Download - Phase Diagrams Binary Eutectoid Systems Iron-Iron-Carbide Phase Diagram Steels and Cast Iron
Phase DiagramsBinary Eutectoid Systems
Iron-Iron-Carbide Phase DiagramSteels and Cast Iron
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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 sugar–water syrup solution is one phase, and solid sugar is another
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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
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• 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?
ISSUES TO ADDRESS...
Phase BPhase A
Nickel atomCopper atom
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
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Solubility Limit Sugar-Water
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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
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• 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., and ).
Aluminum-CopperAlloy
Components and Phases
(darker phase)
(lighter phase)
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Effect of T & Composition (Co)• Changing T can change # of phases:
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-sugarsystem
PHASE EQUILIBRIA
• Free Energy -> 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
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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
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Pressure-Temperature Diagram (Water)• Each of the phases will exist under equilibrium conditions
over the temperature–pressure 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 P–T phase diagram where three phases are in equilibrium, is called a triple point
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Binary Phase Diagrams
• A phase diagram in which temperature and composition are variable parameters, and pressure is held constant—normally 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
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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.
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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).
• PhaseDiagramfor Cu-Nisystem
• 2 phases: L (liquid)
(FCC solid solution)
• 3 phase fields: LL +
wt% Ni20 40 60 80 10001000
1100
1200
1300
1400
1500
1600T(°C)
L (liquid)
(FCC solid solution)
L + liquidus
solid
us
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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:# and types of phases
• Rule 1: If we know T and Co, then we know: --the number and types of 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)
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wt% Ni20
1200
1300
T(°C)
L (liquid)
(solid)L +
liquidus
solidus
30 40 50
L +
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 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
BTB
DTD
tie line
4C3
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• 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
Phase Diagrams:weight fractions of phases
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 +S
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• 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 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
20
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.
• 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
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• 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
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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
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 10020
30
40
50
60
%EL for pure Ni
%EL for pure Cu
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
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: Min. melting TE
2 componentshas a special compositionwith a min. melting T.
Binary-Eutectic 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% Ag20 40 60 80 1000
200
1200T(°C)
400
600
800
1000
CE
TE 8.0 71.9 91.2779°C
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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:
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
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L+
+
200
T(°C)
C, wt% Sn20 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%
40Co
46CL
17C
220SR
C = 17 wt% SnCL = 46 wt% Sn
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• 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 T solubility limit)
TE
(Pb-SnSystem)
L
L: Co wt% Sn
: Co wt% Sn
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• 2 wt% Sn < Co < 18.3 wt% Sn• Result:
Initially liquid + then alonefinally two 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
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• 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
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Lamellar Eutectic Structure
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• 18.3 wt% Sn < Co < 61.9 wt% Sn• Result: crystals and a eutectic microstructure
Microstructures in Eutectic Systems (Pb-Sn): 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 = = 73 wt%
W = 27 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
32
L+L+
+
200
Co, wt% Sn20 60 80 1000
300
100
L
TE
40
(Pb-Sn System)
Hypoeutectic & Hypereutectic
160 m
eutectic micro-constituent
hypereutectic: (illustration only)
175 m
hypoeutectic: Co = 50 wt% Sn
T(°C)
61.9eutectic
eutectic: Co = 61.9 wt% Sn