1. Phase Diagrams
(equilibrium diagrams)
Prepared By
Dr.S.S.Saravanakumar, M.E., Ph.D.,
Associate professor,
Department of Mechanical engineering,
Kamaraj college of engineering and technology,
Virudhunagar.
2. Why we study Phase Diagrams?
Many material systems and alloy systems exist in more than one phases
depending on the conditions of temperature, pressure and
compositions. Each phase will have different microstructure which is
related to the mechanical properties. The development of
microstructure is related to the characteristics of phase diagrams. Thus
the knowledge and understanding of the phase diagrams is very
important for engineers . Proper knowledge and understanding of
phase diagrams will lead to design and control of heating procedures
for developing the required microstructure and properties.
3. Phase (gas, liquid, solid):
Gas (Atoms are not contact with each other)
Liquid( Limited mobility)( contact with each other but free to move)
Solid(should maintain contact with neighbour and “Fixed “in ordered form)
Physics definition for solid: In solid the atom /molecules are arranged in
regular 3 dimensional pattern
Engineering Purpose : Rigid body is called solid
5. What is Phase Diagram?
A graphic representation showing the phase or phases present for a
given composition, temperature and pressure. Also termed equilibrium
diagrams.
A phase diagrams show what phases exist at equilibrium and what
phase transformations we can expect when we change one of the
parameters of the system (T, P, composition)
We will discuss phase diagrams for binary alloys only and will assume
pressure to be constant at one atmosphere. Phase diagrams for
materials with more than two components are complex and difficult to
represent
6. Binary(2 components) Phase Diagrams:
1.Components completely soluble in the liquid state and
a, completelySoluable in solid state: Isomorphous Systems(Type I)
b.Insoluble in the solid state: the eutectic reaction (Type II)
c. Partly soluble in the solid state: the eutectic reaction (Type III)
e. Formation of a congruent-melting intermediate phase (Type IV)
f. The peritectic reaction (Type V)
2. Transformations in the solid state
a. Allotropic change
b. Order-disorder
c. The eutectoid reaction
d. The peritectoid reaction
7.
8. Interpolation of phase diagram
In a binary system(Cu-Ni) cooling experiments are conducted on some
of the alloys and the results are given in table. Plot the phase diagram,
label the phases draw the cooling curve for the alloy of 50%cu (50%Ni)
and mark the micro structure change on cooling. At 1275°C find
amount of phases and composition of phases.
S.No Pure Cu 80%Cu+
20%Ni
60%Cu+
40%Ni
40%Cu+
60%Ni
20%Cu+
80%Ni
Pure Ni
Ts 1085°C 1180°C 1280°C 1350°C 1400°C 1453°C
Te 1085°C 1140°C 1220°C 1290°C 1360°C 1453°C
9. Binary Isomorphous Systems
Iso - same
Morphous - Structure
complete solid solubility of the two components (both in the liquid and
solid phases)
10.
11. congruent-melting alloys
It begins and ends solidification at a constant temperature with no
change in composition, and its cooling curve will show a horizontal line.
Such alloys are known as congruent-melting alloys
The end product of a congruent melting alloy shall have only one phase
13. Two Metals Completely Soluble in the Liquid State
and Completely Insoluble in the Solid State (Type II)
Technically, no two metals are completely insoluble in each other.
However, in some cases the solubility is so restricted that for practical
purpose they may be considered insoluble.
A-Cd(cadmium ) B-Bi(Bismuth)
14. A is added to B, the temperature for the beginning of solidification for
those alloys is also lowered. Therefore, since each metal lowers the
freezing point of the other, the line connecting the points showing the
beginning of solidification, the liquidus line, must show a minimum.
This is shown by the upper dotted line in the above figure, showing a
minimum at point E, known as the eutectic point, for a composition of
40A-60
It can be seen that over a wide range of compositions, a portion of the
cooling curve that shows the end of solidification occurs at a fixed
temperature
15. The lower horizontal line at TE, shown dotted in the above figure, is
known as eutectic temperature. In one alloy, the eutectic composition
40A-60B, complete solidification occurs at a single temperature, the
eutectic temperature. Although the freezing of the eutectic
composition alloy thus resembles that of a pure metal, it is not
congruent melting alloy since the resulting solid is composed of two
phases
16.
17. TE is called the eutectic temperature and 40A-60B the eutectic composition
The phase diagram consists of four areas. The area above the liquidus line is
a single-phase homogeneous liquid solution, since the two metals are
soluble in the liquid state (labeled as Liquid solution). The remaining three
areas are two-phase areas.
Since the two metals are assumed to be completely insoluble in the solid
state, when freezing starts, the only solid that can form is a pure metal. Thus,
every alloy when completely solidified must be a mixture of the two pure
metals. Thus the area below FEG line in above diagram will be a mixture of
two solid pure metals A and B (labeled as Solid A + Solid B).
18. It is common practice to consider alloys to the left of the eutectic
composition as hypoeutectic alloys and those to the right as
hypereutectic alloys.
19.
20. Alloy 1 (Eutectic Composition)
Let us assume that a small amount of pure metal A solidified. This leaves the
remaining liquid richer in B; the liquid composition has shifted slightly to the right.
