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Showing papers on "Directional solidification published in 2006"



Journal ArticleDOI
TL;DR: In this article, the Stokes-Darcy-Brinkman model is applied to some analytically tractable flows through adjacent porous and pure-fluid domains and is compared systematically with the multiple-domain Stokes−Darcy model.
Abstract: The single-domain Darcy–Brinkman model is applied to some analytically tractable flows through adjacent porous and pure-fluid domains and is compared systematically with the multiple-domain Stokes–Darcy model. In particular, we focus on the interaction between flow and solidification within the mushy layer during binary alloy solidification in a corner flow and on the effects of the chosen mathematical description on the resulting macrosegregation patterns. Large-scale results provided by the multiple-domain formulation depend strongly on the microscopic interfacial conditions. No satisfactory agreement between the single- and multiple-domain approaches is obtained when using previously suggested conditions written directly at the interface between the liquid and the porous medium. Rather, we define a viscous transition zone inside the porous domain, where the Stokes equation still applies, and we impose continuity of pressure and velocities across it. This new condition provides good agreement between the two formulations of solidification problems when there is a continuous variation of porosity across the interface between a partially solidified region (mushy zone) and the melt.

286 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena, and demonstrate that these models differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions.
Abstract: Recent improvements in detectors combined with the eminent brightness and collimation offered with modern synchrotron sources open the way forin situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching regular video microscopy. Here, the authors present observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena. From directional solidification experiments it has been found that the tendency for crystal fragments to detach by remelting of branch roots in the mush dendrite network strongly depends upon the relative buoyant and settling motions of crystal fragments and mush liquid, respectively. At the copper concentrations studied (20 to 30 wt pct), primary aluminum dendrites are lighter than the melt and solidification experiments parallel and anti-parallel to gravity show significant differences in detachment tendency. The experimental results compare well with three different models proposed for fragmentation at different mush locations; however, the results also demonstrate that these models differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions. Under particular conditions it has been found that crystal fragmentation could lead to an alternating mesoscale segregation.

169 citations


Journal ArticleDOI
TL;DR: In this paper, the microsegregation inherited from the directional solidification and quenching (DSQ) of an experimental third generation single crystal superalloy is characterised using electron probe microanalysis (EPMA).
Abstract: The microsegregation inherited from the directional solidification and quenching (DSQ) of an experimental third generation single crystal superalloy is characterised using electron probe microanalysis (EPMA). A statistical treatment of the data is used to estimate the composition of the solid in the mushy zone. Comparison of the experimental data with the predictions from a numerical model indicates that the results cannot be rationalised without acknowledging that back diffusion is occurring; it is demonstrated that the magnitudes of the diffusion coefficients which need to be invoked are consistent with recent experimental measurements of the coefficients deduced from diffusion couples.

162 citations


Journal ArticleDOI
TL;DR: In this article, the (Nb)/Nb3Si eutectic alloys in binary and ternary systems were unidirectionally solidified in an optical floating zone melting furnace and the effect of solidification rates on microstructures of the alloys was investigated.

145 citations


Journal ArticleDOI
TL;DR: In this article, microstructural selection during the directional solidification of binary TiAl alloys grown from Ti-43Al-3Si (at.%) seeds was examined, and it was found that successful crystal growth of TiAl was dependent upon the undercooling necessary for β nucleation.

145 citations


Journal ArticleDOI
TL;DR: In this paper, the columnar-to-equiaxed transition (CET) in directional solidification of alloys is simulated using the phase-field method, which relies on the solution of a solute conservation equation and an equation for the propagation of the phase field on the scale of developing microstructure.

143 citations


Journal ArticleDOI
TL;DR: In this article, a new concept for growing a polycrystalline silicon ingot suitable for solar cells by the casting method was proposed, which induced dendrite growth along the crucible wall in the initial stage of growth.

93 citations


Journal ArticleDOI
TL;DR: In this article, a numerical study of the fluid flow during directional solidification of a binary alloy (Pb85wt%Sn) in presence of a forced convection was presented.

