Chapter

Solidification

Prepared by

Prof. Naman M. Dave

Assistant Professor,

Mechanical Engg. Dept.

Gandhinagar Institute of Technology.

MATERIAL SCIENCE &

METALLURGY

2131904

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Contents

• Introduction

• Solidification of Pure Metals

• Nucleation

• Homogeneous or Self Nucleation

• Heterogeneous Nucleation

• Growth of Nucleus

• Effects of Structure on Mechanical Properties

• Methods to control the grain structure resulting from

solidification

• Solidification Defects

Prof. Naman M. Dave

Introduction • The “Grain Structure” of a material shows shape and size of the

grains (crystals) which form the bulk material.

• It is characterized by grain boundaries, grain shape and grain size.

Grain type can be

controlled by

controlling nucleation

and growth phenomena

which occur during

solidification of the

liquid metal.

Dendrites grow

outward until they

contact the

neighboring dendrites

and generate grain

boundaries.

Introduction

• There are different types of grains such as

columnar-2, dendritic, equiaxed-3 or a

combination of these types.

Prof. Naman M. Dave

What is Solidification ? • The process of transformation of a substance from liquid to solid state in which

the crystal lattice forms and crystals appear.

• Volume shrinkage or volume contraction

Solidification

• in pure metals and eutectic alloys takes place at constant temperature,

• in solid solution alloys proceeds over a temperature range.

• Crystallization / Solidification occurs in two stages- (1) Nucleation (2) Growth

Solidification occurs

• by the nucleation of very small (crystals),

• which grow under the thermal and crystallographic conditions existing during

solidification. Grain growth stops when complete melt has been solidified.

Dendritic Growth

Prof. Naman M. Dave

Solidification of Pure Metals

• Pure metals melt and solidify at a single temperature which may be

termed as Melting point or Freezing point (FP).

• If a number of temperature measurements are taken at different

times, while pure metal is cooled under equilibrium conditions from

the molten state till it solidifies, a Time-Temperature plot will look

like Fig.(a)

• If a pure metal cools

rapidly when it is very

pure and does not

contain any impurity

as nucleus to start

crystallization, it may

cool as per Fig.(b)

Prof. Naman M. Dave

Nucleation

• Nucleation is the beginning of a phase transformation.

• Nucleation is marked by the appearance in the molten metal of tiny

regions called Nuclei which grow to solid crystals (by further deposition

of atoms).

• Nucleation may involve:

a) The Assembly of proper kinds of atoms by diffusion.

b) The Structural change into one or more unstable intermediate structures.

c) The Formation of critical sized particle i.e., Nuclei of the new solid

phase.

Nucleation of the supercooled grains depends upon two factors

Free energy available from the

solidification process

Depends upon the volume of the

particle formed.

• The free energy change Δ Fv per unit

volume of metal transformed (i.e.,

Solidified) will be

It is negative; because free energy decreases

Energy required to form a

Liquid-solid interface.

• Creation of A new interface

(surface) is associated

• With free energy increase

proportional to the surface

• Area of the particle and this free

energy increase is equal to

• Particles formed, in the melt have

some surface area.

• Solid-liquid phases possess a surface

in between the two.

• Such a surface has a positive free

energy γ per unit area associated with it.

Total free energy change for a particle of radius r,

Nucleation

Nucleation • Critical particle radius and critical free energy can be calculated by

maximizing equation

Prof. Naman M. Dave

Particles having radius less

than rc

• tend to redissolve and

thus lower the free

energy.

• Such particles are called

EMBRYOS.

Particles having radius

more than rc

• tend to grow and also

lower free energy.

• Such particles are

known as NUCLEI.

• Fig. shows that as the particle radius increases, the free energy Δf also

increases till the particle grows to a critical radius rc.

• Thereafter an increase in particle radius accompanies with decrease in

free energy and so much so, that the free energy becomes negative also.

Nucleation

Homogeneous or Self Nucleation

• The graph here shows the nucleation

rate as a function of temperature.

• With decreasing temperature, there is

an increase in nucleation rate due to

rapid decrease of Free Energy.

