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Constitutional supercooling and corresponding microstructure transition triggered by high magnetic field gradient during directional solidification of Al-Fe eutectic alloy
Abstract
Directional solidification experiments of eutectic AlFe alloys were carried out under different high magnetic field gradients. High magnetic field gradient changed the microstructure selection during solidification process, induced the solidification microstructure to undergo eutectic instability, cellular transition, single-phase instability, and dendrite transition, which is similar to the planar-cellular-dendritic crystal growth pattern transition usually induced by an increasing growth velocity without magnetic field. The single phase was proved to be formed through an independent nucleation mechanism during single-phase instability. The effect of high magnetic field gradient on solidification was analyzed. Through the coupling effect of magnetic force and Lorentz force on solute migration and diffusion during solidification, high magnetic field gradient triggered a constitutional supercooling at the front of solid-liquid interface, which led to the growth pattern transition. A solidification model of eutectic alloy under high magnetic field gradient was proposed, and the microstructure selection map of AlFe alloy under gradient magnetic field was qualitatively drawn for the first time. This work proved that an alloy material with a microstructure usually obtained at high growth velocity can be obtained at a low growth velocity by applying a high magnetic field gradient, and suggested a new potential method for the future controlling of alloy structures and properties.
... Al-Cu 0-10 Eutectic spacing; microstructure evolution, orientation of Al 2 Cu; interface shape [14][15][16][17] Nb-Fe 1-5 Phase separation [18] Cu-Ag 0-200 mT, 0-31.2 TEM convection in the melt; microstructure, electrical resistivity, magnetoresistance [19,20] Al-Si 0-12 Distribution of primary silicon under uniform and gradient magnetic field; arm spacing, eutectic lamellar spacing; refinement of primary silicon [21][22][23] Al-Mg 0, 8.8 Solute segregation [24] Al-Fe 0-12 Distribution of primary phase [25][26][27][28] Al-Ni 0-2 Microstructural evolution [29] Al-Zn 0, 5 Dendrite morphology and growth orientation [30] Sn-Bi 0-0.55 TEM convection in the melt; microstructural evolution and solute distribution; refinement of arm spacing [31][32][33] ...
... In particular, recent studies have focused on the use of a gradient high SMF to control the microstructure of ETAs during solidification, with the aim of solving solute segregation issues and designing new microstructures. Solidification of ETAs, including Al-Si [21], Al-Mg [24], Al-Fe [25,26,28], and Mn-Sb [8,60], has been performed under the gradient high SMF. These investigations demonstrated that both solutes and primary particles could be successfully controlled through the magnetic force, as shown in Figure 8. ...
... supercooling caused by gradient high SMF, as shown in Figure 8e-g [28]. In Mn-89.7wt.%Sb ...
Processing metallic alloys under a static magnetic field (SMF) has garnered significant attention over the past few decades. SMFs can influence both the thermodynamics and kinetics of the solidification process by introducing extra force and energy. Eutectic-type alloys (ETAs) are commonly used as research materials under SMFs due to their featured microstructures. This review aims to present theoretical and experimental results regarding ETAs under SMFs, from post-analysis to in situ observation, to demonstrate the effects of magnetic phenomena such as magnetic braking, thermoelectric magnetic convection, magnetic gradient force, and magnetic energy on the thermodynamics and kinetics of microstructural evolution. In this paper, we adopt a hybrid approach between a review and an overview to comprehensively examine the effect of SMFs on the solidification process. Firstly, we provided a concise review of the historical research on the SMF’s impact on solidification in the literature. Next, we elucidated the basic physical principles of an SMF in material processing, followed by an introduction of numerous laboratory and industrial experiments that have utilized SMFs. Finally, we summarized the effects of SMFs on solidification in the past and provide insights into future research directions.
... Various techniques have been explored to mitigate the negative effects of Fe-rich phases in Al alloys and to increase the production of recycled Al alloys. These methods include 1) diluting the Fe concentration by adding high-purity Al; 2) using physical fields, such as ultrasonic and electromagnetic fields, to refine the Fe-rich phase during alloy solidification [14][15][16]; 3) modifying the morphology of the Fe-rich phase through the addition of the rare earth (RE) elements and neutralizing elements like Mn and Cr [17][18][19]; 4) increasing the cooling rate [20,21]. However, it is essential to note that Fe is not always a harmful element in Al alloy, and intentionally adding an appropriate amount of Fe can enhance the yield strength, tensile strength, and high-temperature mechanical properties of alloys [22,23]. ...
Hard and brittle Fe-rich phases are formed during the solidification of recycled Al-Cu alloys, significantly decrease the mechanical properties of the alloy. In this study, optical microscopy (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), X-ray diffraction (XRD), synchrotron X-ray imaging and thermodynamic calculations were used to investigate the microstructure evolution influenced by ultrasonic melt processing (USMP) and cooling rate during solidification. The results show that increasing the cooling rate after USMP significantly refines the α-Al grain size and inhibited the growth of Fe-rich phase, Al2Cu and pores. The three-dimensional (3D) morphologies of the Fe-rich phase and Al2Cu changed from coarse dispersion to fine compact, their local thickness and mean radius decrease. The observed effects are attributed to the fact that both USMP and increased cooling rate reduce α-Al grain size, leading to the growth of the eutectic Fe-rich phase and Al2Cu within the confined spaces between the α-Al dendrites during the late solidification stage. The cooling rate of 1.2Fe alloy increases from 0.1 °C/s to 1.5 °C/s, there is a rise in the number of Fe-rich phases, accompanied by a decrease in their dimensions. Meanwhile, the increased cooling rate inhibits the diffusion of Fe elements, which in turn promotes the growth of the Fe-rich phase. Also, an increased increasing the cooling rate after USMP can inhibit the growth of pores, especially in 0.7FeU alloy, where the pores basically disappear.
... Interestingly, from the ingots, the tendency of CrFeB phase dendritic growth was more and more evident. It was related to the migration and diffusion of solute during solidification, which was in line with the classical theory of "constitutional supercooling" proposed by Thiller [29]. In Cr0 ingot, the FeB phase presented in large plate-like shape (Figure 1a). ...
The effect of the Cr element on the corrosion behavior of as-spun Fe 72−x Cr x B 19.2 Si 4.8 Nb 4 ribbons with x = 0, 7.2, 21.6, and 36 in 3.5% NaCl solution were investigated in this work. The results show that the glass formability of the alloys can be increased as Cr content (c Cr) is added up to 21.6 at.%. When c Cr reaches 36 at.%, some nanocrystals appear in the as-spun ribbon. With increasing c Cr content, the corrosion resistances of as-spun Fe-based ribbons are continually improved as well as their hardness properties; during the polarization test, their passive film shows an increase first and then a decrease, with the highest pitting potential as c Cr = 7.2 at.%, which is confirmed by an XPS test. The dense passivation film, composed of Cr 2 O 3 and [CrO x (OH) 3−2x , nH 2 O], can reduce the number of corrosion pits on the sample surface due to chloride corrosion and possibly be deteriorated by the overdosed CrFeB phase. This work can help us to design and prepare the highly corrosion-resistant Fe-based alloys.
