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SE2 Structural Materials
Phase Diagram
Dr. Yu Qiao
Department of Structural Engineering, UCSD
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Introduction
•
Phase: A region in a material that differs in structure
and function from other regions.
• Phase diagrams:
 Represents phases present in metal at different conditions
(Temperature, pressure and composition).
 Indicates equilibrium solid solubility of one element in
another.
 Indicates temperature range under which solidification
occurs.
 Indicates temperature at which different phases start to
melt.
8-2
1
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Temperature, pressure, composition…
Jade steel: 1-1.5%
carbon – hard yet brittle
Core steel: < 0.2%
carbon – tough yet soft
Jacket steel
Samurai sword
(http://getasword.com)
8-2
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Phase Diagram of Pure Substances
•
•
•
Pure substance exist as solid, liquid and vapor.
Phases are separated by phase boundaries.
Example : Water, Pure Iron.
•
Different phases coexist at triple point.
Example: 8.3
8-3 After W. G. Moffatt, et al., “The Structure and Properties of Materials,” vol I: “Structure,” Wiley, 1965, p.151
2
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Cooling Curves
Thermal arrest : heat lost = heat supplied by
solidifying metal
Iron at 1 atm pressure
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Binary Isomorphous Alloy Systems
•
Binary alloy
Mixture of
two systems
Two component
system
• Isomorphous system: Two elements completely soluble
in each other in liquid and solid state.
• Example: Cu-Ni solution.
Composition at liquid
and solid phases at any
temperature can be
determined by drawing
a tie line.
8-5
Adapted from “Metals Handbook,” vol. 8, 8th ed., American society of Metals, 1973, p. 294.
3
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Binary Isomorphous Alloy Systems
The four Hum-Rothery rules for the solid solubility of one
element in another are:
1. The crystal structure of each element of the solid solution
must be the same.
2. The size of the atoms of each of the two elements must not
differ by more than fifteen percent.
3. The elements should not form compounds with each other;
there should be no appreciable difference in the
electronegativities of the two elements.
4. The elements should have the same electron valence.
Example: 8.8
8-5
Adapted from “Metals Handbook,” vol. 8, 8th ed., American society of Metals, 1973, p. 294.
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Phase Diagram from Cooling Curves
•
Series of cooling curves at different metal composition
are first constructed.
• Points of change of slope of cooling curves (thermal
arrests) are noted and phase diagram is constructed.
• More the number of cooling curves, more accurate is
the phase diagram.
20% Ni
8-6
4
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Binary Eutectic Alloy System
•
In some binary alloy systems, components have limited
solid solubility.
Example : Lead-Tin
(Pb-Sn) alloy.
• Eutectic composition freezes
at lower temperature than all
other compositions.
• This lowest temperature is
called eutectic temperature.
Liquid
Eutectic temperature
Cooling
α solid solution + β solid solution
8-9
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Binary Eutectic Alloy System
•
In some binary alloy systems, components have limited
solid solubility.
40% Tin
Liquid
Eutectic temperature
Cooling
α solid solution + β solid solution
8-9
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Various Eutectic Structures
•
Structure depends on factors like minimization of free
energy at α / β interface.
• Manner in which two phases nucleate and grow also
affects structures.
Figure 8.14
8-11
After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.
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The Lever Rule
•
The Lever rule gives the weight % of phases in any two
phase regions.
Wt fraction of solid phase
= Xs = w0 – w1
ws – w1
Wt fraction of liquid phase
= X1 = ws – w0
ws – w1
Example: 8.10
8-7
Content of B in liquid
phase at T
Content of B in
solid (at T)
6
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Binary Eutectic Alloy System
Problem 8.15
Problem 8.17
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Slow Cooling of 60% Pb – 40% Sn alloy
• Liquid at 3000C.
• At about 2450C first
solid forms – proeutectic
solid.
• Slightly above 1830C
composition of alpha
follows solidus and
composition of sn varies
from 40% to 61.9%.
• At eutectic temperature,
all the remaining liquid
solidifies.
• Further cooling lowers alpha Sn content and beta Pb.
8-10
From J. Nutting and R. G. Baker, “Microstructure of Metals,” Institute of Metals, London, 1965,p.19.
7
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More Complicated Phase Diagrams
7000 yrs ago: Arsenical bronze
(Mesopotamia; today’s Iran)
6500 yrs ago: Bronze (today’s Serbia;
The “Old Europe” civilization)
Lower melting point than Cu
Higher hardness/strength than Cu
Bronze: Copper + Tin
Pike
Halberd
Satisfactory stiffness & toughness
High resistance to oxidization
[Cu]=5/6
[Cu] = 4/5
Kao-Gong-Ji (~500 B.C.E):
[Cu] = 6/7: Bell
[Cu] = 1/2: Mirror
[Cu] = 5/7:
Arrowhead
(http://www.mrl.ucsb.edu/~edkramer)
[Cu] = 3/4: Falchion
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Steel
Two phased, lamellar structure of
alternating layers of  phase (88
wt%) and cementite (12 wt%).
