Properties of Metals
5. Uses of Metals
Learning outcomes
- I can identify important uses of metals in everyday life.
- I can explain how metal properties determine their applications.
- I can compare different metals used for specific purposes.
- I can evaluate the advantages and limitations of different metals.
- I can connect material selection to engineering and design challenges.
Why Are Metals So Important?
Metals are among the most widely used materials in modern society.
They are found in:
- buildings
- vehicles
- electrical systems
- electronics
- tools
- cookware
- medical equipment
- aircraft
- bridges
- jewellery
- machinery
Metals are useful because they can provide combinations of properties such as strength, electrical conductivity, thermal conductivity, malleability, ductility, durability, and corrosion resistance.
However, different metals have different properties.
Choosing a metal therefore requires matching its properties to the requirements of the application.
Structure Determines Properties
Many important metal properties can be explained using metallic bonding.
A metal contains:
- positive metal ions
- delocalized electrons
The electrostatic attraction between them holds the structure together.
This structure helps explain why many metals are:
- electrically conductive
- thermally conductive
- strong
- malleable
- ductile
These properties then determine what the metal can be used for.
A useful reasoning chain is:
structure → property → application
Electrical Wiring
One of the most important uses of metals is carrying electric current.
Metals conduct electricity because their delocalized electrons can move through the metallic structure.
Copper and aluminium are especially important electrical conductors.
Copper for Electrical Wiring
Copper is widely used for electrical wiring.
Important properties include:
- excellent electrical conductivity
- high ductility
- good corrosion resistance
- useful mechanical strength
Ductility means copper can be drawn into long, thin wires.
Therefore, copper combines two particularly useful properties:
conductivity + ductility
Why Not Use Silver for All Wiring?
Silver is an even better electrical conductor than copper.
However, silver is much more expensive.
Using silver for ordinary household wiring would usually provide little practical benefit compared with its additional cost.
This demonstrates an important engineering principle:
The material with the best single property is not necessarily the best material for the application.
Cost, availability, durability, and manufacturing must also be considered.
Aluminium for Power Cables
Aluminium is also an electrical conductor.
Its electrical conductivity is lower than copper's for the same cross-sectional area, but aluminium has an important advantage:
low density
For overhead power transmission, reducing cable mass is extremely useful.
Aluminium therefore provides a useful combination of:
- electrical conductivity
- low density
- relatively low cost
- corrosion resistance
Copper vs Aluminium Conductors
For electrical systems, engineers might compare:
Copper
Advantages:
- very high conductivity
- ductile
- reliable electrical connections
- good corrosion resistance
Limitations:
- relatively dense
- generally more expensive than aluminium
Aluminium
Advantages:
- low density
- conductive
- relatively inexpensive
- corrosion resistant in many environments
Limitations:
- lower conductivity per unit cross-sectional area than copper
- different mechanical and connection requirements
The best choice depends on the application.
Metals in Buildings
Buildings require materials that can support large loads.
Steel is one of the most important construction materials in the world.
Steel is an alloy based mainly on iron.
Depending on its composition and processing, steel can provide:
- high strength
- toughness
- durability
- predictable engineering properties
- relatively economical production
Steel Structures
Steel is used in:
- beams
- columns
- bridges
- reinforcement
- roofs
- towers
- industrial buildings
Steel structures can carry large loads while allowing engineers to design relatively slender structural components.
However, steel has limitations.
These include:
- high density
- possible corrosion
- loss of strength at sufficiently high temperatures
Therefore, steel structures may require coatings, fire protection, or other engineering measures.
Reinforced Concrete
Concrete is strong under compression but relatively weak under tension.
Steel is strong in tension.
Combining the two produces reinforced concrete.
Steel reinforcing bars are placed inside concrete.
The materials complement each other:
concrete → strong in compression
steel → strong in tension
This demonstrates how engineers combine materials rather than expecting one material to perform every function.
Metals in Bridges
Bridges experience:
- compression
- tension
- bending
- vibration
- repeated loading
- weather exposure
Steel is commonly used because it can provide high strength and toughness.
Engineers must also consider:
- corrosion
- fatigue
- maintenance
- temperature changes
- cost
- expected lifetime
Material selection is therefore part of the entire design process.
Metals in Cars
Cars contain many different metals because no single metal has all the necessary properties.
Metals may be used in:
- body structures
- engines
- electric motors
- wiring
- wheels
- suspension
- batteries
- braking systems
Different components require different combinations of properties.
Steel in Cars
Steels are widely used in vehicle structures.
