Showing papers on "Nucleation published in 1976"
TL;DR: In this paper, the stacking fault energy is shown to consist of both volume energy and surface energy contributions, and when the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that fault energy associated with the simultaneous dissociation of an appropriate group of dislocations can be zero or negative.
Abstract: Consideration of the martensitic nucleation process as a sequence of steps which take the particle from maximum to minimum coherency leads to the hypothesis that the first step in martensitic nucleation is faulting on planes of closest packing. It is further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur in such a way as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the stacking fault energy is shown to consist of both volume energy and surface energy contributions. When the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that the fault energy associated with the simultaneous dissociation of an appropriate group of dislocations (e.g. a finite tilt boundary segment) can be zero or negative. This condition leads to the spontaneous formation of a martensitic embryo. For the specific case of the fcc → hcp martensitic transformation in Fe-Cr-Ni alloys, the defect necessary to account for spontaneous embryo formation at the observedM
s temperatures may consist of four or five properly spaced lattice dislocations. Such defects are considered to be consistent with the known sparseness of initial martensitic nucleation sites.
628 citations
TL;DR: In this article, the authors derive theories for nucleation, coagulation and droplet growth by reformulating the dynamics in terms of "clusters" and show that these theories agree quantitatively with computer simulations of the nucleation kinetics in the lattice gas model, and also give a much better account of recent nucleation experiments on CO2.
Abstract: Starting from a master-equation description of dense gases, binary mixtures, etc., we derive theories for nucleation, coagulation and droplet growth by reformulating the dynamics in terms of ‘clusters’. This treatment is shown to agree quantitatively with computer simulations of the nucleation kinetics in the lattice gas model, and also gives a much better account of recent nucleation experiments on CO2 than previous approaches. The possible definitions of clusters, the appropriate coordinates for their description and their relation to bulk equilibrium properties of the system are outlined, and the effects of fluctuations on the properties of the clusters are investigated. Near the critical point the crossover between ‘classical’ and ‘non-classical’ expressions for the droplet formation energy is described in terms of ‘scaling laws’. In contrast to the Cahn-Hilliard-Langer theories of nucleation based on the concept of a coarse-grained free energy, no significant changes of static or dynamic beh...
527 citations
TL;DR: In this article, the first compound nucleated in planar binary reaction couples is the most stable congruently melting compound adjacent to the lowest temperature eutectic on the bulk equilibrium phase diagram.
Abstract: What is the first compound that nucleates in planar solid silicon–transition‐metal binary couple reactions whose members form bulk equilibrium compounds? We propose, for couples annealed at low temperatures, the following simple rule: The first compound nucleated in planar binary reaction couples is the most stable congruently melting compound adjacent to the lowest‐temperature eutectic on the bulk equilibrium phase diagram. The predictions of this rule are compared with experimental results.
437 citations
TL;DR: In this article, a model of dynamic ductile and brittle fracture was developed for wave propagation in one-and two-dimensional geometries, and the model features were taken mainly from detailed observations of samples partially fractured during impacts, but the functional forms are consistent with theoretical results where applicable.
Abstract: Computational models of dynamic ductile and brittle fracture are developed for wave propagation in one‐ and two‐dimensional geometries. The model features have been taken mainly from detailed observations of samples partially fractured during impacts, but the functional forms are consistent with theoretical results where applicable. Basic features of the models are the nucleation and growth (hence, the acronym NAG for the models) of voids or cracks, and the stress relaxation resulting from the growing damage. The results of the calculations include number and sizes of cracks, voids, or fragments as a function of position in the material. The NAG analysis presents the nucleation law, determined from experiment, and two growth laws: both growth and nucleation are functions of stress and stress duration. Procedures for treating cracks with a range of sizes and orientation are presented with the method for computing the stress relaxation that accompanies growth of damage. Brittle fracture is essentially aniso...
350 citations
IBM1
TL;DR: In this article, a simple theoretical model and experimental observations made on deposits of Ga(As, P) on GaAs, suggest that it is desirable for the film thickness to be small.
300 citations
TL;DR: In this article, the general mechanism of martensitic nucleation by faulting from groups of existing dislocations, as proposed in Part I, is applied to the fcc → bcc, bcc → fcc, hcp → hcp, and related transformations, including mechanical twinning.
