1.
Unit – 3, Solidification, Phase
Diagram, Iron Carbon system
Prof. A. V. Dube

2.
UNIT-III Solidification, Phase Diagram, Iron Carbon system
Solidification: Mechanism of solidification, Homogenous and
Heterogeneous Nucleation, crystal growth. Cast metal structures.
Phase diagram: Diffusion in solids, Fick’s laws of diffusion,
Solid solutions, Hume-Rothery rules, substitution, and interstitial
solid solutions, Gibbs phase rule, construction of equilibrium
diagrams, equilibrium diagrams involving complete and partial
solubility, lever rule. Numericals on phase diagrams.
Iron Carbon system: Iron carbon equilibrium diagram
description of phases, Solidification of steels and cast irons and
invariant reactions

3.
Basics of unit 3

4.
System
A system is a set of interacting or inter dependant components
parts forming a complex / intricate whole is called system. Or
the substances that are isolated and unaffected by their
surrounding are known as system
Or A part of the universe under study is called system.
Phase
A phase is defined as homogeneous, physically distinct and
mechanically separable part of the system under study.
Variable
A particular phase exists under various conditions of
temperature, pressure and concentration. These parameters are
called as the variables of the phase

5.
Component
• The component of the system may be
elements, ions or compounds. They refer to the
independent chemical species that comprise
the system is called as component.
• Cu-Au

6.
Solid solution.
• It is an alloy in which, the atoms of solute are distributed
in the solvent and has the same structure as that of the
solvent.
• Solid Solution have different compositions with similar
structure and are like liquid solutions as sugar in water.
• The atoms of one element become a part of the space
lattice of the other element, thus forming a solid solution.
• The element present in the alloy in the largest proportion is
referred as base metal or parent metal or solvent and the
elements are referred as alloying elements or solute.

7.
Substitutional Solid Solution

8.
• The solute may incorporate into the solvent
crystal lattice substitutionally by replacing a
solvent particle in the lattice.
• Solute atoms sizes are roughly similar to
solvent atoms.
• Due to similar size solute atoms occupy vacant
site in solvent atoms

9.
In interstitial solid solution, the atoms of alloying
elements occupy the interstitial sites of the base metal.

10.
• Many substances exist in more than one stable
crystalline form. The various forms have the same
composition but different crystal structures.
• Change in crystal structure is observed due to either
change in pressure or temperature or both. Such a
change of structure is called polymorphism.
(Observed in element and chemical composition)

11.
• Some metals, as well as nonmetals, may have
more than one crystal structure, a phenomenon
known as polymorphism.
• When found in elemental solids, the condition is
often termed allotropy.

12.
• Enantiotropy
Enantiotropy forms are mutually transformable
reversibly at some temperature.
• Examples: Fe, Zr, Ti, etc.
• Monotropy
Monotropic forms are irreversible in the solid state
and cannot be transformed one into the other.
Monotropic transition occurs at a temperature
above the melting point of the material .
• Examples: Phosphorus, Alumina etc.

13.
Mechanism of solidification

14.
• Casting Process.
• Solidification involves two steps. 1) Nuclei of a
solid phase (crystallite) form from the liquid. 2)
Solid crystallites begins to grow as an atoms
until complete liquid solidifies.
• When the metal is cooled below its melting
point, nuclei begin to form in different parts of
the melt at the same time.
• The rate of formation of nuclei depends upon the
degree of undercooling or super cooling and on
the presence of impurities which mainly
facilitate nucleation

15.
• At any temperature below the melting point, a nucleus has
to be of a certain minimum size so that it will grow. This size
is called as critical size of nucleus.
•The critical size is maximum near the melting point, but
there are less chances of forming such a large nucleus.
•If the size of nuclei is smaller than it gets dissolved by the
vigorous bombardment of neighbouring atoms and cannot
grow. These are known as embryos.
Some level of undercooling is done to promote the growth of
nuclei. Thus, with undercooling i.e. lowering of temperature,
the vibration of atoms decreases and the chances of survival
of nuclei increases

16.
It means, some degree of undercooling is necessary to start
solidification i.e. nucleation and its growth.
•Growth of nuclei occur by
diffusion process which is a
function of temperature.
Therefore, the rate of
nucleation (R.N.) and grain
growth (G.G.) are functions
of temperature.

17.
Homogenous and Heterogeneous
Nucleation
Nucleation is the beginning of a phase transformation. It is
marked by the appearance in the molten metal of tiny
regions called nuclei of the new phase which grow to solid
crystals until the transformation is complete.

18.
• Prenucleants are assumed to exist in the liquid
metal. In many processes, homogeneous
nucleation is assumed to occurs
Homogeneous or Self Nucleation
Homogeneous nucleation is a type of the nucleation
process where formation of nuclei starts in the interior of a
uniform substance such as pure liquid metals.

19.
• Homogeneous nucleation occurs when where there
are no special objects inside a phase which can cause
nucleation.
• It is the slower process and occurs with much more
difficulty.
Heterogeneous Nucleation
• Heterogeneous nucleation consisting of dissimilar
elements or ingredients; not having uniform quality.
• Heterogeneous nucleation is a type of nucleation
which starts at the surfaces, imperfections and
severely damaged regions.

20.
• As the presence of impurities in molten metal
lowers the liquid-solid interface energy, the
amount of supercooling (undercooling)
required to start nucleation will be less.
• Hence, nucleation process takes place easily.
• Heterogeneous nucleation requires the ability
of liquid metal to wet the foreign particle

21.
Solidification of Pure Metals and Alloy
•A pure metal solidifies at a
constant temperature equal to
its freezing point i.e. the same
as its melting point.
•The grain structure of pure
metal cast structure solidified
in a square mould as shown in
fig .
•At mould walls metal cools
rapidly due to exposure to
ambient temperature. This
result in shell of fine equiaxed
grain in the chill zone.

