Properties of Metals
4. Alloys
Learning outcomes
- I can define an alloy and describe how alloys are formed.
- I can compare pure metals and alloys.
- I can explain how alloying changes material properties.
- I can identify common alloys and their applications.
- I can evaluate why alloys are often preferred over pure metals.
What Is an Alloy?
An alloy is a mixture containing a metal and one or more other elements.
The additional elements may be:
- other metals
- non-metals
For example:
brass = copper + zinc
bronze = copper + tin
steel = iron + carbon
Alloys are created because changing the composition of a metal can change its physical and chemical properties.
In many applications, an alloy has more useful properties than the corresponding pure metal.
Pure Metals
A pure metal contains only one metallic element.
Examples include:
- pure copper
- pure aluminium
- pure iron
- pure gold
In a simplified particle model, atoms in a pure metal are approximately similar in size and arranged in a regular metallic structure.
The metal is held together by metallic bonding: electrostatic attraction between positive metal ions and delocalized electrons.
Why Can Pure Metals Be Soft?
In the simplified model of a pure metal, the particles are similar in size and arranged in regular layers.
When a force is applied, layers can sometimes move relative to one another relatively easily.
Metallic bonding continues to hold the structure together as this happens.
This helps explain why many pure metals are:
- malleable
- ductile
- relatively easy to shape
However, this can also mean that a pure metal is too soft for some engineering applications.
What Happens in an Alloy?
An alloy contains atoms of more than one element.
These atoms are often different sizes.
The different-sized atoms disturb the regular arrangement of the metal atoms.
This can make it more difficult for layers and defects in the crystal to move.
As a result, many alloys are:
harder and stronger than the corresponding pure metal.
Pure Metal vs Alloy
Consider the simplified structures.
Pure metal
- mostly one type of atom
- relatively regular arrangement
- similarly sized atoms
- layers can often deform relatively easily
Alloy
- contains more than one element
- less uniform arrangement
- atoms may have different sizes
- movement through the structure can become more difficult
This structural difference can produce major changes in material properties.
How Are Alloys Formed?
Many alloys are produced by heating their components until they can be mixed thoroughly, then cooling the mixture so that a solid metallic material forms.
A simplified process is:
1. Select the elements.
2. Heat the materials as required.
3. Mix them in controlled proportions.
4. Allow the material to cool and solidify.
5. Further process or heat-treat the alloy if required.
Industrial alloy production requires careful control of composition, temperature, cooling, and processing because these can strongly affect the final properties.
Alloying Changes Properties
Adding another element can change properties such as:
- hardness
- strength
- toughness
- ductility
- corrosion resistance
- melting behaviour
- electrical conductivity
- thermal conductivity
- colour
- density
The exact effect depends on:
- which elements are added
- how much is added
- how the material is processed
Therefore, alloy design is an important part of materials science.
Hardness and Strength
One of the most familiar effects of alloying is increased hardness or strength.
Different-sized atoms can interfere with movement within the crystal structure.
In a more advanced description, alloying atoms can make the movement of dislocations more difficult.
Dislocations are defects in a crystal structure that allow deformation to occur.
Making their movement more difficult can make the material harder and stronger.
Alloys Are Still Metallic Materials
Alloying does not usually remove metallic bonding.
The structure still contains metallic bonding and mobile electrons.
Therefore, alloys often retain metallic characteristics such as:
- electrical conductivity
- thermal conductivity
- metallic appearance
- strength
- ability to be shaped
However, these properties can differ significantly from those of the pure metals.
Alloying Can Reduce Conductivity
Pure copper is an excellent electrical conductor.
Adding other elements to copper can increase strength but often reduces electrical conductivity.
Why?
The less regular atomic structure interferes with the movement of conduction electrons.
Therefore, alloying often involves a trade-off:
greater strength
but perhaps:
lower electrical conductivity
This is an important engineering idea.
Steel
One of the most important alloys is steel.
Steel is primarily:
iron + carbon
Small amounts of carbon can significantly change the properties of iron.
Compared with pure iron, many steels can be:
- harder
- stronger
- more resistant to deformation
This makes steel one of the most widely used engineering materials.
