Effective Nucleation Rate Research Articles

Research on Microstructure and Mechanical Properties of Rheological Die Forging Parts of Al-6.54Zn-2.40Cu-2.35Mg-0.10Zr(-Sc) Alloy.

High-strength aluminum alloy (mainly refers to the 7xxx series) is the optimum material for lightweight military equipment. However, this type of aluminum alloy is a wrought aluminum alloy. If it is directly formed by traditional casting methods, there will inevitably be problems such as coarseness, unevenness, looseness, and hot cracking in the structure, which will greatly affect the final performance of the part. Based on the internal cooling with annular electromagnetic stirring (IC-AEMS) method, a new technology of rheological die forging is developed in this paper, and the scale-reduced parts of a brake hub of Al-6.54Zn-2.40Cu-2.35Mg-0.10Zr aluminum alloy were prepared. The influence of IC-AEMS and the addition of rare element Sc on the structure and mechanical properties of the parts was studied. An optical microscope and scanning electron microscope (SEM) were used to observe the microstructure evolution, energy dispersive spectroscopy (EDS) was used to analyze the phase distribution and composition, and the mechanical properties of the parts were tested by uniaxial tensile tests. The results show that the addition of Sc element can effectively refine the grains and improve the strength and elongation of the material; the application of IC-AEMS improves the cooling rate of the melt, increases the effective nucleation rate, and the grains are further refined. Through process optimization, scale-reduced parts of a brake hub with good formability and mechanical properties can be obtained, the ultimate tensile strength is 597.2 ± 3.1 MPa, the yield strength is 517.8 ± 4.3 MPa, and the elongation is 13.7 ± 1.3%.

Study on Formation of Microstructure in Rheo-Diecastings of Semi-Solid A380 Aluminum Alloy Slurry

The formation of the microstructure in the rheo-diecastings were studied by the experiment of rheo-diecasting with semi-solid A380 Aluminum alloy slurry. The results show that the grain diameter and shape factor of primary α-Al grains decreased and increased along the rheo-diecasting direction, respectively, making the primary α-Al grains gradually distributing uniformly. The combination of injection pressure and narrow gate of the runner and ingate was the main reason for the evolution of the primary α-Al grains, moreover, the collision and friction among primary α-Al grains during the filling were helpful for the refinement and spheroidization of the primary α-Al grains. The residual liquid solidified into the secondary solidification microstructure accompanied by the evolution of the primary α-Al grains. The accumulation of the fragments of the primary α-Al grains, the secondary α2-Al nuclei or grains which formed in previous positions, the effective nucleation rate of the residual liquid, and the collision and fraction among the secondary α2-Al grains and between the secondary α2-Al grain and the primary α-Al grains made the secondary solidification microstructure of the residual liquid present multiplex and complicate characteristics. In this paper, the formation of the microstructure in the rheo-diecastings were studied by the experiment of rheo-diecasting. During the rheo-diecasting, the grain diameter and shape factor of primary α-Al grains decreased and increased, respectively along the rheo-diecasting direction, which made the primary α-Al grains gradually distribute uniformly. The drifting of the fragments of the primary α-Al grains and secondary α2-Al nucleus, and the effective nucleation rate of the residual liquid played an important effect on the morphology of the secondary solidification structure.

Characterising the Bushe-Poladpur Contact across the Western Deccan Traps and Implications for Mapping the K-Pg Boundary?

