Structure chill zone

Solidification. When the ingot or casting solidifies, there are three main possible microstructures that form (see Figure 7.5). We will describe here only the final structures the thermodynamics of the liquid-solid phase transformation have been described previously in Chapter 2. The outside layer of the ingot is called the chill zone and consists of a thin layer of equiaxed crystals with random orientation. 

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

The position of the transition was located by visual observation and optical microscopy. The distance from the chill zone of the sample was measured with a ruler. It is noted in Figure 1 that the CET is not sharp, showing an area where some equiaxed grains co-exist with columnar grains. As was reported before, the size of the transition area is in the order of up to 10 mm (Ares et al., 2007, 2010). The grain structure was inspected by visual observation under Arcano optical microscopy. 

Fig, 6.2 Possible casting structures (a) wholly columnar except for chill zone (b) partially columnar, partially equiaxed (c) wholly equiaxed Heine et al. ). 

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