Increasing Nucleation Rate Research Articles

Effect of boron addition on microstructure and mechanical properties of TiC/Ti6Al4V composites

TiC/Ti6Al4V composites with different boron additions were successfully fabricated, and the effects of the boron content on the microstructure, mechanical properties of composites were investigated. The results show that the dendritic TiC in the composites is remarkably refined when the boron content is less than 0.06wt.%. The average primary axial length of TiC dendrite is decreased from 150μm to 50μm according to the increasing of the boron content from 0.01wt.% to 0.06wt.%. And TiC dendrite gradually changes from the coarse dendrite to the fine dendrite or granular, but the abrasive property is obviously reduced with the increase of boron content. On the other hand, when the boron content further increases from 0.1wt.% to 0.6wt.%, the dendritic TiC can not be refined obviously, the fiber-shape TiB begins to appear and the abrasive property of composites is improved. The compressive strength and hardness (HRC) of composites are obviously increased in the whole addition range. The refinement mechanism of boron is attributed to the combined effects of the increase in nucleation rate at the constitutionally supercooled zone in front of the solid–liquid interface and the reduction in growth rate of TiC. The improvement of its abrasive property is mainly attributed to the existence of large size TiC and fiber-shape TiB during the abrasion.

Effect of magnetic field on the carbide precipitation during tempering of a molybdenum-containing steel

The influence of a high magnetic field on the carbide precipitation during the tempering of an Fe–2.8C–3.0Mo(wt%) steel was investigated. As-quenched steels were tempered at 200°C for various times with and without the presence of 12-T magnetic field. The applied field effectively promoted the precipitation of the relatively high-temperature monoclinic χ-Fe5C2 carbide, compared to the usual ε-Fe2C and η-Fe2C carbides precipitated without magnetic field. It is believed that the effect of applying a magnetic field is due to the reduction in the Gibbs free energy of the relatively higher magnetization phase. The denser distributions of the metastable carbides are attributed to the increased nucleation rate due to additional transformation force. The dispersed precipitation strengthening compensated for the decrease of hardness due to the loss of supersaturation of carbon atoms in the matrix.

Crystallization mechanism and its correlation with structural and soft magnetic properties of FeSiBPCu nanocrystalline alloys

Correlation of crystallization mechanism with the structural and soft magnetic properties of FeSiBPCu nanocrystalline alloys has been investigated on the basis of Johnson–Mehl–Avrami kinetic model, transmission electron microscopy, and extended random anisotropy model. Pre-existent α-Fe clusters in melt-spun alloys with a size much smaller than critical nucleus size crystallize at a steadily increasing nucleation rate owing to the chain effect of rising constituent fluctuation. This unique crystallization mechanism leads to the formation of fine and uniform nanocrystallites and therefore superior soft magnetic properties. In contrast, pre-existent α-Fe clusters with a size approximate to the critical nucleus size nucleate gradually at the initial stage of crystallization process driven by thermal activation over the energy barrier of nucleation, and pre-existent α-Fe nanocrystallites with a size larger than critical nucleus size grow directly at the initial stage of crystallization process. Grains formed at the initial stage enlarge further in the subsequent crystallization process, which gives rise to a continuous decrease of the nucleation rate and the deterioration of soft magnetic properties.

Influence of Carbon Content on the Microstructure and Precipitates of the V-N Microalloyed Steels

The microstructure and precipitates of the V-N microalloyed steels whose carbon content were respectively 0.27% and 0.35%, were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). It is shown that the microstructures of the test steels are all composed of ferrite and pearlite, the area percent of ferrite decreases from 49% to 40% and the grain size also descends from 8.96μm to 8.61μm with the increase of carbon content. TEM results show that the precipitates in two kinds of steels all include a large amount of 10~20nm dispersion distribution irregular flake VC, the part of fibrous VC that grows toward to intragranular ferrite along the grain boundary in the local area, and only a small amount of spherical VN or V (C, N). When the carbon content increases from 0.27% to 0.35%, the number of the spherical VN or V(C,N) increases obviously and the size of it varies from 20~100nm to 45~105nm, while the number of flake VC and fibrous VC decreases significantly and the length of fibrous VC shortens from several micrometers to nanometer size. Experimental results indicate that most of the spherical VN or V (C, N) firstly appear in austenite, then the flake VN and fibrous VN with precipitation strength effect emerge in ferrite during the following γ→α transformation and cooling process. The increase of carbon content can lead to the increase of driving force that VN or V (C, N) firstly appear in austenite, which results in the significant increase of ferrite nucleation rate and the refinement of microstructure.

