Showing papers on "Nucleation published in 1981"

Phase transformations in metals and alloys

Abstract: Thermodynamics and Phase Diagrams Equilibrium Single-Component Systems Binary Solutions Equilibrium in Heterogeneous Systems Binary Phase Diagrams Influence of Interfaces on Equilibrium Ternary Equilibrium Additional Thermodynamic Relationships for Binary Solutions Computation of Phase Diagrams Kinetics of Phase Transformations Exercises References Further Readings Diffusion Atomic Mechanisms of Diffusion Interstitial Diffusion Substitutional Diffusion Atomic Mobility Tracer Diffusion in Binary Alloys Diffusion in Ternary Alloys High-Diffusivity Paths Diffusion in Multiphase Binary Systems Exercises References Further Readings Crystal Interfaces and Microstructure Interfacial Free Energy Solid=Vapor Interfaces Boundaries in Single-Phase Solids Interphase Interfaces in Solids Interface Migration Exercises References Further Readings Solidification Nucleation in Pure Metals Growth of a Pure Solid Alloy Solidification Solidification of Ingots and Castings Solidification of Fusion Welds Solidification during Quenching from the Melt Metallic Glasses Case Studies of Some Practical Castings and Welds Exercises References Further Readings Diffusional Transformations in Solids Homogeneous Nucleation in Solids Heterogeneous Nucleation Precipitate Growth5 Overall Transformation Kinetics-TTT Diagrams Precipitation in Age-Hardening Alloys Precipitation of Ferrite from Austenite Cellular Precipitation Eutectoid Transformations Massive Transformations Ordering Transformations Case Studies Exercises References Further Readings Diffusionless Transformations Characteristics of Diffusionless Transformations Martensite Crystallography Theories of Martensite Nucleation Martensite Growth1 Premartensite Phenomena Tempering of Ferrous Martensites Case Studies Exercises References Further Readings Solutions to Exercises Compiled by John C. Ion

A molecular theory for the solid–liquid interface

Abstract: We present an order parameter theory of the solid–liquid interface which uses structural information about the uniform liquid phase. The order parameters are the coefficients of a Fourier expansion of the nonuniform density in terms of reciprocal lattice vectors characteristic of the uniform solid phase. The theory provides explicit formulas for the interfacial density profile and the surface free energy. Connection is made with recent theories of freezing and of the liquid–vapor interface. Two special cases of particular interest are considered: the flat interface, for which numerical simulations have been attempted, and the spherical ball, which is important in theories of liquid–solid nucleation.

The three activation energies with isothermal transformations: applications to metallic glasses

Abstract: In crystallization or precipitation, the activation energy for nucleation of particles,En, for growth of particles,Eg, and for the total process,Ec, can all be determined independently by experiment. Formulae relating these three values are developed. Cases of linear and parabolic growth are included for different nucleation rates and morphologies. The formulae are tested experimentally on amorphous alloys (metallic glasses) based on Fe-Ni. The agreement between the theory and the experiments is very satisfactory.

Control of Calcium Carbonate Nucleation and Crystal Growth by Soluble Matrx of Oyster Shell

Abstract: A calcium-binding soluble protein extracted from oyster shell suppresses calcium carbonate nucleation and decreases the rate of crystal growth in vitro. These findings suggest that soluble matrix may regulate shell growth.

Effects of morphology on the mechanical behavior of a dual phase Fe/2Si/0.1C steel

Abstract: A study has been made on the effects of morphology on the mechanical behavior of a dual phase Fe/2Si/0.1C steel. The coarse dual phase structure obtained by continuously annealing in the two phase region directly from the austenite region results in poor elongation ductility with relatively high strength. However, upon obtaining a fine fibrous or fine globular dual phase structure by following different transformation paths, significant improvements occur in elongation ductility without much sacrifice in strength. The poor elongation ductility of the coarse dual phase structure is due to the initiation of cleavage cracks in the ferrite region where maximum localized stress concentration took place. But, in steels with both fine fibrous or globular morphologies, fracture occurred by void nucleation and coalescence after large amounts of plastic deformation.

Grain refinement in 7075 aluminum by thermomechanical processing

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.

