Phase transformations, casting microstructures

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. 

As-cast Ti-3Al-2Si alloy has a typical polycrystalline microstructure of a solid solution of P-transformed Ti (a-Ti) with single particles of the secondary silicides (Fig. la). Additions of Zr promote the precipitation of the silicide (Fig. lb) and arising the first portion of eutectic (Fig.lc). Ti-3Al-2Si-5Zr and Ti-3Al-2Si-15Zr are two-phase alloys consisting of a-Ti and Ti5Si3 and a-Ti and (Ti,Zr)2Si phases, respectively (Table 1). 

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