Showing papers on "Directional solidification published in 1992"

Semisolid solidification of high temperature superconducting oxides

Abstract: Experiments are reported on two techniques for melt‐texture processing Ba2YCu3O6.5 by directional solidification from a semisolid melt containing particles of BaY2CuO5 and a copper‐rich liquid. One of these employs an electric resistance furnace with ambient or oxygen enriched atmosphere; the other is a laser‐heated furnace operating at 1.3 atm oxygen. Solidification interface morphologies and other structural features were examined in quenched specimens. Depending on growth rate and temperature gradient, three different types of growth morphologies of the growing 123 phase were observed: ‘‘faceted plane front,’’ ‘‘cellular dendritic’’ or ‘‘equiaxed blocky.’’ The interface temperature decreased markedly with increasing growth rate for the faceted plane front specimens. In the remaining specimens, solidification took place over a range of temperatures. The temperature of the ‘‘root’’ of the solidification front dropped, but temperature of the solidification front ‘‘tip’’ did not. A solidification model is ...

Diffusion solidification model on Y-system superconductors

Abstract: The unidirectional solidification of the zone melt method was performed in order to clarify the growth mechanism on Y-system superconductors A sharp faceted interface of YBa2Cu3Oy (123) crystals was obtained in the sample grown at the low growth rate of 1 mm/h The volume of the 211 phase changed drastically from liquid to 123 crystal These results lead to the idea that the necessary solute for peritectic reaction is provided through a liquid Based on this idea, we developed a simple solidification model that is in good agreement with the experimental results

Grain Defect Formation During Directional Solidification of Nickel Base Single Crystals

Abstract: Grain defect formation during solidification of nickel base single crystals has been studied in a series of alloys over a wide range of imposed thermal gradients and growth rates. Solute-driven convective instabilities, associated with high levels of Re and W and/or low levels of Ta, promote the formation of “freckle” defects as well as dendrite fragmentation and nucleation of misoriented grains. As the imposed positive thermal gradients at the solid-liquid interface decrease, grain nucleation events become more frequent until a general columnar to equiaxed transition is encountered. Transitions in solidification behavior and are shown to be consistent with changes in dendrite morphology and operative regimes of grain defect formation are characterized in terms of primary dendrite arm spacings. Suuerallovs 1992 Edited by S.D. Antolokch, R:W. Stusrud, R.A. MacKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom The Minerals, Metals & Materials Society, 1992

Control of grain alignment to fabricate long Y‐Ba‐Cu‐O superconductors with high current density

Abstract: A directional solidification method has been developed to grow YBa2Cu3Ox samples in excess of 55 mm in length with uniform grain alignment and a‐b planes oriented parallel to the sample length. Using a furnace configuration that yields a negative lateral gradient, the YBa2Cu3Ox grains have been forced to nucleate and grow along the axis instead of at an angle to the sample length. X‐ray pole figures analyses along the entire length at different sections show that the a‐b planes are aligned parallel to the sample length with deviations less than 5° along the growth direction. Transport critical current density (Jc) measurements over the entire length and at different sections of the sample reveal a uniform current density in excess of 7000 A/cm2 at 1.5 T and 77 K.

Boundary-conforming mapping applied to computations of highly deformed solidification interfaces

Abstract: A new boundary-conforming mapping is developed for the calculation of highly deformed cellular solidification interfaces in a model of directional solidification of a binary alloy. The mapping is derived through a variational formulation that is designed so that the grid penetrates the grooves between cells along the interface without causing a loss of ellipticity of the mapping equations. A finite element/Newton method is presented for simultaneous solution of the free boundary problem described by the solutal model of directional solidification and the mapping equations. Results are compared to previous calculations and demonstrate the importance of accurate representation of the interface shape for understanding the solution structure.