Why Does Carbon Change Iron?
Carbon atoms are much smaller than iron atoms.
Some carbon atoms can occupy spaces within the iron crystal structure.
This affects how easily parts of the structure can move.
The result can be a harder, stronger material.
However, increasing carbon content can also affect properties such as ductility and toughness.
The goal is not simply to add as much carbon as possible.
The composition must match the intended application.
Different Types of Steel
Steel is not one single material.
There are many different steels with different compositions and properties.
Examples include:
- low-carbon steel
- medium-carbon steel
- high-carbon steel
- stainless steel
- tool steels
Different compositions allow engineers to select steel for specific applications.
Stainless Steel
Stainless steel is an iron-based alloy containing chromium, often along with other elements such as nickel.
Chromium helps the surface form a thin protective oxide layer.
This greatly improves corrosion resistance in many environments.
Stainless steel is widely used for:
- cutlery
- sinks
- kitchen equipment
- medical instruments
- food-processing equipment
- chemical equipment
- architecture
Why Use Stainless Steel?
Ordinary steel can rust when exposed to oxygen and water.
Stainless steel is designed to resist corrosion much more effectively.
Therefore, it is useful when a material must combine:
strength + corrosion resistance
This is a good example of alloying being used to modify both physical and chemical properties.
Brass
Brass is mainly an alloy of:
copper + zinc
Brass can have useful combinations of:
- strength
- corrosion resistance
- workability
- attractive appearance
Applications include:
- musical instruments
- valves
- fittings
- screws
- decorative objects
- some electrical components
Brass vs Pure Copper
Pure copper is highly conductive and very ductile.
Adding zinc produces brass.
Brass is generally harder and stronger than pure copper, although its electrical conductivity is lower.
This demonstrates a common alloying trade-off:
improved mechanical properties
in exchange for:
reduced conductivity
Bronze
Bronze is traditionally an alloy mainly containing:
copper + tin
Bronze was historically extremely important because it offered useful properties compared with pure copper.
Bronze can be:
- harder than copper
- durable
- corrosion resistant
- suitable for casting
Applications include:
- sculptures
- bearings
- bells
- medals
- historical tools and weapons
The Bronze Age
The development of bronze was an important technological advance in human history.
Pure copper is relatively soft.
By adding tin, people produced a harder material suitable for many tools and other objects.
This illustrates an ancient form of materials engineering:
changing composition to improve properties.
Aluminium Alloys
Pure aluminium has many useful properties:
- low density
- corrosion resistance
- malleability
- conductivity
However, pure aluminium can be too soft for some structural applications.
Aluminium can be alloyed with elements such as:
- magnesium
- silicon
- copper
- zinc
depending on the required properties.
These alloys can provide much greater strength while maintaining relatively low density.
Why Aluminium Alloys Are Useful in Aircraft
Aircraft materials must often provide:
- high strength
- low mass
- durability
- corrosion resistance
Pure aluminium is lightweight but may not provide enough strength for many structural components.
Certain aluminium alloys provide a better combination of:
low density + strength
This demonstrates why engineers often prefer an alloy over a pure metal.
Titanium Alloys
Titanium alloys are widely used where a combination of high strength and relatively low density is important.
Titanium alloys can also have excellent corrosion resistance.
Applications include:
- aircraft
- spacecraft
- high-performance engineering
- medical implants
- chemical-processing equipment
Their performance can be excellent, although cost and manufacturing difficulty may limit their use.
Gold Alloys
Pure gold is very soft.
Jewellery made from completely pure gold can scratch or deform relatively easily.
Therefore, gold is often alloyed with other metals.
Alloying can:
- increase hardness
- increase durability
- change colour
- alter cost
For example, different alloy compositions can produce:
- yellow gold
- white gold
- rose gold
Sterling Silver
Pure silver is also relatively soft.
Sterling silver is an alloy that typically contains mostly silver with a smaller amount of another metal, commonly copper.
Alloying improves hardness and durability.
This makes sterling silver more suitable for:
- jewellery
- cutlery
- decorative objects
Again:
pure metal → desirable appearance
alloy → improved mechanical properties
Solder
Solders are alloys designed to melt at useful temperatures so that they can join other metal components.
