Abstract
The tin-rich eutectic in the Sn-Fe system has been investigated using precision resistance thermometry with a resolution of <0.1 mK (0.0001°C). The eutectic occurred at 231.9600°C (IPTS, 1968), 8.1 ± 0.3 mK below the freezing temperature of pure tin at a concentration of 10.1 ± 1 atomic ppm Fe. The distribution coefficient was estimated from the shapes of the freezing and melting curves and was very close of 1 for Fe concentrations above 1 ppm, being approx. 0.97 at the eutectic. The liquidus depression was much more than expected for such dilute solutions when they show ‘regular’ behaviour, and this was taken to indicate clustering in liquid tin. The size of the clusters— ca. 30 atoms—agrees with estimates made from viscosity data. Hypereutectic alloys behaved just like high purity tin except that the temperature was depressed 8.1 mK. It was independent of freezing rate, and was highly reproducible, in contrast to normal euteotics such as Al-CuAl 2.
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