Abstract The Bushe-Poladpur contact is one of the most important stratigraphic boundary separating the middle Lonavala Sub-group from upper Wai Sub-group in the Western Deccan Traps. In this paper, physical volcanology textural and petrochemical finger printing, crystal size distribution and shape preferred orientation studies (SPO) is integrated with paleomagnetic and AMS studies to correlate lavas across five localities transcending the Bushe-Poladpur boundary, south of Pune city. The flows belonging to the Bushe Formation are highly inflated, compound pahoehoe lavas, with plagioclase-olivine-phyric, coarse grained diktytaxitic textures. The flows belonging to the Poladpur Formation are transitional between pahoehoe and aa (rubbly pahoehoe lavas), with or without multi-tiered entablature showing fine-grained, olivine-phyric glassy textures. CSD studies indicate that the Bushe lavas have lower plagioclase crystals per unit area (1.11x10-4/μm2), low average effective nucleation rates (2.60x 10-12/μm3s) and high average effective growth rates (1.22x10-5μm2/s) for intact crust time than the Poladpur lavas that have higher plagioclase crystals per unit area (8.31x10-4/μm2), high average effective nucleation rates (3.46x10-11/μm3s) and lower average effective growth rates (2.24x10-6μm2/s) akin to their emplacement dynamics and cooling histories. There is a stark contrast in the magnetic susceptibilities (Bushe- 24759.8 x 10-6 SI; Poladpur-29178.02 x10-6 SI) across the Bushe-Poladpur contact. There is a general similarity between the SPO of the primary silicates (plagioclase) to the AMS derived directions. The divergence in AMS directions across the Bushe-Poladpur contact in the study area can be attributed to complex interplay of local paleotopography and slope, lava flow field dynamics, inflation and inversion of topography, style and emplacement mechanism of flow units. Generalisations and over simplifications of delineating lavas and deciphering their flow direction on a single criteria (tool) or single ghat (road section), outcrop or sample will have disastrous consequences while mapping large and complex lava flow fields. Thus, a multi-disciplinary approach needs to be the norm for characterising, correlating and mapping lavas across formations that transcend the K-Pg boundary in highly dissected ancient continental flood basalt province like the Deccan Traps.

The effect of mixing on silver particle morphology in flow synthesis

Silver particles, prepared in a T-mixer under different flow rates, were selected to study the influences of mixing on particle shape evolution. Mixing effects on particle growth were visualized by scanning electron microscopy (SEM) and analysed quantitatively by sedimentation coefficient distributions derived from analytical centrifugation (AC). The mixing time under different flow rates was determined by the Villermaux-Dushman method to quantify the mixing quality. Based on the finding of a mixing-induced shape transformation from plates to dendrites, an extended growth mechanism involving mixing effects was proposed. Slow mixing leads to a non-uniform distributed reactant mixture and low effective supersaturation. This causes preferential growth of high-energy facets resulting in plate-like particles with broad, multimodal sedimentation distributions. In contrast, fast mixing, corresponding to uniform reactant mixture and thus high effective supersaturation and nucleation rate, leads to dendritic products, and narrow but bimodal sedimentation distributions.

Spontaneous cavitation in liquid n-butane at negative and positive pressures

The method of pulsed liquid superheating on a wire heater in a tension wave has been used to investigate the kinetics of spontaneous boiling-up of liquid n-butane. Experiments were conducted in the pressure range from −6.8 to 2.55MPa. Stretchings in a liquid were created by the inversion of a compression pulse at the liquid–vapor interface. The values of limiting superheatings for n-butane have been determined, as well as the effective nucleation rates J = 1020−1022s−1m−3 and the value of the derivative of the nucleation rate with respect to the temperature GT=dlnJ/dT. A satisfactory agreement between classical nucleation theory and experiment in the derivative GT has been established, which is a weighty argument in favor of realization of the homogeneous nucleation mechanism in experiment, with a systematic understatement of superheatings (stretchings) achieved by experiment with respect to their theoretical values. It has been noted that this may be connected with the fact that classical nucleation theory does not take into account the size dependence of the surface tension of critical bubbles.

