Showing papers on "Directional solidification published in 1987"

Advanced Superalloy Airfoils

Abstract: Significant progress has recently been made with regard to the elements that compose a modern gas turbine airfoil. These aspects include alloy, casting, coating, and internal cooling design. Current single-crystal super-alloy turbine airfoils represent a significant improvement in temperature capability over their predecessors, polycrystal and columnar crystal alloys. Single-crystal superalloy technology has combined the design of alloys for exclusive use in single-crystal form with advances in directional solidification to produce single-crystal castings with complex internal cooling passages. In addition, metallic and ceramic coatings have been developed to provide even greater temperature capability.

Thermosolutal convection during directional solidification. II. Flow transitions

Abstract: The effect of thermosolutal convection on the solute segregation in crystals grown by vertical directional solidification of binary metallic alloys or semiconductors is considered. Numerical results are obtained using finite differences in a two‐dimensional, time‐dependent model that assumes a planar crystal–melt interface. The configuration is assumed to be periodic in the horizontal direction with a given period, and the possibility of multiple flow states sharing the same period is examined. The results are summarized in bifurcation diagrams of the nonlinear states associated with the critical points of linear theory. The use of a time‐dependent numerical scheme results in gaps in the bifurcation diagram where presumed unstable states exist that cannot be computed by this procedure. As the solutal Rayleigh number is varied, multiple steady states, time‐periodic states, and quasiperiodic states may occur. This case is compared to the simpler case of thermosolutal convection with linear vertical gradients and stress‐free boundaries, for which a rather complete numerical treatment is possible through the use of a simple spectral representation of the nonlinear solution. Retaining a finite number of terms in the expansion results in a set of coupled nonlinear algebraic equations for the Fourier coefficients, which is solved by a quasi‐Newton method. The linear stability of the nonlinear solutions is also determined from a numerical computation of the eigenvalues of the Jacobian matrix. The resulting bifurcation diagram shows qualitative similarity to the bifurcation diagram computed for the solidification system.

Petrov‐Galerkin methods for natural convection in directional solidification of binary alloys

Abstract: A Petrov-Galerkin finite element method is presented for calculation of the steady, axisymmetric thermosolutal convection and interface morphology in a model for vertical Bridgman crystal growth of nondilute binary alloys. The Petrov-Galerkin method is based on the formulation for biquadratic elements developed by Heinrich and Zienkiewicz and is introduced into the calculation of the velocity, temperature and concentration fields. The algebraic system is solved simultaneously for the field variables and interface shape by Newton's method. The results of the Petrov-Galerkin method are compared critically with those of Galerkin's method using the same finite element grids. Significant improvements in accuracy are found with the Petrov-Galerkin method only when the mesh is refined and when the formulation of the residual equations is modified to account for the mixed boundary conditions that arise at the solidification interface. Calculations for alloys with stable and unstable solute gradients show the occurrence of classical flow transitions and morphological instabilities in the solidification system.

Testing shape selection in directional solidification.

Abstract: We report results from an experiment on the directional solidification of pivalic acid. In addition to the usual cellular interface patterns, we observe a new, metastable dendritic form that can be interpreted in the light of recent ideas about pattern formation.

Study of solid-liquid interfaces during solidification, using ultrasonic waves.

Abstract: We have studied the onset and evolution of instabilities at the solid-liquid interface of tin undergoing directional solidification, as a function of growth velocity, temperature gradients, and impurity content. The initiation of an instability that we identify as "cellular" growth, and its passage to pseudosteady state, have been investigated. Within this steady state a new oscillatory phenomenon has been found, indicative of changes in the interface configuration that are quasiperiodic in time.

Morphological and thermosolutal instabilities inside a deformable solute boundary layer during directional solidification. I.— Theoretical methods

Abstract: The linear perturbation analysis of morphological and thermosolutal instabilities at the growth front during directional solidification of a binary alloy is generalized to the case where bulk convection exists in the basic state, as is encountered in most cases, in practice. the model assumes a finite and deformable solute boundary layer DELTA . The compatibility condition is obtained in analytical form for the marginal case of exchange of stabilities. The present model is shown to be consistent with literature results in the limiting case of infinite DELTA .

Cellular-dendritic transition in directionally solidified binary alloys

Abstract: The microstructural development of binary alloys during directional solidification is studied. Cellular growth data for the Al-Cu and Pb-Sn binary alloy systems are analyzed in order evaluate the criteria of Kurz and Fisher (1981) and Trivedi (1984) for cellular-dendritic transition. It is observed that the experimental growth values do not correlate with the Kurz and Fisher or Trivedi data.

Results from a compound semiconductor crystal growth experiment in a low gravity environment

Abstract: A directional solidification experiment in the compound semiconductor lead tin telluride has been carried out in a low-gravity environment on board the Space Shuttle during a flight in October, 1985. Optical etching techniques and concentration profiles from wavelength dispersive electron microscopy indicate that there was a significant amount of convection during the solidifcation of the sample. A preliminary hypothesis of this result, that even at gravity levels on the order of 0.0001 g, the convective forces can be significant compared to mass transport due to diffusion, is being tested.