Nucleation in glass and differential thermal analysis

Abstract: The crystal nucleation of Li2O-2SiO2 glass is studied by differential thermal analysis. A simple method, based on the shift of the differential thermal analysis (DTA) crystallization-peak temperature, for evaluating the influence of temperature and time of heat-treatments on bulk nucleation, is described. The experimental results are in good agreement with results obtained by other workers.

Rubber-Modified Epoxies: Transitions and Morphology.

Abstract: A variety of different morphologies, and therefore mechanical properties, can be obtained from a single rubber-modified epoxy formulation. The volume fraction, domain size, and the number of particles of phase-separated rubber are determined by the competing effects of incompatibility, rate of nucleation and domain growth, and the quenching of morphological development by gelation. These factors can be varied by the butadience/acrylonitrile ratio of the reactive rubber, the temperature of cure, and the gelation time. These ideas have been exploited to control the development of morphology of these amorphous systems. Phase separation was investigated by electron microscopy, viscometry, and dynamic mechanical analysis.

The Evolution of Enhanced Surface Geometries for Nucleate Boiling

Abstract: This paper surveys the evolution of special surface geometries that promote high-performance nucleate boiling. Early work by Jakob and Fritz in 1931 showed that emery paper roughening or machined grooves provided only temporary performance increase. However, this improvement dissipated after a few days to the flat surface value. There was little sustained interest in this unique, but apparently unuseful, phenomenon until the mid-1950s. During the period 1955-1965, supporting fundamental studies of the character and stability of nucleation sites provided a basis for renewed efforts to develop a high area density of stable, artificially formed nucleation sites whose performance does not deteriorate with time. Beginning in 1968 industrial research produced patented technology that achieved the long-sought goal. In 1980 at least six high-performance nucleate boiling surfaces were commercially available. The technology reported in this paper represents a dramatic advance in the field of heat transfer.

Kinetics of silica polymerization

Abstract: The polymerization of silicic acid in dilute aqueous solutions was studied experimentally and theoretically. Two basic processes were studied: the formation of colloidal particles by homogeneous nucleation, and their further growth by deposition of dissolved silica upon them. It was found that the state of ionization of the silica surface controls the rate of polymerization. The rate of deposition of dissolved silica on the surface of amorphous silica is proportional to the surface density of ionized silanol groups. The extent of surface ionization also determines the value of the surface tension, and thus also the rate of homogeneous nucleation. Added salts accelerate both molecular deposition and homogeneous nucleation by increasing the extent of surface ionization and decreasing the solubility of silica. Except for fluoride, salt ions were found not to have any specific catalytic effect. Fluoride was confirmed to be a powerful catalyst. Aluminum and boron were found to inhibit the reaction at pH 8. A successful quantitative theory of the homogeneous nucleation of colloidal silica particles was developed. The Lothe-Pound factor was shown to be about 3.34 × 1025 (kg H2O)−1. The nucleation model and results of our data analysis are incorporated in a computer code which models the homogeneous nucleation and growth of colloidal silica particles. This code is able to reproduce much of our data to within experimental error, and may be used to generate useful predictions for conditions that are typical of geothermal brines. The concentration of “adsorbed silica” on the surface of amorphous silica was determined, and was found to increase with the concentration of dissolved silica in the ambient solution. It is probably a reaction intermediate between dissolved silica and solid amorphous silica.

Modification of Al-Si eutectic alloys with Na

Abstract: Quenching experiments have been carried out on cast Al-Si eutectic alloys to show that additions of Na affect both the nucleation and growth of the Si phase. The Na not only changes the growth morphology of the Si from the platelike to the fibrous form but also prevents Si nucleating ahead of the eutectic growth front. It is the latter effect, resulting in a change in the average growth velocity, which is responsible for the much finer spacings and larger undercoolings found in modified cast structures. The mechanism for the change in Si growth morphology is discussed and it is suggested that the change may be the result of non-faceted Si growth at high velocities or in the presence of Na.

Chemisorption and oxide formation on metals: Oxygen–nickel reaction

Abstract: The reaction of oxygen with atomically clean nickel surfaces is reviewed. The reaction consists of three stages—chemisorption, oxide nucleation and lateral growth to coalescence, and thickening of coalesced oxide. Chemisorption is dissociative and nonreconstructive in general. However, a reconstructed chemisorbed phase is detected on Ni (110). NiO nucleates and grows laterally as islands displacing the chemisorbed phase, and finally coalescing to passivate the nickel surface. The conditions and rates at which the coalesced oxide thickens have not been well defined.

