Directional solidification of lead-copper immiscible alloys in a cyclic gravity environment
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INTRODUCTION
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study employing Cu-Pb alloys has been initiated to examine the possibility of improving the performance of battery grids by increasing their electrical conductivity. Incorporating a continuous copper phase into the otherwise essentially pure lead grid would satisfy this requirement; however, it is an essential condition that the Cu phase does not become involved in the electrochemical processes occurring at the grid surface. Previous studies tr'2'31 have shown that aligned composite structures of monotectic composition (Cu-37 wt pct Pb) consisting of irregular lead fibers in a copper matrix can be achieved using directional solidification techniques. Aligned composites of this composition were initially employed in this study, and selective dissolution of the Cu phase from the surface with the intent of leaving a protective lead coating over the rest was attempted. [4] Due to the large volume fraction of Cu, this proved unsuccessful and indicated the need to consider alloys with higher lead concentrations. As shown in Figure 1, a liquid miscibility gap exists in the Cu-Pb system between - 3 7 and 86 wt pct Pb. Composite growth of these alloys by directional solidification techniques is thus complicated by coalescence and subsequent separation due to density differences of the representative liquids. Additionally, in this system, the denser L~ (lead) phase does not wet the solid copper phase, t2'3'51 further impeding aligned growth. It was envisaged that the liquid density difference could be overcome and composite growth facilitated by solidifying in a microgravity environment. Consequently, to gain some insight into this problem, directional solidification of hypermonotectic alloys was carried out on the KC-135 aircraft which, when flying its parabolic trajectories, provides alternating periods of microgravity (20 to 30 seconds, - 1 0 -3 g) and macrogravity (~ 1 minute, 2 g maximum).
Alloys with compositions of 36, 70, and 90 wt pct lead were prepared from high-purity constituents which were placed in fused silica tubes (5 mm diameter) whose inner surfaces were coated with colloidal graphite and sealed at both ends with graphite plugs. The alloys were heated in a vertical tube furnace at 1250 ~ for 24 hours and rapidly cooled in air by removing the tubes from the furnace. The tubes containing the homogenized samples were transferred to the directional solidification furnace for carrying out ground-based and in-flight runs. In the ground-base experiments, a thermocouple was inserted into the melt and maintained at a fixed position (approximately the midpoint of the sample) as the furnace was translated. This allowed determination of temperature v s time and permitted calculation of the temperature gradient at the solid-liquid interface (160 ~ in the sample. Prior to translation the entire sample was held for 15 minutes in the furnace hot zone -250 ~ above the consolute temperature (995 ~ The in-flight experiments were conducted in an identical manner to those on the ground, although it is
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