phase equilibrium engr 2110 – chapter 9 dr. r. r. lindeke
TRANSCRIPT
Phase Equilibrium
Engr 2110 – Chapter 9
Dr. R. R. Lindeke
Topics for Study
Definitions of Terms in Phase studies Binary Systems
Complete solubility (fully miscible) systems Multiphase systems
Eutectics Eutectoids and Peritectics Intermetallic Compounds The Fe-C System
Some Definitions
Component of a system: Pure metals and or compounds of which an alloy is composed, e.g. Cu and Ag or Fe and Fe3C. They are the solute(s) and solvent
System: A body of engineering material under investigation. e.g. Ag – Cu system, NiO-MgO system (or even sugar-milk system)
Solubility Limit: The maximum concentration of solute atoms that may dissolve in the Solvent to form a “solid solution” at some temperature.
Phases: A homogenous portion of a system that has uniform physical and chemical characteristics, e.g. pure material, solid solution, liquid solution, and gaseous solution, ice and water, syrup and sugar.
Microstructure: A system’s microstructure is characterized by the number of phases present, their proportions, and the manner in which they are distributed or arranged. Factors affecting microstructure are: alloying elements present, their concentrations, and the heat treatment of the alloy.
Single phase system = Homogeneous system Multi phase system = Heterogeneous system or mixtures
Definitions, cont
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 H2O (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
Phase Equilibrium: A stable configuration with lowest free-energy (internal energy of a system, and also randomness or disorder of the atoms or molecules (entropy).Any change in Temperature, Composition, and Pressure causes an increase in free energy and away from Equilibrium thus forcing a move to another ‘state’
Equilibrium Phase Diagram: It is a “map” of the information about the control of microstructure or phase structure of a particular material system. The relationships between temperature and the compositions and the quantities of phases present at equilibrium are represented.
Definition that focus on “Binary Systems”
Binary Isomorphous Systems: An alloy system that contains two components that attain complete liquid and solid solubility of the components, e.g. Cu and Ni alloy. It is the simplest binary system.
Binary Eutectic Systems: An alloy system that contains two components that has a special composition with a minimum melting temperature.
Definitions, cont
With these definitions in mind:ISSUES TO ADDRESS...
• When we combine two elements... what “equilibrium state” would we expect to get?• In particular, if we specify... --a composition (e.g., wt% Cu - wt% Ni), and --a temperature (T ) and/or a Pressure (P)
then... How many phases do we get? What is the composition of each phase? How much of each phase do we get?
Phase BPhase A
Nickel atomCopper atom
Gibb’s Phase Rule: a tool to define the number of phases and/or degrees of phase changes that can be found in a system at equilibrium
For any system under study the rule determines if the system is at equilibrium
For a given system, we can use it to predict how many phases can be expected
Using this rule, for a given phase field, we can predict how many independent parameters (degrees of freedom) we can specify
Typically, N = 1 in most condensed systems – pressure is fixed!
where:
F is # degrees of freedom of the system (independent parameters)
C is # components (elements) in system
P is # phases at equil.
N is # "noncompostional" parameters in system (temp &/or Press
F C P N
ure)
Looking at a simple “Phase Diagram” for Sugar – Water (or milk)
Effect of Temperature (T) & Composition (Co)
• Changing T can change # of phases:
D (100°C,90)2 phases
B (100°C,70)1 phase
See path A to B.• Changing Co can change # of phases: See path B to D.
Adapted from Fig. 9.1, Callister 7e.
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-sugarsystem
Phase Diagram: A Map based on a System’s Free Energy indicating “equilibuim” system structures – as predicted by Gibbs Rule
• Indicate ‘stable’ phases as function of T, P & Composition, • We will focus on: -binary systems: just 2 components. -independent variables: T and Co (P = 1 atm is almost always used).
Phase Diagram --for Cu-Ni system
• 2 phases are possible: L (liquid)
(FCC solid solution)
• 3 ‘phase fields’ are observed: LL +
wt% Ni20 40 60 80 10001000
1100
1200
1300
1400
1500
1600T(°C)
L (liquid)
(FCC solid solution)
L + liquidus
solid
us
An “Isomorphic” Phase System
wt% Ni20 40 60 80 10001000
1100
1200
1300
1400
1500
1600T(°C)
L (liquid)
(FCC solid solution)
L +
liquidus
solid
us
Cu-Niphase
diagram
Phase Diagrams:
• Rule 1: If we know T and Co then we know the # and types of all phases present.
