Metals: Classification, Sources & Properties
Classifying Metals
Metals are one of the most versatile material families available to designers. At GCSE level they are divided into three broad groups: ferrous metals, non-ferrous metals, and alloys. Understanding this classification is the starting point for selecting the right material for any product.
Ferrous metals contain iron as their principal element. The iron content makes most of them magnetic and gives them a tendency to rust when exposed to moisture and oxygen. The most common ferrous metals encountered in GCSE Design & Technology are:
- Cast iron — very hard and brittle, excellent compressive strength; used for engine blocks and heavy machine bases.
- Mild steel — low carbon content (0.1–0.3%); tough, ductile and easy to weld; the most widely used engineering metal, found in car bodies, structural beams and sheet-metal products.
- Medium carbon steel — 0.3–0.6% carbon; stronger and harder than mild steel but less ductile; used in axles, crankshafts and railway track.
- High carbon steel — 0.6–1.4% carbon; very hard but brittle; used for cutting tools, springs and piano wire.
- Stainless steel — iron alloyed with at least 10.5% chromium; resists corrosion because a thin oxide layer forms on the surface; used in kitchen sinks, cutlery and surgical instruments.
- High-speed steel (HSS) — contains tungsten, chromium and vanadium; retains its hardness at high temperatures; used for drill bits and lathe cutting tools.
- High-tensile steel — alloyed to achieve exceptional tensile strength; used in suspension bridges, pre-stressed concrete and aircraft undercarriages.
Non-Ferrous Metals
Non-ferrous metals contain little or no iron. They do not rust (though some corrode in other ways), they are generally non-magnetic, and many have lower densities than steel. Key non-ferrous metals include:
- Aluminium — low density (2.7 g/cm³), good corrosion resistance, excellent conductor; used in aircraft structures, drink cans and window frames.
- Copper — excellent electrical and thermal conductor; malleable and ductile; used in electrical wiring, water pipes and printed circuit boards.
- Zinc — low melting point, good corrosion resistance; mainly used to galvanise steel (coating it to prevent rust) and in die casting.
- Tin — soft, low melting point, very good corrosion resistance; used to coat steel food cans (‘tin cans’) and as a component of solder.
Alloys are mixtures of two or more elements where at least one is a metal, engineered to combine the best properties of their constituents. Brass (copper + zinc) is harder and more workable than pure copper and resists corrosion well; it is used for taps, door handles and musical instruments. Duralumin (aluminium + copper + magnesium) is far stronger than pure aluminium while remaining lightweight, making it essential in aerospace.
Increasing the carbon content of steel increases its hardness but reduces its ductility. Mild steel has around 0.1–0.3% carbon and is highly ductile; high carbon steel has up to 1.4% carbon, making it hard and brittle. Adding alloying elements such as chromium produces stainless steel, which resists corrosion through a self-forming oxide layer.
Sources: Metals from Ores
All primary metals are extracted from ores — naturally occurring mineral deposits found in the Earth’s crust. Iron ore (haematite) is smelted with coke and limestone in a blast furnace to produce pig iron, which is then refined into steel. Aluminium is extracted from bauxite by electrolysis, a far more energy-intensive process. Because ores take millions of years to form, metals are a finite, non-renewable resource. At current extraction rates, reserves of some metals such as indium and gallium are projected to be depleted within decades.
Mining causes significant ecological damage: open-cast mining strips away topsoil and destroys habitats; tailings (waste rock) can leach heavy metals into waterways; smelting releases carbon dioxide and sulphur dioxide, contributing to climate change and acid rain. Socially, mining operations in lower-income countries are sometimes associated with poor working conditions, child labour and community displacement. These concerns form part of the lifecycle assessment of any metal product. Designers can reduce this footprint by specifying recycled metals wherever possible — recycled aluminium uses only 5% of the energy needed to smelt virgin aluminium from bauxite.
For exam questions on sustainability and metals, you should be able to link the finite nature of ores to the importance of recycling, and identify specific ecological harms of extraction (habitat loss, water pollution, carbon emissions) rather than giving vague answers about “harming the environment”.
Physical and Mechanical Properties
When selecting a metal, designers consider two categories of property. Physical properties describe how a material behaves without forces being applied:
- Melting point — the temperature at which the solid becomes liquid. Tungsten has the highest melting point of any metal (3,422 °C); tin melts at only 232 °C. High melting points are required for furnace components; low melting points enable die casting.
- Thermal conductivity — how readily heat passes through the material. Copper and aluminium are excellent thermal conductors; stainless steel is a poorer conductor, which is why it is used for saucepan handles when the body is copper.
- Electrical conductivity — copper has the highest electrical conductivity of commonly used metals, making it the standard choice for wiring. Aluminium is used where weight matters (overhead power cables).
- Density — mass per unit volume (g/cm³). Aluminium (2.7 g/cm³) is about one-third the density of steel (7.85 g/cm³), making it preferable where weight reduction is critical.
Mechanical properties describe how a material responds to applied forces:
- Tensile strength — resistance to being pulled apart. High-tensile steel cables support suspension bridges precisely because of this property.
- Toughness — the ability to absorb impact energy without fracturing. Mild steel is tough; cast iron is not — it shatters under impact.
- Malleability — the ability to be shaped by compression (hammering, rolling) without cracking. Gold and lead are the most malleable metals; copper and aluminium are also highly malleable.
- Ductility — the ability to be drawn out into a wire without breaking. Copper’s excellent ductility is central to its use in electrical wiring.
- Hardness — resistance to surface scratching or indentation. High carbon steel and HSS are very hard; this is why they are used for cutting tools that must scratch or cut other materials.
Malleability and ductility are frequently confused. Malleability is the ability to be shaped by compressive force (hammering or rolling); ductility is the ability to be drawn into a wire by tensile force. A metal can be malleable without being very ductile — lead is highly malleable but only moderately ductile.
Pure metals often have a single outstanding property but fall short in others — pure aluminium is lightweight and corrosion-resistant but lacks the strength needed for structural applications. By adding small amounts of other elements, designers can engineer an alloy that combines the best qualities of its constituents: duralumin (aluminium + copper + magnesium) achieves tensile strength close to mild steel while retaining aluminium’s low density, making it the material of choice for aircraft frames. This engineering flexibility is why alloys dominate modern manufacturing far more than pure metals.