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Valence electron concentration (VEC) analysis of high-entropy alloys (HEAs) has been extended to a scheme using a two-parameter plot by introducing the period of the periodic table as the second variable. The extension aimed to include HEAs with a hexagonal close-packed (hcp) structure comprehensively in the framework of VEC analysis, along with other HEAs with body- and face-centered cubic (bcc and fcc) structures. A statistical analysis using a VEC–period chart was performed for 261 single-phase exact- and near-equiatomic HEAs with bcc, hcp, or fcc structure acquired from the literature. Supplemental experiments were conducted in the present study for an hcp-Co35Cr20Fe15Ru20V10 alloy (Ru20 alloy) predicted by Thermo-Calc software and the TCHEA6 database. The experiments revealed that the water-quenched Ru20 alloy annealed at 1600 K for 1 h exhibited a single hcp structure. The formation of the hcp-Ru20 HEA was attributed to the inclusion of a minimum amount of 20 at%Ru, as an hcp forming element, from a 4d-transition metal (TM). The Ru20 alloy with (VEC, Period) = (7.65, 3.2) was located immediately below the conventional fcc-HEAs plotted along the segment of (VEC, Period) = (8–9.5, 3). The VEC-period chart revealed a strict boundary of the hcp/fcc regions in the VEC–period chart between the Ru20 alloy and fcc-HEAs. It is practically difficult to prepare hcp-HEAs comprising 3d-TM only through conventional solidification from a melt and subsequent annealing, as required. [ABSTRACT FROM AUTHOR]

The utilization of the laser powder bed fusion (L-PBF) method in additive manufacturing experiments has revealed the intriguing potential to induce morphological evolutions within the solidification microstructure by manipulating the thermal conditions. Transitions of this nature can have a substantial impact on the ultimate texture and material properties of the product. This study utilizes numerical investigations to examine the microstructure evolution of the AlSi10Mg alloy, which is fabricated using the L-PBF process, under different thermal conditions. In order to investigate the changes in the microstructure, we employ a parameter phase-field (PF) model. This model effectively replicates the natural development of nuclei from inoculant particles and accurately simulates the transitions in the morphology. The PF model's validity is determined through the numerical investigation of morphological transitions during directional solidification of the AlSi10Mg alloy. The model's predictions are then compared to the analytical Hunt model. The formation of a columnar microstructure with a significantly reduced secondary dendrite arm spacing is observed when the ratio of the temperature gradient (G) to the solidification rate (R) is increased. [ABSTRACT FROM AUTHOR]

A comparative analysis of brasses alloys, namely lead-free CuZn (CB771) and lead containing CuZn (CB770), was conducted in this article. The results of the comparative analysis and experimental investigations aimed to provide comprehensive knowledge about the thermophysical properties and solidification characteristics of these alloys. Thermodynamic simulations using Thermo-Calc software and modifications in the chemical composition of the CB771 alloy were employed to approximate its characteristics to those of the lead containing CuZn alloy. Thermal-derivative analysis of the alloys and a technological trial were carried out to determine their solidification characteristics, fluidity, and reproducibility. The casting trials were conducted under identical conditions, and the results were compared for a comprehensive analysis. Additionally, a solidification process simulation was performed using MagmaSoft software to match the thermophysical properties. The aim of this research was to achieve maximum consistency between the simulation results and experimental investigations. [ABSTRACT FROM AUTHOR]

We report on the incidence of cellular breakdown in silicon wafers implanted with indium and pulsed laser melted at varying laser fluence. We predict from heat flow calculations that as laser fluence is decreased, (a) the melt depth decreases, (b) the solidification velocity increases, and (c) the solidification front should be more stable with respect to cellular breakdown (based on the binary alloy solidification theory), which should result in observed incidence of cellular breakdown at a relatively shallower depth below the surface. However, secondary ion mass spectrometry (SIMS) data show that cellular breakdown at lower fluence is observed deeper, indicating that the interface became unstable earlier in its solidification. The SIMS data show significant In evaporation, with greater evaporation occurring as fluence increases. Calculations show that surface evaporation can reduce the bulk liquid concentration during solidification, reducing the degree of instability. The longer melt durations after irradiation at higher fluence give more time for evaporation, which may result in a relatively larger delay in the onset of breakdown compared to lower fluence by suppressing the bulk liquid concentration and suppressing the growth of unstable perturbation. Under certain conditions, this effect could dominate over the role of reducing the interface speed, which would tend to accelerate amplification of a perturbation, which leads to breakdown. Calculations qualitatively support this interpretation. [ABSTRACT FROM AUTHOR]

A novel ternary eutectic Al-27%Cu-5%Si cast alloy was investigated to understand its solidification behavior and microstructural evolution. The studied alloy was cast, remelted and subjected to a rapid cooling in ice-water mixture, and also cooled slowly in air. The solidification sequence of the alloy was studied using Thermo-CalC software with a subsequent DSC measurement. Micro- and nano-hardness tests were conducted at room temperature to assess the change of hardness associated with the microstructural changes due to the different cooling paths the alloy was subjected to. Thermodynamic analysis showed that the equilibrium solidification path of the eutectic alloy consists of three stages: precipitation of Al 2 Cu primary phase at 527.6 °C, evolution of binary eutectic (α-Al + Al 2 Cu) at 525.9 °C and the formation of ternary eutectic (α-Al + Al 2 Cu + Si) at 521.7 °C. DSC analysis of the alloy suggests only one endothermic peak at 536.4 ± 0.2 °C due to the melting of the alloy, which also corresponds to the ternary eutectic transition. The quenched alloy has a well-refined and uniform distribution of bimodal eutectic microstructure when compared with the slowly cooled alloy. The hardness of the quenched alloy was observed to be 16% higher than the slowly cooled alloy. The increased microhardness of the quenched alloy is attributed to the solid solution strengthening of the alloying elements and homogeneous dispersion of fine Si particles, and partly due to the higher volume fractions of both θ-Al 2 Cu and Si phases. [ABSTRACT FROM AUTHOR]