83 citations


Journal ArticleDOI
TL;DR: In this article, the effect of a high axial magnetic field on the microstructure in a directionally solidified Al-Al2Cu eutectic alloy has been investigated experimentally.

72 citations


Journal ArticleDOI
TL;DR: In this paper, the influence of dendrite arm spacing and solute redistribution on the corrosion resistance of samples of aluminum alloys was investigated by using optical and electronic microscopy techniques.
Abstract: In general, aluminum alloys provide the most significant part of all shaped casting manufactured. An optimum range of properties can be obtained as a function of different cooling rate processes, such as sand, plaster, investment, permanent molds and die castings. It is well known that the dendritic network affects not only the mechanical properties but also the corrosion resistance. However, the literature is scarce on reports concerning the influences of dendrite arm spacing on corrosion resistance and mechanical behavior. The aim of this study is to investigate the influence of as-cast microstructure features, i.e., dendrite arm spacing and solute redistribution on the corrosion resistance of samples of aluminum alloys. In order to investigate the electrochemical behavior of solute and solvent of different aluminum systems, samples with the same order of magnitude of dendritic spacings were analyzed to permit comparison between Al–10 wt% Sn and Al–20 wt% Zn alloys. A casting water-cooled assembly promoting upward directional solidification was used in order to obtain controlled casting samples of these alloys. In order to characterize the dendritic structure, longitudinal sections from the directionally solidified specimens were analyzed by using optical and electronic microscopy techniques. The corrosion resistance was analyzed by both the electrochemical impedance spectroscopy technique and Tafel extrapolation method conducted in a 3% NaCl solution at room temperature. Although both systems present an Al-rich dendritic matrix, different responses to corrosive action as a function of dendritic spacing have been detected.

Journal ArticleDOI
TL;DR: In this paper, the floating zone method was used to solidify three eutectic systems with columnar grains of parallel lamellae with interlamellar spacings that adhere to the lambda-squared (R) C relationship.
Abstract: The ZrC-ZrB2, ZrC-TiB2, and TiC-TiB2 metal diboride-metal carbide pseudo-binary eutectic systems have been successfully solidified directionally by means of the floating zone method. The first and third of these eutectics possess a morphology consisting of columnar grains of parallel lamellae with interlamellar spacings that adhere to the lambda-squared (R) C relationship, while the second is noted to solidify in a Chinese calligraphy-like morphology of broken and deformed lamellae. This phenomenon may be related to solid solution effects, but it did not prevent the interlamellar spacings from following the aforementioned law. The calligraphic effect's values are larger than those of the other two systems, and the constant, C, is accordingly about an order of magnitude larger. All three of these cubic-hexagonal systems exhibited identical epitaxial relationships. 24 references.

Journal ArticleDOI
TL;DR: In this paper, the process of solidification with a supercooled layer is analytically described on the basis of two joint theories of directional and bulk crystallization, and the complex structure of the non-equilibrium mushy layer is completely recognized.

Journal ArticleDOI
TL;DR: In this article, a method for the theoretical study of the directional solidification process in a binary melt with a two-phase zone in which the supercooling is noticeable is presented.
Abstract: The processes of crystallization of melts and solutions are usually described by the Stefan frontal model. In application of this model, it is necessary to determine the impurity concentration and temperature of the substance in both the melt and the growing crystal. In addition, the phase-transition boundary is also unknown, and the phase-transition temperature (liquidus) in the two-component system depends on the local impurity concentration and is determined from the phase diagram [1]. As is well known, the solidification of binary melts is sufficiently often accompanied by arising additional supercooled zones, i.e., domains in the liquid phase ahead of the crystallization front, with a temperature in these zones being lower than the liquidus temperature. One of the mechanisms governing the appearance of the supercooling in a melt was indicated in [2] and is referred to as the concentration supercooling. Due to bulk nucleation in the supercooled domain, the spontaneous generation of solid-phase elements in the form of dendrites or particles can begin. Thus, in the concentration-supercooling zone, the substance can exist in both solid and liquid states. This domain is called the two-phase zone. Numerous papers are devoted to investigating the processes of solidification with supercooled domains (see, e.g., [1, 3‐5]). However, in many studies, the twophase zone is considered in the quasi-equilibrium approximation [6]. This implies that the supercooling is completely destroyed due to the release of the crystallization latent heat by growing solid-phase elements. Thus, the substance temperature in the zone is equalized attaining the liquidus temperature. This fact essentially simplifies solving the problem, however, the solution presents no information on the internal structure and the topology of the zone. In addition, in the general case, the supercooling disappears not entirely. Therefore, in constructing a theory of the solidification in the presence of the two-phase domain supercooling, it is necessary to include into the theoretical scheme kinetic factors responsible for the generation of new-phase elements in the zone. In this paper, we offer a method for the theoretical study of the directional solidification process in a binary melt with a two-phase zone in which the supercooling is noticeable. We consider a zone as a suspension of interacting crystalline particles arising and growing in a supercooled melt. The nucleation rate for solid particles is determined by the Frenkel’‐Zel’dovich formula [7‐9]

