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ENGI 7007: Marine Materials
Chapter 9
Dr. Amy Hsiao
Spring 2012
Chapter 9
Phase Diagrams–Equilibrium
Microstructural Development
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Figure 9.1 Single-phase microstructure of commercially pure molybdenum, 200×. Although there are many grains in this
microstructure, each grain has the same uniform composition. (From ASM Handbook, Vol. 9: Metallography and Microstructures,
ASM International, Materials Park, OH, 2004.)
Figure 9.2 Two-phase microstructure of pearlite found in a steel with 0.8 wt % C, 650 ×. This carbon content is an average of the
carbon content in each of the alternating layers of ferrite (with <0.02 wt % C) and cementite (a compound, Fe3C, which contains 6.7
wt % C). The narrower layers are the cementite phase. (From ASM Handbook, Vol. 9: Metallography and Microstructures, ASM
International, Materials Park, OH, 2004.)
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Figure 9.3 (a) Schematic representation of the one-component phase diagram for H2O. (b) A projection of the phase diagram
information at 1 atm generates a temperature scale labeled with the familiar transformation temperatures for H 2O (melting at 0°C
and boiling at 100°C).
Figure 9.4 (a) Schematic representation of the one-component phase diagram for pure iron. (b) A projection of the phase diagram
information at 1 atm generates a temperature scale labeled with important transformation temperatures for iron. This projection
will become one end of important binary diagrams, such as that shown in Figure 9.19
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COMPONENTS AND PHASES
• Components:
The elements or compounds which are mixed initially
(e.g., Ni and Cu)
• Phases:
The physically and chemically distinct material regions
that result (e.g., a and b).
Cu-Ni Alloy
System
COMPONENTS, PHASES, SYSTEMS
• Components:
The elements or compounds which are mixed initially
(e.g., Al and Cu)
• Phases:
The homogeneous portion of a system that has uniform physical and
chemical characteristics; distinct material regions result (e.g., a and b).
System:
AluminumCopper
Alloy
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Phase Diagrams
Solubility Limit – the maximum concentration of solute atoms that may
dissolve in the solvent to form a solid solution for a given alloy system at a
specific temperature
Effect of T & Composition (Co)
• Changing T can change # of phases:
• Changing Co can change # of phases:
path A to B.
path B to D.
B (100°C,70)
1 phase
D (100°C,90)
2 phases
watersugar
system
Temperature (°C)
100
80
L
60
(liquid)
+
L
(liquid solution
40
i.e., syrup)
20
0
0
S
(solid
sugar)
A (20°C,70)
2 phases
20
40
60 70 80
100
Co =Composition (wt% sugar)
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Equilibrium
• A system is at equilibrium if its free energy is at a minimum
under a specified combination of temperature, pressure, and
composition. The system is stable and does not change with
time. A change in T, P, or C for a system in equilibrium will
result in an increase in the free energy and possibly a
spontaneous change to another state whereby the free
energy is lowered.
Figure 9.5 Binary phase diagram showing complete solid solution. The liquid-phase field is labeled L, and the solid solution is
designated SS. Note the two-phase region labeled L + SS.
Complete solid solution – a binary system where the two components are completely soluble in each
other in both the solid and the liquid states. The liquidus is the upper boundary of the two-phase
region above which a single liquid phase is present and the solidus is the lower boundary of the twophase region below which the system has completely solidified.
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Figure 9.6 The compositions of the phases in a two-phase region of the phase diagram are determined by a tie
line (the horizontal line connecting the phase compositions at the system temperature).
At a given temperature and composition (state point) within the two-phase region,
an A-rich liquid coexists in equilibrium with a B-rich solid solution. The compositions
of each is given by the intersection point with the liquidus (for liquid phase) and
solidus (for solid phase). This tie line connects the two phase compositions.
Figure 9.8 Various microstructures characteristic of different regions in the complete solid-solution phase diagram.