Useful properties include:
- strength
- toughness
- formability
- relatively low cost
- ability to absorb energy through controlled deformation
Modern vehicles often use several different types of steel in different locations.
The goal is not simply to make the vehicle as rigid as possible.
Engineers must balance:
strength + mass + safety + manufacturing + cost
Aluminium in Vehicles
Aluminium alloys can be used to reduce vehicle mass.
Advantages include:
- low density
- corrosion resistance
- useful strength when alloyed
Reducing mass can improve vehicle efficiency.
However, aluminium may:
- cost more than some steels
- require different manufacturing methods
- have different repair requirements
Again, material selection involves trade-offs.
Metals in Aircraft
Aircraft provide one of the clearest examples of materials engineering.
An aircraft must be strong but also lightweight.
Every additional kilogram affects aircraft performance.
Important properties include:
- low density
- high strength
- fatigue resistance
- corrosion resistance
- temperature resistance
Aluminium Alloys in Aircraft
Pure aluminium has low density but is relatively soft.
Aluminium alloys can provide much greater strength while maintaining relatively low mass.
This gives many aluminium alloys a useful:
strength-to-mass ratio
They have therefore been important aircraft materials.
Titanium in Aerospace
Titanium and titanium alloys are used in demanding aerospace applications.
Important properties include:
- high strength
- relatively low density
- excellent corrosion resistance
- useful performance at elevated temperatures
However, titanium has limitations.
It is:
- relatively expensive
- more difficult to process than many common metals
Therefore, engineers use it where its performance justifies the additional cost.
Jet Engines
Jet engines operate under extremely demanding conditions.
Components may experience:
- very high temperatures
- rapid rotation
- large stresses
- repeated heating and cooling
- oxidation and corrosion
Special alloys are required.
Nickel-based superalloys are particularly important in hot sections of many turbine engines.
These alloys are designed to retain useful strength at temperatures where many ordinary metals would perform poorly.
Metals in Cookware
Metals are commonly used for cooking because many are good thermal conductors.
Important cookware metals include:
- aluminium
- copper
- stainless steel
Each has advantages and limitations.
Copper Cookware
Copper has excellent thermal conductivity.
This allows heat to spread quickly through the pan.
Advantages:
- rapid heat transfer
- responsive temperature control
Limitations:
- expensive
- relatively heavy
- reactive with some foods if the cooking surface is unlined
Copper cookware may therefore be lined with another material.
Aluminium Cookware
Aluminium is:
- lightweight
- a good thermal conductor
- relatively inexpensive
This makes it useful for cookware.
However, pure aluminium is relatively soft.
Aluminium cookware may therefore use alloys or surface treatments such as anodizing to improve durability.
Stainless Steel Cookware
Stainless steel is valued for:
- corrosion resistance
- strength
- durability
- relatively unreactive cooking surfaces
However, stainless steel is generally a poorer thermal conductor than aluminium or copper.
A common engineering solution is to combine materials.
A pan might contain:
stainless steel + aluminium + stainless steel
The aluminium improves heat transfer.
The stainless steel provides durability and corrosion resistance.
Metals in Electronics
Electronic devices contain many metals.
Examples include:
- copper
- gold
- silver
- aluminium
- tin
- nickel
Different metals perform different functions.
Copper:
conductive tracks and wires
Gold:
specialized corrosion-resistant contacts
Tin-based alloys:
solder connections
Aluminium:
structures, heat management, and some electrical applications
Why Is Gold Used in Electronics?
Gold is expensive, so it is not used everywhere.
However, it has valuable properties:
- good electrical conductivity
- excellent corrosion resistance
- chemical stability under many operating conditions
Thin layers of gold can therefore be useful on certain electrical contacts where reliable connections are important.
This is a case where a costly material can be justified because only a small amount is needed.
Metals for Heat Transfer
Many engineering systems need to transfer thermal energy efficiently.
Examples include:
- radiators
- heat exchangers
- computer cooling systems
- refrigeration systems
Copper and aluminium are commonly used because they conduct thermal energy effectively.
Aluminium is particularly useful for cooling fins because it combines:
- thermal conductivity
- low density
- relatively low cost
- ease of forming
Metals in Plumbing
Copper has traditionally been widely used for water pipes.
Useful properties include:
- corrosion resistance in many water systems
- ductility
- durability
- ease of joining
- useful strength
Stainless steel can also be used where greater corrosion resistance or strength is required.
However, modern plumbing systems may also use non-metal materials.
This demonstrates that metals compete with polymers, ceramics, composites, and other materials.