... During solidification, the bulges of the solidification interface gradually transform into cellular crystals and primary dendrites. With the continuous extension of the bulge tips into the supercooled liquid phase, the lateral bulges transform into secondary dendrites at the solidification interface of primary dendrites [51]. Simultaneously, Nb and Mo elements are enriched at the solid-liquid interface, leading to the precipitation of the Laves phase in the inter-dendritic regions. ...
... Magnetic field has been widely used to material research as a kind of cold physical field [7], which can apply magnetising energy to the atomic scale of matter in a contactless form [8], and has distinguished effect on solute transport, solid phase transformation and crystal orientation of the alloy during solidification [9][10][11][12]. Therefore, the application of magnetic field in the directional solidification process of metal materials can not only control the solidification process but also improve the microstructure and performance. ...
In order to reveal the effect of the magnetic field on the microstructure and the martensitic transformation of directional solidified MnNi alloy, four specimens were prepared by applying different magnetic fields during the directional solidification. The results show that the external magnetic field induces the formation of α-Mn phase, changes the orientations of dendrites as well as increases the dislocation density. The external magnetic field eliminates the fco→fct2 phase transformation and retains the fct→fcc transformation, meanwhile has little effect on the antiferromagnetic transition. The internal friction from twin boundary and phase transformation of directionally solidified MnNi alloy is 3–4 times that of as-rolled MnNi alloy, showing excellent high damping capacity in a wide temperature range.
... For instance, a variety of appealing phenomena can be achieved by applying the MF in casting, including magnetic orientation [23,24], magneto-thermodynamics [25] and magnetohydrodynamics (MHD) [25][26][27]. Therefore, many types of MF, including static MF [28][29][30][31][32], travelling MF [33,34], rotating MF [35][36][37] and pulsed electromagnetic fields [38][39][40], have been adopted in metallic materials processing. ...
Additive manufacturing (AM) offers unprecedented design freedom and manufacturing flexibility for processing complex components. Despite the numerous advantages of AM over conventional manufacturing methods, there are still some issues and bottlenecks that hinder the wide-scale industrial adaptation of AM techniques. The emerging field-assisted additive manufacturing (FAAM) is a designation that combines different auxiliary energy fields (e.g., ultrasound, magnetism, etc.) to overcome limitations in AM by benefiting from the intrinsic advantages of auxiliary fields. This work provides an up-to-date and dedicated review of FAAM in metallic materials, assisted by mainstream auxiliary magnetic, acoustic, mechanical, and thermal fields, as well as some emerging fields. The work principle and interaction mechanism between the field and the deposited metallic materials are elucidated. The auxiliary fields can affect the melt pool convection and dynamics, alter the temperature profile and thermal history during material solidification and induce stress or plastic deformation to the deposited materials. Hence, the effects of the auxiliary fields on the melt pool dynamics, solidification kinetics, densification behaviour, microstructure and texture, mechanical properties and fatigue performance are reviewed and discussed in detail. The perspectives on the research gap and further development trends of FAAM are also discussed.
... Control of microstructure is essential based on the understanding of solidification behavior in liquid metal processing to obtain high strength and good ductility [1][2][3] . The solidification behavior of liquid metal is influenced by various factors, such as composition, heat flow in the solidifying system, and quality of liquid metal. ...
Grain refinement is a crucial issue in metallic materials. One of the emerging techniques to obtain equiaxed grains is to apply an electric current to the liquid metal during solidification. With this view, in this paper, the effect of electric current on the solidification behavior in various cavity shapes of mold was investigated. Cylinder-, cube-, and cuboid-shaped cavities designed to have similar cavity volume were used. By applying an electric current during the solidification of liquid aluminum, the grains were effectively refined with a grain size of approximately 350 µm for all three types of cavities. The circulating flow of liquid aluminum was observed to have a similar shear rate intensity in all three types of cavities, which is known to be sufficiently high (over hundreds of s⁻¹) to induce dendrite fragmentation resulting newly generated nuclei. Dispersion of nuclei on unsolidified aluminum appeared differently according to the shape of the cavity, which influences final shape of refined zone. The area fraction of refined zone was affected by the relative relationship between the solidification completion time and the electric current application time. This study will provide insight to control of process parameters when electrically-assisted solidification is applied to a real product with a complex shape.
The effect of static magnetic field (SMF) on the solid–liquid interfacial free energy of Al–Cu alloy system was experimentally determined by an improved grain boundary groove method. The value of the solid α-Al-liquid Al–Cu system was found to increase from (157.16±14.14) × 10⁻³J•m⁻² to (206.65±18.60) × 10⁻³J•m⁻², with an increasing rate of 31.5%, while that of the solid CuAl2-liquid Al–Cu system decreased by 46.17% from (280.70±25.26) × 10⁻³J•m⁻² to (151.11±13.56) × 10⁻³ J•m⁻² with SMF. The influence of SMF on the value of Al-Cu alloy system was explained as the formation of magnetic dipole on the solid-liquid interface, which was supposed to change the surface tension.
Although nanostructured (NS) and ultrafine-microstructure (UFM) materials possess excellent mechanical properties, how to manufacture bulk NS and UFM materials with minimum cost via direct casting remains challenging. Here, we report an efficient strategy to prepare bulk UFM alloys via the direct solidification method for eutectic high-entropy alloy (EHEA). As a proof of concept, we designed CoCrFeNiNb0.45 EHEA and achieved complete bulk UFM ingot with weight of 2.5 kg, demonstrating an ultrafine-lamellar microstructure. The as-cast alloy shows outstanding thermal stability and hardness up to 1100 °C.
We present a review of the principal developments in the evolution and synergism of solute and particle migration in a liquid melt in high-gradient magnetic fields and we also describe their effects on the solidification microstructure of alloys. Diverse areas relevant to various aspects of theory and applications of high-gradient magnetic field-controlled migration of solutes and particles are surveyed. They include introduction, high-gradient magnetic field effects, migration behavior of solute and particles in high-gradient magnetic fields, microstructure evolution induced by high-gradient magnetic fieldcontrolled migrations of solute and particles, and properties of materials modified by high-gradient magnetic field-tailored microstructure. Selected examples of binary and multiphase alloy systems are presented and examined, with the main focus on the correlation between the high-gradient magnetic field-modified migration and the related solidification microstructure evolution. Particular attention is given to the mechanisms responsible for the microstructure evolution induced by highgradient magnetic fields.
