The document discusses nucleation and crystallization processes. It explains that nucleation refers to the initial formation of nano-sized crystallites from molten material as the first step in solidification. The critical radius is the minimum size needed for a crystal embryo to become a stable nucleus and continue growing. Segregation occurs as solute elements are more soluble in liquid than solid, causing compositional variations within castings.

1. NUCLEATION
&
CRYSTALLIZATION

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3. INTRODUCTION
• “The formation of first nano-sized crystallites from
molten material”.
• In a broader sense, the term nucleation refers to the
“initial stage” of formation of one phase from
another phase.
• So nucleation is a phenomenon associated with the
phase transformation.
• As water begins to freeze, nano-sized ice crystals
forms 1st.

4. PROCESS THERMODYNAMICS
• At the thermodynamic melting or freezing
temperature, the probability of forming stable,
sustainable nuclei is extremely small.
• Therefore, solidification does not begins at
thermodynamic melting or freezing temperature.
• If the temperature is decreased from the freezing
point of the material, the liquid is considered as
under-cooled.

5. PROCESS THERMODYNAMICS
• A material is solidify when the liquid cools to just
below its freezing temperature because the energy
associated with the crystalline structure of the solid
is less than the energy of the liquid.
• This energy difference between the liquid and the
solid is the free energy per unit volume and is the
“driving force” for solidification.

6. PROCESS ENERGETIC
• When the solid
forms, a solid-liquid
interface is created.
A surface free
energy associated
with this interface.
The larger the solid
the greater the
increase in the
surface energy.
An interface is created when a solid forms
from the liquid

7. AN INTERFACE IS CREATED WHEN A SOLID FORMS FROM
THE LIQUID

8. PROCESS FREE ENERGY
• The total energy change is given as:
∆G=4/3πr3 ∆Gv + 4πr2σsl
4/3πr3=the volume of a spherical embryo of radius r
4πr2=the surface area of spherical embryo
σsl=surface energy of solid liquid interface
∆Gv = free energy change per unit volume.

9. • The “Top Curve” shows the parabolic variations of total surface energy.
• The “Bottom Curve” shows the total volume free energy change.
• The “Middle Curve” shows the net variation of ∆G.

10. NUCLEATION
• The total free energy of solid-liquid system changes
with the size of the solid.
• The solid is an “Embryo”, if it’s radius is less than the
critical radius and is a “Nucleus” if its radius is
greater than the critical radius.
• “The critical radius is the minimum size of the crystal
that must be formed by the atoms clustering
together in a liquid before the solid particle is stable
and begins to grow.”

11. The total free energy of the solid-liquid system changes with the size of
the solid. The solid is an embryo if its radius is less than the critical
radius, and is a nucleus if its radius is greater than the critical radius

12. INITIATION OF GROWTH
• The new solid is then stable and sustainable since
nucleation has occurred, and growth of solid
particle which are now called a “Nucleus” begins.

13. GROWTH
• The growth process which follow nucleation
determine the following crystallographic structure of
the solid.
• The mode of the growth, both of individual grains
and of general mass of solid, depends upon
thermal conditions in the solidification zone.

14. SEGREGATION
• Segregation means “to isolate something”
according to specific laws.
• Here, Segregation means “Change in the average
composition of the metal as one moves from place
to place in an ingot”.
• The liquid which are frozen to form industrial alloys
usually contain many impurity elements.
• These impurity elements are frequently eliminated
during smelting and refining operation.
• The refractory lining of furnaces and the gases in
the furnace atmospheres may be the sources of
these impurities.

15. SEGREGATION
• Elements enter the liquid metal in the form of
dissolved gases.
• The various elements dissolved in the liquid
metals can and often do react with each other
to form compounds.
• In many cases, these compounds are less dense
than the liquid and rise to the surface and join
the slag which float on the top of the liquid
metals.
• On the other hand, it is quiet possible for small
impurity particles to exist in the liquid.

16. SEGREGATION
• When an alloy is frozen, a rule applies is that solute
elements, whether present as alloying elements or
as impurities, are more soluble in liquid state than in
the solid state.
• This fact usually leads to the segregation of solute
elements in finished castings.

17. SEGREGATION
• There are two basic ways of looking at the resulting
non-uniformity of the solute.
• As freezing point progresses, the solute concentration in a
casting tend to rise in those regions which solidify in the last
(centre of ingot).
• In general, segregation means “ to change in the average
composition of the metal as one moves from place to
place in an ingot.

18. SEGREGATION
• Gravitational effects are often a factor in producing
the segregation.
• The crystal which form freely in the liquid often have
a different density from that of liquid. As a result,
they may either “Rise” towards the surface of the
casting or “Settled” towards the bottom.

19. MACRO-SEGREGATION
• The basic factor is the accumulation of rejected
solute by transport over long distances through the
casting.
• The other factor is the movement of growing
crystals from their nucleation sites from gravity or
turbulence and long range capillary flow of liquids
during the final stages of freezing.

20. MICRO-SEGREGATION
• It results from accumulation of rejected solute
between the growing crystals and its failure due to
either inadequate time to diffuse into the main
body of residual liquid.
• The micro-segregation of alloying elements and
impurities can effect the strength and the ductility.

21. GRAVITY SEGREGATION
• Macro-segregation are frequently influenced by the
mass movement of precipitated phases due to
difference in density compared with their parent
liquid.
• Metals crystals growing independently in the melt
tend to sink and produce a corresponding upward
displacement of solute enriched liquid.
• Gravity concentration is mainly encountered in
heavy sections.