Fluid flow in solidifying monotectic alloys
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I.
I N T R O D U C T I O N AND M O T I V A T I O N
IN order to visualize phenomena
associated with the solidification of alloy systems, transparent alloy systems have been employed as models for actual metallic alloys for some time. tq In practice, the microstructure and compositional features of a given metallic casting determine, to a certain extent, its physical properties and, ultimately, its utility. An understanding of the processes operative during solidification of metallic alloys is a useful way to identify controlling parameters. Such knowledge can eventually be used to optimize the production of cast metallic alloys. This work describes phenomena observed during the directional solidification of the transparent alloy systems succinonitrile-water (SCN-W) and succinonitrile-ethanol (SCN-E). Both these systems serve as models for m o n o t e c t i c metal alloys. The primary focus of this study is on the nature of fluid flow in the melt, its relation to alloy composition, and its influence on mass transport. Observations made using holographic and shadowgraph techniques, although primarily qualitative, are supplemented by quantitative data obtained from processed two-wavelength holographic measurements. We used two-wavelength holography to obtain temperature and composition profiles. In contrast to the single-wavelength technique, the use of an additional laser affords the opportunity to simultaneously measure temperature and concentration without the use of thermocouples in the melt. Figure 1 shows a generic phase diagram for a monotectic system. It has been subdivided into seven compositional regions. The primary sources of fluid motion in this system are buoyancy and thermocapillary forces. These forces act together in regions III through A. ECKER, Scientist, is with Dornier System Gmbh, P.O. Box 1360, D-7990, Friedrichshafen, Federal Republic of Germany. D.O. FRAZIER, Scientist, is with the Space Science Laboratory E575, Marshall Space Flight Center, Huntsville, AL 35812. J. IWAN D. ALEXANDER, Scientist, is with the Center for Microgravity and Materials Research, The University of Alabama in Huntsville, Huntsville, AL 35899. Manuscript submitted March 13, 1987. METALLURGICAL TRANSACTIONS A
V, where phase separation results in the formation of droplets in the immiscible two-phase regions (this occurs at the solid-liquid interface and in the bulk melt). Buoyancy forces arise due to density differences between the droplet and the host phase, but thermocapillary forces (associated with temperature gradients in the droplet surface) may predominate.f2; In other regions, buoyancy resuits from density gradients caused by temperature and compositional gradients in the single-phase bulk melt. The effects of buoyancy associated with the latter are evident in regions IV and V, owing to the existence of volumes of water- or ethanol-rich liquid. Fluid flow dominates mass transport. Thus, compositional profiles in the melt differ significantly from purely diffusive profiles. The following sections describe the exp
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