Comparison: metals vs alloys; composites

Chemistry · Senior Secondary (HKDSE) · 5 min read · Updated: 21 Aug 2026

Metals, Alloys, and Composites: Engineered for Performance

Hello! Ever wondered why the steel frame of a skyscraper is so strong, why a trumpet is made of shiny brass instead of pure copper, or how modern aircraft can be so light yet incredibly tough? The answers lie in the amazing world of metals, alloys, and composite materials.

In these notes, we're going to explore the microscopic structures of pure metals and see how creating alloys and composites allows materials scientists to overcome the limitations of pure substances to design materials with custom-tailored properties.

1. The Structure of Pure Metals: A World of Order

Before we dive into how metals are structured, let's have a quick refresh on what holds them together.

Quick Review: Metallic Bonding

Imagine a box of marbles submerged in honey. The marbles are like the positive metal ions, and the honey is like a "sea" of freely moving delocalised electrons.

This "sea of electrons" holds all the positive ions together in a strong but flexible electrostatic attraction known as metallic bonding.
This structure explains why metals conduct electricity (delocalised electrons can move freely) and are malleable/ductile (layers of ions can slide past each other without breaking the metallic bond).

The Crystal Lattice: A Metal's Blueprint

Metal atoms arrange themselves in a regular, repeating three-dimensional pattern called a crystal lattice. The smallest repeating unit of this pattern is called the unit cell.

How Metal Atoms Pack Together

In metallic crystals, atoms pack tightly together in distinct packing geometries:

Style 1: Close-Packed Structures (Maximum Packing Efficiency)

In close-packed arrangements, each sphere touches 12 immediate neighbours, giving a coordination number of 12.

Hexagonal Close-Packed (hcp): Stacking sequence is \( \text{ABAB}\dots \). Found in metals like magnesium (\(\text{Mg}\)) and zinc (\(\text{Zn}\)).
Cubic Close-Packed (ccp): Also known as Face-Centred Cubic (fcc). Stacking sequence is \( \text{ABCABC}\dots \). Found in metals like copper (\(\text{Cu}\)), aluminium (\(\text{Al}\)), and silver (\(\text{Ag}\)).

Style 2: Open Structure

Some metals pack in a slightly less dense arrangement:

Body-Centred Cubic (bcc): Has one atom at each of the 8 corners and one central atom. The coordination number is 8. This open structure is found in metals like iron (\(\text{Fe}\)) and sodium (\(\text{Na}\)).

2. What are Alloys? A Recipe for Better Metals

The Basic Idea

An alloy is a mixture of a metal with at least one other element (which can be another metal or a non-metal).

Crucially, an alloy is a mixture, not a chemical compound. The atoms are not bonded in a fixed stoichiometric ratio. Metallic bonding is maintained throughout, but the lattice now contains atoms of differing atomic radii.

Lattice Distortion: Why Alloys are Harder

In a pure metal, the atoms are of identical size, forming uniform, smooth layers that easily slip over one another under stress.

When foreign atoms of different sizes are introduced into the lattice:

The regular arrangement of the crystal lattice becomes distorted.
The distorted lattice prevents the layers of metal ions from sliding over each other easily.
This makes the alloy significantly harder and stronger than the parent pure metal.

3. Metals vs. Alloys: Property Comparisons

Property 1: Hardness and Mechanical Strength

Pure Metals: Softer and more ductile/malleable because uniform layers slip easily.
Alloys: Harder, stronger, and less malleable due to lattice distortion impeding layer slippage.

Property 2: Electrical Conductivity

Pure Metals: High electrical conductivity; delocalised electrons travel with minimal resistance through the orderly lattice.
Alloys: Lower electrical conductivity than pure parent metals. The structural irregularities and foreign atoms scatter the conduction electrons, increasing electrical resistance.

Property 3: Corrosion Resistance

Many pure metals (e.g., pure iron) corrode easily in the presence of air and water.
Alloying elements can form a stable, adherent oxide layer on the surface (such as chromium in stainless steel), greatly enhancing corrosion resistance.

Everyday Alloys

Steel: Mainly iron (\(\text{Fe}\)) with a small percentage of carbon (\(\text{C}\)). Much harder and stronger than pure iron; used in construction and structural frames. Stainless steel contains added chromium (\(\text{Cr}\)) and nickel (\(\text{Ni}\)) to prevent rusting.
Brass: An alloy of copper (\(\text{Cu}\)) and zinc (\(\text{Zn}\)). Harder than copper, corrosion-resistant, and acoustically resonant; used for plumbing fittings and musical instruments.
Bronze: An alloy of copper (\(\text{Cu}\)) and tin (\(\text{Sn}\)). Hard, tough, and wear-resistant; used in sculptures, bearings, and medals.

