Microstructural development in undercooled lead- tin eutectic alloys
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I.
INTRODUCTION
UNDERCOOLING an alloy melt weft below its equilibrium freezing temperature prior to solidification is now an established means of promoting novel microstructures with potentially unique material properties.m In contrast to powders or flakes, which require compaction, the controlled nucleation of a large highly undercooled melt might lead to a "near-net-shape" product with a uniform and directional microstructure characterized by a fine dendritic network, metastable phases, or other desirable attributes. Such processing might be extended to the microgravity environment of space where large samples could be processed. Furthermore, density-driven convective flow in the alloy melt would essentially be eliminated and the melt would not need to be physically contained, a factor which should facilitate achieving large undercoolings in high melting temperature and/or reactive materials while maintaining their purity. Nucleation and growth of an undercooled melt is far removed from the steady-state conditions upon which most theories are based. In this regard, consideration must be given to the dynamics of the process in which the approach to steady state and the ever-changing kinetics of the process are taken into account. Thus, before such processing becomes a reality, there exists the need to explore the underlying theory by conducting an initial series of unit-gravity experiments. The intent of this work is, therefore, to investigate undercooled lead-tin alloys of eutectic composition with the goal of gaining insight regarding microstructural
FAY HUA, Graduate Research Assistant, is with the Center for Microgravity Research and Applications, Vanderbilt University, Nashville, TN 37235. R.N. GRUGEL, Associate Scientist, is with the Universities Space Research Association, c/o Marshall Space Flight Center, Huntsville, AL 35812. This article is based on a presentation made in the symposium entitled "Microgravity Solidificatiort, Theory and Experimental Results" as a part of the 1993 TMS Fall meeting, October 17-21, 1993, Pittsburgh, PA, under the auspices of the TMS Solidification Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
development prior to processing in a microgravity environment. II.
EXPERIMENTAL PROCEDURE
A. Model Development
The solidification model is Newtonian in nature, follows that of Levi and Mehrabian, t21 and is developed with the following assumptions. (1) The latent heat serves to heat up the undercooled eutectic sample and its surroundings. Cooling of the sample is due to steady and uniform heat loss from the surface due to convection and is represented by an overall heat-transfer coefficient, h, that was experimentally determined and assumed to be constant. (2) The solidification interface grows in a stable radial configuration from the point of nucleation. (3) Uniform temperatures are assumed in the sample as calculation of the Blot number is much less than 0.1. (4) Solidification velocity is dependent on the undercooling at the interface, V = K A 7 ~, where K is a consta
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