Why Do Liquid Metals Resist Solidification?

NASA funded research has yielded the first proof of why liquid metals resist the transformation into solids and in the process have validated a 50 year old hypothesis.

This phenomenon was observed by German physicist Fahrenheit in the 1700’s when he was working on his temperature scale. He noted that water did not immediately freeze, rather it remained as a supercooled liquid for a period, before freezing.

In the 1950, General Electric researchers showed that liquid metals exhibit a similar behaviour. The “undercooling” behaviour was explained by a fundamental mismatch in the way the atoms arrange themselves in the liquid and solid phases.

The research that used NASA’s electrostatic levitator to perform tests researchers in times gone by could have only dreamed about. Experiments removed the chance of contaminants that could induce crystal formation as the sample is not in contact with anything.

By exposing a titanium-zirconium-nickel alloy to an Advanced Photon Source and mapping atomic locations using x-rays, the researchers were able to confirm Frank’s hypothesis from 1952, that the a nucleation barrier was responsible for the observed resistance to solidification.

The process involved the transformation from a liquid to an icosahedral (pattern with 20 triangular phases) quasicrystal structure via nucleation, which requires energy, prior to crystallisation. The isocahedral crystal structure formation is responsible for the nucleation barrier hypothesised by Frank.

The research work is part of NASA’s ongoing work pertaining to solving advanced materials problems on Earth and for spacecraft applications.

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