phase diagrams, solid solutions, phase transformations · a phase is a homogenous, ... • in metal...
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Phase Diagrams, Solid Solutions, Phase
Strengthening, Phase Transformations
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• Components: The elements or compounds that are mixed initially (Al and Cu).
• Phases: A phase is a homogenous, physically distinct and mechanically separable
portion of the material with a given chemical composition and structure ( and ).
(darker phase)
(lighter phase)
Components and Phases
Aluminum-CopperAlloy
Solid solution A solution that contains two or more types of atoms or ions that are dispersed uniformly throughout the material
Solute The impurities that may occupy regular lattice sites in the crystal or interstitial sites
Alloys A material made from multiple elements that exhibits properties of a metallic material
Stainless steels Alloys that usually contain iron (Fe), carbon (C), chromium (Cr), nickel (Ni), and some other elements
Single-phase alloy An alloy consisting of one phase
Multiple-phase alloy An alloy that consists of two or more phases
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Figure 10.2
Chapter 10: Solid Solutions and Phase Equilibrium
Phase Equilibria: Solubility Limit• Solution – solid, liquid, or gas solutions, single phase
• Mixture – more than one phase
Question: What is the solubility limit for sugar in water at 20°C?
Answer: 65 wt% sugar. At 20°C, if C < 65 wt% sugar: syrup At 20°C, if C > 65 wt% sugar: syrup + sugar
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• Solubility Limit:
Maximum concentration for which only a single phase solution exists.
Sugar/Water Phase Diagram
Su
gar
Tem
per
atu
re (
°C)
0 20 40 60 80 100 C = Composition (wt% sugar)
L (liquid solution
i.e., syrup)
Solubility Limit L
(liquid) +
S
(solid sugar)20
40
60
80
100
Wat
er
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Equilibrium
• A system is at equilibrium if its free energy is at a minimum, given a specified combination of temperature, pressure and composition.
• The (macroscopic) characteristics of the system do not change with time — the system is stable.
• A change in T, P or C for the system will result in an increase in the free energy and possible changes to another state whereby the free energy is lowered.
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Conditions for Unlimited Solid Solubility
Hume-Rothery rules Size factor – The atoms or ions must be of similar size, with no more
than a 15% difference in atomic radius, in order to minimize the lattice strain.
Crystal structure – The materials must have the same crystal structure.
Valence – The ions must have the same valence. Electronegativity – The atoms must have approximately the same
electronegativity.
Chapter 10: Solid Solutions and Phase Equilibrium
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Figure 10.3
Chapter 10: Solid Solutions and Phase Equilibrium
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Solubility and Solid Solutions
Polymeric systems Copolymer: A polymer that is formed by combining two or more
different types of monomers, usually with the idea of blending the properties affiliated with individual polymers.
For example, acrylonitrile (A), butadiene (B), and styrene (S) monomers can be made to react to form a copolymer known as ABS.
Chapter 10: Solid Solutions and Phase Equilibrium
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Solid-Solution Strengthening
Solid-solution strengthening Increasing the strength of a metallic material
via the formation of a solid solution.
Chapter 10: Solid Solutions and Phase Equilibrium
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Figure 10.8
Chapter 10: Solid Solutions and Phase Equilibrium
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Isomorphous Phase Diagrams
Phase diagram Indicate phases as a function of Temp., Comp. and Pressure
(under equilibrium condition) Binary phase diagram
A phase diagram for a system with two components. Isomorphous phase diagram
A phase diagram in which the components display unlimited solid solubility.
Chapter 10: Solid Solutions and Phase Equilibrium
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Figure 10.9
Chapter 10: Solid Solutions and Phase Equilibrium
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• Rule 1: If we know T and Co, then we know: --how many phases and which phases are present.
• Examples:
wt% Ni20 40 60 80 1000100 0
110 0
120 0
130 0
140 0
150 0
160 0T(°C)
L (liquid)
(FCC solid
solution)L +
liquidus
solidus
A(1100,60)
B(1
250,
35)
Cu-Niphase
diagram
A(1100, 60): 1 phase:
B(1250, 35): 2 phases: L +
Determination of phase(s) present
Melting points: Cu = 1085°C, Ni = 1453 °C
Solidus - Temperature where alloy is completely solid. Above this line, liquefaction begins.Liquidus - Temperature where alloy is completely liquid. Below this line, solidification begins.
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• Rule 2: If we know T and Co, then we know: --the composition of each phase.
• Examples:
wt% Ni20
1200
1300
T(°C)
L (liquid)
(solid)L +
liquidus
solidus
30 40 50
TA A
DTD
TBB
tie line
L +
433532CoCL C
Cu-Ni system
Phase Diagrams: composition of phases
At TA = 1320°C:
Only Liquid (L) present CL = C0
( = 35 wt% Ni)
At TB = 1250°C:
Both and L present
At TD = 1190°C:
Only Solid () present C = C0 ( = 35 wt% Ni)
CL = C liquidus ( = 32 wt% Ni)
C = C solidus ( = 43 wt% Ni)
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• Rule 3: If we know T and Co, then we know: --the amount of each phase (given in wt%).
