Theoretical density was approached at the equilibrium O2 partial pressure needed to maintain the Cr2O3 phase (Po2=2×10−12 atm). The presence of N2 in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O2 activity, correlated with the changing predominant volatile species in the Cr-O system.

Journal ArticleDOI
TL;DR: In this article, the authors describe hillock growth as proceeding by surface diffusion to reduce compressive stresses caused by differential thermal expansion between film and substrate, and the effects of oxygen atmospheres are accounted for in terms of the energetics of hillock nucleation.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the nucleation rate and the rate of crystal growth in a-Ge films, paying particular attention to the effects of residual gases, and the activation energies for the relevant processes were obtained.
Abstract: The authors studied the nucleation rate and the rate of crystal growth in a-Ge films, paying particular attention to the effects of residual gases. Activation energies for the relevant processes were obtained.

Journal ArticleDOI
TL;DR: In this article, it was shown that the properties of the small clusters govern the free energy functions in the neighborhood of the peak cluster concentration, and that these in turn affect the over-all height of the free-energy barrier for nucleation.
Abstract: A consideration of the molecular aspects of ion induced nucleation shows that, in contrast to accepted theory, small clusters are a segment of the over‐all nucleation size spectrum. No direct relationship exists between ion solvation energy and the free energy barrier for nucleation. Instead, it is established that the properties of the small clusters govern the free energy functions in the neighborhood of the peak cluster concentration, and that these in turn affect the over‐all height of the free energy barrier for nucleation. The fact that ion‐molecule reactions are taking place during the course of the usual ion induced nucleation experiment is clearly demonstrated. Assumptions that nucleation generally occurs directly about ions of the carrier gas are disproven.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the condensation of ethanol by homogeneous nucleation of liquid droplets in a supersonic nozzle with continuous static pressure measurements on the nozzle centerline and light scattering measurements.
Abstract: Condensation of ethanol by homogeneous nucleation of liquid droplets has been studied in a supersonic nozzle. Continuous static pressure measurements on the nozzle centerline and light scattering measurements permit the condensation process to be resolved in detail for comparison with theory. Nucleation rates found in the present work are in general agreement with previous results obtained in diffusion and expansion cloud chambers. It is seen that the predictions of the classical theory of homogeneous nucleation or that of the statistical mechanical theory with the assumptions of Dunning roughly agree with the experimental results. Various terms entering different versions of nucleation theory are given quantitatively. The pressure variations in the flow found beyond the onset point of condensation are used to study droplet growth processes. It is shown that a simple kinetic growth law with a constant mass condensation coefficient describes the condensation process well.

Journal ArticleDOI
TL;DR: This paper derived a thermodynamic definition of the standard free energy to form a cluster which is used in nucleation theory and showed that the Lothe-Pound correction factor is based on a misconception concerning the standard states involved.
Abstract: We have derived a precise thermodynamic definition of the standard free energy to form a cluster which is used in nucleation theory. The results [Eq. (9)] have a form differing slightly from the form usually used in nucleation theory and show that the Lothe-Pound correction factor is based on a misconception concerning the standard states involved.

Journal ArticleDOI
TL;DR: In this article, it was shown that nuclei develop from sub-grains situated in regions where there is a rapid change of orientation, either due to the original grain-boundaries or to transition bands introduced inside grains by deformation.
Abstract: Recent experimental evidence on the process of nucleation is reviewed. This suggests that nuclei develop from sub-grains situated in regions where there is a rapid change of orientation, either due to the original grain-boundaries or to transition bands introduced inside grains by deformation. Development of large sub-grains capable of growth at grain-boundaries may be stimulated either by a coalescence of sub-grains adjacent to old grain-boundaries, or at grain edges, by a process of re-establishing the equilibrium angles distorted by deformation. Further understanding of nucleation will require investigation both of the nature of the cold-worked state in polycrystalline materials and also of the mobility of general low-angle boundaries (θ less than 15 °).

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of silicate on the transformation of lepidocrocite to goethite and found that the presence of Si merely retard the nucleation stage of the transformation.
Abstract: The mechanism of the transformation of lepidocrocite (γFeOOH) to goethite (αFeOOH) has previously been established and the effect of silicate on the transformation was investigated Rather than completely inhibiting the reaction, as had been suggested, the presence of Si was found to merely retard the nucleation stage of the transformation There was found to be no decrease in the dissolution rate of the lepidocrocite due to surface adsorption of Si Si has no effect if introduced after the nucleation stage, and under conditions of pH and temperature where the dissolution rate of the lepidocrocite largely determines the rate of transformation, the presence of Si has a reduced effect The results show that Si is adsorbed and incorporated into the goethite structure, and due to its retarding effect on the nucleation, larger crystals of goethite are formed, many of which are twinned It is inferred from the results that the apparent stability of lepidocrocite occurring in soils in association with goethite cannot be attributed solely to the presence of Si in the soil system

Journal ArticleDOI
L.E Tanner1
TL;DR: In this paper, the transformation to long-range order in stoichiometric Ni 2 V was studied in detail over a wide range of subcritical temperatures, and the alloy was subjected to both continuous cooling and isothermal aging treatments.