Influence of Nitrogen Content on the Microstructure and Precipitate of the Medium-Carbon Vanadium Microalloyed Steel

The microstructure and precipitate of the two kinds of medium-carbon vanadium microalloyed steels whose nitrogen contents were 0.0035% and 0.012% respectively, were studied by image analysis and transmission electron microscope (TEM). The results show there are the large amount of 10~20nm dispersion distribution irregular flake VC precipitates within the ferrite, the part of clustered fibrous VC precipitates with the diameter of 4~13nm that grow toward to ferrite intracrystalline along the grain boundary with some angle in the local area, and only a very small amount of 20 ~ 50nm spherical particles V (C, N) in the low-nitrogen steel. However, in the high-nitrogen steel, the precipitates are divided into two stages: the first stage is the part of 30~80nm spherical particles V(C, N) which precipitation in austenite, the second stage is flakiness VC which precipitation in ferrite during the following γ → α phase transformation and cooling process. Compared with the low-nitrogen steel, the number of precipitates in decreased significantly and the size increased obviously in the high-nitrogen steel. The substantial increase of nitrogen content leads to the rapid increase of driving force that V (C, N) precipitation in austenite. A lot of V (C, N) that precipitation before phase transformation results in the significant increase of ferrite nucleation rate, which leads to the microstructure of high-nitrogen steel fined obviously.

Effects of Two-Step Annealing on the Magnetic Properties of Iron-Based Nanocrystalline Soft Magnetic Material

In the present paper, the effects of two-step annealing on the soft magnetic properties of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 melt spun ribbons were studied. The two-step annealing comprises low-temperature preannealing (first step) for an α-Fe(Si) phase nucleation in an amorphous matrix, and a subsequent high-temperature annealing (second step) for the growth of precipitated crystallites. Compared with a nanocrystalline alloy obtained by conventional one-step annealing at 550 °C for 1 h, the grain size of α-Fe（Si）crystalline phase in the Fe73.5Cu1Nb3Si13.5B9 alloy decreased greatly after it is pre-annealed at 480°C for 1 h followed by annealing at 550°C for 40 min. It has been shown that the soft magnetic properties can be significantly improved by applying two-step annealing. The decrease of grain size of α-Fe(Si) crystalline phase may result from the nucleation rate increase during the crystallization of pre-annealed.

Semisolid microstructure evolution of AZ91D alloy under high shear rate

Experimental work was undertaken to investigate the effect of rotation speed on the solidification microstructure of AZ91D alloy using a twin-screw slurry maker. Then numerical simulation was used to investigate the effect of rotation speed on the flow filed and temperature field of the melt. It was found that increasing the rotation speed could give rise to substantial grain refinement, which can be attributed to the increase of effective nucleation rate caused by the extremely uniform temperature and dispersing any potential clusters of nucleation agents in the bulk liquid under dispersive mixing.

Rapid synthesis of epitaxial ZnO films from aqueous solution using microwave heating

Rapid deposition of epitaxial ZnO films has been demonstrated with a low temperature aqueous solution technique utilizing microwave heating. X-Ray diffraction analysis of films deposited on single crystal (111) MgAl2O4 substrates revealed the ZnO to deposit epitaxially with an orientational relationship of ZnO [110]‖MgAl2O4 [2] & ZnO [010]‖MgAl2O4 [10]. By altering conditions of solution pH, ammonia concentration, heating rate, and hold time, the growth rate and morphology of the ZnO films can be modified. Using a pH 11, 1 mol L−1ammonia growth solution, an approximately 2 micron thick continuous ZnO film can be deposited in only 5 minutes at 90 °C. Compared to similar ZnO deposition conditions using conventional heating, the use of microwave heating resulted in substantially higher nucleation and growth rates. Using an explanation based on classical nucleation theory and predicted variations in ZnO solubility with temperature, the increase in nucleation and growth rate is attributed to the high heating rates achieved with microwave heating.