Abstract: The general mechanism of martensitic nucleation by faulting from groups of existing dislocations, as proposed in Part I, is applied to the fcc → bcc, bcc → fcc, bcc → hcp, and related transformations, including mechanical twinning. Where thermodynamic data are available, the conditions at the observedMs temperatures are consistent with nucleation from a defect composed of four or five properly spaced lattice dislocations. Examples of nucleation by faulting on the planes predicted are found in published electron microscopy. The faults are observed at the types of sites where the required dislocation groups are expected. These include grain boundaries, incoherent twin boundaries, and inclusion particle interfaces. Having defined the function of a nucleation site, mechanisms of strain induced nucleation and autocatalysis are then considered.
254 citations
TL;DR: A theoretical study of capillarity driven growth has been performed for several regimes of importance to the ripening of noble metal particles supported on flat, partially wetted, metal oxide substrates as discussed by the authors.
227 citations
TL;DR: In this paper, a systematic theory for nucleation and growth of mixture droplets out of vapors with two different chemical species, like H2SO4-H2O aerosol formation, was developed.
225 citations
TL;DR: In this paper, the growth of martensitic fault embryos in the fault plane, the development of their interfacial structure, and the thickening of the embryos normal to the fault planes are examined as possible rate limiting steps in the total Martensitic nucleation process.
Abstract: The growth of martensitic fault embryos in the fault plane, the development of their interfacial structure, and the thickening of the embryos normal to the fault plane are examined as possible rate limiting steps in the total martensitic nucleation process. Growth of the embryos in the fault plane appears the most probable rate limiting step, capable of accounting for both the observed isothermal and athermal kinetic behavior depending on the parameters (such as activation volume) which control the motion of the transformational dislocations. The thermally activated nucleation of dislocation loops responsible for lattice invariant deformations is a possible rate limiting step for some isothermal transformations, though such deformations are not required for all martensitic transformations. Embryo thickening by the nucleation of discrete loops of transformation dislocations appears improbable in bulk material; instead, a plausible pole mechanism for embryo thickening is presented which incorporates existing “forest” dislocations intersected by embryos growing in the fault plane. Lattice softening phenomena may lower the critical chemical driving force for nucleation, but are not essential for martensitic nucleation by the proposed faulting mechanism.
198 citations
IBM1
TL;DR: In this article, the authors identify interstitial loops with ½ 〈110〉 Burgers vectors and an ideal loop is rhombus-shaped with line senses in 》112〉 directions.
Abstract: Oxygen-rich precipitates in silicon, and the generation of dislocations at the interfaces between the precipitates and the matrix were observed. The precipitates are square-shaped plates with 〈110〉 sides and {100} habit planes. Prismatic dislocation loops are generated in silicon by the mechanism of prismatic punching. The loops are identified as interstitial loops with ½ 〈110〉 Burgers vectors and 〈110〉 axis. An ideal loop is rhombus-shaped with line senses in 〈112〉 directions. Nucleation of loops follows the mechanism of Ashby and Johnson (1969): a shear loop is nucleated on an initial slip plane and completes a rhombus-shaped prismatic loop by repeated cross-slips. To our knowledge, the loop nucleation process is identified for the first time for a system completely under internal stress.
154 citations
TL;DR: In this article, a two-dimensional (axisymmetric) numerical cloud model with parameterized microphysics for water drops and ice particles is described, which is used to simulate three different conditions of ice generation as a function of temperature.
Abstract: A two-dimensional (axisymmetric) numerical cloud model with parameterized microphysics for water drops and ice particles is described. The parameterized liquid-phase processes include condensation, evaporation, autoconversion of small drops to large ones, and collection of small drops by large ones. The solid-phase processes include heterogeneous sorption nucleation, homogeneous contact nucleation, deposition, sublimation, riming and melting. Both liquid and solid particles may precipitate. The model was used to simulate three different conditions of ice generation as a function of temperature: 1) based on classical concepts of the activity of heterogeneous ice-forming nuclei—suggesting continental cumuli; 2) based on field observations of much greater concentrations of ice particles at warmer temperatures than indicated measurements of heterogeneous ice nuclei—suggesting maritime cumuli; and 3) based on nucleus seeding practice when the goal is to stimulate the growth of cumulus clouds. General ...
TL;DR: Nickel surfaces were prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction as discussed by the authors, and the surface was found to chemisorb oxygen to produce a coverage of 0.014 micro g cm 2 in an ordered c(2 × 2) structure.
TL;DR: In this article, the growth kinetics of Ag particles in glass were investigated by means of optical spectroscopy, and two growth mechanisms were identified: Ago centers coagulate at the nuclei, forming particles of some hundred atoms, and the growth is continued much slower, by diffusion of Ag+, the kinetics following the growth theory of Ham.
