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Unit - III
Solidification of Metals and Binary Alloys: Concepts of
nucleation & grain growth; directional-solidification; ...
A phase can be defined as a homogeneous portion of a system that has uniform physical and
chemical characteristics i.e. it...
Phase
• A phase is defined as a homogenous portion
of the system having uniform physical and
chemical characteristics.
• E...
Solubility Limit
• A maximum amount of solute that can be
dissolved in the solvent to form a solid
solution is termed as s...
Phase Equilibria: Solubility Limit
Introduction
– Solutions – solid solutions, single phase
– Mixtures – more than one pha...
8
Effect of T & Composition (Co)
• Changing T can change # of phases:
D (100°C,90)
2 phases
B (100°C,70)
1 phase
path A to...
Solidification ?
➢Solidification, is a phase transition in which a liquid turns into
a solid when its temperature is lower...
➢ Thermodynamically, both liquid and solid have equal energy at melting
point and therefore both are equally stable at mel...
Nucleation and Growth
Transformation
• Embryo - An embryo is a tiny particle of solid that
forms from the liquid as atoms ...
NUCLEATION
➢The first step of metal solidification is the creation of tiny, stable, nuclei in the
liquid metal.
➢ Cooling ...
NUCLEATION & GROWTH
Nucleation and formation of grains
Nucleation – The physical process by which a new phase is produced ...
Crystal Nucleation and Growth
Various stages during solidification
of molten metal. Each small square
represents a unit ce...
Nucleation and Growth of Crystals
• At the solidification temperature,
atoms from the liquid, such as molten
metal, begin ...
TYPES OF NUCLEATION
Nucleation is of two types-
➢ Homogeneous nucleation:
Homogeneous Nucleation – Formation of a critically sized solid from ...
COOLING CURVES
A cooling curve is a graphical plot of the changes in temperature with time for a material
over the entire ...
• Homogeneous Nucleation – Formation of a critically sized
solid from the liquid by clustering together of a large
number ...
Supercooling
During the cooling of a liquid, solidification (nucleation) will
begin only after the temperature has been l...
Metal ∆T, 0C
Antimony 135
Germanium 227
Silver 227
Gold 230
Copper 236
Iron 295
Nickel 319
Cobalt 330
Palladium 332
HOMOGE...
phase transformations
Most phase transformations begin with the formation of numerous small
particles of the new phase th...
Time-Temperature curve for the solidification of a pure metal
 Undercooling(Supercooling)A-B: It is the gap between the t...
➢The first step in the solidification is the formation of nuclei. The nucleus can be
regarded as a small cluster of atoms ...
NUCLEATION
➢The volume free energy ΔGV – free energy
difference between the liquid and solid
Δ GV = 4/3πr3ΔGv (- ve)
➢The ...
➢ If a spherical particle of solid of radius r is to form, an interface must be created
between the solid and the surround...
Surface
free energy
Volume
free energy
Total free
energy
Fig (b) shows the sum of both the terms volume free energy and su...
r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow (to reduce energy)
Homogeneous Nucleation & Energy Effects
GT =...
[2]
[3]
[4]
r*
Heat of fusion ∆Hf (energy release upon solidification)
And
Tm-T= Super cooling
Tm= Melting Temp.
Solidification
TH
T
r
S
m



2
*
Note: HS = strong function of T
 = weak function of T
 r* decreases as T incre...
As T decreases, both r* and ∆G* become smaller;
LIQUID INSTABILITY at LOWER TEMPERATURES.
➢When the temperature is lowered...
➢It is also clear that some degree of undercooling is necessary to start
solidification i.e nucleation. The extend of unde...
Heterogeneous nucleation begins on alien surfaces or particles,
or pre-existing nuclei in the old phase. The nuclei can fo...
HETEROGENEOUS NUCLEATION
Heterogeneous Nucleation
1. Consider the nucleation of a solid from liquid, on a flat surface.
2....
• Heterogeneous Nucleation – Formation of a critically sized
solid from the liquid on an impurity surface.
• Heterogeneous...
Heterogeneous Transformation
• In practice, homogeneous nucleation rarely takes
place and heterogeneous nucleation occurs ...
Heterogeneous Nucleation
1. The undercooling for Heterogeneous nucleation is only few
degrees, unlike few hundreds for Hom...
Nucleation and Grain Growth
• Nucleation
– Homogeneous nucleation: substantial undercooling (0.2Tm)
– Heterogeneous nuclea...
• Free energy-versus - embryo/nucleus radius plot for homogeneous and
heterogeneous nucleation – Schematic.
• The lower ∆G...
Cooling Curve of Alloys
critical radius versus undercooling
Critical Size of Nucleus:The minimum size that must be formed by atoms
clustering toge...
Development of the ingot structure
of a casting during solidification:
(a) Nucleation begins,
(b) the chill zone forms,
(c...
Solidification in square moulds
a) Pure metals
b) Solid-solution alloys
c) Structure obtained by
heterogeneous
nucleation ...
1.Equiaxed zone: A region of randomly oriented grains in the center of a casting
produced as a result of widespread nuclea...
GROWTH
Planar growth Dendritic growth
Dendrite
Directional solidification (DS) and progressive solidification are types
of solidification within castings. Directional so...
➢ Gibb’s phase rule states that under equilibrium conditions, the following
relation must be satisfied.
P + F = C + 2
Wher...
Schematic cooling curve of a pure metal
Freezing starts at B and completes at C and between B and C, the metal is in the l...
In region AB:
P+F=C+1
1+F=1+1
So, F=1 (univarient) Temp. can be varied without changing the liquid phase existing in the
...
Schematic cooling curve of a solid solution alloy
Binary solid solution alloy
From A to B, the alloy is in the liquid stat...
AB: F=2 (bivarient) i.e both temperature and concentration can be varied independently
without changing the liquid phase e...
Schematic cooling curve of a binary eutectic alloy
Binary eutectic alloys
From A to B, the alloy is in the liquid state. F...
Lamellar structure
Lamellar structures or microstructures are composed of fine, alternating layers of different
materials ...
AB: F=2
BC: F=0 (neither temp. nor concentration can be varied without changing the phases
existing in the system. Hence e...
Off-eutectic binary alloy
Eutectic transformation occurs for a definite composition is called eutectic composition. If the...
➢ A to B alloy is in liquid state. Freezing starts at B and either solid 1 or
solid 2 separates out from the liquid depend...
Liquidus is the lowest temperature at which an alloy is completely liquid;
Solidus is the highest temperature at which an ...
Cooling Curve for Pure Metals
• Under equilibrium conditions, all metals exhibit a definite
melting or freezing point.
• I...
Cooling Curve of pure metals
Cooling Curve of Alloys
• In this method, alloys with different compositions are melted and then the
temperature of the mi...
