Chemistry only
Look around the room you are in and find an object made of metal. It could be a lamp, chair leg, pen nib, stapler,
door handle, a musical instrument....... the chances are that the metal object you have is not actually a pure metal but
an alloy. The way we use any material, including metals depends on their properties.
Copper for example is ductile (can
be drawn into a wire) and also a good conductor of electricity so it is used for electrical wires and cables. Aluminium is not as good a
conductor of electricity as copper but it is lightweight so it can be used for overhead power lines; it's also a soft
metal
and can be rolled into thin sheets which are used as cooking foil. Mercury is used in thermometers because it is a liquid
at room temperature and expands evenly on heating.
However the physical properties of metals are fixed; we cannot change their melting and boiling points or their hardness or
strength. However this is not the case with alloys; an alloy is a mixture of a metal with one or more other elements mixed through it;
these other elements could be other metals or
even non-metals and by changing the amount of each substance in the alloy we can change the properties of the alloy until the desired
properties are achieved. The fact that we are able to change the properties of an alloy by changing the mixture of
metals and
non-metals is the main reason why alloys are used over metals with their fixed properties.
Consider pure
gold which is often called 24 carat gold. Gold is a metal used in jewellery for its beauty and
corrosion resistance. However pure gold is also a very soft metal and this undesirable property would limit its use
in rings, necklaces and other jewellery. However when gold is mixed with other metals it forms gold alloys which are much
harder than pure gold. However adding these other metals affects the colour of the
gold. Rose gold for example is a common alloy of gold; as its name
suggests it has a pinkish tint to it. This gold alloy is a mixture of 75% gold, 2.5% silver and 22.5% copper and it is much
harder than pure gold. Another alloy of gold which is commonly used in jewellery is
white gold. This is an alloy
containing gold which is mixed with silver or even platinum or palladium and
as its name suggests white gold is not yellow
but has a silvery colour. The addition of platinum and/or palladium makes this alloy of gold hard and scratch
resistant but also very expensive.
Pure gold is referred to as 24 carat gold, 18 carat
gold is 75% gold with other metals mixed through it; commonly copper and zinc are used.
18 carat gold is much harder than 24 carat gold
and is suitable for use in rings, necklaces and other items of jewellery.
12 carat gold is 50% gold and 9 carat gold
is 37.5% gold. 9 carat gold is commonly used in jewellery but is not as hard
as 18 carat gold or as resistant to corrosion and discolouration but it is less expensive since it contains less gold and more
copper and zinc.
The diagram below illustrates why adding other elements, either metals or non-metals to an existing metal structure alters its properties. Metals are malleable which means that they can be hammered, beaten and rolled in different shapes. This occurs because the layers of ions in a metal lattice structure are able to move and slide over each other if a large enough force is applied; this is outlined in the diagram below:
When other metals or non-metals are added and mixed in with the metal lattice the layers of metal ions present in the metallic lattice are not able to move or slide so freely. This means that the alloy will be harder and stronger than the original metal. Depending on the size of the atoms which are mixed through the metal structure we can end up with alloys which have different types of structures.
Anything which stops these layers of metal atoms sliding when a force is applied will make the alloy stronger than the metal. Alloys can be made simply by melting the main metal until it is a liquid then simply adding in the new metal or non-metal and stirring sufficiently until it dissolves. If the added metal/non-metal atoms are of a similar size to the existing metal atoms they will substitute and replace the existing atoms in the metal structure. This will form an alloy called a substitution alloy. Brass and bronze are examples of substitution alloys. If the added atoms are larger than the existing ones, as shown below, then these new larger atoms will prevent the layers of metal atoms sliding and a much larger force will be needed to move the layers. This will make the alloy much stronger with a larger tensile strength, it can be stretched and pulled before snapping and it will also make it much harder.
If the added atom is much smaller than the existing metal ions in the lattice then it will fit into the gaps between the metal ions, this is called an interstitial alloy. These small atoms such as carbon and boron can bond to the metal ions in the lattice and prevent them from moving as freely. This will result in a much stronger alloy but one which is much less ductile. Steel is an interstitial alloy; it consists of iron with a small amount of carbon added. Steel can be turned into stainless steel by the addition of chromium, this will result in a substitutional/interstitial alloy where the chromium atoms will substitute for iron atoms in the structure and the carbon will fit into the gaps between the metal ions. The addition of the metal chromium increases the strength and corrosion resistance of the alloy. This is outlined in the image above.
Everyone has probably heard a brass band or an orchestra with its brass section playing.
Brass is a widely used alloy
of copper and zinc, it is a substitution alloy. Brass has many common uses which take advantage of its properties of
durability, attractive gold like appearance, good thermal and electrical conductivity and excellent corrosion resistance.
Everyday objects which are made of brass include: locks, hinges, electrical plugs, gears, outdoor taps and fittings e.g. brass is widely used on boats fitting due to
its corrosion resistance properties. Brass is also commonly used in costume jewellery due to it attractive nature and
gold like appearance. Brass is also easy to cast into moulds making it ideal for making statues and models. Brass is also suitable
for use as fitting near flammable gases and liquids as it does not produce a spark when struck.
Copper is used for electrical wires because it is a good conductor of electricity, however brass is used to make the metal
pins in plugs because it is stronger, harder and very resistant to corrosion although it is not as good an electrical
conductor as copper
Bronze is an alloy of copper and tin. Copper is the main metal with a percentage abundance of around 80-90% depending on the type of bronze used. Bronze is used mainly for statues and sculptures. Many Greek and Roman bronze sculptures still survive today due its corrosion resistance.
Steel is an alloy of iron with other elements mixed in with it. There are many different types of steel and each type of steel has a different composition, this ensures that the properties of the particular type of steel are suited to its uses.