Cold-crucible directional solidification of refractory metal-silicide eutectics

Abstract: Microcomposite structures generated by directional solidification of eutectics offer many unique microstructural advantages, including thermodynamic stability, directional alignment, and a fine dispersion of component phases. In an effort to improve the performance of advanced turbine engines, a Czochralski crystal growth method in conjunction with cold-crucible induction melting has been developed for the production of high-temperature eutectic composites. Cold-crucible directional solidification has been successfully applied to the growth of refractory metal-silicide eutectics into directional microcomposites. The eutectics studied include Cr-Cr3Si,Nb-Nb3Si,and V-V3Si, which have melting temperatures ofl,705°C, 1,880°C, and 1,870°C, respectively.

Dynamics of banded structure formation in rapid solidification.

Abstract: Motivated by widespread experimental observations of the ``banded structure'' in rapidly solidified alloys, we have studied numerically by Green's-function method the dynamics of the planar interface under rapid directional solidification condition. We find that the formation of this structure can be explained and partially characterized quantitatively in terms of highly nonlinear ``relaxation oscillations'' of the interface whose spacings are controlled by latent heat diffusion but not by the thermal gradient. A comparison with experiment is presented for Al-Fe alloys.

The effect of gravity modulation on thermosolutal convection

Abstract: In a gravitational field, the opposing effects of components of different diffusivities, for example, temperature and solute, in the density profile in a fluid may produce convective instabilities that exhibit a broad range of dynamical behavior. The effect of time periodic vertical gravity modulation on the onset of these instabilities in an infinite horizontal layer with stress free boundaries is examined. This work is viewed as a first step in expanding previous results in solidification to the full problem of characterizing the effects of gravity modulation in thermosolutal convection during the directional solidification of binary alloys. Calculations carried out both with and without steady background acceleration are presented, the latter results being relevant to microgravity conditions.

Attenuation of ultrasonic waves in cubic metals having elongated, oriented grains

Abstract: The attcntuation of ultrasonic waves propagating in polycrystalline metals is a subject which has received considerable experimental and theoretical attention. Important recent contributions include the work of Hirsekorn and of Stanke and Kino. In this paper, we extend the approach of the latter authors to a particular problem motivated by the cast stainless steels that are utilized in the piping of nuclear reactors. In those materials, large columnar grains often are found as a result of the directional solidification occurring in the casting. To model the effects of these grains on the ultrasonic attenuation, we consider a medium consisting of ellipsoidal grains with the axes of revolution all parallel to a preferred direction and having an arbitrary aspect ratio. In each grain, one of the axes of the cubic lattice is assumed to lie parallel to the preferred direction while the rotation about this axis is random. The unified theory of Stanke and Kino, based on an anisotropic extension of the Ke...

Dynamical wavelength selection by tilt domains in thin-film lamellar eutectic growth

Abstract: Tilt domains are dynamical defects of cellular growth fronts consisting of a group of asymmetric cells traveling laterally along the front. We study the dynamics of these defects in thin-film directional solidification of the CBr 4 -C 2 Cl 6 eutectic alloy. We show that the sweeping of the growth front by tilt domains, at a given growth velocity V, brings it into a well-defined «dynamically selected» state. Once this state is reached, the tilt domains travel with a constant width. A sudden increase (decrease) of the growth velocity transforms constant-width tilt domains to growing (shrinking) ones

Directional Solidification of Salt Water: Deep and Shallow Cells

Abstract: The common observation of cellular substructure ((1 ÷ 10-2)mm) at the sea-ice-ocean interface is explained by modelling the natural solidification of seawater as that of a dilute H2O-NaCl solution. Linear and nonlinear perturbation theories reveal that the one-dimensional planar steady state is morphologically unstable, and a bifurcation to cells occurs in geophysically relevant growth regimes. We compute the range of solidification velocity Vc < V < Va in which the system is unstable for fixed far-field solute concentration C∞, and bound the geophysical observations. The system exhibits weak wavelength selection near Vc and the nonlinear theory shows that the bifurcation to cells is subcritical.