Cooling Curve
• Then these temperatures are used for the construction of the
phase diagrams
Series of cooling curves for different alloys in a completely
soluble system. The dotted lines indicate the form of the ph...
Phase Diagram of Solid Solution
Cooling Curves for Solid Solution
crystal growth and grain formation
• nuclei → crystals → grains
• polycrystalline – solidified metal containing many cryst...
Dendrites
• In metals, the crystals that form in the liquid during freezing generally
follow a pattern consisting of a mai...
Dendrites
Dendrites
• During freezing of a polycrystalline material, many dendritic
crystals form and grow until they eventually bec...
Dendrites
Equilibrium Phase Diagrams
• What is a phase diagram?
• Phase diagram is a “temperature” versus “composition” plot,
displa...
• What is an alloy?
• An alloy is a homogeneous solid solution of two* or more metals,
the atoms of one metal substitutes ...
• Phase diagrams provide valuable information about
melting,
casting,
crystallization,
heat treatment,
phase transformatio...
Definitions
(i) Component
Components are pure metals (or compounds) from which an
alloy is formed.
Cu-Zn, Cu-Ni, Fe-C, Al-...
Phases
A phase is defined as a homogeneous portion of a system that
has uniform physical and chemical characteristics.
(i)...
Binary Phase Diagrams
1. Temp – Composition plots for alloys consisting of two
components
2. Pressure is considered as one...
Binary Isomorphous Systems
Alloys having complete liquid and solid solubility are called
isomorphous systems. Example: Cu-...
• Cooling curve for pure Cu or Ni
• Cooling curve for Cu-40 wt% Ni
• Heating Cu-50 wt% Ni
1280 0C melting begins.
1320 0C ...
• How to determine phase
compositions?
(i) Determination of composition in
single phase region, say at position
“A”.
Ex. T...
(ii). Determine composition of the two
phases of the alloy Cu- 35 wt% Ni
present at 1250 0C:
(a) Composition is to be
dete...
Liquid
Composition, wt% Ni
Temperature,0C
(..cont.,)
e. The perpendicular line dropped from
the liquidus line gives compos...
• How to Determine phase amounts?
Single phase region:
For Cu- 60 wt% Ni alloy at 1100 0C
(see previous Fig region designa...
Lever Rule Derivation based on moment equilibrium
WL R = Wα S
We know that: WL + Wα= 1
∴WL = (1 - Wα)
(1 - Wα) R = WαS
R -...
Determination of phase amounts (..cont.)
Two phase region: (..cont.,)
(iii) From Lever rule,
WL = S / (R + S) =
= (Cα – C0...
Development of Microstructures in Isomorphous Alloys –
Cu-35 wt% Ni alloy is cooled extremely slowly from 1300 0C to allow...
• Cu-35 wt% Ni liquid alloy is cooled
from 1300 0C, see point ‘a’ & the
associated microstructure in Fig.
• No microstruct...
• The compositions of the liquid and ‘α’
phases will follow the liquidus and
solidus lines respectively.
• The fraction of...
• The solidification is complete upon
reaching point ‘d’ at ~1220 0C, the
comp. of solid ‘α’ is ~Cu-35 wt% Ni
(the over al...
Development of Microstructures in Isomorphous Alloys –
• In practical solidification situations, the
cooling rates are rap...
• The castings with cored structure when
heated below the solidus line, the grain
boundaries could melt resulting in loss
...
Development of Microstructures in Isomorphous Alloys – Nonequilibrium Cooling
Lever Rule
• The composition of various phases in a phase
diagram can be determined by a procedure
called the lever rule.
...
Summary of Important Equilibrium Phase Transformations
(c)2003Brooks/Cole,adivisionofThomsonLearning,Inc.ThomsonLearning™isatrademarkusedhereinunderlicense.
Al – Si eutectic all...
Al-Si Alloy Phase Diagram
Al-Si alloys differ from our "standard" phase diagram in that aluminium has zero solid
solubility in silicon at any temper...
coarse flakes of Si in the eutectic promote brittleness within these alloys. Most Al-Si
alloys used have a near-eutectic c...
Typical eutectic microstructures:
(a) needle-like silicon plates in the aluminum silicon eutectic (x100), and
(b) rounded ...
The effect of hardening with phosphorus on the microstructure of hypereutectic
aluminum-silicon alloys:
(a) coarse primary...
The hypereutectic Al-Si alloys containing primary β will provide the wear-
resistance that at one-third the weight of the ...
• In an eutectic reaction, when a liquid solution of fixed composi-
tion, solidifies at a constant temperature, forms a mi...
In eutectic system, there is always a specific alloy,
known as eutectic composition, that freezes at a lower
temp. than a...
 Binary alloy eutectic system can be classed as:
1. One in which, two metals are completely soluble in the liquid
state b...
1. Two metals completely soluble in the liquid state
but completely insoluble in the solid state.
 Technically, no two me...
• Alloy-1: 20% Cd and 80% Bi
 Contrary to alloy 3, in this case crystal of pure Bi form first,
enriching the melt with Cd...
 Alloy-3: 80% Cd and 20% Bismuth.
 As the temperature falls to T1, crystal nuclei of pure Cd
begin to form. Since pure C...
2. Two metals completely soluble in the liquid state, but only partly soluble in the
solid state
 Since most metals show some solubility for each other in the solid
state, this type is the most common and, therefore, t...
 The alloy solidifies as a solid solution until at 183°C, the last layer
of solid to form is of composition C (80.5% Pb-1...
 Eutectoid Transformation:
 Eutectoid reaction is an isothermal reversible reaction in
which a solid phase (usually soli...
The peritectoid reaction is the transformation of two solid
into a third solid.
Peritectic reaction
It is the reaction that occurs during the solidification of
some alloys where the liquid phase reacts with a solid
phase ...
 Peritectoid Transformation:
 The peritectoid reaction is the transformation of two solid
into a third solid.
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
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Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist
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Unit 3-k.srinivasulureddy-MMS-metallurgy & material science-snist

  1. 1. Unit - III Solidification of Metals and Binary Alloys: Concepts of nucleation & grain growth; directional-solidification; dendritic growth and equiaxed grain growth. Phase rule, invariant reactions (eutectic, eutectoid, peritectic, peritectoid; Lever rule, cooling-curves of pure metals, binary alloys. Binary phase diagrams (Aluminum-Silicon and Aluminum-Copper, Copper-Zinc, Copper-Tin, Copper-Nickel. Non ferrous Metals & Alloys: Aluminum based alloys; Cast alloys, Wrought alloys and their applications. Copper based alloys: Brasses, Bronzes and their applications. Magnesium based alloys; Cast alloys, wrought alloys and their applications. Titanium based alloys: aerospace alloys, bio-compatible alloys. Nimonic alloys - their compositions and applications. Heat treatment of Non-ferrous metals, Age hardening, solution hardening.
