Heterogeneous nucleation of sand casting

- Heterogeneous nucleation occurs at surfaces, imperfections, deformed regions, etc. which lowers the critical free energy.

- In the molten metal (castings) usually foreign particles are present as impurities lower the solid liquid interface energy and aid in nucleation and thereby reduce the amount of nucleation undercooling required to actuate.

- The basic requirement for heterogeneous nucleation in the capacity of the liquid metal to wet the foreign particles. Can forming solid impurities critical radius took a smaller increase in the surface energy. Therefore, heterogeneous nucleation can occur with relatively low undercoolings

- After the initial nuclei are formed.
1. more solid can be deposited on the first nuclei
2. More nuclei form,
3. different stage in the melt occurring.

- Has either very little or no supercooling at all in heterogeneous nucleation.

GROWTH:

- A growth following nucleation.

- Growth process determines the final crystallographic structure of the solid.

- Method depends growth of individual grains as well as the general mass of the solid on the thermal conditions prevalent in the solidification zone and the alloy composition.

- Growth is defined as the increase in size of the nucleus.

- The nuclei grow by additions of atoms.

- The nuclei reduce their total energy-free continuous growth

- During growth, material is transferred by diffusion.
a) In the old stage (ie liquid metal)
b) Across the liquid-solid interface.
c) In the nucleus – begins to grow on the grains already formed.

- The growth is controlled by the rate of heat transfer from the casting, since the temperature gradient toward the casting surface, the growth occurring in the opposite direction of heat flow, ie towards the center of the melt.

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