Steel
Annealed: Ferrite (white) + pearlite (dark)
Cast Iron

L+
Quenched: Austenite  Martensite
Carbon content:
Low carbon steel: 0.05-0.3%
Medium carbon steel: 0.3-0.6%
High carbon steel: 0.6-1%
Ultrahigh carbon steel: 1-2%
Cast iron: > 2%
Metals, London, 1964.
8-11 After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of http://www.fgg.uni-lj.si/
8
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Steel
Iron “bloom”
Perfused steel
+Fe3C
+Fe3C
8-11
http://www.m4040.com/
After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Steel
Early Steels: Iron Bloom
Solid state reduction:
T is only
~ 1200 oC
Highly pure iron is directly
reduced from iron ore, without
melting
8-11
Carbon is added through
diffusion later on, during
iron working
After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.
9
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Binary Peritectic Alloy System
•
Peritectic reaction: Liquid phase reacts with a solid
phase to form a new and different solid phase.
Liquid + α
β
cooling
• Peritectic reaction occurs
when a slowly cooled alloy
of Fe-4.3 wt% Ni passes
through Peritectic
temperature of 15170C.
• Peritectic point is invariant.
Figure 8.16
Liquid(5.4 wt% Ni) + δ (4.0 wt% Ni)
cooling
γ 4.3 wt % Ni
8-12
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Peritectic Alloy System
Figure 8.17
Figure 8.18
Prob. 8-21
• At 42.4 % Ag & 14000C
Phases present Liquid
Alpha
Composition
55% Ag
7%Ag
Amount of Phases 42.4 –7
55-42.4
55 – 7
55 - 7
= 74%
= 26%
• At 42.4% Ag and 11860C – ΔT
Phase Present
Beta only
Composition
42.4% Ag
Amount of Phase 100%
• At 42.4% Ag and 11860C + ΔT
Phases present Liquid
Alpha
Composition
66.3% Ag 10.5%Ag
Amount of Phases 42.4 –10.5 66.3-42.4
66.3 – 10.5 66.3–10.5
= 57%
=43%
8-13
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Rapid Solidification in Peritectic System
•
Surrounding or Encasement: During peritectic
reaction, L+ α
β , the beta phase created surrounds
primary alpha.
• Beta creates diffusion barrier resulting in coring.
Figure 8.19
Figure 8.20
8-14
After F Rhines, “ Phase Diagrams in Metallurgy,”McGraw- Hill, 1956, p. 86.
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Binary Monotectic Systems
•
Monotectic Reaction: Liquid phase transforms into
solid phase and another liquid.
L1 Cooling α + L2
• Two liquids are immiscible.
• Example:- Copper – Lead
system at 9550C and 36% Pb.
Table 8.1
Eutectic
Eutectoid
Peritectic
Peritectoid
Monotectic
Figure 8.23
8-15
Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 296.
11
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Intermediate Phases and Compounds
•
Terminal phases: Phases
occur at the end of phase
diagrams.
• Intermediate phases:
Phases occur in a
composition range inside
phase diagram.
• Examples: Cu-Zn
diagram has both
terminal and
intermediate phases.
• Five invariant peritectic
points and one eutectic
point.
Figure 8.25
8-16 “Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 301
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Intermediate Phases in Ceramics
•
In Al2O2 – SiO2 system, an intermediate phase called
Mullite is formed, which includes the compound
3Al2O3.2SiO2.
Figure 8.26
8-17
After A. G. Guy, “Essentials of Materials Science, “McGraw-Hill, 1976
12
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Intermediate Compounds
•
In some phase diagrams, intermediate compound are formed
– Stoichiometric
• Percent Ionic/Covalent bond depends on electronegativeness
• Example:- Mg-Ni phase diagram contains
 Mg2Ni : Congruently melting compound
 MgNi2 : Incongruently melting compound.
Figure 8.27
8-18
Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 314.
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Ternary Phase Diagrams
•
•
Three components
Constructed by using a equilateral triangle as base.
• Pure components at each
end of triangle.
• Binary alloy composition
represented on edges.
Figure 8.28
Temperature can be represented as uniform throughout the
Whole Diagram
Isothermal section.
8-19
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Ternary Phase Diagram (Cont..)
•
Example:- Iron-Chromium-Nickel phase diagrams.
Figure 8.30
8-20
•Isothermal reaction at 6500C
for this system
• Composition of any metal
at any point on the phase
diagram can be found by
drawing perpendicular
from pure metal corner to
apposite side and calculating
the % length of line at that
point
After “Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 425.
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Binary peritectic iridium-osmium (Prob. 8-21)
8-20
After “Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 425.
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