*3.6.3 Inherent nucleation and growth conditions in the liquid melt*

The inherent nucleation and growth conditions in the melt are decided by the presence of foreign particles as well as the solute present in the melt. These solute atoms could be present as trace impurities or may be due to deliberate additions to influence nucleation. Obviously, these will influence/modify the possibilities of nucleation and growth, influencing the cast structure.

To elaborate the above we take help of **Figures 11** and **12**.

The alloys considered in these figures form a continuous range of solid solutions. The **Figures 11** and **12** illustrate the mode of crystallisation and hence the structure of the casting, as governed by the interaction of temperature and compositional gradients in the liquid.

**Figures 11** and **12** depict the effect of temperature gradient and that of liquidus temperature profile, respectively on the structure of the casting. Initially when the melt is at higher temperature the existing temperature gradient is stiff [Ti (in **Figure 11**)] planar growth is encouraged and columnar grain structure is favoured. This is assisted by a slow cooling rate. This continues till the temperature gradient

**31**

**Figure 13.**

*Solidification of Metals and Alloys*

because of the following:

of structure development.

the aid of **Figures 16** and **17**.

solidifying melt.

*DOI: http://dx.doi.org/10.5772/intechopen.94393*

rates, allowing growth to overtake nucleation.

face. The equilibrium temperature is altered.

Such a situation promotes columnar growth.

is sufficiently shallow to generate considerable undercooling which disturbs planar growth and growth proceeds adopting other modes, as explained earlier. **Figure 12** clearly indicates, with a given temperature gradient the alterations of equilibrium temperature profile, which could be due to the alterations in the solute concentration, undercooling is witnessed with liquidus profile TE (ii), with the liquidus profile TE (i) and the given temperature gradient 'T', undercooling is not witnessed and growth proceeds by plane-front growth giving rise to columnar grain structure. From the above, it can be concluded that columnar growth is promoted under stiff temperature gradients. Columnar growth is also favoured at slow cooling rates

slow cooling rates establish low rate of nucleation in comparison to the growth

As seen in **Figure 13** when the cooling rates are slow the solid rejected at the interface get sufficient time to migrate into the melt interior, away from the inter-

It changes from TE (ii) to TE (i) (**Figure 13**). This is parallel to a situation as in **Figure 12** when with TE (i) the extent of undercooling are negligible or absent.

In a foundry the various local factors like the extent of superheat, the extent of heterogeneous nucleation, the mould characteristics, etc. decide the variations in the thermal gradient (G) and the rate of cooling (R). Needless to say, the ratio G/R forms an important parameter to decide the mode of growth and the consequence

**Figure 14** illustrates that as the G/R ratio progressively changes from a high to a low volume the effect of undercooling becomes more and more pronounced. Columnar, plane-front growth gradually gives way to independent nucleation. During freezing the thermal conditions prevailing in the melt continuously change. Thus separate structural zones as shown in **Figure 15** are encountered in the

These zones are consequences of the continuously changing G/R ratio in the melt. Assuming of a lower value of the G/R ratio with the lapse of time results in the increasing extents of undercooling which are instrumental in the separable structural zones as preserved in **Figure 15**. The above can be made more clear with

Both **Figures 16** and **17** provide for an explanation of the mixed structure in a solidifying casting on the basis of the prevailing thermal conditions. To start with, as presented in **Figure 16**, the temperature gradient is stiff. Solidification initially occurs under this marked thermal gradient. This is often sufficient to cause

*Change in equilibrium temperature profile as a consequence of solute concentration crystal variation.*

**3.7 The temperature gradient (G) and rate of cooling (R) ratio (G/R)**

**Figure 12.** *Effect of liquidus temperature profile on extent of supercooling and crystal structure.*

### *Solidification of Metals and Alloys DOI: http://dx.doi.org/10.5772/intechopen.94393*

*Casting Processes and Modelling of Metallic Materials*

dictating the mode of growth.

gradients in the liquid.

solidifying melt. This is also related to the thermal properties of the melt as well as that of the mould. Obviously, the above would influence the cast structure by

The inherent nucleation and growth conditions in the melt are decided by the presence of foreign particles as well as the solute present in the melt. These solute atoms could be present as trace impurities or may be due to deliberate additions to influence nucleation. Obviously, these will influence/modify the possibilities of

The alloys considered in these figures form a continuous range of solid solutions. The **Figures 11** and **12** illustrate the mode of crystallisation and hence the structure of the casting, as governed by the interaction of temperature and compositional

**Figures 11** and **12** depict the effect of temperature gradient and that of liquidus temperature profile, respectively on the structure of the casting. Initially when the melt is at higher temperature the existing temperature gradient is stiff [Ti (in **Figure 11**)] planar growth is encouraged and columnar grain structure is favoured. This is assisted by a slow cooling rate. This continues till the temperature gradient

*Effect of temperature gradient variations on the extent of undercooling that influence the crystal structure.*

*Effect of liquidus temperature profile on extent of supercooling and crystal structure.*

*3.6.3 Inherent nucleation and growth conditions in the liquid melt*

To elaborate the above we take help of **Figures 11** and **12**.

nucleation and growth, influencing the cast structure.

**30**

**Figure 12.**

**Figure 11.**

is sufficiently shallow to generate considerable undercooling which disturbs planar growth and growth proceeds adopting other modes, as explained earlier. **Figure 12** clearly indicates, with a given temperature gradient the alterations of equilibrium temperature profile, which could be due to the alterations in the solute concentration, undercooling is witnessed with liquidus profile TE (ii), with the liquidus profile TE (i) and the given temperature gradient 'T', undercooling is not witnessed and growth proceeds by plane-front growth giving rise to columnar grain structure. From the above, it can be concluded that columnar growth is promoted under stiff temperature gradients. Columnar growth is also favoured at slow cooling rates because of the following:

slow cooling rates establish low rate of nucleation in comparison to the growth rates, allowing growth to overtake nucleation.

As seen in **Figure 13** when the cooling rates are slow the solid rejected at the interface get sufficient time to migrate into the melt interior, away from the interface. The equilibrium temperature is altered.

It changes from TE (ii) to TE (i) (**Figure 13**). This is parallel to a situation as in **Figure 12** when with TE (i) the extent of undercooling are negligible or absent. Such a situation promotes columnar growth.