Modern electronics commonly use lead-free solder compositions based mainly on tin with other elements.
Important properties can include:
- suitable melting range
- ability to flow
- electrical conductivity
- ability to form reliable joints
Here, alloying is used partly to control melting behaviour.
Nickel-Based Superalloys
Some engineering systems operate under extremely demanding conditions.
Jet engines, for example, contain components exposed to:
- high temperatures
- large forces
- rapid rotation
- corrosive gases
Special nickel-based superalloys can maintain useful mechanical properties at temperatures where many ordinary metals would perform poorly.
This demonstrates how sophisticated alloy design allows materials to operate under extreme conditions.
Shape-Memory Alloys
Some alloys have unusual properties.
Shape-memory alloys can return toward a previously defined shape after deformation when their temperature changes.
One well-known family includes nickel-titanium alloys.
Applications can include:
- medical devices
- actuators
- specialized engineering components
This shows that alloying can create properties very different from those of either pure element.
Comparing Pure Metal and Alloy Properties
Consider pure iron and steel.
Pure iron:
- relatively soft
- relatively easy to deform
- useful metallic properties
- susceptible to corrosion
Many steels:
- stronger
- harder
- better suited to structures
- properties can be adjusted through composition and processing
Consider pure copper and brass.
Pure copper:
- excellent electrical conductor
- highly ductile
- relatively soft
Brass:
- harder
- stronger
- often easier to use in certain mechanical applications
- lower electrical conductivity than pure copper
There is rarely a single property that determines which material is better.
Alloys and Corrosion Resistance
Alloying can improve chemical properties as well as physical properties.
Stainless steel is an important example.
Chromium helps produce a protective surface layer.
Some alloys are designed specifically to resist:
- water
- salts
- acids
- high-temperature oxidation
- other corrosive environments
This can greatly increase the useful lifetime of a component.
Alloys and Melting Behaviour
Alloys do not necessarily melt at exactly the same temperature as their pure components.
Changing composition can alter melting behaviour significantly.
This can be useful.
For example, solder must melt at a temperature low enough to join electronic components without damaging them.
Engineers can select alloy compositions that provide suitable melting characteristics.
Alloys and Density
Alloying can also change density.
This matters in applications where mass is important.
Aircraft and spacecraft engineers often seek materials that provide:
high strength-to-mass ratio
A material does not simply need to be strong.
It needs to provide sufficient strength without adding unnecessary mass.
This is one reason aluminium and titanium alloys are important in aerospace engineering.
Alloys and Electrical Resistance
Alloys often have greater electrical resistance than highly conductive pure metals.
This can sometimes be useful.
Certain alloys are deliberately selected for electrical resistance.
For example, resistance wires in heating devices require materials that:
- resist electrical current
- become hot
- tolerate high temperatures
- resist oxidation
Therefore, lower conductivity is not always a disadvantage.
Alloy Properties Depend on Composition
It is incorrect to assume that all alloys have the same properties.
Even alloys based on the same main metal can behave very differently.
Changing the percentage of an alloying element can change:
- hardness
- strength
- ductility
- toughness
- corrosion resistance
- conductivity
- melting behaviour
This allows engineers to tune materials for particular purposes.
Processing Also Matters
Composition is not the only factor controlling alloy properties.
How an alloy is processed can also be extremely important.
Processes may include:
- heating
- cooling
- rolling
- forging
- quenching
- tempering
- ageing
These processes can change the microscopic structure of the material and therefore change its properties.
Two pieces of steel with similar chemical composition can have different properties if processed differently.
Engineering Is About Trade-Offs
An alloy is rarely "better" in every possible way.
Consider adding another element to copper.
Possible advantages:
- increased strength
- increased hardness
- improved wear resistance
Possible disadvantages:
- reduced conductivity
- reduced ductility
- increased cost
- more difficult manufacturing
Engineers must decide which properties matter most for the intended application.
Evaluating Materials: Electrical Cable
Suppose an engineer needs material for a long electrical cable.
Pure copper has:
- excellent electrical conductivity
- high ductility
A copper alloy may be stronger but less conductive.
