Thermal Equilibrium
Diagrams
Contain information about
changes that take place in alloys
Alloys
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Alloys are combinations or mixtures of
elements.
Metals are alloyed to improve on properties
of pure metals such as hardness, strength,
corrosion resistance, etc.
Alloys may behave differently when
combined, some mix easily while others will
only be soluble to a limited extent. Thermal
Equilibrum diagrams seek to interpret how
alloys combine.
Solid Solution Alloys
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Two metals are
completely soluble in
each other. Solid
solution alloys are
normally:
Substitutional solid
solution or,
Interstitial solid solution.
Copper-nickel alloy is a
solid solution .
Cooling curve
This shows how a pure
metal cools from a liquid to
a solid. A graph of
temperature and time shows
that copper is fully liquid at
1500C. As temperature falls
to 1083C, copper begins to
change into a solid. As this
solid-liquid change happens,
the temperature does not
change (the latent heat of
fusion is given up keeping
the temperature constant)
Cooling curve graphs of Copper and Nickel
Thermal Equilibrium diagram for Coppernickel alloy drawn from cooling curves.
Parts of Copper-nickel diagram
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Solidus Line – all points below this line are in
solid form.
Liquidus Line – all points above this line are
in liquid form.
Between the solidus and liquidus lines, all
points are in a mixture of liquid and solid
(pasty) form.
Composition of phases at a particular
temperature (Lever Law)
Ratio = Mass of Solid = AB
Mass of Liquid
BC
= 65-60 = 5
60-48
12
Eutectic alloy
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The two metals are completely soluble in the liquid
state but are completely insoluble in the solid state.
Cadmium-Bismuth is an example of a eutectic alloy.
The phase equilibrum diagram is compiled in the same
way as the solid solution alloy but looks completely
different.
The eutectic point (140C) is the lowest melting point of
the alloy. The alloy changes directly from a liquid to a
solid without going through a pasty stage at this point.
Cadmium-bismuth (eutectic alloy) phase
equilibrum diagram.
Partial Solubility
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When one metal dissolves in another to a limited
extent.
Lead and tin is a partial solubility alloy.
The solvus lines give an indication of the amount of
one metal that will dissolve in the other metal.
Soft solders are based on lead-tin alloys with tinmans
solder (used for electronics) being the eutectic alloy.
Plumbers solder has 70% lead with the pasty phase
allowing the joint to be shaped.
Partial solubility thermal equilibrum
diagram of lead and tin
Intermetallic compounds
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This is a chemical compound formed
between at least two elements. Iron carbide,
or cementite, is a compound of iron and
carbon. It is hard and brittle.
The Allotrophy of Iron
Iron can exist in more than
one form.
 Up to 910 degrees, alpha
iron is in BCC form.
 Between 910 and 1390
degrees, gamma iron
exists in FCC form.
 Above 1390 degrees,
delta iron returns to BCC
form.
Iron Carbon Equilibrum Diagram
Steel part of the Iron Carbon Equilibrum
Diagram
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Eutectoid point is a solid to solid change point where solid
Austenite changes into solid Pearlite.
Microstructures on steel part of the Iron
Carbon Equilibrum Diagram