  2. 2. A phase can be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics i.e. it is a physically distinct from other phases, chemically homogeneous and mechanically separable portion of a system. A component can exist in many phases. E.g.: Water exists as ice, liquid water, and water vapor. Carbon exists as graphite and diamond. ➢A solution (liquid or solid) is phase with more than one component; a mixture is a material with more than one phase. A solution is a mixture where one of the substances dissolves in the other. ➢In mixtures, there are different phases, each with its own atomic arrangement. It is possible to have a mixture of two different solutions! ➢ Solute (minor component of two in a solution) does not change the structural pattern of the solvent, and the composition of any solution can be varied. Ex: Salt water Salt (Solute) in water(Solvent)
  3. 3. Phase • A phase is defined as a homogenous portion of the system having uniform physical and chemical characteristics. • Every pure material is considered to be a single phase. • Each phase is separated by phase boundaries. • A phase may contain one or two component. • A single phase system is called as homogenous and systems with two or more phases are heterogeneous systems.
  4. 4. Solubility Limit • A maximum amount of solute that can be dissolved in the solvent to form a solid solution is termed as solubility limit. • For example, alcohol has unlimited solubility in water, sugar has limited solubility, and oil is insoluble in water. • Cu and Ni are mutually soluble in any amount, while C has limited solubility in Fe. • The addition of solute in excess of this limit results in the formation of two phase solution.
  5. 5. Phase Equilibria: Solubility Limit Introduction – Solutions – solid solutions, single phase – Mixtures – more than one phase • Solubility Limit: Max concentration for which only a single phase solution occurs. Question: What is the solubility limit at 20°C? Answer: 65 wt% sugar. If Co < 65 wt% sugar: syrup If Co > 65 wt% sugar: syrup + sugar. 65 Sucrose/Water Phase Diagram Pure Sugar Temperature(°C) 0 20 40 60 80 100 Co =Composition (wt% sugar) L (liquid solution i.e., syrup) Solubility Limit L (liquid) + S (solid sugar)20 40 60 80 10 0 Pure Water
  6. 6. 8 Effect of T & Composition (Co) • Changing T can change # of phases: D (100°C,90) 2 phases B (100°C,70) 1 phase path A to B. • Changing Co can change # of phases: path B to D. A (20°C,70) 2 phases 70 80 1006040200 Temperature(°C) Co =Composition (wt% sugar) L (liquid solution i.e., syrup) 20 100 40 60 80 0 L (liquid) + S (solid sugar) water- sugar system
  7. 7. Solidification ? ➢Solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. ➢During solidification, the liquid changes in to solid as cooling proceeds. ➢The energy of liquid is less than that of the solid above the melting point. Hence liquid is stable above the melting point. ➢But below the melting point, the energy of liquid becomes more than that of the solid. Hence below the melting point, the solid becomes more stable than the liquid. ➢ At temperatures below the freezing/ melting point, the substance is a solid.
  8. 8. ➢ Thermodynamically, both liquid and solid have equal energy at melting point and therefore both are equally stable at melting point. ➢ Freezing is almost always an exothermic process, meaning that as liquid changes into solid, heat is released. ➢ This heat must be continually removed from the freezing liquid otherwise the freezing process will stop. ➢ The energy released upon freezing is latent heat ➢ Some under-cooling is essential for solidification. ➢ Solidification occurs by two process : nucleation and growth.
  9. 9. Nucleation and Growth Transformation • Embryo - An embryo is a tiny particle of solid that forms from the liquid as atoms cluster together. The embryo is unstable and may either grow in to a stable nucleus or re-dissolve. • Nucleus – It is a tiny particle of solid that forms from the liquid as atoms cluster together. Because these particles are large enough to be stable, nucleation has occurred and growth of the solid can begin.
  10. 10. NUCLEATION ➢The first step of metal solidification is the creation of tiny, stable, nuclei in the liquid metal. ➢ Cooling the liquid below its equilibrium freezing temperature, or undercooling, provides the driving force for solidification. ➢ Once a cluster reaches a critical size, it becomes a stable nucleus and continues to grow. ➢The mold walls and any solid particles present in the liquid make nucleation easier. Cluster of atoms Embryo Nuclei Crystals Grains r > r’ r < r’ r’ = critical radius
  11. 11. NUCLEATION & GROWTH Nucleation and formation of grains Nucleation – The physical process by which a new phase is produced in a material. In the case of solidification, this refers to the formation of tiny stable solid particles in the liquid. Growth - The physical process by which a new phase increases in size. In the case of solidification, this refers to the formation of a stable solid particle as the liquid freezes.
  12. 12. Crystal Nucleation and Growth Various stages during solidification of molten metal. Each small square represents a unit cell. a) Nucleation of crystals at random sites in the molten metal. Note that the crystallographic orientation of each site is different. (b) & (c) Growth of crystals as solidification continuous. d) Solidified metal, showing grains and grain boundaries. Note the different angles at which neighboring grains meet each other
  13. 13. 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. a)Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain boundaries as seen in a microscope.
  14. 14. TYPES OF NUCLEATION
  15. 15. Nucleation is of two types- ➢ Homogeneous nucleation: Homogeneous Nucleation – Formation of a critically sized solid from the liquid by clustering together of a large number of atoms at a high undercooling. ➢ Heterogeneous Nucleation : Formation of a critically sized solid from the liquid on an impurity surface. heterogeneous nucleation occurs in a liquid on the surface of its container, insoluble impurities and other structural materials that lower the critical free energy required to form a stable nucleus. In practice, homogeneous nucleation rarely takes place and heterogeneous nucleation occurs either on the mould walls or on insoluble impurity particles. TYPES OF NUCLEATION
  16. 16. COOLING CURVES A cooling curve is a graphical plot of the changes in temperature with time for a material over the entire temperature range through which it cools. COOLING CURVE WITH UNDERCOOLING Supercooling is the cooling of a liquid below its freezing point without it becoming solid.
  17. 17. • Homogeneous Nucleation – Formation of a critically sized solid from the liquid by clustering together of a large number of atoms at a high undercooling (without an external interface).
  18. 18. Supercooling During the cooling of a liquid, solidification (nucleation) will begin only after the temperature has been lowered below the equilibrium solidification temperature. This phenomenon is termed supercooling (or undercooling.)  The driving force to nucleate increases as T increases Small supercooling  slow nucleation rate - few nuclei - large crystals Large supercooling  rapid nucleation rate- many nuclei - small crystals
  19. 19. Metal ∆T, 0C Antimony 135 Germanium 227 Silver 227 Gold 230 Copper 236 Iron 295 Nickel 319 Cobalt 330 Palladium 332 HOMOGENEOUS NUCLEATION Table: Degree of undercooling (∆T) values for Several metals (Homogeneous Nucleation).
