Negative Segregation Research Articles

On the Alternate Stirring Mode of F-EMS for Bloom Continuous Castings

Local solute transportation behaviors under different alternate stirring parameters of final electromagnetic stirring (F-EMS) and their influences on the internal quality of the as-cast bloom are compared and evaluated based on a developed coupled model of electromagnetism, heat, and solute transport. To this end, plant trials were conducted in Shaoguan Steel, China. Under the action of F-EMS, a negative segregation band in an ellipse shape is observed at the central area of strand cross section, where the minimum carbon segregation degree is decreased from 0.98 to 0.84 as the stirring duration increases from 15 to 35 seconds in the alternate stirring mode, while it is reduced to 0.805 in the continuous stirring mode. The white band and shrinkage cavity are simultaneously observed at strand center under the conditions of continuous stirring mode, and alternate stirring mode with a stirring period of 35 seconds because of the local over-sustaining melt rotation. In contrast, the V-shape porosity belt width and strand center segregation fluctuation range increase from 60 to 90 mm and from 0.12 to 0.30, respectively, as the stirring duration is reduced from 25 to 15 seconds in the alternate stirring mode because of the poor mixing of the local melt at the strand center.

Numerical simulation for macrosegregation in direct-chill casting of 2024 aluminum alloy with an extended continuum mixture model

Abstract An extended continuum mixture model for macrosegregation is applied to predicting Cu and Mg segregation in large-size ingot of 2024 aluminum alloy during direct chill casting (DC). A microsegregation model using the approximate phase diagram data was coupled with macroscopic transport equations for macrosegregation profiles. Then, the impacts of transport mechanisms on the formation of macrosegregation were discussed. It is found that copper and magnesium have a similar segregation configuration from the billet center to surface. Negative segregation is observed in the centerline and subsurface, whereas positive segregation is obtained in the surface and somewhat underestimated positive segregation in the middle radius. Further, the discrepancy between the predicted and experimental results was discussed in detail. The results show that the magnesium to some extent alleviates the copper segregation in ternary alloy, compared with that in binary alloy. The predicted results show good agreement with measured experimental data obtained from literatures.

Precipitation mechanism of Mg2Ni in Mg-Ni-Y studied by STEM, 3DAP and first-principles calculations

The precipitation mechanism of nano-sized and micro-sized Mg2Ni particles during hydrogen-induced decomposition of 18R-type long period stacking ordered phase is investigated. Nano-sized Mg2Ni particles adhere to YH2 particles inside grains, while micro-sized Mg2Ni particles form at grain boundaries due to the segregation of Ni atoms. The interface energies are predicted to be 1.108–1.263 J/m2 for Mg/Mg2Ni, 1.389 J/m2 for YH2/Mg2Ni, and 1.536 J/m2 for YH2/Mg, indicating that the adhesion of nano-sized Mg2Ni to YH2 reduces the total energy of the system. The negative segregation energy of −0.194 eV proves that the segregation of Ni atoms is energetically favorable.

Aptamers as biomarkers for neurological disorders. Proof of concept in transgenic mice.

The act of selecting aptamers against blood serum leads to deep libraries of oligonucleotide sequences that bind to a range of epitopes in blood. In this study we developed an enriched aptamer library by performing positive selection against a pool of blood serum samples from transgenic mice (P301S) carrying the human tau gene and counter selecting against pooled blood serum from negative segregant (wild type) mice. We demonstrated that a large proportion of the aptamer sequences observed with next generation sequence (NGS) analysis were the same from selection round 5 and selection round 6. As a second step, we applied aliquots of the selection round 5 enriched library to blood serum from 16 individual mice for a single round of selection. Each of these individual libraries were characterized through NGS analysis and the changes in relative frequency in aptamers from transgenic mice versus wild type were used to construct a diagnostic fingerprint of the effect of the action of the transgene on the composition of blood serum. This study serves as a model for similar applications with human subjects.

