Distribution Of Solute Elements Research Articles

Numerical Simulation of Macro-Segregation Phenomena in Transition Blooms with Various Carbon Contents

This paper presents a numerical simulation of the steel grade transition from the ladle nozzle to the solidification end of the bloom. The simulation is based on models encompassing fluid flow, solidification, heat transfer, an electromagnetic field, and solute transport. To validate the accuracy of the steel grade transition model, transition blooms of high-carbon steel are sampled. Subsequently, the model is applied to investigating the steel grade transition between medium-carbon steel and low-carbon steel. The findings indicate that the regions exhibiting significant differences between their molten steel flow velocity and bloom casting speed in the strand model are primarily concentrated within 1 m below the meniscus. Additionally, the mushy zone in the strand model possesses a substantial volume. Solute elements continuously permeate the liquid phase from the solid phase through the mushy zone. Consequently, the distribution of solute elements in the transition bloom is primarily influenced by the molten steel flow in the tundish and macro-segregation in the casting process. The segregation degree of each solute element varies among grades with different carbon contents. In the austenite phase, the segregation degree of each element follows the order C > Si > Mo > Mn > Cr > Ni, while in the ferrite phase, the segregation degree is ordered as C > Si = Mn. Considering macro-segregation, the transition bloom partition model proves to be more stringent than the original partition method. This results in longer transition blooms when a significant difference exists between the new and old grades. For example, in Scheme 1, the original plan transition bloom length is 8.88 m, whereas the new plan transition bloom length is 10.88 m. Similarly, in Scheme 2, the original plan transition bloom length is 34.64 m, and the new plan transition bloom length is 35.16 m. Conversely, shorter partition intervals occur when there is an overlap in the composition of the new and old grades. In Scheme 3, the original plan partition interval for the new and old grades is 4.08 m, while the new plan partition interval is reduced to 0.94 m.

Analysis of macrosegregation during slab continuous casting using 3D-longitudinal 2D hybrid model

ABSTRACT To improve the computational efficiency of simulating the macrosegregation process that occurs during continuous casting of steel slabs, a new hybrid method combining three-dimensional and longitudinal two-dimensional models was developed to consider the turbulent flow, heat transfer, solidification, and solute transport phenomena. The reasonability of the performed computational domain division was determined, and the model accuracy was verified experimentally. Using this approach, the solute distribution characteristics and central segregation during continuous casting were examined in detail. The obtained results revealed that recirculation flows in the turbulent flow region play a critical role in the distribution of solute elements near the slab surface. Owing to the redistribution of solute elements at the solid–liquid interface and the movement of the solid–liquid interface towards the slab centre during continuous casting, the solute elements in the slab centre are continuously enriched, ultimately resulting in central segregation.

Segregation behavior, microstructure and properties of TA2/AA7005 composite plates manufactured by horizontal twin-roll casting under the effect of an electromagnetic oscillation field

Three TA2/AA7005 composite plates were manufactured by horizontal twin-roll casting (HTRC) under various external fields, and the formation mechanism of segregation in the cast-rolling clad process as well as the effect of external fields on the segregation behavior, microstructure and properties of the composite plates were studied. In the non-field (NF) plate, due to the high cooling rate and asymmetric heat transfer in the cast-rolling clad process, a central macro-segregation band and an interface segregation band were formed in the aluminum matrix. After applying the electromagnetic oscillation field (EMOF), the electromagnetic oscillation stirring effect was induced in the melt of the cast-rolling zone. The dendrites grown at the solidification front were broken by a shearing force and re-entered the melt. In addition, the temperature gradient at the solidification front decreased, which inhibited the growth of dendrites and avoided blocking the equilibrium distribution of solute elements due to the “bridging” of dendrite arms. After applying the EMOF, the central and interface segregation bands of the composite plate were nearly eliminated and the microstructure became more homogeneous. As a results, under the action of the EMOF, the tensile strength, yield strength, effective elongation, and interfacial bonding strength of the TA2/AA7005 cast-rolling composite plate were 303.8 MPa, 204.7 MPa, 21.6%, and 25.3 N/mm respectively, which were significantly improved in comparation with the NF plate.

Genetic relationship of microstructure and mechanical properties between ferrite/pearlite steel plate and billet

This paper proposed the double-pass compression thermal/force simulation experiment to in situ analyze the phase transformation of continuous casting billet during the controlled rolling and controlled cooling process. The genetic relationship between the central segregation of the billet and solute element distribution of the steel plate, and also the genetic relationship of microstructure and mechanical properties between them were experimentally studied. The results show that the microstructure and mechanical properties between the billet and corresponding batch of steel plate have the genetic relationship, and the link of the relationship is central segregation. The phase transformation of supercooling austenite will be affected by the central segregation of Mn and C elements, and the granular bainite abnormal segregation band is the transformation product of Mn and C element segregation region.