  20. 20. phase transformations Most phase transformations begin with the formation of numerous small particles of the new phase that increase in size until the transformation is complete.  Nucleation is the process whereby nuclei (seeds) act as templates for crystal growth. ✓ Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable supercooling (typically 80-300°C). ✓ Heterogeneous nucleation - form at structural inhomogeneities (container surfaces, impurities, grain boundaries, dislocations) in liquid phase much easier since stable “nucleating surface” is already present; requires slight supercooling (0.1-10ºC ).
  21. 21. Time-Temperature curve for the solidification of a pure metal  Undercooling(Supercooling)A-B: It is the gap between the temp. predicted for the transformation to occur and the temp at which the transformation actually occurs, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid. A-B: Undercooling B-C: Evolution of latent heat after undercooling
  22. 22. ➢The first step in the solidification is the formation of nuclei. The nucleus can be regarded as a small cluster of atoms having the right crystalline arrangement. ➢When the melt is cooled below the melting point, nuclei begin to form in many parts of the melt at the same time. ➢The rate of nuclei formation depends on the degree of undercoolong or supercooling and also on the presence of impurities which considerably facilitate nucleation. ➢At any temperature below the melting point, a nucleus has to be of a certain minimum size, called the critical size, so that it will grow. ➢Particles smaller than the critical size will be dissolved by the vigorous bombardment of neighboring atoms and can’t grow are called embryos.
  23. 23. NUCLEATION ➢The volume free energy ΔGV – free energy difference between the liquid and solid Δ GV = 4/3πr3ΔGv (- ve) ➢The surface energy ΔGs – the energy needed to create a surface for the spherical particles ΔGs = 4πr2γ (+ ve) γ → specific surface energy of the particle Total free energy Change, ΔGT = ΔGV + ΔGs ➢ Embryo’s formed may either form into stable nuclei or may re-dissolve in the liquid. ➢ Beyond the critical radius of the nuclei it will remain stable and growth occurs Variation of free energy of a spherical particle as a function of its radius
  24. 24. ➢ If a spherical particle of solid of radius r is to form, an interface must be created between the solid and the surrounding liquid.  Since ɣ is the energy required to create one unit area of interface, the overall change in free energy, ΔGv, that accompanies the formation of a spherical solid within the liquid is: ➢The surface energy term (i.e energy needed to create interface or surface energy) is always positive but the volume energy term(i.e energy released by volume of solidifying phase or volume energy) will be negative for any phase transformation under consideration. ➢Since the energy needed to create interface varies as r2 and energy released by volume of solidifying phase varies as r3 , the variation of these two terms with increasing value of r is as shown in Fig. ➢Initially for smaller values of r, energy released by volume of solidifying phase is smaller than the energy needed to create interface but it becomes greater for larger values of r, since it varies as r3 . ➢Thus the sum of ΔGV goes through a maximum at some critical radius r*.
  25. 25. Surface free energy Volume free energy Total free energy Fig (b) shows the sum of both the terms volume free energy and surface free energy. Many clusters will form embryo and dissolve and do not make it to critical radius r* Few of them survive past the critical radius r*, then growth continue for the nucleus with decrease in free energy. Since the maximum free energy occurs at r*, we differentiate ∆G, w.r.t., r, & set the expression to zero. Unstable embryos Stable nuclei Variation of free energy of a spherical particle as a function of its radius
  26. 26. r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow (to reduce energy) Homogeneous Nucleation & Energy Effects GT = Total Free Energy = GS + GV Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface)  2 4 rGS  = surface tension Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy)  GrGV 3 3 4 volumeunit energyfreevolume  G
  27. 27. [2] [3] [4] r* Heat of fusion ∆Hf (energy release upon solidification) And Tm-T= Super cooling Tm= Melting Temp.
  28. 28. Solidification TH T r S m    2 * Note: HS = strong function of T  = weak function of T  r* decreases as T increases HS = latent heat of solidification Tm = melting temperature  = surface free energy T = Tm - T = supercooling r* = critical radius
  29. 29. As T decreases, both r* and ∆G* become smaller; LIQUID INSTABILITY at LOWER TEMPERATURES. ➢When the temperature is lowered, the vibrations of atoms gradually decrease, increasing the chances of survival of small clusters and therefore, the critical size of nucleus decreases with decreasing temperature or increasing degree of undercooling.
  30. 30. ➢It is also clear that some degree of undercooling is necessary to start solidification i.e nucleation. The extend of undercooling(i.e temperature A – temperature B) varies from metal to metal and also depends on the impurities present in the metal. *Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration. ➢Hence, at lower temperatures nuclei become progressively smaller in size but the number greatly increases. Growth of nuclei occur by diffusion* process which is also a function of temperature and hence the rate of nucleation(N) and rate of growth(G) are functions of temperature.
  31. 31. Heterogeneous nucleation begins on alien surfaces or particles, or pre-existing nuclei in the old phase. The nuclei can form at preferential sites(Eg. Mould wall, impurities or catalysts) Critical radius of the nucleus (r*) for a heterogeneous nucleation is the same as that for a homogeneous nucleation
  32. 32. HETEROGENEOUS NUCLEATION Heterogeneous Nucleation 1. Consider the nucleation of a solid from liquid, on a flat surface. 2. Assume that both liquid and solid phases “wet” this flat surface; that is both of these phases spread out and cover the surface. 3. Interfacial energies: γSL = Solid/Liquid; γIL = Liquid/surface; γSI = Solid /surface. 4. Taking surface tension force balance: γIL = γSI + γSL cos θ [12]
  33. 33. • Heterogeneous Nucleation – Formation of a critically sized solid from the liquid on an impurity surface. • Heterogeneous nucleation occurs in a liquid on the surface of its container, insoluble impurities and other structural materials that lower the critical free energy required to form a stable nucleus Heterogeneous Nucleation The factors which determine the rate of phase change are: • (1) the rate of nucleation, N (i.e. the number of nuclei formed in unit volume in unit time) and • (2) the rate of growth, G (i.e. the rate of increase in radius with time)
  34. 34. Heterogeneous Transformation • In practice, homogeneous nucleation rarely takes place and heterogeneous nucleation occurs either on the mould walls or on insoluble impurity particles. • A reduction in the interfacial energy(Surface energy) would facilitate nucleation at small values of ∆T. • This occurs at a mould wall or pre-existing solid particle
  35. 35. Heterogeneous Nucleation 1. The undercooling for Heterogeneous nucleation is only few degrees, unlike few hundreds for Homogeneous nucleation. 2. The reason for very small undercooling is that the activation energy (i.e., energy barrier) for nucleation (∆G* ) is lowered when nuclei form on preexisting surfaces or interfaces, since the surface free energy (γ ) is reduced. 3. Therefore it is easier to nucleate at surfaces and interfaces than at other Homogeneous sites.
