Specific heat and related thermophysical properties of liquid Fe-Cu-Mo alloy
WANG HaiPeng
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LUO BingChi
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CHANG Jian
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WEI BingBo
)
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Department of Applied Physics, Northwestern Polytechnical University
, Xi'an 710072,
China
The specific heat and related thermophysical properties of liquid Fe77.5Cu13Mo9.5 monotectic alloy were investigated by an electromagnetic levitation drop calorimeter over a wide temperature range from 1482 to 1818 K. A maximum undercooling of 221 K (0.13 Tm) was achieved and the specific heat was determined as 44.71 Jmol1K1. The excess specific heat, enthalpy change, entropy change and Gibbs free energy difference of this alloy were calculated on the basis of experimental results. It was found that the calculated results by traditional estimating methods can only describe the solidification process under low undercooling conditions. Only the experimental results can reflect the reality under high undercooling conditions. Meanwhile, the thermal diffusivity, thermal conductivity, and sound speed were derived from the present experimental results. Furthermore, the solidified microstructural morphology was examined, which consists of (Fe) and (Cu) phases. The calculated interface energy was applied to exploring the correlation between competitive nucleation and solidification microstructure within monotectic alloy.
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The thermophysical properties of undercooled liquid metals and alloys have aroused great research
interest in the field of materials physics in recent years[16]. Specific heat, one of the most
important thermophysical properties, has a significant influence on developing the current solidification
theory[36]. Due to the metastable state of undercooled alloy melts, the conventional experimental
methods cannot be applied to determining their thermophysical properties. This results in great
difficulty to obtain these important data. Therefore, the thermodynamic research of highly
undercooled metals and alloys has been in the qualitative or semi-quantitative state.
Up to now, the experimental data for undercooled metals and alloys are still very scarce,
although there have been some reports. In the recent literature, only the measurements of some pure
metals and simple binary alloys are available. For monotectic alloys, the second liquid phase L2
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always acts as the leading phase to nucleate owing to the lower liquid/liquid interface energy when
temperature drops to their monotectic temperatures. Once L2 phase begins to nucleate, the local
concentration is favorable for the nucleation of solid phase, resulting in great difficulty to obtain a
large undercooling. Therefore, the investigations on the thermophysical properties of monotectic
alloys are seriously limited due to the lack of fundamental undercooled data. Electromagnetic
levitation can avoid the contamination from crucible walls and obtain a high degree of
undercooling. Meanwhile, the drop calorimetric method is applicable to specific heat measurements[48].
Fe-Cu-Mo ternary alloy is a typical monotectic system and has a good prospect for industrial
applications. The objective of this work is to determine the specific heat of undercooled liquid
Fe77.5Cu13Mo9.5 ternary alloy using the drop calorimetric method in combination with
electromagnetic levitation. The related thermophysical propterties, including the excess specific heat,
thermal diffusivity, thermal conductivity and sound speed, are derived on the basis of the
experimental results. And their temperature dependence is also revealed by theoretical predictions.
Furthermore, the rapid solidification of ternary monotectic alloy is investigated under high
undercooling conditions.
Experimental procedure
Fe77.5Cu13Mo9.5 alloy samples were prepared from 99.999% pure Fe, 99.999% pure Cu and
99.999% pure Mo in an arc melting furnace and each sample had a mass of 0.85 g. The experiments
were performed by an electromagnetic levitation facility, which was evacuated to 105 Pa and
back-filled with a mixed gas of argon, helium and hydrogen in the volume ratio of 6:3:1. During the
experiment, the sample was levitated and melted by rf induction heating. The temperature was
monitored by an infrared pyrometer. A He gas flow refrigerated by liquid nitrogen was blown
toward the sample to achieve high undercooling. When an ideal undercooling was attained, the gas
flow rate was adjusted to maintain the alloy melt at this undercooling for about 5 s. Then, the power
of the rf induction heating was switched off and the levitated sample would drop into an adiabatic
copper calorimeter. The temperature difference between the calorimeter shell and the calorimeter
block was monitored by a Eurotherm 818P4 unit. When the temperature of the calorimeter shell
was higher or lower than that of the calorimeter block, the resistance heating would begin or stop to
ensure that the calorimeter is practically adiabatic. The temperature rise of the adiabatic copper
block was measured by Pt100 resistance.
Under adiabatic conditions, after d (...truncated)