Thermal Equilibrium Diagrams show the
temperatures at which phase changes take place
in alloys of different percentage composition.
A Phase is a portion of a
system that has uniform
physical and chemical
characteristics.
Thermal Equilibrium Diagrams
are made for information gained
form Cooling Curves.
When the temperature of a cooling molten
metal alloy is plotted against time, a cooling
curve is formed.
Remember this only happens
for pure metals.
When cooling a solid pure
metal, change from a solid
to a liquid at constant
temperature.
This is called the Melting
Point Temperature.
The heat that is released in
the transition from a solid
to a liquid is called hidden
or Latent Heat.
LIQUID
Solidification
Starts
Solidification
Ends
SOLID
Latent heat is the quantity of heat
energy absorbed or released when a
substance changes its physical phase
at constant temperature.
The energy that goes in breaks
the bond of the atoms.
Pure Metal
change directly
LIQUID
from Liquid to
Solid at
1 temperature.
SOLID
1500
Temperature 0C
If a metal is 100% pure and
contains no traces of
other elements then
some under cooling may
occur before
solidification begins.
Under cooling is when
the temperature drops
below the liquid to solid
temperature for a short
period.
Undercooling
1100
1000
Time
Fall in temperature stops temporarily at
the freezing point because of Latent heat
Alloys change
from Liquid to
Solid through a
pasty state over a
temperature
range.
LIQUID
Solidification Starts
“PASTY” STATE
Solidification Ends
SOLID
Pure Metal
Alloy
Over a
Temperature
Range
1Temperature
A Solid Solution is when two metals
are completely soluble in each other
in both the liquid and solid states.
When viewed under a microscope, a solid
solution appears like a pure metal.
Iron Carbon
100% Lead
The given table shows the solidification temperatures for various alloys of
Copper and Nickel. The melting point of Copper is 1083°C and Nickel is
1453°C.
Using the graph paper supplied:
(i) Draw the equilibrium diagram according to the given data.
(ii) Label the diagram and describe the main features.
(iii) For the alloy with 50% Nickel determine, from the diagram,
the ratio of the phases at 1250°C.
TEMPERATURE (°C)
1600
1400
1200
1000
800
600
400
200
NICKEL
COPPER
0%
100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
1400
TEMPERATURE (°C)
1200
1000
800
600
400
200
NICKEL
0
0%
10%
20%
30%
40%
50%
60%
70%
COPPER
PRECENTAGE COMPOSITION
80%
90%
100%
Equilibrium diagrams are made from taking
information for cooling of different percentage
of an alloy and putting it on one graph.
This is know as a TERMAL ANALYSIS
100% Copper
100% NICKLE
60% Copper
40% Nickel
40% Copper
60% Nickel
TEMPERATURE (°C)
1600
Liquid Phase
Liquidus Line
1400
1200
1000
800
Solid Phase
600
Phases are defined as regions
that differ from one another on
an equilibrium diagram.
400
Phases defined by composition
(not by state – solid, liquid,
gas)
200
NICKEL
COPPER
0%
100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%

Liquid: the two metals are soluble in each other in the liquid
state.

Liquidus Line: the change from fully liquid to pasty state. Above
the liquidus line, the alloy is liquid. This is the beginning of
solidification.

Liquid and Solid: the alloy is in a pasty form, there are two
phases one Liquid and one Solid.

Solidus Line: the change from pasty to solid. Below the solidus
line, the alloy is cooling and solid. This is the end of
solidification.

Solid: alloy is in solid form.



Equilibrium may be defined as a state of
balance or stability.
Diagrams indicates the phases the alloy is
at different temperatures.
Phases are defined as regions that differ
from one another on an equilibrium
diagram. Phases defined by composition.
27
Equilibrium refers to Balance existing in a system

