Answer: Alloying improves hardness and strength, corrosion resistance, castability and other properties, and can lower the melting point or change colour; pure metals are often too soft, reactive or weak.
Pure metals are usually soft, ductile and easily corroded, and their properties cannot be tuned. Alloying changes these properties. Purposes:
- To increase hardness and tensile strength: e.g. pure iron is soft; steel (Fe + up to about 1.5% C) is much harder. Gold for jewellery is hardened with Cu.
- To increase corrosion resistance: stainless steel (Fe + 18% Cr + 8% Ni) does not rust.
- To lower the melting point: solder (Pb–Sn) melts below either metal; Wood's metal melts below 100 °C, used in fuse wire and sprinklers.
- To improve castability: a good casting expands slightly on solidifying; type metal (Pb–Sb–Sn) is used for this.
- To change colour or appearance: brass (Cu + Zn) has a golden look; aluminium bronze is gold-coloured.
- To change electrical or magnetic properties: nichrome has high resistance for heaters; alnico makes strong permanent magnets.