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Figure 9.9 Cu–Ni phase diagram. (From Metals Handbook, 8th ed., Vol. 8: Metallography, Structures, and Phase Diagrams,
American Society for Metals, Metals Park, OH, 1973, and Binary Alloy Phase Diagrams, Vol. 1, T. B. Massalski, ed., American
Society for Metals, Metals Park, OH, 1986.)
Phase Equilibria
Simple (complete) solid 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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Figure 9.10 NiO–MgO phase diagram. (After Phase Diagrams for Ceramists, Vol. 1, American Ceramic Society,
Columbus, OH, 1964.)
Figure 9.11 Binary eutectic phase diagram showing no solid solution. This general appearance can be contrasted to the opposite
case of complete solid solution illustrated in Figure 9.5.
Eutectic Diagram with no solid
solution – A and B components are
so dissimilar that their solubility in
each other is negligible or nonexistent.
Characteristic features: at relatively
low temperatures, there is a twophase field for pure solids A and B
(since they can’t dissolve in each
other).
Second, the solidus is a horizontal
line at the “eutectic temperature”.
This means that only material with
the eutectic composition is fully
melted at the eutectic temperature.
Any other composition than the
eutectic will not melt immediately,
but must be heated further through
a two-phase region to the liquidus
line.
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Figure 9.12 Various microstructures characteristic of different regions in a binary eutectic phase diagram with no solid solution.
Eutectic Diagram with no solid solution – Eutectic composition is fine-grained. Issues:
limited time for diffusion and “ordering” of A and B atoms into layers, etc.
Figure 9.13 Al–Si phase diagram. (After Binary Alloy Phase Diagrams, Vol. 1, T. B. Massalski, Ed., American Society for Metals,
Metals Park, OH, 1986.)
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Figure 9.14 Binary eutectic phase diagram with limited solid solution. The only difference between this diagram and the one
shown in Figure 9.11 is the presence of solid-solution regions α and β.
Eutectic Diagram with limited solid
solution – A and B components are
partially soluble in each other. It looks
like an intermediate case of the solid
solution binary phase diagram and the
no solid solution binary phase diagram.
The two solid-solution phases, a and b,
are distinguishable and usually have
different crystal structures. So the
crystal structure of a will be that of A,
and A is the solvent which consists of B
atoms in solid solution. In the same
way, the crystal structure of b will be
that of B, where atoms of A act as
solutes in the B crystal lattice.
Figure 9.15 Various microstructures characteristic of different regions in the binary eutectic phase diagram with limited solid
solution. This illustration is essentially equivalent to the illustration shown in Figure 9.12, except that the solid phases are now solid
solutions (α and β) rather than pure components (A and B).
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Figure 9.16 Pb–Sn phase diagram. (After Metals Handbook, 8th ed., Vol. 8: Metallography, Structures, and Phase Diagrams,
American Society for Metals, Metals Park, Ohio, 1973, and Binary Alloy Phase Diagrams, Vol. 2, T. B. Massalski, Ed., American
Society for Metals, Metals Park, OH, 1986.)
Figure 9.17 This eutectoid phase diagram contains both a eutectic reaction (Equation 9.3) and its solid-state analog, a eutectoid
reaction (Equation 9.4).
L(eutectic) cooling
a  b
 (eutectoid) cooling
a  b
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Figure 9.18 Representative microstructures for the eutectoid diagram of Figure 9.17.
Figure 9.19 Fe–Fe3C phase diagram. Note that the composition axis is given in weight percent carbon even though Fe 3C, and not
carbon, is a component. (After Metals Handbook, 8th ed., Vol. 8: Metallography, Structures, and Phase Diagrams, American Society
for Metals, Metals Park, OH, 1973, and Binary Alloy Phase Diagrams, Vol. 1, T. B. Massalski, Ed., American Society for Metals,
Metals Park, OH, 1986.)
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Figure 9.20 Fe–C phase diagram. The left side of this diagram is nearly identical to the left side of the Fe–Fe3C diagram (Figure
9.19). In this case, however, the intermediate compound Fe 3C does not exist. (After Metals Handbook, 8th ed., Vol. 8:
Metallography, Structures, and Phase Diagrams, American Society for Metals, Metals Park, OH, 1973, and Binary Alloy Phase
Diagrams, Vol. 1, T. B. Massalski, Ed., American Society for Metals, Metals Park, OH, 1986.)