High magnetic field (HMF) and solidification processes were changed during the solidification of both Cu-28 mass %Ag and Cu-72 mass %Ag alloys. The results indicated that the eutectic morphology in Cu-Ag alloys was affected by HMF, composition and solidification parameters. The lamellar spacing of Cu-28 mass %Ag alloy solidified by furnace-cooling was refined by the application of HMF owing to the decreased diffusion coefficient in mushy zone. The lamellar spacing in both Cu-28 mass %Ag sample held at the eutectic temperature and Cu-72 mass %Ag sample by slow controlling cooling was increased by HMF, which might be attributed to the dominated thermolectromagnetic convection. The lamellar spacing in Cu-72 mass %Ag alloys was increased compared with that of Cu-28 mass %Ag alloys because of the decreased growth rates. In Cu-28 mass %Ag alloy, however, fluid transverse velocity gradient was dominate rather than the growth rate and the imposition of HMF had reverse influences on the lamellar spacing. The lamellar-rod transition of Cu phase was promoted by HMF because of the increased Cu volume fraction in eutectic component. These results shed light on the dependence of eutectic morphology in Cu-Ag alloys on composition, external high magnetic field and solidification parameters.
Cooling rate is an important and controllable variable in casting processing. The effect of cooling rate on the microsegregation and Laves phase in INCONEL718 superalloy castings was studied by high-temperature–laser confocal scanning microscopy and quantitative metallography in this study. The transformation rate of solid phase with a feature of Gaussian distribution in the solidifications at the cooling rates of 0.10 to 14 K/s is acquired. The solidification time and secondary dendrite arm spacing (SDAS) as a function of cooling rate are analyzed. The amount of Laves phase presents a maximum value at a threshold cooling rate of 3 K/s owing to the opposite effects of cooling rate on the solidification time and SDAS. A modified dimensionless microsegregation index criterion was used for the scaling of solute segregation and Laves phase depending on cooling rates. The prediction of maximal microsegregation and the amount of Laves phase by MSI and experiments provide a guide for cooling rate control in the casting applications.
Efficient metallurgical and processing development requires ability to predict the solidification microstructure of products. Models were developed to calculate possible growth morphologies and phases in order to select the most efficient one for a given solidification condition. It is then possible to predict solidification microstructure selection maps (SMSM) indicating the microstructure as a function of the composition and the growth rate. A summary of the methodology of solidification microstructure prediction is given. Results obtained in a large velocity range for Al-Fe and Al-Cu systems are reviewed with experimental comparison and emphasis on the role played by metastable phase equilibria in the modelling. Finally, an outline of the actual work extending this approach to multicomponent alloys is given. Provisional calculated solidification microstructure maps for the Al-Cu-Si system are presented.
Casting alloys of eutectic composition are of great practical importance. Good microstructural control of these alloys is essential in obtaining the required properties in the cast state. Primary phases present in a eutectic matrix may have either a beneficial or a detrimental effect on these properties. Therefore, their appearance must be carefully controlled. In this review the authors describe the fundamentals and recent advances in the understanding of the extent of eutectic growth – the coupled zone. Because of the inherent difficulties and the present inapplicability of stability analyses to predicting eutectic to eutectic plus dendrite transitions, the simpler ‘competitive growth’ approach is presented in this review. As this involves separate consideration of eutectic and dendrite growth, the theory of these growth forms for temperature gradients is first reviewed. The competitivegrowth principle is then applied, i.e. the principle that the morphologies appearing in the microstructure, under given conditions, will be those having the highest interface temperature or the highest growth rate (depending on whether growth rate or undercooling is the controlled variable). On the basis of this theory, a modified classification of eutectics is proposed. Mter discussion of the experimental techniques available for determining the form and extent of the coupled zone, the experimental results are compared with theory. Conclusions are drawn concerning the reasons for the various forms of coupled zone.
Directionally solidified samples of Mg-32.3wtpct Al eutectic alloy were produced under an argon atmosphere in a vacuum Bridgman-type
furnace to study the eutectic growth with different growth velocities. Typical features such as steady-state lamellar eutectic
growth, lamellar branching at the quenching interface, and the formation of colony structures due to the impurity of the Mg-Al
binary alloy were observed using a JEOL 6301F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The lamellar spacing
of the two eutectic phases was measured on the transverse sections of the samples. It was found that the relationship between
the measured lamellar spacing and growth velocity agreed well with the prediction of the Jackson-Hunt model. Subsequent studies
of Mg-Al eutectic growth were conducted using a numerical model based on the cellular automaton (CA) method. Taking account
of the solute diffusion, constitutional undercooling, and curvature undercooling, modeling of steady-state lamellar eutectic
growth was achieved. A systematic investigation of the eutectic growth morphology and lamellar spacing of the Mg-Al eutectic
was carried out under directional solidification with different undercoolings, initial lamellar spacings, temperature gradients,
and growth velocities. The results showed that under the interaction between solute diffusion and surface energy, the adjustment
of eutectic lamellar spacing was accomplished by nucleation, lamellar branching, lamellar termination, and overgrowth. The
simulated results were consistent with both the experimental results and the Jackson-Hunt eutectic theory.
Directionally solidified (DS) intermetallic and ceramicbased eutectic alloys with an in-situ composite microstructure microstructure containing finely distributed, long aspect ratio, fiber, or plate reinforcements are being seriously examined for several advanced aero-propulsion applications. In designing these alloys, additional solutes need to be added to the base eutectic composition in order to improve their high temperature strength, and provide for adequate toughness and resistance to environmental degradation. Solute addition, however, promotes instability at the planar liquid-solid interface resulting in the formation of two-phase eutectic “colonies.”[1–4] Because morphology of eutectic colonies is very similar to the single-phase cells and dendrites, the stability analysis of Mullins and Sekerka[5] has been extended to describe their formation.[6,7,8] Onset of their formation shows a good agreement with this approach;[9] however, unlike the single-phase cells and dendrites, there is limited examination of their growth speed dependence of spacing, morphology, and spatial distribution.[4,10–11] The purpose of this study is to compare the growth speed dependence of the morphology, spacing, and spatial distribution of eutectic cells and dendrites with that for the single-phase cells and dendrites.
A 12-T magnetic field has been applied to a medium plain carbon steel during the diffusional decomposition of austenite and the effect of a high magnetic field on the distribution of misorientation angles, grain boundary characteristics and texture formation in the ferrite produced has been investigated. The results show that a high magnetic field can cause a considerable decrease in the frequency of low-angle misorientations and an increase in the occurrence of low Σ coincidence boundaries, in particular the Σ3 of ferrite. This may be attributed to the elevation in the transformation temperature caused by the magnetic field and, therefore, the reduction of the transformation stress. The wider temperature range for grain growth offers longer time to the less mobile Σ boundaries to enlarge their areas. Moreover, the magnetic field can enhance the transverse field-direction fiber (〈0 0 1〉∥TFD). It can be assumed that the effects of the field were caused by the dipolar interaction between the magnetic moments of Fe atoms.
