Embed Size (px)
Citation preview
MSE 513: What do we want to
understand?
• Primary goal: get a basic understanding of:
– phase diagrams
– phase transitions
– Microstructure and how it forms
– Diffusion of atoms
– Thermodynamics and how they apply to phase
transitions
– Kinetics of phase transitions
• How do they start and proceed?
1
Why do we care?
• MSE 511: crystal structure
– FCC, BCC, NaCl, CsCl, …
– What happens if we have different compositions?
• Not always one crystal structure
• Can get multiple phases with the same crystal
structure
– What is the role of temperature?
• MSE 512: mechanical properties
– “solid solution” hardening by alloying
– Strengthening by precipitation of second phases
• May also cause brittleness
– How do we control this?2
What is a “phase”?
• A portion of material where properties are essentially
the same.
• Properties:
– State of matter (solid, liquid, gas)
– Crystal structure
– Composition
3
4
Phase Equilibria: Solubility LimitIntroduction
– 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
Pu
re
Su
gar
Tem
pera
ture
(°C
)
0 20 40 60 80 100Co =Composition (wt% sugar)
L(liquid solution
i.e., syrup)
Solubility Limit L
(liquid)
+ S
(solid sugar)20
40
60
80
100
Pu
re
Wate
r
Adapted from Fig. 9.1,
Callister 7e.
5
Effect of T & Composition (Co)
• Changing T can change # of phases:
Adapted from
Fig. 9.1,
Callister 7e.
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
Tem
pera
ture
(°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
6
Phase Equilibria
CrystalStructure
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.
Simple solution system (e.g., Ni-Cu solution)
• Ni and Cu are totally miscible in all proportions.
8
Phase Diagrams
• Indicate phases as function of T, Co, and P.
• 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
Adapted from Fig. 9.3(a), Callister 7e.
(Fig. 9.3(a) is adapted from Phase
Diagrams of Binary Nickel Alloys, P. Nash
(Ed.), ASM International, Materials Park,
OH (1991).
• 2 phases:
L (liquid)
a (FCC solid solution)
• 3 phase fields:
L
L + a
a
wt% Ni20 40 60 80 10001000
1100
1200
1300
1400
1500
1600T(°C)
L (liquid)
a
(FCC solid solution)
9
wt% Ni20 40 60 80 10001000
1100
1200
1300
1400
1500
1600T(°C)
L (liquid)
a(FCC solid solution)
Cu-Ni
phase
diagram
Phase Diagrams:# and types of phases
• Rule 1: If we know T and Co, then we know:--the # and types of phases present.
• Examples:
A(1100°C, 60): 1 phase: a
B(1250°C, 35): 2 phases: L + a
Adapted from Fig. 9.3(a), Callister 7e.
(Fig. 9.3(a) is adapted from Phase
Diagrams of Binary Nickel Alloys, P. Nash
(Ed.), ASM International, Materials Park,
OH, 1991).
B(1
250°C
,35)
A(1100°C,60)
10
wt% Ni
20
1200
1300
T(°C)
L (liquid)
a
(solid)
30 40 50
Cu-Ni
system
Phase Diagrams:composition of phases
• Rule 2: If we know T and Co, then we know:--the composition of each phase.
• Examples:TA
A
35Co
32CL
At TA = 1320°C:
Only Liquid (L) CL = Co ( = 35 wt% Ni)
At TB = 1250°C:
Both a and L
CL = C liquidus ( = 32 wt% Ni here)
Ca = Csolidus ( = 43 wt% Ni here)
At TD = 1190°C:
Only Solid ( a)
Ca = Co ( = 35 wt% Ni)
Co = 35 wt% Ni
Adapted from Fig. 9.3(b), Callister 7e.
(Fig. 9.3(b) is adapted from Phase Diagrams
of Binary Nickel Alloys, P. Nash (Ed.), ASM
International, Materials Park, OH, 1991.)
BTB
DTD
tie line
4Ca3
11
• Rule 3: If we know T and Co, then we know:--the amount of each phase (given in wt%).
• Examples:
At TA: Only Liquid (L)
WL = 100 wt%, Wa = 0
At TD: Only Solid ( a)
WL = 0, Wa = 100 wt%
Co = 35 wt% Ni
Adapted from Fig. 9.3(b), Callister 7e.