For ordinary electrical wiring, conductivity may be more important than maximum strength.
Therefore, relatively pure copper may be preferred.
This shows that alloys are often, but not always, preferable to pure metals.
Evaluating Materials: Bridge
A bridge requires:
- high strength
- toughness
- durability
- suitable cost
- ability to withstand environmental conditions
Pure iron would generally not provide the combination of properties required.
Appropriate steels can provide much more useful combinations of mechanical properties.
Therefore, an alloy is normally preferable for major structural applications.
Evaluating Materials: Jewellery
A jewellery designer may want:
- attractive colour
- corrosion resistance
- hardness
- durability
- workability
Pure gold provides excellent corrosion resistance but is soft.
A gold alloy can be harder and more durable while maintaining the desired appearance.
Therefore, alloying allows the material to be adapted to its purpose.
Evaluating Materials: Aircraft
Aircraft structures require a difficult combination:
- high strength
- low density
- fatigue resistance
- corrosion resistance
- predictable performance
Pure metals rarely provide the optimum combination.
Engineers therefore use carefully designed alloys.
The best material depends on the particular aircraft component and its operating conditions.
Example 1: Identify the Alloy
A material contains:
70% copper
30% zinc
What type of material is it?
It contains a metal mixed with another element.
Therefore, it is:
an alloy
A copper-zinc alloy is generally called:
brass
Example 2: Explain Increased Hardness
A pure metal is alloyed with atoms of a different size.
Explain why the alloy may be harder.
The differently sized atoms disrupt the regular arrangement of the metal lattice.
This can hinder movement of dislocations and make deformation more difficult.
Therefore, more force may be required to change the shape of the material.
The alloy is harder.
Example 3: Choosing Between Copper and Brass
An engineer needs a material for a decorative mechanical fitting.
It should be:
- reasonably strong
- corrosion resistant
- easy to shape
- attractive
Brass may provide a more suitable combination than pure copper because alloying copper with zinc changes its mechanical properties while retaining useful corrosion resistance and workability.
However, if maximum electrical conductivity were required, pure copper would probably be more appropriate.
The best material depends on the design requirements.
Example 4: Choosing a Material for a Saucepan
A saucepan needs:
- thermal conductivity
- strength
- corrosion resistance
- durability
An engineer must compare several materials.
A pure metal may have excellent thermal conductivity but insufficient strength or durability.
An alloy may provide greater strength and corrosion resistance while sacrificing some conductivity.
Material selection therefore involves balancing properties.
A Materials-Selection Strategy
When deciding whether to use a pure metal or an alloy:
1. Identify the application.
What must the material do?
2. Identify the required properties.
For example:
- strength
- hardness
- conductivity
- corrosion resistance
- low density
3. Compare the pure metal with available alloys.
4. Identify how alloying changes the structure.
5. Connect structure to properties.
6. Identify advantages.
7. Identify disadvantages or trade-offs.
8. Consider operating conditions.
Temperature? Moisture? Large forces? Electrical current?
9. Consider practical constraints.
Cost? Availability? Manufacturing?
10. Select the material that provides the most appropriate combination of properties.
Common Mistakes
Mistake 1: Saying an alloy is a compound
An alloy is generally a mixture, not a single compound with a fixed chemical formula.
Mistake 2: Saying alloys contain only metals
Alloys contain a metal plus one or more other elements.
Some alloying elements are non-metals.
Carbon in steel is an important example.
Mistake 3: Saying steel is pure iron
Steel is an iron-based alloy containing carbon and often other elements.
Mistake 4: Saying alloys are always stronger
Many alloys are designed for greater strength, but alloying can be used to change many different properties.
Not every alloy is stronger than every pure metal.
Mistake 5: Saying different-sized atoms simply "break the bonds"
A better explanation is that they disrupt the regular crystal structure and can hinder movement responsible for deformation.
Mistake 6: Assuming harder always means better
Hardness may be useful, but excessive hardness can come with reduced ductility or toughness.
Mistake 7: Assuming alloys always conduct electricity better
Alloying often reduces electrical conductivity compared with a highly conductive pure metal.