Heat-resistant Ni-base single crystal alloy

Abstract: A heat-resistant Ni-base single crystal alloy, suitable as a material for blades and vanes of gas turbines or jet engines, consists of, by weight, 5 to 10% of Co, 5 to 10% of Cr, 9 to 15% of W, 1 to 5% of Mo, 4.5 to 6% of Al, 5 to 10% of Ta, and the balance substantially of Ni with the total proportion of W, Mo and Ta being 17 to 24%, and is produced by directional solidification of a melt of the above-specified composition, followed by solution treatment and aging treatment.

Microstructures of directionally frozen in situ composites of multicomponent eutectics

Abstract: Microstructural variations of binary (Sn-Pb, Sn-Cd, Pb-Cd), ternary (Sn-Pb-Cd) and quaternary (Sn-Pb-Cd-Zn) eutectics which were permitted to solidify directionally were investigated in terms of the controlled solidification rates. It was found that, at an appropriate freezing rate, the structure was lamellar and satisfied the Jackson-Hunt relationship, and furthermore, that the order of the layers present in the solidified eutectics was definite, such that in the case of Sn-Pb-Cd ternary eutectic alloy, for instance, the Sn-Cd layers would not be allowed to be in contact. An explanation for this could be the high interfacial energy between the two.

Periodic Si-CoSi2 eutectic structures

Abstract: Submicron periodicity Si‐CoSi2 structures were fabricated by the directional solidification of eutectic thin films. The films, grown on oxidized Si wafers by laser recrystallization, were prepared without the ablation and cracking previously encountered. Using growth rates of up to 17 cm/s yielded lamellar periods as small as 60 nm. Defects in the lamellar structure were investigated as a function of growth rate, the shape of the solid–liquid interface, and thermal aging.

Ultrasonic technique for the study of solid-liquid interface in metallic alloys

Abstract: We have developed a new ultrasonic technique which realizes the in site or "real time" observation of the solid-liquid interface (SLI) in metallic alloys. In this technique, the location and velocity of the SLI can be determined from the transit time measurements of the echo from the SLI. The roughness of the interface can also be determined from amplitude measurements of the echo. This technique has been successfully applied to the observation of directional solidification in tinalloys.

Microsegregation in dendritic single crystals of nickel-rich alloys

Abstract: Segregation effects caused by unidirectional solidification of nickel-base alloys have been studied. It was found that: the composition in the branches of dendrites and y′-particles is constant along the growth path; the segregation of titanium, aluminium, and molybdenum increases towards the interdendritic region and of nickel and chromium increases towards the base of the dendrites; the occurrence of y–y′ nodules increases in those parts of the castings which solidified in regions remote from the chill plate.MST/237

Directional solidification and densification of permanent magnets having single domain size MnBi particles

Abstract: Directional solidification of Bi and Mn compositions to produce magnetic single domain size MnBi particles with aligned morphologies followed by post heat treating and densification.

Natural convection during directional solidification of an AlZn eutectic

Abstract: A study was made of the buoyancy convection of melt provoked by temperature gradient of an orientation identical to that of the gravity vector. The convection was assessed on the basis of temperature fluctuations in the melt. The critical Rayleigh numbers for periodic and turbulent flow were determined. Both periodic and turbulent flow gives rise to transverse “bands” in the structure of the eutection obtained by directional solidification. The intensity of unstable flow affects the number of eutectic crystallites in the sample. [Russian Text Ignored].

Spatial profile of thermoelectric effects during Peltier pulsing in Bi and Bi/MnBi eutectic

Abstract: The spatial profile of the thermal transients that occur during and following the current pulsing associated with Peltier Interface Demarcation during directional solidification is studied. Results for pure Bi are presented in detail and compared with corresponding results for the Bi/MnBi eutectic. Significant thermal transients occur throughout the sample that can be accounted for by the Peltier effect, the Thomson effect, and Joule heating. These effects are separated and their behavior is studied as a function of time, current density, and position with respect to the solid/liquid interface.

Internal quality improvement method for lh directional solidified ingot

Abstract: PURPOSE:To improve an internal quality for LH directional solidified ingot by pouring in a range of a specified relation for a super heating degree of molten steel at the time of executing the directional solidification from a bottom surface on a surface plate toward an upper part. CONSTITUTION:A mold 1, which is covered by a heat insulating material 3 on the inside wall thereof in substance except a bottom part of the mold 1, is used and the casting ingot is produced to directional solidification from the bottom surface on the surface plate 2 toward the upper part, as keeping the hot temp. of the top part for the casting molten steel 4 by a heat insulating material 5. In this case, the over heating degree for the molten steel DELTAT is as under-mentioned. 85 deg.C