The nature of liquid structures with their changing behaviors remains an unsolved fundamental problem in many fields of science and technology. It has been widely accepted that liquid structures change gradually with temperature and/or pressure. With x-ray diffraction in the melt In-Sn80, however, we have confirmed that a temperature-dependent discontinuous structural change could occur in some binary liquids, which does not fall into any other up-to-date recognized liquid-liquid changes. This finding, together with the recently recognized pressure-induced liquid change, suggests that the conventional view on liquids should be revised.
In this work, different heat treatments were performed in order to study two different bainite morphologies, granular and lath-like, obtained at different cooling rates in a low carbon microalloyed steel. For that purpose, advanced crystallographic characterization and quantitative metallography using SEM, EBSD and TEM were carried out. The evolution of the volume percentage, size, aspect ratio, retained austenite volume percentage and crystallography of the M/A constituents was studied as a function of the cooling rate and classified according to a machine learning algorithm. For the characterization of the crystallography and morphology of the bainitic ferrite matrix, two microstructures were selected as representative of granular and lath-like bainite. The results show that the main differences between granular and lath-like bainite are the size and shape of the M/A constituents, that change progressively from a mixture of large grains, coarse blocks and fine roughly equiaxed grains at low cooling rates to short and long films at high cooling rates. The crystallographic and morphological differences between both microstructures can be explained based on the different driving force and amount of bainitic ferrite formed at the different temperature ranges. These results confirm that the mechanisms by which granular bainite forms are not different from the transformation mechanisms of lath-like bainite.
Using Tb0.27Dy0.73Fe1.95 as a model alloy, we generate (Tb, Dy)Fe2 and (Tb, Dy)Fe3 phases with preferred orientations. We study the mechanism that determines the preferred crystal orientation of a peritectic alloy under high magnetic fields. Adjusting the preparation parameters yields four Tb0.27Dy0.73Fe1.95 alloys with respective 〈110〉, 〈111〉, 〈112〉, and 〈113〉 directions of the peritectic (Tb, Dy)Fe2 phase parallel to the high magnetic field. The <112¯0>, 〈0001〉, <101¯0>, and < 101¯3¯> directions of the corresponding primary (Tb, Dy)Fe3 phase are parallel to the magnetic field. We deduce the orientation relationships (1¯11)//(0001) and [110]//[112¯0] for the (Tb, Dy)Fe3 and (Tb, Dy)Fe2 phases. The growth velocity and high magnetic field do not change the orientation relationships of these two phases. The high magnetic field induces the rotation of the primary (Tb, Dy)Fe3 phase and then affects the orientation of the peritectic (Tb, Dy)Fe2 phase. Magnetic torque induces stacking of the {0001} plane perpendicular to the magnetic field, meaning the c-axis of the primary (Tb, Dy)Fe3 phase aligns parallel to the field. At this time, the {111} plane of the (Tb, Dy)Fe2 phase grows perpendicular to the magnetic field, as reflected in the preferred orientation of the (Tb, Dy)Fe2 phase along the <111> direction.
The giant magnetostriction exhibited by pseudobinary Tb–Dy–Fe compounds has attracted considerable attention for use in magneto-mechanical actuators and sensors. However, simultaneously producing a crystallographic orientation and a morphological alignment of the (Tb,Dy)Fe2 phase along the ⟨111⟩ direction has proven difficult and inhibits further increase in the desired property. This work demonstrates that, by coupling the directional solidification and a high magnetic field, a ⟨111⟩-orientation and -alignment were simultaneously created. In addition, the pores and the defects in the alloys were eliminated, leading to an enhancement of the magnetostrictive performance. Analyses indicate that the controlled growth of the (Tb,Dy)Fe2 crystal was owing to the collaboration of the multiple magnetic field effects on both the liquid and the solid phases during the directional solidification. Specifically, the magnetic torque induced a rotation of the crystals aligning their easy axis of magnetization (i.e., ⟨111⟩) along the magnetic field direction. Further, the Lorentz force stabilized the directional growth of the crystals by suppressing the convection, while the magnetic force exerted a compressive stress on the paramagnetic alloy melt to remove the gases in the melt. As a result, a highly ⟨111⟩-oriented and -aligned and defect-free Tb–Dy–Fe compound was produced. This strategy may also be expanded to other alloy systems whose phases exhibit a magnetic anisotropy and thereby fabricate anisotropic functional compounds.
The morphological instability of solid/liquid (S/L) interface during solidification will result in different patterns of microstructure. In this study, two dimension (2D) and three dimension (3D) in-situ observation of solid/liquid interfacial morphology transition in Al-Zn alloy during directional solidification were performed via X-ray imaging. Under a condition of increasing temperature gradient (G), the interface transition from dendritic pattern to cellular pattern, and then to planar growth with perturbation was captured. The effect of solidification parameter (the ratio of temperature gradient and growth velocity (v), G/v) on morphological instabilities was investigated and the experimental results were compared to classical “constitutional supercooling” theory. The results indicate that 2D and 3D evolution process of S/L interface morphology under the same thermal condition are different. It seems that the S/L interface in 2D observation is easier to achieve planar growth than that in 3D, implying higher S/L interface stability in 2D thin plate samples. This can be explained as the restricted liquid flow under 2D solidification which is beneficial to S/L interface stability. The in-situ observation in present study can provide coherent dataset for microstructural formation investigation and related model validation during solidification.
Systematic understanding on the morphology and magnetic alignment of the primary α-Co phase during non-equilibrium solidification of a Co-B eutectic alloy were carried out. Under an imposed magnetic field, a morphological alignment was found for the primary α-Co phase with its primary dendrite trunk or long axis paralleling to the direction of magnetic field. The primary α-Co phases are rod-like and spherical at relatively high undercooling, and the application of magnetic field is more conducive to obtain such kind of α-Co phases. The magnetic energy, magnetic torque and time required for rotation were analyzed theoretically to evaluate the magnetic alignment and alignment mechanisms. Subsequently, the dipolar forces between particles were calculated, based on which the phenomenon that the primary α-Co particles self-organize as chain-like stacking was described. The present research provides a better understanding of the strong magnetic field effects on non-equilibrium solidification and a potential technology of field-manipulation of ferromagnetic materials.
This paper investigates how applying high magnetic fields influences the crystallographic orientations of the primary and eutectic phases, and their relationship, in a binary eutectic alloy. At 0 T, the primary MnSb phase in hypoeutectic Mn–Sb showed a random orientation, but at 3, 6, 9 and 11.5 T, its c axis was perpendicular to the magnetic field direction. In all cases, the eutectic MnSb phases showed the same orientations as their neighboring primary MnSb phase, on which they nucleated and grew. With high magnetic fields, the c axes of the eutectic and primary MnSb phases were oriented perpendicular to the magnetic field direction. The results show that applying a high magnetic field during solidification is a way of controlling the crystallographic orientation of both the primary and the eutectic phases in eutectic alloys.