Macrosegregation During Steady-State Arrayed Growth of Dendrites in Directionally Solidified Pb-Sn Alloys

Abstract: The paper examines the growth parameter dependence of the longitudinal macrosegregation due to interdendritic thermosolutal convection during directional solidification of Pb-Sn alloys with a steady-state dendritic arrayed morphology. The experimentally observed longitudinal macrosegregation can be described by an empirical parameter, effective partition coefficient (kE), obtained from Cs = kEC0(1 - fs) super KI-1, where the original tin content is C0 and that corresponding to the fraction distance solidified (Fs) is Cs. Channel formation is strongly associated with the onset of thermosolutal convection in the interdendritic melt. The critical effective Rayleigh number for the onset of channel formation is about 0.07.

Unidirectional dendritic solidification under longitudinal resonant vibration

Abstract: Longitudinal steady-state sinusoidal vibration is introduced into the unidirectional dendritic solidification process of Al−3% Mg alloy. The first (470 Hz), second (1050 Hz), and third (1736 Hz) order resonant frequencies of solidification system is utilized to produce strong vibrational response out of less exciting energy. Solidification front temperature is raised by vibration under constant solidification conditions, whereas liquid temperature gradient ahead of solidification front and solidification rate are both reduced. Dendrite fragmentation is the most conspicuous variation of structural morphology, which is characterized by primary stalk disintegration and secondary arm detachment. The first order resonance results in a periodically fractured bamboo-like dendritic structure and most severe solute segregation. Contrary to its grain refinement effect, vibration expedites the coarsening of resultant structures. The alloy mechanical properties are markedly deteriorated under the first order resonance. But they are appreciably improved if the second and third order resonant vibrations are applied.

Salt‐finger instability in an anisotropic and inhomogeneous porous substrate underlying a fluid layer

Abstract: In the directional solidification of a binary liquid, salt‐finger and plume convection are the two major types of fluid motion that occur when the lighter species is rejected from the solid. Fingering convection is usually observed prior to the initiation of plume formation. When the nominally planar liquid‐solid interface is morphologically unstable, solidification is dendritic. Consequently, the melt is separated from the solid by a mushy zone consisting of solid dendrites and interdendritic liquid. This can be considered to be a porous layer with anisotropic and inhomogeneous permeability and, possibly, thermal and solutal diffusivities. A linear stability analysis is empoyed to study salt‐finger instability in a fluid layer bounded above by a rigid wall and below by an anisotropic and inhomogeneous porous medium saturated by the same fluid. The effects of anisotropy and inhomogeneity on the stability characteristics are discussed for a variety of porous media. Results are also presented for a porous medium for which the anisotropy and inhomogeneity of the permeability and thermal and solutal diffusivities have been obtained experimentally during the directional solidification of aqueous ammonium chloride solutions. The nature of the instability modes relevant to the present problem are also discussed. Based on the present results, a relationship is proposed between salt‐finger and plume convection during directional solidification. Methods for inhibiting salt‐finger convection are also suggested, and the feasibility of their implementation is discussed.

Blow up and growth in the directional solidification of dilute binary alloys

Abstract: Numerous asymptotic equations have been derived to describe the evolution of the solid-liquid interface which occurs during the directional solidification of a dilute binary alloy. In the limit of a small distribution coefficient, many of these equations have been of the form [l-4] where α and K are positive constants. More recently the equation has also been derived in this context by a modified asymptotic method,which more accurately preserves some of the original nonlinearities [5]. Both of these equations fall into the class of equations where f(u) is a positive convex function,where r, K and α are constants and where K and α are positive. For (0.1) and (0.2), we demonstraie that if the solution breaks down in finite time, then blowup of the L∞norm occurs. Furthermore, if K is sufficiently large, a=0, and if the initial data is sufficiently small, then solutions exist globally and decay to zero exponentially.Estimates on the growth of the H −1norm are also given. Sufficient conditions for blow up for ...

