Alloys
Section: 9. Metals | Syllabus: Cambridge AS Level Physics 9702
What are Alloys? Alloy: A mixture of two or more metals (or a metal with a non-metal) that has metallic properties. Key Point: Alloys are NOT compounds - they are mixtures. The elements are mixed together but not chemically bonded.
Why Make Alloys? Pure metals often have properties that limit their usefulness. Alloys are made to improve these properties: Increased strength: alloys are usually much stronger than pure metals Increased hardness: alloys are harder and more resistant to wear Improved resistance to corrosion: some alloys resist rusting and corrosion better Lower melting point: some alloys melt at lower temperatures Better appearance: some alloys have more attractive colors or finishes Reduced cost: mixing expensive metals with cheaper ones reduces cost Why are Alloys Stronger than Pure Metals?
Structure of Pure Metals In a pure metal, all atoms are the same size and arranged in regular layers. These layers can slide over each other easily when a force is applied, making the metal soft and malleable.
Pure metal structure: Regular layers of identical atoms → layers slide easily → metal is soft Structure of Alloys In an alloy, atoms of different sizes are mixed together. The different-sized atoms disrupt the regular arrangement and make it much harder for the layers to slide over each other.
Alloy structure: Different-sized atoms disrupt regular layers → layers cannot slide easily → alloy is harder and stronger Common Alloys and Their Uses 1. Steel (Iron + Carbon) Steel is the most widely used alloy in the world.
It is iron mixed with a small amount of carbon (0.1-2%) and sometimes other metals. Type of Steel Composition Properties Uses Mild Steel Iron + 0.1-0.3% carbon Strong, malleable, easily welded, rusts easily Car bodies, machinery, nails, wire, construction girders High Carbon Steel Iron + 0.6-1.5% carbon Very hard, strong, brittle, difficult to weld Cutting tools, drill bits, knives, chisels Stainless Steel Iron + chromium + nickel Hard, strong, resistant to corrosion and rusting Cutlery, surgical instruments, sinks, saucepans, chemical plant equipment 2.
Brass (Copper + Zinc) Composition: 70% copper + 30% zinc (approximately) Properties: harder than copper, does not corrode, attractive gold color, easy to shape Uses: musical instruments, door handles, ornaments, screws, electrical plugs 3.
Bronze (Copper + Tin) Composition: copper + tin (typically 88% Cu + 12% Sn) Properties: harder than copper, resistant to corrosion, good for casting Uses: statues, medals, bells, coins, ship propellers, bearings Historical Note: Bronze was so important in human history that an entire era was named after it - the Bronze Age (around 3000-1200 BCE).
4. Solder (Lead + Tin or Tin + Copper) Composition: traditionally lead + tin (now often tin + copper for safety) Properties: low melting point (around 200-300°C), good electrical conductor Uses: joining electrical components, joining metals in plumbing Safety: Modern solder often uses tin-copper alloys instead of lead-tin to avoid lead poisoning risks.
5. Duralumin (Aluminum Alloy) Composition: aluminum + copper + magnesium + manganese Properties: strong as steel but much lighter, resistant to corrosion Uses: aircraft bodies, spacecraft, bicycle frames, high-speed trains 6.
Amalgam (Mercury + Other Metals) Composition: mercury + silver + tin + copper + zinc Properties: soft when first mixed, hardens when it sets, expands slightly when setting to fill gaps Uses: dental fillings (though being phased out due to mercury concerns) 7.
Pewter (Tin Alloy) Composition: tin + copper + antimony Properties: soft, easy to cast and shape, attractive silvery appearance Uses: decorative objects, tankards, jewelry, ornaments 8. Cupronickel (Copper + Nickel) Composition: 75% copper + 25% nickel Properties: hard, resistant to corrosion from seawater, silvery color Uses: "silver" coins, marine engineering equipment, condenser tubes Comparison: Pure Metals vs.
Alloys Property Pure Metals Alloys Strength Usually softer and weaker Usually harder and stronger Hardness Relatively soft Much harder Melting Point Fixed melting point Usually lower, melts over a range Electrical Conductivity Better conductors Usually poorer conductors Cost May be expensive Often cheaper Uses Limited (e.g., electrical wiring) Very wide range of uses Shape Memory Alloys Modern Development: Some special alloys have unusual properties that make them useful for advanced applications.
Shape memory alloys can "remember" their original shape. If you bend or deform them, heating them makes them return to their original shape. Nitinol (Nickel-Titanium) Composition: approximately 50% nickel + 50% titanium Special property: returns to original shape when heated Uses: Glasses frames (bend back into shape if deformed) Dental braces (use body heat to apply gentle constant pressure) Medical stents (inserted small, expand to correct size in body) Temperature-sensitive switches and valves Key Points Summary An alloy is a mi…
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