Solutal convection induced macrosegregation and the dendrite to composite transition in off-eutectic alloys
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EXPERIMENTAL PROCEDURES
THE ability to solidify two-phase aligned structures from a wide range of compositions near a eutectic composition has importance in the full utilization of this class of composite materials. The early work of Mollard and Flemings t claimed that the range of compositions for which plane front composite growth was possible was restricted not only by constitutional supercooling requirements but also by the requirement that convection be avoided. In fact, their experimental work using vertical upward solidification was limited to the Sn-rich side of the Pb-Sn eutectic where the fluid density, influenced by temperature and composition, always decreases with distance from the interface (height). On the Pb-rich side of the eutectic, the concentration gradient in the liquid can cause the density to increase with distance near the interface and possibly cause convection (here termed solutal convection). Subsequently, it was argued 2,3 that as long as a stagnant boundary layer exists whose thickness is large compared to the eutectic spacing, plane front growth of the composite should be possible in the presence of convection. This was in fact verified by experiments 2,4,5 which demonstrated the growth of Pb-rich Pb-Sn composites. However, Verhoeven, Kingery and Hofer 6 subsequently showed that significant macrosegregation (10 wt pct) down the length of solidified samples occurs in samples only 1 wt pet on the Pb-rich side of the Pb-Sn eutectic, and attributed this macrosegregation to solutal convection. The purpose of this research is to describe the microstructure and macrosegregation produced during upward solidification of off-eutectic alloys 2 to 5 wt pct from the eutectic composition when the liquid density near the interface increases with distance from the interface and to determine if a magnetic field or solidification downward can eliminate solutal convection and hence macrosegregation.
Alloys were prepared from 99.999 pct pure Sn and Pb in evacuated and sealed silica crucibles. These alloys were quenched and swaged into 1 cm diam rods. Pieces from these rods were vacuum cast into 3 mm ID silica tubes and quenched. The directional solidification furnace is constructed of nonmagnetic materials and has a bifilar wound cylindrical resistance heating element with upper and lower sections and circulating water cooled chill zone. A silica tube containing the desired alloy is pulled down through the chill, sealed by O-rings, at constant velocity to accomplish directional solidification. The cooling water comes directly in contact with the silica tube. Using two separate AC proportional temperature controllers for the furnace, and water cooled by a heat exchanger, temperatures are constant to 0.1 ~ over an hour and 1 ~ over several days. The vertical drive is powered by a synchronous motor with a multispeed gear box. Speeds from 0.5 /~m/s to 0.5 m m / s are possible. The tube is attached to the pull shaft using a device which permits the sample to be suddenly pulled down about 3 cm to quench the l
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