• Examples:A(1100°C, 60): 1 phase:
B(1250°C, 35): 2 phases: L +
B (
1250
°C,3
5) A(1100°C,60)
wt% Ni20
1200
1300
T(°C)
L (liquid)
(solid)L +
liquidus
solidus
30 40 50
L +
Cu-Ni system
Phase Diagrams:
• Rule 2: If we know T and Co 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 and L CL = C liquidus ( = 32 wt% Ni here)
C = C solidus ( = 43 wt% Ni here)
At TD = 1190°C:
Only Solid ( ) C = Co ( = 35 wt% Ni)
Co = 35 wt% Ni
adapted from Phase Diagrams of Binary Nickel Alloys, P. Nash (Ed.), ASM
International, Materials Park, OH, 1991.
BTB
DTD
tie line
4C3
Figure 9.31 The lever rule is a mechanical analogy to the mass-balance calculation. The (a) tie line in the two-phase region is analogous to (b) a lever balanced on a fulcrum.
Tie line – a line connecting the phases in equilibrium with each other – at a fixed temperature (a so-called Isotherm)
The Lever Rule
How much of each phase?We can Think of it as a lever! So to balance:
ML M
R S
RMSM L
L
L
LL
LL CC
CC
SR
RW
CC
CC
SR
S
MM
MW
00
wt% Ni
20
1200
1300
T(°C)
L (liquid)
(solid)L +
liquidus
solidus
30 40 50
L + B
TB
tie line
CoCL C
SR
• 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)
W L = 100 wt%, W = 0At TD: Only Solid ( )
W L = 0, W = 100 wt%
Co = 35 wt% Ni
Therefore we define
wt% Ni20
1200
1300
T(°C)
L (liquid)
(solid)L +
liquidus
solidus
30 40 50
L +
Cu-Ni system
TAA
35Co
32CL
BTB
DTD
tie line
4C3
R S
At TB: Both and L
% 733243
3543wt
= 27 wt%
WL S
R +S
W R
R +SNotice: as in a lever “the opposite leg” controls with a balance (fulcrum) at the ‘base composition’ and R+S = tie line length =
difference in composition limiting phase boundary, at the temp of interest
wt% Ni20
1200
1300
30 40 501100
L (liquid)
(solid)
L +
L +
T(°C)
A
35Co
L: 35wt%Ni
Cu-Nisystem
• 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; phase field extends from 0 to 100 wt% Ni.
Adapted from Fig. 9.4, Callister 7e.
• Consider Co = 35 wt%Ni.
Ex: Cooling in a Cu-Ni Binary
4635
4332
: 43 wt% Ni
L: 32 wt% Ni
L: 24 wt% Ni
: 36 wt% Ni
B: 46 wt% NiL: 35 wt% Ni
C
D
E
24 36
• C changes as we solidify.• Cu-Ni case:
• Fast rate of cooling: Cored structure
• Slow rate of cooling: Equilibrium structure
First to solidify has C = 46 wt% Ni.
Last to solidify has C = 35 wt% Ni.
Cored vs Equilibrium Phases
First to solidify: 46 wt% Ni
Uniform C:
35 wt% Ni
Last to solidify: < 35 wt% Ni
Cored (Non-equilibrium) Cooling
Notice:The Solidus line is “tilted” in this non-equilibrium cooled environment
: Min. melting TE
2 componentshas a ‘special’ composition with a min.
melting Temp.
Binary-Eutectic (PolyMorphic) Systems
• Eutectic transition
L(CE) (CE) + (CE)
• 3 single phase regions (L, ) • Limited solubility: : mostly Cu : mostly Ag • TE : No liquid below TE
• CE
composition
Ex.: Cu-Ag system Cu-Agsystem
L (liquid)
L + L+
Co , wt% Ag
20 40 60 80 1000200
1200T(°C)
400
600
800
1000
CE
TE 8.0 71.9 91.2779°C
L+L+
+
200
T(°C)
18.3
C, wt% Sn20 60 80 1000
300
100
L (liquid)
183°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)
+ --compositions of phases:
CO = 40 wt% Sn
--the relative amount of each phase (by lever rule):
150
40Co
11C
99C
SR
C = 11 wt% SnC = 99 wt% Sn
W=C - CO
C - C
=99 - 4099 - 11
= 5988
= 67 wt%
SR+S
=
W =CO - C
C - C=R
R+S
=2988
= 33 wt%=40 - 1199 - 11
Adapted from Fig. 9.8, Callister 7e.