  36. 36. Nucleation and Grain Growth • Nucleation – Homogeneous nucleation: substantial undercooling (0.2Tm) – Heterogeneous nucleation: nucleation agents (5ºC undercooling) • Grain growth – Planar: pure metal – Dendritic: solid solution • Grain size – depends on number of nuclei and cooling rate. • The solidification of metals occur by nucleation and growth transformation. • In nucleation and growth transformation, the nuclei of the solid phase are formed and then they grow. Grain Growth in pure and solid solutions
  37. 37. • Free energy-versus - embryo/nucleus radius plot for homogeneous and heterogeneous nucleation – Schematic. • The lower ∆G* for heterogeneous means that a smaller energy must be overcome during the nucleation process, (than for homogeneous), and therefore, heterogeneous nucleation occurs more readily. NUCLEATION & GROWTH
  38. 38. Cooling Curve of Alloys
  39. 39. critical radius versus undercooling Critical Size of Nucleus:The minimum size that must be formed by atoms clustering together in the liquid before the solid particle is stable and begins to grow.
  40. 40. Development of the ingot structure of a casting during solidification: (a) Nucleation begins, (b) the chill zone forms, (c) preferred growth produces the columnar zone, and (d) additional nucleation creates the equiaxed zone.
  41. 41. Solidification in square moulds a) Pure metals b) Solid-solution alloys c) Structure obtained by heterogeneous nucleation of grains using nucleating agents
  42. 42. 1.Equiaxed zone: A region of randomly oriented grains in the center of a casting produced as a result of widespread nucleation. 2.Columnar zone: A region of elongated grains having a preferred orientation that forms as a result of competitive growth during the solidification of a casting. 3.Chill zone: A region of small, randomly oriented grains that forms at the surface of a casting as a result of heterogeneous nucleation. 4.Dendrite: The treelike structure of the solid that grows when an undercooled liquid solidifies. ➢ Note down all the definitions and try to understand and identify different zones as shown in figure solidified solid and the difference between four .
  43. 43. GROWTH Planar growth Dendritic growth
  44. 44. Dendrite
  45. 45. Directional solidification (DS) and progressive solidification are types of solidification within castings. Directional solidification is solidification that occurs from farthest end of the casting and works its way towards the sprue. Directional solidification can be used as a purification process. Since most impurities will be more soluble in the liquid than in the solid phase during solidification, impurities will be "pushed" by the solidification front, causing much of the finished casting to have a lower concentration of impurities than the feedstock material, while the last solidified metal will be enriched with impurities. This last part of the metal can be scrapped or recycled. Progressive solidification, also known as parallel solidification, is solidification that starts at the walls of the casting and progresses perpendicularly from that surface Directional Solidification Progressive Solidification
  46. 46. ➢ Gibb’s phase rule states that under equilibrium conditions, the following relation must be satisfied. P + F = C + 2 Where P= No. 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, Gibbs phase rule becomes: ✓ P+ F = C + 1 ✓ In the above rule, 1 represents any one variable out of the remaining two i.e temperature and concentration
  47. 47. Schematic cooling curve of a pure metal Freezing starts at B and completes at C and between B and C, the metal is in the liquid plus solid state. Above the temperature indicated by point B, the metal is in the liquid state and below C, it is in the solid state Cooling curve-Pure metal Note: For start of solidification or nucleation, undercooling is necessary; but for simplicity, this is not shown on cooling curves
  48. 48. In region AB: P+F=C+1 1+F=1+1 So, F=1 (univarient) Temp. can be varied without changing the liquid phase existing in the system In region BC: 2+F=1+1 F=0 (nonvarient or invarient system).Temperature can’t be varied without changing the liquid and solid phases existing in the system. If temp. is increased, the metal goes in the liquid state and if decreased, it goes in the solid state. Hence pure metals solidify at constant temp. In region CD: 1+F=1+1 F=1 (Univarient system) Temperature can be changed without changing the solid phase existing in the system.
  49. 49. Schematic cooling curve of a solid solution alloy Binary solid solution alloy From A to B, the alloy is in the liquid state. Freezing starts at B and completes at C, and between B and C, the alloy is in the liquid plus solid state. From C to D, there is no change in the solid state of the alloy
  50. 50. AB: F=2 (bivarient) i.e both temperature and concentration can be varied independently without changing the liquid phase existing in the system BC: F=1(univarient) i.e any one variable out of temp. and composition can be changed independently without altering the liquid and solid phases existing in the system. From this, it is clear that solid solution allolys solidify over a range of temperature. They have incongruent melting points i.e melting starts at one temperature and finishes at another temperature. CD: F=2 P+F=C+1 1+F=2+1, F=3-1=2
  51. 51. Schematic cooling curve of a binary eutectic alloy Binary eutectic alloys From A to B, the alloy is in the liquid state. Freezing starts at B and simultaneously two solids S1 and S2 start separating out from the liquid. This continuous upto C. The alloy gets completely solidified at C and gives a mixture of S1 and S2. From C to D, there is no change in the solidified alloy
  52. 52. Lamellar structure Lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae.
  53. 53. AB: F=2 BC: F=0 (neither temp. nor concentration can be varied without changing the phases existing in the system. Hence eutectic alloys solidify at constant temperature similar to that of pure metals. CD: F=1 ➢Binary eutectic is homogeneous mixture of two solids which forms at constant temperature during cooling and melts at constant temperature during heating. Binary eutectic transformation can be shown as: L S1 + S2 ➢A eutectic reaction is a three-phase reaction, by which, on cooling, a liquid transforms into two solid phases at the same time. It is a phase reaction, but a special one. For example: liquid alloy becomes a solid mixture of alpha and beta at a specific temperature (rather than over a temperature range). Eutectic meaning Easy melting. Constant Temp. ➢Where S1 is one solid and S2 is other solid . This mixture appears in a definite morphological form and is usually lamellar. In certain cases, it may have granular or some other type of morphology. The temperature at which this transformation occurs is called eutectic temperature and is the lowest temperature of transformation in the system.
