Solute Redistribution Research Articles

Thermal Simulation of MnS Evolution in Solidification of a Low‐Carbon Alloy Steel Bloom

The continuous casting process of a newly developed low‐carbon alloy steel is simulated using a thermal simulation method to investigate the evolution of MnS inclusion. The results indicate that MnS mainly appears as aggregated small‐sized spherical shapes (type I MnS) in the chill zone of the bloom, while in columnar zone, it tends to form strip‐like or irregular morphologies (type II MnS), and no type III MnS is observed. The size of MnS increases from the surface to the center of the samples, with the maximum equivalent diameter increasing from 1.2 to 6.0 μm. A model, considering the effects of solute redistribution and cooling rate, is established to calculate the precipitation and growth of MnS in continuous casting bloom, which is achieved by coupling theoretical calculations with the discrete solidification units. The calculated results demonstrate that sulfur segregation is the controlling factor for the precipitation and growth of MnS, which are well consistent with experiments. This work suggests that enhancing the cooling rate in the secondary cooling zone or improving homogenization of this new low‐carbon alloy steel bloom can effectively reduce the growth rate of MnS and mitigate the formation of type II MnS.

Microscopic Mechanisms of Solute Migration and Redistribution at the Solid-Liquid Interface During Directional Solidification of a Cu–Ag Eutectic Alloy Under High Magnetic Fields

Microscopic Mechanisms of Solute Migration and Redistribution at the Solid-Liquid Interface During Directional Solidification of a Cu–Ag Eutectic Alloy Under High Magnetic Fields

Editorial for Special Issue “Casting Alloy Design and Characterization”

Solidification, the genesis of metallic materials, is a complex phenomenon encompassing fluid flow, heat transfer, phase transformation, liquid–solid interface, solute redistribution, gas trapping, and others [...]

Disclosing differential precipitation behavior of a novel high Mg-containing Al-Mg-Zn-Si alloy

The application of traditional Al-Mg-Zn alloy in aerospace field is limited due to poor corrosion resistance for β-phase at grain boundaries and low strength for the lack of T phase. Here we propose a novel high Mg-containing Al-Mg-Zn-Si alloy with a novel alloying strategy of high Mg content combining with a suitable microalloying Si, obtaining small and densely precipitated nanoscale T’ phase in the matrix, co-existing T’ and η’ phases adjacent to grain boundary and T phase at grain boundary. The formation of GPT’ zone in the matrix is dramatically enhanced due to energetically favorable substitution of Al by Si and furtherly facilitates homogeneous growth of T’ phase by solute redistribution. η’ phase is transformed from T’ phase or evolved from GPIIη’ zone along grain boundary due to feasible thermodynamic condition of Mg-poor and Zn-rich domain by distinct Mg and Zn exsolution on grain boundary. The present alloying design makes it possible that differential precipitation behavior of T’ phase and T’ + η’ phases can simultaneously occur in the matrix and adjacent to grain boundary, the alloy has obviously advantaged overall performance over traditional Al-Zn-Mg-(Cu) alloys, which raises a new perspective for developing new high-performance lightweight aluminum alloys.

Understanding the solute segregation and redistribution behavior in rapidly solidified binary Ti-X alloys fabricated through non-equilibrium laser processing

The solute segregation and redistribution during non-equilibrium rapid solidification using laser additive manufacturing (LAM) process directly influence the microstructure morphology and phase distribution, which in turn affects their mechanical properties. In this work, a laser micro-alloying strategy was utilized to preserve the original solidification microstructure in the considered Ti-9Mo, Ti-9Cr, Ti-9Fe, and Ti-9Ni (wt.%) alloys. The addition of different β-stabilizing elements (Mo, Cr, Fe, and Ni) resulted in distinct microstructures: Ti-9Mo and Ti-9Cr alloys exhibited larger grains (~502 μm and ~733 μm) and cellular morphologies due to minimum constitutional undercooling at the solid-liquid interface. Because of the increased constitutional undercooling, the Ti-9Fe grains are significantly refined (~398 μm), showing a dendritic morphology with elongated primary dendrite arms. Ti-9Ni exhibited the highest constitutional undercooling, forming equiaxed dendrites. However, due to the significant consumption of solute atoms by the interdendritic eutectic phase Ti2Ni, the grains did not further refine (~396 μm). Combined with the temperature field simulation, the solidification conditions of the alloys were determined. In addition, based on the solute partitioning coefficients (k), the different solute redistribution and diffusion behaviors at the solid-liquid interface during the laser micro-alloying process of Ti-9Mo with k > 1 and Ti-9Cr with k < 1 were elucidated, providing essential insights into the formation of typical cellular morphology and enhanced Mo enrichment phenomenon in the Ti-9Mo alloy.

