Showing papers on "Directional solidification published in 1980"

Instabilities and pattern formation in crystal growth

Abstract: Several common modes of crystal growth provide particularly simple and elegant examples of spontaneous pattern formation in nature. Phenomena of interest here are those in which an advancing nonfaceted solidification front suffers an instability and subsequently reorganizes itself into a more complex mode of behavior. The purpose of this essay is to examine several such situations and, in doing this, to identify a few new theoretical ideas and a larger number of outstanding problems. The systems studied are those in which solidification is controlled entirely by a single diffusion process, either the flow of latent heat away from a moving interface or the analogous redistribution of chemical constituents. Convective effects are ignored, as are most effects of crystalline anisotropy. The linear theory of the Mullins-Sekerka instability is reviewed for simple planar and spherical cases and also for a special model of directional solidification. These techniques are then extended to the case of a freely growing dendrite, and it is shown how this analysis leads to an understanding of sidebranching and tip-splitting instabilities. A marginal-stability hypothesis is introduced; and it is argued that this intrinsically nonlinear theory, if valid, permits aone to use results of linear-stability analysis to predict dendritic growth rates. The review concludes with a discussion of nonlinear effects in directional solidication. The nonplanar, cellular interfaces which emerge in this situation have much in common with convection patterns in hydrodynamics. The cellular stability problem is discussed briefly, and some preliminary attempts to do calculations in the strongly nonlinear regime are summarized.

Primary dendrite spacing of lead dendrites in Pb-Sn and Pb-Au alloys

Abstract: Directional solidification experiments have been carried out on dendritic alloys of Pb-Sn and Pb-Au as a function of temperature gradient,G, solidification rate,R, and composition. Variation of temperature gradient was carried out at a moderate growth rate, 20 µm/s, and variation of growth rate was carried out at high gradients, around 350 °C/cm. Based on these experiments the primary dendrite spacings, λ, are found to follow the correlation λ =kG −a R -b only at rates above 45 µm/s in Pb-Sn alloys and above 10µm/s in Pb-Au alloys. Below these rates λ becomes essentially constant with rate. Possible causes of this break in the rate data are discussed. The values of the exponentsa andb at the higher rates are found to be significantly different from the values predicted by the recent theoretical model of Hunt.

Ceramic shell mold.

Abstract: Method for making a novel shell mold for use in directional solidification and for casting alloys containing reacting components, wherein a binder comprising a non-fibrous aqueous acidic dispersion of alumina monohydrate and being essentially free of silica, is employed. The resultant shell mold is particularly suitable for the casting of nickel and cobalt based alloys containing relatively reactive constituents such as zirconium, aluminum and titanium.

Process for purifying metallurgical-grade silicon

Abstract: A process is provided for purifying metallurgical-grade silicon for the manufacture of solar cells, wherein the acid treatment of the silicon, known per se, is combined with vacuum evaporation. This may be optionally combined with slag extraction and directional solidification.

Characterization of effects of plane-front solidification and heat treatment on magnetic properties of Bi/MnBi composites

Abstract: Eutectic Bi/MnBi composites (3.2 volume percent MnBi) have been plane-front directionally solidified using the Bridgman-Stockbarger method. For the growth velocity range investigated, the morphology of the composites consists of an aligned ensemble of MnBi rods with mean rod diameters lying below the theoretical size for single domain behavior. Magnetization as a function of temperature, applied field and angle with respect to the solidification direction was used to characterize the magnetic properties. These measurements have been correlated with particle size distributions and volume fraction data. Previous studies have shown that solidification processing conditions and post-solidification heat treatment significantly affect the magnetic properties due to the presence of a nonequilibrium magnetic phase. The nonequilibrium phase has been isolated by varying the processing parameters and the temperature dependence of its magnetization determined. In addition, the transformation kinetics of the nonequilibrium to the equilibrium MnBi phase was investigated by monitoring the magnetization change during isothermal annealing. The intrinsic coercivity of the equilibrium component was found to depend on whether a given percent of equilibrium phase is produced by directional solidification or post-solidification heat treatment anneal.

Directional solidification of cubic structure, single crystal blade

Abstract: A process for shell mould casting of a cubic structure single crystal blade, comprising a cylindrical vane (12) with two symmetrical end plates (13, 14), involves top pouring molten metal into the mould, effecting directional solidification with vertical upward motion of the solidification front and using a selector (16), located at the lower part of the mould, to obtain a single crystal of predetermined orientation and of direction (001) coincident with the vertical The novelty comprises: (a) positioning the mould shells such that the end plates (13, 14) are oriented in a plane parallel to the selected crystal direction (001) and such that the vane generatrices (12 a, b) are parallel to one of the horizontal directions (010, 100) of the selected crystal; (b) locating the outlet (17) of the selector (16) between the end plates (13, 14) in a connection (22) connecting the selector (16) to the lower ends (13 b, 14 b) of the end plates (13, 14); and (c) pouring the melt into the mould at the level of the upper ends (13 a, 14 a) of the end plates (13, 14) Also claimed are processes for mfg a blade with a hollow vane using a core (25) and for mfg several blades, by means of the above casting process

Preventing mold and casting cracking in high rate directional solidification processes

Abstract: Insulating material is applied in selected mold surfaces and in critical thicknesses to prevent cracking of the mold and casting therein during the heating and cooling steps of high rate directional solidification processes.

