1. Alloying a metal is done by combining it with one or
more other metals or non-metals that often improve its
2. The term alloy is used to describe a mixture of atoms in
which the primary constituent is a metal.
3. The primary metal is called the base.
Reasons for Alloying.
• To improve its properties such as Toughness. Example: Iron
is relatively soft, Carbon is brittle but Steel is very tough.
• To reduce the melting point. Example: Lead melts at 327 °c,
Tin melts at 232 °c but Solder melts at approx. 183 °c.
• To prevent rusting. Example: Stainless steel is produced by
alloying iron with chromium and carbon.
The chromium forms an impenetrable oxide layer that
adheres strongly to the iron surface preventing the formation
However, pure aluminium is soft and lacks
strength, but alloyed with small amounts of
copper, magnesium, silicon, manganese, and
other elements gives it very useful properties.
Aluminium is very abundant and is used
in pure form for a lot of different things,
like kitchen foil, mirrors, coins and
Substitutional Alloys are
where the metals combining
have atoms of the same size.
Brass is an example.
Interstitial Alloys are where
one of the alloying elements
combining has atoms of
smaller size. Steel is an
example. Carbon atoms are
much smaller and fit into the
gaps in the Iron atoms.
Classification of Alloys.
Solid Solution Alloys.
Solid Solution Alloys:
When metals combine they sometimes become completely soluble in each other.
Metals which combine in this way are said to form solid solutions. When this type of
alloy solidifies, only one type of crystal is formed. Under a microscope the crystalline
structure of a solid solution alloy looks very like a pure metal. Solid solution alloys
have similar properties to pure metals but have greater strength. They also have
poorer electrical/thermal conductivity, greater hardness but not as elastic as pure
How are Metal Alloys made.
The metals forming the alloy are melted in a furnace in a crucible. They are then
poured into blocks called ingots and allowed to solidify. When a molten metal is
mixed with another molten metal and allowed to solidify an alloy is formed. The
new solid metal will have gone through ‘Phase’ changes to become an alloy.
What is Thermal Equilibrium.
Left click with the mouse on the link above to play the
What are Thermal Equilibrium Diagrams.
Thermal equilibrium diagrams are special diagrams which contain information
about changes that take place in alloys. The temperature at which a particular
alloy changes from liquid to solid is an example of the type of thermal information
contained in a thermal equilibrium diagram.
Cooling Curve of a Pure Metal.
Cooling Curve of a Solid Solution (An Alloy).
Set of Cooling Curves for various
• Solidus Line – All points below this line are in solid
• Liquidus Line – All points above this line are in
• Pasty Region - Between the solidus and liquidus
lines, all points are in a mixture of liquid and solid
• Liquid Region – The alloy is completely liquid in
• Solid Region – The alloy is completely solid in this
Parts of the Thermal Equilibrium Diagram.
Effects of Alloying Elements on Steel Properties:
Carbon: Raising carbon content increases hardness slightly and wear resistance
Manganese: Small amounts of Manganese reduce brittleness and improve
forgeability. Larger amounts of manganese improve hardenability, permit oil
quenching, and reduce quenching deformation.
Silicon: Improves strength, toughness, and shock resistance.
Tungsten: Improves "hot hardness" - used in high-speed tool steel.
Vanadium: Refines carbide structure and improves forgeability, also improving
hardness and wear resistance.
Molybdenum: Improves deep hardening, toughness, and in larger amounts, "hot
hardness". Used in high speed tool steel because it's cheaper than tungsten.
Chromium: Improves hardenability, wear resistance and toughness.
Nickel: Improves toughness and wear resistance to a lesser degree.
Including these elements in varying combinations can act synergistically,
increasing the effects of using them alone.
(a) Draw a thermal equilibrium diagram from the table listed below.
(b) Label the diagram and describe its main features.
(c) For the alloy 50% B, determine from the diagram the ratio of the phases at 400 °C
Liquidus line: for the alloy system this line represents the boundary
between the fully liquid state and the beginning of solidification.
Solidus line: the boundary line that determines the end of
solidification. Below this line, the alloy is completely solid.
Liquid region: the two metals are in a liquid state.
Solid region: the two metals are in a solid state.
Pasty region: between the liquidus and solidus lines, the alloy is in
a partly liquid and a partly solid state.
a) Explain any two of the following:
(ii) Interstitial solid solution;
(iii) The difference between amorphous structures and crystalline structures;
(iv) The degree of brittleness in body-centered cubic (bcc) structures and the
degree of brittleness in face-centered cubic (fcc) structures.
Allotropy is the ability of a material to exist in different forms.
(ii) Interstitial solid solution:
An atom from another element moves into the space between the atoms of the parent metal lattice. This
causes compression of the surrounding atoms and will strengthen the material as it takes a higher stress to
(iii) Amorphous structures do not have a pattern in the arrangement of their atoms but are a more random
structure. Pitch, glass and some plastics have this type of structure. Crystalline structures have atoms that
are bonded together in a pattern that is repeated. Metals with bcc and fcc unit cells are examples of a
(iv) Brittleness in bcc and fcc structures:
In the bcc structure, the structure is arranged with an atom at the corner of a cube and an atom in the centre
of the cube. This structure is associated with brittleness.
In the fcc structure atoms are at the corners of a cube and a single atom in the centre of each face of the cube.
Atoms are more tightly packed which allows metals to be more ductile.
The table shows the solidification temperatures for the various alloys of metal A
and metal B. The melting point of metal A is 1083 ºC and the melting point of
metal B is 1453 ºC.
Using the graph paper supplied:
(i) Draw the thermal equilibrium diagram according to the given data;
(ii) Label the main features of the diagram;
(iii) Determine, from the diagram, the ratio of the phases at 1250ºC for the
alloy of 50% metal B.
% of metal B in
10 20 30 40 50 60 70 80 90
1160 1220 1270 1320 1350 1380 1400 1430 1440
End of solidification
1080 1090 1110 1140 1170 1220 1270 1330 1380
(ii) Label the following features:
Liquidus line, Solidus line, Liquid region, Solid region and Pasty region
(iii) Ratio of phases at 1250 °C for 50% metal B:
Mass of solid is 50-26 = 24 = 3
Mass of liquid 66-50 16 2