This document summarizes different methods to control grain structure in materials, including nucleation and grain refinement. It describes homogeneous and heterogeneous nucleation, and explains that heterogeneous nucleation occurs more easily at surfaces and imperfections. Methods to control grain structure discussed are single crystal technique, directional solidification, and epitaxial growth. The single crystal technique allows a single nucleus to grow into a single crystal for applications requiring specific crystal orientations. Directional solidification uses a temperature gradient to grow grains in a particular direction, producing columnar microstructures. Epitaxial growth matches the orientation of a thin film to the substrate crystallographically.

1. Gandhinagar Institute ofTechnology
Nucleation and Methods to control Grain Structure
Mehta Chintan B.
D1-14
3rd SEM. Mech. D
Guided By:- Prof. Shubham Sarthi
Material Science and Metallurgy (2131904)

2. Nucleation
• Nucleation is the beginning of a phase transformation.
• It is marked by appearance in the molten metal of tiny regions called Nuclei of the
new phase which grow to solid crystals until the transformation is complete.
• It involves assembly of proper kinds of atom by Diffusion, Structural changes and
Formation of crystal sized particle.
• Types of nucleation are as follows:
1. Homogeneous nucleation
2. Heterogeneous nucleation

3. Homogeneous Nucleation
• It is also known as self nucleation.
• In this the formation of nuclei starts in the interior of a uniform substance
such as pure liquid metal.
• It is a slower process.
• It occurs with much more difficulty.
• In this we need to provide supercooling or superheating to occur nucleation.

4. Heterogeneous Nucleation
• It is a type of nucleation which starts at the surfaces, imperfections, and
severely damaged regions.
• In this nucleation we don’t need the supercooling or we need in very less
amount.
• Nucleation process takes place very easily.
• It is a faster place.
• In this nucleation the liquid metal requires ability to absorbs the foreign
particles.
• This particles act as a nucleus forming a low contact angle and must possess
some structural affinity with metal.

5. Heterogeneous Nucleation
• The foreign matter or impurity is wetted
by both solid and liquid.
• The force of equilibrium where three
surfaces meet together is given by:
𝛾𝑓𝑚 𝑠 = 𝛾𝑓𝑚 𝐿 − 𝛾𝐿𝑐𝑜𝑠𝜃
• Where :
𝑓𝑚= Foreign matter
S= solid surface
L= Liquid surface
• The total energy is equal to solid-liquid
energy minus net energy decrease.

6. Methods to control Grain Structure
• A fine grain size in the solidified metal is always desirable to have superior mechanical
properties.
• The fine grains are achieved by combination of a high nucleation rate with slow growth rate.
• Nucleation rate can be increased by increasing cooling.
• To make heterogeneous nucleation nucleating agent is added to achieve grain refinement.
• There are following methods to control grain structure:
1. Single crystal technique
2. Directional solidification
3. Epitaxial growth

7. Single CrystalTechnique
• In this only single nucleus is allowed to grow.
• Single crystal are grown by seeding molten
metal with solid piece of crystal.
• Then the crystal is taken below the freezing
temperature.
• But whole assembly is kept above the freezing
temperature so that nucleation in bulk does
not happen.
• Then the seed is taken out, the new crystal
grows in the same direction.
• This new single crystal is utilized in many
electronic and optical applications.

8. Directional Solidification
• The mould is heated from one end
and cooled from the other end,
• Due to this the temperature gradient
helps in growing the crystals or grains
in a particular direction.
• This produce columnar
microstructure with all grain
boundaries in a longitudinal direction.
• No grain boundaries are present in
transverse direction.

9. Epitaxial Growth
• This method refers to the growing of
material in the same orientation as
that of substrate so as to match with
the substrate crystallographically.
• If the lattice matching between the
film and substrate is good the highly
oriented or crystal thin films can be
grown easily.

10. References
• http://www.Wikipedia.org>wiki>nucleation
• http://www.britannica.com/science/homogeneous-nucleation
• Material science and Metallurgy by Anup Goel (Material Science and
Metallurgy)

11. ThankYou