Practical Evaluation Method of Oxygen Precipitation in the Czochralski Silicon

Oxygen precipitates in Czochralski silicon (CZ-Si) are still considered as the major process-induced defects found in semiconductor device fabrication process because the role of oxygen precipitates in device process such as gettering of metal impurity, prevention of undesired wafer deformation, and degradation of carrier lifetime is quite significant to manufacture high-performance devices. Until now, various theoretical models to predict the oxygen precipitation in CZ-Si with respect to the interstitial oxygen (Oi), vacancy (CV), and the process temperature T have been suggested. However, the complexity and calculation time of a model strongly depend on the accuracy of the model. Thus, it is difficult to apply them on the evaluation of oxygen precipitation regularly in conventional wafer production. Hence, a practical and simple method to evaluate oxygen precipitation should be proposed. In this work, we derived the parameters that indicate the thermal stability and formation rate of the precipitate nuclei by analyzing as-grown oxygen precipitates formed in a p-type CZ-Si. First, we calculated a cumulative distribution function (CDF) of as-grown precipitate density with respect to the thermal history of CZ-Si crystal from the theoretical nucleation rate J(Oi, CV, T) [1]. After that, we fitted the CDF mathematically into the temperature-dependent precipitate density measured after slow ramped anneals with 1 K/min [2] at various stability temperatures followed by a growth anneal at 1000°C for 16 hours. Moreover, the first derivative Je (T) of the CDF was also calculated which was associated with an effective nucleation rate. The extreme point (T C, Je (TC)) on the Je (T) was determined as a new parameter that represents the precipitate behavior at a measured point. Finally, we correlated the parameters to the experimental results measured in various crystals after the test heat cycle started at 800°C for 4 hours and followed by 1000°C for 16 hours. As shown in Fig. 1, we found that both T C and Je (TC) were strongly correlated to the precipitate density, which means that the parameters were possibly applied to evaluate the behavior of oxygen precipitation in a crystal after various thermal processes. Fig. 1 Correlation of the T C (a) and Je (T ­C) (b) to the precipitate density measured after 2-step annealing starts at 800°C for 4 hours and followed by 1000°C for 16 hours. [1] G. Kissinger, D. Gräf, U. Lambert, and H. Richter, J. Electrochem. Soc. 144, 1447 (1997).[2] G. Kissinger and J. Dabrowski, J. Electrochem. Soc. 155, H448 (2008). Figure 1

An effective nucleation rate for oxide precipitation in Czochralski silicon and a simple framework for multi-step anneal calculations

A simple and physically consistent quantitative model of oxide precipitate nucleation in Czochralski-Silicon (Cz-Si) can significantly improve predictions of wafer characteristics during fabrications of semiconductor devices or solar cells. Towards that end, a classical nucleation theory based nucleation rate expression is developed for oxide precipitation in Cz-Si wafers in which vacancy supersaturation is not high. The precipitate morphology is assumed to be thin flat plates. The estimates of critical cluster sizes from new model corroborate the theory that thermal donors or some other embryonic form of oxygen clusters can be precursors of oxide precipitates. The new model predicts effective nucleation rates which are in agreement with previously reported data. The nucleation rates predictions from the new model and diffusion limited growth of precipitate plates of constant thickness are used in a simple framework to calculate precipitate densities after multi-step anneals reported previously in literature. The modeled precipitate densities are found to be in reasonable agreement with corresponding experimental values.

Fine grained microstructure crystallographic feature and its effect on mechanical behavior in Ni–Cr–W superalloy

The microstructure, texture and microsegregation in conventional and fine grained ingots of Ni–Cr–W superalloy were experimentally charactered by metallography, field emission scanning electron microscope equipped with an Electron Backscatter Diffraction and electron probe microanalysis, respectively. Distribution of misorientation angles results indicates that the proportion of low-angle grain boundaries and high-angle grain boundaries is about 15% and 85%, respectively, demonstrating that the particles are genuinely separate grains with mostly high-angle boundaries. Further, inverse pole figure results supply evidence for elucidating the mechanism of grain refinement due to a high effective nucleation rate which is attributed to extremely uniform temperature and composition fields in the bulk liquid at early stages of solidification. Element segregation at the grain boundary of equiaxed fine grain microstructure is smaller than that of the conventional dendrite grain microstructure. The effect of different morphologies of microstructure on yield strength was investigated. The effect of different average grain sizes on micro-hardness and yield strength was also analyzed. Relationship between yield strength and the grain size of this alloy followed the Hall–Petch relationship: σs=700+422d−1/2.

Electrochemical Synthesis of Amorphous Fe-B Alloys for Magnetostrictive Resonant Sensor

Fe-B alloy films were fabricated by an electrochemical deposition method. The effects of deposition parameters including current density, solution concentration, and ratio of precursors on crystallinity and magnetostrictivity were investigated. Amorphous Fe-B alloy films were obtained in the conditions of effective boron incorporation and high nucleation rate by increasing current density, solution concentration, and ratio of iron precursor. The results indicate that amorphous phase formation is related to deposition kinetics and chemical affinity between iron and boron precursors. The magnetostrictive characterization of the deposited amorphous Fe-B alloys demonstrated a potential biosensor platform of the films.