Gas atomization uses a high-speed gas flow to impact alloy melt and crush it into small droplets. During this process, the kinetic energy of the high-speed gas flow is transformed into the surface energy of small droplets, and the droplets are dragged and cooled by the gas. In this study, a FeNiCrBSiC alloy was used to investigate the motion and solidification behavior of alloy droplets during gas atomization. A mathematical model of the cooling and solidification process of droplets is modified and used. The initial velocity of the droplets attributed to the breakup by the high-speed gas flow is considered a factor. The results suggest that the velocity difference between the droplet and the gas is appearing to be smaller, indicating that droplets are more likely to collide with each other; as the velocity difference between the droplet and the gas becomes smaller, the heat transfer coefficient decreases. Thus, the solidification process of the droplet needs a longer flight distance; the relation between the cooling rate and the particle size was determined from measured data and empirical formula and model validation was performed. [ABSTRACT FROM AUTHOR]

The hot-dip 55 wt%Al–Zn-1.6 wt%Si-(0–3)wt.%Mg alloy coatings were experimentally investigated, and the solidification behaviors and hot cracking susceptibility were simulated by means of CALPHAD (CALculation of PHAse Diagrams) method. The scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA) and glow discharge spectrometer method (GDS) were utilized to determine the microstructures and the distribution of elements of the Al–Zn–Mg alloy coatings with different Mg contents. It is discovered that with the increase of Mg content, the percentage of the eutectic microstructure scales up, and the surface quality of the alloy coating is improved. Meanwhile, the bending properties of Al–Zn–Mg coatings with different Mg contents still requires further improvement according to the present bending test. Subsequently, the equilibrium solidification processes of the coatings were calculated using thermodynamic approach. In general, the calculated results reflect the solidified phases and the precipitated temperatures according to theory of equilibrium solidification. However, there still exist discrepancies between the thermodynamic calculation results and the observed experimental results during the practical galvanizing process, because the cooling rates were not taken fully into consideration. Consequently, the kinetic analysis was carried out to obtain the secondary dendrite arm spacing under different cooling rates. The cracking susceptibility index (CSI) was also calculated to predict the hot workability of the Al–Zn–Mg alloy coating. In summary, the appropriate increase of the cooling rate turns out be the effective approach to benefit the microstructure and the corrosion resistance of the Al–Zn–Mg coatings in the practical galvanizing process, and the coating is not suitable for the hot stamping process. • SEM, GDS measurements of the Al–Zn–Mg coatings with different Mg contents were carried out. • The cracking susceptibility of Al–Zn–Mg coating was discovered by means of 90° bending test. • Thermodynamic calculations were carried out to explain the theoretical solidification process. • Kinetic analysis was applied to predict the practical solidification process. [ABSTRACT FROM AUTHOR]

The non-equilibrium phase evolution during laser powder bed fusion (LPBF) of AlSi10Mg alloy is thoroughly characterized and analyzed by means of advanced electron microscopy and analytical simulation of rapid solidification phenomenon. The evolution of microstructural strengthening agents such as inter-cellular eutectic phase and intra-cellular precipitates is presented in correspondence with the local variation of cellular/dendritic solidification patterns within a typical melt pool. The eutectic phase exhibits two different morphologies: lamellar and fibrous. As with the cell size variation, the overall volume fraction of eutectic phase and the lamella spacing is shown to gradually decrease by moving away from the melt-pool boundary (MPB), i.e., through crossing over from a coarse to a fine cellular zone. The eutectic-free regions within the α-Al cells contain a large number density of nano-sized precipitates that are predominantly Si-rich and are either fully or semi-coherent with the Al matrix. The formation of nano-precipitates is linked to the increased (non-equilibrium) solubility limits of α-Al cells due to the rapid solidification effect. For the first time, we identify such nano-precipitates with non-equilibrium crystal structures and morphologies: "Spheres" and "Ellipsoids" with Face Centered Cubic (FCC), and "Plates" and "Needles" with a Diamond Cubic (DC) superlattice structure that emerges from within the Al matrix. The microstructure in the heat-affected zone (HAZ) right underneath the MPB exhibits an absence of cell boundaries and eutectic phases while consisting primarily of large Si-rich and Mg-rich precipitates. Finally, the local variation of nano-hardness across a solidified melt pool is shown to correlate well with the corresponding profile of microstructural refinement, i.e., exhibiting a minimum at the HAZ and a peak at around the melt-pool centerline. The findings here can significantly advance the state of knowledge for the strengthening behavior in an as-built LPBF-processed AlSi10Mg alloy. [ABSTRACT FROM AUTHOR]

In situ synchrotron X-ray radiography observations of the Zr-poisoning phenomenon of an Al-20 wt pct Zn alloy inoculated by Al-5Ti-1B were carried out. The effects of Zr addition on heterogeneous nucleation, grain growth, and final grain size were quantitatively studied and further analyzed using the interdependence model. The experimental results show that the undercooling needed for nucleation increases and the nucleation rate decreases at the early stage of solidification with Zr addition, resulting in fast grain growth, a high solidification rate and increased severity of solute segregation in the solid-liquid coexistence regions. At the same time, the poisoning is a progressive process that is enhanced with the increasing Zr content, holding temperature, and holding time. The nucleation-free zone of the Zr-containing sample, either measured from the radiographs or calculated by the interdependency theory, is larger than that of the Zr-free sample. Our analysis shows that both the increase in the nucleation-free zone and the average interparticle spacing of the most potent available nucleation particles contribute to the increase of the grain size caused by Zr poisoning. [ABSTRACT FROM AUTHOR]

Cu-Fe alloy processed by conventional preparation methods has serious large-scale composition segregation or spatial separation of individual Cu and Fe phases prior to solidification. For immiscible Cu-Fe alloy, a homogeneous melt is the prerequisite to obtain the subsequent uniform solidification microstructure. In this paper, the uniform metastable immiscible Cu 100−x Fe x alloys (x = 10, 20, 30, 40 wt%) were prepared by arc-melting. The results indicated that with increasing melting times, the initial aggregated Fe-rich melt gradually dissolves into the bulk copper matrix and a homogeneous melt is obtained. Herein a thermodynamic model was constructed to discuss the surface energy driven melting process and the formation of uniform Cu-Fe alloy. During melting process, if the radius of detached liquid Fe-rich droplet is less than the calculated critical radius, the surface energy is high enough to compensate for the partial molar excess Gibbs energy of iron, thereby leading to the spontaneously dissolution of detached liquid Fe-rich droplet into the bulk liquid. As a result, the aggregated Fe-rich melt dissolves step by step eventually forming a homogeneous melt. After subsequent solidification, the immiscible Cu-Fe alloy exhibits homogeneous microstructure. The rapid melt quenching experiments are further performed to validate the proposal model. The results provide strong evidence and support for the surface energy driven dissolution model. The findings give us a basic understanding for the evolution of melting process in other immiscible alloys. [ABSTRACT FROM AUTHOR]

