Solidification is the process where a liquid turns into a solid when its temperature is lowered below the freezing point. Nucleation is the first step of solidification, where tiny stable nuclei are created in the liquid metal. There are two types of nucleation: homogeneous nucleation which occurs from clustering of atoms at high undercooling, and heterogeneous nucleation which occurs more commonly on surfaces like the mold walls or impurities. Once a nucleus reaches the critical size, growth will occur as more atoms attach in an exothermic process releasing heat, which must be removed for solidification to continue. Understanding nucleation and solidification is important for industrial casting and welding processes where metal melting and resolidification occurs.

1. Solidification
NUCLEATION AND IT’S TYPES
NAME : RAJ PAREKH
ENROLLMENT NO : 140990119029
SUBJECT : MSM

2. INTRODUCTION
Solidification ?
solidification, is a phase transition in which
a liquid turns into a solid when its temperature is
lowered below its freezing point.
Why to study in detail ?
The solidification of metals and their alloys is
important in various industrial process
 Most of the components produced from
metals are by casting process.
 When a metal is welded a small portion of
metal near the weld melts and resolidifies.

3. SOLIDIFICATION
During solidification, the liquid changes in to solid as cooling proceeds.
The energy of liquid is less than that of the solid above the melting point.
Hence liquid is stable above the melting point.
But below the melting point, the energy of liquid becomes more than that of
the solid.
Hence below the melting point, the solid becomes more stable than the
liquid.

4. Thermodynamically, both liquid and solid have equal energy at melting point
and therefore both are equally stable at melting point.
Freezing is almost always an exothermic process, meaning that as liquid
changes into solid, heat is released.
 This heat must be continually removed from the freezing liquid or the
freezing process will stop.
 The energy released upon freezing is a latent heat and is the entropy
part.
 Some under-cooling will be essential for solidification.
 Solidification occurs by two process : nucleation and growth.
SOLIDIFICATION

5. NUCLEATION
The first step of metal solidification is the creation of tiny, stable, nuclei in
the liquid metal.
 Cooling the liquid below its equilibrium freezing temperature, or
undercooling, provides the driving force for solidification.
 Once a cluster reaches a critical size, it becomes a stable nucleus and
continues to grow.
The mold walls and any solid particles present in the liquid make nucleation
easier.
Cluster of atoms Embryo Nuclei Crystals Grains
r > r’
r < r’ r’ = critical radius

6. NUCLEATION
The volume free energy ΔGV – free energy
difference between the liquid and solid
Δ GV = 4/3πr3ΔGv (- ve)
The surface energy ΔGs – the energy needed to
create a surface for the spherical particles
ΔGs = 4πr2γ (+ ve)
γ → specific surface energy of the particle
Total free energy Change, ΔGT = ΔGV + ΔGs
At low temperatures atoms form small cluster
or groups.
 Embryo’s formed may either form into stable
nuclei or may re-dissolve in the liquid.
 beyond the critical radius of the nuclei it will
remain stable and growth occurs

7. NUCLEATION
Nucleation and formation of
grains

8. TYPES OF NUCLEATION
Nucleation is of two types-
 Homogeneous nucleation:
Homogeneous Nucleation – Formation of a critically sized solid from the
liquid by clustering together of a large number of atoms at a high
undercooling.
 Heterogeneous Nucleation :
Formation of a critically sized solid from the liquid on an impurity surface.
heterogeneous nucleation occurs in a liquid on the surface of its container,
insoluble impurities and other structural materials that lower the critical free
energy required to form a stable nucleus.
In practice, homogeneous nucleation rarely takes place and heterogeneous
nucleation occurs either on the mould walls or on insoluble impurity
particles.

9. TYPES OF NUCLEATION