Metals in Tools
Tools often require:
- hardness
- strength
- toughness
- wear resistance
Steels are commonly used because their properties can be adjusted through:
- composition
- alloying
- heat treatment
A cutting tool must be hard enough to resist wear.
However, a material that is extremely hard but too brittle could fracture.
Therefore, designers must balance:
hardness + toughness
Metals in Medical Applications
Metals and alloys are used in:
- surgical instruments
- implants
- artificial joints
- dental devices
- bone screws
Materials used inside the body require careful selection.
Important properties can include:
- strength
- corrosion resistance
- compatibility with the body
- fatigue resistance
Titanium alloys are important for some implant applications because they combine useful mechanical properties with excellent corrosion resistance and good biocompatibility.
Metals in Jewellery
Jewellery materials must often provide:
- attractive appearance
- corrosion resistance
- workability
- durability
Gold, silver, platinum, and their alloys are widely used.
Pure gold is highly corrosion resistant but soft.
Gold is therefore often alloyed to improve hardness and durability.
Alloying can also change its colour.
Why Not Make Everything from Gold?
Gold has useful properties:
- low chemical reactivity
- corrosion resistance
- good conductivity
- malleability
But it also has major limitations:
- high cost
- high density
- relatively low hardness when pure
Therefore, it would be unsuitable or impractical for many structural applications.
A useful material must match the specific requirements of the design.
Metals in Food and Drink Containers
Aluminium is widely used for beverage cans.
Useful properties include:
- low density
- malleability
- corrosion resistance due partly to its protective oxide layer
- ease of forming
- recyclability
A thin aluminium sheet can be shaped into a lightweight container.
Why Aluminium Does Not Rapidly Corrode
Aluminium is a reactive metal, but it quickly forms a thin layer of aluminium oxide on its surface.
This layer adheres strongly and helps protect the underlying metal from further reaction.
This process is called passivation.
Therefore, aluminium can be both:
chemically reactive
and:
corrosion resistant in many everyday environments
These ideas are not contradictory.
Metals in Ships
Ships require materials that can withstand:
- large mechanical forces
- repeated loading
- seawater exposure
- impacts
- long periods of operation
Steel is widely used because of its strength, toughness, manufacturability, and cost.
However, seawater promotes corrosion.
Engineers therefore use methods such as:
- protective coatings
- corrosion-resistant materials
- sacrificial protection
- cathodic protection systems
The material and the protection system must be designed together.
Metals in Batteries
Many batteries depend on metals and metal compounds.
Examples of elements important in various battery technologies include:
- lithium
- nickel
- cobalt
- zinc
- lead
Their usefulness is closely related to their chemical behaviour, particularly their ability to participate in redox reactions involving electron transfer.
This is an example where a metal's chemical properties, rather than simply its mechanical properties, determine its application.
Metals in Renewable Energy
Metals are essential in many energy technologies.
Wind turbines may require:
- steel structures
- copper wiring
- specialized alloys
Solar installations may use:
- aluminium frames
- copper conductors
- silver in some electrical contacts
Electrical grids require large quantities of conductive metals.
Modern energy systems therefore depend heavily on materials science.
Selecting a Metal
Suppose an engineer needs to select a metal for a component.
The engineer first asks:
What must the component do?
Then identifies the necessary properties.
Possible requirements include:
- high strength
- low density
- electrical conductivity
- thermal conductivity
- corrosion resistance
- hardness
- toughness
- ductility
- high-temperature resistance
- low cost
The best material is the one that provides the most appropriate combination.
Example 1: Electrical Wire
Requirements:
- excellent conductivity
- ductility
- durability
- reasonable cost
Possible choice:
Copper
Why?
Copper provides excellent electrical conductivity and can easily be drawn into wires.
Example 2: Aircraft Component
Requirements:
- low mass
- high strength
- corrosion resistance
Possible materials:
- aluminium alloy
- titanium alloy
The better choice depends on the exact component.
Titanium may provide superior performance in some demanding locations but at greater cost.
Aluminium may be more economical and easier to manufacture.
Example 3: Cooking Pan
Requirements:
- thermal conductivity
- durability
- corrosion resistance
- suitable cooking surface
Possible solution:
multiple materials
For example:
stainless steel surfaces + aluminium core
This combines useful properties rather than relying on one material.
Example 4: Bridge Beam
Requirements:
- high strength
- toughness
- predictable behaviour
- economical production
A structural steel may be suitable.
However, engineers must also consider:
- corrosion protection
- fatigue
- temperature
- maintenance
- expected service life
Choosing "steel" is only the beginning of the design process.