  54. 54. Off-eutectic binary alloy Eutectic transformation occurs for a definite composition is called eutectic composition. If the composition of the alloy differs from this, it is called off-eutectic alloy i.e either hypoeutectic(less than eutectic composition) or hypereutectic(more than eutectic composition)
  55. 55. ➢ A to B alloy is in liquid state. Freezing starts at B and either solid 1 or solid 2 separates out from the liquid depending on whether the alloy is hypoeutectic or hypereutectic. This continues upto C. ➢ The remaining liquid at C solidifies at constant temperature and forms a mixture of S1 and S2. This eutectic transformation starts at C and ends at D. The alloy completely solidifies at D and there is no change from D to E. Solidus and Liquidus Temperatures The start of solidification temperature is called liquidus temperature because above this the metal or alloy is in the liquid state; where as the end of solidification temperature is called solidus temperature because below this , the metal or alloy is in the solid state. Off-eutectic binary alloy
  56. 56. Liquidus is the lowest temperature at which an alloy is completely liquid; Solidus is the highest temperature at which an alloy is completely solid.
  57. 57. Cooling Curve for Pure Metals • Under equilibrium conditions, all metals exhibit a definite melting or freezing point. • If a cooling curve is plotted for a pure metal, It will show a horizontal line at the melting or freezing temperature.
  58. 58. Cooling Curve of pure metals
  59. 59. Cooling Curve of Alloys • In this method, alloys with different compositions are melted and then the temperature of the mixture is measured at certain time intervals while cooling back to room temperature. • A cooling curve for each mixture is constructed and the initial and final phase change temperatures are determined.
  60. 60. Cooling Curve • Then these temperatures are used for the construction of the phase diagrams
  61. 61. Series of cooling curves for different alloys in a completely soluble system. The dotted lines indicate the form of the phase diagram
  62. 62. Phase Diagram of Solid Solution
  63. 63. Cooling Curves for Solid Solution
  64. 64. crystal growth and grain formation • nuclei → crystals → grains • polycrystalline – solidified metal containing many crystals • grains – crystals in solidified metal • grain boundaries – the surfaces between the grains • two major types of grain structures: (1) equiaxed grains – crystals grow about equally in all directions, commonly found adjacent to a cold mold wall (2) columnar grains – long, thin, coarse grains, created when metal solidifies rather slow in the presence of a steep temperature gradient. columnar grains grow perpendicular to the mold surface
  65. 65. Dendrites • In metals, the crystals that form in the liquid during freezing generally follow a pattern consisting of a main branch with many appendages. A crystal with this morphology slightly resembles a pine tree and is called a dendrite, which means branching. • The formation of dendrites occurs because crystals grow in defined planes due to the crystal lattice they create. • The figure shows how a cubic crystal can grow in a melt in three dimensions, which correspond to the six faces of the cube. • For clarity of illustration, the adding of unit cells with continued solidification from the six faces is shown simply as lines. • Secondary dendrite arms branch off the primary arm, and tertiary arms off the secondary arms and etcetera.
  66. 66. Dendrites
  67. 67. Dendrites • During freezing of a polycrystalline material, many dendritic crystals form and grow until they eventually become large enough to impinge upon each other. • Eventually, the interdendritic spaces between the dendrite arms crystallize to yield a more regular crystal. • The original dendritic pattern may not be apparent when examining the microstructure of a material. • However, dendrites can often be seen in solidification voids that sometimes occur in castings or welds, as shown in the next slide..
  68. 68. Dendrites
  69. 69. Equilibrium Phase Diagrams • What is a phase diagram? • Phase diagram is a “temperature” versus “composition” plot, displays several equilibrium phases that are possible to exist at various given specified ranges of temperature, composition, and pressure.
  70. 70. • What is an alloy? • An alloy is a homogeneous solid solution of two* or more metals, the atoms of one metal substitutes for another metal or occupies interstitial positions. Examples: Brass: Alloy of copper and zinc. Bronze: Alloy of copper and tin. Steel: Alloy of iron and carbon. CMSX4: Alloy of Nickel and Co, Cr, Ta, W, Al, Re, Ti, Mo, Hf ______________________ * At least one is a metal. Equilibrium Phase Diagrams
  71. 71. • Phase diagrams provide valuable information about melting, casting, crystallization, heat treatment, phase transformations, & resulting microstructure, …, • Phase diagrams are extremely important because there is strong correlation between the: (i) phase diagrams, (ii) microstructure, & (iii) mechanical properties. Equilibrium Phase Diagrams
  72. 72. Definitions (i) Component Components are pure metals (or compounds) from which an alloy is formed. Cu-Zn, Cu-Ni, Fe-C, Al-Cu, Be-Cu,…… (ii) System A series of possible alloys consisting of the same components is called a system. Example series of the system is: 10%Cu-90%Ni, is a system. 30%Cu-70%Ni, is a system. 60% Cu-40%Ni, is a system. (iii) Solubility Limit A maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution. Ex. Sugar-water Equilibrium Phase Diagrams
  73. 73. Phases A phase is defined as a homogeneous portion of a system that has uniform physical and chemical characteristics. (i) Solid is a phase. (ii) Liquid is a phase. (iii) Gas is a phase. (iv) Every pure material is a phase. (v) Water and ice in a container; Two phases in the container, because they are physically dissimilar (one is solid & other is liquid) but identical chemically!. (vi) Fe exists in two polymorphic forms: (a) α–Fe (BCC), (b) γ – Fe (FCC). Both are different phases Equilibrium Phase Diagrams
  74. 74. Binary Phase Diagrams 1. Temp – Composition plots for alloys consisting of two components 2. Pressure is considered as one atmosphere. 3. Examples: Cu-Ni, Cu-Zn, Fe-Fe3C, e.t.c., Binary Equilibrium Phase Diagrams
  75. 75. Binary Isomorphous Systems Alloys having complete liquid and solid solubility are called isomorphous systems. Example: Cu-Ni. • In Binary phase diagrams, temperature is plotted on vertical axis & composition on horizontal axis. • “L” is a homogeneous liquid solution composed of both copper and nickel. • α, β, γ, γ’ (Greek) letters are used to represent solid solutions. • Liquidus line • Solidus line • Mushy state (L + α ) Liquid α Composition, wt% NiCu Ni Temperature,0C Binary Equilibrium Phase Diagrams