Bottom-Up Preparation Techniques for Nanocrystals of Lipophilic Drugs

Modern drug discovery techniques result in the development of an increasing number of highly lipophilic drug candidates. Their lipophilicity results in a slow dissolution of these drugs in the aqueous gastro-intestinal fluids, resulting in a poor bioavailability. Over the past few decades, many strategies have been developed to increase the dissolution rate of these types of drugs. The application of drug nanocrystals is one technique. The dissolution rate of drugs from nanocrystals is increased due to an increase of the saturation concentration around these particles (Kelvin law) and an increase of the surface area available for dissolution. Preparation techniques for drug nanocrystals can be divided into top-down and bottom-up techniques. Top-down techniques are based on size reduction of relatively large particles into smaller particles, whereas bottom-up techniques consist of the growth of small particles from individual molecules. The driving force for the growth of a crystal from individual molecules is supersaturation. Supersaturation of a drug in a solution can be obtained by decreasing the temperature or addition of an anti-solvent. The size of crystals formed from supersaturated pure drug solutions depends on the balance between the nucleation rate and crystal growth. This balance between nucleation and growth is determined by the extent of supersaturation. At a higher extent of supersaturation, the nucleation rate increases, and, hence, the crystal size decreases, whereas at a lower extent of supersaturation, the growth rate increases, and, hence, the crystal size increases. Therefore, process conditions, such as temperature or the ratio between solvent and anti-solvent and their mixing rate, can be used to control the drug crystal size. When drug crystals are formed in drug composites, the size of the crystals is also determined by the interstitial spaces between matrix molecules. While there are many excellent reviews on top-down techniques (e.g. (1,2)), bottom-up techniques are usually just referred to as “precipitation techniques.” Bottom-up techniques can be considered as the oldest techniques to prepare drug nanocrystals. However, recently, interesting new developments in the field of bottom-up preparation of drug nanocrystals have evolved. This commentary focuses on these techniques. First, an overview of bottom-up preparation techniques of crystalline drug nanoparticles will be given (Table I). Second, the process-related barriers for products prepared by these techniques to reach the market will be discussed. For an introduction on top-down methods to prepare drug nanocrystals and the application thereof, the reader is referred to an excellent recently published review (2). Table 1 Overview of Bottom-Up Techniques to Prepare Drug Nanocrystals

How ambient pressure influences water droplet nucleation at tropospheric conditions

[1] Nucleation theories typically neglect the influence of the ambient pressure on the condensation of droplets from a supersaturated vapor, despite increasing experimental evidence. We have applied a recently presented model that incorporates this effect to the homogeneous nucleation rates of water at tropospheric conditions. We measured experimental pressure dependent nucleation rates of water in helium at low to intermediate pressures (70–200 kPa) and at temperatures from 240–270 K with the laminar flow diffusion chamber. The observed pressure effect shows a clear positive effect (increasing nucleation rate with increasing pressure) at 270 K and a weaker effect with lower temperatures, consistent with the theory. The experimental pressure effect was more pronounced than predicted by theory. The same principle of the pressure effect should also hold for heterogeneous nucleation of water, which implies that water vapor removal by droplet nucleation of water may be suppressed at tropospheric conditions.

Crystallisation Pathways of Polymorphic Triacylglycerols Induced by Mechanical Energy

The aim of these studies is to establish sound scientific principles to guide nucleation rate and the selection of a desired polymorph via the application of mechanical energy – ultrasound (US) irradiation. When delivered to a metastable liquid, before the offset of nucleation and under constant temperature and supercooling conditions, the wave nature of this simple form of energy should be critical for defining different crystallisation pathways of polymorphic materials including polymorph selection. To test this hypothesis, we crystallized a melt-grown trilaurin (LLL), a typical polymorphic triacylglycerols (TGA's), with and without US by using in-situ simultaneous synchrotron radiation time-resolved small-angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS), SAXS/WAXS. Without US application, both polymorphic forms β' and β crystallized. With US treatment of the super cooled melt, the following effects were observed: (a) a marked decrease of induction times (b) an increased nucleation rate, and (c) selective crystallization of only β-form when crystallised at 25 and 30°C with input powers of 20 and 100 W and a sonication time of 2 s. Combining the existing knowledge on the dynamic nucleation of collapsing cavities and a qualitatively developed (P-T) phase diagram for the TGA's, it was possible to describe, for the first time, the behaviour of the most important parameters and the events that characterize the crystallization of these systems. It was shown that the interplay of sonication and the temperature of supercooled melts are critical to the selection of a stable β form.

