Electrical resistivity of pb and pb-10 wt pct sn during solidification
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I.
INTRODUCTION
R E S I S T I V I T Y measurements have been used successfully to study nucleation and growth processes during solid-solid transformations in a variety of alloy systems (e.g., see References 1 through 4). Measuring resistivity, however, is yet to be used to characterize solidification in metals and alloys, although a technique involving the use of acoustic emissions ~Sj was used to track solidification. Specifically, it was tbund that the number of grains in eutectic A1-Si alloy was proportional to the emission count rate. Events occurring at the early stages are very critical in defining the final grain size in castings, and empirical data obtained during solidification (and not postsolidification) are not available. Such data could be useful for improving models that are currently used in numerical simulations of solidification in castings, such as the micro/macro models described by Rappaz and Gandin.161 During solidification of metals, the resistivity changes distinctively when a small amount of solid forms, because the resistivities of the solid metals are typically only 45 to 65 pct of their liquid counterparts (the transition metals, Mn, Fe, Ni, and Co are exceptions). IT1 In this work, we used the technique to study solidification events in Pb and Pb-10 wt pet Sn alloy. For Pb and Sn, the aforementioned ratio of the resistivities is about 50 pct so that we can detect solidification events easily. In addition, Pb and Pb-Sn alloys were chosen because the resistivity in these melts is known as a function of temperature in the range 200 ~ -< T -< 530 ~ and for all concentrations of Sn. ~j II.
EXPERIMENTAL DETAILS
A schematic diagram of the four-probe potentiometer used in this investigation is shown in Figure 1. The setup S.H. LIU, Visiting Scientist, University of Arizona, is Associate Professor, Dalian University of Technology, Dalian, 116024 China. D.R. POIRIER, Professor, and P.N. OCANSEY, Graduate Student, are with the Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721. Manuscript submitted June 9, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
is made of a borosilicate glass in two parts: a capillary tube assembly and a crucible (3.5 cm in diameter). The former consists of a capillary tube, 16-cm long with an inner diameter of 0.1 cm. The capillary tube was joined to the larger tube (0.6 cm inside diameter) with a ceramic slurry, which was dried before use. The capillary tube assembly was used to contain the lead or alloy whose resistivity was measured, while the crucible was used to contain the bulk of the metal and provide electrical continuity to the electrodes. A second capillary with a thermocouple is also shown in Figure 1. About 2.7 kg of Pb (99.99 wt pct) was placed in the crucible and melted in an oven equipped with forced convection for uniform heating. The capillary tube assembly was inserted into the melt through a hole in the roof of the oven until the capillary was fully immersed in the melt, as shown in Figure I. The distance betw
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