The effect of the addition of Be and Sc on the microstructure and mechanical properties of A357 alloy were systematically investigated. The results show that the addition of small amounts of Be and Sc could change the acicular structure of iron-bearing intermetallic compounds to harmless compact Al-Fe-Si and Sc-Fe iron-bearing intermetallic compounds. Compact iron-bearing intermetallic compounds could improve fluidity, causing a reduction in interdendritic shrinkage during solidification. The addition of 0.05 wt % Be enhanced the quality index of the A357 alloy by 11% and increased the notch-yield ratio of fracture toughness by 4.5%. In contrast, the addition of 0.05 wt % Sc increased the quality index and the notch to yield ratio of fracture toughness up to 17% and 9%, respectively. Therefore, the microstructure and mechanical properties of the A357 alloy could be improved by substituting Be with Sc. [ABSTRACT FROM AUTHOR]

An inverse thermal analysis of Alloy 690 laser and hybrid laser-GMA welds is presented that uses numerical-analytical basis functions and boundary constraints based on measured solidification cross sections. In particular, the inverse analysis procedure uses three-dimensional constraint conditions such that two-dimensional projections of calculated solidification boundaries are constrained to map within experimentally measured solidification cross sections. Temperature histories calculated by this analysis are input data for computational procedures that predict solid-state phase transformations and mechanical response. These temperature histories can be used for inverse thermal analysis of welds corresponding to other welding processes whose process conditions are within similar regimes. [ABSTRACT FROM AUTHOR]

Grain size evolution for γ -TiAl based alloys with aluminum content ranging from 43 to 52 at.% were cast at constant superheat and characterized. Experiments show that the sooner the peritectic phase transition occurs, the finer are the equiaxed grains. An accurate estimate of the number of surviving dendrite fragments and fragmentation rate is performed, which can be used to predict the position of the columnar-to-equiaxed transition in the cast ingots. For alloys solidifying with the β pro-peritectic primary phase, the fragmentation rate significantly increases when the onset of the peritectic reaction is earlier. A mechanism based on the peritectic reaction is proposed for the observed grain refinement and is discussed on the basis of the fluid flow, mushy zone permeability and the influence of the peritectic reaction. [ABSTRACT FROM AUTHOR]

Under terrestrial conditions, solidification processes are often affected by gravity effects, which can significantly influence the final characteristics of the grown solid. The low-gravity environment of space offers a unique and efficient way to eliminate these effects, providing valuable benchmark data for the validation of models and numerical simulations. Moreover, a comparative study of solidification experiments on earth and in low-gravity conditions can significantly enlighten gravity effects. The aim of this paper is to give a survey of solidification experiments conducted in low-gravity environment on metal alloys, with advanced post-mortem analysis and eventually by in situ and real-time characterization. [ABSTRACT FROM AUTHOR]

The three-phase, (Mo,Nb) ss /(Mo,Nb) 5 Si 3 /(Mo,Nb) 5 SiB 2 , Bcc/T 1 /T 2 microstructures that develop in Mo–Nb–Si–B alloys have been examined in arc cast and directionally solidified samples to identify the phase sequencing during solidification. A Mo-32.6Nb-19.5Si-4.7B (at.%) alloy was directionally solidified using an optical floating zone (OFZ) furnace in a flowing Ar gas atmosphere at a constant growth rate of 10 mm/h. The microstructure of the directionally solidified alloy is characterized by an elongated T 2 phase surrounded by bcc and T 1 phases with an interwoven morphology From the evaluation of the microstructures in arc cast ingots of several alloys at a constant 32.6 at%Nb composition, the path of the liquidus valleys with decreasing temperature has been determined to intersect at a common point that establishes the solidification reaction as a eutectic. The incorporation of the experimental results into a computational thermodynamic analysis provides insight on the partitioning of components within each phase during solidification. [ABSTRACT FROM AUTHOR]

Fluxing and cyclic superheating technique was used to investigate the rapid solidification behavior of CoCrCuFe x Ni (x = 1.0, 1.5, 2.0, molar concentration) high-entropy alloys in this study. The microstructures of CoCrCuFe x Ni (x = 1.0, 1.5, 2.0) high-entropy alloys solidified at different undercoolings (ΔT) were investigated. Liquid-phase separation leading to Cu-rich and Cu-depleted regions, were obtained in the solidified microstructure from highly undercooled melt. This occurs when the melt undercooling exceeds a critical undercooling (ΔT crit ) of 160 K for CoCrCuFeNi, 190 K for CoCrCuFe 1.5 Ni and 293 K for CoCrCuFe 2 Ni alloy. However, typical dendrites and interdendritic regions were observed in rapid-solidified CoCrCuFe x Ni alloys prepared from melts with a small undercooling (ΔT < ΔT crit ). Conversely, a large amount of Cu-rich spheres and even egg-type structures were observed in alloys solidified from melts with large degree of undercooling, ΔT in excess of the critical value, ΔT crit . A large amount of Cu-rich nano-phases were found in the matrix, possibly, due to the precipitation of Cu-rich phase from the supersaturated solid solution obtained during solidification. The positive enthalpies of mixing between Cu and the other elements in the multi-component alloys resulted in the occurrence of liquid-phase separation prior to the liquid–solid transformation starts. The sluggish diffusion effect of high-entropy alloys and rapid solidification play an important part in the precipitation of nanophase during the solid-state transformation in the Cu-based matrix. Similar to other immiscible alloys, liquid-phase separation occurred when a critical undercooling was exceeded. Differently, nanophases were found in the microstructures of multi-component CoCrCuFe x Ni (x = 1.0, 1.5, 2.0) alloys. [ABSTRACT FROM AUTHOR]