Improvement of carbon segregation in billet by final-EMS, based on segmented continuous casting model

Based on magnetohydrodynamics and solidification theory, a three-dimensional segmented continuous casting model, coupling electromagnetic phenomena, fluid flow, heat transfer and solidification, and solute distribution, was established using COMSOL. Simulation results revealed a positive segregation, with a degree of approximately 1.05, was observed beneath the meniscus, and as the position moved downward, a mild negative segregation, with a degree of approximately 0.97, emerged at a distance of about 10 mm from the wall due to the washing effect on the solidification front. Within the natural convection zone, an asymmetric distribution of carbon segregation between the loose and fixed sides was evident, influenced by the thermal solutal buoyancy. With an increase in the electrical current applied for the final electromagnetic stirring (F-EMS), the Lorentz forces responsible for agitating the molten steel became more pronounced, resulting in a more uniform distribution of carbon. This indicated that the introduction of F-EMS accelerated convection between the molten steel and the mushy zone, facilitating the redistribution of solutes. At an F-EMS current of 250 A–8 Hz, the carbon distribution exhibited the highest uniformity, reducing the degree of segregation at the billet centre from 1.30 to 1.22, a decrease of 0.08. The incorporation of F-EMS effectively addressed centreline segregation issues, and the calculated carbon concentration distribution under both F-EMS and non-F-EMS conditions closely matched experimental measurements.

Austenite formation and solute redistribution during double soaking of a 7Mn-steel

In this work, the influence of secondary soaking parameters on microstructural evolution in medium manganese steels is evaluated with a specific focus on the mechanisms driving austenite formation and solute partitioning at an intermediate secondary soaking temperature of 750 °C. The secondary soaking heat treatment represents the latter step of the double soaking heat treatment where it is used, after an initial intercritical annealing heat treatment, to form additional, solute-lean austenite. Here, data are obtained on a 0.14C-7.17 Mn steel for secondary soaking temperatures between 700 °C and 800 °C, and for times up to 1200 s. Austenite formation during secondary soaking is characterized using dilatometry and XRD, while Mn redistribution is experimentally observed using STEM-EDS. Manganese partitioning during secondary soaking, as well as calculated changes in carbon partitioning, are further explored through a DICTRATM model. Experimental and simulation results suggest that the ferrite-to-austenite transformation during the secondary soaking step may proceed under a partitioning, diffusional transformation at the 750 °C secondary soaking temperature. Manganese partitioning was demonstrated to primarily occur from ferrite to newly formed secondary austenite, while carbon diffusion principally occurred from primary to secondary austenite. Transformation of secondary austenite to martensite upon quenching along with some fraction of primary austenite, whose transformation was attributed to a reduction in C concentration, was shown demonstrated. The results of this study are expected to be of interest to those seeking to design multi-step heat treatments which produce austenite of variable solute concentrations and stabilities.