Abstract: Nucleation and growth kinetics of Ag particles in glass have been investigated by means of optical spectroscopy. The underlying purpose of the work is production of particles for quantum size experiments smaller than previously available. Ago centers are produced by UV irradiation, and some ‰ of them act as nuclei. The particles grow at elevated temperatures by diffusion, and two growth mechanisms were identified. First, the Ago centers coagulate at the nuclei, forming particles of some hundred atoms. Samples with markedly smaller particles and narrow size distributions could not be produced because of the high growth velocity. Subsequently, the growth is continued much slower, by diffusion of Ag+, the kinetics following the growth theory of Ham.
01 Jul 1976
TL;DR: In this article, the effect on constitutive behavior of the presence of rigid particles, embedded in and bonded to a rigid-plastic porous matrix, is examined, and it is shown that the yield function is altered, and that the familiar concept of a yield function as a plastic potential must be used more carefully.
Abstract: In this paper, the effect on constitutive behavior of the presence of rigid particles, embedded in and bonded to a rigid-plastic porous matrix, is examined. It is shown that the yield function is altered, and that the familiar concept of the yield function as a plastic potential must be used more carefully. The results also show how a void nucleation mechanism could destabilize, causing rapid bulk softening and failure.
TL;DR: The combined techniques of ultraviolet photoemission spectroscopy and LEED-AES have been applied to the study of the electronic and atomic structure of the Fe(100) surface during initial stages of chemisorption and oxidation as discussed by the authors.
TL;DR: Co gamma irradiation of 2-hydroxethyl methacrylate in the presence or in the absence of other acrylic monomers was found to constitute an effective technique for the synthesis of hydrophilic functional microspheres in the size range of approxumately 0.3 to 3 mu in diameter.
Abstract: Co gamma irradiation of 2-hydroxyethyl methacrylate in the presence or in the absence of other acrylic monomers was found to constitute an effective technique for the synthesis of hydrophilic functional microspheres in the size range of approximately 0.3 to 3 microns in diameter. The effect of monomer concentration, steric stabilization, and electrostatic interaction on the particle size was investigated. Experimental conditions were determined to obtain desired particle sizes of relatively narrow distribution. It was shown that particles may be formed without intermediate micelles, i.e., by homogeneous nucleation, and the rate of particle formation is affected primarily by the rate of particle coalescence in the initial stages of the reaction. When covalently bound to antibodies these microspheres were successfully used to label murine and human lymphocytes.
TL;DR: In this paper, a model describing this type of grain nucleation has been constructed based on the assumption that the finely dispersed particles restrict subgrain growth to grains of larger than average size.
TL;DR: In this paper, the authors observed crystal nucleation in a Lennard-Jones fluid by molecular dynamics and showed that for a 108-particle system cooled slowly below its triple point, crystallization occurs at kT/e=0.50 and 0.57 for densities Nσ3/V = 0.90.
Abstract: We have observed crystal nucleation in a Lennard‐Jones fluid by molecular dynamics. In a 108‐particle system cooled slowly below its triple point, crystallization occurs at kT/e=0.50 and 0.57 for densities Nσ3/V =0.90 and 0.91, respectively. For larger systems (256 and 500 particles) crystallization has also been achieved. The supercooling limit decreases with increasing system size. These results are compatible with classical nucleation theory.
TL;DR: In this article, the authors describe the arrival at the substrate of a thin film and its formation and growth, including simple interfacial adhesion, interdiffusion between two materials, formation of intermediate layers and mechanical adhesion.
TL;DR: In this paper, the binding energies and optimized structures for various two to six atom clusters of Ti, Cr, Fe, and Ni are calculated in a low spin molecular orbital approximation, and the photoemission spectra for O and CO on a nine atom Fe (100) model cluster are calculated and are found to be similar, within calculational and experimental resolution, to those for the Fe(100) surface.
Abstract: In a low spin molecular orbital approximation, binding energies and optimized structures for various two to six atom clusters of Ti, Cr, Fe, and Ni are calculated. Ti, with few d electrons, shows a preference for tightly packed clusters with positively charged corner atoms while Ni favors open and ringlike clusters with negatively charged corner atoms; Cr and Fe, with nearer to half‐filled d shells, prefer tightly packed configurations, but charge distributions are less predictable. The parameterization and spin models used in the theoretical procedure are considered. The binding energies, structures, and charge distributions are discussed in relation to transition metal cluster catalysis and experimental Fe nucleation studies. The photoemission spectra for O and CO on a nine atom Fe (100) model cluster are calculated and are found to be similar, within calculational and experimental resolution, to those for the Fe(100) surface.