Characterization of microstructure and microhardness of Neutron irradiated Inconel X-750

In this study, we characterized the microstructure evolution and hardness of Inconel X-750 samples that were irradiated in-reactor at several different dose rates and temperatures and at doses up to 84 dpa. The irradiation induced lattice defects, the stability of γ’ precipitates, and the formation of helium bubbles were studied. Detailed statistics regarding the size and density of those features were obtained. The combined effects from irradiation dose, dose rate, and temperature on the disordering and dissolution of precipitates and the formation of bubbles were discussed. The isolated contributions of those microstructural features to the total material strength were calculated based on the Dispersed Barrier Hardening (DBH) model. The spatial distribution of He bubbles, especially the visible large ones (>2 nm), were also found to either correlate or anti-correlate with the spatial distribution of solute elements, e.g., Ti, Fe, and Cr, at doses higher than 75 dpa.

APT and TEM study of behaviour of alloying elements in neutron-irradiated zirconium-based alloys

In this study, APT and TEM analyses were used to characterise two Zr-based alloys neutron-irradiated to 13.7 dpa. The high spatial and chemical resolution of these techniques has enabled the irradiation-induced nanoscale distribution of solute elements to be characterised. The results on both Zircaloy-2 and low-Sn ZIRLO support previous observations of Fe segregation to planar dislocation arrays with enhanced Sn between. In Zircaloy-2, the APT data in this study have also revealed short-range (∼2 nm) clustering of Fe and Sn atoms both within the Fe-enriched planes and in the regions between dislocation arrays. The resultant microstructure is reminiscent of spinodal decomposition. In low-Sn ZIRLO Nb-rich precipitates formed in the Sn-rich planes. The data in this article enhance our understanding of microstructural evolution in Zr alloys under neutron irradiation and are relevant for predicting changes in the properties of Zr alloys in-service in fission reactors.

Quantitative analysis of the Portevin–Le Chatelier effect by combining digital image correlation and dead-weight-type tensile test

Abstract A quantitative evaluation method of a serration deformation behavior by a dead-weight tensile test combined with a strain measurement by a DIC method is proposed. This method applies to binary solid solution aluminum alloys and a ternary cluster strengthened aluminum alloy. In the dead-weight tensile test, the serrated flow was measured as a stress-strain curve with a stepped-shape divided into two stages of a stress rising phase and a strain burst phase. By adopting this tensile test, the serration deformation behavior can be measured with an extremely reproducibility. It was confirmed that an elastic deformation occurs predominantly in the stress rising phase, and this deformation behavior is discussed in relation to the concentration of solid solution elements. Propagation of PLC bands has occurred in the strain burst phase. An activation volume obtained from the strain rate in the PLC bands was in good agreement with a calculated value based on a distribution interval assuming a regular square distribution of solute elements. For the ternary alloy strengthened by clusters, it is also shown that this method can be applied to the qualitative estimation of the change in the residual amount of the solid solution element in the matrix during natural aging.

Relationship between cooling rate, microstructure evolution, and performance improvement of an Al–Cu alloy prepared using different methods

The relationship between the cooling rate and microstructure evolution and the performance improvement of an Al–4.5 wt% Cu alloy prepared using different casting methods was investigated. The microstructure evolution, solute element distribution, and mechanical properties of specimens prepared using different cooling rates were systematically investigated using multi-scale optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile testing, and hardness testing. Upon increasing the cooling rate from 1.65 to 117.3 K s−1, the average grain size, secondary dendrite arm spacing, and width and area fraction of eutectic phases significantly decreased, and the density of the precipitated phase, super-saturation degree of solute elements, and mechanical properties improved. Through a combination of grain-boundary, solid-solution, and precipitation strengthening, the cooling rate indirectly affected the mechanical properties of the heat-treated alloy sheets. The Al−4.5 wt% Cu alloy prepared by twin-roll casting under a cooling rate of 117.3 K s−1 exhibited a strength between 145.33 and 321.79 MPa and an elongation of 11.65%, demonstrating its potential as a high-performance aluminium alloy.