Comparing Steel and Aluminium
Suppose either steel or aluminium alloy could be used for a vehicle component.
Steel
Advantages:
- high strength
- good toughness
- relatively inexpensive
- well-established manufacturing
Limitations:
- high density
- corrosion protection may be needed
Aluminium alloy
Advantages:
- low density
- corrosion resistance
- useful strength
Limitations:
- often more expensive
- different manufacturing and joining requirements
Neither is automatically better.
The decision depends on the design priorities.
Comparing Copper and Aluminium
For an electrical conductor:
Copper
- higher conductivity per cross-sectional area
- denser
- generally more expensive
Aluminium
- lower density
- lower conductivity per cross-sectional area
- often less expensive
For household wiring, copper may be highly practical.
For large overhead transmission lines, aluminium-based conductors can offer important mass and cost advantages.
The application changes the decision.
Properties Can Conflict
One of the greatest challenges in material selection is that improving one property may reduce another.
For example:
increasing hardness may reduce ductility
increasing strength may increase cost
reducing mass may increase material cost
alloying copper may increase strength but reduce conductivity
A material cannot be judged using only one property.
Engineering Trade-Offs
A trade-off occurs when improving one aspect of a design creates a disadvantage somewhere else.
For example, titanium alloys can provide excellent strength and corrosion resistance.
But titanium is relatively expensive.
Steel is less expensive but heavier.
Aluminium is lightweight but may not provide the required properties for every high-temperature or highly loaded component.
Engineering involves selecting the most appropriate compromise.
Cost Matters
Imagine two metals that could both perform the required job.
Metal A:
$5 per kg
Metal B:
$100 per kg
If Metal B provides no important performance advantage for that application, using it would be wasteful.
Material cost becomes especially important when producing:
- millions of vehicles
- kilometres of cable
- large buildings
- bridges
- consumer products
Good engineering considers both performance and economics.
Density Matters
Density is particularly important when objects must move.
Reducing mass can be valuable in:
- aircraft
- spacecraft
- cars
- bicycles
- portable electronics
This is why aluminium, magnesium, and titanium alloys can be attractive despite sometimes costing more than steel.
A lighter design may reduce the energy needed for transportation or improve performance.
Corrosion Resistance Matters
A strong metal is not useful if it rapidly deteriorates in its operating environment.
Engineers must consider exposure to:
- water
- oxygen
- salts
- acids
- high temperatures
- industrial chemicals
Possible solutions include:
- stainless steel
- aluminium
- titanium
- coatings
- galvanising
- sacrificial protection
Sometimes it is cheaper to protect an inexpensive metal than to build the entire structure from a more corrosion-resistant metal.
Strength-to-Mass Ratio
For transportation and aerospace systems, engineers often care about more than strength alone.
They consider:
strength relative to mass
A material that is slightly less strong but much lighter may be preferable.
This is one reason aluminium and titanium alloys are important engineering materials.
Environmental Considerations
Material selection also has environmental consequences.
Engineers may consider:
- energy required for extraction
- energy required for processing
- material availability
- product lifetime
- ability to reuse the material
- recyclability
Metals are often recyclable, but recycling still requires collection, separation, and processing.
Recycling Metals
Many metals can be recovered and reused.
Recycling can reduce:
- demand for newly mined ore
- waste
- some energy requirements
- environmental impacts associated with extraction
Commonly recycled metals include:
- aluminium
- steel
- copper
Designers increasingly consider what happens to materials at the end of a product's useful life.
Evaluating a Material
When evaluating whether a metal is suitable, avoid statements such as:
"Aluminium is good because it is lightweight."
A stronger evaluation considers both advantages and limitations:
"Aluminium alloys can be useful for vehicle components because their low density reduces mass while providing useful strength. However, they may cost more than conventional steel and can require different manufacturing techniques."
An evaluation should consider both sides of the decision.
A Material-Selection Strategy
When solving an engineering materials problem:
1. Identify the application.
What is being designed?
2. Identify the required properties.
Strength?
Conductivity?
Low density?
Corrosion resistance?
3. Identify possible metals or alloys.
4. Connect their structures to their properties where appropriate.
5. Compare the advantages.
6. Compare the limitations.
7. Consider operating conditions.
Temperature?
Moisture?
Repeated forces?
Electric current?
8. Consider manufacturing.
Can the material be shaped, welded, machined, or joined effectively?
9. Consider cost and sustainability.
10. Select the material that provides the most appropriate overall combination.
Common Mistakes
Mistake 1: Assuming all metals have the same properties
Different metals can have very different densities, strengths, conductivities, melting points, and chemical behaviours.
