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

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


1
Phase Diagrams Binary Eutectoid
Systems Iron-Iron-Carbide Phase Diagram Steels
and Cast Iron
  • Weeks 7 - 8

2
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

3
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

4
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
5
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

6
Solubility Limit Sugar-Water
7
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

8
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)
9
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
10
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

11
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

12
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

13
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

14
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.

15
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
16
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)
17
Phase Diagrams composition of phases
Rule 2 If we know T and Co, then we know
--the composition of each phase.
Examples
18
Phase Diagrams weight fractions of phases
Rule 3 If we know T and Co, then we know
--the amount of each phase (given in wt).
Examples
19
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)
20
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.
21
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
22
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
23
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

24
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
25
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
26
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
27
Microstructures in Eutectic Systems I
Co lt 2 wt Sn Result --at extreme ends
--polycrystal of a grains i.e., only
one solid phase.
28
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

29
Microstructures in Eutectic Systems III
Co CE Result Eutectic microstructure
(lamellar structure) --alternating layers
(lamellae) of a and b crystals.
30
Lamellar Eutectic Structure
31
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
32
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
33
Eutectoid Peritectic
  • Eutectic - liquid in equilibrium with two solids
  • L ? ?

34
Eutectoid Peritectic
  • Cu-Zn Phase diagram

35
Iron-Carbon (Fe-C) Phase Diagram
2 important
points
36
Hypoeutectoid Steel
a

g
(a)
a (Ferrite) Fe3C (cementite)
37
Hypereutectoid Steel
a

g
Co
0.76
38
Case Study
  • For a 99.6 wt Fe-0.40 wt C at a temperature
    just below the eutectoid, determine the following
  • composition of Fe3C and ferrite (?)
  • the amount of carbide (cementite) in grams that
    forms per 100 g of steel
  • the amount of pearlite and proeutectoid ferrite
    (?)

39
Case Study
  • Solution

a) composition of Fe3C and ferrite (?)
  1. the amount of carbide (cementite) in grams that
    forms per 100 g of steel

40
Case Study
  • the amount of pearlite and proeutectoid ferrite
    (?)
  • note amount of pearlite amount of g just
    above TE

Co 0.40 wt C Ca 0.022 wt C Cpearlite C?
0.76 wt C
41
Alloying Steel with More Elements
42
Self-Study Task
  • Applications of Phase Diagrams of Alloys in
    Mechanical and Manufacturing Engineering

43
Summary
Phase diagrams are useful tools to determine
--the number and types of phases, --the wt of
each phase, --and the composition of each phase
for a given T and composition of the system.
Alloying to produce a solid solution usually
--increases the tensile strength (TS) --decreases
the ductility.
Binary eutectics and binary eutectoids allow
for a range of microstructures.
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