Segregation Kinetics Research Articles

Solute segregation kinetics and dislocation depinning in a binary alloy

Solute segregation kinetics and dislocation depinning in a binary alloy

Effect of subgrain structure on the kinetics of phosphorus segregation in grain boundaries

The effect of steel microstructure, in particular the grain fragmentation onto subgrains, on the kinetics of phosphorus segregation in grain boundaries is considered. Assuming that intergranular phosphorus redistribution happens much faster than bulk diffusion, the segregation process is controlled by phosphorus transport toward the subgrain surfaces. The diffusion problem for a subgrain, allowing the phosphorus depletion in the subgrain volume, is proposed to describe the segregation kinetics. This model is applied to the pressure vessel steel 15Kh2NMFA-A and weld metal Sv-10KhGNMAA of the nuclear power reactor WWER-1000. It was shown that this model provides better fitting of the experimental data than the conventional Langmuir–McLean equation.

Segregation of a two-phase mixture of incompatible viscous fluids in laminar flow

Investigation of segregation of multiphase fluid sys� tems is of significant scientific and practical interest. In practical terms, the obtained knowledge is in demand in the chemical and oil industries, for which data on the kinetics of the spatial redistribution of the dispersed phase are of considerable applied impor� tance. For example, a change in the specific surface area of emulsions due to coalescence of drops can lead to noticeable variations in chemical reaction rates, including the emulsion polymerization kinetics. This should be taken into account in designing industrial dispersers and mixers. At the same time, in production and transportation of petroleum products containing a certain amount of water, phase separation processes can give rise to discontinuous water domains or con� tinuous water streams along the lower generatrix of pipelines [1, 2]. The presence of dissolved gases, e.g., CO2 and/or H2S, leads to accelerated development of pitting or groove corrosion [3–6], which is fraught with service life reduction and even breakdown of pipelines. In oil pipelines, water dispersions deposit in a flow of a medium. The combination of these processes can change the segregation kinetics of components of the medium in comparison to the deposition kinetics at rest. In particular, this can be caused by variations in the drop collision frequency and drop shape, which, in turn, influence the coalescence conditions and, hence, the deposition velocity. Numerous studies were made of sedimentation of liquid and solid dispersions in flows of air and gases, river streams, ocean currents, and also oil transportation [7–10]. However, this chal� lenge continues to be in the limelight. Specifically, this is highly topical for representative sampling during

Electroless- and Electroplating of Cu(Re) Alloy Films for Self-Forming Ultrathin Re Diffusion Barrier

To inhibit the detrimental diffusion of copper into silicon devices, an effective barrier is strongly demanded. In this study, copper(rhenium) alloy films were successfully electroless- and electroplated on silicon substrates for self-forming an ultrathin rhenium barrier layer. In pure copper films, the interdiffusion of copper and silicon already occurred at 400°C, forming silicides and increasing the electrical resistivity of the films. In comparison, not any silicide formation was observed in the electroless- and electroplated copper(rhenium) alloy films with minor incorporations of rhenium for 0.10 and 0.39 at% until 600 and 500°C, respectively, attributed to the prior segregation of rhenium and the self-formation of rhenium barrier layer at the copper/silicon interface at 400°C. A zero solubility of rhenium in copper and a small solubility in palladium catalysts facilitate the separation of rhenium from copper. The kinetics of rhenium segregation follows uphill diffusion, and the diffusivity of rhenium in copper at 400°C is estimated to be 1.5 × 10−17 cm2/s.

Non-equilibrium grain-boundary segregation of Bi in Cu bicrystals

The observations of grain-boundary segregation of Bi in Cu bicrystals were analyzed. According to equilibrium grain boundary segregation (EGS) model and non-equilibrium grain-boundary segregation (NGS) model, respectively, the segregation kinetics of isothermal annealing at 500 °C and that of isochronal annealing for 24 h of Bi in Cu bicrystals were investigated. By qualitative analysis and quantitative analysis, it is concluded that the grain-boundary segregation of Bi agrees well with the theory of NGS. Based on the kinetics model of NGS, some parameters that are useful to predicting and controlling the Bi-induced embrittlement in Cu alloys are calculated as follows: the diffusion coefficient of Bi-vacancy complexes Dc=7.8×10−5exp[–1.46/(kT)]; the apparent diffusion coefficient of Bi atoms DiA=7.66×10at+bexp[–1.76/(kT)], where a=8.45×10−8 and b=–13.37.