Interflake spacing–growth velocity relationship in Al–Si and Al–CuAl2 eutectic alloys

Abstract: Temperature and interparticle spacing measurements are presented for directionally solidified Al–CuAl2 and Al–Si eutectic alloys. The results show that care must be taken to ensure that steady state heat flow conditions are established during directional solidification and that the correct definition of spacing is used to obtain interparticle spacing–growth velocity relationships that can be used to assess current eutectic solidification models. The results presented for the Al–Si eutectic show a strong temperature gradient dependence of the spacing–growth velocity relationship which is not predicted by the most recent detailed solidification models.MST/1516

Method for directional solidification casting of a titanium aluminide

Abstract: A method for directional solidified vacuum or protective atmosphere casting of gamma titanium aluminide alloys in a mold comprised of a chill and a sidewall means extending from the chill to form a cavity for holding a molten metal, the method comprising, forming a melt of the gamma titanium aluminide alloy in the cavity, the melt being comprised of a metal from the group consisting of niobium, tantalum, tungsten, and molybdenum in an effective amount to reduce oxygen pickup in the melt, the sidewall means having at least an inner liner of a calcia refractory facing the melt. The melt is heated in a thermal gradient sufficient to cause directional solidification of the melt from the chill.

The effect of compositionally-generated elastic stresses on morphological instability during directional solidification

Abstract: The effect of compositionally-generated elastic stresses on the conditions for morphological instability during directional solidification of a dilute, ideal binary alloy is investigated using the Gibbs-Thomson equations for an elastically-stressed solid with zero surface stress in equilibrium with a liquid. It is found that these stresses lead to a small stabilization of the Mullins and Sekerka cellular mode of instability. The steady mode is stabilized by a stress-induced modification to the interfacial concentration of the solid, which in turn alters the amount of solute rejected to inhibit the growth of perturbations. The presence of elastic stresses could generate a new oscillatory instability which is most likely to be found in experiments near absolute stability for materials with segregation coefficients near unity and large solute expansion coefficients.

Method and apparatus to effect a fine grain size in continuous cast metals

Abstract: Method and apparatus used for continuous casting of copper alloy rods and tubes for obtaining a fine grain size therein. Liquidus copper alloy material flows from a reservoir area or crucible into a continuous casting die through spaced apart feed slots disposed in the casting die cap, thereby effecting agitation of the liquidus material, which prevents the formation of thermal gradients large enough to produce gross directional solidification of the alloy at the liquid-solid state transition zone. The desired agitation is effected by altering the number, location, size and angle of the feed slots in accordance with formulas provided for that purpose.

Dendritic growth during directional solidification of hypoeutectic Fe-C-Si alloys

Abstract: The liquid decanting technique has been used to study the morphology of dendrites in directionally solidified Fe-3.08 pct C-2.01 pct Si alloy. The experimental results indicated that the morphology of primary dendrites in the Fe-C-Si system is very similar to those obtained in some transparent metal model systems and in some other metal systems. In order to study the morphological transition between cellular and dendritic growth, directionally solidified samples were quenched in cold water at various stages of solidification and the morphology was examined on the polished and etched surface. It has been found that when the growth velocity decreased from 326.6 to 0.8 μn/s, the average dendrite tip radius increased from 1.12 to 33.1 μm. At a growth velocity of about 0.65 μm/s, a transition from dendritic to cellular growth occurred. Models for dendritic growth proposed by various investigators have been briefly reviewed and compared with the present experimental results. Significant disagreements were found for some of the available theoretical models. Possible explanations have been given for these disagreements.

Intermetallic/Metallic Polyphase In-Situ Composites

Abstract: To evaluate various in-situ reinforcement schemes, a computer controlled containerless directional solidification system has been used to produce NiAl-based polyphase composites containing up to two intermetallic phases and at least one ductile phase. Systems evaluated include Ni-Al-Cr, Ni-Al-Mo, Ni-Al-V ternary systems that form NiAl/α-refractory metal eutectics and a three phase eutectic in the Ni-Al-Cr-Nb system. Initial screening of these in- situ composites has included morphological characterization, four point bend testing, temperature dependent yield strength evaluation and compressive creep testing. Occasional growth defects termed “banding” currently interrupt the continuity of these composite structures and limit the attainment of optimum properties. However, both the creep strength and toughness of NiAl were improved by in- situ reinforcement.