Macrosegregation behavior of solute Cu in the solidifying Al-Cu alloys in super-gravity field

In this research, super gravity field was introduced to investigate the macrosegregation behavior of solute Cu in Al-Cu alloys in super gravity field systematically. And the macrosegregation mechanism was also explored by well-designed experiments. When Al-Cu alloys were solidified in super gravity field, the macrosegregation of solute Cu was generated and the solute Cu increases along the direction of super gravity field. The macrosegregation becomes severer with the increasing gravity coefficient and the solute content. When the Al-4.5wt%Cu alloy was solidified in super gravity field of G = 800, the copper content at the bottom position increases up to 8.48 wt% and that at the up position decreases to only 2.58 wt%, resulting in the positive segregation at the bottom and the negative segregation at the top of the sample. The segregation mechanisms are that solute-rich regions, which have a larger density than the main liquid, sediment toward the bottom of the sample under the effect of super gravity, and at the final solidification stage, super gravity can drive the residual solute-rich liquid to flow toward the bottom of the sample along the dendrite space (channel), which formed the super gravity channel segregation.

Effect of subsurface negative segregation induced by M-EMS on componential homogeneity for bloom continuous casting

The subsurface negative segregation behavior and its effect on componential homogeneity of as-cast bloom within a section size of 320 mm × 425 mm under different stirrer currents were investigated with the aid of numerical simulation. To this end, plant trials were conducted based on a curved bloom caster in Shaoguan steel. The results show that the minimum subsurface negative segregation degree is decreased from 0.977 to 0.858 and its corresponding location moves from 16 mm to 11.9 mm distance from strand surface as the stirrer current is increased from 100 A to 600 A. The carbon content fluctuation range of rolled product cross-section first reduces and then increases because of the integrated effects of two factors: melt temperature at the mold exit center decreases (that is, undercooling is increased), whereas the local segregation degree increases with the increase in the stirrer current. As compared to the normal M-EMS parameters (600 A/2.5 Hz), the fluctuation range of carbon content at the cross-section of rolled product is reduced from 0.038 to 0.012 wt.% under the optimized M-EMS parameters of 200 A/2.5 Hz.

Incorporation of fragmentation into a volume average solidification model

In this study, a volume average solidification model was extended to consider fragmentation as a source of equiaxed crystals during mixed columnar-equiaxed solidification. The formulation suggested for fragmentation is based on two hypotheses: the solute-driven remelting is the dominant mechanism; and the transport of solute-enriched melt through an interdendritic flow in the columnar growth direction is favorable for solute-driven remelting and is the necessary condition for fragment transportation. Furthermore, a test case with Sn-10 wt%Pb melt solidifying vertically downward in a 2D domain (50 × 60 mm2) was calculated to demonstrate the model’s features. Solidification started from the top boundary, and a columnar structure developed initially with its tip growing downward. Furthermore, thermo-solutal convection led to fragmentation in the mushy zone near the columnar tip front. The fragments transported out of the columnar region continued to grow and sink, and finally settled down and piled up in the bottom domain. The growing columnar structure from the top and pile-up of equiaxed crystals from the bottom finally led to a mixed columnar-equiaxed structure, in turn leading to a columnar-to-equiaxed transition (CET). A special macrosegregation pattern was also predicted, in which negative segregation occurred in both columnar and equiaxed regions and a relatively strong positive segregation occurred in the middle domain near the CET line. A parameter study was performed to verify the model capability, and the uncertainty of the model assumption and parameter was discussed.

Determination of gaseous fission product behavior near the cerium dioxide [formula omitted] tilt grain boundary via first-principles study

Grain boundaries (GBs) are the most abundant structural defects in nanostructured nuclear fuels and play an important role in determining fission product behavior, which further affects the performance of nuclear fuels. In this work, cerium dioxide (CeO2) is used as a surrogate material for mixed oxide fuels to understand gaseous fission product behavior, specifically Xe. First-principles calculations are employed to comprehensively study the behavior of Xe and trap sites for Xe near the Σ3 (111)/[11¯0] grain boundary in CeO2, which will provide guidance on overall trends for Xe stability and diffusion at grain boundaries vs in the bulk. Significant segregation behavior of trap sites, regardless of charge states, is observed near the GB. This is mainly ascribed to the local atomic structure near the GB, which results in weaker bond strength and more negative segregation energies. For Xe, however, the segregation profile near the GB is different. Our calculations show that, as the size of trap sites increases, the segregation propensity of Xe is reduced. In addition, under hyper-stoichiometric conditions, the solubility of Xe trapped at the GB is significantly higher than that in the bulk, suggesting higher Xe concentration than that in the bulk. The results of this work demonstrate that the diffusion mechanism of Xe in CeO2 is comparable to that in UO2. The diffusion activation energies of Xe atoms in the Σ3 GB are lower than that in the bulk CeO2. These results suggest that the diffusivity of Xe atoms is higher along the GB than that in the bulk, which enhances the aggregation of Xe atoms near the GB.

