Saturday, July 9, 2016

Asymmetry in Crystallization Behavior

The article discusses the asymmetry between the critical heating and critical cooling rates of Vit 1, a glass-forming liquid. In this context, the word glass is not exactly used to refer to a transparent material used in windows or cups. This glass is an amorphous solid that is not at equilibrium as opposed to a crystal, a solid that is more ordered and at equilibrium.
It was observed that the heating rate required to suppress crystallization was significantly higher than the cooling rate required. The cooling rate was found to be 1 K/s while the heating rate was 200 K/s. This may be attributed to the fact that nuclei formed during cooling and heating are exposed to different growth rates, likely a general feature for metallic systems.
The significance behind bypassing crystallization is to allow the creation of a pure glass or super-cooled liquid; crystallization disrupts the disorder of the glass’ atomic arrangement. It is defined as the point at which the crystalline volume fraction within the melt reaches some small but finite value. Crystallization is identified when there is a rise in temperature, referred to as recalescence, caused by the release of the heat of fusion at the solid/liquid interface during crystallization. This leads to an increased heating rate.
In steady-state nucleation, the number of nuclei formed during the heating of a purely amorphous sample up to the liquidus temperature is the exact same as the number formed during cooling from the liquidus temperature down to the glass transition temperature. It was concluded that if any metallic liquid is quenched and forms an amorphous solid, it has to be heated at an extensively faster rate to combat crystallization.

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