Segregation Profiles Research Articles

Interconnection of thermal parameters, microstructure, macrosegregation and microhardness of unidirectionally solidified Zn-rich Zn–Ag peritectic alloys

In this work, the microstructural evolution of Zn–3.2wt%Ag (hypoperitectic) and Zn–8wt%Ag (hyperperitectic) alloys during transient unidirectional solidification is investigated. The experimental results include solidification thermal parameters such as the growth rate (VL), thermal gradient (GL) and tip cooling rate (Ṫ), which are related to the microstructural interphase spacing (λ) by proposed experimental growth laws. It is shown that, the classical lamellar eutectic growth law λ2V=constant, applies to the growth of the peritectic Zn–Ag alloys examined, despite the different values of the constant associated with each alloy composition. In contrast, it is shown that identical functions of the form λ=constant (GL)−14 (VL)−1/8, and λ=constant (Ṫ-1/3) can be applied to both alloys examined. Positive solute macrosegregation was observed in regions close to the bottom of the castings. The dependence of microhardness (HV) on the length scale of the microstructures (including that of a single phase Zn 0.8wt%Ag alloy: λC− cellular spacing) is examined. An experimental Hall–Petch type power law is proposed relating the resulting microhardness to λC for the single phase alloy, and despite the segregation profiles and the alloying differences of the hypoperitectic and hyperperitectic alloys, the average microhardnesses of these alloys is shown to be essentially constant and similar along the castings lengths.

Segregation during Metal Sampling

In order to obtain accurate chemical compositions in as-cast billets and ingots the sampling methods for the analysis have to give reproducible results with high precision. OES analysis made on samples at certain milling depths does not always show the desired nominal composition, and especially the segregation profiles within the samples can show significant variations. The present work examine the influence of main sampling parameters, such as the volume of the melt, the melt temperature, the mould design and the method of filling the mould, on the segregation. The results point out the importance of the convection in the mould during solidification, and thus the technique of pouring the melt into the sampling mould.

Characterization of 4 in VGF-GaAs single crystals grown in a heater-magnet module

Silicon-doped 4in VGF-GaAs single crystals were grown under the influence of traveling magnetic fields (TMF). A KRISTMAG˜® heater-magnet module (HMM) was used for the simultaneous generation of heat and TMF through a combination of DC and AC control. In this study, we investigated changing structural and electronical properties as well as micro- and macrosegregation of VGF-GaAs single crystals grown with applied TMF.Striations were observed in crystals grown without or too strong TMF. Almost no micro-inhomogeneities were detected when the magnetic flux densities of the TMF were matched to progression of solidification. With utilized TMF, induced melt flow opposed natural convection driven by buoyancy forces. Axial dopant incorporation was enhanced through a reduction of flow velocities and converging melt flow towards the center of the solid–liquid interface. The radial segregation profiles were flattened through a reduction of the concave deflection. While low frequency TMF bended the interface center more convex, the effect of high frequency TMF was more limited to the melt periphery. Furthermore, with large high frequency TMF current shares the impact of the minor three-dimensional asymmetric magnetic field distribution in the HMM became more relevant. Consequently, the application of double-frequency TMF led to a reduction of etch pit density through reduction of interface concavity.

Simulation of particle flow in a bell-less type charging system of a blast furnace using the discrete element method

A three-dimensional model was established by the discrete element method (DEM) to analyze the flow and segregation of particles in a charging process in detail. The simulation results of the burden falling trajectory obtained by the model were compared with the industrial charging measurements to validate the applicability of the model. The flow behavior of particles from the weighing hopper to the top layer of a blast furnace and the heaping behavior were analyzed using this model. A radial segregation index (RSI) was used to evaluate the extent of the size segregation in the charging process. In addition, the influence of the chute inclination angle on the size segregation and burden profile during the charging process was investigated.