Directional solidification of eutectic alloys attracts considerable attention because of microhardness, tensile strength, and electrical resistivity influenced by eutectic structures. In this research, we examined processing of Al–Ni–Fe (Al–6.5wt.%Ni–1.5wt.%Fe) eutectic by directional solidification. The alloy was prepared by vacuum furnace and directionally solidified in Bridgman-type equipment. During the directional solidification process, the growth rates utilized varied from 8.25 μm/s to 164.80 μm/s. The Al–Ni–Fe system showed a eutectic transformation, which resulted in the matrix Al, rod Al 3 Ni, and plates Al 9 NiFe phases. The eutectic spacing between ( λ A l 3 N i − λ A l 3 N i , λ A l 9 N i F e − λ A l 9 N i F e ) was measured. Additionally, the microhardness, tensile strength, and electrical resistivity of the alloy were determined using directionally solidified samples. The effects of growth rates on microhardness, tensile strength, and electrical resistivity for directionally solidified Al–Ni–Fe eutectic alloy were investigated, and the relationships between them were experimentally obtained. It was found that the microhardness, tensile strength, and electrical resistivity were affected by both eutectic spacing and the solidification parameter. [ABSTRACT FROM AUTHOR]

The density of vacuum counter-pressure cast aluminum alloy samples under grade-pressuring condition was studied. The effect of grade pressure difference and time on the density of aluminum alloys was discussed, and the solidification feeding model under grade-pressuring condition was established. The results indicate the grade-pressured solidification feeding ability of vacuum counter-pressure casting mainly depends on grade pressure difference and time. With the increase of grade pressure difference, the density of all the aluminum alloy samples increases, and the trend of change in density from the pouring gate to the top location is first decreasing gradually and then increasing. In addition, in obtaining the maximum density, the optimal grade-pressuring time is different for samples with different wall thicknesses, and the solidification time when the solid volume fraction of aluminum alloy reaches about 0.65 appears to be the optimal beginning time for grade-pressuring. [ABSTRACT FROM AUTHOR]

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb (at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated. The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 µm*s-1 under the temperature gradients of 15-20 K*mm-1, and the primary phase is transformed into an a phase at relatively higher growth rates (V >20 µm*s-1). The mainly S-segregation and β-segregation can be observed in Ti-45AI-5Nb alloy at a growth rate of 10 µm*s-1 under a temperature gradient of 15 K*mm-1. The increase of temperature gradient to 20 K*mm-1 can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β/→ L+β L+β→α/→ α+βR β→α/→ α α→γ/→ α→γ/→ α+γ α→α2+γ/→γR + (α2+γ), in which βR and γR mean the residual β and γ. [ABSTRACT FROM AUTHOR]

Due to the extensive application of Al-Si alloys in the automotive and aerospace industries as structural components, an understanding of their microstructural formation, such as dendrite and (Al+Si) eutectic, is of great importance to control the desirable microstructure, so as to modify the performance of castings. Since previous major themes of microstructural simulation are dendrite and regular eutectic growth, few efforts have been paid to simulate the irregular eutectic growth. Therefore, a multiphase cellular automaton (CA) model is developed and applied to simulate the time-dependent Al-Si irregular eutectic growth. Prior to model establishment, related experiments were carried out to investigate the influence of cooling rate and Sr modification on the growth of eutectic Si. This CA model incorporates several aspects, including growth algorithms and nucleation criterion, to achieve the competitive and cooperative growth mechanism for nonfaceted-faceted Al-Si irregular eutectic. The growth kinetics considers thermal undercooling, constitutional undercooling, and curvature undercooling, as well as the anisotropic characteristic of eutectic Si growth. The capturing rule takes into account the effects of modification on the silicon growth behaviors. The simulated results indicate that for unmodified alloy, the higher eutectic undercooling results in the higher eutectic growth velocity, and a more refined eutectic microstructure as well as narrower eutectic lamellar spacing. For modified alloy, the eutectic silicon tends to be obvious fibrous morphology and the morphology of eutectic Si is determined by both chemical modifier and cooling rate. The predicted microstructure of Al-7Si alloy under different solidification conditions shows that this proposed model can successfully reproduce both dendrite and eutectic microstructures. [ABSTRACT FROM AUTHOR]

The present work investigates the effects of Samarium (Sm) addition on the performance of 6063 Aluminum alloy. The investigation includes aspects related to the solidification behavior, the microstructure and the mechanical properties for the reference alloy in addition to four modified alloys containing 0.1, 0.3, 0.5 and 1.0 wt% Sm. The reactions corresponding to the formation of α -Al, Al 13 Fe 4 , α -AlFeSi and Mg 2 Si are identified through thermal analysis of the reference alloy. The modified alloys experience similar reactions during solidification that, however, occur at slightly different temperatures, implying the modification of the forming phases due to Sm addition. Optical microscopic examination reveals 37 % grain refinement for the modified alloys due to the Sm addition up to 0.5 wt%. Sm addition influences also the Fe-bearing precipitates, causing the formation of granular Al-Sm-Si-Fe-Mg due to the addition of 0.1 wt% Sm. Samarium addition beyond 0.1 wt% induces the formation of Fe-free Al-Sm-Si-Mg intermetallics, having sharp plate-like and zigzag morphology, in addition to Sm-free Al-Fe-Si precipitates. The best mechanical performance, in the T6 condition, is achieved by the (AA6063 + 0.1 wt% Sm) alloy, attaining an increase of 38 % in the hardness and 11 % in the yield strength, in comparison to the unmodified alloy. The significant strengthening of the 0.1 wt% Sm modified alloy in the T6 condition suggests a promoted precipitation of Mg 2 Si during solidification due to the Sm addition. • Sm alters the reaction temperatures during the solidification of modified AA6063. • The grain size decreases due to Samarium addition. • Samarium changes the morphology and composition of Fe-bearing precipitates. • Both the strength and hardness of Sm-modified alloys are enhanced. • Testing of T6 samples suggests promoted precipitation of Mg 2 Si due to Sm addition. [ABSTRACT FROM AUTHOR]

The non-equiatomic FeCoNiAlSi alloy is prepared by the Bridgman solidification (BS) technique at different withdrawal velocities (V = 30, 100, and 200 μm/s). Various characterization techniques have been used to study the microstructure and crystal orientation. The morphological evolutions accompanying the crystal growth of the alloy prepared at different withdrawal velocities are nearly the same, from equiaxed grains to columnar crystals. The transition of coercivity is closely related to the local microstructure, while the saturation magnetization changes little at different sites. The coercivity can be significantly reduced from the equiaxed grain area to the columnar crystal area when the applied magnetic field direction is parallel to the crystal growth direction, no matter what is the withdrawal velocity. In addition, the alloy possesses magnetic anisotropy when the applied magnetic field is in different directions. [ABSTRACT FROM AUTHOR]

