Directional solidification of Cu- Pb and Bi- Ga monotectic alloys under normal gravity and during parabolic flight
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I.
INTRODUCTION
CONTROLLED production of aligned rod morphologies in monotectic systems in a manner analogous to that in the eutectic systems is an attractive goal. While aligned structures have been produced in the most common commercial monotectic system, Cu-Pb, it is not clear by which mechanism these aligned structures are obtained. Therefore, process control to produce desired structures is difficult. The phase diagram of monotectic alloys is characterized by a dome-shaped region where two immiscible liquid metals exist. The importance of wetting on the nature of monotectic solidification has been discussed by Cahn. tl] Cahn's model has been corroborated by Grugel and Hellawell. t21 Earlier, Livingston and Cline t31conducted extensive work on Cu-Pb systems and correlated structure with composition and solidification conditions. The effect of ~avity on the solidification of Cu-Pb alloys has been studiedtl by solidifying (under terrestrial conditions) parallel and antiparallel to the gravity vector. The above studies suggest that the mechanism controlling the aligned eutectic growth differs from that controlling aligned monotectic growth. Although some researchers have suggested a mechanism inferring particle pushing and engulfment behavior in front of a solidifying interface as enunciated by Uhlmann et al.,tsl none has elaborated in detail as to the specifics of the mechanism. In the present study, the effect of the magnitude of gravity during solidification on the microstructure of Cu-Pb and Bi-Ga alloys is examined to better understand the mechanism of formation of aligned structures in these systems. II.
EXPERIMENTAL
The Cu-Pb samples were prepared by melting 99.9 pct pure lead shots and 99.9 pct pure copper pellets under argon atmosphere. The samples for directional solidification were vacuum pulled from the melt into 4 mm and 10 mm B.K. DHINDAW (permanent position: Professor at liT Kharagpur, India), Visiting Scholar, and D.M. STEFANESCU, Professor, are with the Department of Metallurgical Engineering, The University of Alabama, P.O. Box G, Tuscaloosa, AL 35487. A.K. SINGH, formerly Graduate Student, Department of Metallurgical Engineering, The University of Alabama, is with Inland Steel, East Chicago, Indiana. P.A. CURRERI is a Materials Scientist with NASA/Marshall Space Flight Center, Huntsville, AL 35812. Manuscript submitted December 7, 1987. METALLURGICALTRANSACTIONSA
I.D. quartz tubes and quenched in air. Bi-Ga samples were prealloyed by mixing the appropriate weights of 99.999 pct pure Ga and Bi metals in a quartz tube which was connected to a high vacuum system with the capability to backfill with inert gas. At a pressure of 10-6 torr, the tube was heated by a torch to a temperature above the Bi-Ga consolute temperature. The ampoule was agitated continuously during heating to achieve a homogeneous mixture. The molten sample was then quenched in liquid nitrogen. Directional solidification was accomplished in one of two Bridgman type directional solidification furnaces. The first is a Space App
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