10/8/2009
Prof. A.K.M.B. Rashid
Department of MME
BUET, Dhaka
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Concept of alloying
Classification of alloys
Introduction to the phase diagram
Uses and limitations of phase diagrams
Classification of phase diagrams
Construction of phase diagrams
Reference:
1. Avner. Introduction to Physical Metallurgy, Ch. 5 and 6.
© Rashid, DMME, BUET . 2009
MME 291, Lec 03: Introduction to phase diagrams
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‰ An alloy is a substance that has metallic properties and is
composed of two or more chemical elements, of which at least
one is a metal.
‰ Alloying elements are deliberately
introduced into a metal to enhance
properties (especially, mechanical
properties)
60% Ni
Monel
Note the difference
z Alloy and alloy system
z Alloying element and impurity element
Alloy systems
z Binary system (Fe-C system, Cu-Zn system)
z Ternary system (Fe-C-Mn system, Al-Si-Mg system)
© Rashid, DMME, BUET . 2009
Cu
Wt % Nickel
Property
Ni
Direction
Tensile strength
Yield Strength
% Elongation
Electrical Conductivity
MME 291, Lec 03: Introduction to phase diagrams
Up
Up
Down
Down
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‰ Classifications based on:
X Structure (e.g., austenitic stainless steel)
yp of p
phase diagram
g
((e.g.,
g , eutectic alloys)
y )
Y Type
‰ Alloy can be homogeneous or mixture.
‰ In the solid state, there are three possible phases exist:
X Pure metal
y phase
p
or compound
p
Y Intermediate alloy
Z Solid solution
‰ If the alloy is a mixture, then any combination of
the above three phases is possible in the solid state.
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Intermediate Alloy Phases, or Compounds
‰ Intermediate chemical composition; different crystal structure
‰ Expressed by chemical formula
‰ Congruently melting, exhibiting definite melting point (like metals)
X Intermetallic compounds (Mg2Pb, Mg2Sn, ....)
Ñ
Ñ
Ñ
Ñ
formed between chemically dissimilar metals
have strong ionic/covalent bond
nonmetallic properties (poor ductility, poor conductivity)
complex strucutre
Y Interstitial compounds (Fe3C, Fe4N, TiC, .....)
Ñ formed between transition metals and H/O/C/N/B
Ñ small atoms of nonmetals go into the interstitices of metals
Ñ metallic properties, narrow composition range, extremely hard, high melting point
Z Electron compounds (AgZn, FeAl, Cu3Si, ....)
Ñ form structure similar to constituting elements
Ñ have a definite ratio of valence electrons
Ñ properties similar to solid solutions (wide composition range, high ductility, low hardness
Solid Solutions
‰ Solution in the solid state where two kinds of atoms
are combined
bi d in
i one crystall lattice.
l i
Distortion resulted during formation of solution causes
an increase in strength of the alloy. This is the primary
basis for strengthening of a metal by alloying.
‰ Solubility of solute in solution depends on temperature
and pressure of the system, and method of forming.
‰ Unsaturated, saturated, and supersaturated solutions
© Rashid, DMME, BUET . 2009
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Solid Solutions
‰ Substitutional and interstitial solid solutions
Interstitial
Substitutional
Hume Rothery Rule for forming
Hume-Rothery
substitutional solid solution
X
Y
Z
[
Crystal structure factor
Relative size factor
Chemical affinity factor
Relative valence factor
© Rashid, DMME, BUET . 2009
Example:
Cu – Ni alloy ⇒ Completely soluble
Cu – Zn alloy ⇒ Partially soluble
Cu – Pb alloy ⇒ Completely insoluble
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Summary of Possible Alloy Structures
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z We have seen how the structure of materials control
their properties.
z So it is important that we must know
X about the structure of a material that has been
developed during its manufacture, and
Y the method of controlling (and/or modifying) the
structure to enhance its properties.
z Phase diagram is an important tool for materials
scientists that tells which phases are stable in a system
under specified conditions (e.g. of temperature, overall
composition, pressure)
© Rashid, DMME, BUET . 2009
MME 291, Lec 03: Introduction to phase diagrams
z Phase diagram is basically a map that
presents the domains of stability of
phases and the limits of stability of
phases in a graphical form.
z Reading the map will tell you, at the
state when it comes to equilibrium,
P 09
liquid
solid
gas
liquid-gas
equilibria
Temperature
Typical phase diagram for
one component system
1. what phases are present,
2. the state of those phases, and
3 the relative quantities of each phase
3.
phase.
z Reading a phase diagram will also tell what phase
transformations we can expect when we change one of the
parameters of the system (T, P, X).
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Components
‰ Chemically recognisable species that are mixed to
f
form
the
h alloy.
ll
z In Brass: Cu, Zn (element)
z In steels: Fe, C (element)
z In ceramics: SiO2, Al2O3 (compound)
‰ Binary alloy contains 2 components, ternary 3, etc.
Phase
‰ A phase is a homogenous, physically distinct and
mechanically separable portion of the material with
a given chemical composition and structure.
