Showing papers on "Directional solidification published in 1989"

Parity-breaking transitions of modulated patterns in hydrodynamic systems.

Abstract: A model for subcritical parity-breaking bifurcation of a periodic pattern is introduced. It is found that nucleated regions of the new asymmetric state propagate with a well-defined velocity in a direction determined by their parity, and leave in their wake a pattern with an altered wavelength. Successive passage of these ``parity bubbles'' enables the system to relax to a selected wavelength. The possible relevance of these findings to recent observations of ``solitary modes'' in directional solidification and hydrodynamic experiments is discussed.

Computer simulation of grain growth and macrostructure development during solidification

Abstract: A Monte Carlo computer simulation technique, previously applied to the simulation of a number of solid state processes involving microstructural evolution, has been employed to simulate and represent pictorially grain growth and grain interactions during solidification. By careful control of the number, location, and time of origin of grain nuclei it has been possible to simulate a wide range of features of grain structure in castings. Also, in support of an existing statistical argument, it has been demonstrated that the columnar to equiaxed transition occurs when the volume fraction of equiaxed grains ahead of the columnar interface is 0·50.MST/912

Texture processing of bulk YBa2Cu3O6+x by zone melting

Abstract: Zone melting has been used to produce texture in bulk YBa 2 Cu 3 O 6+ x . By controlling the directional solidification of the molten zone, oriented structures are produced that consist of domains of aligned grains, with some misalignment between domains. Scanning electron microscopy and optical microscopy show a high degree of texture produced by this technique. Magnetization measurements show a large increase in hysteresis at 77 K compared to unaligned polycrystalline YBa 2 Cu 3 O 6+ x . The use of BaCuO 2 as a solvent to aid the texturing process is discussed.

Solitary Tilt Waves in Thin Lamellar Eutectics

Abstract: During directional solidification of thin layers (50 μm) of the transparent CBr4-C2Cl6 lamellar eutectic alloy, we observed tilt waves, a type of instability consisting in small domains of tilted lamellae moving transversely along the growth front. This instability is a way the system adjusts its wavelength after a small perturbation, without creating or destroying lamellae.

Thermosolutal Convection during Dendritic Solidification of Alloys: Part II. Nonlinear Convection

Abstract: A mathematical model of thermosolutal convection in directionally solidified dendritic alloys has been developed that includes a mushy zone underlying an all-liquid region. The model assumes a nonconvective initial state with planar and horizontal isotherms and isoconcentrates that move upward at a constant solidification velocity. The initial state is perturbed, nonlinear calculations are performed to model convection of the liquid when the system is unstable, and the results are compared with the predictions of a linear stability analysis. The mushy zone is modeled as a porous medium of variable porosity consistent with the volume fraction of, interdendritic liquid that satisfies the conservation equations for energy and solute concentrations. Results are presented for systems involving lead-tin alloys (Pb-10 wt pct Sn and Pb-20 wt pct Sn) and show significant differences with results of plane-front solidification. The calculations show that convection in the mushy zone is mainly driven by convection in the all-liquid region, and convection of the interdendritic liquid is only significant in the upper 20 pct of the mushy zone if it is significant at all. The calculated results also show that the systems are stable at reduced gravity levels of the order of 10−4 g 0 (g 0=980 cm·s−1) or when the lateral dimensions of the container are small enough, for stable temperature gradients between 2.5≤G l≤100 K·cm−1 at solidification velocities of 2 to 8 cm·h−1.

Hydrodynamic and morphological stability of the unidirectional solidification of a freezing binary alloy - a simple-model

Abstract: In this paper we consider the effect of a model boundary-layer flow on the hydrodynamic and morphological stability of a simple model of the solidification of a binary alloy. We conduct a linear analysis and develop asymptotic solutions for large Schmidt number and large Reynolds number. We also present numerical solutions for data appropriate to a lead–tin alloy. We show that for modes parallel to the free-stream velocity the flow is responsible for the appearance of travelling waves and, for common values of the material parameters, may stabilize the morphological stability of the interface. However the morphological stability of modes perpendicular to the free-stream velocity is unaffected by the presence of the flow. The hydrodynamic stability of the boundary layer is very weakly affected by the presence of the interface, which we attribute to the large Schmidt numbers associated with real crystal growth situations.

Steady-state cellular growth during directional solidification.

Abstract: We study cellular structures moving at constant velocity, in a symmetric model of directional solidification. This is done by employing Newton's method to solve a discretized version of the integro-differential equation for the solid-liquid boundary. Our results indicate that there is a continuous band of allowed wavelengths and that there is a generic fold in the solution diagram. This fold provides a maximum allowed wavelength for any given velocity. Also, we argue that the mechanism of microscopic solvability serves to fix the tip shape at fixed wavelength. The implications of our results for wavelength and shape selection are briefly discussed.

Destabilization of a flat nematic-isotropic interface.