  76. 76. • Cooling curve for pure Cu or Ni • Cooling curve for Cu-40 wt% Ni • Heating Cu-50 wt% Ni 1280 0C melting begins. 1320 0C completely liquid. • What phases are present? Ex.1. Cu-60 wt% Ni, at “A”, at 1100 0C, is located within the α region, therefore, only α phase will be present. Ex.2.Cu-35 wt% Ni, at “B” 12500C, will have two phases: α & Liquid phases in equilibrium. Liquid α Composition, wt% NiCu Ni Temperature,0C Binary Equilibrium Phase Diagrams
  77. 77. • How to determine phase compositions? (i) Determination of composition in single phase region, say at position “A”. Ex. The position “A” is located in the single phase region called “α”, and the composition at “A” is 60 wt% Ni & 40 wt% Cu. Liquid α Composition, wt% NiCu Ni Temperature,0C Binary Equilibrium Phase Diagrams
  78. 78. (ii). Determine composition of the two phases of the alloy Cu- 35 wt% Ni present at 1250 0C: (a) Composition is to be determined for the specific alloy is located at position ‘B’, lying with in the (α + L) region. (b) Draw a tie line from liquidus to solidus line through the point ‘B’. c. Draw a line perpendicular to the axis of composition through the point of intersection of the tie line with liquidus. d. Similarly, draw another line perpendicular to the axis of composition, through the point of intersection of the tie line with solidus. Liquid Composition, wt% Ni Temperature,0C Binary Equilibrium Phase Diagrams
  79. 79. Liquid Composition, wt% Ni Temperature,0C (..cont.,) e. The perpendicular line dropped from the liquidus line gives composition of the liquid phase Cu-31.5 wt% Ni. f. The perpendicular line dropped from the solidus line gives composition of the α solid phase Cu-42.5 wt% Ni. Binary Equilibrium Phase Diagrams
  80. 80. • How to Determine phase amounts? Single phase region: For Cu- 60 wt% Ni alloy at 1100 0C (see previous Fig region designated by ‘A’), only α phase is present; hence the alloy is completely 100% α Two phase region: (i) For the region designated by ‘B’ at 1250 0C for Cu-35 wt% Ni alloy, both α phase and ‘L’ liquid phase are present. (ii) Fraction of each of the α phase & ‘L’ phases can be computed by Lever Rule. Liquid Composition, wt% Ni Temperature,0C Binary Equilibrium Phase Diagrams
  81. 81. Lever Rule Derivation based on moment equilibrium WL R = Wα S We know that: WL + Wα= 1 ∴WL = (1 - Wα) (1 - Wα) R = WαS R - WαR = WαS R = WαR +WαS R = Wα(R + S) Wα= 𝐑 𝐑+𝐒 WL= 𝐒 𝐑+𝐒 Where, WL = Weight fraction of liquid. Wα = Weight fraction of α solid solution. CL = Composition of liquid. Cα = Comp of α solid solution. B
  82. 82. Determination of phase amounts (..cont.) Two phase region: (..cont.,) (iii) From Lever rule, WL = S / (R + S) = = (Cα – C0) / (Cα-CL) WL = (42.5 – 35) / (42.5 – 31.5) = 0.68 = Weight fraction of liquid ‘L’ (iv) Wα = R / (R + S) = (C0 – CL) / (Cα – CL) = (35 – 31.5) / (42.5 – 31.5) = 0.32 = Weight fraction of α solid Liquid Composition, wt% Ni Temperature,0C Binary Equilibrium Phase Diagrams
  83. 83. Development of Microstructures in Isomorphous Alloys – Cu-35 wt% Ni alloy is cooled extremely slowly from 1300 0C to allow diffusion to complete the process of readjustment of composition. Equilibrium Cooling Liquid α Composition, wt% NiCu Ni Temperature,0C
  84. 84. • Cu-35 wt% Ni liquid alloy is cooled from 1300 0C, see point ‘a’ & the associated microstructure in Fig. • No microstructural or compositional change will be realized until we reach the liquidus line at point ‘b’, ~12600C. • At point ‘b’, the first solid ‘α’ begins to form, which has a composition dictated by the tie line drawn at this temperature is Cu-46 wt% Ni, but the composition of liquid is still ~ Cu-35 wt% Ni. • With continued cooling, both compositions and relative amounts of each of the phases will change. Development of Microstructures in Isomorphous Alloys – Equilibrium Cooling
  85. 85. • The compositions of the liquid and ‘α’ phases will follow the liquidus and solidus lines respectively. • The fraction of the ‘α’ phase will increase with continued cooling. • Overall alloy composition (Cu-35 wt% Ni) remains unchanged during cooling even though there is redistribution of copper and nickel between the phases. • At 1250 0C, point ‘c’, the composition of liquid and ‘α’ phases are Cu-32 wt% Ni, and Cu-43 wt% Ni respectively. Development of Microstructures in Isomorphous Alloys – Equilibrium Cooling
  86. 86. • The solidification is complete upon reaching point ‘d’ at ~1220 0C, the comp. of solid ‘α’ is ~Cu-35 wt% Ni (the over all alloy comp), while the last remaining liquid is Cu-24 wt% Ni. • Upon crossing the solidus line, the remaining liquid solidifies; • The final product is a polycrystalline α-phase solid solution, that has a uniform Cu-35 wt% Ni composition, since it was cooled extremely slowly under equilibrium conditions by allowing diffusion to complete. • Subsequent cooling to RT will not produce any microstructural or compositional changes (alterations). Development of Microstructures in Isomorphous Alloys – Equilibrium Cooling
  87. 87. Development of Microstructures in Isomorphous Alloys – • In practical solidification situations, the cooling rates are rapid enough to cause nonequilibrium microstructural development. • The adjacent Figure shows development of microstructure during the nonequilibrium solidification. • Note that the solidus line got shifted towards right side represented by dashed line. The center of the grain is rich in high m.pt element (Ni) and the area near the GB is rich with low m.pt element (Cu), leading to formation of cored structure, resulting in poor mechanical properties. Nonequilibrium Cooling
  88. 88. • The castings with cored structure when heated below the solidus line, the grain boundaries could melt resulting in loss of mechanical properties. • Therefore, the castings are given a homogenization treatment by heating far below the equilibrium solidus temperature and held for longer durations to minimize the segregation by allowing the atomic diffusion. Development of Microstructures in Isomorphous Alloys – Nonequilibrium Cooling
  89. 89. Development of Microstructures in Isomorphous Alloys – Nonequilibrium Cooling
  90. 90. Lever Rule • The composition of various phases in a phase diagram can be determined by a procedure called the lever rule. • Example: Calculate the relative proportions of the phases in a Cu-Ag alloy of eutectic composition just below the eutectic temperature. Ls s Ls L CC CC LS L or CC CC LS S        00 %2.23 2.919.7 2.919.71             CC CCE
  91. 91. Summary of Important Equilibrium Phase Transformations
  92. 92. (c)2003Brooks/Cole,adivisionofThomsonLearning,Inc.ThomsonLearning™isatrademarkusedhereinunderlicense. Al – Si eutectic alloy ll ll ll ll ll ll ll ll ll ll ll ll ll ll
  93. 93. Al-Si Alloy Phase Diagram
  94. 94. Al-Si alloys differ from our "standard" phase diagram in that aluminium has zero solid solubility in silicon at any temperature. This means that there is no beta phase and so this phase is "replaced" by pure silicon (you can think of it as a beta phase which consists only of silicon).So, for Al-Si alloys, the eutectic composition is a structure of alpha+Si rather than alpha+beta.