Phase selection, a common and important phenomenon in rapid solidification, determines the solidification sequence and the mechanical/physical properties. In this work, a Co-20 at%B hypereutectic alloy was undercooled by the melt fluxing technique and the microstructure was characterized by the back-scattering diffraction technique. A transition from hypereutectic to hypoeutectic was found at a critical undercooling of ΔT = 119 K. When ΔT < 119 K, a primary directional dendritic β-Co3B phase surrounded by the regular α-Co + β-Co3B lamellar eutectics was found. When ΔT > 119 K, the above hypereutectic microstructure changes into hypoeutectic structure with the α-Co phase as the primary phase. According to dendrite growth model, the transition from hypereutectic to hypoeutectic can be ascribed to the higher growth velocity of the α-Co phase than the β-Co3B phase, i.e., the growth-controlled mechanism. The current work shows also that there is a coupled zone skewed to the β-Co3B phase in the Co-B alloys system.
The effect of high magnetic fields on the solute diffusion behavior in the mushy zone of a Mn–Sb alloy has been investigated. A hypoeutectic Mn–Sb alloy was heated in high magnetic fields to yield a mixture of primary MnSb dendrites and Mn/Sb liquid to simulate the mushy state, annealed isothermally for various times, and quenched to room temperature. Without a magnetic field or with a 1-T field, the quenched alloy had a microstructure that was composed of MnSb particles that were dispersed in MnSb/Sb eutectics. At higher magnetic fields, a multiple-phase mixed microstructure that consisted of fine MnSb particles, bulk Sb, and MnSb dendrites and eutectic was observed at relatively short annealing times. The amounts of fine MnSb particles and bulk Sb decreased with an increase in annealing time but increased with an increase in magnetic-flux density. The appearance of the multiple-phase mixed microstructure can be attributed to solute interception in the mushy zone, which can change the solute distribution among the dendrite arms and which is caused by a high magnetic field that is based on the Lorentz force. The application of a high magnetic field to the mushy state of the alloys is proposed as an effective route for the in-situ control of solute diffusion in the mushy zone.
Eutectic Al-Ni alloys are widely faced as materials to be considered for advanced structural components. Nevertheless, still there is a lack of research on microstructural aspects of the eutectic Al-6.3 wt%Ni alloy and important points need to be addressed, such as, the morphology of the unsteady solidified eutectic, size and distribution of phases under different solidification cooling rates and the effects of the resulting microstructural features on mechanical properties. As such, in the present study, an Al-6.3 wt%Ni alloy is directionally solidified under a wide range of cooling rates and the resulting microstructure is shown to be formed by eutectic colonies. Three general microstructural features characterize the colony: a fine central zone composed of fibrous α-Al + Al3Ni eutectic, a boundary coarse zone formed by lamellar eutectic, and an Al-rich zone delimiting the colony. A quantitative analysis relating solidification thermal parameters to Al3Ni and colony spacings is outlined. Furthermore, the evolution of tensile properties as a function of these spacings was examined, and the highest strength and elongation of 160 MPa and 15%, respectively, are associated with an ultrafine bimodal structure formed by eutectic colonies with 55 μm in spacing containing very fine fibers (300 nm in spacing) and lamellae with 750 nm in spacing.
We investigated applying a magnetic field (up to 6 T) during directional solidification of a hypereutectic Al–8 wt. %Fe alloy, finding that it dramatically affected the final microstructure. A eutectic area appeared at the top of the samples, and as the magnetic flux density increased, the eutectic area clearly enlarged. In addition, the Al3Fe phase was twisted and fractured, and some phases aggregated and distributed randomly in the samples. We also investigated the volume fraction distribution of the Al3Fe phase, revealing that applying the magnetic field during solidification caused dramatic disorder in the solute and phase distributions. The magnetic force induced by the interaction between the magnetic field gradient and the magnetic materials appeared to be the main reason not only for the occurrence and enlargement of the eutectic area but also for the movement of Fe-enriched zones during directional solidification. Otherwise, the deformation and fracture of the Al3Fe phase, the morphological instability in the interface between the eutectic area and the Al3Fe phase, and the random distribution of the aggregated Al3Fe phase appeared to come from the thermoelectric magnetic force/thermoelectric magnetic convection under the magnetic field.
The conditions for fully eutectic growth in Al-Fe alloys at a temperature gradient of 20 K mm-1 are reported for ranges of composition from 2.2 to 6.1 wt % Fe and of growth velocity from 0.03 to 10 mm sec-1. All six main classes of growth structure (i.e. Al-Al3Fe or Al-Al6Fe eutectics either alone or together with primary αAl or Al3Fe) were obtained, some of them reported for the first time for steady-state conditions. Observed concentration-dependences both of the limiting growth velocity for primary Al3Fe and of the interphase spacing for the fully eutectic Al-Al6Fe displacing it are in good agreement with theory. Hardness levels for the Al-Al6Fe eutectic as a function of concentration are similar to those for αAl dendritic structures grown in much thinner sections under splat-cooling conditions. The significance of some observed transitions in growth morphology for eutectic cells, Al6Fe eutectic rods and αAl dendrites is discussed.
Eutectic solidification is very important in the development of new materials in which the periodic multiphase structures may have a remarkable or enhanced functionality. The morphology evolution during eutectic solidification is investigated experimentally using slab-geometry slides of succinonitrile-(D)camphor (SCN-DC) transparent organic eutectic material. By specifically focusing on the effect of pulling velocity on microstructure in directional growth, the temperature gradient and the thickness are kept the same in all the experiments. It is found that eutectic seeds first occur in the grain boundary channel or the specimen side-wall groove. And the growth of eutectic seeds is both parallel to the direction of temperature gradient and along the liquid/solid interface at the same time. At a low pulling velocity (0.064-0.44 mu m/s), the macroscopic growth morphology is flat, and the inner microstructure is rod-shaped, which is parallel to the growth direction. It is obvious that the eutectic spacing becomes smaller with the increase of pulling velocity. At a high pulling velocity (0.67-1.56 mu m/s), the macroscopic growth morphology becomes cellular. However, the inner microstructure is still rod-shaped, but its distribution is radially outward. And the eutectic spacing decreases as pulling velocity increases.
