Recent Developments in Undercooling Experiments

Meanwhile, a group at the Jet Propulsion Lab developed an electrostatic levitator that could levitate drops as large as 2 mm in diameter in an ultrahigh vacuum or a controlled atmosphere using an active feedback system to position the sample. A similar instrument was acquired by the MSFC and was equipped with lasers to melt the nonmetal as well as metal samples and with diagnostics for measuring properties of molten materials in their 

In dealing with a single component system, the Gibbs phase rule tells us the melting point is a function of pressm-e only. Since most of the processes we will be dealing with take place at ambient pressrue we will concern oruselves with T, the temperature at which a plane front solid can remain indefinitely with its melt. 

Even though the release of latent heat raises the temperature of an undercooled melt back to the melting point, this heat has already been removed so that solidification is very rapid. Therefore, the ability to imdercool a melt is very useful in developing rapid solidification processing for producing metastable states or other novel microstructures. 

In the absence of any nudeation site, homogeneous nudeation will eventually occur as the atoms cluster together to form embryonic nuclei. There is a critical radius for these 

Nudeation events are found in many aspects of materials processing besides the solidification of a melt. A nudeation event is required for precipitates to form in precipitation-hardened alloys, for the decomposition of immisdble systems, for new grains to form in a casting, for graphite flakes to form in cast iron, phase selection in alloy solidification and in crystal growth, etc. Understanding of the nudeation process is key to being able to control it in order to achieve the desired result in a process. 

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