Solidification plane front

Figure 22.1 (a) Bar-shaped specimen after plane-front solidification. Grayscale... 

As shown by Fig. 22.16, the concentrations of solute atoms are significantly reduced in the material that is solidified early in the solidification process when k < 1. One-dimensional plane-front solidification can therefore be used as a method of purification. However, purification is carried more effectively out by modifying the process and adopting a zone-melting technique. 

Considerable purification is achieved during zone melting. The final transient at the end begins when the leading end of the zone reaches the end of the specimen. At that point, the solidification becomes very similar to plane-front solidification. Additional passes produce further purification and very small solute concentrations in the first part of the specimen. An asymptotic limit exists, however, as taken up in Exercise 22.2. 

As a reasonable approximation, the dendritic structure may be represented by the diagram in Fig. 22.4. The solidification in the interdendritic space can be described by constructing the cell (shown dashed) and assuming that solidification proceeds in a manner similar to the plane-front solidification of a bar (as discussed in Section 22.1). Under typical casting conditions, kf k. Therefore, the segregation... 

FIGURE 6.32 Plane front solidification of an isomorphous binary alloy system, (a) As the... 

Nonuniform composition occurs not only in a linear plane front solidification process, but can occur in almost any nucleation-and-growth processes (solid/liquid or solid/solid). In such cases, compositional partitioning between the growing nuclei and the parent matrix phase leads to a characteristic cored microstructure, as shown in Figure 6.33. Coring can have undesirable effects on materials properties, and so it is often removed by heat treating the microstructure at a temperature just below the transformation temperature to allow solid-state diffusion to homogenize the composition. 

During a plane front solidification process, partitioning of species between the solid and liquid phases can take place, which results in a nonuniform compositional profile at the end of the solidification process. The severity of the compositional nonuniformity depends on the degree of solute partitioning between the solid and the liquid as well as the rate of solidification relative to the rate of solid-state diffusion. If solid-state diffusion is fast, then the composition gradient in the solid can be quickly erased, leading to a more uniform composition profile. However, in most cases, solid-state diffusion is slow compared to solidification, and thus these nonuniform composition gradients tend to be frozen in. The Scheil equation provides a way to model the nonuniform concentration profile that arises in such a plane front solidification process ... 

Local freezing temperature along the growing sample. The temperature ahead of the solidification interface must increase at least at the mte shown by Gi to stabilize the plane front growth interface. If the thermal gradient is insufficient, as illustrated by slope Gz, the growth interface breaks down into a dendritic structure with a mushy zone (mixture of solid dendrites and interdendritic fluid) that extends to the point where the temperature is above the local freezing point. 

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