MATERIAL SCIENCE

1. Dr. S & S. S Gandhy Government Engineering
College, Surat
Affiliated with GTU
Subject : Material science and metallurgy (3131904)
Presentation on
Topic : “Solidification of metals and alloys”
Prepared by:
210230119005 :
210230119006 :
210230119008 :
210230119009 :
Ehtesham Momin
Dhaval Kidecha
Brijesh Gauswami
Upasana Singh
GUIDED BY:
DR. LALIT S. PATEL

2. Content
 Introduction
 1. Condition for crystallization to occur.
 2. Formation of nuclei and grain structure (overview).
 3. Nucleation and Growth of crystals
3.1. sub topic 1 – Nucleation
 3.2. sub topic 2 - Growth of nucleus
 4. Solidification Defects
 5. Solidification of Alloys
 6. Gibbs phase rule
 7. Hume rothery rule
References

3. Introduction
1.The transformation of metal/alloys from the liquid state to the solid
crystalline state is called solidification or crystallization.
2. During solidification the disordered structure of the liquid state (atoms)
transforms to an orderly characteristic arrangements of the crystals i.e. an
orderly array of atoms!
3. When the disordered( high entropy state ) structure is frozen as it is, the
change is called ‘glass transition ‘ ,which is a transition state between
disordered & ordered( lest entropy state ) conditions of phases.
4. Thus, Crystallization is a term used to refer to the transformation to a
crystalline state starting from either a liquid or a glassy state.

4. 1. Condition for crystallization to occur
Solidification/crystallization proceeds under the conditions in which system is
converted to a more stable thermodynamic sates with lesser free energy, or
thermodynamic potential E: i.e. when the free energy of the crystal is less than that of
the liquid phase.
Here, free energy or thermodynamic potential is the portion of the total energy that is
reversibly changed upon changes in temperature or is available as work.
If, H = total energy of the system
T = absolute temperature
S = entropy,
Then, for a small change in volume during the transformation
E = H – TS.
The variation of free energy of liquid and solid with respect to the temperature is shown
in figure (a)&(b) in next slide:

6.  At temperature Ts, liquid metal is more stable with less free energy below
this temperature solid is more stable, having less free energy.
 The solidification process does not begin at this temperature, i.e Ts.
 Solidification proceeds only under conditions in which there is a difference
in free energy(Δe) as a result of lesser free energy of the solid metal in
comparison to that of the liquid metal; i.e. if Δe = EL – Es should be positive.
 So, if the difference Δe is negative, solidification will not start under this
condition. For example , at some temperature Tk as shown in figure (a).
 Consequently, the solidification process is initiated & proceeds only when
metal is supercooled below the equilibrium temperature Ts of solidification.
 The difference between the temperature Ts and temperature Tk at which
solidification proceeds under given conditions is called the ‘degree of
supercooling’; ΔT = Ts - Tk [ See Figure ].

7. 2. Formation of nuclei and grain structure (overview).
The Process of Solidification begins with the formation of
crystalline nuclei of Embryos so called "Centre of -
Crystallization", & proceeds with their Growth.
The stable crystalline nuclei, capable to Grow further, appear in
entire liquid Volume of a metal when it is supper-Cooled below
Ts, such crystallization Nuclei are said to be "Crystal“.
 As a result of this process, the growing Crystals, having a
regular geometric shape Initially take on an irregular shape
after Solidifying. They are called "Crystallites" Or "grains" with
irregular grain boundaries.
The structure obtained in metals or alloy By such a process of
crystallization is Known as "grain structure".

8. Nucleation and Growth of crystals
 At the solidification temperature, atoms from the liquid, such as
molten metal, begin to bond together and start to form crystals.
 The moment a crystal begins to grow is know as nucleus and the point
where it occurs is the nucleation point.
 When a metal begins to solidify, multiple crystals
begin to grow in the liquid.
 The final sizes of the individual crystals depend
on the number of nucleation points.
 The crystals increase in size by the progressive
addition of atoms and grow until they impinge upon
adjacent growing crystal.