TL;DR: In this paper, the phase separation, devitrification and micro-hardness of the above glasses were investigated in relation to the starting materials and the composition of the glasses, and the results showed that the glasses made from gels are more homogeneous than those made from oxide mixtures.
Abstract: Nucleation and crystallization behaviour of glasses in SiO2-La2O3, SiO2-La2O3-Al2O3 and SiO2-La2O3-ZrO2 systems were investigated using glasses prepared by the fusion of “gels” and the mixtures of oxides in solar/image furnace. Two methods for the preparation of multicomponent homogeneous non-crystalline products in the form of gels were developed. The phase separation, devitrification and micro-hardness of the above glasses were investigated in relation to the starting materials and the composition. The results show that the glasses made from gels are more homogeneous than those made from oxide mixtures. The phase separation characteristics of glasses made from gels are markedly different from those of glasses made from a mixture of oxides. The addition of Al2O3 to the binary SiO2-La2O3 glasses improves the homogeneity but reduces the micro-hardness and the devitrification tendency, whereas the addition of ZrO2 causes a considerable increase in micro-hardness and enhances the devitrification. The rates of nucleation and crystallization of glasses of different compositions made from gels are much higher than those made from the mixture of oxides. The formation of the high temperature crystal form, (Β-La2Si2O7 is more evident with the crystallization of gel-glasses. When the rate of nucleation is low, (in the case of glasses from the mixture of oxides), the curve representing the relation between the micro-hardness and the time of heat-treatment shows a distinct minimum, whereas this minimum is not obtained with the gel-glasses. With most of the gel-glasses, the micro-hardness rises very sharply with the length of heat-treatment. The curve showing the relation between the micro-hardness and the volume fraction of the dispersed crystalline phase also gives a distinct minimum which can be explained on the basis of the fracture mechanism consisting of the processes of crack nucleation and of crack propagation around the dispersed crystalline particles.
TL;DR: In this article, the growth of platinum particles supported on flat alumina substrates has been measured in the temperature range 600-800°C, in two different oxidizing atmospheres having oxygen partial pressures of 2 and 20 kPa respectively.
TL;DR: The sintering behavior of boehmitic alumina gels during the transformation to the stable α phase has been studied using dilatometry, transmission electron microscopy, X-ray analysis and differential thermal analysis as mentioned in this paper.
Abstract: The sintering behaviour of boehmitic alumina gels during the transformation to the stableα phase has been studied using dilatometry, transmission electron microscopy, X-ray analysis and differential thermal analysis. The specimens for transmission electron microscopy were prepared from gel specimens, sintered to various predetermined temperatures, using an ion-beam thinning technique. The transmission electron microscope study and X-ray analysis have revealed a characteristic sintering behaviour which is associated with theθ toα phase transition. The transformation to theα phase occurs by a nucleation and growth process. During the growth process considerable redistribution of the fine porosity existing within the transition alumina matrix occurs, in the form of large elongated interconnected pores trapped within the nucleatingα grains. These pores grow rapidly to a size approximately one hundred times that of theθ grains. This process results in a rapid fall-off in sintering rate at the end of the transformation. A study of theθ/α interphase interface by transmission electron microscopy has led to the development of a model that accounts satisfactorily for the redistribution of the porosity.
TL;DR: In this paper, the authors extended the kinetic treatment of glass formation by the introduction of continuous cooling (CT) curves to estimate the cooling rates required to form glasses of various materials.
Abstract: The kinetic treatment of glass formation is extended by the introduction of continuous cooling (CT) curves to estimate the cooling rates required to form glasses of various materials. The CT curves may be constructed from isothermal time-temperature-transformation curves following the approach originally suggested by Grange and Kiefer. The modified analysis is used to evaluate the effects of nucleating heterogeneities on glass formation. It is found that for the concentrations of such heterogeneities found in most liquids, those characterized by contact angles greater than about 100° have a negligible effect on the cooling rate required to form glasses. Heterogeneities with smaller contact angles, can, however, have a significant effect on glass formation, with the critical cooling rate increasing with decreasing contact angle. The effects on glass formation of changes in the contact angle of nucleating heterogeneities are also compared with the effects of changes in the thermodynamic barrier to nucleation (in the crystal-liquid surface energy).