Figure 9.25 (a) A relatively complex binary phase diagram. (b) For an overall composition between AB 2 and AB4, only that binary
eutectic diagram is needed to analyze microstructure.
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Figure 9.27 Al–Cu phase diagram. (After Binary Alloy Phase Diagrams, Vol. 1, T. B. Massalski, Ed., American Society for Metals,
Metals Park, OH, 1986.)
Figure 9.28 Cu–Zn phase diagram. (After Metals Handbook, 8th ed., Vol. 8: Metallography, Structures, and Phase Diagrams,
American Society for Metals, Metals Park, OH, 1973, and Binary Alloy Phase Diagrams, Vol. 1, T. B. Massalski, Ed., American
Society for Metals, Metals Park, OH, 1986.)
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Figure 9.30 A more quantitative treatment of the tie line introduced in Figure 9.6 allows the amount of each phase
(L and SS) to be calculated by means of a mass balance (Equations 9.6 and 9.7).
Figure 9.33 Microstructural development during the slow cooling of a eutectic composition.
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Microstructural Development During Slow Cooling
Microstructural Development During Slow Cooling
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Microstructural Development of Hypoeutectic Composition During
Slow Cooling
Microstructural Development During Slow Cooling
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Figure 9.34 Microstructural development during the slow cooling of a hypereutectic composition.
Figure 9.35 Microstructural development during the slow cooling of a hypoeutectic composition.
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Figure 9.36 Microstructural development for two
compositions that avoid the eutectic reaction.
Figure 9.37 (left) Microstructural development for white cast iron (of composition 3.0 wt % C) shown with the aid of the Fe–Fe3C
phase diagram. The resulting (low-temperature) sketch can be compared with a micrograph in Figure 11.1a.
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Figure 9.38 Microstructural development for eutectoid steel (of composition 0.77 wt % C). The resulting (low-temperature) sketch
can be compared with the micrograph in Figure 9.2.
Figure 9.39 Microstructural development for a slowly cooled hypereutectoid steel (of composition 1.13 wt % C).
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Figure 9.40 Microstructural development for a slowly cooled hypoeutectoid steel (of composition 0.50 wt % C).
Figure 9.41 Microstructural development for gray cast iron (of composition 3.0 wt % C) shown on the Fe–C phase diagram. The
resulting low-temperature sketch can be compared with the micrograph in Figure 11.1b. A dramatic difference is that, in the actual
microstructure, a substantial amount of metastable pearlite was formed at the eutectoid temperature. It is also interesting to
compare this sketch with that for white cast iron in Figure 9.37. The small amount of silicon added to promote graphite
precipitation is not shown in this two-component diagram.
Figure 11.1 (right) Typical microstructures of (a) white iron (400 ×), eutectic carbide (light constituent) plus pearlite (dark constituent);
(b) gray iron (100 ×), graphite flakes in a matrix of 20% free ferrite (light constituent) and 80% pearlite (dark constituent).
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Figure 11.1 (continued) (c) ductile iron (100 ×), graphite nodules (spherulites) encased in envelopes of free ferrite, all in a matrix of pearlite;
and (d) malleable iron (100 ×), graphite nodules in a matrix of ferrite. (From Metals Handbook, 9th ed., Vol. 1, American Society for Metals,
Metals Park, OH, 1978.)
Practice Problem: Cu-Ni Phase Diagram
•
A copper-nickel alloy of
composition 70 wt% Ni-30
wt% Cu is slowly heated from
a temperature of 1300 °C.
– At what temperature does
the first liquid phase form?
– What is the composition of
this liquid phase?
– At what temperature does
complete melting of the
alloy occur?
– What is the composition of
the last solid remaining
prior to complete melting?
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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.