L+
+
200
T(°C)
C, wt% Sn
20 60 80 1000
300
100
L (liquid)
L+
183°C
• For a 40 wt% Sn-60 wt% Pb alloy at 220°C, find... --the phases present: Pb-Sn
system
EX: Pb-Sn Eutectic System (2)
+ L--compositions of phases:
CO = 40 wt% Sn
--the relative amount of each phase:
W =CL - CO
CL - C=
46 - 40
46 - 17
= 6
29= 21 wt%
WL =CO - C
CL - C=
23
29= 79 wt%
40
Co
46
CL
17
C
220SR
C = 17 wt% SnCL = 46 wt% Sn
• Co < 2 wt% Sn• Result: --at extreme ends --polycrystal of grains i.e., only one solid phase.
Microstructures In Eutectic Systems: I
0
L+ 200
T(°C)
Co, wt% Sn10
2%
20Co
300
100
L
30
+
400
(room Temp. solubility limit)
TE
(Pb-SnSystem)
L
L: Co wt% Sn
: Co wt% Sn
• 2 wt% Sn < Co < 18.3 wt% Sn• Result:
Initially liquid Then liquid + then alonefinally two solid phases
polycrystal fine -phase inclusions
Microstructures in Eutectic Systems: II
Pb-Snsystem
L +
200
T(°C)
Co , wt% Sn10
18.3
200Co
300
100
L
30
+
400
(sol. limit at TE)
TE
2(sol. limit at Troom)
L
L: Co wt% Sn
: Co wt% Sn
• Co = CE • Result: Eutectic microstructure (lamellar structure) --alternating layers (lamellae) of and crystals.
Microstructures in Eutectic Systems: III
160 m
Micrograph of Pb-Sn eutectic microstructure
Pb-Snsystem
L
200
T(°C)
C, wt% Sn
20 60 80 1000
300
100
L
L+ 183°C
40
TE
18.3
: 18.3 wt%Sn
97.8
: 97.8 wt% Sn
CE61.9
L: Co wt% Sn
45.1% and 54.8% -- by Lever Rule
Lamellar Eutectic Structure
• 18.3 wt% Sn < Co < 61.9 wt% Sn• Result: crystals and a eutectic microstructure
Microstructures in Eutectic Systems: IV
18.3 61.9
SR
97.8
SR
primary eutectic
eutectic
WL = (1-W) = 50 wt%
C = 18.3 wt% Sn
CL = 61.9 wt% SnS
R + SW = = 50 wt%
• Just above TE :
• Just below TE :C = 18.3 wt% Sn
C = 97.8 wt% SnS
R + SW = = 72.6 wt%
W = 27.4 wt%
Pb-Snsystem
L+200
T(°C)
Co, wt% Sn
20 60 80 1000
300
100
L
L+
40
+
TE
L: Co wt% Sn LL
L+L+
+
200
Co, wt% Sn20 60 80 1000
300
100
L
TE
40
(Pb-Sn System)
Hypoeutectic & Hypereutectic Compositions
160 m
eutectic micro-constituent
hypereutectic: (illustration only)
from Metals Handbook, 9th ed., Vol. 9, Metallography and Microstructures, American Society for Metals, Materials Park, OH, 1985.
175 m
hypoeutectic: Co = 50 wt% Sn
T(°C)
61.9eutectic
eutectic: Co = 61.9 wt% Sn
“Intermetallic” Compounds
Mg2Pb
Note: an intermetallic compound forms a line - not an area - because stoichiometry (i.e. composition) is exact.
Adapted from Fig. 9.20, Callister 7e.
An Intermetallic Compound is also an important part of the Fe-C system!