TL;DR: In this article, Avrami analysis of data on each polymer gives n ∼ 3 (predetermined nucleation, spherulitic growth), over the range of crystallization temperatures investigated.
TL;DR: In this article, it was found that the inhomogeneous nature of plastic flow on the scale of the grain size can result in the nucleation of microcracks in marble.
TL;DR: In this article, an explicit expression for the line energy of straight dislocations in orthorhombic polyethylene crystals was obtained and it was shown that the screw dislocation is the perfect dislocation with the lowest line energy whilst the 〈0, 0, c〉 edge has negative line tension.
Abstract: Explicit expressions are obtained for the line energy of straight dislocations in orthorhombic polyethylene crystals. It is found that the 〈0, 0, c〉 screw dislocation is the perfect dislocation with the lowest line energy whilst the 〈0, 0, c〉 edge has negative line tension and is therefore unstable. The possible dissociation of perfect dislocations into partials is discussed, and it is shown that both (110) and (310) twin dislocations can arise from such dissociations. Numerical calculations also show that homogeneous nucleation of 〈0, 0, c〉 dislocations, and twin dislocations, has a very high probability of occurrence under a stress of the order of the yield stress of polyethylene crystals. These results help to explain the readiness of polyethylene crystals to deform by c axis slip, twinning, and monoclinic transformation modes. The supression of c axis slip by irradiation and cross-linking is also explained by the very high line tension of the 〈0, 0, c〉 screw dislocations.
TL;DR: In this paper, the authors used the classic Johnson-Mehl-Avrami expression for the rate of crystallization of nucleation and growth-controlled reactions combined with the heat flow equation including a term for the release of latent heat.
Abstract: The kinetics of crystallization of a magma body can be described in terms of the classic Johnson-Mehl-Avrami expression for the rate of crystallization of nucleation and growth-controlled reactions combined with the heat flow equation including a term for the rate of release of latent heat. Although present data for the rates of nucleation and growth in geologic systems are insufficient to warrant quantitative calculations, finite difference solutions of these equations using reasonable values for these rates predict qualitatively many of the observed features in natural systems. These observed features include the variation in mineral composition and crystal morphology with distance from a surface of cooling and fraction crystallized, the form of the fraction crystallized versus time curve, and the relationships between the rates of nucleation and growth and the fraction crystallized. More detailed calculations will require accurately determined values for the appropriate rates of nucleation and growth and a knowledge of the process by which nucleation takes place.
TL;DR: In this paper, the authors examined pit nucleation and repassivation on iron and nickel by potentiostatic transient measurements and found that very short times for the nucleation of corrosion pits of some ms to some 100 ms for prepassivated specimens have been observed.
Abstract: Pit nucleation and repassivation on iron and nickel has been examined by potentiostatic transient measurements. Very short times for the nucleation of corrosion pits of some ms to some 100 ms for prepassivated specimens have been observed. Addition of picrate to the electrolyte can stop pit nucleation for well prepassivated specimens at all potentials completely, but cannot prevent the corrosion of already existing pits. The results are explained by the adsorption- and film breaking theory but give contradiction to the penetration mechanism. The repassivation of corrosion pits in an early stage of development takes Δ Tr = 1 ms to some 100 ms according to the pitting time Δ tp and the pit current density. A relation Δ Tr = k(Δtp)2 has been found. The transport of the accumulated aggressive anions from the pit electrolyte to the bulk solution is postulated as the rate determining process for repassivation.
TL;DR: In this article, the effects of strain rate, applied potential, pH, temperature, and hydrogen sulfide concentration on sulfide cracking of mild steel were examined at constant strain rates by means of an Instron-type machine.
Abstract: Effects of strain rate, applied potential, pH, temperature, and hydrogen sulfide concentration on sulfide cracking of mild steel were examined at constant strain rates by means of an Instron-type machine. Fractographic observations were also carried out on a scanning electron microscope. Kinetic studies were also made to elucidate the hydrogen electrode reaction mechanism for mild steel and the effect of hydrogen sulfide on the reaction. Amount of hydrogen absorbed in mild steels stressed at a constant strain rate was measured and relationship of hydrogen embrittlement of mild steel with hydrogen electrode reaction and hydrogen absorption in hydrogen sulfide solutions was investigated. Fracture surfaces were transgranular quasi-cleavage and nucleation sites of cracks appeared to be carbides in the pearlite, inclusions and/or grain boundaries. The elementary reactions of hydrogen electrode reaction in acidic solutions are proton discharge and recombination of adsorbed hydrogen atoms and the rate d...