Modeling of collision and coalescence of droplets during microgravity processing of Zn-Bi immiscible alloys
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I.
INTRODUCTION
THE
production of uniform, homogeneous microcomposites from materials having different densities, such as liquid-phase miscibility gap systems or fine dispersions of solid particles in a host matrix, is desirable for many applications, for example, as engineering and electronic materials, ductile high-temperature superconductors, and self-lubricating bearings, ll.z,3~ Microcomposite materials are composite materials which have a homogeneous, finely dispersed phase in a host matrix, in which the occlusions of the dispersed phase are on the order of microns in size. On Earth, such systems separate rapidly in the molten phase because of sedimentation and buoyancy, and strictly ground-based studies of microcomposite processing methods have been rather limited. However, because of the predicted properties of microcomposite materials, there has been a great deal of interest in the study of microcomposite processing methods in microgravity environments, where the effects of sedimentation and buoyancy are greatly reduced. Microgravity environments which have been used for these studies include drop towers, sounding rockets, Skylab, and the Space Shuttle. One class of materials in which there has been a great deal of interest is bimetallic composite materials. An immiscible alloy of two metals can be produced from a liquid-phase miscibility gap system. Over 500 binary metallic miscibility gap systems have been identified, f41 The phase diagram of liquid-phase miscibility gap materials is characterized by the presence of a region in which two liquid phases are in equilibrium with one an-
J.R. ROGERS, Doctoral Student, and R.H. DAVIS, Associate Professor, are with the Department of Chemical Engineering/Center for Low-Gravity Fluid Mechanics and Transport Phenomena, University of Colorado, Boulder, CO 80309-0424. Manuscript submitted March 13, 1989. METALLURGICAL TRANSACTIONS A
other. When heated above the miscibility gap, the components become miscible and are in a true solution. When the material is cooled into the miscibility gap, the components are no longer miscible, and two liquid phases develop. By a process of nucleation and growth by diffusion, small droplets rich in one material develop in a matrix rich in the other material. Microgravity processing studies of immiscible alloys involve a number of steps. The preliminary steps are performed on Earth. The raw materials are mixed in the composition of interest, heated to a temperature well above the miscibility gap, and solidified by rapid cooling in order to reduce segregation of the liquid phases due to buoyancy. Magnetic or ultrasonic mixing methods may be employed to help produce a uniform dispersion. The resultant material can be analyzed to determine the initial particle size distribution. This base material is then prepared for experiments in microgravity. Samples are machined to fit into test cells, which are approximately 1 centimeter in diameter and, at most, a few centimeters in length. Once in the microgravity environment, the liquid dis
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