10 kW Lepel high frequency generator. Helium gas was used to cool
the specimen by varying the flow rate. Helium was used to orevent
oxidation of the sample and control the temperature.
The temperature of the levitated sample was measured by a two-color
pyrometer (Ratioscope III) and was continuously recorded. The pyro-
meter, covers a temperature range from 750°C to 1850°c, accuracy 6°C
+0,75% of temperature reading, repeatability 1°c + 0,1% of temperature
; reading. The temperature could also be altered by changing the
ist current through the coil. If the current increases, the sample will
rise from the force of the magnetic field and therefore not be heated
as much; the result is a reduction of temperature. Similarly, a lower
current would cause the sample to become hotter. The melting point of
1. each levitated sample, however, was used as a reference point to
and calculate the supercooling of that sample. The current at which the
onsi- sample would be heated above the melting point and levitated was
00 KC, determined experimentally. At a desired temperature, the levitation
power was cut off, allowing the sample to fall through the plastic
diaphragm into the quenching medium. The quenching medium placed
directly below the levitation chamber was water, Woods metals
(52,5% Bi + 32,0% Pb + 15,5% Sn), splat cooler, aluminium wheel,
vacuum sucking unit or copper olate. The alloy was prepared from
high purity nickel (99,99%) and tin (99,99%). Each series of alloy
was prepared in a vacuum arc melting furnace, weights of the samples
were between 0,3 and 1,0 gram, with a diameter of about 5,0 mm. The
composition of the alloy used in this study is eutectic composition.
The quenched samples were mounted, ground and polished, etchants
used were Marble and Kalling reagents.
The lamellar eutectic spacing was measured in a few instances, using
linear intercept analysis. Figure 2 shows lamellar eutectic spacing
as a function of supercooling.
The average size of the Ni-rich phase within the irregular eutectic
was measured using linear intercept analysis. Figure 3 shows size
of the Ni-rich phase as a function of supercooling.
A portion of "irregular" eutectic was measured by computerized imace
analysis. Results are given on figure 4.
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