Casting method and mold device for crushing balls

Abstract: PURPOSE: To prevent shrinkage and to improve the yield of casting by laminating granular mold forming materials and metals for metallic molds, and charging molten metal from the runner and ingate provided on the granuler mold forming material side. CONSTITUTION: Both half split molds 3, 4 of a mold device 1 laminate respectively shell materials 3a, 4a and metals 3b, 4b for metallic molds in the thickness direction. Molds 5 for grinding balls are formed in the boundary part of the shell materials 3a, 4a and the metals 3b, 4b, with the half part located on the shell material side and the other half on the metal side. Both dies 3, 4 are assembled in such a way that the shell material 3a, 4a sides come to the inner side, and the surfaces of the materials 3a, 4a are a parting surface 2. The materials 3a, 4a form a runner 6 and an ingate 7 communicating with each mold 5. The molten metal flowed into the molds 5 is cooled from the metal 3b, 4b sides, and the materials 3a, 4a sides continuous to the ingate 7 solidify after this and cause directional solidification; therefore, the molten metal solidifies without causing shrinkage in each mold 5. COPYRIGHT: (C)1982,JPO&Japio

SPAR VI Technical Report for Experiment 76-22: Directional Solidification of Magnetic Composites

Abstract: Samples of eutectic Bi/MnBi were directionally solidified during a low-g interval aboard the SPAR 6 flight and in a l-g environment under identical furnace velocity and thermal conditions. The Bi/MnBi eutectic is characterized by a regular rod eutectic whose morphology may be sensitive to thermo-solutal convection and by its components, MnBi, which is ferromagnetic. Morphological analyses on samples show statistically smaller interrod spacings and rod diameters with respect to samples grown under identical solidification furnace conditions in l-g. An adjustment between the interrod spacing, growth velocity, and total undercooling at the solidification interface is proposed. Morphological analyses on samples grown in l-g indicate little difference between results for different growth orientations with respect to the gravity vector. The magnetic properties are significantly affected, however, by the presence of a nonequilibrium magnetic phase and the nonequilibrium phase transforms to the equilibrium ferromagnetic phase during isothermal heat treatment.

Directional Solidification of Nonoxide Eutectics

Abstract: : The directional solidification of the eutectic composition in the systems SiC-B4C, TiC-TiB2, ZrC-ZrB2 and ZrC-TiB2 was studied. Microstructure, crystallography, hardness, fracture toughness and wear were determined. In several of the studied system mechanical properties exceeded that of the end members.

Transient and diffusion analysis of Hg Cd Te

Abstract: Solute redistribution during directional solidification of HgCdTe is addressed. Both one-dimensional and two-dimensional models for solute redistribution are treated and model results compared to experiment. The central problem studied is the cause of radial inhomogeneities found in directionally solidified HgCdTe. A large scale gravity-driven interface instability, termed shape instability, is postulated to be the cause of radial inhomogeneities. Recommendations for future work, along with appropriate computer programs, are included.

Floating-zone melting with viscous torque control

Abstract: Two control devices were used in an RF furnace as well as in an arc image furnace in order to stabilise the liquid zone during crucible-free zone melting. The control principle is based on the relation between liquid-zone dimensions and a viscous frictional torque built up within the liquid by a laminar forced convection flow. This flow is due to the relative rotation of the two parts of the treated rod. This method is quite effective in the treatment of bar samples greater than 8 mm in diameter with rotational speeds between 100 and 600 rev min-1. It was applied in the crystal growth and directional solidification of eutectic alloys with growth rates ranging from 2 mm h-1 to 500 mm h-1.

Strengthening of oxidation-resistant Fe-Cr-Al-Y alloys by directional solidification of eutectic compositions

Abstract: Two eutectic alloys having solid-solution Fe-Cr-Al-Y matrixes reinforced by fibres of (Fe, Cr)7C3 have been identified and aligned by directional solidification. Alloy A (60·2Fe-33·2Cr-3·2Al-2·6C-0·8Y), which has a ferritic matrix and has good cyclic oxidation resistance to 1373 K, has better creep properties than existing ferritic alloys. Its stress-rupture properties are strongly influenced by the dimensions of the fibres which are subject to a phase transformation; at 973 K the optimum creep properties are obtained with a mean fibre radius of 2·5 μm. Alloy B (60·2 Fe-18·2Cr-15·0 Ni-3·2Al-0·8Y-2·6C), which has an austenitic matrix, has poorer oxidation resistance but preliminary tests show the stress-rupture properties to be superior to those of current iron-base materials, approaching the behaviour of nickel-base superalloys such as Nimonic 115.