To restore the liquid composition to equilibrium value, B will solidify. If slightly too
much B is solidified, the liquid composition will have shifted to the left, requiring A
to solidify to restore equilibrium. Therefore, at constant temperature, the liquid
solidifies alternately pure A and pure B, resulting in extremely fine mixture usually
visible only under the microscope. This is known as eutectic mixture shown at (4) in
the above figure.
21. Eutectic reaction
The change of this liquid of composition E into two solids at constant
temperature is known as the eutectic reaction and may be written as
Since solidification of eutectic alloy occurs at constant temperature, its
cooling curve would be the same as that for a pure metal or any
congruent melting alloy. The eutectic solidification however is not
congruent as there is a difference in composition between the liquid
and the individual solid phases.
22. Alloy 2 (Hypoeutectic Alloys)
Remains a uniform liquid solution as shown at (1) until the liquidus line
temperature T1 is reached
At this point the liquid L1, is saturated in A, and as the temperature is dropped
slightly, the excess A must solidify as shown at (2)
the amount of liquid is gradually decreasing. When the alloy reaches xE.
23. The conditions existing just a fraction of a degree above TE are:
When solidified, the alloy will consist of 67 percent of grains of primary or proeutectic A (which
formed between T1 and TE or before the eutectic reaction) and 33 percent eutectic (A + B) mixture
as shown at (5).
Every alloy to the left of the eutectic point E, when solidified, will consist of grains of proeutectic A
and the eutectic mixture. The closer the alloy composition is to the eutectic composition, the more
eutectic mixture will be present in the solidified alloy.
Phase Liquid Solid A
Chemical
Composition
40A-60B 100A
Relative amount
(TExE / TEE) x 100 = 33
percent
(xEE / TEE) x 100 = 67
pecent
24. Alloy 3 (Hypereutectic Alloys)
when the liquidus line is reached at temperature T3 the liquid deposits crystals of
pure B instead of A as shown at (2).
As the temperature is decreased, more and more of B will solidify, leaving the liquid
richer in A
The remaining liquid now solidifies into the eutectic (A + B) mixture as shown at (5)
After solidification the alloy will consist of 75 percent grains of primary B or
proeutectic B and 25 percent eutectic (A + B) mixture.
25. In Si –Ag system melting point of Silicon is 1414°C and the melting
point of Ag (Silver) is 962°C.
An alloy of 20% Si is solidified in constant temperature of 363°C.Plot
the phase diagram.
(1) Label the phases
(2) Draw the cooling curve an alloy of 50% Si and mark the micro
structure change.
26. Two Metals Completely Soluble in the Liquid State
but only Partly Soluble in the Solid State (Type III)
31. Congruent-melting Intermediate Phase (Type IV)
When one phase changes into another phase isothermally (at constant
temperature) and without any change in chemical composition, it is
said to be congruent phase change or congruent transformation. All
pure metals solidify congruently
32. In the above figure, the intermediate alloy phase is shown as a vertical line. Since it is a
compound, it is indicated as AmBn, where m and n are subscripts which indicate the
number of atoms combined in the compound.
33.
34. The Peritectic Reaction (Type V)
In the peritectic reaction a liquid and a solid react isothermally to form
a new solid on cooling. The reaction is expressed in general as under.
35. It can be seen in the above figure that the compound AmBn, 70A-30B, when heated to the
peritectic temperature, point G, decomposes into two phases, liquid and
solid A. Therefore, it can be said that it is an incongruent melting intermediate alloy.
36.
37. Reverse of the eutectic reaction
The Peritectic Reaction
It can be seen that the peritectic
reaction is just the reverse of the
eutectic reaction, where a single
phase formed two new phases on
cooling. The Eutectic Reaction
38. Transformations in the Solid State
There are several equilibrium changes and reactions which take place
entirely in the solid state. They are known as transformations in the
solid state.
a. Allotropic change
b. Order-disorder
c. The eutectoid reaction
d. The peritectoid reaction
39. Allotropic change
Several metals may exist in more than one type of crystal structure depending upon their
temperature. This property of a metal is called allotropy. Iron, manganese, cobalt and tin have this
property. On equilibrium diagram, allotropic change is indicated by a point on the vertical line for
pure metals.
40. Order-disorder
Ordinarily in the formation of a substitutional type of solid solution the solute atoms do not occupy
any specific position but are distributed at random in the lattice structure of the solvent. The alloy is
said to be in a disordered condition. Some of these random solid solutions, if cooled slowly, undergo
a rearrangement of the atoms where the solute atoms move into definite positions in the lattice.
This structure as shown in figure below is now known as ordered solid solution or superlattice.
The effect of ordering on mechanical properties is negligible. However there is a significant reduction in
electrical resistance.
41. The Eutectoid Reaction
This is a common reaction in the solid state. It is very similar to eutectic
reaction but does not involve the liquid. In this case, a solid phase
transforms on cooling into two new solid phases. The general equation
may be written as under
42.
43. The Peritectoid Reaction
This is a fairly common reaction in the solid state and appears in many
alloys. The peritectoid reaction may be written in general as under.