(Fig. 9.3(b) is adapted from Phase Diagrams of
Binary Nickel Alloys, P. Nash (Ed.), ASM
International, Materials Park, OH, 1991.)
Phase Diagrams:
weight fractions of phases
wt% Ni
20
1200
1300
T(°C)
L (liquid)
a
(solid)
30 40 50
Cu-Ni
system
TAA
35Co
32CL
BTB
DTD
tie line
4Ca3
R S
At TB: Both a and L
% 733243
3543wt
= 27 wt%
WL S
R +S
Wa R
R +S
12
• Tie line – connects the phases in equilibrium with
each other - essentially an isotherm
The Lever Rule
How much of each phase?
Think of it as a lever (teeter-totter)
ML Ma
R S
RMSM L a
L
L
LL
LL
CC
CC
SR
RW
CC
CC
SR
S
MM
MW
a
a
a
a
a
00
wt% Ni
20
1200
1300
T(°C)
L (liquid)
a
(solid)
30 40 50
BTB
tie line
CoCL Ca
SR
Adapted from Fig. 9.3(b),
Callister 7e.
13
wt% Ni20
1200
1300
30 40 50110 0
L (liquid)
a
(solid)
T(°C)
A
35Co
L: 35wt%Ni
Cu-Ni
system
• Phase diagram:Cu-Ni system.
• System is:--binary
i.e., 2 components:
Cu and Ni.
--isomorphousi.e., complete
solubility of one
component in
another; a phase
field extends from
0 to 100 wt% Ni.
Adapted from Fig. 9.4,
Callister 7e.
• ConsiderCo = 35 wt%Ni.
Ex: Cooling in a Cu-Ni Binary
4635
4332
a: 43 wt% Ni
L: 32 wt% Ni
L: 24 wt% Ni
a: 36 wt% Ni
Ba: 46 wt% NiL: 35 wt% Ni
C
D
E
24 36
14
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
Adapted from Fig. 9.6(a), Callister 7e. Adapted from Fig. 9.6(b), Callister 7e.
Te
nsile
Str
en
gth
(M
Pa
)
Composition, wt% NiCu Ni0 20 40 60 80 100
200
300
400
TS for pure Ni
TS for pure Cu
Elo
ng
atio
n (
%E
L)
Composition, wt% NiCu Ni0 20 40 60 80 100
20
30
40
50
60
%EL for pure Ni
%EL for pure Cu
More complicated: Multiple solid phases
• Example: water (ice) + salt
18
Tem
per
atu
re
% salt
Water
Ice +salt water
Salt water +Pure salt
Ice + salt
19
: Min. melting TE
2 componentshas a special composition
with a min. melting T.
Adapted from Fig. 9.7,
Callister 7e.
Binary-Eutectic Systems
• Eutectic transition
L(CE) a(CaE) + (CE)
• 3 single phase regions
(L, a, )
• Limited solubility: a: mostly Cu
: mostly Ag
• TE : No liquid below TE
• CE
composition
Ex.: Cu-Ag system
Cu-Ag
system
L (liquid)
a L + aL+
a
Co , wt% Ag20 40 60 80 1000
200
1200T(°C)
400
600
800
1000
CE
TE 8.0 71.9 91.2779°C
20
L+aL+
a +
200
T(°C)
18.3
C, wt% Sn
20 60 80 1000
300
100
L (liquid)
a183°C
61.9 97.8
• For a 40 wt% Sn-60 wt% Pb alloy at 150°C, find...--the phases present: Pb-Sn
system
EX: Pb-Sn Eutectic System (1)
a +
--compositions of phases:
CO = 40 wt% Sn
--the relative amount
of each phase:150
40Co
11Ca
99C
SR
Ca = 11 wt% Sn
C = 99 wt% Sn
Wa=C - CO
C - Ca
=99 - 40
99 - 11=
59
88= 67 wt%
SR+S
=
W =CO - Ca
C - Ca
=R
R+S
=29
88= 33 wt%=
40 - 11
99 - 11
Adapted from Fig. 9.8,
Callister 7e.