4. Composite Materials: Combining Strengths

What is a Composite Material?

A composite material consists of two or more distinct materials with significantly different physical or chemical properties. Unlike alloys (which are atomic-scale mixtures), the constituent materials in a composite remain separate and distinct at a macroscopic or microscopic level within the finished structure.

The Two Essential Phases

Composites are engineered by combining two main phases:

Matrix Phase: The continuous phase that surrounds, binds, and protects the reinforcement. It holds the reinforcement in place and distributes applied loads (e.g., polymer resin, cement, or ceramic).
Reinforcement (Dispersed Phase): The strong, rigid material embedded in the matrix to provide high tensile strength and stiffness (e.g., glass fibres, carbon fibres, or steel rods).

Why Use Composites? (Synergy of Properties)

Neither component on its own has all the desired properties, but combined, they produce superior performance:

The reinforcement gives high tensile strength and rigidity.
The matrix provides bulk, compressive resistance, ductility, and protection against environmental degradation.

Common Examples of Composites

Reinforced Concrete:
- Matrix: Concrete (strong under compression, but brittle and weak under tension).
- Reinforcement: Steel reinforcing bars/mesh (high tensile strength).
- Use: Bridges, high-rise buildings, and dams.
Glass-Fibre Reinforced Plastic (GRP / Fibreglass):
- Matrix: Polymer resin (lightweight, flexible, corrosion-resistant).
- Reinforcement: Woven glass fibres (high tensile strength).
- Use: Boat hulls, water storage tanks, and car body panels.
Carbon-Fibre Reinforced Polymer (CFRP):
- Matrix: Epoxy resin.
- Reinforcement: High-strength carbon fibres.
- Properties & Use: Exceptional strength-to-weight ratio and stiffness; used in aerospace components, high-performance racing cars, and premium sporting goods (e.g., tennis rackets and bicycle frames).

5. Summary: Metals vs. Alloys vs. Composites

Pure Metals: Regular crystal lattice (hcp, ccp/fcc, or bcc), high electrical/thermal conductivity, malleable, but often too soft for demanding mechanical applications.
Alloys: Atomic-level mixtures of metals with other elements; lattice distortion prevents layer slip, giving higher strength and hardness but reduced electrical conductivity.
Composites: Macroscopic combination of a binder (matrix) and structural reinforcement (fibres/particles), delivering optimised properties such as a high strength-to-weight ratio.

References

Science Education – Chemistry — Education Bureau (EDB), HKSAR
Updates for the Chemistry Curriculum Compulsory Part Topic III Metals & Topic XIV Materials Chemistry — Curriculum Development Council / EDB

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is metallic bonding and what microscopic features account for a metal's electrical conductivity and malleability?

Metallic bonding is the strong electrostatic attraction between positive metal ions and a "sea" of delocalised electrons. Free movement of delocalised electrons enables electrical conductivity, while uniform layers of ions sliding past each other without breaking bonds accounts for malleability.

What are the differences in stacking sequence and coordination number between close-packed structures and body-centred cubic structures in pure metals?

Close-packed structures (hcp: \(\text{ABAB}\dots\) and ccp/fcc: \(\text{ABCABC}\dots\)) have a coordination number of 12 for maximum packing efficiency, whereas body-centred cubic (bcc) is an open structure with a coordination number of 8.

What is an alloy, and how does lattice distortion explain why alloys are harder than pure metals?

An alloy is a mixture of a metal with other elements; foreign atoms of different sizes distort the regular crystal lattice, preventing layers of ions from sliding past one another easily and thus increasing hardness and strength.

Why do alloys exhibit lower electrical conductivity than their pure parent metals?

The lattice distortion and presence of foreign atoms in an alloy scatter conducting delocalised electrons, increasing electrical resistance.

What are the key constituents and main uses of common alloys such as steel, brass, and bronze?

Steel is iron with carbon (plus Cr/Ni in stainless steel) used in construction; brass is copper and zinc used in musical instruments and fittings; bronze is copper and tin used in sculptures and bearings.

How does a composite material differ from an alloy at the structural level?

While alloys are atomic-level mixtures, composites consist of distinct, separate phases (matrix and reinforcement) that remain physically and chemically distinguishable at macroscopic or microscopic levels.

What are the respective roles of the matrix phase and the reinforcement phase in a composite material?

The reinforcement phase provides high tensile strength and stiffness, while the matrix phase binds and surrounds the reinforcement, distributes applied loads, and protects it from environmental damage.

What are the matrix and reinforcement components of reinforced concrete, fibreglass (GRP), and carbon-fibre reinforced polymer (CFRP)?

Reinforced concrete combines a concrete matrix with steel rebar; GRP uses a polymer resin matrix with woven glass fibres; CFRP uses an epoxy resin matrix with high-strength carbon fibres.

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