Composite growth in hypermonotectic alloys
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I.
INTRODUCTION
THE binary miscibility gap systems of interest are characterized by (1) a region where two distinctly different liquids are in thermodynamic equilibrium and (2) the presence of the monotectic reaction, LI = SI + LI. Though similar in form to the well-studied eutectic, two possibilities exist at the monotectic reaction: Ln can either wet or not wet S~. Employing controlled directional solidification techniques, the microstructure of the former case exhibits a uniform, hexagonal close-packed array of aligned La fibers, whereas in the latter, L n droplets collecting at the interface are pushed and eventually physically incorporated, leading to a somewhat irregular structure. This has been theoretically discussed by Chadwickl~j and CahnI21 and experimentally verified by Livingston and Cline E3j and Grugel and Hellawell. f4] Furthermore, the transition from wetting to nonwetting has been shown to change abruptly through the selected addition of a ternary component, both in organic [5'61 and metal t4J systems. Uniform solidification of hypermonotectic alloys has been hampered by the inherent, usually large, density differences between the Z I and Ln phases. This leads to rapid separation, coalescence, and, consequently, a highly inhomogeneous structure. Note, however, that this can be minimized by employing rapid solidification techniques, t7,8~ With the advent of processing in a microgravity environment, it was envisioned that elimination of gravity-induced sedimentation would result in a uniform composite of finely dispersed L n (eventually Sti) in the S~ matrix. This generated considerable interest; t8-651 unfortunately, the experiments which examined gravity, or lack of it, as a processing variable still resulted in highly macrosegregated structures (References 14 through 19, 21, 24, 28, 30, 32, 37, 39, 45, 50, 52, 59, 61, and 63 through 65). A number of explanations for these resuits have been proposed, including phase diagram accuracy (References 15 and 17), insufficient mixing (References 14, 15, 17, 21, 24, and 63), residual or inR.N. GRUGEL, Research Assistant Professor, is with the Department of Materials Science and Engineering, Center for Microgravity Research and Applications, Vanderbilt University, Nashville, TN 37235. Manuscript submitted June 27, 1990. METALLURGICAL TRANSACTIONS B
duced convection (References 16, 18, 21, 24, 39, 45, 52, 59, 61, and 63), thermal gradient effects (References 16, 17, 21, 28, 30, 32, 37, 39, 45, 52, and 63), droplet coalescence (References 17, 18, 21, 24, 28, 30, 32, 37, 39, 45, 52, 61, and 63), and preferential wetting (References 16 through 19, 24, 28, 30, 32, 37, 52, 59, 63, and 64) of the container by one of the liquid phases. The intent of this study is to eliminate the former four factors and to take advantage of the latter two. II.
EXPERIMENTAL PROCEDURE
A. Model Systems
This study utilized models based on the organic succinonitrile (SCN)-glycerine (GLC) and succinonitrileethanol (EtOH) systems. Both systems exhibit a miscibility gap, Figure 1. SC
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