Alloys and Metal Properties
~8 min read
- Definition: Alloy = homogeneous mixture of two or more metals (or metal + non-metal).
- Common alloys: Brass (Cu + Zn), Bronze (Cu + Sn), Steel (Fe + C), Stainless steel (Fe + Cr + Ni), Duralumin (Al + Cu + Mg + Mn).
- Purpose: Stronger, harder, corrosion-resistant or lower melting point than pure metals.
Alloys combine the best properties of their constituents. CDS/OTA tests common alloys, their composition and the everyday uses each supports.
Why Alloy?
- Pure metals are often too soft, too reactive or too prone to corrosion for engineering use.
- Alloying changes hardness, tensile strength, ductility, corrosion resistance, melting point and electrical resistance.
- Examples — pure gold is too soft; mixing with copper makes 22-carat jewellery durable. Pure iron rusts and is soft; carbon and chromium give steel and stainless steel.
Common Alloys
| Alloy | Composition | Use |
|---|---|---|
| Brass | Cu ~70% + Zn ~30% | Utensils, decorative items, fittings, instruments |
| Bronze | Cu ~88% + Sn ~12% | Statues, medals, bearings, bells |
| Steel | Fe + C (0.2-2%) | Construction, rails, machinery |
| Stainless steel | Fe + Cr (18%) + Ni (8%) + C | Cutlery, surgical instruments, kitchenware |
| Duralumin | Al + Cu (4%) + Mg + Mn | Aircraft body, light strong components |
| Magnalium | Al + Mg | Aircraft and automobile parts |
| German silver | Cu + Zn + Ni (no silver) | Utensils, decorative items, resistance wires |
| Solder | Pb + Sn (typically 50-50 or 60-40) | Joining electrical wires; low MP |
| Amalgam | Mercury + other metal (Hg + Ag for dental) | Dental fillings |
| Gunmetal | Cu + Sn + Zn | Gears, bearings, valves |
| Nichrome | Ni + Cr + Fe | Heating elements in iron, toaster, heater |
| 22-carat gold | Au 91.6% + Cu/Ag | Jewellery |
Steel — A Closer Look
- Mild steel: 0.05-0.25% C — soft, ductile, used in nails, wires, sheets.
- Medium-carbon steel: 0.3-0.6% C — rails, machinery.
- High-carbon steel: 0.6-1.5% C — cutlery, knives, springs, drills.
- Stainless steel: 18% Cr + 8% Ni — resists rust; used in surgical instruments, kitchenware.
- Tool steel: includes tungsten, vanadium for hardness at high temperature.
CDS/OTA PYQ Examples
Q: Brass is an alloy of:
(a) Cu + Sn (b) Cu + Zn (c) Fe + C (d) Al + Cu
Answer: (b) Copper + Zinc.
Q: Stainless steel contains:
(a) Fe + C (b) Fe + Cr + Ni (c) Al + Cu (d) Cu + Sn
Answer: (b) Fe + Cr (18%) + Ni (8%).
Q: Duralumin is used in aircraft because it is:
(a) Heavy and strong (b) Light and strong (c) Cheap and brittle (d) Magnetic
Answer: (b) Light (Al-based) and strong.
Q: Amalgam is an alloy of:
(a) Mercury and another metal (b) Tin and lead (c) Iron and carbon (d) Copper and zinc
Answer: (a) Mercury + another metal — used in dental fillings.
Q: German silver does NOT contain:
(a) Copper (b) Zinc (c) Nickel (d) Silver
Answer: (d) Silver — name is misleading.
Drill Alloys and Metal Properties for CDS/OTA
CDS/OTA-pattern items on Alloys and Metal Properties with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is pure gold not used for jewellery?
Pure (24 carat) gold is too soft. Mixing with copper or silver (22 carat or lower) gives the durability needed for daily wear.
Why is stainless steel used in surgical instruments?
Resists rust, can be sterilised by autoclaving (high steam pressure), is non-reactive and easily polished.
Why is solder a lead-tin alloy?
Both metals melt at low temperature; the alloy melts even lower (eutectic ~183 °C for 63/37 Sn-Pb), so it can be melted with a soldering iron without damaging delicate components.