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CHEMISTRY · Stage 7 · Chapter 3: Metals and Non-Metals

Chemistry 3.3 Amazing alloys — Steel, Brass, Bronze and Particle Structure

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3.3 Amazing alloys — Steel, Brass, Bronze and Particle Structure — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define alloy and steel, compare the properties of alloys with the elements in them and explain why alloys are harder than pure metals.Alloy = a mixtur…

  • Mixture, not compound
  • Why alloys are needed
  • Pure aluminium
  • Alloy 7075
  • Types of steel
  • Gold jewellery

Section 1 of 6What an alloy is

  • Alloy = a mixture of a metal with one or more other elements, usually other metals.
    • Alloys are made by melting the metals together and letting the mixture cool.
  • Mixture, not compound
    • the parts are not chemically combined in a fixed ratio, so the proportions can vary.
  • Why alloys are needed
    • pure metals are often too soft, too weak or too reactive.
    • Pure iron is soft and rusts, and pure copper, gold and aluminium bend easily.

Section 2 of 6Aluminium and its alloys

  • Pure aluminium
    • it is light for its size, which suits aeroplanes because lighter aeroplanes burn less fuel, but it is not strong and not hard.
    • It breaks easily and is easy to scratch and dent.
  • Alloy 7075
    • it is about 90% aluminium with 6% zinc, 2% magnesium, 1% copper and 1% other metals.
    • Relative hardness rises from 23 for pure aluminium to 150.
    • Strength when pulled rises from 8 MPa to 572 MPa.
  • Duralumin
    • aluminium with copper is strong and light, and is used in aircraft.

Section 3 of 6Steels

  • Steel = an alloy of iron with small amounts of carbon, and in some cases other metals.
    • Iron is plentiful and cheap, but iron alone is too soft and bendy to be useful.
  • Types of steel
    • each has its own properties and uses.
AlloyOther elementsPropertiesUses
Low carbon steelCarbonStrong, easily shapedBridges, buildings, ships, vehicles
Manganese steelManganese, carbonHard, toughMining equipment, railway points
Stainless steelChromium, nickel, carbonDoes not rustKnives and forks, surgical instruments

Section 4 of 6Other common alloys

AlloyMade fromProperties and uses
BrassCopper and zincHard, shiny, easy to shape; taps, instruments
BronzeCopper and tinHard, resists corrosion; statues, bells, ship parts
SolderTin and lead (or lead-free)Low melting point; joining electrical wires
PewterTin with antimony and copperUsed for tankards
AmalgamMercury with silver and tinDental fillings
  • Bronze
    • it is an ancient alloy first used about 6000 years ago to make tools and weapons, and it is harder than the stone used before.
    • Bronze motors and gears do not need oiling because bronze has low friction with other metals.
  • Gold jewellery
    • pure gold is 24 carat but too soft, so 18 carat gold contains 75 per cent gold mixed with copper or silver.

Section 5 of 6Why alloys are harder

  • Pure metal
    • the atoms are the same size and arranged in layers that slide over each other easily, so the metal is soft and not very strong.
  • Alloy
    • atoms of another element have a different size and disturb the regular layers.
    • In steel, smaller carbon atoms get between the iron atoms.
    • The layers cannot slide as easily, so more force is needed to deform the material.
    • This makes the alloy harder and stronger.
  • Diagrams
    • a pure metal shows identical circles in neat rows, and an alloy shows circles of different sizes mixed in.
  • Chemical properties
    • an alloy may also differ chemically, since pure iron rusts but stainless steel does not.
  • Conductivity
    • an alloy keeps metallic bonding and still conducts electricity, but usually less well than the pure metal.

Section 6 of 6Choosing alloys

  • Aircraft
    • they need strong but low-density materials, so aluminium alloys are chosen.
  • Ship propellers
    • they must resist sea water, so bronze is used.
  • Comparing materials
    • use strength, density, cost, corrosion resistance and appearance, and justify a choice with at least two.
  • Smart alloys
    • shape memory alloys such as nitinol return to their original shape when heated and are used in spectacle frames and medical stents.
      • (nickel and titanium)

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