Alloy Ti44Al6Nb1.0Cr2.0V0.15Y0.1B is newly designed and melted by vacuum consumable melting method, and is then prepared by cold crucible directional solidification (CCDS) technology. Thermo-compression experiment is carried out on CCDS billets at 1373-1573 K. Experimental results show that, when compressing direction (CD) is perpendicular to columnar grains, higher deformation active energy of Q = 689.8 kJ/mol can be acquired which means excellent creep resistance in radial direction; the microstructural evolution with different lnZ parameters is investigated by electron back scattering diffraction and the deformation mechanism is discussed; the ordering temperature of B2/β phase is deduced to be in the range of 1200-1250 °C. When the CD is parallel to the columnar grains, a much higher peak stress is acquired which means more excellent creep resistance in the axial direction; the shearing deformation feature is presented and the schematic of deformation mechanism is given. [ABSTRACT FROM AUTHOR]

Ti44Al6Nb1.0Cr2.0V alloy was directionally solidified (DS) with cold crucible under different input powers (40 kW, 45 kW, 48 kW and 50 kW; corresponding to temperature gradient of 15.2 K/mm, 21.0 K/mm, 22.9 K/mm, 25.5 K/mm) and the same pulling velocity (8.33 μm/s). The results show that all the DS samples have the β-solidification characteristics and when the power is 45 kW or 48 kW, they have the well-DS macrostructure. After DS, the lamellar width becomes smaller and more homogeneous from the as-cast 1650 ± 1200 nm to less than 710 ± 376 nm. The higher the power is, the smaller the lamellar width it has. The original casting microcracks can be eliminated after DS and the percentage of small angle lamellas is higher for the well-DS samples. For the DS samples, the nanoindentation hardness in lamella region and the lamellar width yield the relationships of H N = 16.12 d − 0.127 and r 2 = 0.992 . When the as-cast and heat treatment (HT) conditions are considered, the relationships are changed as H N ' = 16.09 d ' − 0.127 and r ' 2 = 0.971 . The nanoindentation hardness of B 2 phase and block γ phase keep almost the same respectively under the different conditions. RT tensile property is improved to average UTS 589.4 MPa and average elongation 1.16% after DS (P = 45 kW) comparing with the original as-cast 498.8 MPa and 0.53%. Trans-granular and trans-lamella fracture is the main fracture mode for the DS samples. After annealing ( at 1250 °C, 1300 °C, 1350 °C), lamellar width is coarsen to more than 2002 ± 1286 nm; B 2 phase segregation is reduced obviously, but some new microcracks form; HT samples exhibits a poor RT tensile property, in which the HT2 sample has a relatively higher property and it yields the average UTS 440.6 MPa and average elongation 0.45%. [ABSTRACT FROM AUTHOR]

Textured materials usually promise an enhanced physical effect along their preferred orientation. However, how to orientate the material, especially from a fissile alloy, is still a great challenge. The transition-metal-based alloy, MnCoSi, is a potential low-cost magnetostrictive material, which application is limited by the poor mechanical property and reduced magnetostriction in polycrystal. Here, we demonstrate that, through the approach of high magnetic field solidification with a slow cooling rate, some textured and dense MnCoSi 1− x alloys are obtained, which exhibit giant magnetostriction and enhanced manufacturing performance. Meanwhile, by introducing vacancies of Si element, the metamagnetic transition of MnCoSi alloy is adjusted, giving rise to the room-temperature and reversible magnetostriction under reduced driving magnetic field, which is of significance for the practical applications. [ABSTRACT FROM AUTHOR]

The determination of the solidification characteristics of alloys using differential scanning calorimetry (DSC) is difficult because of the unknowns associated with the kinetic of phase transformations and the thermal resistance between the sample and the temperature measuring device. This paper shows how appropriate assumptions coupled with a thermodynamic software package and an accurate mathematical analysis of a power-compensated DSC, can enable a direct comparison between the experimental and the theoretical heat evolutions obtained during the solidification of a multiphase alloy. This comparison is helpful in order to assess the thermodynamic database and to validate the different assumptions made in the solidification model. [ABSTRACT FROM AUTHOR]

We present a solidification model for equiaxed globular microstructure based on an analytical solution of the diffusion fields in the liquid with cross-diffusion terms for multicomponent alloys. It is demonstrated that the solute profiles for any element can be defined after computation of the eigenvalues and the eigenvectors of the liquid diffusion matrix. The interface velocity and the solute profiles are compared and validated with a numerical solution developed with a front tracking approach. A comprehensive application to the Al–7 wt.%Si–1 wt.%Mg alloy shows the effect of interdiffusion phenomena on the solidification path. This application is developed with an integral approach for the computation of the liquid composition far from the interface. It appears to be the only relevant choice in order to compute the expected driving force. Analytical solutions of the diffusion profile can also be computed for planar and cylindrical geometries. Coupling of this microsegregation model with thermodynamic equilibrium calculations is finally proposed and discussed. [ABSTRACT FROM AUTHOR]

Magnesium die-casting alloys based on the Mg-Al (magnesium-aluminum) system have been used increasingly in many industrial applications for the main purpose of weight savings. However, relatively low conductivity of conventional Mg-Al casting alloys such as AZ91 often limits the application areas and attempts to improve the conductivity without losing excellent casting capabilities and other properties of the Mg-Al based alloys that have been made recently. In this research, aluminum content was restricted to a comparatively low level for improving the conductivity and lanthanum (La) was added to compensate for reduced casting capabilities. As the La content was increased, the mold filling ability measured using a fluidity serpentine test mold was gradually increased, however the hot cracking susceptibility was observed to be initially increased and then decreased. The relationship between solidification characteristics and castabilities of Mg-3Al-xLa alloys was determined based on the thermal and cooling curve analysis results and microstructural characterization. [ABSTRACT FROM AUTHOR]

Owing to the effect of the transformation-induced plasticity (TRIP) or the twinning-induced plasticity (TWIP), Fe-Mn-based alloys have excellent mechanical properties and attract more attention as a promising structural material. Thin strip continuous casting was considered as an effective method to produce high-Mn steels, which were hard to produce by traditional methods. In this paper, the effect of different cooling rates by changing the trip thickness on the microstructure of Fe-15 wt-%Mn alloy prepared under sub-rapid solidification was investigated. The experimental results showed that α′-martensite and ε-martensite directly formed in the sub-rapidly solidified strips. The formation of ε-martensite in the 1·5 mm and 1 mm thickness strips suppressed with increasing cooling rate, while some nanocrystallines were obtained in these strips. The size of martensite phases was very fine in all strips, and the ε-martensite still existed after the heat treatment at 1000°C for 2 h. [ABSTRACT FROM AUTHOR]