21
L+
a +
200
T(°C)
C, wt% Sn
20 60 80 1000
300
100
L (liquid)
a
L+a
183°C
• For a 40 wt% Sn-60 wt% Pb alloy at 200°C, find...--the phases present: Pb-Sn
system
Adapted from Fig. 9.8,
Callister 7e.
EX: Pb-Sn Eutectic System (2)
a + L
--compositions of phases:
CO = 40 wt% Sn
--the relative amount
of each phase:
Wa =CL - CO
CL - Ca
=46 - 40
46 - 17
=6
29= 21 wt%
WL =CO - Ca
CL - Ca
=23
29= 79 wt%
40Co
46CL
17Ca
220SR
Ca = 17 wt% Sn
CL = 46 wt% Sn
Intermetallic Compounds
Mg2Pb
Note: some intermetallic compound forms a line - not an
area - because stoichiometry (i.e. composition) is exact.
Adapted from
Fig. 9.20, Callister 7e.
Congruent
melting point:
LS with same
composition
22
23
Eutectoid & Peritectic
• Eutectic - liquid in equilibrium with two solids
L a + cool
heat
intermetallic compound - cementite
cool
heat
• Eutectoid - solid phase in equilibrium with two solid
phases
S2 S1+S3
a + Fe3C (727ºC)
cool
heat
• Peritectic - liquid + solid 1 solid 2 (Fig 9.21)
S1 + L S2
+ L (1493ºC)
24
Eutectoid & Peritectic
Cu-Zn Phase diagram
Adapted from
Fig. 9.21, Callister 7e.
Eutectoid transition +
Peritectic transition + L
Sn-Au phase diagram:
• Identify the eutectics,
eutectoids, peritectics,
and congruent melting
points by their
temperature and
composition ranges.
• Write the reactions on
cooling.
25
Sn-Cu phase diagram (brass):
• A typical ancient brass is
about 88 wt-% Cu, with the
remaining portion tin (Sn).
• What phases are present?
• How much of each phase?
• Describe phases on cooling.
26
Cu-Zn (brass) phase diagram:
• Identify the eutectics,
eutectoids, peritectics,
and congruent melting
points by their
temperature and
composition ranges.
• Write the reactions on
cooling.
27
Ti-Al
28
Ni-Al
29
Ti-Ni
30
Al-Zn
31
Ni-Cr
32
Cu-Pb
33
Cu-Al phase diagram:
• Identify the eutectics,
eutectoids, peritectics,
and congruent melting
points by their
temperature and
composition ranges.
• Write the reactions on
cooling.
34
Al2O3-Cr2O3
35
MgO-Al2O3
36
Compare to Mg-Pb:
2 eutectics with an
intermediate phase
ZrO2-CaO
37
Other types of reactions
38
http://www1.asminternational.org/asmenterprise/apd/help/intro.aspx
What is microstructure?
• Material properties are not just determined by what
phases are present!
• Quantity of each phase
• Size distribution of each phase
• Interfaces between phases
– Strong? Weak? High energy? Mobile?
39
Examples of microstructure
42
Nb-Nb3Si
Annealed brass Pb-Sn solderNi-Al superalloy
Y1.34Gd0.6Eu0.06O3
(Ceramic scintillator)Nb-Nb3Si
Nb-Si
43
44
• 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 ).
Aluminum-
Copper
Alloy
Components and Phases
a (darker
phase)
(lighter
phase)
Adapted from
chapter-opening
photograph,
Chapter 9,
Callister 3e.
Example: Microstructure and creep resistance in Mg alloys
45Moreno et al., Scripta Mat 2003
Mg-Al
solid
solution
Mg
Mg-RE
compound
Al-RE
compound
• Mg alloys much lighter than Al
• Can make stronger by alloying with Al
(solid-solution hardening)
• Poor creep resistance
• New phases can help with this.
Ni-Al phase diagram and superalloy
microstructure
46
Ni
Ni +
Ni3Al
47
• Co < 2 wt% Sn
• Result:
--at extreme ends
--polycrystal of a grains
i.e., only one solid phase.
Adapted from Fig. 9.11,
Callister 7e.
