Supercooled liquid constitutional

Figure 9.2. Constitutional supercooling in alloy solidification (a) phase diagram (b) solute-enriched layer ahead of the solid/liquid interface (c) condition for a stable interface (d) condition...

Figure 3.19. Schematic illustration of constitutional supercooling (C ). Tj is the liquid temperature gradient, is the growth temperature gradient, and is the actual temperature gradient.

The rates of crystallization vary markedly from polymer to polymer (Table 10-4). Polymers such as poly(ethylene terephthalate) that crystallize slowly can in fact be obtained in almost completely amorphous form by rapid supercooling from the melt. An amorphous form has never been achieved with the very rapidly crystallizing poly(ethylene), even when it is supercooled from the melt with liquid nitrogen. The crystallization rate depends on polymer constitution and configuration. Symmetric polymers crystallize rapidly. Bulky substituents and chain units lower the crystallization rate. 

Since the Constitutional Supercooling Criterion [1] and the analysis of Nullins and Sekerka [2] the study of the morphological stability of a planar solidification front during unidirectional growth of a binary alloy has been extensively developed [3]. Most of the authors consider that the melt is motionless so that heat and matter are transported by diffusion only. Convection in the melt can yet be actually important [4,5]. The influence of natural convection has recently been taken into account for a viscous liquid phase [6,7],... 

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 addition of an i oculant will only be effective provided it remains uniformly distributed through the melt and does not become contaminated or melt. The growth of the nucleus also requires that some supercooling exists in the liquid. This will usually be constitutional supercooling, although at the beginning of the solidification... 

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