  95. 95. coarse flakes of Si in the eutectic promote brittleness within these alloys. Most Al-Si alloys used have a near-eutectic composition since this gives a lower melting point and makes them cheaper to cast. If these alloys are to be of any great use, we must improve their properties somehow and deal with the brittle Si flakes. No etching is required as Si appears grey whilst the alpha phase appears white. by adding a very small impurity of 0.01%Na the microstructure of the alloy is changed and its properties are greatly improved. un-doped alloy doped alloy
  96. 96. Typical eutectic microstructures: (a) needle-like silicon plates in the aluminum silicon eutectic (x100), and (b) rounded silicon rods in the modified aluminum-silicon eutectic (x100).
  97. 97. The effect of hardening with phosphorus on the microstructure of hypereutectic aluminum-silicon alloys: (a) coarse primary silicon, and (b) fine primary silicon, as refined by phosphorus addition (x75).
  98. 98. The hypereutectic Al-Si alloys containing primary β will provide the wear- resistance that at one-third the weight of the steel. Since the part to be produced is cylindrical in shape, centrifugal casting (Figure) will be a unique method for producing it. A typical alloy used to produce aluminum engine components is Al-17% Si. From Al-Si binary phase diagram (Figure), the total amount of primary β that can form is calculated at 578o C, just above the eutectic temperature: %0.5100 6.1283.99 12.617Primary%    NU
  99. 99. • In an eutectic reaction, when a liquid solution of fixed composi- tion, solidifies at a constant temperature, forms a mixture of two or more solid phases without an intermediate pasty stage. This process reverses on heating. Eutectic
  100. 100. In eutectic system, there is always a specific alloy, known as eutectic composition, that freezes at a lower temp. than all other compositions. At the eutectic temp. two solids form simultaneously form a single liquid phase. The eutectic temp. & composition determine a point on the phase dia, called the eutectic point.
  101. 101.  Binary alloy eutectic system can be classed as: 1. One in which, two metals are completely soluble in the liquid state but are insoluble in each other in the solid state. 2. two metals are completely soluble in the liquid state but are partly soluble in each other in the solid state.
  102. 102. 1. Two metals completely soluble in the liquid state but completely insoluble in the solid state.  Technically, no two metals are completely insoluble in each other. However, in some cases the solubility is so restricted that for practical purposes they may be considered insoluble.
  103. 103. • Alloy-1: 20% Cd and 80% Bi  Contrary to alloy 3, in this case crystal of pure Bi form first, enriching the melt with Cd.  The composition of the melt (or liquid) moves to right until Ultimately the point E is reached and the remaining liquid solidi- fies as eutectic (40% Cd and 60% Bi). • Alloy-2: 40% Cd and 60% Bi (eutectic alloy)  No solidification occurs until the melt reaches the eutectic temperature (140°)  At the eutectic temperature, the two pure metals crystallize together to give a characteristically line aggregate known as eutectic.  Eutectic consists of alternate layers of Cd and Bi which form at the eutectic temperature (140°C in this case).
  104. 104.  Alloy-3: 80% Cd and 20% Bismuth.  As the temperature falls to T1, crystal nuclei of pure Cd begin to form. Since pure Cd is deposited, it follows that the liquid becomes richer in Bi; the composition of liquid move s to left 3’ and as indicated by the diagram, no further Cd deposits until temperature falls to T2.  At T2 more Cd is deposited and dendrites begin to develop from the already formed nuclei.  The growth of the Cd dendrites, on the one hand, and the consequent enrichment of the remaining liquid in Bi, on the other, continues until the temperature has fallen to 140°C, the eutectic temperature in this case.  The remaining liquid then contains 40% Cd and 60% Bi, the eutectic composition.
  105. 105. 2. Two metals completely soluble in the liquid state, but only partly soluble in the solid state
  106. 106.  Since most metals show some solubility for each other in the solid state, this type is the most common and, therefore, the most common alloy system.  Metals such as Pb-Sn and Pb-Sb are partly soluble in each other in the solid state.  Fig. shows the Tin-Lead equilibrium diagram with micro-structures (of course) obtained under non-equilibrium condition of solidification. I. Tin will dissolve up to maximum of 2.6% Pb at the temperature, forming the solid solution α. II. Lead will dissolve up to a maximum of (100-80.5) i.e. 19 .5% tin at the eutectic temperature, giving the solid solution β. III. Slope of BA and CD indicate that the solubility of Pb in Sn (α) and that of Sn in Pb (β) decrease as temperature falls  Consider an alloy of composition Z (70% Pb-30% Sn). As the melt temperature falls to T1, dendrites of composition Y will deposit.
  107. 107.  The alloy solidifies as a solid solution until at 183°C, the last layer of solid to form is of composition C (80.5% Pb-19.5% Sn).  The remaining liquid which has the eutectic composition (38% Pb-62% Sn) then solidifies by depositing, in the form of a eutectic, i.e., alternate layers of α and β, of compositions B and C respectively.  If cooled slowly to room temperature the compositions of the solid solutions α and β will follow the line BA and CD, i.e., α will become progressively poorer in lead and β in tin.  Take another alloy of composition Z' (95% Pb-5% Sn). When cooled slowly, solidification starts at R and is complete at P, the resultant solid being a homogeneous single phase, the β solid solution.  As the alloy cools, the solvus line is reached at point Q. The β solution is now saturated in tin. Below this temperature, under conditions of slow cooling, the excess tin must come out of solution. Since tin is soluble in lead, the precipitate does not come out as the pure metal tin, but rather the α solid solution.
  108. 108.  Eutectoid Transformation:  Eutectoid reaction is an isothermal reversible reaction in which a solid phase (usually solid solution) is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of component in the system.
  109. 109. The peritectoid reaction is the transformation of two solid into a third solid.
  110. 110. Peritectic reaction
  111. 111. It is the reaction that occurs during the solidification of some alloys where the liquid phase reacts with a solid phase to give a solid phase of different structure. Assuming very slow rates of cooling, the peritectic reaction will occur only in those Pt-Ag alloys that Contain between 12 and 69% silver (Ag). Consider a liquid (melt) of composition Z, i.e., containing 25% Ag. Solidification commences at T1 and dendrites of α, initially of composition W, begin forming. Selective crystallization of α continues down to Tp, the peritectic temperature; when the alloy reaches. this temperature, it is composed of solid α-dendrites of composition B and liquid of composition D in the proportion α : liquid = RD : RB.
  112. 112.  Peritectoid Transformation:  The peritectoid reaction is the transformation of two solid into a third solid.

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