Journal ArticleDOI
TL;DR: In this article, a combination of analyzing method: the in-situ phase identification system, morphological observation by high-temperature laser scanning confocal microscopy and observation of microstructure at room temperature by OM, SEM and micro diffraction-system, is suggested to analyze the phase transformation during welding process.
Abstract: Kinetic information about the phase transformation, during solidification process and solid-state transformation is essential to the material processing, such as welding. In our research group, in-situ phase identification system consisting of undulator beam and imaging plate have recently been used. The welding torch is driven by stepping-motor in the system. Those make possible that phase transformation can be identified in real-time under the condition of directional solidification and the spatial resolution of 100 × 500 µm. The time-resolution is 0.3125 seconds. In the present work, combination of analyzing method: the in-situ phase identification system, morphological observation by high-temperature laser scanning confocal microscopy and observation of microstructure at room temperature by OM, SEM and micro diffraction-system, is suggested to analyze the phase transformation during welding process. Phase transformation process of hypereutectoid carbon steel, during welding was analyzed as an example of combination observation.

Journal ArticleDOI
TL;DR: In this paper, the effect of a temperature-dependent solute diffusion coefficient on the model of unidirectional solidification of a binary melt with a steady-state two-phase zone is studied.

Journal ArticleDOI
TL;DR: In this paper, a directionally solidified eutectic NiAl-W alloy was employed as a source for NiAl nanopore arrays, W-nanowire arrays and W-nodes.

Journal ArticleDOI
TL;DR: In this paper, a model for steady convection in a mushy layer of a binary mixture is presented and solved for industrial and geophysical solidification problems such as segregation in metallic castings and expulsion of brine from growing sea ice.

Journal ArticleDOI
05 Nov 2006
TL;DR: In this article, an evaluation was made of liquid to solid transformation during the solidification of an austenitic stainless steel based on an investigation of the material's solidification mode and microstructure.
Abstract: An evaluation was made of liquid to solid transformation during the solidification of an austenitic stainless steel based on an investigation of the material's solidification mode and microstructure. The solidification mode was studied using directional solidification at low growth rates and differential thermal analysis. The alloy analyzed here showed solidification, with the liquid transforming completely into austenite; however, depending on the solute segregation level, it could present the formation of austenite and ferrite. In addition, most of the alloying elements in this steel showed a partition coefficient of less than 1.

