For formation of nucleuses of critical size there is not enough energy, exuded during atoms transfer from liquid to crystals. It compensates these energetic consumptions for two thirds, connected with surface energy or for half of energy, released owing to atoms transfer from liquid to crystal phase.
Nucleus critical size is decreasing with increasing of overcooling. The value of energy fluctuation, necessary for nucleus formation, also depends on the overcooling level. The bigger ΔТ, the less is work of nucleus formation. Experiments with metals confirm this.For example, relative stability (metastability) of molten metals significantly decreases with increasing the overcooling level and, if it is high, it is impossible to keep melt from crystallization. If the overcooling level is low, the liquid is so stable that the conception of metastability area and its interface can be used. If the temperature is higher than interface, nicely cleaned from impurities metastable liquid is stable and it is hard to crystallize it without nucleuses insertion. The interface of metastability of big amount of pure metals corresponds to overcooling to tens or even hundreds degrees. If the temperature is lower than metastability interface, the crystallization will begin quickly.
The speed of crystals formation is valued as the quantity of nucleuses, formed in time unit in liquid volume unit. Since phase fluctuations set and critical value of nucleus are changed with decreasing the temperature, the number of crystallization centers also changes: with increasing overcooling, the speed of nucleuses formation increases.
The speed of nucleuses formation for metals increases droningly with overcooling increasing, fig. 1.
The relation of nucleuses formation speed for nonmetallic materials with change of overcooling level is more difficult: the curve has both ascending (1) and descending (2) branches, fig. 1, b. Its occurrence is determined by increasing the viscosity of melt during overcooling. The formation of crystallization centers connected with atoms transfer from liquid phase to nucleus grouping. Such transfer becomes more complicated with temperature decreasing and the speed of crystallization centers formation decreases. Under conditions of high overcooling levels it can be so small that during cooling liquids transfer to amorphous bodies without crystallization.