Segregation of solute elements and their effects on the strength of Al Σ5 (210) [001] symmetrical tilt grain boundary in 2219 alloys

Through the first-principles calculations based on quantum mechanics, systematic study has been made into the segregation of solute elements (Cu/Mn/Fe/Si/Zn/Mg/Zr/V/Ti) and their effects on the Al Σ5 (210) [001] symmetrical tilt grain boundary (STGB) in 2219 alloys, which provides a theoretical basis for optimizing the distribution of solute elements. The calculation results show that Zr and Mg atoms have a spontaneous tendency to segregate in the direction of Al Σ5 (210) [001], while other atoms, Ti atoms in particular, are more likely to stay in the grains. At the same time, the first-principles computational tensile test (FPCTT) shows that the GB segregation of most solute elements is beneficial to improve the bonding strength of the materials. V element can improve the separation energy and enhance the bonding strength to the greatest extent, while Mg and Zn elements would reduce the separation energy and lead to the decrease of the theoretical tensile strength. The analysis results of charge density and density of states show the embrittlement induced by Mg atoms and strengthening effect of V atoms depend very much of two factors: atomic size and charge density. The strengthening effect of V atoms lies in the resonance hybrid occurring in s-electrons and p-electrons. While the embrittlement induced by Mg atoms is due to the combination of GB expansion and charge density depletion. In the same way, we can try to reasonably explain the influence mechanism of other elements on the macroscopic properties of the alloys from the atomic and electronic level.

Partitioning of Solute Elements and Microstructural Changes during Heat-treatment of Cold-rolled High Strength Steel with Composite Microstructure

The partitioning of solute elements during intercritical annealing and the effects of partitioning on ferrite transformation during slow cooling after intercritical annealing in a 0.17% C–1.5% Si–1.7% Mn (mass%) steel were investigated by a new FE-EPMA (field emission electron probe microanalysis) technique. This new technique enables highly accurate measurement of the C distribution. During the intercritical annealing, C and Mn concentrated into austenite, while Si concentrated into ferrite. The distribution of Mn in austenite was inhomogeneous, and austenite with small Mn content was transformed into ferrite during slow cooling. This ferrite transformation proceeded in the NPLE (negligible partitioning local equilibrium) mode. Two kinds of ferrite were produced due to slow cooling, one being intercritically-annealed ferrite, and the other transformed ferrite. The transformed ferrite had larger Mn content than the intercritically-annealed ferrite. Furthermore, the transformed ferrite was classified into the ferrite grown epitaxially from the intercritically-annealed ferrite and that nucleated in the austenite with relatively small Mn content. Prior microstructure and distribution of solute elements before cooling are determined by the intercritical annealing conditions, and then control the ferrite transformation. Precise control of the ferrite transformation is effective for stable production of cold-rolled high strength steel with composite microstructure.

Distribution of Al Element of Ti–6Al–4V Joints by Fiber Laser Welding

In the process of laser welding, the uneven distribution of solute elements caused by element burning loss and flow of molten pool affects the quality of joints. In this paper, butt welding experiments were conducted on the 3 mm thick Ti–6Al–4V specimens with different preset ratios of Al and Si powders by using 4 kW fiber laser. The distribution of Al solute element and its influence on the microstructure and mechanical properties of the final weld joint were investigated. The results showed that the self-diffusion of Al element and the flow of molten pool affects the alloy elements distribution in laser welding. And the microhardness of the welded joint with Ti–6Al–4V and 90% Al + 10% Si powders was significantly higher than that with only Ti–6Al–4V, with the difference of about 130HV. At the same time, in the joint with 90% Al and 10% Si powders, the acicular α’ size was finer, and basketweave microstructure was present as well. This research is helpful to better understand the distribution of Al solute element and its influence on the joint quality during laser welding of Ti–6Al–4V alloy, which provides a certain reference for improving the weld or surface properties of Ti–6Al–4V alloy during laser processing.

Numerical modelling of fluid flow and macrosegregation in a continuous casting slab with asymmetrical bulging and mechanical reduction

The various influence factors of macrosegregation are complex and have been researched widely due to their undesirable effect on continuous casting slab. Based on an Eulerian approach, a multiphase solidification model combining turbulent fluid flow, heat transfer, microstructure evolution, solute transport with back diffusion and shell deformation were developed in this work to investigate the fluid flow and macrosegregation in continuous casting slabs under the effects of shell bulging and mechanical reduction. In this model, five phases of the slab were considered: the liquid phase, inter-dendritic melt phase of equiaxed grains, solid phase of equiaxed grains, inter-dendritic melt phase of columnar dendrites, and solid phase of columnar dendrites. The predicted temperature, shell thickness and solute element distribution were verified by the results of thermal infrared imaging, nail-shooting experiments, macrostructure analysis, and carbon-sulphur analysis. In this model, the asymmetrical bulging between two adjacent supporting rollers was considered, and its effect on the fluid flow and macrosegregation of the slab was investigated. The calculation results show that the positive centreline segregation considering the asymmetrical bulging profiles was more serious than that considering the regular sinusoidal shell profiles. Using this model, the slab macrosegregation was investigated with different reduction mechanisms in the mushy zone; a large reduction applied just before the solidification end could significantly reverse the flow of solute-enriched melt and reduce the macrosegregation. These results were also verified by an industrial application.

