One of aluminium's
weaknesses is it's lack of strength is its pure form. To get around this and preserve aluminium's low density and lightweight other elements are added to the metal to "pin" dislocations reducing
ductility but increasing strength. By this method some aluminium alloys can be as strong as steel. Adding different elements achieves slightly different effect but almost all alloys are stronger than the original aluminium metal.
Copper, an important part of some aluminium alloys
Adding copper to aluminium increases aluminium's strength, and hardness and also makes it heat treatable. Under a classification system that is currently in place all aluminium alloys are give a four digit number. Those with copper come in the form 2XXX. Alternatively adding magnesium causes increased tensile strength, resistance to marine corrosion and ease at which welding can occur. The code for these alloys begins with a 5.
Manganese also forms important aluminium alloys.
There are three further common elements which can feature in aluminium alloys. Manganese is often added to give increased strength and resistance to
corrosion. The addition of silicon lowers the melting point and improves castability, and alloys with zinc have increased strength and hardness. What makes these alloys really special is they retain the lightweight property of aluminium whilst gaining the extra properties that aluminium itself lacks.
A dislocation is pinned by a different element in an alloy, increasing strength
But how do the addition of these materials strengthen aluminium? It is all to do with aluminium's
structure which has dislocations which make it ductile, and malleable. Even though these properties are sometimes very useful often strength is more important and alloys use other elements (usually of different sizes) to stop the dislocations from moving. This is done by disrupting the regular crystal structure and therefore making it harder for atoms to slip past each other.
Most of the uses that aluminium has are in fact as part of an alloy rather than as a pure metal. Pure aluminium simply isn't strong enough for usage in aeroplanes, cars, trains and buildings.
Summary: Alloys of aluminium are more useful that aluminium itself, as they give real strength to the material, by pinning dislocations whilst maintaining aluminium's other properties. Using different materials in alloys gives slightly different effects, so alloys can be hand picked for their specific job.
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