Effect of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3−δ thin films

The effects of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF-6428) were investigated. (001)-oriented LSCF-6428 thin films were deposited on lattice matched (110)-oriented NdGaO3 (NGO) substrates by pulsed laser deposition (PLD). Using the synchrotron technique of total reflection X-ray fluorescence (TXRF), it was found that the kinetics of Sr surface segregation was enhanced when annealing at 800°C in a high-CO2 partial pressure, as compared to a similar anneal in a CO2-free atmosphere, with the oxygen partial pressure being constant in both cases. Hard X-ray photoelectron spectroscopy (HAXPES) measurements showed that the contribution of the surface carbonate to surface oxide phases increased significantly for the sample annealed in the high-CO2 atmosphere. Atomic force microscopy (AFM) studies showed enhanced surface phase formation during the high-CO2 partial pressure anneal. Density functional theory (DFT) calculations provide a thermodynamic basis for the enhanced kinetics of surface segregation in the presence of atmospheric CO2.

Correlation Between MnS Precipitation, Sulfur Segregation Kinetics, and Hot Ductility in C-Mn Steel

In a C-Mn steel without Nb, the mechanism of hot ductility loss and recovery has been understood. The specimens were solution treated at 1673 K (1400 °C), subsequently cooled to 1273 K (1000 °C) at a rate of 1 or 20 K/s and finally held at the temperature. At a rate of 1 K/s, the segregation concentration of sulfur at prior austenite grain boundaries decreases gradually with holding time. At the rate of 20 K/s, the segregation concentration of sulfur shows a convex profile in a time versus segregation concentration plot. Such segregation behaviors of sulfur are deeply related to the MnS reaction during cooling to 1273 K (1000 °C) or holding at the temperature. The high-temperature intergranular fracture observed in this steel is due to the sulfur segregated at the grain boundaries. The recovery of hot ductility results from the combination between the decrease in sulfur segregation concentration governed by the MnS reaction and the overall decohesion at the interface of the MnS particles which act as a strong sink of the free sulfur tending to segregate to the grain boundaries.

Numerical study of particle segregation in a coal beneficiation fluidized bed by a TFM–DEM hybrid model: Influence of coal particle size and density

Abstract Particle segregation behavior in a coal beneficiation fluidized bed (CBFB) is numerically studied using a TFM–DEM hybrid model, in which the gas and the dense solid phases are modeled using a Eulerian–Eulerian or two fluid model (TFM), while the beneficiated coal particles are modeled as a dilute phase by the discrete element method (DEM). For validation purpose, the numerical model was setup using geometric and operating conditions similar to a laboratory experimental model with the bed thickness set to one particle diameter to save computational cost. For a fixed gas injection velocity, the influence of particle size and density of the beneficiated samples was studied. It was found that the particles would segregate along the bed height due to the density differences with the degree of segregation being strongly influenced by particle size. Obvious segregation occurs for the coarse samples (6.7 mm and 4.3 mm) and little segregation occurs for the particles smaller than 3 mm. The flow patterns and segregation kinetics were qualitatively comparable with those observed in physical experiments conducted under similar conditions. On this basis, the underlying mechanisms governing particle segregation have been explained in terms of the hydrodynamic forces acting on individual particles. It was demonstrated that the segregation of coarse particles was mainly controlled by the balance between gravity and the local pressure force, while fine particles were more strongly affected by the direct drag forces from the gas phase and the continuum solid phase, thus making them difficult to separate.

Electronic Structure Measurements of Heteroepitaxial Solid Oxide Fuel Cell Cathode Thin Films

Synchrotron based total reflection x-ray fluorescence (TXRF) and hard x-ray photoelectron spectroscopy (HAXPES) technique have been utilized to measure the compositional changes and evolution of surface chemistry of strontium doped lanthanum cobalt iron oxide (LSCF) annealed at 800 °C, as a function of strontium dopant concentration and gas composition. TXRF is a useful way to depth-profile the surface composition of material at elevated temperatures in real time. The capability of HAXPES to overcome surface contamination issues helps better understand the segregation phenomena. This study demonstrates that the formation of Sr carbonate results in enhanced kinetics of Sr surface segregation in LSCF thin films with presence of CO2 when annealing at 800 °C and lowering the Sr dopant concentration results in a decrease Sr surface segregation.