Sixty nine Nb–Si based alloys with Al, B, Cr, Fe, Ge, Hf, Mo, Sn, Ta, Ti, V, W, Zr additions that were produced using cold hearth based solidification processing were used to study the macrosegregation of Si. The study used chemical analysis data for the compositions of the cast alloys and phases present in their microstructures to calculate various material parameters and the partitioning of solutes. The material parameters were based on the melting temperatures and enthalpies of melting of sd and sp electronic configuration alloying elements in Nb. The key material parameters controlling the macrosegregation of Si were the alloy melting temperature T m , the melting temperature of the sd elements of the alloy T m sd , the enthalpy of melting of the sd elements Δ H m sd , the ratio of the melting temperatures of the sd and sp elements in the alloys T m sd / T m sp and the “alloy entropy of fusion” Δ H m / T m . The macrosegregation of Si was explained using the concepts of “alloy entropy of fusion”, and “alloy enthalpy of melting” Δ H m and T m and by considering (i) the stability of S/L interfaces in terms of constitutional supercooling theory and the growth of S/L interfaces in terms of their structure and (ii) the partitioning of solutes between the Nb ss and the intermetallics present in the microstructures of the alloys. High Si macrosegregation was observed in alloys that had high Δ H m / T m and T m sp values and low T m sd / T m sp and T m . [ABSTRACT FROM AUTHOR]

Directional solidification experiments have been performed on Sn-36 at.%Ni peritectic alloy in a constant temperature gradient at different growth velocities. Experimental result shows that a 'sawtooth' morphology forms on secondary dendrite arms during the migration of secondary dendrites in the presence of tertiary dendrite arms. A theoretic model is therefore proposed to describe the formation of this 'sawtooth' morphology in the peritectic solidification with tertiary dendrite arms taken into consideration. The migration of secondary dendrite arms is caused by remelting/solidification at the hot/cold sides of a liquid pool between secondary dendrite arms, which is a form of temperature gradient zone melting. And, the 'sawtooth' morphology is ascribed to the difference in remelting velocity at the hot side of liquid pool during the migration of secondary dendrite arms due to the presence of tertiary dendrite arms. In addition, the proceeding of peritectic reaction can accelerate the formation of 'sawtooth' morphology. [ABSTRACT FROM AUTHOR]

Over the last 20 years, time-resolved high-resolution x-ray imaging techniques have become established as methods of choice for in situ studies of solidification microstructure formation in metallic alloys. Using high-brilliance x-radiation delivered at modern synchrotrons, combined with the most recent advances in x-ray cameras and optics, it is now possible to capture real-time video sequences with projection image resolutions down to 100 nm at frame rates of 20 Hz. This article presents some illustrative examples of the current capabilities of in situ x-ray video microscopy in terms of radiographic and topographic imaging as a platform for studying microstructure pattern formation, growth dynamics, and other topics related to metallic alloy solidification. [ABSTRACT FROM AUTHOR]

In this work, a parametric analysis of the radial solute distribution in crystals grown by the vertical Bridgman method is performed in order to find optimal conditions for growing crystals with improved chemical homogeneity. The analytical formulas from the literature are used to estimate the convecto-diffusive parameter and the radial segregation. The parametric analysis performed by varying the growth rate and the diffusion coefficient can predict the conditions for obtaining a convective regime in the melt. As theoretically shown, the crystals grown in these conditions are characterized by small radial variations of the solute concentration. [ABSTRACT FROM AUTHOR]

The solidification microstructures of three nine-element Zr-Ni–based AB2 type C14/C15 Laves hydrogen storage alloys are determined. The selected compositions represent a class of alloys being examined for usage as an MH electrode in nickel metal-hydride batteries that often have their best properties in the cast state. Solidification is accomplished by dendritic growth of hexagonal C14 Laves phase, peritectic solidification of cubic C15 Laves phase, and formation of cubic B2 phase in the interdendritic regions. The B2 phase decomposes in the solid state into a complex multivariate platelike structure containing Zr-Ni–rich intermetallics. The observed sequence C14/C15 upon solidification agrees with predictions using effective compositions and thermodynamic assessments of the ternary systems, Ni-Cr-Zr and Cr-Ti-Zr. Experimentally, the closeness of the compositions of the C14 and C15 phases required the use of compositional mapping with an energy dispersive detector capable of processing a very high X-ray flux to locate regions in the microstructure for quantitative composition measurement and transmission electron microscope examination. [ABSTRACT FROM AUTHOR]

The optical microscopy is a traditional, very important tool for the study of materials, in particular of metallic alloys. Even though electron microscopy techniques have evolved to forms increasingly more sophisticated, optical microscopy remains as a tool indicated for the study of a broad spectrum of topics linked to the physical metallurgy, such as solidification, phase transformations products, grain structures, fracture, among others. The most important application of light microscopy is the determination of the structural phases present and the constitution of the bulk of the metal. These observations are of practical importance because the structure and constitution have a strong influence on the behavior of the material. In this work, we present numerous examples of optical microscopy applied to the study of metallic alloys. In particular, the phase transformations taking place in Cu-based shape memory alloys, and the analysis of structures formed during directional solidification of dilute alloys. In all the cases, the specimens were carefully prepared for optical examination in a Reichert MeF2 Optical Microscope, with a photographic system put on. The studied alloys and the metallographic procedures are mentioned opportunely. [ABSTRACT FROM AUTHOR]

Abstract: Fe–C–V and Fe–C–V–Si alloys of various C, V and Si compositions were investigated in this work. It was found that the phases present in both of these alloy systems were alloyed ferrite, alloyed cementite, and VC x carbides. Depending on the alloy composition the solidified microstructural constituents were granular pearlite-like, lamellar pearlite, or mixtures of alloyed ferrite+granular pearlite-like or granular pearlite-like+lamellar pearlite. In addition, it is shown that in Fe–C–V alloys the C/V ratio influences (a) the type of matrix, (b) the fraction of vanadium carbides, f v and (c) the eutectic cell count, N F. In Fe–C–V alloys, a relationship between the alloy content corresponding to the eutectic line was experimentally determined and can be described by where Ce and Ve are the carbon and vanadium composition of the eutectic. Moreover, in the Fe–C–V alloys (depending on the alloy chemistry), the primary VC x carbides crystallize with non-faceted or non-faceted/faceted interfaces, while the eutectic morphology is non-faceted/non-faceted with regular fiber-like structures, or it possesses a dual morphology (non-faceted/non-faceted with regular fiber-like structures+non-faceted/faceted with complex regular structures). In the Fe–C–V–Si system, the primary VC x carbides solidify with a non-faceted/faceted interface, while the eutectic is non-faceted/faceted with complex regular structures. In particular, spiral eutectic growth is observed when Si is present in the Fe–C–V alloys. In general, it is found that as the matrix constituent shifts from predominantly ferrite to lamellar pearlite, the hardness, yield and tensile strengths exhibit substantial increases at expenses of ductility. Moreover, Si additions lead to alloy strengthening by solid solution hardening of the ferrite phase and/or through a reduction in the eutectic fiber spacings with a decrease in the alloy ductility. [Copyright &y& Elsevier]