Methods of segregation analysis applied to simulated multicomponent multiphase microstructures

Abstract Advanced microstructure simulation models can predict solute segregation in 2D and 3D space, which often leads to outputs of large arrays of concentration values with a multitude of detailed information that hinders straightforward evaluations. The target of this study is to establish a common evaluation method for both simulation results and experimental data as a standard for quantitative comparison and validation. For this purpose, a methodology is adopted from experimental segregation analysis to transform the multi-dimensional data into meaningful 1D segregation profiles, which can easily be plotted and discussed. As an application example, a directional solidification experiment on an AZ31 magnesium alloy is selected and the solidification process is simulated using the phase-field method. The subsequently obtained 1D segregation profiles are compared to measured segregation profiles. As part of the study, two common sorting methods are evaluated with respect to their applicability to recover the general segregation behavior and the solidification path, as well as to handle numerical noise.

Correlation between carbide reaction and anomalous interfacial segregation of solutes under tensile stress in heat-resistant steels

Depending on the phosphorus bulk content, the phosphorus segregation profile at the grain boundary/carbide interface of heat-resistant ferritic steels is composed of a convex profile or two maximum peaks and one minimum peak during rupture test at 650°C. In the higher phosphorus bulk content steel, the smaller pre-formed non-equilibrium M3C carbides, which arise from the higher phosphorus segregation concentration at the carbide interface and thus its carbide growth retardation, are massively dissolved into the matrix after the first maximum peak. While the dissolved carbon is consumed in forming the equilibrium M7C3 carbides, the carbon segregates at the same time to the grain boundary/formed M7C3 carbide interface and the matrix/formed M7C3 carbide interface, resulting in a strong repulsive segregation between carbon and phosphorus. However, because the transformation of the thermally unstable M3C carbides to the M7C3 carbides is very early finished in the lower phosphorus bulk content steel, such a repulsive segregation is not observed. Meanwhile, the decrease in phosphorus segregation concentration after the second maximum segregation peak in the former steel or a plateau in segregation concentration of the latter steel is due to the formation of equilibrium MX carbides which results in the increase in total interface area. Due to the finer and denser MX particles formed on the M7C3 carbide surface of the former steel, the phosphorus segregation concentration decreases abruptly with increasing rupture time after the second maximum. The difference in rupture behavior of the steels is understood from the viewpoint of such a correlation.

Internal Gettering of Iron at Extended Defects

The role of iron in crystalline silicon solar cells has been extensively investigated, yet the interaction mechanisms with structural defects have not been fully understood. In this work we have investigated a multicrystalline silicon ingot made in a small scale (1.5kg) vertical gradient freeze (VGF) furnace with the addition of 50 ppma Fe in the polysilicon feedstock. The minority carrier lifetime was qualitatively measured by photoluminescence (PL). Grain morphology and -orientation were determined by electron backscatter diffraction (EBSD). The comparison between the PL and EBSD maps shows high lifetime areas near the grain boundaries, which is explained by an internal gettering mechanism. Furthermore, it is evident that Fe segregates slightly at coincidence site lattice (CSL) boundaries – especially at Σ3, while it segregates heavily at random grain boundaries. These results indicate the dependence of Fe segregation towards defect on grain boundary character. A method to qualitatively and quantitatively identify the segregation profiles and depleted region thickness is developed by image analysis.

Experimentally-aided Simulation of Directional Solidification of Steel

Dendritic microstructures are most dominant patterns in solidified alloys. The microstructural features of these structures control the segregation profiles of solute elements in the interdendritic regions, thus determining the mechanical properties of cast structures. In this study, a 2D model of solid/liquid interface instability in a low carbon steel was introduced, using the multi-phase-field software code MICRESS® combined with an in-situ study of solidification in a laser-scanning confocal microscope. The use of a moving-frame boundary condition and a linear temperature gradient within the simulation allows further optimization of the solidification studies in the laser-scanning confocal microscope. By analysing the shape of the delta-ferrite grain boundary at the solid/liquid interface, in-situ and at temperature, it was possible to experimentally determine the Gibbs-Thomson coefficient and the solid/liquid interfacial energy of the alloy. The interface mobility of the solid/liquid interface was calibrated in the model so as to reproduce the experimentally measured interface velocity at the onset of interface instability. The proposed model was used to describe the morphological transitions from planar to cellular to dendritic modes during solidification and solute segregation under a variety of processing conditions such as cooling rate and temperature gradient. The importance of this approach is that the verified model has been used to extend the prediction of microstructural development to cooling rates well beyond what can be achieved experimentally and into the regime pertinent to high-speed continuous casting. Significant microstructural differences that arise as a result of varying processing conditions are discussed.