Furnace atmosphere effects on casting of eutectic superalloys

Abstract: Control of furnace atmosphere is a key factor in the use of silica-bonded alumina shell molds for the directional solidification of eutectic superalloys reinforced with tantalum monocarbide whiskers. The use of a furnace atmosphere which is simultaneously oxidizing to aluminum in the eutectic alloy and reducing to silica phases in the mold results in the formation of an alumina barrier layerin situ at the metal/mold interface and an absence of silica phases in the mold region adjacent to this barrier layer. The presence of this microstructure permits castings of eutectics at metal temperatures up to 1750°C.

Swedish Materials Science Experiment in the Texus 1 and 2 Rockets

Abstract: Texus is a sounding rocket project for materials science experiments in low gravity A 40-kg experiment module is included in the reusable payload The experiments on the first and second Texus flights in 1977 and 1978 were 12 metal solidification experiments, two involving directional solidification in a gradient furnace and ten *'thermal analyses" of small samples in so-called mirror furnaces The gradient furnace is designed for studies of samples with melting points <1000°C The furnace consists of a tubular resistive heating element inside which the sample is placed and a phase-change heat sink which extracts heat from one end of the sample to get a unidirectional solidification The mirror furnace is designed for thermal analysis experiments below 1000° C The radiation from two halogen lamps focused by ellipsoidal mirrors on the sample provides the heating

Iterative nonlinear thermal analysis of directional solidification using a fast elliptic solver

Abstract: The material characteristics of directionally solidified superalloy crystals require carefully controlled solidification rates, temperature field distributions, and maximum temperature gradients. Experimentally, these can be determined by vacuum casting of cylindrical rods. Corresponding analytical studies yield nonlinear equation systems due to radiative boundary conditions and latent heat effects, which in the past required lengthy finite-difference or finite-element solutions. For the quasi-steady freezing encountered in long cylindrical alloy bars, we employed an efficient iteration scheme based on a fast elliptic equation solver previously reported in the literature. This paper discusses the principal aspects of the computational scheme and gives results showing the two-dimensional temperature fields encountered in solidification processes subject to various input conditions.

Castings with directional solidification - where empty moulds are heated to above m.pt. of metal to be cast, then filled and cooled from their bases

Abstract: The entire surfaces of the moulds, including their bases, are heatedd to above the m.pt. of the metal to be cast, and the moulds are then filled with that metal. The filled moulds are conveyed through a crystallisation zone equipped with side heaters reduced stepwise towards the top of the moulds. In the crystallisation zone, the moulds rest on a cooled surface so solidification starts from the base of each mould and proceeds upwards as the moulds travel through the zone. The pref. plant consists of a furnace with heaters in its side walls, where heat is transferred to different vertical zones of the moulds, so the latter are heated and cooled stepwise in accordance with a program and the floor of the crystallisation zone is formed by a water-cooled box. Accurate control of temp. gradients in moulds, is achieved, so columnar crystal structures are obtd. Useful in mfr. of turbine blades or magnets etc.

Directional solidification at ultra-high thermal gradient

Abstract: A high gradient controlled solidification (HGC) furnace was designed and operated at gradients up to 1800 C/cm to continuously produce aluminum alloys. Rubber '0' rings for the water cooling chamber were eliminated, while still maintaining water cooling directly onto the solidified metal. An HGC unit for high temperature ferrous alloys was also designed. Successful runs were made with cast iron, at thermal gradients up to 500 C/cm.

Gerichtete Erstarrung. Verfahren zur Herstellung anisotroper Werkstoffe mit hoher Gefügeperfektion

Abstract: Seit einigen Jahren hat die gerichtete Erstarrung in der Industrie als Verfahren zur Herstellung von Werkstoffen mit speziellen Eigenschaften einen festen Platz eingenommen. In diesem Artikel werden gerichtet erstarrte Gusprodukte vorgestellt, und die Bedeutung der Erstarrungsbedingungen fur das Erstarrungsgefuge wird erlautert. Mit der Beschreibung eines selbstgebauten Gradientenofens soll dem eventuellen Benutzer die Moglichkeit geboten werden, den Zusammenhang zwischen Ofenkonstruktion, Erstarrungsprozes und Gefugequalitat zu verstehen. Directional Solidifaction a Manufacturing Process for Anisotropic Materials with high Quality Microstructures During the last few years, directional solidification has become a practical route to the production of materials with special properties. In this paper, some of these anisotropic cast products are presented. The effects of growth conditions on the microstructure are explained. A detailed description of a gradient furnace suitable for directional solidification, is given, allowing the potential user to understand the relations which exist between furnace design, the solidification conditions, and the quality of the microstructure.