In this study Ni-Ni3Si eutectic in situ composites are obtained by Bridgman directional solidification technique when the solidification rate varies from 6.0 mu m/s to 40.0 mu m/s. At the low solidification rates the lamellar spacing is decreased with increasing the solidification rate. When the solidification rate is higher than 25 mu m/s, the lamellar spacing tends to be increased, because the higher undercooling makes the mass transport less effective. The adjustments of lamellar spacing are also observed during the directional solidification process, which is consistent with the minimum undercooling criterion. Moreover, the transitions from planar interface to cellular, then to dendritic interface, and finally to cellular interface morphologies with increasing velocity are observed by sudden quenching when the crystal growth tends to be stable. Copyright
Recently, the studies on the effects of high magnetic fields on solidification processes have been paid much attention both from the fundamental and applied points of view. With the aid of the enhanced Lorentz force and magnetization effect caused by the remarkably increased magnetic field intensity, several interesting phenomena, such as the control of fluid flow and particle migration in a melt, crystal orientation, and phase alignment, have been obtained. Moreover, the magnetic force induced by the interaction of magnetization and high magnetic field gradient has been evidenced to show significant effects on the microstructure evolution of alloys. In this paper, the recent development of the control of the solidification process by high magnetic fields is reviewed from the view point of uniform magnetic fields and magnetic field gradients.
Coupling with the thermodynamic database and incorporating the effects of local non-equilibrium and off-diagonal diffusional interactions, an extended morphological stability analysis was performed for a planar interface upon rapid solidification of concentrated multi-component alloys. Compared with the previous work, the model is able to clarify the effects of concentration dependence and diffusional interactions on the interface stability. Taking the Al–Mg–Zn alloy as an example, the neutral stability, i.e. the critical Zn concentration (with Mg concentration fixed) for the breakdown of the planar interface, was studied. The stability mechanisms for the neutral Zn concentration subjected to different Mg concentrations and the off-diagonal diffusion effect were clarified. In particular, for high Mg concentrations, a stage of absolute instability dependent on the Mg diffusion effect but independent of the Zn concentration happens. Although the off-diagonal diffusion is several orders of magnitude smaller than the solute diffusion coefficients, it can significantly change the Zn concentration for the neutral stability conditions.
Most theoretical work on dendrite growth has focused on dilute binary alloys, while most industrial alloys are concentrated multi-component systems. By incorporating the local non-equilibrium effects both at the interface and in the bulk liquid, the thermodynamic database and diffusional interaction, a model was developed for dendrite growth in undercooled concentrated multi-component alloys. An experimental study of dendrite growth in undercooled Ni–18 at.% Cu–18 at.% Co melts was carried out and the measured interface velocities (V) were well predicted by the present model over the whole undercooling range (ΔT = 30–313 K). During dendrite growth the partition coefficients change non-monotonically due to interaction between the species and changes in the dendrite tip radius. Interaction between the species also leads to a lower interface velocity and larger ΔT and V as the ΔT–V relation plateaus. The previous definition of constitutional undercooling, i.e. the sum of the contributions of each solute, is not applicable to concentrated multi-component alloys. The controlling mechanisms during dendrite growth are discussed with respect to the results of the calculations.
The effect of a high axial magnetic field (up to 12 T) on the microstructure in a directionally solidified Al–Al2Cu eutectic alloy has been investigated experimentally. The results show that a high magnetic field decreases the eutectic spacing and degenerates the lamellar structure into a wavy one at a low growth speed. X-ray diffraction, selected-area electron diffraction and high-resolution electron microscopy analyses indicate that the field changes the preferred orientation. The Al2Cu crystal is oriented with the 〈0 0 1〉-crystal direction along the solidification direction (i.e., the magnetic field direction). At a pulling velocity of 0.5 μm/s, the magnetic field (B ⩾ 4T) is responsible for the segregation; which consists of Al striations on the longitudinal section and Al-rich zones on the transverse section. The effects of the field may be attributed to the orientation of the Al2Cu and the Al crystals and the decrease of the diffusion coefficient caused by the magnetic field.
The eutectic Sn–0.7 wt%Cu alloy is considered an important alternative to replace the classic eutectic Sn–Pb alloy, used to join metallic surfaces in electronic devices. The stable Sn–Cu eutectic is composed of a mixture of a tin-rich phase and fibrous Cu6Sn5 intermetallic particles. The morphology, size and distribution of stable and metastable intermetallic particles may affect the mechanical properties of the alloy. The distribution of these intermetallics is characterized by the interphase spacing, which depends on thermal parameters such as the growth rate (v) and the cooling rate (T˙) during solidification. The aim of this study is to investigate the microstructural evolution of a eutectic Sn–0.7 wt%Cu solder alloy during transient solidification. The resulting microstructural morphology depends on v and T˙, and in the case of soldering processes the control of these parameters is essential for the design of the final microstructure. A gradual cellular to dendritic transition was observed to occur for growth rates ranging from 0.3 to 0.5 mm s−1 and cooling rates from 0.9 to 1.5 K s−1. The cellular region was shown to be characterized by aligned eutectic colonies, and experimental growth laws relating cellular, dendritic and interphase spacings to both v and T˙ have been proposed.
As one of the semiconductor-metal eutectic (SME) composites, Si-TaSi2 eutectic composite has many characters such as the high melting point of TaSi2 material, the large density of TaSi2 fibers incorporated into the Si matrix, three-dimensional array of Schottky junctions grown in the composite spontaneously. So it is an ideal candidate for field emission array cathodes. In this paper, the directionally solidified Si-TaSi2 eutectic in situ composite for field emission is prepared by means of the electron beam floating zone melting (EBFZM) technique on the basis of Czochralski (CZ) method. The Si-TaSi2 eutectic in situ composite, which has high-aligned and uniformly distributed TaSi2 fibers in the Si matrix, can be obtained when the solidification rate changes from 0.3 to 9.0 mm/min. As the solidification rate is increased, both the fibers’ diameter and inter-rod spacing are decreased, while the fibers’ density and the volume fraction are increased. Moreover, the transition from a planar interface to cellular interface and then to planar interface morphologies with increasing velocity is observed with the zero power method.
In this work, Mn–Sb alloys were melted and held in various magnetic field gradients for various times. The corresponding distributions of the alloying elements in the alloys in liquid state were revealed by a quenching method. It was found that the primary MnSb showed a relatively uniform distribution in the alloy quenched in the magnetic field gradient of BdB/dz=0T2/m, while it was mainly distributed in the lower and upper part of the alloys quenched in negative and positive magnetic field gradients, respectively. The experimental results show that it is possible to in situ control the distributions of alloying elements in alloys in the liquid state using high gradient magnetic fields if there is a difference in magnetic susceptibility.