Segregation and mixture profiles in dense, inclined flows of two types of spheres

We study dry flows of two types of spheres down an inclined, rigid, bumpy bed in the absence of sidewalls. The flow is assumed to be steady and uniform in all but the direction normal to the free surface, collisions between particles are dissipative, and the sizes and masses of the particles are not too different. We restrict our analysis to dense flows and use an extension of kinetic theory to predict the concentration of the mixture and the profile of mixture velocity. A kinetic theory for a binary mixture of nearly elastic spheres that do not differ by much in their size or mass is employed to predict profiles of the concentration fraction of one type of sphere. We also determine the ratio of the radii and of the masses of the two species for which there is no segregation. We compare the predictions of the theory to the results of numerical simulations.

Innovative measurement of spatial segregation: Comparative evidence from Hong Kong and San Francisco

The spatial distribution of households of different socioeconomic groups in urban areas has drawn longstanding attention from scholars because residential location patterns have important impacts on social outcomes and the economic efficiency of cities. Recent comparative work on this topic has yielded some insight into the causes and consequences of segregation patterns, but much of this comparison is indirect. An explicitly spatial version of the entropy index has recently been developed that facilitates comparison, as it allows for the disaggregation of segregation levels by scale and income (Reardon and O'Sullivan, 2004; Reardon, 2009; Reardon and Bischoff, 2011). This paper applies these new measurement techniques to two metropolises; Hong Kong and San Francisco. Although overall segregation levels are similar, the shape of the segregation profile across geographic scales and the income distribution is quite different. The paper also includes a script for calculating spatial ordinal segregation indices in ArcGIS.

<b>The genetic diversity of strawberry (<i>Fragaria ananassa</i> Duch.) hybrids based on ISSR markers</b> - doi: 10.4025/actasciagron.v35i4.16737

The strawberry is an important agricultural crop in Brazil. However, most of the commercial genotypes currently in cultivation in Brazil were developed in other countries with environmental adaptations often inadequate for the regional conditions. In this work, inter-simple sequence repeat markers were used to determine the genetic variability and the loci segregation profiles of 84 strawberry hybrids obtained from a genetic breeding program at the ‘Empresa de Pesquisa Agropecuária de Minas Gerais.’ The hybrids were produced from crosses involving the following progenitors: ‘Toyonoka’ x ‘Sweet Charlie’, ‘Camino Real’ x ‘Sweet Charlie’, ‘Oso Grande’ x ‘Sweet Charlie’, ‘Oso Grande’ x ‘Toyonoka’, ‘Dover’ x ‘Oso Grande’, and ‘Camino Real’ x ‘Toyonoka’. Fourteen genotypes were randomly sampled for each hybrid combination and evaluated. The results showed that the genetic profiles of the hybrids from each test cross were very diverse, most likely due to the high heterozygosity of the genome of each progenitor involved, which might indicate the presence of adequate genetic diversity among the hybrids to allow for the selection of new cultivars with agronomic traits that are more suitable to environmental conditions in Brazil.

Experimental Validation of the Carbon Diffusion Model for Transformation of Ferritic-Pearlitic Microstructure into Austenite during Continuous Annealing of Dual Phase Steels

Modeling of the transformation of the starting ferritic-pearlitic microstructure into austenite during heating in continuous annealing process was the objective of the work. Kinetics of this transformation was predicted by solving Avrami equation as well as carbon diffusion equation with a moving boundary. Mathematical and numerical models describing austenitic phase transformation were created for the 1D and 2D domains. Developed models were solved using the Finite Difference, as well as the Finite Element Method. Results of the numerical simulations include austenite volume fraction and carbon segregation profiles in the austenite. The former were compared with the experimental data obtained in laboratory simulations of the continuous annealing. Developed and validated model was applied to simulation of the austenitic transformation during annealing of DP steels.

