Showing papers on "Directional solidification published in 1986"

Solidification of an alloy from a cooled boundary

Abstract: We Present a mathematical model for the region of dendritic or cellular growth which often forms during the solidification of alloys. The model treats the region of mixed phase (solid and liquid) as a continuum whose properties vary with the local volume fraction of solid. It is assumed that transports of heat and of solute are by diffusion alone, and the model is closed by a condition of marginal equilibrium. Results are obtained for the unidirectional solidification of an alloy from a plane wall. The spatial variations of solid fraction are highly suggestive of the types of morphology that can occur, and a wealth of different structures are found as the physical parameters are varied. Although the model ignores gravity entirely, the results can be applied to the solidification from below of an alloy which is initially less dense than its eutectic. Predictions for the growth rate of the mixed-phase region agree well with existing experimental measurements of ice growing from aqueous salt solutions.

Analysis of solute distribution in dendrites of carbon steel with δ/γ transformation during solidification

Abstract: Solute distribution in dendrites during solidification of carbon steel was analyzed by unidirectional solidification experiments and mathematical analysis. The characteristic of the mathematical analysis is that diffusion of solutes in solid and redistribution of solutes at solid/liquid andδ/γ interfaces are taken into consideration. Based on the observed and calculated results, it was found that phosphorus was redistributed fromγ-phase toδ-phase, and that manganese was slightly redistributed fromδ-phase toγ-phase. Therefore the concentrated region of phosphorus can be separated from that of manganese duringδ/γ transformation in the case of slow cooling. Moreover, it was concluded that rapid diffusion inδ-phase and the redistribution duringδ/γ transformation played an important role in the variation of the interdendritic concentrations of solutes with lower carbon concentration.

Interaction of Flows with the Crystal-Melt Interface

Abstract: The coupling between a crystal-melt interface and fluid flow is investigated. The solidification boundary conditions at the crystal-melt interface are described. The influences of morphological and double-diffusive instabilities during directional solidification on the interface are studied. The experiments by Glicksman and Mickalonis (1982) and Fang et al. (1985) which examine the relationship between the hydrodynamic state of the phase melt and phase-change interface are analyzed. The Rayleigh-Benard problem, and the effect of crystal-melt interaction on the Rayleigh number are examined. Various applications for melt-flow interactions with solid-liquid interface to engineering and welding are discussed.

Interfaces in Oriented Al2O3-ZrO2 (Y2O3) Eutectics

Abstract: Oriented samples of Al2O3-ZrO2 (Y2O3) eutectics consisting of an alumina matrix with zirconia dispersoids were grown by directional solidification. Preferred growth directions and epitaxial relations were determined from X-ray and electron diffraction analyses. Imaging of interfaces was performed by high-resolution transmission electron microscopy on oriented platelets. Semicoherent interfaces were observed with faceting along crystallographic planes of both phases.

Cellular instabilities in directional solidification

Abstract: The March 1987 Les Houches workshop on “Propagation in Systems far from Equilibrium” allowed us to compare the state of research in a number of different fields where instabilities occur. It appeared clearly that the field of solidification has been much less systematically explored than those, for example, of Rayleigh-Benard instabilities or convection in mixtures. Many questions have yet to receive an answer. The aim of the present text is to examine some of them in the light of the results of this Workshop. However, we shall begin by a brief review of the literature on the subject.

Directional solidification with buoyancy in systems with small segregation coefficient

Abstract: We derive an evolution equation governing the cellular structure of a binary alloy having a small segregation coefficient. This equation, applicable to long-wave instabilities of a planar interface, incorporates coupled morphological instabilities and buoyancy effects. The presence of buoyancy can inhibit the onset of the cellular structure by effectively magnifying the segregation and diffusion.

Si‐TaSi2 in situ junction eutectic composite diodes

Abstract: Nearly ideal diodes have been fabricated using the in situ junctions present in Si‐TaSi2 semiconductor‐metal eutectic composites. The composites, prepared by directional solidification at the eutectic composition, have a high density of TaSi2 rods in a quasi‐single crystalline P‐doped, n‐type Si matrix. Analysis of the diodes using current‐voltage and capacitance‐voltage techniques yields a TaSi2/Si Schottky barrier height of 0.62 eV and evidence that the voltage‐dependent depletion zones can be made comparable to the interrod spacing to produce a ‘‘pinch‐off’’ condition.

Method and apparatus for epitaxial solidification

Abstract: Disclosed are a mold, apparatus and method for obtaining articles of controlled crystallographic orientation using solidification from seeds. The starter section of a directional solidification mold is adapted to both contain a seed and receive molten metal which is flowed over and about a seed to heat and partially melt it. A selector section of the mold has reduced cross section compared to the starter section so that only epitaxially solidified metal will be formed in the article section. A barrier layer resistive to molten metal is applied to portions of the seed to facilitate its removal and reuse.