Mistake 2: Choosing a material based on one property
Real engineering decisions usually require several properties to be considered.
Mistake 3: Saying the strongest metal is automatically the best
Strength may be less important than density, conductivity, corrosion resistance, or cost.
Mistake 4: Assuming pure metals are always better
Alloys often provide more useful combinations of properties.
Mistake 5: Assuming alloys are always better
For some applications, a relatively pure metal may provide a property such as exceptionally high electrical conductivity.
Mistake 6: Ignoring cost
A material may perform extremely well but be economically impractical.
Mistake 7: Ignoring corrosion
Material behaviour must be considered in its actual environment.
Mistake 8: Confusing hardness with strength
Hardness describes resistance to indentation, scratching, or localized permanent deformation. Strength describes the ability to withstand applied stresses without failure or excessive deformation.
Mistake 9: Ignoring mass
Density is critical in transportation and aerospace design.
Mistake 10: Ignoring manufacturing
A material must not only have suitable properties; it must also be practical to form, join, repair, and manufacture.
Did You Know?
A modern aircraft, car, smartphone, or building does not use just one "best" metal.
Instead, engineers place different materials exactly where their particular properties are useful.
Copper may carry electrical current.
Aluminium may reduce mass.
Steel may provide structural strength.
Titanium may operate in demanding environments.
Gold may protect a critical electrical contact from corrosion.
The central question in materials engineering is therefore not:
"Which metal is best?"
It is:
"Which material has the best combination of properties for this particular job?"
Key Terms
- Application: A practical use for a material.
- Material selection: Process of choosing a suitable material for a particular design.
- Electrical conductivity: Ability to allow electric charge to move through a material.
- Thermal conductivity: Ability to transfer thermal energy.
- Strength: Ability to withstand applied stress without failure or unacceptable deformation.
- Hardness: Resistance to scratching, indentation, or localized permanent deformation.
- Toughness: Ability to absorb energy before fracturing.
- Malleability: Ability to be hammered or rolled into sheets.
- Ductility: Ability to be drawn into wires or undergo tensile deformation.
- Density: Mass per unit volume.
- Corrosion resistance: Ability to resist chemical deterioration.
- Alloy: Mixture containing a metal and one or more other elements.
- Trade-off: Situation where improving one property or feature involves accepting a disadvantage elsewhere.
- Strength-to-mass ratio: Comparison of a material's strength with its mass or density.
- Materials engineering: Application of knowledge about material structure and properties to engineering design.
Metal → Property → Application
Copper
High electrical conductivity + ductility
→ electrical wiring
High thermal conductivity
→ heat exchangers
Aluminium
Low density + corrosion resistance + malleability
→ cans and lightweight structures
Low density + useful strength when alloyed
→ vehicles and aircraft
Steel
High strength + toughness + relatively low cost
→ buildings, bridges, vehicles and machinery
Stainless steel
Strength + corrosion resistance
→ medical instruments, cookware and chemical equipment
Titanium alloys
High strength-to-mass ratio + corrosion resistance
→ aerospace and medical applications
Gold
Corrosion resistance + conductivity
→ specialized electrical contacts
Low reactivity + attractive appearance
→ jewellery
Key Takeaways
- Metals are used throughout transportation, construction, electronics, medicine, energy, manufacturing, and everyday products.
- The usefulness of a metal depends on its properties.
- Metallic bonding helps explain conductivity, malleability, ductility, and many mechanical properties.
- Copper is widely used for electrical wiring because it combines high conductivity with ductility.
- Aluminium is useful when low density is important.
- Steel is widely used where strength, toughness, manufacturability, and cost are important.
- Stainless steel is useful where strength and corrosion resistance are required.
- Titanium alloys provide useful strength-to-mass ratios and corrosion resistance but can be expensive.
- Gold's corrosion resistance makes small quantities useful in some electrical contacts.
- Thermal conductivity makes metals useful in cookware and heat-transfer systems.
- Different metals are often combined within one product because no single material provides every desired property.
- Alloys allow engineers to modify the properties of metals.
- The material with the best single property is not necessarily the best overall choice.
- Engineers must consider advantages and limitations.
- Important design considerations include strength, density, conductivity, corrosion resistance, temperature resistance, manufacturability, cost, durability, and sustainability.
- Material selection often involves trade-offs.
- A useful engineering reasoning chain is:
application → required properties → candidate materials → advantages and limitations → trade-offs → material selection → design performance.