The three-dimensional (3-D) microstructure of an Al–Si base alloy is reconstructed to visualize and characterize the 3-D geometry of primary Si. Octahedral, plate-like and other 3-D morphologies of primary Si are observed and quantitatively characterized. The octahedral morphology is predicted to occur when the ratio of the crystal growth velocities along [001] and [111] directions is equal to √3 (and not 1.5 as previously reported by other researchers). A digital library of 3-D primary Si particles is created for realistic microstructure simulations. [Copyright &y& Elsevier]

Abstract: Rapid solidification is one of the most important techniques to produce nanocrystalline rare-earth–transition metal–boron (RE–TM–B) hard magnetic materials. To achieve high performance on these NdFeB-based alloys, compositional modification and microstructure optimization have been frequently employed. In this short review, various substitutions and doping elements have been discussed regarding to their behaviors in adjusting the individual or combined hard magnetic properties as well as the microstructure based on our recent results. It has been demonstrated that Pr and Dy enhance coercivity j H C, whereas Sm reduces j H C due to their effects on intrinsic properties. Co improves the thermal stability as well as the microstructure. Introducing Fe65Co35 is a possible approach to enhance the magnetization and maximum energy product (BH)max. As a doping element, Ta was found to play an important role to obtain an appropriate combination of magnetic properties for this type of alloys. [Copyright &y& Elsevier]

Abstract: The electrochemical corrosion behaviour of as-prepared and structurally relaxed Zr48Cu36Ag8Al8 glassy alloy samples was investigated in 1M H2SO4. Results show that the active–passive transition behaviour of Zr48Cu36Ag8Al8 glassy alloy depends on the amount of free volume (FV) present in the metallic glassy alloy. Poor passivation due to active dissolution of Cu was displayed by the Zr48Cu36Ag8Al8 glassy alloy when it was in a high energy metastable state with relative high amount of FV. It is shown that by reducing the amount of FV in the glass, the passivation behaviour is significantly improved. [Copyright &y& Elsevier]

Abstract: A novel quaternary CuAlNiBe shape memory alloy (SMA) Cu–11.5Al–3.8Ni–0.4Be (wt%) was designed and synthesized in this study. The single crystal of this alloy was fabricated by means of a high temperature gradient directional solidification furnace with a selective growing crystallizer. After solid solution treatment, the phase transformation points (including M s, M f, A s and A f) were determined by DSC; the bending shape memory performance, the maximal recoverable deformations, the tensile strength and the elongation percentage were measured. Its maximum recoverable strain reached 10% which approximately equals to or exceeds the reported maximum recoverable strain for polycrystalline TiNi alloy. The number of bend-relaxing times arrived at 750. Both results implied that this single crystal alloy may become a promising candidate to challenge TiNi SMA. [Copyright &y& Elsevier]

Abstract: Experiments have been carried out to analyze the columnar to equiaxed transition during the horizontal directional solidification of Sn–Pb hypoeutectic alloys as a function of solidification parameters, heat transfer coefficients, temperature gradients, growth rates, cooling rates and composition Co. For this purpose, a water-cooled solidification experimental apparatus was developed, and specimens were solidified under unsteady state heat flow conditions. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal variables considered. The results have supported a criterion proposed based on a critical value of cooling rate which for a particular binary alloy system is independent of the solute concentration in the hypoeutectic range. The observation of the macrostructures has indicated that the columnar-to-equiaxed transition occurred in a zone rather than in a sharp plane parallel to the chill wall and that resulting thermo-solutal convection seems to favor the structural transition which occurs for a critical cooling rate of about 0.048K/s and temperature gradients in the range between 0.25 and 0.90K/mm for any of four alloy compositions examined. A comparative analysis between some results of this work and those from the literature proposed to analyze the CET during upward and downward vertical solidification of Sn–Pb hypoeutectic alloys is conducted. [Copyright &y& Elsevier]

Abstract: Bulk metallic glass (BMG) composites containing ductile phase were prepared by rapid water quenching of a homogenous Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 melt. The microstructure and composition of the materials were investigated by X-ray diffraction (XRD), optic microscopy (OM), and scanning electrion microscopy (SEM) attached with energy-dispersive spectroscopy (EDS). The secondary dendrite arm spacing (SDAS) in the composites was detected and the solidification parameters of this amorphous composites were obtained. The result shows that the composition of the dendrite phase is depleted in Ni, Cu and enriched in Zr, Nb and Ti, which strongly deviates from the nominal composition. SDAS is gradually increasing from the surface to the centre of the specimen, which inverses proportionally to cooling rate in Newton cooling type. With the increasing diameter of the samples, the secondary arm spacing gradually increases, except the specimen with the diameter of Φ25mm. Meanwhile, the relationship between SDAS and sample diameter or thickness is given quantitatively. [Copyright &y& Elsevier]