Microstructures
in Eutectic Systems: I
0
L+ a
200
T(°C)
Co, wt% Sn10
2
20Co
300
100
L
a
30
a+
400
(room T solubility limit)
TE
(Pb-SnSystem)
aL
L: Co wt% Sn
a: Co wt% Sn
48
• 2 wt% Sn < Co < 18.3 wt% Sn
• Result: Initially liquid + a
then a alone
finally two phases
a polycrystal
fine -phase inclusions
Adapted from Fig. 9.12,
Callister 7e.
Microstructures
in Eutectic Systems: II
Pb-Sn
system
L + a
200
T(°C)
Co , wt% Sn10
18.3
200Co
300
100
L
a
30
a+
400
(sol. limit at TE)
TE
2(sol. limit at Troom)
L
a
L: Co wt% Sn
a
a: Co wt% Sn
49
• Co = CE
• Result: Eutectic microstructure (lamellar structure)
--alternating layers (lamellae) of a and crystals.
Adapted from Fig. 9.13,
Callister 7e.
Microstructures
in Eutectic Systems: III
Adapted from Fig. 9.14, Callister 7e.
160m
Micrograph of Pb-Sn eutectic microstructure
Pb-Sn
system
L
a
200
T(°C)
C, wt% Sn
20 60 80 1000
300
100
L
a
L+a
183°C
40
TE
18.3
a: 18.3 wt%Sn
97.8
: 97.8 wt% Sn
CE
61.9
L: Co wt% Sn
50
Lamellar Eutectic Structure
Adapted from Figs. 9.14 & 9.15, Callister
7e.
52
• 18.3 wt% Sn < Co < 61.9 wt% Sn
• Result: a crystals and a eutectic microstructure
Microstructures
in Eutectic Systems: IV
18.3 61.9
SR
97.8
SR
primary a
eutectic a
eutectic
WL = (1-Wa) = 50 wt%
Ca = 18.3 wt% Sn
CL = 61.9 wt% SnS
R + SWa= = 50 wt%
• Just above TE :
• Just below TE :
Ca = 18.3 wt% Sn
C = 97.8 wt% SnS
R + SWa= = 73 wt%
W = 27 wt%Adapted from Fig. 9.16,
Callister 7e.
Pb-Sn
systemL+200
T(°C)
Co, wt% Sn
20 60 80 1000
300
100
L
a
L+a
40
a+
TE
L: Co wt% Sn LaLa
53
Microstructures in Eutectic Systems: IV
50wt%Sn-50wt%Pb alloy.
From http://pwatlas.mt.umist.ac.uk/internetmicroscope///micrographs/microstructures/more-
metals/lead/lead-50-tin_z7.html
This alloy has a composition close to the eutectic. When cooled slowly from the molten state, it
begins to solidify at approximately 210°C by the nucleation and growth of lead rich dendrites.
The remaining liquid becomes tin rich with decreasing temperature until it solidifies as the
eutectic at the eutectic temperature (183°C).
The lead rich dendrites are the large dark phase. At high magnifications, the fine dispersion of
lead rich and tin rich phases in the eutectic can be seen.
The solubility of tin in the lead rich dendrites decreases rapidly below the eutectic temperature,
and tin precipitates can be seen within the dendrites.
Primary α Primary α plus eutectic α+β Primary α
54
L+aL+
a +
200
Co, wt% Sn20 60 80 1000
300
100
L
a TE
40
(Pb-Sn System)
Hypoeutectic & Hypereutectic
Adapted from Fig. 9.8,
Callister 7e. (Fig. 9.8
adapted from Binary Phase
Diagrams, 2nd ed., Vol. 3,
T.B. Massalski (Editor-in-
Chief), ASM International,
Materials Park, OH, 1990.)
160 m
eutectic micro-constituentAdapted from Fig. 9.14,
Callister 7e.
hypereutectic: (illustration only)
Adapted from Fig. 9.17,
Callister 7e. (Illustration
only)
(Figs. 9.14 and 9.17
from Metals
Handbook, 9th ed.,
Vol. 9,
Metallography and
Microstructures,
American Society for
Metals, Materials
Park, OH, 1985.)
175 m
a
a
a
aa
a
hypoeutectic: Co = 50 wt% Sn
Adapted from
Fig. 9.17, Callister 7e.