Observations of solute redistribution obtained using atom probe tomography examinations of bulk Zircaloy neutron irradiated at nominally 410°C

Atom Probe Tomography (APT) examinations were performed on bulk regions of alpha-annealed and beta-treated Zircaloy-2 following irradiation in the Advanced Test Reactor (ATR) to neutron fluences of 2.9–3.1 × 1025 n/m2 (E>1 MeV). Irradiation at higher temperatures of nominally 410 °C resulted in about 10–20% less hardening than observed in the literature for irradiations at 260–326 °C. Clustering of Fe, Sn, and Cr solute into approximately spherical shaped clusters at features consistent with the size and shape of 〈a〉 dislocation loops was observed. In some cases, these clusters were observed to form in layers along the basal plane orientation in the location of 〈a〉 loops. The formation of clusters at the dislocation loops produces the high barrier strength that is needed for a hardening model to more closely correlate with the measured irradiation hardening. Individual clusters not located at loops are an additional hardening barrier. The average size, Number Density (ND), and composition of Fe and Sn clusters were generally similar for alpha-annealed and beta-treated Zircaloy-2 with the exception of differences in Sn cluster ND. Local variation in the type and distribution of clusters is observed that is correlated with local differences in microstructure, precipitates, and local microchemistry.

Multi-scale and multi-physics simulation of central segregation in an equiaxed dendritic mushy zone during continuous casting of steel

Centreline segregation during continuous casting of steel is highly detrimental to the end-product mechanical properties. In this study, a 3D multi-scale and multi-physics model is developed and validated that directly predicts alloy segregation occurring in peritectic steel grades having centreline equiaxed grain morphology. The model consists of four models: (1) a 1D macroscale heat transfer and solidification model, (2) a 3D mesoscale dendritic solidification model, (3) a 3D mesoscale dendritic fluid flow model, and (4) a 3D mesoscale solute transport and redistribution model. The use of a mesoscale domain where the solid and liquid phases are separately discretized allows for consideration of physical phenomena affecting segregation that are classically hidden by the averaging procedures of fully continuum approaches. Thus the new model is able to account for the multiple phenomena occurring during continuous casting while also directly considering the stochastic effects resulting from grains having randomly-placed nuclei as well as interactions between the liquid and solid phases. The results demonstrate that solute partitioning combined with intra-dendritic fluid flow leads eventually to liquid channels enriched with solute that result in centreline segregation. The predicted composition in these discrete liquid channels shows excellent agreement with a Mn centreline segregation profile experimentally-measured via microscopic X-ray fluorescence of a commercially-cast steel slab. Finally, the effects of alloy composition, and soft reduction on the degree of centreline segregation are examined.

Origin of thermal deformation induced crystallization and microstructure formation in additive manufactured FCC, BCC, HCP metals and its alloys

The additive manufacturing (AM) technology has received the widespread attention in the industrial application because of its unique ability to achieve the complex shape and ideal performance. Unfortunately, the impact of the material natural characteristics, such as the crystal structure and alloying, on the microstructure formation at the nanoscale has never been revealed in AM metals. Here, we use large-scale molecular-dynamics simulations to study the rapid thermal deformation processes of additively-manufactured face-centered-cubic (FCC), body-centered-cubic (BCC), hexagonal close-packed (HCP) metals and their alloys (FCC Cu, CuAl0.1, BCC Fe, FeAl0.1, and HCP Mg, MgAl0.1) in the molten pool, in an attempt to elucidate the intrinsic physical mechanisms controlling the crystal nucleation and growth under the complex thermal stress. The results show that the arcuate solid-liquid interface migrates towards the liquid phase until the complete crystallization, in good agreement with the experimental observation. The solidification rate follows a unified slow-to-fast law in the three types of metals, due to the competition between the compressive thermal stress and fast atomic motion. It is noteworthy that the critical radius of the crystal nucleus in the FCC and BCC metals is smaller than that in the HCP metal. The FCC metal exhibits the weak nucleation ability and growth rate, and yet the HCP metal shows the fast growth rate and stable columnar crystal nucleation. Especially, due to the dual regulation of the steep thermal gradient and solute redistribution on the growth driving force, the alloying increases the constitutive supercooling of the solidification front and inhibits the growth of columnar crystals. This trend is particularly prominent in FCC alloys, which is manifested by the obvious amorphous phase and discrete nucleation point in the upper part of the melt pool. The nucleation barrier free energy descends in order of FCC, BCC, and HCP pure metals, but the alloying would increase this energy in the corresponding alloys. The current work gives an insight into the atomic mechanism of the nucleation and growth in AM FCC, BCC, and HCP metals.

