Phase Diagrams Binary Eutectoid Systems Iron-Iron-Carbide Phase Diagram Steels and Cast Iron

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Phase DiagramsBinary Eutectoid
SystemsIron-Iron-Carbide Phase DiagramSteels
and Cast Iron
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What is Phase?

• The term phase refers to a separate and
  identifiable state of matter in which a given
  substance may exist.
• Applicable to both crystalline and
  non-crystalline materials
• An important refractory oxide silica is able to
  exist as three crystalline phases, quartz,
  tridymite and cristobalite, as well as a
  non-crystalline phase, silica glass, and as
  molten silica
• Every pure material is considered to be a phase,
  so also is every solid, liquid, and gaseous
  solution
• For example, the sugarwater syrup solution is
  one phase, and solid sugar is another

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Introduction to Phase Diagram

• There is a strong correlation between
  microstructure and mechanical properties, and the
  development of microstructure of an alloy is
  related to the characteristics of its phase
  diagram
• It is a type of chart used to show conditions at
  which thermodynamically distinct phases can occur
  at equilibrium
• Provides valuable information about melting,
  casting, crystallization, and other phenomena

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ISSUES TO ADDRESS...
When we combine two elements...
what equilibrium state do we get?
In particular, if we specify... --a
composition (e.g., wt Cu - wt Ni), and --a
temperature (T )
then... How many phases do we get? What
is the composition of each phase? How much of
each phase do we get?
Phase B
Phase A
Nickel atom
Copper atom
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Solubility Limit

• At some specific temperature, there is a maximum
  concentration of solute atoms that may dissolve
  in the solvent to form a solid solution, which is
  called as Solubility Limit
• The addition of solute in excess of this
  solubility limit results in the formation of
  another compound that has a distinctly different
  composition
• This solubility limit depends on the temperature

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Solubility Limit Sugar-Water
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Microstructure

• the structure of a prepared surface of material
  as revealed by a microscope above 25
  magnification
• The microstructure of a material can strongly
  influence properties such as strength, toughness,
  ductility, hardness, corrosion resistance,
  high/low temperature behavior, wear resistance,
  etc

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Components and Phases
Components The elements or compounds
which are present in the mixture (e.g.,
Al and Cu) Phases The physically and
chemically distinct material regions that
result (e.g., a and b).
Aluminum- Copper Alloy
b
(lighter
phase)
a
(darker
phase)
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Effect of T Composition (Co)
path A to B.
Changing T can change of phases

• Changing Co can change of phases

path B to D.
water- sugar system
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PHASE EQUILIBRIA

• Free Energy -gt a function of the internal energy
  of a system, and also the disorder of the atoms
  or molecules (or entropy)
• A system is at equilibrium if its free energy is
  at a minimum under some specified combination of
  temperature, pressure, and composition
• A change in temperature, pressure, and/or
  composition for a system in equilibrium will
  result in an increase in the free energy
• And in a possible spontaneous change to another
  state whereby the free energy is lowered

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Unary Phase Diagram

• Three externally controllable parameters that
  will affect phase structure temperature,
  pressure, and composition
• The simplest type of phase diagram to understand
  is that for a one-component system, in which
  composition is held constant
• Pure water exists in three phases solid, liquid
  and vapor

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Pressure-Temperature Diagram (Water)

• Each of the phases will exist under equilibrium
  conditions over the temperaturepressure ranges
  of its corresponding area
• The three curves (aO, bO, and cO) are phase
  boundaries at any point on one of these curves,
  the two phases on either side of the curve are in
  equilibrium with one another
• Point on a PT phase diagram where three phases
  are in equilibrium, is called a triple point

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Binary Phase Diagrams

• A phase diagram in which temperature and
  composition are variable parameters, and pressure
  is held constantnormally 1atm
• Binary phase diagrams are maps that represent the
  relationships between temperature and the
  compositions and quantities of phases at
  equilibrium, which influence the microstructure
  of an alloy.
• Many microstructures develop from phase
  transformations, the changes that occur when the
  temperature is altered

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Phase Equilibria
Simple solution system (e.g., Ni-Cu solution)
Crystal Structure electroneg r (nm)
Ni FCC 1.9 0.1246
Cu FCC 1.8 0.1278

• Both have the same crystal structure (FCC) and
  have similar electronegativities and
  atomic radii (W. Hume Rothery rules)
  suggesting high mutual solubility.