9. 3.1. Nucleation
• It is defines as the process that occurs in the formation of a crystal from
a solution, a liquid, or a vapour, in which a small number of ions, atoms
or molecules become arranged in a pattern characteristic of a
crystalline solid, forming a site upon which additional particles are
deposited as the crystal grows.
• There are two types of Nucleation :
1. Homogeneous Nucleation
2. Heterogeneous Nucleation

10. 3.1.1 Homogeneous Nucleation :
It occurs without any interruption of foreign
particles below the freezing point at the temperature that are not visible in
water bodies.
Example : 1. In gas: Creation of liquid droplets in saturated vapour.
2. In liquid: Formation of gaseous bubbles, crystals (e.g. ice
formation from water), or glassy region.
3. In solid: Nucleation of crystalline, amorphous, and even vacancy
clusters in solid.

11. 3.1.2 Heterogeneous Nucleation:
It is the process in which the crystallization is
aided by the presence of impurities and those impurities are called catalyzers.
Example: Free surface, wall of a container, embedded particle, interface
boundary in a liquid or solid.
• Heterogeneoous Nucleation is faster than homogeneous nucleation
process.

12. 3.2 Growth of Nucleus
 Once a stable nucleus is formed as discussed above , the second phase of
solidification called Growth starts.
 In general growth may be defined as increase in the size of the previously
formed nucleus. The previously formed nucleus is mostly two dimensional in
nature ; which then grow by addition of new atoms diffusing towards it &
getting attached to the same by strong metallic bonding layer by layer.
 Once, these newly added atoms complete this second layer, another layer of
atoms is laid on this two dimensional nucleus formed in similar fashion, and
this process continues in that growth direction till the liquid is available and
assemble.
 During the growth phase of solidification the atoms get transferred to the
locations required for bonding by diffusion which follows the Arrhenius law as
given below:
Rate = C x exp(-Q/RT)

13. 4.Solidification Defects
 Solidification defects, such as porosity and hot cracking, are commonly
observed in metal-based additive manufacturing processes.
 Their existence drastically affects the mechanical and physical properties of
additive manufactured materials, which in turn hinders their full potential for
advanced applications.
 There are large number of defects that can be produced during solidification .
A few of them are discussed here along with remedy to prevent them.
1.shrinkage defect
2.Gas porocity defect

14. 5.Topic – Solidification of Alloys
Alloy is a material formed by mixing two or more metals or a metal with
metalloid, the addition of certain elements called alloying elements/solutes
improves strength & corrosion resistance
The Constitutional Super cooling means composition changes due to
diffusion of solutes along with changes in temperatures
During alloy solidification, a volume contraction
accompanies the transition first from liquid to solid,
and then in the solid as the temperature
decreases further.

15. Solidification of pure metal

16. Solidification of Pure metals Pure metals possess:-
i) Excellent thermal and electrical conductivity.
• ii) Higher ductility, melting point, lower yield point and tensile
strength
• iii) Better corrosion resistance, as compared to alloys. Pure metals
melts and solidify at a single temperature which may be termed as
Melting point or Freezing point (FP).

18. 6.

22. 7. Hume-Rothery rule
The Hume-Rothery rules are a set of basic rules describing the conditions
under which an element could dissolve in a metal, forming a solid solution.
There are two sets of rules, one which refers to substitutional solid solutions,
and another which refers to interstitial solid solutions.
Substitutional Solid Solution Rule for substitutional solid solutions,
the Hume-Rothery rules are:
1. The atomic radii of the solute and solvent atoms must differ by no more
than 15%
2. The crystal structures of solute and solvent must match.
3. Maximum solubility occurs when the solvent and solute have the same
valency. Metals with lower valency will tend to dissolve metals with higher
valency.
4. The solute and solvent should have similar electronegativity. If the
electronegativity difference is too great, the metals will tend to form
intermetallic compounds instead of solid solutions.

23. References
• DENTAL MATERIALS CLINICAL APPLICATIONS Pankaj Datta
• BASIC DENTAL MATERIALS John J.Manappallil
• PHILLIPS’ SCIENCE OF DENTAL MATERIALS by Kenneth J. Anusavice
• Material science and metallurgy by OP KHANNA