The effect of enhanced gravity levels on microstructural development in Pb-50 wt pct Sn alloys during controlled directi
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I.
INTRODUCTION
IT is well established~ that controlled directional solidification of metals and alloys can significantly enhance the material properties of, for example, turbine blades. Unfortunately, composition or temperature gradients121 in the liquid ahead of the interface can be unstable with respect to Earth's gravity and serve to initiate convection currents which result in severe macrosegregation and, subsequently, inferior material properties. It has been demonstratedtaj that convection can be minimized, and a uniform microstructure maintained, by processing in the microgravity environment of space. However, for numerous reasons, space is unlikely to become a commercial processing arena for such products. 141 In contrast, evidence has been presented which implies that centrifugation of a solidifying melt can yield microstructures similar to those attainable by space processing techniques. Miiller and co-workers t5-91 solidified crystals of Ga, GaSb, GaSb:Te, and InSb:Te, using a centrifuge to generate accelerations from 1 to 10 g. Their experimental and theoretical work suggests that singlecell convective flows in the melt can reverse direction at high accelerations. This reversal coincides with a transition from unsteady flow, wherein temperature fluctuations and doping striations were present, to steady flow without thermal fluctuations or doping striations. Rodot and co-workers t~~ coupled Bridgman growth of PbTe:Ag and Pbo.83Sno.~TTe:Ag alloys with accelerations ranging from 1 to 10 g, and it was found that the distribution of Ag dopant over the sample length improved at higher accelerations. In view of the aforementioned studies, the intent of this work is to assess the effect of centrifugation on microstructural development during the controlled directional solidification of Pb-50 wt pet Sn alloys. CORBETT C. BATTAILE, Graduate Student, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. R.N. GRUGEL, Research Associate Professor, A.B. HMELO, Research Assistant Professor, and T.G. WANG, Professor and Director, are with the Center for Microgravity Research and Applications, Vanderbilt University, Nashville, TN 37235. Manuscript submitted March 29, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. EXPERIMENTAL PROCEDURE AND RESULTS The centrifuge and experimental procedure have been described in detail elsewhere, l~2,t3~ Briefly, a directional solidification furnace was mounted on bearings near the end of a 1.2-m rigid arm which rotated about a central axis; a maximum acceleration of 20 g at the solid/liquid interface was possible. Thirty sliding electrical contacts and two rotary fluid lines served as junctions to transfer power, thermocouple signals, and cooling liquids from the furnace assembly to control and data acquisition equipment. Equal proportions of Sn (99.85 pet) and Pb (99.9 pet) were melted together, thoroughly stirred, and cast into 5-mm-i.d. quartz tubes. The alloy rods were subsequently removed and remelted in closed-end 6-mm-
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