T(°C) L (liquid)
130 0
L: 35 wt% Ni
a: 46 wt% Ni
• Consider
Co = 35 wt%Ni.
Cu-Ni
system
A
35
32
--isomorphous
i.e., complete
solubility of one
component in
another; a phase
field extends from
0 to 100 wt% Ni.
L: 35wt%Ni
B
C
46
43
D
24
L: 32 wt% Ni
36
120 0
a: 43 wt% Ni
E
L: 24 wt% Ni
a: 36 wt% Ni
a
(solid)
110 0
20
30
35
Co
40
50
wt% Ni
Practice Problem: Cu-Ni
Phase Diagram
Is it possible to have a Cu-Ni alloy that,
at equilibrium, consists of an a phase
of composition 37 wt% Ni-63 wt% Cu,
and also a liquid phase of composition
20 wt% Ni-80 wt% Cu? If so, what will
be the approximate temperature of
the alloy? If this is not possible,
explain why.
Ca=37 wt% Ni
Cliq=20 wt% Ni
Not possible; there is not a temperature at
which both phases with the given
compositions can be in equilibrium
together.
Only possibilities: ~1200C: Ca=30 wt% Ni
Cliq=20 wt% Ni
OR ~1240C: Ca=37 wt% Ni, Cliq=27 wt% Ni
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The Lever Rule
x  x0
ma
 b
ma  mb xb  xa
mb
ma  mb

x0  xa
xb  xa
• Given a solid solution binary phase diagram of an 50:50 A-B alloy weighing 1 kg, calculate the
amount in each phase when the temperature of the alloy is lowered until the liquid solution
composition is 18 wt% B and the solid-solution composition is 66 wt%B.
•The alloy is reheated to a temperature at which the liquid composition is 48 wt% B and the s-s
composition is 90 wt % B. Calculate the amount in each phase.
Lever Rule
For a 40 wt% Sn – 60 wt% Pb alloy at 150°C, what phases are present and what are
the compositions of the phases? Calculate the mass fraction of each phase.
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EX: Pb-Sn Eutectic System (1)
• For a 40 wt% Sn-60 wt% Pb alloy at 150°C, find...
--the phases present: a + b
CO = 40 wt% Sn
Ca = 11 wt% Sn
Cb = 99 wt% Sn
--the relative amount
of each phase:
C - CO
S
= b
ma=
R+S
Cb - Ca
Pb-Sn
system
T(°C)
--compositions of phases:
300
L (liquid)
L+ a
a
200
150
61.9
R
100
99 - 40
59
=
= 67 wt%
99 - 11
88
C - Ca
mb = R = O
Cb - Ca
R+S
L +b b
183°C
18.3
97.8
S
a + b
=
=
0 11 20
Ca
60
80
C, wt% Sn
40
Co
99100
Cb
40 - 11
29
=
= 33 wt%
99 - 11
88
EX: Pb-Sn Eutectic System (2)
• For a 40 wt% Sn-60 wt% Pb alloy at 200°C, find...
--the phases present: a + L
CO = 40 wt% Sn
Ca = 17 wt% Sn
CL = 46 wt% Sn
--the relative amount
of each phase:
CL - CO
46 - 40
=
ma =
CL - Ca
46 - 17
6
=
= 21 wt%
29
Pb-Sn
system
T(°C)
--compositions of phases:
300
L+a
220 a
200
R
L (liquid)
L +b b
S
183°C
100
CO - Ca
23
=
mL =
= 79 wt%
CL - Ca
29
a + b
0
17 20
Ca
40 46 60
80
Co CL C, wt% Sn
100
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Microstructural Development of Hypoeutectic Composition During
Slow Cooling
Microstructures
in Eutectic Systems: IV
• 18.3 wt% Sn < Co < 61.9 wt% Sn
• Result: a crystals and a eutectic
T(°C)
L: Co wt% Sn
300
a L
L
a
L
Pb-Sn
system a
microstructure
L+ a
R
200
TE
L+ b b
S
S
R
20
18.3
CL = 61.9 wt% Sn
S
ma=
= 50 wt%
R+S
mL = (1- Wa) = 50 wt%
C a = 18.3 wt% Sn
primary a
eutectic a
eutectic b
0
C a = 18.3 wt% Sn
• Just below TE :
a+b
100
• Just above TE :
40
60
61.9
80
100
97.8
C b = 97.8 wt% Sn
m a = S = 73 wt%
R+S
m b = 27 wt%
Co, wt% Sn
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For a 2 wt% Sn – 98 wt% Pb alloy, what phases are present and what are the compositions of the
phases of the alloy at 100 ºC? Calculate the mass fraction of each phase.