Peculiarity of Grain Boundary Diffusion of Fe and Co in Cu

In this article new experimental evidences of anomalous grain boundary diffusion (GBD) of Fe and Co in Cu were describe. To demonstrate that the brief describing of results of grain boundary diffusion in Cu with following formulation of rules which can be established on the base of the analysis of the results is presented. Experimental results which are described here concern the attempt to change the effect negative segregation by microalloying by sulfur which did not change the situation and the diffusion through foil which allowed to demonstrate the absence of accelerated GB diffusion without specific sample preparation. It is shown that GBs do not give the additional effect to the flux of Fe and Co through the foil. The extended model of surface tension gradient as an additional driving force is described.

Mechanism of Macrosegregation Formation in Continuous Casting Slab: A Numerical Simulation Study

Solidified shell bulging is supposed to be the main reason for slab center segregation, while the influence of thermal shrinkage rarely has been considered. In this article, a thermal shrinkage model coupled with the multiphase solidification model is developed to investigate the effect of the thermal shrinkage, solidification shrinkage, grain sedimentation, and thermal flow on solute transport in the continuous casting slab. In this model, the initial equiaxed grains contract freely with the temperature decrease, while the coherent equiaxed grains and columnar phase move directionally toward the slab surface. The results demonstrate that the center positive segregation accompanied by negative segregation in the periphery zone is mainly caused by thermal shrinkage. During the solidification process, liquid phase first transports toward the slab surface to compensate for thermal shrinkage, which is similar to the case considering solidification shrinkage, and then it moves opposite to the slab center near the solidification end. It is attributed to the sharp decrease of center temperature and the intensive contract of solid phase, which cause the enriched liquid to be squeezed out. With the effect of grain sedimentation and thermal flow, the negative segregation at the external arc side (zone A1) and the positive segregation near the columnar-to-equiaxed transition at the inner arc side (position B1) come into being. Besides, it is found that the grain sedimentation and thermal flow only influence solute transport before equiaxed grains impinge with each other, while the solidification and thermal shrinkage still affect solute redistribution in the later stage.

Modeling the final sintering stage of doped ceramics: mutual interaction between grain growth and densification

Applying the thermodynamic extremal principle, a model for grain growth and densification in the final stage of sintering of doped ceramics was derived, with segregation-dependent interfacial energies and mobilities (or diffusivities). The model demonstrated an interdependence between the driving forces of grain growth and densification during sintering evolution, observed because the surface energy contributes positively to the driving force of grain growth while the GB energy negatively to the driving force of densification. The model was tested in alumina as a host system, and calculations demonstrate that dopants with more negative GB (or surface) segregation enthalpy or which cause lower GB diffusion coefficient can induce higher relative densities at a given grain size. Comparatively studying yttria- and lanthana-doped alumina, the lanthana doping showed significantly enhanced sintering attributed to the larger La3+ radius causing a more negative GB segregation energy. This present model is expected to help dopant designing to improve control over sintering.

Modelling of ingot size effects on macrosegregation in steel castings

A mixed columnar dendritic-equiaxed three-phase solidification model that considers the secondary dendrite arm spacing (SDAS), nucleation and growth of equiaxed crystals, sedimentation of equiaxed crystals, growth of columnar dendritic trunks, and columnar-to-equiaxed transition (CET) was employed to study the formation of macrosegregation in steel ingots of different sizes. A reported experimental case (55,000kg ingot) was used to verify the rationality of the current three-phase model. The predicted segregation patterns were in good agreement with the experimental results. The formation of macrosegregation in a series of steel ingots with different sizes (50kg, 400kg, 3300kg, 6000kg, 25,000kg, 55,000kg, and 97,000kg) was studied using this solidification model. The results showed that the severity of macrosegregation increased with the increasing ingot weight. The thermal-solutal buoyancy and sedimentation of the dendritic equiaxed grains played dominated roles in determining the final segregation patterns in the different ingots. Additionally, the maximum negative segregation increased until the ingot weight exceeded 25,000kg, which was caused by limited packing of the dendritic equiaxed grains.