Cu-Ni system
• Examples:
At T B: Both and L
At T A: Only Liquid (L) WL = 100wt%, W = 0
At T D: Only Solid ( ) WL = 0, W = 100wt%
Co = 35wt%Ni
wt% Ni
20
1200
1300
T(°C)
L (liquid)
(solid)
L +
liquidus
solidus
30 40 50
TA A
DTD
TBB
tie line
L +
433532CoCL C
R S
Phase Diagrams: weight fractions of phases
WL SR S
W RR S
RR S
SR S
WL C CoC CL
= 27wt % RR S
W Co CLC CL
SR S
WL C CoC CL
WL SR S
W RR S
43 3543 32
73wt %
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Figure 10.15
Chapter 10: Solid Solutions and Phase Equilibrium
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Figure 10.16
Chapter 10: Solid Solutions and Phase Equilibrium
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Figure 10.17
Chapter 10: Solid Solutions and Phase Equilibrium
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Importance of Phase Diagrams
• There is a strong correlation between microstructure and mechanical properties, and the development of alloy microstructure is related to the characteristics of its phase diagram.
• Phase diagrams provide valuable information about melting, casting, crystallization and other phenomena.
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Microstructure
• In metal alloys, microstructure is characterized by the number of phases, their proportions, and the way they are arranged.
• The microstructure depends on: – Alloying elements– Concentration– Heat treatment (temperature, time, rate of
cooling)
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Eutectic• A eutectic or eutectic mixture is a mixture of two or
more phases at a composition that has the lowest melting point.
• It is where the phases simultaneously crystallize from molten solution.
• The proper ratios of phases to obtain a eutectic is identified by the eutectic point on a binary phase diagram.
• The term comes from the Greek 'eutektos', meaning 'easily melted.‘
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•The phase diagram displays a simple binary system composed of two components, A and B, which has a eutectic point.
•The phase diagram plots relative concentrations of A and B along the X-axis, and temperature along the Y-axis. The eutectic point is the point where the liquid phase borders directly on the solid α + β phase; it represents the minimum melting temperature of any possible A B alloy.
•The temperature that corresponds to this point is known as the eutectic temperature.•Not all binary system alloys have a eutectic point: those that form a solid solution at all
concentrations, such as the gold-silver system, have no eutectic. An alloy system that has a eutectic is often referred to as a eutectic system, or eutectic alloy.
•Solid products of a eutectic transformation can often be identified by their lamellar structure, as opposed to the dendritic structures commonly seen in non-eutectic solidification.
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2 componentshas a special compositionwith a min. melting T.
Binary-Eutectic Systems
• 3 single phase regions (L, , )
• Limited solubility: : mostly Cu : mostly Ag
• TE : No liquid below TE
: Composition at temperature TE
• CE
Cu-Ag system
L (liquid)
L + L+
C , wt% Ag20 40 60 80 1000
200
1200T(°C)
400
600
800
1000
CE
TE 8.0 71.9 91.2779°C
cooling
heating
• Eutectic reaction
L(CE) (CE) + (CE)
Ag) wt%1.29( Ag) wt%.08( Ag) wt%9.71( L
c10f08Pb-Sn Phase Diagram
Liquidus
SolidusSolidus
Solidus
Solvus Solvus
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Lamellar Eutectic StructureA 2-phase microstructure
resulting from the solidification of a liquid having the eutectic composition where the phases exist as a lamellae that alternate with one another.
Formation of eutectic layered microstructure in the Pb-Sn system during solidification at the eutectic composition. Compositions of α and β phases are very different. Solidification involves redistribution of Pb and Sn atoms by atomic diffusion.
Pb-rich
Sn-rich
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Pb-Sn Microstructures
The dark layers are Pb-rich α phase, the light layers are the Sn-rich β phase.
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Iron-Carbon System• Pure iron when heated experiences 2
changes in crystal structure before it melts.
• At room temperature the stable form, ferrite ( iron) has a BCC crystal structure.
• Ferrite experiences a polymorphic transformation to FCC austenite ( iron) at 912 ˚C (1674 ˚F).
• At 1394˚C (2541˚F) austenite reverts back to BCC phase ferrite and melts at 1538 ˚C (2800 ˚F).
• Iron carbide (cementite or Fe3C) an intermediate compound is formed at 6.7 wt% C.
• Typically, all steels and cast irons have carbon contents less than 6.7 wt% C.
• Carbon is an interstitial impurity in iron and forms a solid solution with the phases
c10f28Iron-Carbon System
Though carbon is present in relatively low concentrations, it significantly influences the mechanical properties of ferrite: (a) α ferrite, (b) austenite.
c10f29ab
4 Solid Phases
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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
+
(Fe) C, wt% C
1148°C
T(°C)
727°C = Teutectoid
4.30
Result: Pearlite = alternating layers of and Fe3C phases,
not a separate phase.
120 m0.76
B
AL+Fe3C
Fe3C (cementite-hard) (ferrite-soft)
↔
c10f30Pearlite Eutectoid reaction:
↔ + Fe3C
Austenite – 0.76 wt% CFerrite - 0.022 wt% CCementite - 6.70 wt% C
Redistribution of carbon by diffusion
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