Showing papers on "Nucleation published in 1980"

Void Nucleation Effects in Biaxially Stretched Sheets

Abstract: The effects of void nucleation occurring during the deformation history on forming limit curves are considered for both in-plane and punch stretching employing a constitutive model of a porous plastic solid. Both plastic strain controlled and stress controlled nucleation processes are simulated by a two parameter void nucleation criterion. For in-plane stretching, plastic strain controlled nucleation can have, in certain circumstances, a significantly destabilizing effect on the forming limit curve. However, within the framework of plane stress theory which neglects the enhance­ ment of the hydrostatic stress due to necking, a stress controlled nucleation process is not found to be significantly destabilizing. In punch stretching a ductile rupture criterion, which limits the maximum volume fraction of voids, as well as the ap­ pearance of a well defined thickness trough, is adopted as a localized necking criterion. Only plastic strain controlled void nucleation is considered here in out-ofplane stretching. The resulting forming limit curves have the same shape as those obtained previously with void nucleation neglected.

Fatigue of metallic materials

Abstract: 1. Introduction. 2. Cyclic Stress-Strain Response. Mechanical properties. Microstructure. Theories of hardening and softening. 3. Fatigue-Crack Nucleation. Stress near surface. Sites of crack initiation. Near-surface dislocation structures. Surface relief and its relation to near-surface dislocation structures. Mechanisms of crack nucleation. Factors influencing crack nucleation. 4. Fatigue-Crack Propagation. Kinetics of crack growth. Fracture mechanics for fatigue cracks. Quantitative description of fatigue-crack propagation rate and threshold. Properties of plastic zone. Models of fatigue-cracks propagation and thresholds. Effects of miscellaneous factors on the crack-propagation rate. 5. Fatigue-Life Curves. Fatigue-life curve a versus f. Fatigue-life curve a versus f. Transformation of fatigue-life curves. Influence of cycle asymmetry. Hysteresis energy and fatigue life. Fatigue limit. Curves of constant damage. S/N curves of precracked bodies. Influence of temperature on fatigue life. 6. Notched Behaviour. Stress and strain concentration. Influence of notches on fatigue life. 7. Fatigue Life for Random Loading. Cyclic plasticity. Analysis of random variations of stress and strain. Prediction of fatigue life. References. Subject Index.

Kinetics of nucleation in near-critical fluids

Abstract: A simple set of rate equations is proposed to describe nucleation and growth of droplets in metastable, near-critical fluids. These equations are used in conjunction with steady-state nucleation theory to compute completion times for phase separation in binary mixtures. Reexamination of available experimental data provides little if any evidence for the major failure of conventional nucleation theory that has been postulated on the basis of these data.

A Detailed Microphysical Model Within a Two-Dimensional Dynamic Framework: Model Description and Preliminary Results

Abstract: A two-dimensional anelastic cloud model which incorporates detailed treatments of the water and ice phase is presented. The liquid phase processes considered include condensation, quasi-stochastic coalescence, fallout and breakup, while the ice phase processes include diffusional and accretional growth of ice particles. Results of two cloud simulations are presented. The first case assumes an atmosphere with maritime cloud condensation nuclei (CCN) activation characteristics and considers the warm rain processes only. It was found that with the appearance of precipitation, the model-predicted supersaturations within updraft regions often reach values larger than 5% with respect to water. The second case assumes an atmosphere with continental CCN characteristics and includes the ice phase processes leading to the formation of graupel. The results of the second case illustrate the importance of cloud vertical motions in transporting ice particles from preferential nucleation regions in the upper po...

On the formation of thermally sprayed alumina coatings

Abstract: A model for the fomation of thermally sprayed alumina coatings is proposed. The spreading and crystallization of liquid droplets on impact with the substrate are analysed and the thermal history of individual particles related to the kinetics of nucleation of γ-Al2O3 to other forms. The results suggest that under the usual spraying conditions undercooling of the liquid droplets is such that γ-Al2O3 nucleates in preference to α-Al2O3 and the cooling rate after solidification is sufficiently rapid to prevent transformation to δ-Al2O3 or α-Al2O3. Transformation of initially formed γ-Al2O3 to α-Al2O3 appears to be possible only if the lamellae formed on impact are thicker than about 10 μm if the substrate is heated to about 1000° C, or if the thickness is greater than about 20 μm on an unheated substrate. The α-Al2O3 generally observed in thermally sprayed coatings is the result of crystallization from pre-existing nuclei arising from incomplete melting of the feed material.