• After a certain fall in temperature, the

activation energy for diffusion

becomes significant and role of free

energy is no more in picture.

• This results into decrease in the

nucleation rate with further drop in the

temperature.

• The maximum nucleation rate is

observed at a temperature below

melting point.

• Variation of nucleation rate with temperature

Prof. Naman M. Dave

A

B

C

Heterogeneous Nucleation • The formation of nuclei within its own melt with the help of foreign

substances or substrates is known as heterogeneous nucleation.

• The phase transformation takes place with the help of impurities.

• If a metal is to solidify on a foreign substrate it is essential that the

surface of the substrate should be wet by a liquid metal. Once this

condition is satisfied. next the liquids solidify easily on the substrate.

• When angle of contact θ is

small. interface between solid

and substrate has a low surface

energy.

• Hence. the total free energy for

formation of stable nucleus is

also decreased and critical radius

of the nucleus will be smaller as

per the given equation.

Heterogeneous Nucleation • When the contact angle is small, nucleation will occur at a small

amount of under-cooling.

• If the contact angle is large, a greater amount of under-cooling is

necessary. If θ = 180°. the liquid metal does not easily solidify on the

substrate, since solid metal and substrate interface energy is high.

• Sometimes, nucleating agent is added

to molten metal to act as a catalyst

This substrate may be a compound.

i.e.. insoluble in metal which will

produce a small contact angle.

• Some substances in fine sizes are

added in small quantities to the

molten metal in order to promote

heterogeneous nucleation and growth

of crystals These are called inoculants.

Prof. Naman M. Dave

Growth of Nucleus • Growth follows Nucleation.

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

• Growth may be defined as the increase of the nucleus in size.

• The nuclei grow by addition of atoms.

• The nuclei reduce their total free energy by continuous growth.

• During growth, material is transferred by diffusion.

• The rate of transfer obeys Anhenius equation with the activation energy

determined by the rate limiting step in the transfer process.

• Growth starts on the grains already formed.

• Thus, in general, hath rates of nucleation & growth depends upon the

degree of supercooling. Prof. Naman M. Dave

Growth of Nucleus • The specific heat is the heat required to change temperature of unit weight

of the material by one degree. The specific heat must be removed first,

either by radiation into the surrounding atmosphere or by conduction into

the surrounding mold, until the liquid cools to the freezing temperature.

• The latent heat of fusion, (which represent the energy that is evolves as

the disordered liquid structure transforms to a more stable crystal

structure must be removed from the solid-liquid interface before

solidification completely.

• The manner in which the latent heat is removed determines the growth

mechanism & final structure.

• There are two possible ways for growth:

1. Planar Growth

2. Dendritic Growth

Prof. Naman M. Dave

Growth of Nucleus • Planar Growth

• The temperature of the liquid metal is greater than the freezing temperature;

and the temperature of the solid formed is at or below the freezing

temperature.

• The latent heat of fusion must be removed by conduction from solid-liquid

interface through the solid to the surrounding for solidification to continue.

Growth of Nucleus • Any small protuberance (a small projection) which begins to grow on the

interface is surrounded by liquid metal above the freezing temperature.

• The growth of the protuberance then stops, until the remainder of the

interface catches up, This growth mechanism, known as planar growth,

occurs by the movement of a smooth solid-liquid interface into the liquid.

Growth of Nucleus • Dendritic Growth:

• When nucleation is poor, the liquid freezing temperature before the solid

undercools to a temperature below the forms.

• Under these conditions, a small solid protuberance called a dendrite, which

forms at the interface, is encouraged to grow.

• As the solid dendrite grows, the

latent heat of fusion is conducted

into the undercooled liquid, raising

the temperature of the liquid towards

the freezing temperature.

Growth of Nucleus • Initially grown dendrites are called primary arms. The secondary and

tertiary dendrite are can also form on the primary arms to speed up the

evolution of the latent heat.

• Dendritic growth continues until the undercooled liquid warms to the

freezing temperature.

• Any remaining liquid then solidifies by

planar growth.