Abstract: We present experiments on the directional solidification of a moving nematic-isotropic interface. We study the bifurcation and marginal stability of the interface in a system of 4,4'-n-octylcyanobiphenyl (8CB) with the impurity hexachloroethane ${\mathrm{C}}_{2}$${\mathrm{Cl}}_{6}$ added. In the velocity, temperature-gradient parameter space, we can trace the marginal stability line, in qualitative agreement with theory. A quantitative analysis shows that three-dimensional effects, such as solute-driven convection and the thickness of the sample, must be considered.

Magnetic anisotropy of Nd15Fe77B8 flakes made by twin-roller quenching

Abstract: Rapid solidification by twin‐roller quenching of iron‐neodymium‐boron alloys produces flakes which show a strong (00l) texture normal to their surface by x‐ray diffraction. The strong crystal orientation is argued to be due to both directional solidification in a thermal gradient and uniaxial deformation of the solid phase in the twin rollers. Magnetization studies on individual flakes show intrinsic coercivities of 14 kOe and a nearly 50% higher remanance for field normal to the flake surface (Br =9.5 kG) than in the flake plane (Br =6.5 kG). The perpendicular (BH)max is 16 MG Oe.

Development of solidification microstructures in the presence of fibers or channels of finite width

Abstract: Directional solidification studies in a transparent succinonitrile system have been carried out in the presence of single or multiple fibers. The solute-rich region and the solute bands which form as the interface approaches the fibers are discussed. The microstracture formation in small interfiber spacings has been examined and it is shown that the development of microstructure depends on (a) the interfiber spacing, (b) the angle between the two fibers and (c) the orientation of fibers with respect to the growth direction. Experimental studies have also been carried out to examine systematically the microstructural development in well-characterized rectangular channels of controlled widths. Under given experimental conditions of growth rate, temperature gradient and composition, it is shown that the microstructure changes from dendritic to cellular to a nearly planar interface as the width of the channel is reduced. Furthermore, for certain channel widths, half-dendritic or half-cellular structures have also been observed. The various implications of these observations for the fundamental understanding of microstructural evolutions and for the applied aspects of solidification of composite materials are discussed.

Fluid flow in solidifying monotectic alloys

Abstract: Use of a two-wavelength holographic technique results in a simultaneous determination of temperature and composition profiles during directional solidification in a system with a miscibility gap. The relationships among fluid flow, phase separation, and mass transport during the solidification of the monotectic alloy are discussed. The primary sources of fluid motion in this system are buoyancy and thermocapillary forces. These forces act together when phase separation results in the formation of droplets (this occurs at the solid-liquid interface and in the bulk melt). In the absence of phase separation, buoyancy results from density gradients related to temperature and compositional gradients in the single-phase bulk melt. The effects of buoyancy are especially evident in association with water- or ethanol-rich volumes created at the solid-liquid growth interface.

Method for directional solidification of single crystals

Abstract: The invention provides a method for growth of single crystals from the melt by directional solidification in vertical bottom seeded crucibles. The crucible confining the melt is insulated radially and from above. The heat is supplied to the melt by a heater submerged in the melt or an auxiliary heater above the crucible containing the melt. A small portion of the melt is enclosed between the submerged heater and the growing crystal. The confined melt in this region is thermally stratified and therefore stagnant.

Convective instabilities during directional solidification

Abstract: Results of a ground-based experimental program that supports a low-gravity space processing Spacelab experiment are reported. The phenomena which precipitate pluming and thus freckling in a metal alloy are studied in detail using a metal analog (ammonium chloride and water) and the sequential events leading to massive channeling and convection are optically documented. The pluming is shown not to be a random burst of unstable fluid from a preferred channel but, rather, a natural occurrence resulting from a fundamental (Rayleigh-Benard) fluid dynamic instability at the density inversion interface. Rayleigh numbers are calculated for the instability and a critical Rayleigh number is determined.

Directional solidification: Transition from cells to dendrites.

Abstract: The evolution of cells and dendrites on a solidification front is analyzed numerically by a Green's-function method. We obtain a consistent picture of the pattern-forming processes in the whole experimentally accessible range of the phase-diagram cell spacing versus growth rate. We find a smooth transition from cells to free dendrites confirming suspected scaling relations. The selection of dendritic spacings appears to occur through a tail instability.

Microstructural and magnetization studies of sintered and melt-textured Y-Ba-Cu-O

Abstract: For bulk applications, high‐Tc superconductors generally require high critical current density, Jc. This paper reports two methods of producing high‐Tc ceramic wires. The first is an extrusion and sintering route where Jc is routinely 600–1000 A cm−2. The second is by melt texturing or directional solidification where we have achieved in excess of 7000 A cm−2 by direct measurement at 77 K in zero field. However, the ease with which wire may be extruded and sintered is attractive and functioning devices have already been manufactured using this route. Fabrication of devices is facilitated by a unique processing route that has resulted in the strongest bulk ceramic yet reported (bend strength >200 MPa). The melt‐texturing route and the problems associated with it are examined and compared with the extrusion and sintering route. The processing and the microstructures associated with the directionally solidified YBa2Cu3Ox will be examined in detail as will the resultant superconducting properties, in particul...

Shallow cells in directional solidification.