For a 15 wt% Sn – 85 wt% Pb alloy, what phases are present and what are the compositions of
the phases of the alloy at 100 ºC? Calculate the mass fraction of each phase.
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Microstructural Development During Slow Cooling
Microstructures
in Eutectic Systems: I
• Co < 2 wt% Sn
• Result:
--at extreme ends
--polycrystal of a grains
i.e., only one solid phase.
T(°C)
L: Co wt% Sn
400
L
a
L
300
a
200
L+ a
(Pb-Sn
System)
a: Co wt% Sn
TE
a+ b
100
0
Co
10
20
30
Co, wt% Sn
2
(room T solubility limit)
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Microstructures
in Eutectic Systems: II
• 2 wt% Sn < Co < 18.3 wt% Sn
• Result:
L: Co wt% Sn
T(°C)
400
 Initially liquid + a
 then a alone
 finally two phases
 a polycrystal
 fine b-phase inclusions
L
L
a
300
L+a
a
200
TE
a: Co wt% Sn
a
b
100
a+ b
0
10
20
30
Co
Co , wt%
2
(sol. limit at T room)
18.3
(sol. limit at TE)
Pb-Sn
system
Sn
Microstructural Development During Slow Cooling
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Microstructures
in Eutectic Systems: III
• Co = CE
• Result: Eutectic
microstructure (lamellar structure)
--alternating layers (lamellae) of a and b crystals.
T(°C)
L: Co wt% Sn
300
Micrograph of Pb-Sn
eutectic
microstructure
L
Pb-Sn
system
L+a
a
200
Lb b
183°C
TE
100
ab
0
20
18.3
40
b: 97.8 wt% Sn
a: 18.3 wt%Sn
60
CE
61.9
80
160 m
100
97.8
C, wt% Sn
HYPOEUTECTIC & HYPEREUTECTIC
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Practice Problems
A 40 wt% Pb – 60 wt% Mg alloy is heated to a temperature within the a +
liquid phase range. If the mass fraction of each phase is 0.5, then estimate
(a) the temperature of the alloy (b) the compositions of the two phases.
Answer: (a) ~540ºC (b) Ca=26 wt % Pb, CL=54 wt % Pb
A 45 wt% Pb – 55 wt% Mg alloy is rapidly quenched to room temperature
from an elevated temperature in such a way that the high-temperature
microstructure is preserved. This microstructure is found to consist of the
a phase and Mg2Pb, having respective mass fractions of 0.65 and 0.35,
respectively. Determine the approximate temperature from which the alloy
was quenched.
0.65 
c b  c0
cb  ca

81  45
 ca  25.6wt % Pb
81  ca
T ~ 350 °C
~360C
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Recap: Binary-Eutectic Systems
has a special composition
with a min. melting T.
2 components
Cu-Ag
system
T(°C)
1200
Ex.: Cu-Ag system
L (liquid)
• 3 single phase regions
1000
(L, a, b)
L+ a
a
• Limited solubility:
779°C
800
T
E
a: mostly Cu
8.0
b: mostly Ag
600
• TE : No liquid below TE
a  b
400
• CE : Min. melting TE
composition
200
• Eutectic transition
L(CE)
0
20
40
L +b b
71.9 91.2
100
60 CE 80
Co , wt% Ag
a(CaE) + b(CbE)
Lamellar Eutectic Structure
66
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Chapter 10
Kinetics–Heat Treatment
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Figure 10.1 Schematic illustration of the approach to equilibrium. (a) The time for solidification to go to completion is a strong
function of temperature, with the minimum time occurring for a temperature considerably below the melting point. (b) The
temperature–time plane with a transformation curve. We shall see later that the time axis is often plotted on a logarithmic scale.