22.
• The grain growth through the mould away from
wall is columner. This is known as columner
zone.
Solidification process Pure metals (cooling curve) Alloy - copper-nickel alloy
system

23.
• The actual freezing time is called as local
solidification time in casting. During this time
the latent heat of fusion of metal is released
into the surrounding mould.
• Due to chilling action of the mould wall, a thin
skin of solid metal is initially formed at the
interface.
• This thickness of skin increases to form a shell
around the molten metal as the solidification
progresses towards the centre of the cavity.

24.
• Alloys
• Most of the alloys freeze over a temperature range
rather than at a single temperature.
• The start of freezing is similar to that of pure metal. A
thin skin is formed at the mould wall due to large
temperature gradient at the surface. After this, the
freezing progresses similar to pure metal.
• During the solidification of metal alloy where both
liquid and solid phases exist known as mushy zone. It
is an important factor during solidification.
• This zone present in freezing temperature range. The
columner dendritic structure of grain growth is seen
in this zone.

25.
• Chill Zone
• Chill zone contains narrow band of randomly
oriented grains.
• The metal at the mould wall cools to the
freezing temperature at first due to higher heat
dissipation.
• The mould wall provides the surfaces at which
heterogeneous nucleation takes place.

26.
• Columnar Zone
• Columnar zone contains elongated grains
oriented in specific crystallographic directions.
• The grain growth in this region takes place in
the direction opposite to heat flow.
• Heat is removed from casting by the mould
material and grain grows perpendicular the
mould wall.
• This columnar zone has anisotropic properties
and formation of this zone is a growth
controlled process

27.
• Equiaxed Zone
• This zone forms in the center of the casting or
ingot.
• It contains randomly oriented grains that are
relatively round or equiaxed.
• The formation of equiaxed zone is a nucleation
controlled process and has isotropic properties.
• The grain structure resulting from the
solidification of molten metal decides the
strength, hardness, toughness, and etc. properties
of a component
•

28.
Gibb’s Phase Rule
The Gibb’s phase rule states that under equilibrium conditions, the following
relation must be satisfied.
𝑃 +𝐹=𝐶 +2 ……. (1.1)
Where,
P = Number of phases existing in a system under consideration.
F = Degree of freedom i.e. the number of variables such as temperature, pressure
or concentration (i.e. composition) that can be changed independently without
changing the number of phases existing in the system.
C = Number of components (i.e. elements) in the system, and 2 represents any
two variables out of the above three i.e. temperature, pressure and concentration.
Most of the studies are done at constant pressure i.e. one atmospheric pressure
and hence pressure is no more a variable. For such cases, Gibb’s phase rule
becomes:
𝑃 +𝐹=𝐶 +1 ……. (1.2)
In the above rule, 1 represents any one variable out of the remaining two i.e.
temperature and concentration.

29.
Cooling curve of a pure metal Cooling curve of solid solution alloy
According to phase rule applied at different regions.
Region AB:
P + F = C + 1
1 + F = 1 + 1
F = 1
Region BC:
P + F = C + 1
2 + F = 1 + 1
F = 0
The meaning of F = 1 is that the temperature will be different
without changing the liquid phase existing in the system.
The meaning of F = 0 is that the temperature will not be
different without changing the liquid and solid phases existing
in the system. If the temperature increased, the metal goes in
the liquid state and if decreased, it goes in the solid state.
Hence, pure metals solidify at constant temperature.

30.
Hume-Rothery’s Rules for Solid Solubility
• Atomic Size
• Alloying elements having similar atomic size as that
of the base metal matrix have better solid solubility.
• The greater the difference in size between the atoms
of the two metals involved, the smaller will be the
range over which they are soluble.
• If atom diameter differ by more than 15% of that of
the solvent metal then solid solubility is generally
extremely small. But if diameter differ by less than
7% then, other things being equal, they will be likely
to form solid solution in all proportions.
• Any difference in atomic size will of course produce
some strain in the resultant crystal structure.

31.
• Electrochemical Properties/ Chemical Affinity
• The greater the chemical affinity of two metals,
the more restricted is their solid solubility and
greater is the tendency of formation of compound
. Generally , wider the separation of element in
the periodic table, greater is their chemical
affinity.

32.
• Relative Valency
• A metal of lower valency is more likely to
dissolve one of higher valency than vice versa
always assuming that other conditions are
favourable.
• This holds true particularly for alloys of the
monovalent metals copper, silver and gold
with many metals of higher valency

33.
• Crystal Structure
• Metal having same crystal structure will have
greater solubility.
• This gives rise to the formation of crystals in
the solid state. The atoms oscillate about fixed
locations and are in dynamic equilibrium rather
than statically fixed.
• From the above, it can be concluded that two
metals are most likely to form substitutional
solid solutions over a wide range of
compositions. If their atomic sizes are about
equal and if their electrochemical properties are
similar.

34.
• These conditions are fulfilled when two metals
are very close together in the periodic
classification of the elements. Usually side by
side in the same period (nickel and copper) or
one above the other in the same group (silver
and gold).
• Also if singly they crystallize in the same
pattern this will assist simple disordered
substitution.
• Pairs of metals which fulfil these conditions and
dissolve in each other in all proportions in the
solid state.

35.
LEVER RULE

36.
Phase Diagram/Equilibrium Diagram
• Isomorphous System – CSS and Liquid (L+S)
• Eutectic System - CSL and complete insoluble in
solid
• Partial Eutectic System -two metals have
complete solubility in liquid state and partial
solubility in the solid state.
• Layer Type system- Complete insolubility in L
and solid.