Grain boundary embrittlement by Mn and eutectoid reaction in binary Fe–12Mn steel

The grain boundary embrittlement in a binary Fe–12Mn is due to the grain boundary segregation of Mn. During tempering at 400°C (higher than the equilibrium eutectoid reaction temperature 247°C), reverted austenite particles were formed at lath and grain boundaries through the equilibrium reaction of lath martensite to ferrite+austenite. Surprisingly, hydrostatic pressure, which is induced by the transformation of epsilon martensite to austenite during heating at the tempering temperature, resulted in the nonequilibrium eutectoid reaction producing α-Mn precipitates at the interface between lath martensite and the transformed austenite during the tempering. The segregation concentration kinetics of Mn formed a convex profile due to the active grain boundary precipitation of the reverted austenite particles and the α-Mn particles, which act as a sink for the segregated Mn. Finally, the convex segregation profile of Mn corresponded to the concave profile of intergranular fracture strength.

Atomistic simulations of grain boundary segregation in nanocrystalline yttria-stabilized zirconia and gadolinia-doped ceria solid oxide electrolytes

Hybrid Monte Carlo–molecular dynamics simulations are carried out to study defect distributions near Σ5(310)/[001] pure tilt grain boundaries (GBs) in nanocrystalline yttria-stabilized zirconia and gadolinia-doped ceria. The simulations predict equilibrium distributions of dopant cations and oxygen vacancies in the vicinity of the GBs where both materials display considerable amounts of dopant segregation. The predictions are in qualitative agreement with various experimental observations. Further analyses show that the degree of dopant segregation increases with the doping level and applied pressure in both materials. The equilibrium segregation profiles are also strongly influenced by the microscopic structure of the GBs. The high concentration of oxygen vacancies at the GB interface due to lower vacancy formation energies triggers the dopant segregation, and the final segregation profiles are largely determined by the dopant–vacancy interaction.

Anomalous surface segregation profiles in ferritic Fe-Cr stainless steel

The iron-chromium alloy and its derivatives are widely used for their remarkable resistance to corrosion, which only occurs in a narrow concentration range around 9 to 13 atomic percent chromium. Although known to be due to chromium enrichment of a few atoms thick layer at the surfaces, the understanding of its complex atomistic origin has been a remaining challenge. We report an investigation of the thermodynamics of such surfaces at the atomic scale by means of Monte Carlo simulations. We use a Hamiltonian which provides a parameterization of previous ab initio results and successfully describes the alloy's unusual thermodynamics. We report a strong enrichment in Cr of the surfaces for low bulk concentrations, with a narrow optimum around 12 atomic percent chromium, beyond which the surface composition decreases drastically. This behavior is explained by a synergy between (i) the complex phase separation in the bulk alloy, (ii) local phase transitions that tune the layers closest to the surface to an iron-rich state and inhibit the bulk phase separation in this region, and (iii) its compensation by a strong and non-linear enrichment in Cr of the next few layers. Implications with respect to the design of prospective nanomaterials are briefly discussed.

A Novel Sample Structure for the Measurement of Indium Segregation Profiles in GaAs/InGaAs/GaAs Heterostructures

A Novel Sample Structure for the Measurement of Indium Segregation Profiles in GaAs/InGaAs/GaAs Heterostructures

Adsorbate-induced segregation: First-principles study for C/Pt25Rh75(100)

