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Transcript of Plastic Deformation of Polycrystalline Solidsmse235.cankaya.edu.tr/uploads/files/Lecture10-Phase...
‘’PHASE DIAGRAMS’’
IE-114 Materials Science and General Chemistry
Lecture-10
There is a strong correlation between microstructure and
mechanical properties.
Phase diagrams provides information about:
- The development of microstructure during heating or
cooling can be understood from the phase diagrams.
- Melting, casting, crystallization, etc.
Importance of Phase Diagrams
Solubility Limit
At some specific temperatures ,
there is a maximum concentration
of solute atoms that may dissolve in
the solvent to form a solid solution;
this is called solubility limit.
Phase-Diagram of Water-Sugar System
Question: What is the solubility limit at 20oC?
Solubility limit increases with T:
T = 20oC, solubility limit = 65 wt% sugar.
T = 100oC, solubility limit = 80 wt% sugar.
Answer: 65wt% sugar.
If Co < 65wt% syrup
If Co > 65wt% syrup + sugar.
Phase: Homogenous portion of a system that has uniform
physical and chemical characteristics
Aluminum-Copper Alloy
Component: Components are pure metals/or compounds of which
alloy is composed. e.g. Brass (Cu-Zn) ; components are Cu and Zn, Fe
and C in
carbon steel, H2O and NaCl in salted water
Solid Solution: A solid solution consists of solute atoms, which
occupy either substitutional or interstitial positions in the solvent
lattice (, , etc,..)
Every pure material is considered to be a
phase; so also is every solid, liquid and
gaseous solution
For example if a substance can exist in
two or more polymorphic forms (BBC and
FCC) each of these structures is a separate
phase because their physical properties
are different.
Homogeneous system:
A single phase system
Mixture or heterogeneous system:
System of two or more phases.
Most of the metallic alloys,ceramics, polymeric and composite
systems are heterogeneous.
Phase Equilibria:
A system is said to be at equilibrium when the free energy, which is
the internal energy and randomness of the atoms, is at minimum
under some specified combination of temperature, pressure and
composition.
Equilibrium Phase Diagrams
Phase diagram is also called equilibrium or constitutional
diagram.
These diagrams define the relationship between the temperature
and compositions or quantities of phases at equilibrium. External
pressure could also be another parameter affecting the phase
distribution but it remains constant at 1 atm in most of the
applications.
- Isomorphous Binary Phase Diagrams
- Eutectic Binary Phase Diagrams
1) Isomorphous Binary Systems:
Binary systems are composed of two components and they are isomorphous since there is a complete solubility of liquids and solids.
Example: Cu-Ni
There are 2 phases :
- L (liquid)
- α-solid solution
There are 3 phase fields :
liquid, L+α, α
Liquidus line:The line separating L and α+L phases.
Solidus line: The line separating
α+L and α phases is called.
Cu Ni wt.% Ni
If we know T and Co, then we know the number and types of
phases present.
Examples:
1) Number and types of phases
1) Cu-35Ni Alloy at 1250oC
(Co = 35 wt.%Ni)
2 phases: α+L
2)Cu-60Ni Alloy at 1100oC
(Co = 60 wt.%Ni)
1 phase: α
From the phase diagrams we can learn the followings:
-Number and types of phases that are present at different temperatures for a fixed composition
-Composition of the phases
-Fractions of the phases
If we know T and Co, then we know the composition of each phase.
2) Composition of phases:
For Cu-35Ni Alloy (Co=35wt%Ni)
at T
A:
-Phases: 1(only Liquid)
-Composition of the alloy:
CL = Co (=35 wt%Ni, 65 wt%Cu)
at T
B:
-Phases: 2 (Liquid +α)
-Composition of the alloy:
CL = Cliquidus (32wt%Ni, 68 wt% Cu)
Cα = Csolidus (43wt%Ni, 57wt%Cu)
at T
D:
-Phases: 1 (only α)
-Composition of the alloy:
Cα=Co (35 wt%Ni, 65 wt%Cu)
3) Weight fractions (or percentage) of phases:
If we know T and Co, then we know the amount of each phase
(given in wt%).
WL S
R S
W R
R S
%731003243
3543wtx
= 27wt%
x100
x100
• Binary System (2 components)
•Isomorphous i.e., complete solubility of one
component in another;
phase field extends from
0 to 100wt% Ni.
Microstructural development during
cooling a Cu-Ni alloy
Binary Eutectic Systems
There are three phases : Liquid, and
Cu Ag
max. solubility of Cu
in Ag (8.8 wt%)
Eutectic line. This line shows the minimum
temperature for the liquid
phase existence.
Max.