Mistake 8: Ignoring composition
Changing the percentage of alloying elements can substantially change properties.
Mistake 9: Ignoring processing
Heat treatment and mechanical processing can significantly affect alloy properties.
Did You Know?
The development of alloys has repeatedly changed human technology.
Bronze helped produce harder tools than those made from pure copper.
Steel transformed construction, transportation, machinery, and manufacturing.
Modern aluminium, titanium, and nickel-based alloys make high-performance aircraft and other advanced technologies possible.
The underlying idea is remarkably consistent:
change the composition → change the structure → change the properties → create a material better suited to a particular purpose.
Key Terms
- Alloy: Mixture containing a metal and one or more other elements.
- Pure metal: Material containing only one metallic element.
- Alloying element: Element deliberately added to a metal to modify its properties.
- Metallic bonding: Electrostatic attraction between positive metal ions and delocalized electrons.
- Hardness: Resistance to scratching, indentation, or permanent deformation.
- Strength: Ability to withstand forces without failure.
- Toughness: Ability to absorb energy and resist fracture.
- Ductility: Ability to be drawn into wires or undergo tensile deformation.
- Malleability: Ability to be hammered or rolled into sheets.
- Corrosion resistance: Ability to resist chemical deterioration by the environment.
- Dislocation: Defect in a crystal structure whose movement contributes to plastic deformation.
- Materials science: Study of how the structure, composition, processing, and properties of materials are related.
Common Alloys
Steel
Main components: iron + carbon
Uses:
- buildings
- bridges
- vehicles
- machinery
Important properties:
- strength
- hardness
- versatility
Stainless steel
Main components:
iron + chromium, often with nickel and other elements
Uses:
- kitchen equipment
- medical instruments
- buildings
- industrial equipment
Important properties:
- strength
- corrosion resistance
Brass
Main components:
copper + zinc
Uses:
- musical instruments
- fittings
- valves
- decorative objects
Important properties:
- workability
- strength
- corrosion resistance
Bronze
Traditionally:
copper + tin
Uses:
- sculptures
- bearings
- medals
- bells
Important properties:
- hardness
- durability
- corrosion resistance
Aluminium alloys
Main component:
aluminium
Uses:
- aircraft
- vehicles
- bicycles
- engineering structures
Important properties:
- low density
- useful strength
- corrosion resistance
Titanium alloys
Main component:
titanium
Uses:
- aerospace
- medical implants
- high-performance engineering
Important properties:
- high strength-to-mass ratio
- corrosion resistance
Structure–Property Connection
Pure metal
similar atoms arranged relatively regularly
↓
layers and dislocations can move relatively easily
↓
material may deform relatively easily
Add alloying atoms
↓
different atoms disrupt the regular structure
↓
movement of dislocations can become more difficult
↓
deformation becomes more difficult
↓
hardness and strength may increase
This is one of the most important structure-property relationships in materials science.
Key Takeaways
- An alloy is a mixture containing a metal and one or more other elements.
- Alloying is used to modify material properties.
- The additional elements can be metals or non-metals.
- Pure metals contain one metallic element.
- Pure metals often have relatively regular atomic structures.
- Alloying introduces different atoms into the structure.
- Different-sized atoms can disrupt the regular arrangement.
- This can hinder movement within the crystal and make deformation more difficult.
- Many alloys are therefore harder and stronger than their corresponding pure metals.
- Alloying can also change corrosion resistance, conductivity, melting behaviour, density, colour, ductility, and toughness.
- Steel is mainly iron alloyed with carbon.
- Stainless steel contains chromium and has improved corrosion resistance.
- Brass is mainly copper and zinc.
- Bronze is traditionally mainly copper and tin.
- Aluminium alloys provide useful combinations of low density and strength.
- Titanium alloys are useful where high strength-to-mass ratio and corrosion resistance are important.
- Alloys often provide a more useful combination of properties than pure metals.
- However, an alloy is not automatically better for every application.
- Alloying may improve one property while reducing another.
- Engineers therefore evaluate trade-offs when selecting materials.
- Composition and processing both affect alloy properties.
- The central materials-science relationship is:
composition → structure → properties → performance → application.