In Metallurgy, Equilibrium means, that the cooling
of a metal or an alloy is so slow that all the
changes that might take place get the chance to do
so.
To achieve equilibrium to take
place enough time for complete
diffusion is required.
Metallurgy is the study of metals and
their properties
Diffusion is the processes by which a
substance spread out through another
substance.
Atoms move in a solid metal in this manner.
HIGH CONSTANTRATION
LOW CONSTANTRATION
Atom move form high concentration
to low concentration
How Many Phases Are There?
PHOTOS
A SINGLE PHASE
TWO-PHASES
A Phase is a portion of a system that
has uniform physical and chemical
characteristics.
Phases are defined as regions that
differ from one another on an
equilibrium diagram.
Phases defined by composition.
NOT BY STATE (solid, liquid , gas)
Nickel
Copper
Copper
Nickel
Copper
Nickel
iii)For the alloy with 50% Nickel determine, from
the diagram, the ratio of the phases at 1250°C.
Tie Line
1400
1250°C
TEMPERATURE (°C)
1200
1000
800
600
400
200
NICKEL
0
0%
10%
20%
26%
30%
40%
50%
60%
66%70%
COPPER
PRECENTAGE COMPOSITION
80%
90%
100%
A
26%
Ratio of phases at 1250℃ for 50% Nickel
The ratio of Solid to Liquid is:
B
C
66%
1400
1250°C
TEMPERATURE (°C)
1200
1000
800
600
400
200
NICKEL
COPPER
0
0%
10%
20%
26%
30%
40%
50%
60%
66%70%
35%
PRECENTAGE COMPOSITION
Ratio of phases at 1250°C for 35% Nickel
80%
90%
100%
1400
1250°C
TEMPERATURE (°C)
1200
1000
800
600
400
200
NICKEL
0
0%
10%
20%
26%
30%
40%
50%
60%
66%70%
COPPER
PRECENTAGE COMPOSITION
Ratio of phases at 1250ºC for 60% Nickel
80%
90%
100%
In a eutectic alloy the two
metals are completely
soluble in the liquid phase
but are insoluble in the
solid phase.
The given table shows the solidification temperatures for various
alloys of Cadmium and Nickel.
Using the graph paper supplied:
i. Draw the equilibrium diagram according to the given
data.
ii. Label the diagram and describe the main features.
TEMPERATURE (°C)
1600
1400
1200
1000
800
600
400
200
BISMUTH
0%
CABMIUM 100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
TEMPERATURE (°C)
PRECENTAGE COMPOSITION
400
TEMPERATURE (°C)
350
300
250
200
150
100
50
0
BISMUTH
0%
10%
20%
30%
40%
50%
60%
70%
CADMIUM
PRECENTAGE COMPOSITION
80%
90%
100%
400
350
Liquidus Line
Liquid Phase
TEMPERATURE (°C)
300
Eutectic
Point
250
200
Liquid Phase
and Solid Phase
Liquid Phase
and Solid Phase
150
100
Solid Phases
50
0%
BISMUTH
CADMIMUM 100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
TEMPERATURE (°C)
400
350
Liquid Phase
300
Eutectic
Point
250
200
Liquid and
Solid Bismuth
Liquid and
Solid Cadmium 150
Solid Cadmium
and Eutectic
100
BISMUTH
CADMIUM
Solid Bismuth
and Eutectic
50
0%
100%
20%
80%
40%
60%
60%
40%
80%
20%
PRECENTAGE COMPOSITION
Solid Cadmium
100%
0%
Green Line
Eutectic Alloy
Solid Bismuth
Eutectic
∘ Cadmium Atoms
∙ Bismuth Atoms
X 1 000
X 10 000 000
The point where the liquid alloy
changes to solid without going
through a liquid/solid state is
called the eutectic point.
Some metals in an alloy only partially
dissolve in each other. Lead/Tin Alloy
(Solder) is an example of this.
The equilibrium diagram for this type of alloy is
called a Partial Solubility thermal equilibrium
diagram.
It is a combination of the solid solution and
eutectic diagrams and is a little more complex.
Eutectic Alloys
Solid Solution Alloys
Solid Solution Alloys
Partial Solubility Alloy
400
Liquidus Line
Liquid
and Solid
TEMPERATURE (°C)
300
Liquid Phase
Eutectic
Point
Liquid
and Solid
200
100
Solid Phases
TIN
LEAD
0%
100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
400
Liquidus Line
Liquid
and Solid
Liquid Phases
Solid
Solution
TEMPERATURE (°C)
300
Liquid
and Solid
200
Solid
Solution
Solidus Line
SOLVUS LINES
100
Solid
TIN
LEAD
0%
100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
400
α Solid
and Liquid
TEMPERATURE (°C)
α Solid
Solution
Liquid Phases
300
β Solid
and Liquid
200
β Solid
Solution
α and β
Solid Solution
100
TIN
LEAD
0%
100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
Tin
Lead
Copy the given lead-tin diagram into your answer book
and answer all of the following:
(i) Identify the lines labelled A, B and C;
(ii) Explain what each line represents;
(iii) For the alloy with 30% tin determine, from the diagram, the
composition of the phases at 250°C;
(iv) Indicate clearly on your diagram the eutectic point.
Liquidus line
Solidus line
Solvus line
A – Liquidus line. For the alloy system this line
represents the boundary between the fully liquid
state and the beginning of solidification.
B – Solvus line. The transition line from one solid
form to another solid form of an alloy is called
the solvus line. This line indicates the maximum
amount of tin
which can be dissolved in the lead.
C – Solidus line. The boundary line that determines
the end of solidification. Below this line the alloy
is completely solid.
To determine
composition of
the phases
A
B
A – Solid composition consisting of 8% tin and 92% lead.
B – Liquid composition consisting of 43% tin and 57% lead.
Eutectic Point
TEMPERATURE (°C)
1600
Liquid Phase
Liquidus Line
1400
1200
1000
800
Solid Phase
600
Phases are defined as regions
that differ from one another on
an equilibrium diagram.
400
Phases defined by composition
(not by state – solid, liquid,
gas)
200
NICKEL
COPPER
0%
100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
400
350
Liquidus Line
Liquid Phase
TEMPERATURE (°C)
300
Eutectic
Point
250
200
Liquid Phase
and Solid Phase
Liquid Phase
and Solid Phase
150
100
Solid Phases
50
0%
BISMUTH
CADMIMUM 100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%
400
α Solid
and Liquid
TEMPERATURE (°C)
α Solid
Solution
Liquid Phases
300
β Solid
and Liquid
200
β Solid
Solution
α and β
Solid Solution
100
TIN
LEAD
0%
100%
20%
80%
40%
60%
60%
40%
PRECENTAGE COMPOSITION
80%
20%
100%
0%


You should know that a pure metal has one clearly
defined melting point.
Mixtures tend to solidify over a temperature range,
that is they start to solidify at one temperature and
do not complete the process until they reach a lower
point.

A mix of 80% A with 20% B will solidify over a
different range than a mix of 20% A with 80% B.

A phase diagram allows us to present all this
information in a single (sometime simple) diagram.



Equilibrium may be defined as a state of
balance or stability.
Diagrams indicates the phases the alloy is
at different temperatures.
Phases are defined as regions that differ
from one another on an equilibrium
diagram. Phases defined by composition.
68