Figure 10.2 (a) On a microscopic scale, a solid precipitate in a liquid matrix. The precipitation process is seen on the atomic scale
as (b) a clustering of adjacent atoms to form (c) a crystalline nucleus followed by (d) the growth of the crystalline phase.
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Figure 10.4 The rate of nucleation is a product of two curves that represent two opposing factors (instability and diffusivity).
•
Figure 10.5 The overall transformation rate is the product of the nucleation rate, N, (from Figure 10.4) and the growth rate, G
•
(given in Equation 10.1).
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Figure 10.6 A time–temperature–transformation diagram for the solidification reaction of Figure 10.1 with various percent
completion curves illustrated.
Figure 10.7 TTT diagram for eutectoid steel shown in relation to the Fe–Fe3 C phase diagram (see Figure 9.38). This diagram
shows that, for certain transformation temperatures, bainite rather than pearlite is formed. In general, the transformed
microstructure is increasingly fine grained as the transformation temperature is decreased. Nucleation rate increases and
diffusivity decreases as temperature decreases. The solid curve on the left represents the onset of transformation (~1%
completion). The dashed curve represents 50% completion. The solid curve on the right represents the effective (~99%) completion
of transformation. This convention is used in subsequent TTT diagrams. (TTT diagram after Atlas of Isothermal Transformation and
Cooling Transformation Diagrams, American Society for Metals, Metals Park, OH, 1977.)
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Figure 10.8 A slow cooling path that leads to coarse pearlite formation is superimposed on the TTT diagram for eutectoid steel.
This type of thermal history was assumed, in general, throughout Chapter 9.
Figure 10.9 The microstructure of bainite involves extremely fine needles of ferrite and carbide, in contrast to the lamellar
structure of pearlite (see Figure 9.2), 250×. (From ASM Handbook, Vol. 9: Metallography and Microstructures, ASM International,
Materials Park, OH, 2004.)
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Figure 10.10 The interpretation of TTT diagrams requires consideration of the thermal history “path.” For example, coarse
pearlite, once formed, remains stable upon cooling. The finer-grain structures are less stable because of the energy associated with
the grain-boundary area. (By contrast, phase diagrams represent equilibrium and identify stable phases independent of the path
used to reach a given state point.)
Figure 10.11 A more complete TTT diagram for eutectoid steel than was given in Figure 10.7. The various stages of the timeindependent (or diffusionless) martensitic transformation are shown as horizontal lines. M s represents the start, M50 represents
50% transformation, and M90 represents 90% transformation. One hundred percent transformation to martensite is not complete
until a final temperature (Mf) of −46°C.
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Figure 10.12 For steels, the martensitic transformation involves the sudden reorientation of C and Fe atoms from the fcc solid
solution of γ-Fe (austenite) to a body-centered tetragonal (bct) solid solution (martensite). In (a), the bct unit cell is shown relative
to the fcc lattice by the 100α axes. In (b), the bct unit cell is shown before (left) and after (right) the transformation. The open
circles represent iron atoms. The solid circle represents an interstitially dissolved carbon atom. This illustration of the martensitic
transformation was first presented by Bain in 1924, and while subsequent study has refined the details of the transformation
mechanism, this diagram remains a useful and popular schematic. (After J. W. Christian, in Principles of Heat Treatment of Steel,
G. Krauss, Ed., American Society for Metals, Metals Park, OH, 1980.)
Figure 10.13 Acicular, or needlelike, microstructure of martensite 100×. (From ASM Handbook, Vol. 9: Metallography and
Microstructures, ASM International, Materials Park, OH, 2004.)
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Figure 10.15 TTT diagram for a hypereutectoid composition (1.13 wt % C) compared with the Fe–Fe3 C phase diagram.
