In one line
Alloys are made to improve properties of a pure metal, such as hardness, strength, corrosion resistance, melting point, castability or colour, to suit a particular use.
Explained simply
An alloy is a metallic mixture (homogeneous solid solution or mixture) of two or more elements, at least one of which is a metal. Pure metals are often too soft, rust easily or melt at the wrong temperature. Mixing in another element is like adding ingredients to improve a recipe. Pure iron is soft; add a little carbon and you get steel, which builds bridges.
How it works
Purposes of making alloys, with examples:
- To increase hardness and strength: foreign atoms of different size distort the crystal lattice and block the sliding of atom layers. Steel (Fe + C) is much harder than pure iron; adding Cu to gold makes jewellery gold (e.g., 22-carat) harder.
- To lower the melting point: solder (Sn–Pb) melts lower than either metal and is used to join electrical parts; Wood's metal (Bi, Pb, Sn, Cd) melts at about 70 °C and is used in fire sprinkler plugs and safety fuses.
- To increase corrosion resistance: stainless steel (Fe with at least about 11–12% Cr, often Ni) forms a passive chromium oxide film; brass and bronze resist corrosion better than copper in many conditions.
- To improve castability: an alloy that expands slightly on solidifying fills the mould sharply; type metal (Pb–Sb–Sn) was used to cast printing type.
- To modify colour: brass (Cu–Zn) is yellow; aluminium bronze (Cu–Al) is golden and used in imitation jewellery and coins.
- To change electrical or magnetic properties: nichrome (Ni–Cr) has high resistance and resists oxidation, used in heating elements; alnico (Al–Ni–Co) makes permanent magnets.
- To reduce weight with strength: duralumin (Al with Cu, Mg, Mn) is light and strong for aircraft parts.
Key formula
No formula. To study how composition and temperature control an alloy's phases, we use the phase rule and phase diagrams (next topics).
Quick example
Kitchen sinks and spoons are stainless steel rather than plain steel because the Cr film prevents rusting in wet, salty, acidic conditions.
Common mistakes
- Saying alloying always increases melting point. Often it lowers it (solder).
- Defining an alloy as a compound; it is a mixture or solid solution, not a fixed-formula compound (though intermetallic compounds can form within some alloys).
- Giving purposes without an example; marks are usually given per purpose-with-example.
How MU asks it
"What are alloys? Explain the purposes of making alloys with examples." (3–5 marks). For full marks: definition, at least 5 purposes each with one named alloy and its composition or use, and a line on why hardness increases (lattice distortion).