Bi–MnBi composite has been directionally solidified to investigate the influence of a strong magnetic field on its microstructure, growth characteristics and magnetic properties. Experimental results show that MnBi eutectic fiber is aligned, regularized and coarsened during directional solidification under an axial strong magnetic field. In particular, the alignment of irregular eutectic fiber under a magnetic field may offer a new method to prepare regular fiber composite and extend the range of uses of such composite. It has also been found that the strong magnetic field has modified the growth mode of the MnBi fiber and enhanced the transition from non-faceted to faceted. The distribution of the solute Mn in front of the MnBi fiber at the liquid–solid interface has been measured and the result shows that the magnetic field has enhanced the enrichment of the solute Mn in front of the MnBi fiber. Moreover, magnetic measurement reveals a pronounced promotion of the magnetic anisotropy and the remanence (Br) under the magnetic field. Based on these experimental results, a simple theory of solute enrichment in front of a ferromagnetic fiber and fiber coarsening under a strong magnetic field has been proposed.
Co–24.0at.% Sn eutectic alloy melt was undercooled to different degrees below the equilibrium eutectic temperature. The dependence of solidification behavior on undercooling was established based on the experimental results of the solidification microstructure, crystal orientation and crystal growth velocity. In the entire undercooling range studied (0–203K), coupled eutectic growth of the α-Co and β-Co3Sn2 phases invariably takes place during the rapid solidification stage. The eutectic solidification interface advances in the alloy melt in seaweed morphologies rather than the well-known dendritic mode due to the weak interface energy anisotropy. But a critical undercooling of 175K exists, from which the eutectic solidification interface changes from a fractal into a compact seaweed pattern, accompanied with an abrupt increase of growth velocity. At high undercoolings the enhancement of the interface tip stability with the rise of crystal growth velocity should be responsible for the growth mode transition.
Ag–Cu eutectic alloy was undercooled by the glass flux method and the solidification structure was investigated. It is revealed that when undercooling is not more than 70 K, the large difference in composition between two eutectic phases and very large thermal diffusion coefficient of the liquid result in cellular growth of the lamellar eutectics from the nucleation site. The variation in interface temperature during rapid solidification gives rise to a systematic change in microstructure within the sample. With the distance along the growth direction increasing, the finest lamellar spacing across the cellular eutectic rises, which indicates a gradually decreasing growth velocity of the primary eutectics. The primary lamellar eutectics near the nucleation site solidify under conditions far from equilibrium, and therefore are supersaturated with solute, and then partially remelted and ripened into anomalous eutectics. As undercooling increases, the area of the anomalous eutectics enlarges.
Effect of a 0.2-T static magnetic field on the microstructure of a direct chill cast Al-9.8wt%Zn alloy slab was investigated.
The static magnetic field transferred the microstructure from a mixture of equiaxed and columnar grains with the primary trunks
growing in directions to twinned lamellar feathery grains with the primary and secondary arms growing in directions.
The application of the static magnetic field results in the reduction of the heat discharge and solute mixing capacity through
a damping effect on convection and thus a delay of the melt transformation to solid and a request to reduce the liquid/solid
interface energy through reducing the interface area due to the loss of undercooling. The delay and the request account for
the growth direction change and the formation of lamellas. The difference between the Al and Zn atomic radii and the related
incoming flow facilitate the formation of the twins.
The solidification structure of Ag–Cu eutectic alloy at different undercoolings was investigated. A critical undercooling of 76K is revealed, at which a transition from cellular to dendritic growth of lamellar eutectic occurs, accompanied with an abrupt rise of growth velocity. It is suggested that the large difference in composition between the two eutectic phases and the very large thermal diffusion coefficient of the liquid result in the cellular growth of lamellar eutectic at low undercoolings.
The levitation in air of water, other diamagnetic substances and even living organisms was recently achieved by using the extremely strong magnetic field provided by a Bitter-type hybrid magnet. We too have succeeded in levitating water, but in the lower fields of an ordinary 10 T superconducting magnet. To achieve this we make use of gravitational and magnetically induced buoyancy forces in the host paramagnetic atmosphere (pressurized air or oxygen), rather than simply the diamagnetic force on the levitating object, to balance the gravitational force. This permits the magnetic levitation in air of paramagnetic as well as diamagnetic substances, which was widely believed to be impossible. The physics underlying this effect is essentially the same as that of magnetohydrostatic ore separation, where a ferromagnetic fluid is used. Because our process can levitate subtances at a stable position in an atmosphere, we have named it `magneto-Archimedes levitation'.
The solidification microstructures for Al-0.5–4 at.% Fe alloys under constrained growth conditions have been calculated using analytical models of the growth kinetics of dendritic, eutectic and plane front interface morphologies of stable and metastable phases. Laser remelting experiments are carried out on an Al-4at.% Fe alloy with a low beam velocity (10 mm/s) in order to complete previous experimental results on the solidification microstructures obtained at intermediate growth rates by Bridgman experiments and at high growth rates by rapid laser resolidification. Comparison of predicted with experimentally determined solidification microstructure maps shows satisfactory agreement in view of the limited knowledge of the thermophysical properties of this system. These maps are useful for the interpretation of microstructures and phases forming under medium to high solidification rates and for the understanding and development of rapid solidification processing. Further the modelling is useful for improving available phase diagram information.
Three binary Sn–Cu solder alloys of near-eutectic composition have been directionally solidified at different growth rates. The competition between primary tetragonal Sn cells/dendrites and eutectic is interpreted with the coupled zone concept. It is also found that Sn–Cu is a weakly irregular eutectic system with Cu6Sn5 leading the eutectic, but two different eutectic morphologies (coarse and fine) form simultaneously during eutectic growth. At higher growth rates, the eutectic interface breaks down into a cellular eutectic with the fine eutectic in the centre of the cells and the coarse one at the cell boundaries. This is explained by the segregation of Pb impurities ahead of the eutectic interface.
The effects of high magnetic fields on the solidification microstructure of Al–Si alloys were investigated. Al–7.2wt%Si and
Al–11.8wt%Si alloys were solidified in various high magnetic fields at different cooling rates. The secondary dendrite arm
spacing (SDAS) of the primary Al dendrites and the lamellar spacing (LS) of the eutectics were measured. It was found that
the application of a high magnetic field could decrease the SDAS of the primary Al dendrites in Al–7.2wt%Si alloys and the
LS of the eutectics in Al–11.8wt%Si alloys. The effects of the high magnetic field on the SDAS decreased with increasing
cooling rate. The decrease in the SDAS and LS can be attributed to the decrease of the solute diffusivity in the liquid ahead
of the solid/liquid interface during the growth of the dendrite and eutectic. This decrease is caused by the high magnetic
field which can damp the convection and avoid its contributions to the diffusion.