Growth morphology and defects in plasma-deposited a-Si:H films

Abstract: Structural studies of plasma-deposited a-Si:H are presented that show that a major class of defect is an anisotropic density fluctuation. Studies of the hydrogen environment suggest that an inhomogeneous hydrogen distribution is associated with these fluctuations. From considerations of the deposition chemistry and nucleation theory a model is proposed to describe the film growth process and its relationship to defects.

Experimental effects of helium on cavity formation during irradiation—a review

Abstract: Cavity (void) formation and swelling in non-fissile materials during neutron irradiation and charged particle bombardments are reviewed. Helium is the most important inert gas and is primarily active as a cavity nucleant. It also enhances formation of dislocation structure. Preimplantation of helium overstimulates cavity nucleation and gives a different temperature response of swelling than when helium is coimplanted during the damage process. Helium affects, and is affected by, radiation-induced phase instability. Many of these effects are explainable in terms of cavity nucleation on submicroscopic critical size gas bubbles, and on the influence of the neutral sink strength of such bubbles. Titanium and zirconium resist cavity formation when vacancy loops are present.

``Critical clusters'' in a supersaturated vapor: Theory and Monte Carlo simulation

Abstract: A new thermodynamic analysis is given for the equilibrium between a liquid cluster and the surrounding supersaturated gas phase in afinite constant volume. It is shown that for constant total density and intermediate volume this equilibrium is stable, although it is unstable for very large volume. We show that observation of the critical cluster sizel* then yields information on the surface free energy of the liquid cluster. The accuracy of previous approximate prescriptions for obtaining the free energy of physical clusters is investigated. As an application, the theory is used to analyze Monte Carlo simulations of the two-dimensional lattice gas model at low temperatures. We obtain cluster surface area, diffusivity, and free energy for clusters with 26≥l≥500. It is found that the capillarity approximation is inaccurate forl≥100, but the free energy of small clusters ishigher than the result of classical nucleation theory, in contrast to what one expects from Tolman-like corrections. We interpret these results, deriving low-temperature series expansions for very small clusters, thus showing that the capillarity approximation both underestimates the surface energy and overestimates the surface entropy of very small clusters. Finally, we use our results to give a speculative explanation of recent nucleation experiments. The dependence of the cluster diffusivity on cluster size is tentatively explained in terms of a crossover between two mechanisms yielding different power laws.

Thermal gradient control for enhanced laser induced crystallization of predefined semiconductor areas

Abstract: A method for laser induced conversion of large predefined areas of amorphous or polycrystalline semiconductor material, disposed nonepitaxially upon a substrate, into large single crystal areas, by controlling the lateral heat flow out of the melted regions of the areas for causing their recrystallization from a single nucleation site and for suppressing the formation of competitive nucleation sites at their edges.

The formation of silicides from thin films of some rare-earth metals

Abstract: The formation of silicides from thin films of the rare‐earth (or related) elements Y, Tb, and Er, on both (100) and (111) Si substrates, has been investigated simultaneously with backscattering and x‐ray diffraction. The silicon‐rich compounds of the type R‐ESi2−n form almost directly with no, or only poorly distinct formation of other silicides at temperatures from about 300 to 500 °C. For all three metals, the reactions with (111) Si require temperatures some 100 °C higher than the reactions with (100) Si, a difference in behavior which is quite important considering the relatively low reaction temperatures. The reactions of Er and Tb with (100) Si are quite sudden, indicating that nucleation is probably the controlling mechanism.

현무암을 이용한 Glass-Ceramics

Abstract: Crystallization phenomena of glasses of fused natural basalt rocks were studied by DTA, X-ray phase analysis, electron microscopy, and other techniques. Crystallization was catalyzed by the addition of either chromite ore or P₂O 5 , both up to 5 wt %. Various heat treatments were used, and their influences on controlling the microstructures and properties of the products were studied to develop high strength glass-ceramic material of the CaO-Al₂O₃(Fe₂O₃)-MgO(FeO)-SiO₂ system from the domestic basalts. Magnetite precipitates were found to be a nucleation initiator in every case of the crystallization. Diopside, anorthite, clinoenstatite and monticellite were identified as silicate crystalline phases contained in the crystallized products. The crystallite size was in the range of 0.1-2.5㎛. The fine crystallites were approximately cubic, but large crystallites were either plate or needle shape. The thermal expansion coefficient, microhardness and modulus of rupture of glass-ceramics were ranged from 78.5 to 81.8×10 -7 ㎝/㎝/℃, from 820 to 930㎏/㎟, and from 1800 to 2800㎏/㎠, respectively.