Journal ArticleDOI
TL;DR: In this article, the phase transformation temperatures and the effects of alloy composition on the liquidus temperature of Ru-containing superalloys were determined by differential thermal analysis (DTA), and the partitioning behavior of individual constituents under the influence of alloy chemistry was characterized using a quantitative segregation mapping technique combined with a Scheil type analysis.
Abstract: Ruthenium-containing multicomponent Ni-base superalloys with large variations in refractory alloying elements (Re, Ru, Ta, and W) have been investigated with respect to solidification, segregation characteristics, and the tendency to develop grain defects during directional solidification. Phase transformation temperatures and the effects of alloy composition on the liquidus temperature were determined by differential thermal analysis (DTA). The liquidus temperatures for most Ru-containing superalloys are generally higher than those of current commercial single-crystal superalloys. The partitioning behavior of individual constituents under the influence of alloy chemistry was characterized using a quantitative segregation mapping technique combined with a Scheil-type analysis. Whereas ruthenium partitioned preferentially to the dendrite cores during soldification, segregation of Ru is much less pronounced than Re and W. A higher degree of rhenium segregation was observed in Ru-containing superalloys. For the fixed processing conditions and moderate levels of Ru+Re, single-crystal solidification occurred without freckle formation or convection-induced breakdown of the solidification front. However, with high levels of Ru (9.6 ∼ 14.1 wt pct) and Re (7.2 wt pct), grain defects or the complete breakdown of single-crystal solidification was observed. Results from segregation and DTA analyses were used to estimate the corresponding Rayleigh numbers present during solidification of the experimental alloys. The Rayleigh criterion is effective for predicting the conditions under which the grain defect formation occurs during directional solidification of Ru-containing superalloys.

Journal ArticleDOI
TL;DR: In this paper, the crystal growth in the rapid solidification process was revealed in detail and the peak profile was systematically analyzed in order to acquire the essential information for controlling the weld microstructure.
Abstract: Characterizing of directional solidification mode of Fe-Cr-Ni alloys during welding was performed using intense synchrotron radiation probe. Consequently, the crystal growth in the rapid solidification process was revealed in detail and the peak profile was systematically analyzed in order to acquire the essential information for controlling the weld microstructure. Then, the crystallization timing of the primary and secondary phases was discussed. Furthermore, the possibility of the lattice rotation to commensurate (i) each dendrite and (ii) crystallites in the dendrites is suggested.

Journal ArticleDOI
TL;DR: Using synchrontron microradiography, temperature gradient zone melting (TGZM) was observed in Sn-13 wt pct Bi alloy in real time during directional solidification as mentioned in this paper.
Abstract: Using synchrontron microradiography, temperature gradient zone melting (TGZM) was observed in Sn-13 wt pct Bi alloy in real time during directional solidification A significant amount of remelting was measured on the cold sides of the dendrite arms, whereas added solidification on the hot sides of the dendrite arms was observed during dendrite growth Kinetics of TGZM was measured based on the real-time observations TGZM had a significant effect on dendrite morphology during continuous cooling and holding within the solidification range The presence of tertiary dendrite arms enhanced the rate of TGZM Remelting also led to the disintegration of some secondary dendrite arms

Journal ArticleDOI
TL;DR: In this article, in situ characterization of directional solidification process during welding was carried out using the time resolved X-ray diffraction technique utilizing intense synchrotron radiation, and the behavior of dendrites in steels under welding conditions of a practical manufacturing process were investigated using the TRXRD method for in-situ weld observation with the uniquely-sensitive two-dimensional pixel detector.
Abstract: In situ characterization of directional solidification process during welding was carried out using the time resolved X-ray diffraction technique utilizing intense synchrotron radiation. Then, behaviour of dendrites in steels under welding conditions of a practical manufacturing process were investigated using the TRXRD method for in-situ weld observation with the uniquely-sensitive two-dimensional pixel detector. Consequently, the crystal growth during the rapid cooling was caught in detail and employed a systematic peak profile analysis in order to acquire the essential information for controlling the weld microstructure. Our results would suggest the microstructure formation process of low alloy in directional solidification during rapid cooling. Simultaneously, we discuss the possibility of detecting the nucleation.

Journal ArticleDOI
TL;DR: It was demonstrated that the temperature gradient influences interface undercooling in the same way as the growth rate and may be used as an additional parameter to control fiber spacing and diameter.
Abstract: The effects of the solidification parameters, such as growth rate and temperature gradient, on the distance and diameter of Re nanowires have been examined. Both the spacing and diameter increase with decreasing growth rate and temperature gradient, respectively. The ratio of fiber spacing to diameter is 9.1. In addition, it was demonstrated that the temperature gradient influences interface undercooling in the same way as the growth rate and may be used as an additional parameter to control fiber spacing and diameter.

