4.2A - Metals and metallic alloys

Grain size:

  • The grain size can vary and be determined by the heat treatment.

  • Reheating a solid metal or alloy allows the grains to change their structure.

  • Slow cooling forms larger grains

  • Rapid cooling (quench) forms smaller grains.

  • Directional properties can be formed by selectively cooling one area of the solid.

  • Grain size in metals can affect the properties:

    • tensile strength – coarse grains the stronger a metal

    • toughness – large or coarse grains the tougher the metal

    • ductility – coarse grains the more ductile a metal

    • brittleness – the smaller the grains the more brittle the metal

    • flexibility/malleability – coarse grains the more malleable a metal

    • can affect density



Modifying physical properties

Alloying

Alloys are a mixture that contains at least one metal

Alloys compared to pure metals:

  • Alloying increases strength and hardness

  • Alloying reduces malleability and ductility, of alloys compared to pure metals.

  • This is due to the presence of “foreign” atoms which interfere with the movements of atoms in the crystals during plastic deformation


Work hardening (Cold working or Strain Hardening)

Is the process of toughening the a metal through plastic deformation.

Metals work-harden after being plastically deformed, much like when you twist a soft drink can back and forward and then it get gets stiff and then eventually fails (breaks).


Design context

Ferrous Alloys contain iron (makes them magnetic). Non-ferrous contains no iron.


Ferrous Alloys

  • Mild Steel – Carbon content of 0.1 to 0.3% and Iron content of 99.7 – 99.9%. Used for engineering purposes and in general, none specialised metal products.

Non-ferrous Alloys

  • Aluminium – An alloy of aluminium, copper and manganese. Very lightweight and easily worked. Used in aircraft manufacture, window frames and some kitchen ware. Pure Aluminium can be used in drink cans.


Tempering

Tempering is a heat treatment process

  • Usually carried out after hardening of a metal to:

    • increase its toughness and ductility

    • decrease hardness and brittleness

  • Remember a hard material will have low toughness and vice versa.