T(°C)
61.9
eutectic
eutectic: Co =61.9wt% Sn
55
Etched rod-like microstructure of a directional solidified Cu-La alloy showing poor selectivity in etching the matrix without affecting the fibers.
Etched Ag fibers in a directionallysolidified eutectic.
Courtesy of Hong-Bin Bei, ORNL
57
• Ca changes as we solidify.
• Cu-Ni case:
• Fast rate of cooling:Cored structure
• Slow rate of cooling:Equilibrium structure
First a to solidify has Ca = 46 wt% Ni.
Last a to solidify has Ca = 35 wt% Ni.
Cored vs Equilibrium Phases
First a to solidify:
46 wt% Ni
Uniform Ca:
35 wt% Ni
Last a to solidify:
< 35 wt% Ni
58
• Why do we care?
1. Different composition different lattice parameter
internal stress leading to cracks!
2. Different average composition different phase fractions
• Fast rate of cooling:Cored structure
• Slow rate of cooling:Equilibrium structure
First a to solidify has Ca = 46 wt% Ni.
Last a to solidify has Ca = 35 wt% Ni.
Cored vs Equilibrium Phases
First a to solidify:
46 wt% Ni
Uniform Ca:
35 wt% Ni
Last a to solidify:
< 35 wt% Ni
59
Iron-Carbon (Fe-C) Phase Diagram
• 2 important points
-Eutectoid (B): a +Fe3C
-Eutectic (A):L +Fe3C
Adapted from Fig. 9.24,Callister 7e.
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
a+Fe3C
L+Fe3C
(Fe) Co, wt% C
1148°C
T(°C)
a727°C = Teutectoid
A
SR
4.30
Result: Pearlite = alternating layers of a and Fe3C phases
120 m
(Adapted from Fig. 9.27, Callister 7e.)
R S
0.76
Ce
ute
cto
idB
Fe3C (cementite-hard)
a (ferrite-soft)
60
Hypoeutectoid Steel
Adapted from Figs. 9.24
and 9.29,Callister 7e.
(Fig. 9.24 adapted from
Binary Alloy Phase
Diagrams, 2nd ed., Vol.
1, T.B. Massalski (Ed.-in-
Chief), ASM International,
Materials Park, OH,
1990.)
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
a+ Fe3C
L+Fe3C
(Fe) Co, wt% C
1148°C
T(°C)
a727°C
(Fe-C
System)
C0
0.7
6
Adapted from Fig. 9.30,Callister 7e.
proeutectoid ferritepearlite
100 mHypoeutectoid
steel
R S
a
wa =S/(R+S)
wFe3
C
=(1-wa)
wpearlite = wpearlite
r s
wa =s/(r+s)
w =(1- wa)
a
aa
61
Hypereutectoid Steel
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
a +Fe3C
L+Fe3C
(Fe) Co, wt%C
1148°C
T(°C)
a
Adapted from Figs. 9.24
and 9.32,Callister 7e.
(Fig. 9.24 adapted from
Binary Alloy Phase
Diagrams, 2nd ed., Vol.
1, T.B. Massalski (Ed.-in-
Chief), ASM International,
Materials Park, OH,
1990.)
(Fe-C
System)
0.7
6 Co
Adapted from Fig. 9.33,Callister 7e.