Microstructural development for the slow cooling of this alloy was shown in Figure 9.39. (TTT diagram after Atlas of Isothermal
Transformation and Cooling Transformation Diagrams, American Society for Metals, Metals Park, OH, 1977.)
Figure 10.16 TTT diagram for a hypoeutectoid composition (0.5 wt % C) compared with the Fe–Fe3C phase diagram.
Microstructural development for the slow cooling of this alloy was shown in Figure 9.40. By comparing Figures 10.11, 10.15, and
10.16, one will note that the martensitic transformation occurs at decreasing temperatures with increasing carbon content in the
region of the eutectoid composition. (TTT diagrams after Atlas of Isothermal Transformation and Cooling Transformation
Diagrams, American Society for Metals, Metals Park, OH, 1977.)
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Figure 10.17 Tempering is a thermal history [T = f n(t)] in which martensite, formed by quenching austenite, is reheated. The
resulting tempered martensite consists of the equilibrium phase of α-Fe and Fe3 C, but in a microstructure different from both
pearlite and bainite (note Figure 10.18). (After Metals Handbook, 8th ed., Vol. 2, American Society for Metals, Metals Park, OH,
1964. It should be noted that the TTT diagram is, for simplicity, that of eutectoid steel. As a practical matter, tempering is generally
done in steels with slower diffusional reactions that permit less-severe quenches.)
Figure 10.18 The microstructure of tempered martensite, although an equilibrium mixture of α-Fe and Fe3C, differs from those for
pearlite (Figure 9.2) and bainite (Figure 10.9). This micrograph produced in a scanning electron microscope (SEM) shows carbide
clusters in relief above an etched ferrite. (From ASM Handbook, Vol. 9: Metallography and Microstructures, ASM International,
Materials Park, OH, 2004.)
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Figure 10.19 In martempering, the quench is stopped just above Ms . Slowcooling through the martensitic transformation range
reduces stresses associated with the crystallographic change. The final reheat step is equivalent to that in conventional tempering.
(After Metals Handbook, 8th ed., Vol. 2, American Society for Metals, Metals Park, OH, 1964.)
Figure 10.20 As with martempering, austempering avoids the distortion and cracking associated with quenching through the
martensitic transformation range. In this case, the alloy is held long enough just above M s to allow full transformation to bainite.
(After Metals Handbook, 8th ed., Vol. 2, American Society for Metals, Metals Park, OH, 1964.)
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Heat Treatment of 1045 Steel
•
•
Heat treatment scheme #1: Take sample out of the furnace and quench immediately in water.
 expected microstructure?____________________________________________
•
Heat treatment scheme #2: Take sample out of the furnace, cool in air for 1 minute, then quench immediately in water, hold in
water for 3 minutes.
 expected microstructure?___________________________________________
•
•
•
•
•
•
•
•
Heat treatment scheme #3: Take sample out of the furnace, cool in air for 3 minutes, then quench immediately in water, hold in
water for 3 minutes.
 expected microstructure? ___________________________________________
Heat treatment scheme #4: Take sample out of the furnace, cool in 400ºC furnace for 5 minutes, then quench immediately in
water, hold in water for 3 minutes.
 expected microstructure?____________________________________________
Heat treatment scheme #5: Take sample out of the furnace, quench immediately in water, hold in water for 3 minutes, place
sample back in 400ºC furnace for 5 minutes, then quench in water for 3 minutes.
 expected microstructure? ____________________________________________
•
Heat treatment scheme #6: Take sample out of the furnace, cool at room temperature for 10 minutes, then quench in water, hold
in water for 3 minutes.
 expected microstructure? ___________________________________________
•
•
Heat treatment scheme #7: Take sample out of the furnace and quench immediately in water.
 expected microstructure? ___________________________________________
Figure 10.24 Hardenability curves for various steels with the same carbon content (0.40 wt %) and various alloy contents. The
codes designating the alloy compositions are defined in Table 11.1. (From W. T. Lankford et al., Eds., The Making, Shaping, and
Treating of Steel, 10th ed., United States Steel, Pittsburgh, PA, 1985. Copyright 1985 by United States Steel Corporation.)