Effect of trace zinc on the microstructure and mechanical properties of extruded Mg-Gd-Y-Zr alloy

In this work, the Mg-8.5Gd-2.5Y-0.3Zr (wt. %) and Mg-8.5Gd-2.5Y-0.5Zn-0.3Zr (wt. %) alloys were subjected to indirect extrusion. The microstructure and mechanical properties of the two alloys were carefully investigated to clarify the effect of trace Zn, in terms of solid solution Zn atoms and γ′ phases, on the Mg-Gd-Y-Zr alloy. The results indicate that the addition of trace Zn stimulates the precipitation of γ′ phases and suppresses the grain boundary segregation during preheating treatment conducted before extrusion. During extrusion, the dynamic recrystallization (DRX) occurs via both discontinuous DRX (DDRX) and continuous DRX (CDRX) mechanisms. The addition of Zn impedes the DRX process, resulting in a lower recrystallization fraction and a strong fiber texture in the Zn containing alloy. The dynamic precipitation occurs along with the DRX. Trace Zn modifies the distribution of dynamic precipitates, resulting from the varied distribution of solute elements. Finally, it is found that the addition of trace Zn notably enhances the mechanical strength and stimulates the occurrence of yield point in the as-extruded alloy.

High-efficiency continuous casting of GCr15 bearing steel bloom based on cooperative control technique of complex electromagnetic stirring and soft reduction

GCr15 bearing steel exhibits comparatively serious center macro-segregation in the continuous casting of bloom with the increase of casting speed. In the present work, the influence of complex electromagnetic stirring (M + F-EMS) and mechanical soft reduction (MSR) on the center macro-segregation in the continuous casting of 220 × 260 mm blooms of GCr15 bearing steel have been comparatively investigated to increase casting speed in order to ensure a good internal quality. Based on numerical simulation and experiments, M + F-EMS and MSR have been comprehensively evaluated and compared by combination of industrial trials. The results show that center carbon segregation first decreases and then increases with the increase of casting speed in both processes without optimization. For M + F-EMS process, when casting speed increases from 0.75 to 0.85 m · min−1, the average degree of center carbon segregation decreases from 1.2 ∼ 1.26 to 1.18 ∼ 1.25 by asymmetrical optimization; with regard to combination of M + F-EMS and MSR process, when casting speed increases from 0.75 to 0.9 m · min−1, the average degree of center carbon segregation decreases from 1.2 ∼ 1.26 to 1.08 ∼ 1.1 and the solute element distribution becomes homogeneous by optimization. In comparison, significant reduction of the center macro-segregation with the increase of casting speed can be achieved for combination of M + F-EMS and MSR process, however, it is infeasible for M + F-EMS process in the present technology situation.

Solute Clusters/Enrichment at the Early Stage of Ageing in Mg–Zn–Gd Alloys Studied by Atom Probe Tomography

Three-dimensional distribution of solute elements in an Mg–Zn–Gd alloy during ageing process is quantitatively characterized by three-dimensional atom probe (3DAP) tomography. Based on the radius distribution function, it is found that Zn–Gd solute pairs in Mg matrix appear mainly at two peaks at early stage of ageing, and the separation distance between Zn and Gd atoms could be well rationalized by the first-principle calculation. Moreover, the fraction of Zn–Gd solute pairs increases first and then decreases due to the precipitation of long-period stacking ordered (LPSO) structures. Both the composition of the structural unit in LPSO structure and the solute enrichment around it are quantified. It is found that Zn and Gd elements are synchronized in the LPSO structure, and solute segregation of pure Zn or Gd is not observed at the transformation front of the LPSO structure in this alloy. In addition, the crystallography of transformation front is further determined by 3DAP data.

