Equiaxed zone

Figure 22.5 Classic grain structure of casting (or ingot) containing chill zone, columnar zone, and equiaxed zone. From Bower and Flemings [5].

Figure 4.5 Section through a solidification ingot showing the various solidification zones. The equiaxed zone forms near the mold walls. In the interior is the columnar zone, where crystal growth is in the direction of heat flow. (After Lalena and Cleary, 2005. Copyright John Wiley Sons, Inc. Reproduced with permission.)...

Probably the most important factor in determining subsequent properties is the relative proportions of the columnar and equiaxed zones. The chill zone is normally only a small number of grains thick and has a very limited influence. 

This nucleation theory has been modified to allow for crystal multiplication by the fragmentation of the initial nuclei. Crystal multiplication ly the remelting of dendrite arms resulting from growth fluctuations (produced, for instance, ty convection) was observed ty Jackson et al. This is shown in Fig. 6.4 and was used as a basis for a theory of equiaxed zone development (see Section 6.5). Biloni ° has established that both mechanisms are operable in the generation of the chill zone, with their relative importance depending on the casting conditions. The crystals in the chill zone are normally equiaxed and of random orientation. 

Explanations of the extent and nature of the eolumnar zone begin, as described above, with the development from the chill crystals. The main controlling factor is the appearance of the equiaxed zone. 

Effective distribution coefficient, 43, 51, 54 Embryos, 16 Entropy of melting, 4, 15 Equiaxed zone, 95, 103 theories of the development of, 105... 

The crystals in the equiaxed zone usually have a grain size larger than that of the chill zone and their orientation is effectively random. As shown in Figs. 6.7 and 6.8, the formation of an extensive equiaxed... 

I liner bands of V-segregatesalso occur which consist of fairly uniform Ncgregalc distributions on the surfaces of cones with their points downwards. These occur at the ingot centre in the equiaxed zone. 

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