Kinetic studies on the formation of zeolite ZSM-5

Abstract: Zeolite ZSM-5 was synthesized from sodium tetrapropylammonium aluminosilicate gels and tetrapropylammonium silicate gel. Effects of varying the ratio SiO2/Al2O3 and alkalinity in the starting gels and the reaction temperature were studied. The alkalinity was found to affect the rate of nucleation more than the rate of crystal growth. An optimum alkalinity exists in the synthesis of ZSM-5; the optimum value is closely related to the value of SiO2/Al2O3 ratio in the batch composition. The apparent activation energies for nucleation and crystal growth are 38 and 46 kJ mol–1, respectively, for the aluminium-free silicate system and 25 and 29 kJ mol–1, respectively, for the aluminosilicate system with SiO2/Al2O3= 70. The results are consistent with a complicated solution mechanism of crystallization.

The β→αTransformation in Poly crystalline SiC: III, The Thickening of α Plates

Abstract: The final stages of the β→α polymorphic phase transformation in conventionally sintered and hot-pressed SiC were studied by optical and transmission electron microscopy. The interface between the α and β regions of partially transformed grains invariably contains thin α lamellae adjacent and parallel to coherent β twins (or β stacking faults). These planar defects serve as nucleation sites for the transformation, with growth of α occurring by the motion of partial dislocations nucleated at the intersection of coherent and incoherent twin boundaries. Various structural phenomena in SiC, including faulting, mi-crosyntaxy, and long-period polytypism, are discussed in terms of this transformation mechanism.

Grain refinement in magnetically stirred GTA welds of aluminum alloys

Abstract: The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.

The recrystallization of nickel-base superalloys

Abstract: The effects of recrystallization on the γ′ distribution in four nickel-base superalloys of varying γ′ volume fraction (Nimonics PE16, 80A and 115, and Udimet 720) have been studied by transmission electron microscopy. These effects are explained in terms of high solubility and diffusivity in the recrystallization interface, and it is suggested that high diffusivity assumes greater importance as the amount of solute dissolved in the boundary increases. Some attention is given to the nucleation of recrystallization. It is shown that in one of the alloys (Udimet 720), nucleation at grain boundaries involves subgrain coalescence. Subsequent growth of the nucleus occurs by strain-induced boundary migration.

Helium Bubble Behaviour in b.c.c. Metals Below 0.65$T_m$

Abstract: We have considered the methods available for distinguishing between the rate-controlling mechanisms for gas bubble migration in metals at temperatures below 0.6 T m . We show that several combinations of mechanism, gas behaviour and rate-controlling process give rise to similar power laws describing the rate of growth of populations of bubbles by migration and coalescence. We have therefore extended the model developed by Gruber (1967) to take account of the condition of constant gas pressure in the bubbles, which is likely to obtain at low temperatures in the absence of continuous irradiation damage, and the additional possibility that the nucleation of a surface ledge can control the migration rate of faceted bubbles. The experimental growth rates of helium bubbles, which we have measured in niobium, niobium-zirconium alloys and vanadium, are shown to be consistent with bubble migration by a surface diffusion mechanism controlled by the surface diffusion coefficient for small bubbles but by ledge nucleation for larger bubbles. The bubble size above which the (slow) ledge nucleation process controls growth is sensitively affected by the ledge energy. We show that the addition of zirconium to niobium can alter the ledge energy by an order of magnitude by cleansing the bubble faces of oxygen. Subsequent segregation of Zr-O complexes to the bubbles further alters the ledge energy. The bubble growth rate, and hence the swelling and embrittlement behaviour of the material under these conditions, is therefore very sensitive to the material purity and to segregation effects either induced thermally or accelerated by transmutation and irradiation damage. We find that the ledge energy on the (100) face of pure niobium is ca . 11 x 10 -11 J/m, which is decreased to ca . 4 x 10 -11 J/m by the segregation of Zr-O to the surfaces. The ledge energy at a similar surface in niobium containing 400/10 6 oxygen is as low as 1.2 x 10 -11 J/m. In vanadium we find a ledge energy of 3.4 x 10 -11 J/m. These ledge energies result in the effective cessation of bubble growth at bubble sizes in the range 2-20 nm.