Solubility
of Ag in Cu
8 wt% Ag at
7790C
SOLVUS
SOLIDUS LIQUIDUS
Melting point
of pure Cu Melting point
of pure Ag
Eutectic composition: 71.9wt%Ag, 28,1wt%Cu
Eutectic temperature: 779oC
Eutectic reaction: Liquid (71.9%) (8%Ag) + (91.2% Ag) heating
cooling
For a 40wt%Sn-60wt%Pb alloy at 150oC, find:
1) the phases present
2) the compositions of the phases
For a 40wt%Sn-60wt%Sn alloy at 150oC, find:
--the phases present: +
--the compositions of the phases:
C = 11wt%Sn, 89wt%Cu
C = 99wt%Sn, 1wt%Cu
--the relative amounts of each phase:
x100
x100
Microstructural Development During
Cooling of Pb-Sn Alloys
• 18.3wt%Sn < Co < 61.9wt%Sn
• Room temperature microstructure: crystals and a eutectic
microstructure
Cooling of an Alloy Having Eutectic Composition
alternating layers of and crystals.
The alloy having eutectic composition is called eutectic alloy
Hypoeutectic and Hypereutectic Alloy
For some alloy systems, discrete intermediate compounds rather
than solid solutions may be observed in phase diagrams. For
example; Mg-Pb system. These are called intermetallic compounds.
The compound Mg2Pb is shown as a vertical line on the diagram
rather than a phase region since it exists precisely at the composition
defined.
Intermetallic Compounds
Eutectoid and Peritectic Reactions
Eutectoid reaction: 5600C and 74 wt% Zn-26 wt% Cu
Peritectic reaction: 5980C and 78.6 wt% Zn-21.4 wt% Cu
γ + cooling
heating
+ L cooling
heating
Consider Cu-Zn system.
Phase transformations can be classified according to whether or not
there is any change in composition.
Congruent and Incongruent Phase
Transformation
Phase transformations in which there is
no changes in composition are called as congruent transformations. The
opposite is incongruent
transformation. Allotropic
transformations are congruent as well as
melting pure metals. Eutectic, eutectoid
or melting alloy systems are incongruent
transformations.
This is the most important system in manufacturing since primary
structural materials are essentially Fe-C alloys, such as, steel and
cast iron.
Iron Carbon System
STEEL IS an ALLOY OF;
Iron-Iron Carbide Equilibrium Phase Diagram
Phases and phase mixtures present in iron alloys;
• Ferrite (α)
• Cementite (Fe3C)
• Pearlite (ferrite + cementite)
• Austenite (γ)
• -ferrite
• Ledeburite (austenite + cementite)
Definition and Properties of Phases 1) Ferrite : -iron, Solid Solution, max. Carbon solubility 0.022%wt. at 727oC
BCC structure, SOFT
2) Cementite : Iron carbide(Fe3C), contains 6.67% wt. C
Orthorhombic structure, HARD and BRITTLE
3) Pearlite : Phase mixture (ferrite+cementite), Lamellar structure, contains ~0.8% wt. C
Produced from austenite decomposition
4) Austenite : -iron, Solid solution, stable at higher temperatures (>727oC)
Max. Carbon solubility is 2.14%wt. at 1147oC, FCC structure
HIGH TOUGHNESS
5) Ledeburite: Eutectic phase mixture(austenite+Fe3C), seen in cast irons
Contains 4.3 %wt. Carbon, forms at 1147oC
6) -ferrite : Solid solution, max. carbon solubility is 0.1%wt. At 1493oC
Invariant reactions in Fe-Fe3C Phase diagram
1) At 1493oC, 0.18 %wt C (PERITECTIC REACTION)
Liquid(l, 0.5%C)+-ferrite(,0.1%C) Austenite(, 0.18%C) cooling
heating
2) At 1147oC, 4.30 %wt C (EUTECTIC REACTION)
Liquid(l, 4.30 %C) Austenite(, 2.14 %C) + Cementite(Fe3C,6.67%C) cooling
heating
3) At 727oC, 0.77 %wt C (EUTECTOID REACTION)
Austenite(, 0.77 %C) Ferrite(, 0.022 %C) + Cementite(Fe3C,6.67%C) cooling
heating
PEARLITE
LEDEBURITE
CAST IRONS STEELS
Hypoeutectoid
Steel
Eutectoid
Steel
Hypereutectoid
Steel
Eutectoid Steel
Hypoeutectoid Steel
Hypereutectoid Steel
Effect of Carbon on Mechanical Properties
EXAMPLE1: For an annealed (cooled in equilibrium conditions after
austenitization) hypoeutectoid steel;
1) Determine the composition of the steel (C wt.%) if quantitative
metallographic analyses revealed 22% secondary ferrite (assume
solubility of carbon in ferrite is nil)
2) Calculate the weight fraction ratio of secondary ferrite and cementite
present in pearlite.
3) Determine the total amount of carbon dissolved in secondary ferrite at
room temperature for the corresponding steel if the total weight of the
steel part in kgs is equal to 80.(Room temperature solubility of carbon
in ferrite is 0.008%)
EXAMPLE2: The mass fraction of eutectoid cementite in iron carbon alloy
is 0.109. On the basis of this information, is it possible to determine the
composition of the alloy? If so, what is its composition?