Kinetics of phase separation in polymer mixtures: A molecular dynamics study

We present detailed results from a molecular dynamics (MD) simulation of phase-separation kinetics in polymer mixtures. Our MD simulations naturally incorporate hydrodynamic effects. We find that polymeric phase separation (with dynamically symmetric components) is in the same universality class as segregation of simple fluids: the degree of polymerization only slows down the segregation kinetics. For d = 2 polymeric fluids, the domain growth law is L(t) ∼ t(ϕ) with ϕ showing a crossover from 1/3 → 1/2 → 2/3. For d = 3 polymeric fluids, we see the crossover ϕ = 1/3 → 1. Our MD simulations do not yet access the inertial hydrodynamic regime (with L ∼ t(2/3)) of phase separation in 3-d fluids.

In Vivo Kinetics of Segregation and Polar Retention of MS2-GFP-RNA Complexes in Escherichia coli

The cytoplasm of Escherichia coli is a crowded, heterogeneous environment. From single cell live imaging, we investigated the spatial kinetics and heterogeneities of synthetic RNA-protein complexes. First, although their known tendency to accumulate at the cell poles does not appear to introduce asymmetries between older and newer cell poles within a cell lifetime, these emerge with cell divisions. This suggests strong polar retention of the complexes, which we verified in their history of positions and mean escape time from the poles. Next, we show that the polar retention relies on anisotropies in the displacement distribution in the region between midcell and poles, whereas the speed is homogeneous along the major cell axis. Afterward, we establish that these regions are at the border of the nucleoid and shift outward with cell growth, due to the nucleoid’s replication. Overall, the spatiotemporal kinetics of the complexes, which is robust to suboptimal temperatures, suggests that nucleoid occlusion is a source of dynamic heterogeneities of macromolecules in E. coli that ultimately generate phenotypic differences between sister cells.

Grain boundary segregations in nanocrystalline alloys

The dependence of grain boundary segregations and alloy composition in the bulk of the grain on the grain size was considered. At moderate temperatures, there is a critical grain size for which the integrated degree of alloy decomposition reaches its maximum; this size increases as the average (over the sample) impurity concentration decreases. In complex alloys, the type of the component that segregates on the boundaries depends on the grain size. For segregation kinetics, an approximate analytical solution was obtained based on the assumption that the grain boundary is an ideal drain for the impurity. The equilibrium grain size exists below the critical temperature and its temperature dependence is nonmonotonic in the general case. A segregation mechanism of stabilization of nonequilibrium grain boundaries in nanocrystalline alloys obtained by severe plastic deformation was suggested.

Kinetics of phosphorus segregation at grain boundaries of low-alloy low-carbon steel

In a temperature range of 280–320°C, the mechanism and kinetics of segregation of impurities in steels have yet remained insufficiently studied. Under these conditions diffusion of impurities in the bulk of steel grains practically ceases, and for describing the kinetics of the process it is incorrect to use the Langmuir-McLean equation. In this work we put forward two new approaches to describe the mechanism and kinetics of phosphorus segregation in steels: a model of sequential changes in the state of phosphorus based on first-order reactions and a model of diffusion redistribution of phosphorus between boundaries of carbide precipitates, structure defects, and boundaries of steel grains. A comparative analysis of the suggested models has been conducted, and estimates of the kinetics of segregation based on them have been made; these estimates have been compared with the experimental results obtained in the temperature range of 280–320°C for test times to ∼20 years. It has been shown that these models fairly well describe the experimental kinetics of phosphorus segregation in boundaries of steel grains.

Local equilibrium in the dissolution and segregation kinetics of Ag on Cu(1 1 1) surface

The local equilibrium in the kinetic processes of dissolution and segregation of Ag on Cu(111) surface is addressed. The measured dissolution and segregation kinetic data of Ag on Cu(111) surface at temperature of 450°C are well fitted by the modified Darken model. The segregation parameters, i.e. segregation energy, interaction and diffusion parameters, in the Cu(111)(Ag) binary system are obtained upon fitting. Using the obtained segregation parameters, the discontinuous transition of Ag surface concentration against the bulk concentration of the surface neighboring layers deduced from the local equilibrium model is quantitatively interpreted.