Suppression of Cavity Formation in Ceramics: Prospects for Superplasticity

Abstract: Ceramics exhibit macroscopic stress/strain rate relations that should lead to superplastic extension. However, premature fracture is normally encountered, due to the formation and growth of grain-boundary cavities. Thus, cavity nucleation and growth were analyzed in an attempt to identify microstructures and/or strain-rate regimes that would suppress cavity evolution and hence allow superplasticity. Analysis of cavity nucleation indicates that fine-grained materials devoid of grain-boundary amorphous phases and inclusions should sustain substantial deformation rates without nucleating cavities, especially if solid-solution additions that encourage rapid grain-boundary diffusion (while not excessively decreasing surface energy) are identified. The analysis of void growth indicates that high relative surface diffusivities are also desirable, indicating that alloy additions that do not depress (and probably enhance) the relative surface diffusivities must be selected.

The effect of Fe and Mg on crystallization in granitic systems

Abstract: Single-step and multistep undercooling experiments using both Fe,Mg-free and Fe,Mgbearing model granitic compositions were conducted to investigate the influence of mafic components on the crystallization of granitic melts. Crystallization of granite and granodiorite compositions in the system NaAlSi3O6-KAlSirOr-CaAlzSi2Os-SiOr-H2O produces assemblages containing one or more of the following phases: plagioclase, alkali feldspar, qloartz, silicate liquid, and vapor. The observed phase assemblages are generally in good agreement with equilibrium data reported in the literature on the same bulk compositions. With the addition of Fe and Mg to these bulk compositions six new phases participate in the equilibria (orthopyroxene, clinopyroxene, biotite, hornblende, epidote, and magnetite). However, crystalline assemblages produced in phase equilibrium and crystal grofih experiments brought to the same fnal P-Z-X6,. conditions are in general not equivalent. In crystal-growth experiments, nucleation of the feldspars and quartz is greatly inhibited in the presence of Fe and Mg. Indeed, plagioclase is the only tectosilicate to nucleate in the granodiorite composition. Mafic phases nucleate and grow outside of their thermal stability fields as defined by the equilibrium phase diagrams. This contrast in mineral assemblages between the equilibrium and crystal growth experiments is in marked contrast to the results obtained for Fe- and Mg-free compositions. Perhaps the addition of Fe and Mg has caused a breakdown of the Si-O framework in the melt, thereby promoting the more rapid nucleation of the inoand phyllosilicates rather than the framework silicates. Border zones of granitic plutons, com-only rich in mafic minerals, may result from the more rapid nucleation of mafic phases from the silicate liquid. These zones are thought to develop by early crystallization along the walls of the pluton. Our results suggest the mafic phases should nucleate more quickly than the feldspars and quartz and thus should enrich the early crystallization products in ferromagnesian minerals.

Melt crystallization of polypropylene: Effect of contact with fiber substrates

Abstract: The isothermal crystallization of isotactic polypropylene at different temperatures in the presence of fibrous substrates has been investigated. It is shown that preferential transcrystalline growth occurs at the fiber surface and that changes in nucleation density in the bulk material adjacent to the fibers also occur, the extent of which is dependent on temperature and fiber volume fraction. The effects are discussed in terms of the diffusion of heterogeneities in the bulk due to interaction and the adsorption on the fibers.

Numerical Simulation of Ice-Phase Convective Cloud Seeding

Abstract: A two-dimensional time-dependent cloud model which covers a region 19.2 km × 19.2 km in the x and z directions with 200 m grid intervals, has been used to simulate silver iodide (AgI) seeding effects on strong convective clouds. The model is a set of conservation equations for momentum, energy and mass (air and water contents). One extra conservation equation is applied to trace the seeding agent which advects and diffuses along the flow field and interacts with the supercooled cloud fields. Contact and deposition nucleation are simulated. Only inertial impact and Brownian collection are considered as possible mechanisms for contact nucleation. Most of the AgI particles work as deposition or sorption nuclei in this study. Three different soundings are tested. Most of the effort is used in testing sounding H1, from Miles City, Montana, 29 July 1975. Seeding at a different place (see H1/P1), at a different time (case H1/T1), and with different amounts of AgI (cases H1/M1 and H1/M2) are simulated. T...

Nucleation and growth of creep cavities in a Type 347 steel

Abstract: The nucleation and growth of intergranular creep cavities has been studied in a Type 347 austenitic steel tested at 550 and 650°C over a limited range of stress. It was found that the number of cavities per unit volume is linearly related to the time-dependent creep strain, being independent of stress or temperature over the conditions tested. The results indicate that cavity nucleation is controlled by a deformation process. Studies on cavity growth show that the cavity diameter increases approximately linearly with time, with a stress dependence for growth similar to that for secondary creep. These results are consistent with a diffusion growth model when a simple deformation-controlled nucleation law is incorporated. Using this growth model the observed stress and time dependence for the total void volume can be predicted, and the rupture life has been estimated reasonably well using a simple fracture criterion.