📖 In-Depth Theory
Why Alloys Are More Useful Than Pure Metals
Pure metals often have properties that limit their usefulness:
Pure iron is TOO SOFT — deforms easily under stress.
Pure gold is TOO SOFT for jewellery — scratches easily.
Pure aluminium is TOO WEAK for aircraft.
ALLOYS are mixtures of metals (or metal + non-metal) — mixing changes properties.
WHY ALLOYS ARE HARDER:
In a pure metal, layers of atoms slide over each other easily (metallic bonding — sea of electrons).
In an alloy, different-sized atoms DISRUPT the regular lattice — layers can't slide as easily.
Result: HARDER, STRONGER, MORE RESISTANT to deformation.
EXAMPLE — iron and steel:
Pure iron: soft, weak, few uses.
Add carbon (0.1–1.5%): STEEL — harder, stronger, less malleable.
Different amounts of carbon give different properties:
Low carbon steel (<0.3%): tough, ductile, used in car body panels.
High carbon steel (>0.6%): harder, brittle, used in cutting tools.
Important Alloys
STEEL:
Iron + carbon (+ other elements for specialist steels).
Stainless steel: iron + chromium (~18%) + nickel (~8%) — resists corrosion, used in cutlery, surgical instruments, kitchen equipment.
High-speed steel: iron + tungsten — maintains hardness at high temperatures, used in drill bits and cutting tools.
BRONZE:
Copper + tin — harder than copper alone.
Used historically for weapons and tools; now: bearings, sculptures, bells, musical instruments (cymbals).
BRASS:
Copper + zinc — harder than copper, good acoustic properties.
Used in: musical instruments (trumpets, trombones), taps, valves, decorative fittings.
GOLD ALLOYS:
Pure gold (24 carat) is too soft for jewellery.
18 carat gold = 75% gold + 25% silver/copper/zinc.
9 carat gold = 37.5% gold — cheaper, more durable.
Different alloys give different colours: yellow, white, rose gold.
ALUMINIUM ALLOYS:
Pure aluminium: light but weak.
Aluminium + copper/magnesium/silicon: much stronger, still lightweight.
Used in aircraft, bicycle frames, cars.
Alloy Design and Properties
CHOOSING ALLOYS:
Properties can be tuned by varying composition:
More carbon in steel → harder but more brittle.
More chromium in stainless steel → better corrosion resistance.
More zinc in brass → harder but less malleable.
COMMERCIAL APPLICATIONS:
COINS: cupronickel (copper + nickel) — durable, resistant to corrosion.
'GOLD' in electronics: gold is used in connectors — excellent conductor, doesn't corrode, but expensive.
SHAPE MEMORY ALLOYS: nitinol (nickel + titanium) — return to original shape when heated, used in medical stents and eyeglass frames.
PURITY AND CARATS:
Gold purity is measured in CARATS:
24 carat = 100% gold (pure).
18 carat = 75% gold.
9 carat = 37.5% gold.
Higher carat = softer and more expensive.
⚠️ Common Mistake
Alloys are HARDER than pure metals because different-sized atoms DISRUPT the regular lattice — they prevent layers sliding over each other. An alloy is NOT simply a compound — the components are not chemically combined in fixed ratios. Stainless steel contains iron + CHROMIUM (not carbon) for corrosion resistance.