A Decision Support System That Considers Risk and Site Specificity in the Assessment of Irrigation Water Quality (IrrigWQ)

Irrigators are increasingly challenged to maintain or even increase production using less water, sometimes of poorer quality, and often from unconventional sources. This paper describes the main features of a newly developed software-based Decision Support System (DSS), with which the fitness for use (FFU) of water for irrigation (IrrigWQ) can be assessed. The assessment considers site-specific factors, several non-traditional water constituents, and the risk of negative effects. The water balance components of a cropping system and the redistribution of solutes within a soil profile are assessed with a simplified soil water balance and chemistry model. User-friendly, colour-coded output highlights the expected effects of water constituents on soil quality, crop yield and quality, and irrigation infrastructure. Because IrrigWQ uses mainly internationally accepted cause–effect relationships to assess the effect of water quality constituents, it is expected to find universal acceptance and application among users. IrrigWQ also caters for calculating so-called Water Quality Requirements (WQRs). WQRs indicate the threshold levels of water quality constituents for irrigation at specified levels of acceptability or risk. WQRs assist water resource managers in setting site-specific maximum threshold levels of water quality constituents that can be tolerated in a water source before impacting negatively on successful irrigation.

Development and Numerical Testing of a Model of Equiaxed Alloy Solidification Using a Phase Field Formulation

A computational framework is developed to understand the transient behavior of isothermal and non-isothermal transformation between liquid and solid phases in a binary alloy using a phase-field method. The non-isothermal condition was achieved by applying a thermal gradient along the computational domain. The bulk solid and liquid phases were treated as regular solutions, along with introducing an order parameter (phase field) as a function of space and time to describe the interfacial region between the two phases. An antitrapping flux term was integrated into the present phase-field model to mitigate the amount of solute trapping, which is characterized by the non-equilibrium partitioning of the solute. The governing equations for the phase field and the solute composition were solved by the cell-centered finite volume method using the open-source computational tool OpenFOAM. Simulations were carried out for the evolution of equiaxed dendrites inside an undercooled melt of a binary alloy, considering the effect of various computational parameters such as interface thickness, strength of crystal anisotropy, stochastic noise amplitude, and initial orientation. The simulated results show that the solidification morphology is sensitive to the magnitude of anisotropy as well as the amplitude of noise. A strong influence of interface thickness on the growth morphology and solute redistribution during solidification was observed. Incorporating antitrapping flux resulted in the solute partitioning close to the equilibrium value. Simulations show that the grain shape is unaffected by changes to crystallographic orientation with respect to the Cartesian computational grid. Thermal gradients exerted discernible effects on the solute distribution and the dendritic growth pattern. Starting with multiple nucleation events the model predicted realistic polycrystalline solidification and as-solidified microstructure.

An interpolation-based transient solidification conditions model for numerical calculation of grain growth during laser welding

The weld microstructure is highly related to the performance of welded joints during the laser welding. The temperature gradient and pulling velocity are critical solidification conditions which play a major role in determining the grain evolution in the weld formation. Therefore, an interpolation-based transient solidification conditions model is proposed in this paper for numerical calculation of the grain growth during laser welding. The transient temperature gradient obtained in the model is compared with that from the previous method and the accuracy of transient temperature gradient is improved. The corresponding phase field (PF) model based on the obtained transient solidification conditions is developed to calculate the evolution process of grain growth. It is found that the morphology and spacing of the primary dendrite arm from the numerical calculation agree well with the experimental results. In the competitive growth stage, some grains are suppressed, which provides more space for neighboring grains to grow coarser. The solute redistribution and insufficient solute diffusion demonstrate significant effects on the solute concentration distribution. The proposed method is of great importance for controlling microstructure and improving the laser welding quality.