Detection and Analysis of Fine Particles Acting as the Nucleation Site of Aluminum Grains Using FIB-TEM

The paper describes a technique development using focused ion beam and transmission electron microscopy to find and analyze fine particles which acted as the heterogeneous nucleation substrate for aluminum grains. An aluminum alloy was melted, sheared intensively by a melt conditioning unit, and cast by a laboratory scale horizontal twin roll caster. The intensively sheared alloy showed low segregation of solute elements on the surface as well as at the centerline, probably by enhanced nucleation of aluminum grains from fine oxide particles. In order to verify the possibility of the heterogeneous nucleation of aluminum grains, several techniques using focused ion beam and transmission electron microscopy were tried to find any oxide particles in the grains. Finally, a technique using the lifting-out of a lamella combined with the serial milling by focused ion beam detected a fine particle at the center of one aluminum grain and transmission electron microscopy confirmed the particle as an oxide. The intense nucleation of aluminum grains on fine dispersed oxides in the intensively sheared aluminum alloy can induce uniform distribution of solute elements and consequently minimize the severe centerline and surface segregations.

Composition Homogenization Evolution of Twin-Roll Cast 7075 Aluminum Alloy Using Electromagnetic Field

Abstract Many microstructural defects such as well-developed dendritic crystal and severe segregation have been found in 7075 aluminum alloy strips by continuous twin-roll casters. Morphology and phase composition of 7075 alloy strips were tested in this paper. Electromagnetic field of 0.13 T was applied during a twin-roll casting (TRC) process to study the difference in solute element distribution compared to conventional process. The most soundest method by which the microstructure of slabs was greatly refined and equiaxed, and segregation bands narrowed down efficiently, was an alternating oscillating process with industrial frequency current and frequency of 386 A and 50 Hz, respectively. In addition, t (AlZnMgCu) phase with high density was formed at (sub) grain boundaries, which was refined and homogenized consecutively in accordance with direct chill (DC) electromagnetic field, half-wave oscillating field and alternating oscillating field; moreover, netlike precipitates completely disappeared at oscillating TRC processes.

Microstructural evolution of 7075 Al gas atomized powder and high-pressure cold sprayed deposition

This study investigates the microstructure evolution of as-atomized 7075 aluminum powder during high pressure cold spray deposition. The as-received powder and the deposition were characterized via scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The as-received powder was composed of two different particle types, differentiated by their grain boundary structure and solute element distribution. Significant solute element segregation was observed at grain boundaries for both particle types. The powder also contained nanoscale subgrain and dislocation structures. High pressure cold spray resulted in the formation of a high quality deposition with limited porosity and inter-particle voids. The deposition was characterized by two distinct regions: particle/particle boundaries and particle interiors. Particle/particle boundaries showed a high degree of deformation resulting in the formation of an ultra-fine grain structure and a low density of low angle grain boundaries. The formation of the ultra-fine grain structure is attributed to dynamic recrystallization and recovery processes. Particle interiors were characterized by larger grains containing a high density of low angle grain boundaries and dislocation structure. Large grains in the deposition contained precipitates distributed both within the grain interior and at grain boundaries. Precipitates in small grains were distributed primarily at grain boundaries. The effect of these precipitates on the microstructure evolution was also discussed.

Effect of Rotating Magnetic Field (RMF) on Segregation of Solute Elements in CuNi10Fe1Mn Alloy Hollow Billet

The effect of rotating magnetic field (RMF) on macro/microsegregation of solute elements is investigated experimentally, and a comprehensive three-dimensional mathematical model is built, and the finite element package ANSYS® is employed to calculate the distribution of temperature and liquid fraction along the radial direction in horizontal continuous casting of CuNi10Fe1Mn alloy hollow billet. The results show that RMF makes the temperature field and liquid fraction uniform. The original inhomogeneous columnar grain macrostructure turns into homogeneous equiaxed grain structure, and the macro/microsegregation of Ni, Fe, and Mn elements are restrained effectively with the application of RMF. Moreover, the action mechanism of RMF is discussed to explain its effect on improving the distribution of solute elements.

Characteristics and Formation Mechanism of Segregation During the Solidification of Aluminum Alloy with Ultrasonic Radiation

An industrial experiment was carried out to study the characteristics and formation mechanism of segregation in semi-continuous direct-chill (DC) casting ofΦ300mmaluminum alloybillet with ultrasonic radiation of different powers. Results show that both microsegregation and macrosegregation of solute elements Zn, Mg, Cu are improved remarkably when ultrasonic vibration is applied to A1-Zn-Mg-Cu alloy. Meanwhile, the higher power of ultrasonic vibration is, the more homogeneous distribution of solute elements becomes, and then the more effective improvement of segregation is achieved. With a 240W ultrasonic treatment (UST), the intra-grain solution degree of elements Zn, Mg and Cu increases by 35.8%, 41.8% and 40.2% respectively while the macrosegregation ratio decreases by 10.3%, 7.4%, 6.7%. Physical and chemical effects caused by ultrasonic field exert a positive impact on the solute elements diffusion, grain refinement, the depth of sump and solution degree et al, which contribute to the improvements of segregation.