Segregation behavior of nickel-based superalloy after electron beam smelting

The segregation behavior of alloy elements in nicked-based superalloy ingots during electron beam smelting was investigated. Three series of experiments were carried out with different parameters. The micromorphology of the cross section of the ingot and the distribution of alloy elements in the ingot were analyzed. The results show that the melt solidified at the bottom and formed a region of equiaxed grain at the beginning of solidification; the columnar crystal is formed in the middle of ingot with the proceeding of solidification; the region with large grains is formed at the end of the solidification. The segregation of alloy elements is weak during solidification after the nickel-based superalloy is smelted for 10 min with a power of 9 kW. That is, Ni, Ti, Mo and Al show positive segregation with effective segregation coefficient value of 0.994, 0.982, 0.995 and 0.884, respectively; Cr, Co and W show negative segregation with effective segregation coefficient value of 1.051, 1.005 and 1.013, respectively. With the increasing of smelting power and time, the segregation degree of Ni, Cr, Ti, W and Mo is increased, the segregation degree of Al is decreased and the segregation degree of Co shows little change in the ingot.

Effects of heavy reduction technology on internal quality of continuous casting bloom

ABSTRACTHeavy reduction technology of the bloom is realised without modifying the hydraulic equipment if a crown roll is used. Experimental results of GCr15 showed that the ratio of reduction amount to broadening amount increased from 2.0 (flat roll reduction) to 4.4 (crown roll reduction), which indicated that reduction efficiency is significantly improved. It is evident that the enriched molten steel in the mushy zone will be squeezed out if the crown roll heavy reduction technology is adopted. Thus, centre segregation and V segregation is suppressed or even eliminated. The average centre carbon segregation index is reduced from 1.74 to 1.06, and negative segregation may occur. Meanwhile, the centre shrinkage cavity is clearly decreased and the centre porosity grade is remarkably reduced to 1.0. The calculation results of elastic–plastic FEM model showed that heavy reduction at the solidification end is very beneficial to improve the centre shrinkage cavity.

First-principles study of phosphorus embrittlement in austenitic steels with κ-carbide precipitates

Ab initio density functional theory calculations were performed to study the effect of phosphorus on the structural, electronic, and magnetic properties of bulk, surface, and interface of κ-carbide (Fe3AlC) and fcc Fe. The aim was to shed light on the role of phosphorus in austenitic steels with κ-carbide particles, where its presence promotes both intergranular and transgranular embrittlement. The embrittling potency energy calculated as a difference between phosphorus binding energies at the Fe3AlC or fcc Fe surfaces and at Fe3AlC/Fe interface, indicates that phosphorus promotes embrittling behavior. Phosphorus at the interface has negative segregation energy which strongly depends on the phosphorus location as well as its concentration. The calculated cleavage energy decreases sharply for κ-carbide with substituted phosphorus as well as for the interface with segregated phosphorus; both contribute to the embrittlement. These results elucidate the microscopic reasons why phosphorus favors crack nucleation in κ-carbide, and why a crack propagates along the boundary between κ-carbide and austenite.

Gravity influence on solidification and segregation of CoCrFeNiCu/Y high entropy alloy

The CoCrFeNiCu/Y high entropy alloys (HEAs) are melt and solidified at a temperature gradient of 15 K/mm with growth velocities between 0.1 and 1 mm/sunder the solidification direction of along and opposite to the gravity field. The two typical solidification conditions can cause the chemical-composition difference in the light element, Y, at the solidification interface. The calculation results of segregation of alloy elements show that Co, Cr, and Fe are negative segregation element while Ni, and Y are positive segregation element in the CoCrFeNiY HEA. The results are analyzed from the view-point of the gravity-induced changes of element-partition coefficients during the directional solidification of the CoCrFeNiCu/Y HEA.

Effect of Macrosegregation on the Microstructure and Mechanical Properties of a Pressure-Vessel Steel

Macrosegregation refers to the chemical segregation, which occurs quite commonly in the large forgings such as nuclear reactor pressure vessel. This work assesses the effect of macrosegregation and homogenization treatment on the mechanical properties of a pressure-vessel steel (SA508 Gr.3). It was found that the primary reason for the inhomogeneity of the microstructure was the segregation of Mn, Mo, and Ni. Martensite, and coarse upper bainite with M-A (martensite-austenite) islands have been obtained, respectively, in the positive and negative segregation zone during a simulated quenching process. During tempering, the carbon-rich M-A islands decomposed into a mixture of ferrite and numerous carbides which deteriorated the toughness of the material. The segregation has been substantially minimized by a homogenizing treatment. The results indicate that the material homogenized has a higher impact toughness than the material with segregation, due to the reduction in M-A island in the negative segregation zone. It can be concluded that the microstructure and mechanical properties have been improved remarkably by means of homogenization treatment.