proeutectoid Fe3C
60 mHypereutectoid steel
pearlite
R S
wa =S/(R+S)
wFe3C=(1-w a)
wpearlite = wpearlite
sr
wFe3C=r/(r+s)
w =(1-w Fe3C)
Fe3C
63
Example: Phase Equilibria
For a 99.6 wt% Fe-0.40 wt% C at a temperature just
below the eutectoid, determine the following
a) composition of Fe3C and ferrite (a)
b) the amount of carbide (cementite) in grams that
forms per 100 g of steel
c) the amount of pearlite and proeutectoid ferrite (a)
64
Phase Equilibria
Solution:
g 3.94
g 5.7 CFe
g7.5100 022.07.6
022.04.0
100xCFe
CFe
3
CFe3
3
3
a
a a
a
x
CC
CCo
b) the amount of carbide
(cementite) in grams that
forms per 100 g of steel
a) composition of Fe3C and ferrite (a)
CO = 0.40 wt% C
Ca = 0.022 wt% C
CFe C = 6.70 wt% C3
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
a + Fe3C
L+Fe3C
Co, wt% C
1148°C
T(°C)
727°C
CO
R S
CFe C3Ca
65
Phase Equilibria
c. the amount of pearlite and proeutectoid ferrite (a)
note: amount of pearlite = amount of just above TE
Co = 0.40 wt% C
Ca = 0.022 wt% C
Cpearlite = C = 0.76 wt% C
a
Co Ca
C Ca
x 100 51.2 g
pearlite = 51.2 g
proeutectoid a = 48.8 g
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
a + Fe3C
L+Fe3C
Co, wt% C
1148°C
T(°C)
727°C
CO
R S
CCa
Defects and phase diagrams:
• There is a close relationship between point defect
types and phase diagrams.
• Not all solid solutions are the same!
• Important for understanding diffusion, phase
transformations, other properties…
66
Substitutional alloys:
• Simplest type: one atom
substitutes for another.
• Typically occurs when
atoms are similar sized,
have similar chemical
properties.
• Example: Cu-Ni
67
Cu-Ni
Nickel atomCopper atom
Interstitial alloys:
• Example: Fe-C
• One atom type much
smaller than the other,
sits “between” the other
atoms.
• Austenite: FCC
• Ferrite: BCC / BCT
• Quiz: which phase has
higher solubility for C?
Which phase does C
prefer to sit in? Why?
68
Interstitial alloys: FCC sites
• Two main types of sites:
– Octahedral sites
• Each interstitial has 6
neighbors
• Favored by C in FCC Fe
– Tetrahedral sites
• Red: octahedral region
• Blue: tetrahedral region
69
Interstitial alloys: BCC sites
• Two main types of sites:
– Octahedral sites
• Each interstitial has 6
neighbors
• Favored by C in BCC
Fe
– Tetrahedral sites
70
71
• Martensite:--(FCC) to Martensite (BCT)
• to M transformation..-- is rapid!
-- % transf. depends on T only.
(Adapted from Fig.
10.20, Callister, 7e.
Martensite: Fe-C System
(Adapted from Fig. 10.21, Callister, 7e.
(Fig. 10.21 courtesy United States
Steel Corporation.)
Martensite needles
Austenite
60
m
xx x
x
x
xpotential
C atom sites
Fe atom
sites
(involves single atom jumps)
Fe
3C
(ce
me
ntite
)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
+L
+ Fe3C
a+ Fe3C
L+Fe3C
Co, wt% C
T(°C)
Other types of solutions: NiAl
• Example: NiAl has CsCl
structure.
• Ni, Al strongly prefer to be
neighbors.
• Unlike CsCl, this phase
has a wide solubility.
• Al-rich: “anti-site” defects
– Al sits on Ni sites
• Ni-rich: “constitutional
vacancies”
– Missing Al atoms
72
Why do we care?
Can control properties through microstructure!
73
74
Alloying Steel with More Elements
• Teutectoid changes: • Ceutectoid changes:
Adapted from Fig. 9.34,Callister 7e. (Fig. 9.34
from Edgar C. Bain, Functions of the Alloying
Elements in Steel, American Society for Metals,
1939, p. 127.)
Adapted from Fig. 9.35,Callister 7e. (Fig. 9.35
from Edgar C. Bain, Functions of the Alloying
Elements in Steel, American Society for Metals,
1939, p. 127.)
TE
ute
cto
id(°
C)
wt. % of alloying elements
Ti
Ni
MoSi
W
Cr
Mn
wt. % of alloying elements
Ce
ute
cto
id(w
t%C
)
Ni
Ti
Cr
SiMn
WMo
75
• 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.
Summary
Important terminology
• Phase diagrams:
– Solidus
– Liquidus
– Tie line
– Lever Rule
– Solid solution
– Solubility
– Eutectic
– Intermetallic
– Congruent melting
– Other types: eutectoid, peritectic, …
• Fe-Fe3C phase diagram:
– Austenite
– Ferrite
– Pearlite
– Martensite
– Cementite
76