Porphyroblast crystallization kinetics: the role of the nutrient production rate

The mechanisms that govern porphyroblast crystallization are investigated by comparing quantitative textural data with predictions from different crystallization models. Such numerical models use kinetic formulations of the main crystallization mechanism to predict textural characteristics, such as grain size distributions. In turn, data on porphyroblast textures for natural samples are used to infer which mechanism dominated during their formation. Whereas previous models assume that the rate-limiting step for a porphyroblast producing reaction is either transport or growth, the model advanced in this study considers the production of nutrients for porphyroblasts as a potentially rate-limiting factor. This production reflects the breakdown of (metastable) reactants, which at a specific pressure (P) and temperature (T) depends on the bulk composition of the sample. The production of nutrients that potentially contribute to the formation of porphyroblasts is computed based on thermodynamic models. The conceptual model assumes that these nutrients feed into some intergranular medium, and products form by nutrient consumption from that medium, with rates depending on reaction affinity. For any sequence of P-T conditions along a P-T-t path, the numerical model first computes an effective supersaturation (reff) of the product phase(s), then an effective nucleation rate (J), and finally the amount of (porphyroblast) growth. As a result, the model is useful in investigating how the textural characteristics of a sample (of given bulk composition) depend on the P-T-t path followed during porphyroblast crystallization. The numerical model is tested and validated by comparing simulation results with quantitative textural data for garnet porphyroblasts measured in samples from the Swiss Central Alps.

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.

Development of the Grain Size Distribution During the Crystallization of an Amorphous Solid

ABSTRACTWe present an overview of the theory developed over the last few years to describe the crystallization of amorphous solids. The microstructure of the crystallizing solid is described in terms of the grain size distribution (GSD). We propose a partial differential equation that captures the physics of crystallization in random nucleation and growth processes. The analytic description is derived for isotropic and anisotropic growth rates and allows for the analysis of different stages of crystallization, from early to full crystallization. We show how the timedependence of effective nucleation and growth rates affect the final distribution. In particular, we demonstrate that for cases described by the Kolmogorov-Avrami-Mehl-Johnson (KAMJ) model applicable to a large class of crystallization processes a lognormal type distribution is obtained at full crystallization. The application of the theory to the crystallization of silicon thin films is discussed.

Switching Kinetics in Cesium Nitrate: Poly (Vinyl Alcohol) Composite Film

The effect of pulse amplitude on the switching properties of cesium nitrate: poly (vinyl alcohol) composite film at room temperature has been studied. The switching response of the composite film was studied using the bipolar square pulses. The switching current transients were analyzed using finite grain model (FGM) and infinite grain media (IGM). The Matlab programming based upon the assumptions of FGM and IGM theories is used to fit the experimental data with the theoretical. The microscopic parameters such as effective dimensionality, switching time, nucleation rate, and domain wall velocity have been estimated. The polarization current and domain wall velocity exhibited exponential dependence on the pulse amplitude.

Effect of high shear rate on solidification microstructure of semisolid AZ91D alloy

To investigate the effects of rotation speed and shearing time on morphology of semisolid AZ91D alloy, experimental work was undertaken using a twin-screw slurry maker. The results show that increasing the rotation speed and reasonable time can give rise to substantial grain refinement during continuous shearing stage, which can be attributed to the increasing of effective nucleation rate caused by the extremely uniform temperature due to high shear rate and high degree of turbulence. Comparing with low rotation speed at the same thermal condition, the analysis indicates that the microstructures obtained at high rotation speed are homogenous spherical and fine grains instead of dendritic or rosette and exhibits uniform distribution in the eutectic matrix.