Phase transformation kinetics of Ge 23Se 67Sb 10 glass

Using differential scanning calorimetry (DSC) technique, crystallization kinetics of the Ge 23Se 67Sb 10 glass upon isochronal annealing has been studied, on the basis of an analytical phase transformation model. Kinetic analysis in terms of some recipes shows that the isochronal crystallization is governed by continuous nucleation with increasing nucleation rate upon transformation, three dimensional interface-controlled growth and impingement due to anisotropic growth. This is compatible with the fit using the analytical phase transformation model, i.e. the effective activation energy Q has been determined as 152.6 kJ/mol and the growth exponent n as 4.3.

Production, Kinetic Study and Properties of Fe-Based Glass and Its Composites

Fe-based glassy powders with four different compositions of varying copper content were produced by mechanical alloying of elemental powder mixtures. The thermal stability as well as the crystallization kinetics was investigated using differential scanning calorimetry in both isochronal and isothermal modes. The isochronal and isothermal activation energies have a similar value (∼560–570 kJ/mol). In addition, the Johnson–Mehl–Avrami (JMA) analysis shows that the transformation is diffusion controlled three-dimensional process, and the crystallization proceeds with increasing nucleation rate. Metal matrix composites were synthesized through powder metallurgy methods by uniaxial hot pressing and subsequent extrusion of commercially pure Fe powders, blended with Fe-glass reinforcement. The resultant properties of the composites strongly depend on the composition of the glassy reinforcement.

Toward Further Understanding of Lysozyme Crystallization: Phase Diagram, Protein−Protein Interaction, Nucleation Kinetics, and Growth Kinetics

A detailed study on the phase behavior and mechanism of lysozyme crystallization is presented. The nucleation and crystal growth rates, crystal morphologies, solubility, second virial coefficients, and cloud-point temperatures under different solution conditions were experimentally measured and theoretically analyzed. The nucleation rates were found to increase with the protein concentration and sodium chloride concentration when crystallization occurred in the solid−liquid coexistence region, and the dependence of nucleation rate on protein concentration was identified to bifurcate into two groups around the liquid−liquid coexistence boundary. The suppression of the nucleation in the protein-rich phase was investigated in terms of protein−protein interactions determined by Raman microscopy and the self-assembly of the protein molecules. The concentration distribution in the region in between the growing nucleus and the protein-rich phase was derived to explain the increase of nucleation rate in the liqui...

Numerical Analysis of the Effect of Carbon Monoxide Addition on Soot Formation in an Acetylene/Air Premixed Flame

The effect of carbon monoxide addition on soot formation in an acetylene/air premixed flame was investigated by detailed numerical simulation. This work focused on both the temperature effect and chemical effect of carbon monoxide addition on soot formation by comparing the results of flames with different CO contents. We find that the addition of carbon monoxide consistently reduces the formation of soot. The soot volume fraction and nucleation rate increase until a threshold temperature is reached and then decrease as the temperature increases. Considering that soot formation took place at the active site by H-abstraction mechanism, the addition of CO promotes the formation of soot. The concentration of H radicals increases and the concentration of OH radicals decreases because of the increased forward rate of the reaction OH+CO=CO2+H. For soot formation to occur by the C-addition mechanism, the degradation rates of C2H2 tends to decrease and this promotes the formation of soot along with CO addition. On the other hand, the addition of CO may greatly reduce the volume fraction of C2H2 in fuel resulting in a lower surface growth rate.

Annular electromagnetic stirring—a new method for the production of semi-solid A357 aluminum alloy slurry

A new method for producing semisolid slurry, annular electromagnetic stirring (A-EMS), to refine and spheroidize grains was exploited. Experimental work was undertaken to investigate the effects of cooling rate, stirring power and stirring time on the solidification behavior of A357 alloy using A-EMS. It was found that increasing the cooling rate and stirring power gave rise to substantial grain refinement, which could be attributed to the increase of effective nucleation rate caused by the extremely uniform temperature and composition fields in the bulk liquid during the initial stage of solidification. Results showed that a fully grain refined spherical structure could be obtained using proper processing conditions within 10 s.