Eutectoid: solid phase in equilibrium with two solid phases
S2 S1+S3
+ Fe3C (727ºC)
intermetallic compound - cementite
cool
heat
Peritectic: liquid + solid 1 in equilibrium with a single solid 2 (Fig 9.21)
S1 + L S2
+ L (1493ºC)
cool
heat
Eutectic: a liquid in equilibrium with two solids
L +
Eutectoid & Peritectic – some definitions
coolheat
Eutectoid & Peritectic
Cu-Zn Phase diagram
Adapted from Fig. 9.21, Callister 7e.
Eutectoid transition +
Peritectic transition + L
Iron-Carbon (Fe-C) Phase Diagram• 2 important points
-Eutectoid (B):
+Fe3C
-Eutectic (A): L + Fe3C
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
+Fe3C
+
L+Fe3C
(Fe) Co, wt% C
1148°C
T(°C)
727°C = Teutectoid
A
SR
4.30
Result: Pearlite = alternating layers of and Fe3C phases
120 m
(Adapted from Fig. 9.27, Callister 7e.)
R S
0.76
Ceu
tect
oid
B
Fe3C (cementite-hard) (ferrite-soft)
Max. C solubility in iron = 2.11 wt%
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 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
+ Fe3C
L+Fe3C
(Fe) Co , wt% C
1148°C
T(°C)
727°C
(Fe-C System)
C0
0.76
proeutectoid ferritepearlite
100 m Hypoeutectoidsteel
R S
w =S/(R+S)wFe3C =(1-w)
wpearlite = wpearlite
r s
w =s/(r+s)w =(1- w)
Hypereutectoid Steel
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
1148°C
T(°C)
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
proeutectoid Fe3C
60 mHypereutectoid steel
pearlite
R S
w =S/(R+S)wFe3C =(1-w)
wpearlite = wpearlite
sr
wFe3C =r/(r+s)w =(1-w Fe3C )
Fe3C
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 ()
b) the amount of carbide (cementite) in grams that forms per 100 g of steel
c) the amount of pearlite and proeutectoid ferrite ()
Solution:
g 3.94
g 5.7 CFe
g7.5100 022.07.6
022.04.0
100xCFe
CFe
3
CFe3
3
3
x
CC
CCo
b) the amount of carbide (cementite) in grams that forms per 100 g of steel
a) composition of Fe3C and ferrite ()
CO = 0.40 wt% C
C = 0.022 wt% C
CFe C = 6.70 wt% C3
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
+ Fe3C
L+Fe3C
Co , wt% C
1148°C
T(°C)
727°C
CO
R S
CFe C3C
Solution, cont:c) the amount of pearlite and proeutectoid ferrite ()
note: amount of pearlite = amount of just above TE
Co = 0.40 wt% C
C = 0.022 wt% C
Cpearlite = C = 0.76 wt% C
Co CC C
x 100 51.2 g
pearlite = 51.2 gproeutectoid = 48.8 g
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
+ Fe3C
L+Fe3C
Co , wt% C
1148°C
T(°C)
727°C
CO
R S
CCLooking at the Pearlite:11.1% Fe3C (.111*51.2 gm = 5.66 gm) & 88.9% (.889*51.2gm = 45.5 gm)
total = 45.5 + 48.8 = 94.3 gm
Alloying Steel with More Elements
• Teutectoid changes: • Ceutectoid changes:
Adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 127.)
Adapted 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
Mo SiW
Cr
Mn
wt. % of alloying elements
Ce
ute
cto
id (
wt%
C)
Ni
Ti
Cr
SiMn
WMo
Looking at a Tertiary Diagram
When looking at a Tertiary (3 element) P. diagram, like this image, the Mat. Engineer represents equilibrium phases by taking “slices at different 3rd element content” from the 3 dimensional Fe-C-Cr phase equilibrium diagram
What this graph shows are the increasing temperature of the euctectoid and the decreasing carbon content, and
(indirectly) the shrinking phase field
From: G. Krauss, Principles of Heat Treatment of Steels, ASM International, 1990
Fe-C-Si Ternary phase diagram at about 2.5 wt% SiThis is the “controlling Phase Diagram for the ‘Graphite‘ Cast Irons
• 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 (cause) a range of microstructures.
Summary