Wavelength selection in directional solidification

Abstract: Cell-spacing selection in directional solidification is investigated. An integral equation describing steady-state cells in the limit where the solute diffusion length is much larger than the cell spacing is derived and solved numerically by Newton's method. With surface tension and no crystalline anisotropy present the spatial periodicity of a one-dimensional array of cells with cusp singularities is found to be determined uniquely by a solvability condition. The inclusion of crystalline anisotropy has no other effect to shift the value of the selected spacing.

Mechanical properties of conventionally cast, directionally solidified, and single-crystal superalloys

Abstract: In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and l...

Microstructural variations induced by gravity level during directional solidification of near-eutectic iron-carbon type alloys

Abstract: The effects of gravity on the microstructure of directionally solidified near-eutectic cast irons are studied, using a Bridgman-type automatic directional solidification furnace aboard a NASA KC-135 aircraft which flies parabolic arcs and generates alternating periods of low-g (0.01 to 0.001 g, 30 seconds long) and high-g (1.8 g, 1.5 minutes long). Results show a refinement of the interlamellar spacing of the eutectic during low-g processing of metastable Fe-C eutectic alloys. Low-g processing of stable Fe-C-Si eutectic alloys (lamellar or spheroidal graphite) results in a coarsening of the eutectic grain structure. Secondary dendrite arm spacing of austenite increases in low-g and decreases in high-g. The effectiveness of low-gravity in the removal of buoyancy-driven graphite phase segregation is demonstrated.

Steady and oscillatory cellular morphologies in rapid solidification

Abstract: Finite-amplitude cellular solidification fronts are calculated for morphologically unstable rapidly solidifying metals and semiconductors. A two-dimensional directional solidification model is used which accounts for nonequilibrium segregation of dopant. Finite-element approximations of temperature, concentration, and melt-solid interface shape are used in conjunction with computer-implemented perturbation techniques and a fully implicit time integration scheme to analyze the dependence of melt-solid interface morphology on solidification rate and dopant concentration. A sequence of transitions between cellular interfaces with different spatial and temporal oscillation frequencies is found. Stable time-periodically oscillating families of cellular melt-solid interfaces are observed as Hopf bifurcations both off of the planar family and off of oscillatory families. The role of nonequilibrium dopant segregation in driving oscillatory interface morphologies is elucidated with use of calculations for rapidly solidified antimony-doped silicon.

Effect of fluid flow and hafnium content on macrosegregation in the directional solidification of nickel base superalloys

Abstract: Nickel base superalloys of the MAR-M200 type with additions of hafnium from 0 to 2.6 pct were directionally solidified in investment molds with an abrupt change in cross-section. Segregation patterns of several alloying elements (Hf, W, Ti, and Co) were determined and related to partition ratios, hafnium content, and casting geometry. The segregation is more pronounced near the section change, and hafnium substantially increases the segregation tendency of these alloying elements. Variation in eutectic volume percent was related to the observed macrosegregation. A model based on the flow of interdendritic liquid to feed solidification shrinkage was Actapted to describe the macrosegregation behavior of these complex superalloys. The model accurately predicts the effect of casting geometry on macrosegregation.

Yield Strength of Al–CuAl2 Lamellar-Eutectic Composites at High Temperatures

Abstract: Preparation de deux types de monocristaux Al−CuAl 2 eutectiques a espacement interlamellaire different par solidification dirigee. Comparaison de leur resistance mecanique par des essais de compression entre 523 et 673 K a des vitesses de deformation de 4×10 −5 s −1 a 3×10 −3 s −1 . La limite elastique est independante de l'espacement interlamellaire, et est aussi independante de l'orientation lamellaire

The Mullins-Sekerka instability in directional solidification of quasi-azeotropes

Abstract: We investigate the stability of the planar front during directional solidification of a quasi-azeotrope. We show that the corresponding shape of the phase diagram induces a strong reduction of the scale of the bifurcation curve which makes its complete exploration feasible in standard directional solidification experiments. We show that the finite concentration of azeotropic mixtures results in a «chemical capillary term» in the equation relating interface concentrations. This effect, which turns out to be negligibly small for metallic azeotropes, should be significant for the 3 He- 4 He azeotrope where it might give rise to a qualitative change of the bifurcation curve. We study the nature (normal or inverted) of the bifurcation with the help of a third order amplitude expansion. We show that, for metallic alloys, thanks to the reduction of scale of the instability diagram, the normal bifurcation regime ― which cannot be easily reached with dilute mixtures ― becomes accessible to standard experiments Etude de la stabilite du front plan lors de la solidification dirigee d'un quasi-azeotrope. La forme du diagramme de phase donne lieu a une forte reduction d'echelle de la courbe de bifurcation qui rend possible son exploration complete. Comme les azeotropes ne sont pas des melanges dilues, il apparait un «terme capillaire chimique» dans l'equation reliant les concentrations a l'interface. Cet effet est tres faible dans les azeotropes metalliques, mais devrait etre important dans le melange 3 He- 4 He et pourrait donner lieu a un changement qualitatif de la forme de la courbe de bifurcation

