Nucleation

Question 1

Nucleation

Answer 1

The physical process by which a new phase is produced in a material

Question 2

Critical Radius (r*)

Answer 2

The minimum size that must be formed by atoms clustering together in the liquid before the solid particle is stable and begins to grow

Question 3

Undercooling

Answer 3

The temperature to which the liquid metal most cool below the equilibrium freezing temperature before nucleation occurs

Question 4

Homogeneous Nucleation

Answer 4

Formation of a critically sized solid from the liquid by clustering together of a large number of atoms at a high undercooling (without an external interface)

ΔGv is the driving force for the nucleation

The nucleation is a balance between the free energy available from the driving force, and the energy consumed in forming new interface.

Question 5

Heterogeneous Nucleation

Answer 5

Formation of a critically sized solid from the liquid on an impurity surface

• Smaller undercooling required than homogeneous nucleation
• r* only depends on ΔT

Question 6

Sources of nucleation sites

Answer 6

• Dislocations
• Grain boundaries
• Dust particles
• Secondary phase particles
• Mould walls and cracks