Hot tearing in the AA1050 alloy was investigated in real time and in situ using acoustic emission (AE) and cooling curve analysis techniques and a ring mold. Activities involving AE have been detected in three zones of the solidification curve. The characteristic signals for hot tearing were an AE energy of over 600 e.u. and an average frequency of ~125 ± 15 kHz in zone II. For hot cracking, the AE energy was over 650 e.u. and the average frequency was ~128 ± 17 kHz in zone III. The hot-tear start temperature ranged from 636 °C to 653 °C; the nonequilibrium solidus T′ S, from 556 °C to 614 °C; the fraction solid at hot-tear onset from 0.71 to 0.99; and the Clyne–Davis hot-tear susceptibility coefficient (HSC) from 0.25 to 0.81. The HSC correlated inversely with a total energy of solidification cracking ( Etotal) $$ {\text{HSC}} \cong 167\left( {E_{\text{total}} } \right)^{ - 0.8}. $$ A hot-tear susceptibility factor (HSF) = (pct Fe)·(cooling rate (CR))2 was related to the HSC and T′ S as HSC = 0.002 HSF + 0.3 and T′ S = −0.3 HSF + 617. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the effect of alloy composition on the formation of porous Ni catalysts prepared by chemical dealloying of rapidly solidified Al–Ni alloys has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and N2 adsorption experiments. The experimental results show that rapid solidification and alloy composition have a significant effect on the phase constituent and microstructure of Al–Ni alloys. The melt spun Al–20at.% Ni alloy consists of α-Al, NiAl3 and Ni2Al3, while the melt spun Al–25 and 31.5at.% Ni alloys comprise NiAl3 and Ni2Al3. Moreover, the formation and microstructure of the porous Ni catalysts are dependent upon the composition of the melt spun Al–Ni alloys. The morphology and size of Ni particles in the Ni catalysts inherit from those of grains in the melt spun Al–Ni alloys. Rapid solidification can extend the alloy composition of Al–Ni alloys suitable for preparation of the Ni catalysts, and obviously accelerate the dealloying process of the Al–Ni alloys. [Copyright &y& Elsevier]

Abstract: We present a two-dimensional granular model for the mechanical behavior of an ensemble of globular grains during solidification. The grain structure is produced by a Voronoi tessellation based on an array of predefined nuclei. We consider the fluid flow caused by grain movement and solidification shrinkage in the network of channels that is formed by the faces of the grains in the tessellation. We develop the governing equations for the flow rate and pressure drop across each channel when the grains are allowed to move, and we then assemble the equations into a global expression that conserves mass and force in the system. We show that the formulation is consistent with dissipative formulations of non-equilibrium thermodynamics. Several example problems are presented to illustrate the effect of tensile strains and the availability of liquid to feed the deforming microstructure. For solid fractions below , we find that the fluid is able to feed the deformation at low strain, even if external feeding is not permitted. For solid fractions above , clusters of grains with “dry” boundaries form and fluid flow becomes highly localized. [Copyright &y& Elsevier]

Abstract: Thermal analysis with cooling curve has been used for investigation of microstructure evolution with composition variation during solidification process in Mg–Zn–Y–Zr alloys (denoted as ZW alloys). The obtained results reveal that two pseudobinary eutectic reactions appear one after another during solidification due to the different Zn/Y ratio (ranged from 0.35 to 3.33). In ZW62 alloy, a peritectic reaction happens before I-phase formation through pseudobinary eutectic reaction distinguishing it from other alloys. The changes of characteristic temperature points for phase transformation are studied in detail. Microstructure observation shows dominating secondary phases in the alloys differ from one another according to various Zn/Y ratios. Divorced massive eutectic Z-phase is formed during solidification in a low Zn/Y ratio; W-phase comes out when Zn/Y ratio grows to 0.85; if the Zn/Y ratio increases further to a certain level, I-phase forms by a peritectic reaction through W-phase and the liquid. [Copyright &y& Elsevier]

The paper presents the results of a study concerning the influence of the thermo-physical characteristics of coolers on the solidification of hot spots for cast parts. The analysis of the solidification part was achieved by the method of the real solidification module. The use of four different cooler types was analyzed, made of different materials (cast iron, steel, aluminium, copper). When used as exterior coolers, non-ferrous alloys (based on aluminium and copper) ensure a considerably greater cooling coefficient of the parts surface, due to their significantly larger heat accumulation coefficient. [ABSTRACT FROM AUTHOR]

In order to explore the effect of the presence of SiC particles on dendritic coherency during solidification of Al-Si based metallic alloys, a factorial fractional two levels experimental design was implemented to allow an identification of the main effects of the particle content, silicon content, grain refinement, and cooling rate on the solid fraction at coherency. This solidification parameter was determined for Al-3wt%Si and Al-7%Si alloys, and Al-3%Si/SiCp and Al-7%Si/SiCp metal matrix composites. The cooling process during solidification was monitored by performing cooling curve measurements at two radial locations within samples poured into cylindrical molds at two cooling rates. The experimental cooling curves were numerically processed by the Fourier thermal analysis method to know the evolution of solid fraction as a function of time. The effect of grain size was included using samples with or without grain size refinement. The grain refinement was obtained by adding predetermined quantities of TiAlB master alloy. It was found that presence of SiC particles affects the coherency point of the metal matrix composites increasing the solid fraction at coherency. However this effect is relatively small when compared to the effect of grain refinement, cooling rate, and Si content on dendritic coherency of experimental probes. [ABSTRACT FROM AUTHOR]

Solid-liquid interfacial free energy σ[subSL] plays a key role in a wide range of metallurgical phenomena. Pioneer estimates by Turnbull et al. in the 1950s from undercooling measurements were primarily for pure metals on the assumption of homogeneous nucleation and estimates from dihedral angle measurements require an independent measurement or estimate of σ for at least one of the other interfaces involved. Gündüz and Hunt in 1985 reported the first direct measurements of σ[subSL]. for three euteçtic alloy systems from the profiles of grain boundary grooves stabilized by a known radial temperature gradient, embodying a methodology applied prior to 1985 only to transparent systems or pure metals at or near to ambient temperatures. The significance of these and sulsequent measurements and their application to solidification theory will be critically evaluated. [ABSTRACT FROM AUTHOR]

A meso-scale front-tracking model (FTM) of nonequilibrium binary alloy dendritic solidification has been extended to incorporate Kurz, Giovanola, and Trivedi (KGT) dendrite kinetics and a Scheil solidification path. Model validation via comparison with thermocouple measurements from a solidification experiment, in which natural convection is limited by design, is presented. Via solution of the flow field due to natural thermal buoyancy, it is shown that resultant liquid-phase convection creates conditions in which equiaxed solidification is favored. Comparison with simulations in which casting solidification is diffusion controlled show that natural convection has greatest effect at intermediate times, but that at early and late stages of columnar solidification, the differences are relatively small. It is, however, during the time of greatest divergence between the simulations that the authors' predictive index for equiaxed zone formation is enhanced most by convection. Finally, the columnar-to-equiaxed transition is directly simulated, in directional solidification controlled by diffusion. [ABSTRACT FROM AUTHOR]