Apparatus for anticipation of structure of casting alloys and particularly the degree of spheroidization of cast iron

Abstract: The apparatus has a supply system 1 and measuring system 2, connected via suitable electrodes to the mould 3. This mould 3 has a cylindrical shape and a cylindrical pouring cup. The cavity of the mould 3 has been divided by vertical walls 4 and 5 into three parts in such a way that the wall 4 divides the mould 3 into two halves and the wall 5, in turn, divides one of the halves also into two equal parts. The electrodes of the supply system 1 and measuring system 2 have been mounted in the bottom of the mould 3 in the portions delimited by the wall 5. The shape of the mould ensures a directional solidification of the specimens contained therein. Owing to this a compact specimen is obtained without shrinkage porosities and contraction cavities over the measuring length. The structure of the alloy is anticipated from the curve of variations of voltage signal during the period from the moment of pouring of liquid metal into the mould 3 to the moment of solidification of the specimen.

'Separated Eutectic' Structure of Unidirectionally Solidified Al–Si Eutectic Alloy Caused by Forced Convection Flow

Abstract: Etude sur l'alliage Al-11,7% at. Si prepare par solidification dirigee dans des conditions d'ecoulement de convection force du liquide en tete de l'interface solide-liquide

Solidification cells at low velocity: The moving symmetric model.

Abstract: This paper is the first in a series of theoretical studies of singular cells in the small solute Peclet number limit (P\ensuremath{\rightarrow}0) of two-dimensional models of directional solidification. In this limit solute diffusion in the frame of the moving front is nearly Laplacian in which case solidification cells and Saffman-Taylor fingers are closely related. Here Langer's moving symmetric model in the absence of temperature gradient (which also describes solidification in a channel of width \ensuremath{\lambda} at unit undercooling) is considered. A boundary integral equation describing steady-state cells is derived and it is shown that in the P\ensuremath{\rightarrow}0 limit this equation can be expressed in terms of a single dimensionless parameter \ensuremath{\sigma}\ensuremath{\propto}${d}_{0}$l/${\ensuremath{\lambda}}^{2}$. The endpoint singularity is studied analytically and physically admissible solutions are found numerically to only exist for a discrete set of values of \ensuremath{\sigma}. The small P dependence of \ensuremath{\sigma} is also examined.

Method for the directional solidification of metal melts

Abstract: The present invention relates to a method for producing columnar solidified metal melts by pouring a metal melt into a mold with subsequent solidification in a directional temperature field, with a higher heat dissipation on the upper or lower side of the crucible than on the other sides.

Structural transitions in directionally solidified graphitic cast irons

Abstract: Magnesium fading in conjunction with directional solidification has been used to determine the conditions for the structural transition from flake to compacted to nodular graphite in a hypoeutectic and a hypereutectic cast iron. Results are presented in terms of predominance-area diagrams and the conditions for stability are given. Magnesium fading has also been studied during the experiments and a mass transfer coefficient has been determined.MST/474

Ceramic Eutectic Composites

Abstract: Methods of growing ceramic eutectics are reviewed. The basic microstructure of the obtained oxide eutectics can be generalized from the relative interfacial area/unit volume and the volume fraction of the minor phase for the fibrous and lamellar ideals. Crystallographic investigation of oxide eutectics show the interfacial planes and solidification directions are all low Miller indices regardless of the crystal structures of the phases. The orientation relations of directionally solidified oxid eutectics appear to result from two factors: (i) the minimization of the lattice misfit and strain (ii) the ionic charge balance at the interface between two phases. The oxide eutectics without colony boundaries can be expected to possess superior mechanical properties. The lamellar ZrC-ZrB2 ZrC-TiB2, and SiC-B4C eutectics were directionally solidified. All of the investigated carbide-boride eutectics exhibit superior mechanical properties when compared with the individual constituents.

Method for controlling directional solidification by exothermic core

Abstract: PURPOSE:To control efficiently directional solidification and to obtain a defectless casting by holding an exothermic core having a projecting part in the lower part in the riser part of a casting mold prior to casting. CONSTITUTION:The exothermic core 3 is formed by solidifying a powdery exothermic agent and the shape thereof is provided with a projecting part 4 in a circular conical shape into the lower part of the core body. The core is made into the shape having at least the projecting part 4 extending downward. Such core 3 is held in the riser part 1 of the casting mold by a holding body 5. Then the exothermic effect is generated from the point of the time when the riser surface arrives at the projecting part 4 of the core 3 in casting. The exothermic effect is kept generated continuously according to the shape of the core 3 as the riser surface rises and therefore the replenishing effect of the riser is continuously maintained and the directional solidification is attained. Shrinkage 6 is generated in the lower part of the core 3 and the generation of a defect such as secondary shrinkage in a casting 2 is obviated.