ALLOYS

Steel

An alloy is a mixture of two or more elements in solid solution in which the major

component is a metal.

WHAT ARE ALLOYS?

Iron Carbon

Most pure metals are soft and not very useful in their pure state.

Are their any exceptions?

Alloys

Therefore in order to increase desirable properties like strength, hardness and corrosion resistance we mix two or more pure metals together to give us an alloy.

ALLOYS

Hardness

HSS

Corrosion Resistance

Stainless Steel

Strength

Brass

Everyday examples of alloys include:

1. Bronze which is an alloy of Copper and Tin. Bronze is harder than pure copper.

2. Steel is an alloy of Iron and Carbon Steel is stronger than Iron.

3. Solder is an alloy of Tin and Lead.

Another example of course is aluminium alloy wheels.

ALLOYS

ALLOYS

Electron

Intermetallic Compounds

Interstitial

Substitutional

Solid Solution Alloys

Interstitial

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

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 metals.

Solid Solution Alloys Properties

2 Types of Solid Solutions

Substitude Atom Parent Atom Alloying Atom In Interstice

Parent Atom

Substitutional Solid Solution Alloys

Interstitial Solid Solution Alloys

Atoms of both metals are of similar size.

Direct substitution takes place.

Substitution normally at random.

Substitutional Solid Solution Alloys

Substitude Atom

Parent Atom

If substitution is ordered, it is called a SUPERLATTICE

Substitutional Solid Solution Alloys

Nickel Copper

Interstitial Solid Solution Alloys

Parent metal atoms are bigger than atoms of alloying metal.

Smaller atoms fit into spaces, (Interstices), between larger atoms.

Alloying Atom In Interstice

Parent Atom

INTERMETALLIC COMPOUNDS

Whereas many metals are ductile (literally, they can be drawn into wires) intermetallics tend to be brittle.

For this reason pure intermetallics tend to be unattractive as engineering materials.  

Properties of Intermetallics

However they come into their own at high temperatures when they do become ductile. As a result some of the low density intermetallics, such as the titanium aluminides, are becoming attractive in a range of applications such as gas turbine blades (for jet aircraft and gas fired power stations)

Properties of Intermetallics

Gas Fired Turbine (Power Stations)

Jet Aircraft Engine

Also for rapidly accelerating parts in internal combustion engines (such as rocker arms and turbo chargers).

Properties of Intermetallics

TurbochargerTwin

TurbochargerThe whole point of forcing the air/fuel mixture into an engine is to allow it to burn more fuel and make more power with the same engine capacity.

TurbochargerA turbocharger is used to force air/fuel mixture into an engine at a pressure greater then the natural atmospheric pressure of around 14.5 PSI.

TurbochargerThe way a turbo works is the exhaust coming out of the engine is pushed through a turbine. Compressing the

TurbochargerFor one thing when you compress air (with a turbo) it gets hotter.

The problem with hotter air is that it contains less oxygen than cooler air, so there is less oxygen to help burn extra fuel that’s going into the engine

IntercoolerThis is why many turbocharged cars use “intercooling” of various types, to cool the pressurised air back down into the engine.

Intercooler

TurbochargerThis is why many turbocharged cars use “intercooling” of various types, to cool the pressurised air back down into the engine.

Properties of Intermetallics

Low density intermetallics are very usefull for high temterature

and rapidly accelerating parts such as gas turbine blades,

rocker arms and turbo chargers

These intermetallic compounds have higher melting point than either of the parent metal.

This higher melting point indicates the high strength of the chemical bond in intermetallic compounds.

Intermetallic

What are Intermetallic Compounds

Intermetallics are compounds of two (or more) metallic elements that are held together by

metal bonds. They show long range ordering, in other words they have a regularly repeating

pattern. Compounds consist of a fixed ratio of atom.

Intermetallic compounds are also know as “Intermediate Compounds” or “Intermediate Phases”

What are Intermetallic Compounds

1. Compounds consist of a fixed ratio of atom, e.g. Nickel aluminide (Ni3Al)

2. Compounds show long range ordering, in other words they have a regularly repeating pattern.

Intermetallics Standard Alloy

1. Electron Compounds

2. Interstitial Compounds

Types of Intermetallic Compounds

These compounds are very similar in structure to solid solutions. They take their name from the fact that compounds from according the valency electron ratio between the metals concerned.

Electron Compounds

An example of electron compound would be an alloy of the elements Magnesium and Tin which combine to form an intermetallic compound Mg2SN.

Metallic compounds form a crystal lattice with the atoms of the alloying metals taking up specific positions within the lattice. These compounds are usually hard and brittle.

Electron Compounds Example

Mg2 SNMagnesium 2 Atoms Tin 1 Atom

Interstitial compounds, as the name suggests form between metals, or metals and non- metallic elements, with atom sizes very similar to those that form interstitial solid solution. One set of atoms fit into the spaces, or interstices, between the larger atoms.

Interstitial Compounds

Iron Carbide (Fe3C) or CEMENTITE which is important in the study of Iron- Carbon diagrams is an example of an interstitial compound.

Interstitial Compounds Example

What are the atoms are in a molecule of Cementite and how many of each are there?

Fe3 CIron 3 Atoms Carbon 1 Atom

Iron Carbide

Iron 26

Carbon 6

Cu Al2 is another example of an interstitial compound which is significant in AGE HARDENING aluminium alloys.

Interstitial Compounds Example