A Method Based on Radiative Cooling for Detecting Structural Changes in Undercooled Metallic Liquids

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Abstract
We introduce a structure-sensitive parameter for undercooled melts which can be measured in containerless processing experiments. We have established that the ratio, R(T), of hemispherical total emissivity eT(T) to constant-pressure specific heat cp(T) can serve as an indicator which is sensitive to any changes in short range atomic order in undercooled metallic melts. R(T) ≡ eT(T)/cp(T) values for nickel, zirconium, and silicon have been obtained using the high temperature electrostatic levitator while the levitated melts were undergoing purely radiative cooling into the deeply undercooled region. R(T) plots for undercooled liquid nickel and zirconium indicate no significant change in short-range structure from their melting temperatures to 15% undercooling. In contrast, liquid silicon shows marked shortrange structural changes beginning above its melting temperature and extending throughout the undercooled region. The short-range structure of liquid silicon is related to the highly-directional covalent bonding which characterizes its solid form. The nickel and zirconium data show that eT varies linearly with T, in support of metal emissivity theories.

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