Abstract: The existing theory on two-dimensional transitions (appropriate to thin parallel-plate geometries) is presented in such a way that it is possible to identify easily conditions for the onset of shallow cells. Conditions are given under which succinonitrile-acetone mixtures should undergo supercritical bifurcation in experimentally accessible ranges. These results suggest a means for the quantitative test of the Mullins and Sekerka (1964) model and its weakly nonlinear extensions.

Directional solidification cells with grooves for a small partition coefficient.

Abstract: Using the asymptotic matching procedure of Dombre and Hakim, we determine properties of the steady-state cellular structures with deep narrow grooves observed in directional solidification experiments. The method is valid in the experimentally relevant region of small partition coefficient and finite P\'eclet number. An extension of the Scheil equation for the grooves is given, and the importance of conservation in determining the groove closing is pointed out.

Cusp instability in cellular growth

Abstract: During directional solidification of alloys, a cellular structure can be observed above a critical pulling speed. Under certain conditions, the cusps behind the cells can present a secondary instability characterized by a periodic emission of liquid droplets. These droplets are transported by the solid, but also migrate towards the solid-liquid interface by temperature gradient zone migration. We study the conditions of appearance of the droplet instability and discuss the information concerning growth mechanisms that can be deduced from a study of the migration. J. Phys. France 50 (1989) 3007-3019 1er OCTOBRE 1989, Classification Physics Abstracts 61.50C 81.10F 81.30F

Directional solidification near minimum C∞: two-dimensional isolas and multiple solutions

Abstract: Etude des instabilites faiblement non lineaires bi-dimensionnelles pour |C ∞ −C ∞ | petit. Description simultanee des bifurcations de vitesse basse et elevee. Quand le rapport de conductivite thermique est petit (grand) les deux bifurcations sont super critiques (sous-critiques) et de cellules superficielles (profondes) emergent

Low pressure casting method for light alloy casting

Abstract: PURPOSE:To enable the casting of high quality having no structural defect and the improvement in productivity by promoting the directional solidification of the molten metal inside a cavity by the combination of the thermal conductivity of at least >=two kinds of different materials and the thermal capacity based on the mass CONSTITUTION:A sand core 86a or 86f is arranged at the specified part of a cavity 42 formed inside a casting die 32 A die closing is then executed by displacing an upper die 36, sliding dies 38a, 38b and 40a, 40b by approaching them to a lower die 34 Then, the molten metal melting an Al alloy is filled up inside a cavity 42 from the sprue 45 formed on a nozzle 46 via a stoke 47 Now, after holding the pressurizing state for the specified time a compressed air is drawn to solidify the molten metal For the lower die 34 and upper die 36 and sliding dies 38a, 38b, 40a, 40b the material of different thermal conductivity is used respectively and also the mass is composed by combining selectively The heat radiation of the molten metal is adjusted on each respective part and the solidification is executed with having the directivity in the order of the upper die 36 sliding die 38a, 38b, 40a, 40b and the lower die 34

Growth Of CdTe Crystals By The Heat Exchanger Method (HEMN)

Abstract: The poor thermal conductivity of CdTe has prevented controlled directional solidification and, therefore, completely single crystal growth is not achieved on a routine basis. The Heat Exchanger Method (HE(TM)) has been adapted for the growth of 600 g, 5.5 cm diameter and 1300 g, 7.5 cm diameter CdTe ingots. Emphasis was placed on achieving controlled directional solidification. Unseeded crystal growth was carried out using presynthesized CdTe as meltstock and without the use of a separate Cd or Te reservoir to control stoichiometry. It was demonstrated that only two grains were formed and the structure was maintained during growth. Twinning was minimized and large twin-free samples could be obtained. The EPD values of CdTe were in the range of 103 to 5x105/cm2. Most of the material was p-type, but CdTe with 105 ohm-cm resistivity was grown. An absorption coefficient as low as 0.07/cm was measured showing that the crystals were suitable for CO2 laser modulator applications.

Solid particle sepn. from silicon melt - by time-variable crucible rotation during directional solidification

Abstract: A process is claimed for sepg. solid particles (esp. of C and SiC) from silicon melts (esp. obtained by carbothermic redn. of quartz, in which the melt (3) is subjected, immediately after redn. and holding at above the melting pt., to directional upward solidification to form an ingot, the melt being moved during solidification. The process involves rotating the crucible (1) with time-variable speed about its axis to produce flow currents (8,10) within the melt (3) and between the melt and the crystallised silicon (11). Pref., the time-variable rotation may be achieved by accelerating and braking the rotating crucible (1) and/or by periodic interruption of crucible rotation. Pref. the interruptions comprise stop phases between rotation phases of durations which are a multiple of (pref. at least double) the stop phase durations. Pref. the rotational speed is 5 rpm., the rotation durations are 2.5 mins. and the stop phase durations are 0.5 min.. The rotational speed and the rotation durations are pref. reduced with decrease in melt height during crystallisation. The finished ingot is pref. ground to remove the outer skin and bottom contg. the solid particles. USE/ADVANTAGE - The process allows inexpensive sepn. of solid particles even from large melt volumes, this facilitating mfr. of cheap silicon for mass prodn. of solar cells.