Study of ferroelectric properties of spray pyrolysis deposited cesium nitrate films

Cesium nitrate (CsNO 3) films were prepared by ultrasonic spray pyrolysis technique at different substrate temperatures ( T s) and their ferroelectric and switching properties were examined. The morphology of the deposited films was studied using FESEM. The ferroelectric properties were optimized based on remanent polarization as a function of substrate temperature. The switching response was studied using pulse width technique. The switching parameters such as effective dimensionality, switching time, and nucleation rate were extracted using finite grain model (FGM). The width of reading pulse shows significant effect on switching transients.

Time-evolution of grain size distributions in random nucleation and growth crystallization processes

We study the time dependence of the grain size distribution $N(\mathbf{r},t)$ during crystallization of a $d$-dimensional solid. A partial differential equation, including a source term for nuclei and a growth law for grains, is solved analytically for any dimension $d$. We discuss solutions obtained for processes described by the Kolmogorov-Avrami-Mehl-Johnson model for random nucleation and growth (RNG). Nucleation and growth are set on the same footing, which leads to a time-dependent decay of both effective rates. We analyze in detail how model parameters, the dimensionality of the crystallization process, and time influence the shape of the distribution. The calculations show that the dynamics of the effective nucleation and effective growth rates play an essential role in determining the final form of the distribution obtained at full crystallization. We demonstrate that for one class of nucleation and growth rates, the distribution evolves in time into the logarithmic-normal (lognormal) form discussed earlier by Bergmann and Bill [J. Cryst. Growth 310, 3135 (2008)]. We also obtain an analytical expression for the finite maximal grain size at all times. The theory allows for the description of a variety of RNG crystallization processes in thin films and bulk materials. Expressions useful for experimental data analysis are presented for the grain size distribution and the moments in terms of fundamental and measurable parameters of the model.

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.

Explosive cavitation in superheated liquid argon

The kinetics of explosive boiling-up of liquid argon has been investigated at negative pressures created by the reflection of a compression pulse 3-5 mus long from the free surface of a liquid by the method of liquid pulse heating on a thin platinum wire (with a rate of temperature increase of about 1 Kmus). The limiting superheats T(*) (stretches p(*)), the effective nucleation rate J(*), and the derivative G(T)=(d ln JdT)(T=T(*) ) have been determined by experimental data on the thermal perturbation of a wire probe and the results of solution of the problem on the initial stage of explosive boiling-up of a liquid. The experimental data are compared with homogeneous nucleation theory.

Low superheat pouring with a Shear Field in rheocasting of aluminum alloys

A new rheocasting process, Low Superheat Pouring with a Shear Field (LSPSF), was developed to produce semi-solid slurry for the rheo-forming process. The LSPSF process is one of controlled nucleation and limited growth techniques in which effective nucleation rate is controlled by passive mixing and localized rapid cooling near the liquidus temperature; dendrite growth is limited by a much slower cooling process. The experimental results of rheocast Al-Cu alloy 201 and secondary diecasting alloy A380 demonstrate that LSPSF process is capable of producing high-quality slurry in a matter of seconds. It is concluded that maximizing nucleation and nuclei survival directly lead to the grain refinement of the rheocasting microstructure, and high nuclei density combined with a slower cooling afterwards leads to a globular structure of primary phases.

Fabrication and transport properties of directionally solidified Y123 thin fibre

Among the high Tc superconductive oxides, RE123 superconductors are expected to be useful for high criticalcurrent applications under high magnetic fields. The superconducting current lead for lowtemperature instruments is a suitable application because it can drastically reduce the heatload. A directional solidification process is applied to make a long single-domain RE123crystal. To achieve the continuously grown single-domain Y123 crystal, it is found that thinfibres, 0.5 mm in diameter, can be grown continuously at higher growth rates compared tofat rods 2 mm in diameter. This could be explained by the difference in the effectivenucleation rate in the liquid region ahead of the growth interface; that is, theeffective nucleation volume of the thin fibres, which is smaller than that of thefat rods, leads to the smaller nucleation rate while the nucleation frequency isalmost the same under the same growth conditions. The thin fibre maintains theJc property athigher than 1.5 × 104 A cm−2 while the thin fibre could be grown continuously with high pulling rates compared to theusual rods with a few millimetres in diameter. This high productivity will be an advantagefor the thin fibre for the current lead applications.