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Figure 10.25 Coarse precipitates form at grain boundaries in an Al–Cu (4.5 wt %) alloy when slowly cooled from the single-phase
(κ) region of the phase diagram to the two-phase (θ + κ) region. These isolated precipitates do little to affect alloy hardness.
Figure 10.26 By quenching and then reheating an Al–Cu (4.5 wt %) alloy, a fine dispersion of precipitates forms within the κ
grains. These precipitates are effective in hindering dislocation motion and, consequently, increasing alloy hardness (and strength).
This process is known as precipitation hardening, or age hardening.
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Figure 10.27 (a) By extending the reheat step, precipitates coalesce and become less effective in hardening the alloy. The result is
referred to as overaging. (b) The variation in hardness with the length of the reheat step (aging time).
Figure 10.28 (a) Schematic illustration of the crystalline geometry of a Guinier–Preston (G.P.) zone. This structure is most effective
for precipitation hardening and is the structure developed at the hardness maximum shown in Figure 10.27b. Note the coherent
interfaces lengthwise along the precipitate. The precipitate is approximately 15 nm × 150 nm. (From H. W. Hayden, W. G. Moffatt,
and J. Wulff, The Structure and Properties of Materials, Vol. 3: Mechanical Behavior, John Wiley & Sons, Inc., NY, 1965.) (b)
Transmission electron micrograph of G.P. zones at 720,000×. (From ASM Handbook, Vol. 9: Metallography and Microstructures,
ASM International, Materials Park, OH, 2004.)
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Figure 10.29 Examples of cold-working operations: (a) cold-rolling a bar or sheet and (b) cold-drawing a wire. Note in these
schematic illustrations that the reduction in area caused by the cold-working operation is associated with a preferred orientation
of the grain structure.
Figure 10.30 Annealing can involve the complete recrystallization and subsequent grain growth of a cold-worked microstructure.
(a) A cold-worked brass (deformed through rollers such that the cross-sectional area of the part was reduced by one-third). (b)
After 3 s at 580°C, new grains appear. (c) After 4 s at 580°C, many more new grains are present. (d) After 8 s at 580°C, complete
recrystallization has occurred. (e) After 1 h at 580°C, substantial grain growth has occurred. The driving force for this growth is the
reduction of high-energy grain boundaries. The predominant reduction in hardness for this overall process had occurred by step
(d). All micrographs have a magnification of 75×. (Courtesy of J. E. Burke, General Electric Company, Schenectady, NY.)
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Figure 10.31 The sharp drop in hardness identifies the recrystallization temperature as ~290°C for the alloy C26000, “cartridge
brass.” (From Metals Handbook, 9th ed., Vol. 4, American Society for Metals, Metals Park, OH, 1981.)
Figure 10.32 Recrystallization temperature versus melting points for various metals. This plot is a graphic demonstration of the
rule of thumb that atomic mobility is sufficient to affect mechanical properties above approximately
1/3 to 1/2 Tm on an
absolute temperature scale. (From L. H. Van Vlack, Elements of Materials Science and Engineering, 3rd ed., Addison-Wesley
Publishing Co., Inc., Reading, MA, 1975.)
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Figure 10.33 For this cold-worked brass alloy, the recrystallization temperature drops slightly with increasing degrees of cold
work. (From L. H. Van Vlack, Elements of Materials Science and Engineering, 4th ed., Addison-Wesley Publishing Co., Inc., Reading,
MA, 1980.)
Figure 10.34 Schematic illustration of the effect of annealing temperature on the strength and ductility of a brass alloy shows
that most of the softening of the alloy occurs during the recrystallization stage. (After G. Sachs and K. R. Van Horn, Practical
Metallurgy: Applied Physical Metallurgy and the Industrial Processing of Ferrous and Nonferrous Metals and Alloys, American
Society for Metals, Cleveland, OH, 1940.)
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