Influence of P and C Intergranular Segregation During Manufacturing and Ageing on the Fracture Toughness of Nuclear Pressure Vessel Steels

Mechanical tests on Charpy and CT specimens of a low alloy MnNiMo steel under two conditions, as received and thermally aged, revealed a shift of the ductile to brittle transition temperature. In this paper, an approach based on the combination of local fracture mechanics and segregation kinetics is proposed in order to describe this shift.

Anomalous Segregation Kinetics of Phosphorus and Carbon Governed by Carbide Reactions in 2.25Cr-1.5W Heat-resistant Steel

Anomalous Segregation Kinetics of Phosphorus and Carbon Governed by Carbide Reactions in 2.25Cr-1.5W Heat-resistant Steel

Ultra-fast sulphur grain boundary segregation during hot deformation of nickel

Sulphur grain boundary segregation during hot-compression of nickel (5.4wt.ppmS) is monitored using Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy. The deformation conditions (temperature/deformation rate) investigated are: 550°C/0, 550°C/3×10−5s−1, 550°C/3×10−4s−1 and 450°C/3×10−5s−1. It is shown that plastic deformation accelerates the kinetics of sulphur grain boundary segregation by a factor of ∼103 to a few 105, depending on the deformation conditions. Very high levels of segregation (∼0.8 monolayer of sulphur) are obtained after very low deformation (∼5%). In addition a linear dependence of the segregation level with time and deformation is demonstrated. The segregation kinetics during plastic deformation is proportional to the deformation rate and almost independent of temperature. Several metallurgical mechanisms are discussed and confronted with the experimental results: dislocations dragging, pipe diffusion, dislocation collection/diffusion and acceleration by excess vacancies. It appears that the models developed in this work on the basis of the two latter mechanisms (dislocation collection/diffusion and acceleration by excess vacancies) predict the experimental data correctly.

Kinetics of nonsymmetrical impurity segregation at thin film boundaries

Theoretical and numerical analysis is presented for kinetics of impurity segregation at the boundaries of a thin layer of substance dividing two different materials. Time dependences of boundary impurity concentrations and concentration profiles within the film have been obtained. Depending on the energy of impurity interaction with the boundaries and the temperature, some criteria were established to predict probable realization of segregation process. The time of exhausting of areas adjusting segregation layers and the width of the exhausted zone near the boundaries was evaluated.

Surface segregation of silver atoms on Au–Ag alloys according to data of laser-heating induced temperature potential shifts, XPS and conventional electrochemical methods

Process of the surface segregation of Ag atoms at the interface of Au–Ag alloy electrodes with the surface inactive electrolyte (NaF) was studied in the potential region of ideal polarizability by the methods of potential shifts induced by laser heating, impedance and cyclic voltammetry. It was shown that impedance measurements on electrodes with the mechanically renewed surface make it possible to obtain well reproducible data on the kinetics of enrichment of the electrode surface layer with silver atoms. As follows from analysis of capacitance vs. potential dependences measured at different moments after the electrode renewal, these dependences are well described by the model of “common diffuse layer”. The use of this model allowed us to calculate the time dependences of the electrode surface coverage by silver atoms after renewal. Specific features of the influence of the electrode potential on the kinetics of the surface segregation of Ag atoms at the Au–Ag electrode surface were demonstrated and analyzed within the framework of the bulk diffusion model. Data for the metal/solution interface were supplemented by the XPS data for Au–Ag/vacuum interface.

Modeling of Surface Segregation Effects Observed on Renewed Electrodes of Eutectic Alloys

Experimental data on the transients of the electric double layer (EDL) capacitance measured after the mechanical renewal of the surface of electrodes of eutectic Sn-Pb and Ag-Bi alloys with small concentrations of second components at the interface with sodium fluoride solutions are analyzed and generalized. The observed transients are associated with the surface segregation of second components of alloys at the boundary with the solution. Phenomenological models of the electrode/solution interface which make it possible to calculate the surface coverage of the metal phase with different alloy components in different times after the renewal are proposed and substantiated. It is shown that the kinetics of surface segregation of lead and bismuth atoms at the alloy electrode interfaces with electrolyte solutions can be described in the first approximation by a model of nonsteady-state surface diffusion.