Cooling Rate to Make Metallic Glasses from Melts
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It has been accepted that any material can vitrify if cooled from the molten state to the glass transition temperature, Tg, at a cooling rate fast enough to prevent crystallization. The slowest a material can be cooled down to Tg without crystallization, the better the glass forming ability (GFA) is.
However, in practice, it is very challenging to make metallic glasses from melts as this process requires very high cooling rates (e.g. 105-106 K/s) and such a high cooling rate still results in a crystal percentage more than 0.1% in many cases. In general, only a material with a maximum allowed fraction of crystals, Xc, between 0.1% and 0.0001%, is conventionally classified as a glass. 
Figure 3070 shows the critical cooling rate (Rc) and critical casting thickness (dc) for formation of various metallic glass alloys from melts as a function of reduced glass transition temperature (Trg). In general, as the reduced glass transition temperature increases, the required cooling rate for glass formation decreases, and thus it is possible to form thicker glasses at slower cooling rates.
Figure 3070. Critical cooling rate and critical casting thickness for formation of various metallic glass alloys from melts as a function of reduced glass transition temperature.
Note that only some techniques can be used to achieve the cooling rates which produce glass alloys as listed on page3041.
 Nascimento MLF, Souza LA, Ferreira EB, Zanotto ED. Journal of Non-Crystalline Solids 2005;351:3296.
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