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

1. Chapter
Solidification
Prepared by
Prof. Naman M. Dave
Assistant Professor,
Mechanical Engg. Dept.
Gandhinagar Institute of Technology.
MATERIAL SCIENCE &
METALLURGY
2131904

2. Please do not blindly follow
the presentation files only, refer
it just as reference material.
More concentration should
on class room work and text
book-reference books.

3. 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

4. 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.

5. Introduction
• There are different types of grains such as
columnar-2, dendritic, equiaxed-3 or a
combination of these types.
Prof. Naman M. Dave

6. 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

7. 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

8. 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.

9. 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

10. 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.

11. • 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

12. 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

13. 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.

14. 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

15. 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

16. 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

17. 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.

18. 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.

19. 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.

20. 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

21. 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

22. 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
Prof. Naman M. Dave

23. 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

24. 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.

25. 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

26. 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

27. 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

28. Solidification
Defects
Prof. Naman M. Dave