We develop a cluster-expansion (CE) formalism for the first-principles study of substrate-adsorbate interaction in order to study adsorbate-induced segregation. The simulation lattice, the configuration space, and the basis functions of the CE are bisected into separate but interacting subsystems. The method is applied to the C-contaminated Pt${}_{25}$Rh${}_{75}$$(100)$ surface. For such catalytic surfaces, it is essential to understand the change in the segregation profile of the substrate due to the presence of adsorbates. In accordance with experiments, we find that even a small amount of C impurities leads to a considerable decrease in the Pt segregation to the topmost substrate layer. We discuss the coupling of the substrate with the adsorbate layers in the CE Hamiltonian, the $T=0\phantom{\rule{4pt}{0ex}}\mathrm{K}$ stability diagram of C/Pt${}_{25}$Rh${}_{75}$$(100)$, and our prediction of the temperature-dependent segregation profile for different C contaminations.

Effects of solidification rate on segregation and fluid flow tendency in mushy zone of directionally solidified superalloy Inconel 718

The microstructure and composition of the interdendritic liquid along the mushy zone of superalloy Inconel 718 that was directionally solidified at various solidification rates between 2 and 100 μm s−1 have been investigated by SEM and EDAX techniques. The interdendritic liquid segregation profiles along the mushy zone are presented. The liquid density difference and Rayleigh number in the interdendritic liquid were calculated and analysed as well. It was found that when the solidification rates increased in the range 10–70 μm s−1, segregation of Nb decreased, but segregation of Mo was most serious at 20 μm s−1. The liquid density difference increased the most for rates from 20 to 40 μm s−1 as temperature decreased. The maximum relative Rayleigh number was highest at 10°C below the liquidus temperature at 20 μm s−1, which indicated the conditions where fluid flow most easily occurred for Inconel 718. The relative Rayleigh number synthetically considers the factors affecting fluid flow and can give a reasonable prediction for fluid flow tendency.

Anomalies in segregation kinetics of solutes under tensile stress in heat resistant steel

In heat resistant steels, tensile stress accelerates the decomposition of preexisting smaller carbides and the growth of larger carbides. Consequently, the dissolved carbon is enriched in the matrix during the decomposition. Under the tensile stress, the massive diffusion of the enriched carbon results in more active segregation to the grain boundary/carbide interfaces for the growth of the larger carbides than the carbide-free grain boundaries. The segregation kinetics of carbon forms a convex profile during the growth process of carbides. A concave segregation profile of chromium corresponds to the convex profile of carbon. This is attributed to the diffusivity of chromium much lower than that of carbon which makes the chromium as the rate-controlling factor for the carbide growth. Due to the repulsive segregation behavior between carbon and phosphorus, the concave segregation profile of phosphorus results from the convex segregation profile of carbon formed during the active carbide growth. A convex segregation profile of sulfur is also formed owing to the formation reaction of MnS particles, but it is independent of the segregation behaviors of the other solutes.

Boundary stresses due to impacts from dry granular flows

Field data and laboratory experiments suggest that bedrock wear from debris flows is largely due to particle–bed impacts, rather than solely due to abrasion by sliding, and that the associated bedrock erosion rates are dependent on the particle size distribution in the debris flow. Here we use Discrete Element Method (DEM) simulations with an established contact mechanics model to explore grain‐size influences on contact forces associated with particle–bed impacts in sheared granular mixtures. We first compare DEM simulations with experimental observations obtained from shallow granular flows in rotating drums of diameters 0.56 m and 4.0 m. Our simulations reproduce, without parameter tuning, experimentally measured segregation, boundary pressures, and height profiles. We perform additional simulations systematically varying particle size distributions in binary mixtures. We show that local time‐averaged boundary pressures in thin flows are essentially the normal component of the weight of the flow, independent of particle size distribution. However, other statistical measures of boundary forces scale with mass‐averaged particle size. We demonstrate that this is because individual particle–bed impacts, rather than impacts from multiple particle collisions, dominate the largest contact forces. We show that these largest impact forces vary as the square of grain size and the 1.2 power of impact velocity as predicted from the contact mechanics model underlying the DEM. These results support the particle size dependence of a recently proposed bedrock incision model and suggest that next steps for a predictive bedrock incision model require the statistics of the largest impact velocities.