Contribution made by double-sized TiC particles addition to the ductility–strength synergy in wire and arc additively manufactured Al–Cu alloys

An Al–Cu matrix composite reinforced with uniformly-distributed micron- and nano-double-sized TiC particles (MNDS-TiCps) was successfully fabricated using a wire and arc additive manufacturing (WAAM) process. The combined effect of the MNDS-TiCps on the precipitation of the θ″ phase, the structural evolution of the grain boundaries, and solidification dynamics of the deposited Al–Cu matrix composites were subsequently investigated. The micron-sized TiC particles in the molten pool generate nucleation undercooling (ΔTnu, ∼0.32 K) which inhibits grain boundary segregation due to constitutional undercooling and promotes the redistribution of the Cu solute in the Al matrix. The phase composition at the grain boundaries changes from θ-Al2Cu to α-Al + θ-Al2Cu and the non-coherent interface between the α+θ transition zone and θ grain boundary is transformed into a coherent interface between the α+θ grain boundary and Al matrix, i.e. (211)θ-Al2Cu//(111)Al. The decrease in free energy within the Al matrix provides energy to facilitate the growth of nuclei on the surface of the nano-sized particles. A semi-coherent interface is thus detected between the TiC and precipitates, characterized by a crystal orientation relationship of (200)TiC//(310)θ″-Al2Cu. The dislocations that provide pipe-diffusion paths for solute Cu constitute a driving force that coarsens the precipitates, promoting the precipitation of θ″ precipitates. Multi-site co-deformation combined with dislocation increment inhibits stress damage to the micro-interface. The strength and elongation increase by 51.0 % and 118 %, respectively, compared to specimens without TiC. This work provides a novel perspective for tailoring WAAM-deposited Al–Cu alloys by achieving favorable structural evolution in the grain boundaries, inducing the precipitation of precipitates, and yielding outstanding synergy between ductility and strength.

Non-Fickian solute transport in three-dimensional crossed shear rock fractures at different contact ratios

Solute transport in fractured rocks plays a crucial role in many subsurface processes, such as geothermal resource extraction and carbon capture and storage. This study conducted simulations of non-Fickian solute transport at varying contact ratios and voids in crossed shear fractures. Results indicated that increasing the contact ratio of the crossed fractures intensified the “tailing” phenomenon of non-Fickian solute transport, and has a negative relationship with the solute outflow concentration. Additionally, the anisotropy of intersections and the variations in their void structure both influence the redistribution and mixing of fluid and solute, therefore, we propose a method to determine the solute mixing patterns at the intersection. Ultimately, while the volume and magnitude of the separated vortex domain in the flow field can influence the non-Fickian solute transport, the “tailing” phenomenon is more significantly affected by its magnitude.

How the underwater environment affects the melt pool solidification during underwater laser direct metal deposition of HNS steel?

Underwater laser direct metal deposition (UDMD) can be employed to repair and maintain offshore engineering structures. The external factors inherent in the UDMD process could have a significant effect on melt pool solidification, but the corresponding mechanism is poorly understood. In the current research, a series of mathematical models based on the finite element method, lattice Boltzmann method, and cellular automaton were proposed to fill the relative knowledge gap. Results indicated that the enhanced powder carrier gas and elevated ambient pressure in UDMD process can flatten the deposition track, and its aspect ratio increased from the original 3.21 to 5.10. Furthermore, the driving effect of the Marangoni convection on the solute redistribution was calculated, and the nitrogen concentration in the downstream region of the DMD melt pool increased by 16.7 % compared to that in the upstream area. Meanwhile, the dendrite morphology between the upstream and downstream was different. In the UDMD melt pool, the changed melt pool thermodynamics (e.g., reduced peak temperature and elevated thermal gradient) and the external factors reinforced the Marangoni convection. In this case, samples fabricated by UDMD exhibited relatively uniform solute distribution and dendrite morphology, which are expected to possess better comprehensive mechanical properties and corrosion resistance.

Hyperspectral imaging combined with artificial intelligence techniques to explore the drying behavior of natural Lonicerae Japonicae Flos extracts

The process of drying solute-containing droplets can lead to dynamic redistribution of solutes. Tracking morphological changes and obtaining drying kinetics will help optimize the spray drying process, but there are few techniques available for measuring the spatio-temporal concentration of solutes in drying droplets. In this study, hyperspectral imaging was used as a non-invasive method to simultaneously obtain information on the morphology and moisture content changes of droplets, and was applied to investigate the drying process of Lonicerae Japonicae Flos extract. The Faster R-CNN algorithm was employed to locate the target droplet and identify its size through the series of droplet images recorded by a hyperspectrometer. Droplet moisture content prediction model was established using PLS and ANN algorithms, with the ANN showing better prediction accuracy. The hyperspectral imaging combined with artificial intelligence algorithms as a promising method can be used for investigating the drying kinetics of solutions with different drying methods.