• Ni and Cu are totally miscible in all
  proportions.

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Phase Diagrams
Indicate phases as function of T, Compos, and
Press. For this course -binary systems
just 2 components. -independent variables T
and Co (P 1 atm is almost always used).
Phase Diagram for Cu-Ni system
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Phase Diagrams and types of phases
Rule 1 If we know T and Co, then we know
--the number and types of phases present.
Examples
A(1100C, 60)
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Phase Diagramscomposition of phases
Rule 2 If we know T and Co, then we know
--the composition of each phase.
Examples
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Phase Diagramsweight fractions of phases
Rule 3 If we know T and Co, then we know
--the amount of each phase (given in wt).
Examples
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The Lever Rule

• Tie line connects the phases in equilibrium
  with each other - essentially an isotherm

How much of each phase? Think of it as a
lever (teeter-totter)
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Ex Cooling in a Cu-Ni Binary
Phase diagram Cu-Ni system.
System is --binary i.e., 2
components Cu and Ni. --isomorphous
i.e., complete solubility of one
component in another a phase field
extends from 0 to 100 wt Ni.
Consider Co 35 wtNi.
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Cored vs Equilibrium Phases
Ca changes as we solidify. Cu-Ni case
First a to solidify has Ca 46 wt Ni. Last a
to solidify has Ca 35 wt Ni.
Fast rate of cooling Cored structure
Slow rate of cooling Equilibrium structure
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Mechanical Properties Cu-Ni System
Effect of solid solution strengthening on
--Tensile strength (TS)
--Ductility (EL,AR)
--Peak as a function of Co
--Min. as a function of Co
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Eutectic System

• A eutectic system is a mixture of chemical
  compounds or elements that has a single chemical
  composition that solidifies at a lower
  temperature than any other composition

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Binary-Eutectic Systems
has a special composition with a min. melting T.
2 components
Cu-Ag system
T(C)

Ex. Cu-Ag system

1200
3 single phase regions

L (liquid)

a, b
(L,
)

1000
a
L

a
Limited solubility

b

L

779C
b

800

TE
a

mostly Cu

8.0
71.9
91.2
b

mostly Ag

600
TE
No liquid below TE

a

b

400
Min. melting TE
CE
composition

200
80
100
20
40
60
0
CE
Co
,
wt Ag
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EX Pb-Sn Eutectic System (1)
For a 40 wt Sn - 60 wt Pb alloy at 150C,
find... --the phases present
a b
Pb-Sn system
--compositions of phases
CO 40 wt Sn
Ca 11 wt Sn
Cb 99 wt Sn
--the relative amount of each phase
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EX Pb-Sn Eutectic System (2)
For a 40 wt Sn - 60 wt Pb alloy at 220C,
find... --the phases present
a L
Pb-Sn system
--compositions of phases
CO 40 wt Sn
Ca 17 wt Sn
CL 46 wt Sn
--the relative amount of each phase
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Microstructures in Eutectic Systems I
Co lt 2 wt Sn Result --at extreme ends
--polycrystal of a grains i.e., only
one solid phase.
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Microstructures in Eutectic Systems II

• 2 wt Sn lt Co lt 18.3 wt Sn
• Result
• Initially liquid ?
• then ? alone
• finally two phases
• a polycrystal
• fine ?-phase inclusions

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Microstructures in Eutectic Systems III
Co CE Result Eutectic microstructure
(lamellar structure) --alternating layers
(lamellae) of a and b crystals.
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Lamellar Eutectic Structure
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Microstructures in Eutectic Systems (Pb-Sn) IV
18.3 wt Sn lt Co lt 61.9 wt Sn Result a
crystals and a eutectic microstructure
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Hypoeutectic Hypereutectic
300

L
T(C)
a
L

a
b
b
L

(Pb-Sn
200


TE
System)
a

b
100

Co, wt Sn
20
60
80
100
0
40
eutectic
61.9
eutectic Co 61.9 wt Sn
160 mm
eutectic micro-constituent