Effects of alloying elements X (X=Zr, V, Cr, Mn, Mo, W, Nb, Y) on ferrite/TiC heterogeneous nucleation interface: first-principles study

The segregation behavior of alloying elements X (X=Zr, V, Cr, Mn, Mo, W, Nb, Y) on the ferrite (100)/TiC(100) interface has been investigated using first principles method, and the work of separation and interface energy of ferrite/TiC interfaces alloyed by these elements were also analyzed. The results indicated that all these alloying additives except Y were thermodynamically favorable because of the negative segregation energy, showing that they have the tendency to segregate to the ferrite/TiC interface. When the Fe atom in the ferrite/TiC interface is replaced by Y, Zr, or Nb, the adhesive strength of the interface will be weakened due to the lower separation work, larger interfacial energy, and weaker electron effects. However, the introduction of Cr, Mo, W, Mn and V will improve the stability of the ferrite/TiC interface through strong interaction between these elements and C, and Cr-doped interface is the most stable structure. Therefore, the Cr, Mo, W, Mn and V in ferrite side of the interface can effectively promote ferrite heterogeneous nucleation on TiC surface to form fine ferrite grain.

Effects of EMS Induced Flow on Solidification and Solute Transport in Bloom Mold

The flow, temperature, solidification, and solute concentration field in a continuous casting bloom mold were solved simultaneously by a multiphysics numerical model by considering the effect of in-mold electromagnetic stirring (M-EMS). The mold metallurgical differences between cases with and without EMS are discussed first, and then the solute transport model verified. Moreover, the effects of EMS current intensity on the metallurgical behavior in the bloom mold were also investigated. The simulated solute distributions were basically consistent with the test results. The simulations showed that M-EMS can apparently homogenize the initial solidified shell, liquid steel temperature, and solute element in the EMS effective zone. Meanwhile, the impingement effect of jet flow and molten steel superheat can be reduced, and the degree of negative segregation in the solidified shell at the mold corner alleviated from 0.74 to 0.78. However, the level fluctuation and segregation degree in the shell around the center of the wide and narrow sides were aggravated from 4.5 mm to 6.2 mm and from 0.84 to 0.738, respectively. With the rise of current intensity the bloom surface temperature, level fluctuation, stirring intensity, uniformity of molten steel temperature, and solute distribution also increased, while the growth velocity of the solidifying shell in the EMS effective zone declined and the solute mass fraction at the center of the computational outlet (z = 1.5 m) decreased. M-EMS with a current intensity of 600 A is more suitable for big bloom castings.

Investigation of the Solidification Behavior of NH4Cl Aqueous Solution Based on a Volume-Averaged Method

Based on solidification theory and a volume-averaged multiphase solidification model, the solidification process of NH4Cl-70 pct H2O was numerically simulated and experimentally verified. Although researchers have investigated the solidification process of NH4Cl-70 pct H2O, most existing studies have been focused on analysis of a single phenomenon, such as the formation of channel segregation, convection types, and the formation of grains. Based on prior studies, by combining numerical simulation and experimental investigation, all phenomena of the entire computational domain of the solidification process of an NH4Cl aqueous solution were comprehensively investigated for the first time in this study. In particular, the sedimentation of equiaxed grains in the ingot and the induced convection were reproduced. In addition, the formation mechanism of segregation was studied in depth. The calculation demonstrated that the equiaxed grains settled from the wall of the mold and gradually aggregated at the bottom of the mold; when the volume fraction reached a critical value, the columnar grains stopped growing, thus completing the columnar-to-equiaxed transition (CET). Because of solute partitioning, negative segregation occurred at the bottom region of the ingot concentrated with grains, whereas a wide range of positive segregation occurred in the unsolidified, upper part of the ingot. Experimental investigation indicated that the predicted results of the sedimentation of the equiaxed grains in the ingot and the convection types agreed well with the experimental results, thus revealing that the sedimentation of solid phase and convection in the solidification process are the key factors responsible for macrosegregation.