Modulating Meltpool Dynamics and Microstructure using Thermoelectric Magnetohydrodynamics in Additive Manufacturing

Meltpool modulation in Selective Laser Remelting Additive Manufacturing via an oscillating magnetic field generates Thermoelectric Magnetohydrodynamics (TEMHD) flow. Numerical predictions show that the resulting microstructure can be significantly altered. A multi-scale numerical model captures the meso-scale melt pool dynamics coupled to microscale solidification showing the microstructure evolution and solute redistribution. The results highlight the complex interaction of the various physical phenomena and also show the method’s potential to disrupt the epitaxial growth defect. The model predictions are supported by preliminary experimental results that demonstrate the dependency of the melt pool depth on magnetic field orientation. The results highlight how a time-dependent field has the potential to provide an independent control mechanism to tailor microstructures.

Role of silicon on formation and growth of intermetallic phases during rapid Fe–Zn alloying reaction

The study of liquid metal embrittlement in Fe–Zn systems is challenging because of the high temperature and vapor pressure of Zn, which hinders in-situ investigations with sufficiently high spatial resolution. This is typically associated with subsecond processing steps and the coexistence of a solid substrate and a liquid Zn phase, which renders direct observations at the microstructural scale difficult. In this study, we comprehensively investigate the reactions occurring during the rapid heating and cooling stages of Fe–Zn systems using synchrotron X-ray diffraction. The phase transformation is analyzed for specimens with different interfacial structures, with focus on changes in the coating microstructure above and below the peritectic temperature (782 °C) of the Fe–Zn system. Advanced high-strength steel (AHSS) variants with 1.5 wt% Si content show a prominent destabilizing effect at the onset of Fe–Zn intermetallic compound formation and a simultaneous deceleration of the liquid Zn depletion rate compared with other AHSS alloys containing 0 wt% Si. Furthermore, the addition of Si suppresses the formation of the Γ phase, which is due to turbulence in the outburst Zn at temperatures above 773 K. Consequently, the fraction of Γ phase compounds with accumulated Zn deceases as the exposure time of the liquid Zn to the ferritic matrix increases owing to the solute redistribution of Si in the liquid Zn during rapid heating. Meanwhile, the absence of silicon in the substrate causes the formation of a ζ phase with a low melting point, which delays the formation of liquid Zn via an increase in the melting point of the Zn layer in the early stage of heating. Additionally, the amount of residual liquid Zn decreases due to the rapid depletion of Zn during the liquid initiation stage via an equilibrium Fe/Zn binary phase transformation. The results of this study provide a deeper understanding of the Fe–Zn intermetallic phase transformation and the sensitivity to liquid metal embrittlement of third-generation AHSSs based on silicon content.

Irradiation damage reduces alloy corrosion rate via oxide space charge compensation effects

Radiation effects in materials often compound and accelerate other detrimental phenomena such as embrittlement, oxidation and creep. However, irradiation can also decrease the oxidation rate, for instance with ZrNb alloys nuclear fuel cladding. In this study, we rationalize this observation on Zr-0.5Nb alloy by introducing a mechanism based on oxide space charge modification, resulting from irradiation enhanced Nb clustering. This mechanism is investigated using a multiscale approach: from the macroscale, to determine post-irradiation oxidation kinetics, to the atomic scale, using in-situ atom probe tomography sample oxidation, to observe elemental solute redistribution across the oxide/metal interface. The mechanism is further supported by high resolution transmission electron microscopy characterization and density functional theory calculations. A point defect model is proposed to account for oxide space charge effects and their changes under irradiation. This integrated, multiscale experimental and modeling approach challenges the current paradigm on irradiation effects and how they can potentially improve materials performance in extreme environments.