The local solidification conditions and mechanisms associated with the flake-to-fiber growth mode transition in Al–Si eutectic
alloys are investigated here using Bridgman-type gradient-zone directional solidification. Resulting microstructures are examined
through quantitative image analysis of two-dimensional sections and observation of deep-etched sections, showing three-dimensional
microstructural features. Several microstructural parameters were investigated in an attempt to quantify this transition,
and it was found that the particle aspect ratio is effective in objectively identifying the onset and completion velocity
of the flake-to-fiber transition, whereas traditional spacing parameters are not effective indicators of the transition. For
a thermal gradient of 7–14K/mm, the transition was found to occur in two stages, appearing over velocity regimes from 0.10
to 0.50mm/s and from 0.50 to 0.95mm/s. The initial stage is dominated by in-plane plate breakup and rod formation within
the plane of the plate, whereas the second stage is characterized by the onset of out-of-plane silicon rod growth, leading
to the formation of an irregular fibrous structure. The boundary between the two stages is marked by widespread fibrous growth
and the disappearance of the remnant flake structure, indicating a transition in the structural feature that governs the relevant
diffusion length, from inter-flake spacing to inter-rod spacing.
The stability of the shape of a moving planar liquid‐solid interface during the unidirectional freezing of a dilute binary alloy is theoretically investigated by calculating the time dependence of the amplitude of a sinusoidal perturbation of infinitesimal amplitude introduced into the planar shape. The calculation is accomplished by using gradients of the steady‐state thermal and diffusion fields satisfying the perturbed boundary conditions (capillarity included) to determine the velocity of each element of interface, a procedure justified in some detail. Instability occurs if any Fourier component of an arbitrary perturbation grows; stability occurs if all components decay. A stability criterion expressed in terms of growth parameters and system characteristics is thereby deduced and is compared with the currently used stability criterion of constitutional supercooling; some very marked differences are discussed.
The preparation and characterization of metastable phases of the Al–Fe alloy system by mechanical alloying are reported. In Al-rich (up to 10 at.% Fe) alloys, the supersaturated f.c.c. solid solution of Fe in Al (up to 1 at.% Fe) is formed. Almost complete amorphization is confirmed in the composition range 17–33 at.% Fe. The metastable disordered b.c.c. solid solution of about 10 nm in grain size has also been formed by ball-milling for over 180 h in Fe-rich (above 50 at.% Fe) alloys. Examination of lattice parameter and magnetization have shown that the composition range and degree of disorder are comparable to those formed by crushing and sputter deposition.
An as-quenched structural steel is tempered at 600 and 650 °C for 1 h without and with a 14-T magnetic field. The magnetic field can effectively prevent the directional growth of cementite along martensite plate boundaries and twin boundaries by increasing both the cementite/ferrite interfacial energy and the magnetostrictive strain energy. Finally, particle-like cementite is obtained. Moreover, the magnetic field can obviously retard the formation and growth of `distortion-free' regions in the matrix, though without having any noticeable effect on the orientation distribution of the `distortion-free' part. Investigating this subject contributes to the understanding of the way a magnetic field influences phase transformation in solid metallic materials.
The linear perturbation theory of Mullins and Sekerka for the stability of a planar interface is extended to the case of large thermal peclet numbers. It is shown that an absolute stability criterion for a planar interface exists for undercooled melts also. In light of these results, the conventional constitutional supercooling criterion is reexamined and a more general planar interface stability criterion is proposed which is valid for low as well as high growth rate conditions. The results of this stability analysis are applied to dendritic growth from pure undercooled melt.RésuméNous avons étendu la théorie linéaire de la perturbation de Mullins et Sekerka pour la stabilité d'une interface plane au cas des grands nombres de Péclet. Nous montrons qu'il existe un critère de stabilité absolue pour une interface plane dans un matériau surfondu. A la lumière de ces résultats, nous réexaminons le critère de surfusion constitutionnelle classique et nous proposons un critère plus général pour la stabilité d'une interface plane, critère valable aussi bien pour les faibles que pour les fortes vitesses de croissance. Nous appliquons les résultats de cette analyse de la stabilité à la croissance dendritique dans un corps pur surfondu.ZusammenfassungDie lineare Störungstheorie von Mullins und Sekerka für die Stabilität einer ebenen Grenzfläche wird auf den Fall groβer thermischer Pecletzahlen erweitert. Es wird gezeigt, daβ ein absolutes Stabilitätskriterium für eine ebene Grenzfläche auch für unterkühlte Schmelzen besteht. Im Hinblick auf diese Ergebnisse wird das herkömmliche Kriterium der konstitutionellen Unterkühlung neu betrachtet. Es wird ein allgemeineres Kriterium für die Stabilität einer ebenen Grenzfläche vorgeschlagen, welches für Wachstumsbedingungen mit kleiner und groβer Geschwindigkeit gilt. Die Ergebnisse dieser Stabilitätsanalyse werden auf das dendritische Wachstum aus einer reinen unterkühlten Schmelze angewendet.
A quantitative analysis is made of the redistribution of solute resulting from solidification in the absence of convection of a binary solution for transient and steady state conditions. Diffusion in the liquid is taken into account and shown to be of importance in determining the solute distribution in both the liquid and the solid. It is shown that the distribution for both normal freezing and zonemelting depends on the rate of solidification. When the speed of solidification is increased abruptly, a band of high solute concentration is formed in the solid; the reverse occurs when the speed is decreased abruptly. Values for the length of the constitutionally supercooled zone of liquid adjacent to a growing solid-liquid interface are calculated.
Bulk layered synthetic Mn–Sb composites, with a gradient structure in morphology and composition towards the surfaces, were fabricated directly in situ under high magnetic field gradient conditions. Both of the primary phases of Mn–Sb binary system, i.e. MnSb and Sb, were found to segregate simultaneously from desired domains with a gradient interface in morphology in those alloys with hypo-eutectic composition. The volume fraction of the segregated phases strongly depended on the product of the magnetic flux density and its gradient.
The effects of magnetic fields (of 0–5 T magnetic flux density) on iron electrodeposition were investigated in terms of current efficiency, morphology and crystal orientation. The AFM images showed that the shape of iron grains was angular in no magnetic field and roundish in magnetic fields. The occurrence of preferred orientation parallel to the substrate plane was influenced by an electric field (overpotential) and not by a magnetic field (MHD effect). By X-ray pole figure measurement, however, it was found that the biaxial texture evolution proceeded in a magnetic field while the uniaxial texture formed in no magnetic field.
We study the morphological instability of the planar solid/liquid interface for a unidirectionally-solidified dilute binary mixture. We use a model developed by Boettinger et al. (1985, 1986), Aziz (1982), and Jackson et al. (1980), which allows for nonequilibrium effects on the interface through velocity-dependent segregation and attachment kinetics. Two types of instabilities are found in the linear stability analysis: (1) a cellular instability, and (2) an oscillatory instability driven by disequilibrium effects. Merchant and Davis (1990) characterized these instabilities subject to the frozen-temperature approximation (FTA). The present work relaxes the FTA by including the effects of latent heat and the full temperature distribution. Thermal effects slightly postpone the onset of the cellular instability but dramatically postpone the onset of the oscillatory instability; however, the absolute-stability conditions, at which at high speed the cellular and oscillatory instabilities are suppressed, remain unchanged from the FTA.