Undercooling constitutional

Composition profile (upper) and local melting temperature (lower) of material being processed by the traveling zone method. If the material in the molten zone is completely mixed, its composition will be Co/k and it will have the same melting temperature as the newly formed solid when steady-state growth is achieved. Since the material in the zone is not constitutionally undercooled, interfadal breakdown is not a problem. 

The chill crystals nucleate on or near the mould wall. Observations of undercoolings make it clear that they are the result of heterogeneous nucleation. The formation of the chill zone was initially explained " in terms of the copious nucleation considered to occur in the thermally undercooling region adjacent to the mould wall. The extent of nucleation was largely determined by the thermal conditions at the mould wall, but also by the efficiency of the mould wall as a substrate for heterogeneous nucleation and by the existence of effective nucleants in the chilled liauid layer. The existence of localised constitutional supercooling can also play a p art. Overall, the numbers of crystals in the chill zone depend on the superheat of the liquid, the temperature of the mould, the thermal properties of the metal and the mould, as well as on the nucleation potency of the mould wall or of particles in the liquid. In extreme cases it would be possible for chill crystals formed initially to subsequently remelt. 

The magnitude of the undercooling ahead of an interface when constitutional supercooling exists. 

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