Crystallization behavior and consolidation of gas-atomized Al84Gd6Ni7Co3 glassy powder

Abstract Gas-atomized Al84Gd6Ni7Co3 glassy powder displays a double-step devitrification behavior characterized by the precipitation of fcc Al in the first stage of the crystallization process, followed by the formation of intermetallic phases in the following crystallization event. Investigation of the crystallization kinetics reveal that the initial part of the glass-to-fcc Al transformation occurs by diffusion controlled growth with an increasing nucleation rate whereas the later stage of the transformation is dominated by the growth of the formed nuclei rather than by the generation of new nuclei. Highly dense bulk samples were produced by hot extrusion through the combined devitrification and consolidation of the glassy precursor. The consolidated samples exhibit remarkable high strength of about 1130 MPa combined with 3% fracture strain.

A new viewpoint to the mechanism for the effects of melt overheating on solidification of Pb-Bi alloys

The effects of melt overheating treatment on solidification of Pb-Bi alloys were studied from the viewpoint of liquid-liquid structure change (LLSC). Anomalous temperature dependence of internal friction, electrical resistivity, and entropy of liquid Pb-Bi alloys suggested that discontinuous LLSC occurred within about 520–740 °C, based on which the solidification experiments were carried out with different states of Pb-Bi melts. The results revealed that the LLSC affected the solidification behavior and microstructures significantly, that is, the enlarged undercooling, increased nucleation rate, and refined and improved morphologies were brought about when solidifying from the melt experienced LLSC. It is assumed that the LLSC changed the energy constitution of the melt system, and further affected the effective partition coefficient, thermodynamics, and kinetics of crystal growth, then finally altered the solidification behavior and solidified microstructures. This work brings a novel insight into the effect of melt overheating treatment on solidification, by which it could be more effective to manipulate melts.

EFFECT OF ZIRCONIUM ADDITION ON MICROSTRUCTURE AND MECHANICAL PROPERTY OF TiC/Ti6A14V COMPOSITES

TiC/Ti6A14V composites with different Zr additions were prepared successfully in a consumable vacuum arc furnace equipped with a water-cooled copper crucible and the effect of the Zr content on the microstructure and mechanical property of 15 vol.% TiC/Ti6A14V composites was investigated by XRD, SEM and hardness testing. The results show that when the level of Zr addition is less than 4 wt.%, the morphology of the primary TiC in the composites is dendrite, and the petal-shape, piece-shape or palpus-shape eutectic TiC separates out around the primary TiC . The average size of the primary TiC decreases and the amount of eutectic TiC increases gradually with increasing Zr content. The effects of Zr on morphology of the primary TiC weaken with further addition of Zr . And the hardness (HRC) of composites was obviously increased in the whole range of Zr addition. The refinement mechanism of Zr was attributed to the combined effects of increase in nucleation rate at the constitutionally supercooled zone ahead of the solidification front and reduction in growth rate.

A Study on Nucleation for Protein Crystallization in Mixed Vessels

This study investigated mechanisms of secondary nucleation for the crystallization of the protein hen egg white lysozyme. A parametric study of the secondary nucleation of lysozyme in a stirred tank crystallizer was performed, testing the influence of stirrer speed, the size and volume of added seed crystals, solution temperature, supersaturation, and sodium chloride concentration on measured nucleation rates. The measured rates were highly variable, which made assessment of the influence of experimental parameters difficult. Only stirrer speed exhibited significant influence over experimental variability (10-fold increase in nucleation rates with a 50% increase in stirring speed). These results were interpreted by comparison with parameter influence that is commonly observed with secondary nucleation of nonproteins. A novel tubular crystallizer was used to assess the influence on secondary nucleation of fluid shear forces acting on lysozyme crystals. It was observed that fluid shear forces acting on mounted lysozyme crystals do not notably influence secondary nucleation. Overall, the results suggest that attrition is an important source of secondary nuclei in lysozyme crystallization and that secondary nucleation caused solely by fluid shear forces is not.