Solidification NUCLEATION AND IT’S TYPES

NAME : RAJ PAREKHENROLLMENT NO : 140990119029SUBJECT : MSM

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.

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.

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

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

NUCLEATIONThe 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

NUCLEATION

Nucleation and formation of grains

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.

TYPES OF NUCLEATION