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.