10.08.5 Alloys
Syllabus
Most metals in everyday use are alloys.
Bronze is an alloy of Copper and Tin. Brass is an alloy of Copper and Zinc.
Gold used as jewellery is usually an alloy with Silver, Copper and Zinc.
The proportion of Gold in the alloy is measured in carats. 24 carat being 100% (pure Gold), and 18 carat being 75% Gold.
Steels are alloys of Iron that contain specific amounts of Carbon & other metals. High Carbon steel is strong but brittle. Low Carbon steel is softer and more easily shaped. Steels containing chromium and nickel (stainless steels) are hard and resistant to corrosion. Aluminium alloys are low density.
Students should be able to:
recall a use of each of the alloys specified
interpret and evaluate the composition and uses of alloys other than those specified given appropriate information.
What does this mean?
Pure Metals v Alloys
An alloy is a mixture of a metal element with at least one more element (usually other metals).
We don't use pure metals much because they are too soft.
So, most metals we see round us are alloys - even the gold in most jewelry
24 carat Gold means gold that is 24/24 parts Gold - 100%.
It looks nice but wears away.
18 carat gold would be 18/24 Gold, = 3/4 = 75%
12 carat gold would be 12/24 Gold, = 1/2 = 50%
etc
Why are alloys stronger and harder than Pure Metals?
In a pure metal the atoms line up in layers well because they are all the same size.
But in an alloy with some of the atoms much bigger or smaller than the others - the pattern is disrupted.
In a pure metal it's very easy for one layer to slide over another.
But not in an alloy.
So, because the layers can't easily slide it's much more difficult to bend or reshape an alloy.
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