© Rashid, DMME, BUET . 2009
MME 291, Lec 03: Introduction to phase diagrams
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What and how many phases materials
possess?
z Solid, liquid, or gas, (and plasma)?
z Is it possible to have more than one solid phases?
Iron, being an allotropic material, has more than one solid
phases:
‰ When iron first freezes from its liquid state, it is BCC (δ-iron)
‰ As it cools it changes to FCC (γ-iron)
‰ Upon further cooling it changes to BCC (α-iron)
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Microstructure
Microstructure of Al-Cu Alloy
z The properties of an alloy
depend not only on proportions
of the phases but also on how
they are arranged structurally
at the microscopic level.
z Thus, the microstructure is
specified by the number of
phases, their proportions, and
their arrangement in space.
space
α phase
(darker)
Microstructure of Cast Iron
z Phase diagrams will help us to
understand and predict the
microstructures like the one
shown in this page
© Rashid, DMME, BUET . 2009
β phase
((lighter)
g )
MME 291, Lec 03: Introduction to phase diagrams
pearlite
(finger
print)
graphite
(grey)
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Equilibrium state and Metastable state
‰ A system is at equilibrium if at constant
T pressure andd composition
T,
iti the
th system
t is
i
stable, not changing with time.
‰ The equilibrium state always has the
minimum free energy.
z Equilibrium state requires sufficient time to
achieve. When this time is too longg ((due to
slow kinetics), another state along the path
to the equilibrium may appear to be stable.
This is called a metastable state.
z A system at a metastable state is trapped in a local minimum
of free energy, which is not the global one.
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z Selection of alloys showing enhanced characteristics in:
5
5
5
5
Brazing
B
i and
d soldering
ld i
Diffusion problems
Corrosion
Electrical resistivity
z Manipulation of phase transformations of materials to
control their properties
‰ Phase diagrams are also known as the equilibrium diagrams.
‰ Rate of phase transformations is missing.
‰ TTT (Temperature-Time-Transformation) diagrams are a
complement to phase diagrams.
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One component (unary) phase diagrams
‰ Also known as
P T diagrams.
P-T
di
‰ The simple case is Water.
z
z
z
How many single-phase
regions?
How many two-phase
regions?
Is there any three-, or
more-phase regions?
Gibb’s Phase Rule:
F = C–P+2
F = # variables
C = # components
P = # phases
© Rashid, DMME, BUET . 2009
Unary phase diagram of water
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Binary (two-component) phase diagrams
‰ How does mixing of A into B effect the bond energies
andd the
th melting
lti temperature
t
t
off the
th resultant
lt t alloy?
ll ?
‰ Interaction of A and B resulted three bonds: A-A, B-B
and A-B bonds.
‰ In ideal cases:
(A-B) = x (A-A) + (1-x) (B-B)
TAlloy = TA + x (TB – TA)
where x is mole fraction of A in B
Must follow
Hume Rothery Rule
‰ Example: Copper - Nickel, Silicon - Germanium
Î Completely miscible/soluble phase diagrams
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Binary (two-component) phase diagrams
Ni-Cu phase diagram (completely miscible)
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Binary (two-component) phase diagrams
Ge-Si phase diagram (completely miscible)
Binary (two-component) phase diagrams
‰ When
A-B < 0.5 (A-A + B-B)
then
TAlloy < TA , TB
‰ Example: Lead - Tin,
Tin Gold - Silicon,
Silicon Copper - Silver
Î Eutectic phase diagrams
X Completely immiscible/insoluble phase diagrams
Y Partially miscible/soluble phase diagrams
© Rashid, DMME, BUET . 2009
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Binary (two-component) phase diagrams
Au-Si phase diagram (Completely insoluble)
Binary (two-component) phase diagrams
Cu-Ag phase diagram (Partially soluble)
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Binary (two-component) phase diagrams
Pb-Sn phase diagram (Partially soluble)
Binary (two-component) phase diagrams
Au-Ge phase diagram (Partially soluble)
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Binary (two-component) phase diagrams
‰ When
A-B > 0.5 (A-A + B-B)
then
TAlloy > TA , TB
‰ Example:
E
l
G lli
Gallium
- Arsenic,
A
i Aluminium
Al i i
- Antimony
A ti
Î Intermetallic compound formation
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Binary (two-component) phase diagrams
Al-Sb phase diagram
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Theoretical Construction
z By applying thermodynamic principles
z Use of software like Thermocalc
Experimental Methods
X Thermal analysis
z Generation of cooling curves (temperature vs. time) for a number of alloys
of the alloy system to obtain arrest points (temperatures where a change in
slope is observed)
z Solid-state phase changes are difficult to obtained in this method
Y Metallographic
M ll
hi method
h d
z Heating samples of an alloy to different temperatures, and quench them
after equilibrium to retain the high-temperature structure
z Observe the structure microscopically
z Rapidly cooled samples do not always retain high-temperature structures;
considerable skill is required to interpret the microstructure correctly
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