Prof. Naman M. Dave

Methods to control the grain structure

resulting from solidification • To produce the castings with isotropic properties and improved

strength grain size strengthening, the solidification of casting should be controlled in way to produce a large number of small equiaxed grains.

• To improve strength of casting the dendrites should be as small as possible.

• Following explains some of the methods to control the grain structure during solidification:

Inoculation

Rapid Solidification

Directional Solidification

Single Crystal Technique

Prof. Naman M. Dave

Methods to control the grain structure

resulting from solidification

• Inoculation: By using (adding) appropriate inoculating agents, or

grain refining agents a wide spread (well distributed) nucleation can

be solidification that result in fine grain structure.

Mold Wall

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Methods to control the grain structure

resulting from solidification

• Rapid Solidification: By encouraging rapid solidification, a

very small spacing of secondary dendrite arms may be achieved.

The rate of solidification for any given metal can be influenced

by the size of the casting, the mold material and the casting

process.

• Thick casting solidifies slowly than thin casting. Mold materials

having a high density, thermal conductivity & heat capacity

produce more rapid solidification.

• Metal mold casting process gives the highest strength castings

due to rapid solidification. Ceramic molds (insulating nature)

give the slowest cooling & the lowest strength castings.

Prof. Naman M. Dave

Methods to control the grain structure

resulting from solidification • Directional Solidification: In many

applications, a small equiaxed grain

structure in the casting is not desirable.

Castings used for blades and vanes in

turbine are such applications

• The mold is heated from one end and

cooled from the other, producing a

columnar . microstructure with all of the

grain boundaries running in the

longitudinal direction of the part.

• In such solid, there are no grain

boundaries in the transverse direction.

• Single Crystal Technique: In this technique, only

one columnar grain becomes able to grow to the

main body of the casting due to helical connection.

• Properties better than DS technique. no grain

boundaries at all but has its crystallographic planes

& directions in an optimum orientation.

Solidification Defects

• The excess of dissolved hydrogen forms bubbles that may

be trapped in the solid metal during solidification,

producing gas porosity.

• The porosity may be spread uniformly throughout the

casting or may be trapped between dendrite arms.

1. Gas Porosity

• Many metals dissolve a large amount of gas when they are liquid; e.g. aluminum

dissolves hydrogen.

Remedies

• Keeping the liquid temperature low,

• By adding materials to the liquid to continue with the gas and form a solid,

Prof. Naman M. Dave

Solidification Defects • When OXYGEN gets dissolved in liquid steel during steel-making process, it

combines with carbon which is an alloying element, and carbon monoxide [CO]

gas bubbles get trapped in the steel casting.

• The dissolved oxygen can be completely eliminated if aluminum is added before

start of solidification.

• The aluminum combines with oxygen, producing solid alumina (Al2O3).

• In addition to eliminating gas porosity, the tiny Al2O3 inclusions prevent the grain

growth by pinning grain boundaries.

The completely deoxidized steel known

as killed steel or fine grained steel'

In a partially deoxidized steel, by addition

of small amount aluminum, a rimmed

steel. is produced in which enough CO

gas precipitated to offset the solidification

shrinkage

Remedies

Prof. Naman M. Dave

SOLIDIFICATION DEFECTS CAUSES REMEDIES

2. Shrinkage:

• Almost all materials are

denser in the solid state than

in the liquid state.

• During solidification, the

materials contract, or shrink,

by about 2% to 7%.

• Volumetric

contraction both in

liquid and solid state.

• Poor casting design.

• Adequate provision for

evacuation of air and gas

from the mold cavity.

• Increase of permeability of

mould and cores Proper

feeding of liquid metal is

required

• Proper casting design

Interdendritic shrinkage: • Liquid metal may be

unable to flow from a

riser

• through the fine

dendritic network to

the solidifying metal.

• Fast cooling rates

• Dendrites may be shorter,

permitting liquid to flow

through dendritic network

• Cavity: When solidification

begins casting and shrinkage

occurs in bulk

• Pipe: If one surface (usually

top) solidifies more slowly

than the others

• Extra reservoir

liquid metal is

placed adjacent &

connected to

casting

Prof. Naman M. Dave

Solidification

Defects

Prof. Naman M. Dave