Journal ArticleDOI
TL;DR: In this article, a detailed theoretical study has been made of the nucleation model for wall motion in ferroelectrics, and general formulae for absolute wall velocities in various field ranges have been derived.
Abstract: Detailed theoretical study has been made of the nucleation model for wall motion in ferroelectrics. Kinetics of the wall motion has been worked out both analytically and by simulation. The wall velocity v is expressed in terms of nucleation rate I and sideways growth velocity u s of nuclei on the wall by the formula v = c w 0 I or v =1.4 c ( u s I ) 1/2 according as w 0 2 I / u s ≫1 or w 0 2 I / u s ≪1. ( c : minimum advancement of wall, w 0 : width of critical nucleus.) The nucleation rate I and the sideways growth velocity u s have been calculated on the basis of the theory of absolute reaction rates. It is shown that at very high fields two-dimensional nuclei can not nucleate, instead one-dimensional nuclei are formed, whose nucleation rate depends on field following a power law. From these results, general formulae for absolute wall velocities in various field ranges have been derived.

Journal ArticleDOI
TL;DR: In this article, the authors studied the kinetics of crystal growth of a number of sparingly soluble salts and found that the rate of growth of barium sulphate showed a third order dependence on concentration when the crystals were small.

Journal ArticleDOI
TL;DR: In this article, the authors determined the fracture strength of Li2O-SiO2 glass-ceramics as a function of nucleation treatment and volume fraction of crystals present.
Abstract: Variations in the thermal expansion coefficients, elastic moduli, and fracture strengths of Li2O-SiO2 glass-ceramics were determined as a function of nucleation treatment and volume fraction of crystals present. Strength enhancement was attributed to a decrease in the mean free path between crystals as crystallization proceeds. It is postulated that the eventual reduction in strength in some glass-ceramics is caused by the development of localized cracks at the crystal-glass interface as a result of the volumetric changes which occur during crystallization.

Journal ArticleDOI
TL;DR: In this paper, the evaluation of atom fluxes to stable clusters of differing sizes is analyzed for the problem of nucleation and growth of thin films on a substrate. And the physical reasons for the discrepancy, recently noted by Zinsmeister (1971), between current approaches to thin film nucleation theory are discussed.
Abstract: The evaluation of atom fluxes to stable clusters of differing sizes is analysed for the problem of nucleation and growth of thin films on a substrate. The physical reasons for the discrepancy, recently noted by Zinsmeister (1971), between current approaches to thin film nucleation theory are discussed.

Journal ArticleDOI
TL;DR: In this paper, a molecular model applicable to prenucleation water clusters is described, which is applied to water clusters having clathrate-like structures composed of five-membered rings.
Abstract: A molecular model applicable to prenucleation water clusters is described. As an illustration the model is applied to water clusters having clathrate‐like structures composed of five‐membered rings. This work was motivated by the apparent inadequacies of the corrected liquid drop model which (in addition to applying bulk properties to small clusters) predicts nucleation rates which may be as much as 1017 larger than experiment. We present the ``energy of formation'' at a temperature of 277°K for our molecular model for clusters ranging in size from 5 to 57 molecules. These results agree qualitatively with experiment and, we believe, provide a motivation for further development of the molecular approach.

Journal ArticleDOI
01 May 1972
TL;DR: The morphology of extended chain lamellae and fibrous polytetrafluoroethylene has been analyzed by electron microscopy and compared to polyethylene as discussed by the authors, and the crystal growth mechanism of initial folding on molecular nucleation followed by chain extension is applicable to both polymers and seems to be a general mechanism for crystallization of prepolymerized linear macromolecules.
Abstract: The morphology of extended chain lamellae and fibrous polytetrafluoroethylene has been analyzed by electron microscopy and compared to polyethylene. The crystal growth mechanism of initial folding on molecular nucleation followed by chain extension is applicable to both polymers and seems to be a general mechanism for crystallization of prepolymerized linear macromolecules. Differences arise from the branching mechanism of the spherulites. Annealing the striated surface created on fracture could be shown to lead to folded chain “ripples” of time and temperature dependent thickness. Fibrous crystals of smooth and shish-kebob-type appearance were grown from gaseous tetrafluoroethylene.

Journal ArticleDOI
TL;DR: In this paper, the dependence of the work required for the formation of a spherically shaped cluster of the new phase on the electric field strength is found, and the analysis shows that this work may be either reduced or enhanced by the electric fields depending on the ratio of the dielectric constants of the old and the new phases.

Journal ArticleDOI
TL;DR: Secondary nucleation at low supersaturation for the ammonium sulfate-water system was studied using a seeded continuous crystallizer as discussed by the authors, which showed strong dependence on stirring rate and essentially linear dependence on solids concentration, indicate that secondary nuclei are generated by collisions of the seed crystals with the impeller blades.
Abstract: Secondary nucleation at low supersaturation for the ammonium sulfate-water system was studied using a seeded continuous crystallizer. Counter size distribution measurements in the subsieve size range indicated that nuclei are formed by microattrition of seed crystals and that the growth rate of the nuclei is very small. Nucleaion rate was correlated with the characteristics of the secondary environment. The results of this correlation, which shows strong dependence on stirring rate and essentially linear dependence on solids concentration, indicate that secondary nuclei are generated by collisions of the seed crystals with the impeller blades.