Journal ArticleDOI
TL;DR: In this paper, the influence of thermosolutal convection on the tertiary dendrite arm spacing was experimentally examined in the downward vertical unsteady-state directional solidification of Al-Cu and Al-Si hypoeutectic alloys.

Journal ArticleDOI
TL;DR: In this paper, cell spacing and solidification thermal variables were determined for three dilute Pb-Sb alloys, which were solidified under unsteady state heat flow conditions.

Journal Article
TL;DR: In this paper, a new facility has been developed to investigate the directional solidification of transparent aqueous solutions forming mushy layers in a quasi-two-dimensional system, where experiments have been conducted on NaCl-H2O solutions by translating a Hele-Shaw cell at prescribed rates between fixed heat exchangers providing a temperature gradient of approximately 1°C mm−1.
Abstract: A new facility has been developed to investigate the directional solidification of transparent aqueous solutions forming mushy layers in a quasi-two-dimensional system. Experiments have been conducted on NaCl–H2O solutions by translating a Hele-Shaw cell at prescribed rates between fixed heat exchangers providing a temperature gradient of approximately 1°C mm−1. The mush–liquid interface remained planar at all freezing velocities larger than 8 μm s−1, while steepling occurred at lower velocities. No significant undercooling of the mush–liquid interface was detected at freezing velocities up to 12 μm s−1. Mathematical predictions of the steady-state temperature profile and mushy-layer thickness as functions of freezing rate are in excellent agreement with experimental measurements.

Journal ArticleDOI
01 Oct 2006-EPL
TL;DR: In this paper, the growth directions of cells and dendrites into the melt vary with the pulling velocity from the direction of the thermal gradient to that of a principal crystalline axis.
Abstract: In directional solidification, the growth directions of cells and dendrites into the melt vary with the pulling velocity from the direction of the thermal gradient to that of a principal crystalline axis. We experimentally determine them in a succinonitrile alloy in the whole available range of variables. Data reveal a non-linear collapse onto a master curve that synthetizes the orientational response of cells and dendrites to the growth conditions.

Journal ArticleDOI
TL;DR: In this paper, the effect of grain boundary fraction on hot tearing during directional solidification was explored and the increase of GB fraction was found to reduce the hot tearing tendency, which was attributed to the uniform distribution of strain due to the presence of more GBs and the stronger GB cohesion because of the larger bridging areas.

Journal ArticleDOI
TL;DR: In this article, a comparison between upward and downward transient metal/mold heat transfer coefficients is conducted, based on comparisons between experimental data and theoretical temperature profiles furnished by a numerical solidification model, which applies finite volume techniques.
Abstract: Aluminum alloys with silicon as a major alloying element consist of a class of alloys which provides the most significant part of all shaped castings manufactured. This is mainly due to the outstanding effect of silicon in the improvement of casting characteristics, combined with other physical properties such as mechanical properties and corrosion resistance. In general, an optimum range of silicon content can be assigned to casting processes. For slow cooling rate processes (sand, plaster, investment), the range is 5 to 7 wt%; for permanent molds, 7 to 9%; and for die castings, 8 to 12%. Since most casting parts are produced considering there is no dominant heat flow direction during solidification, it seems to be adequate to examine both upward and downward growth directions to better understand foundry systems. The way the heat flows across the metal/mold interface strongly affects the evaluation of solidification and plays a remarkable role in the structural integrity of castings. Gravity or pressure die casting, continuous casting, and squeeze casting are some of the processes where product quality is more directly affected by the interfacial heat transfer conditions. Once information in this area is accurate, foundrymen can effectively optimize the design of their chilling systems to produce sound castings. The present work focuses on the determination and evaluation of transient heat transfer coefficients from the experimental cooling curves during solidification of Al 5, 7, and 9 wt% Si alloys. The method used is based on comparisons between experimental data and theoretical temperature profiles furnished by a numerical solidification model, which applies finite volume techniques. In other words, the resulting data were compared with a solution for the inverse heat conduction problem. The necessary solidification thermodynamic input data were obtained by coupling the software ThermoCalc Fortran interface with the solidification model. A comparison between upward and downward transient metal/mold heat transfer coefficients is conducted.