alloys

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 a good conductor of electricity so it is used for electrical wires and cable. 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. An alloy is a mixture of a metal with one or more other elements, these could be other metals or even non-metals. By changing the mixture of metals and non-metals we can change the properties of the alloy until the desires 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.

Gold alloys

ring of power Consider pure gold, which is often called 24 carat gold. Pure gold is a metal used in jewellery for its beauty and corrosion resistance. However pure gold is 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 alloys which are much harder than pure gold. However adding these other metals affects the colour of the gold. Rose gold, as its name suggests has a pinkish tint to it, this alloy is a mixture of 75% gold, 2.5% silver and 22.5% copper but 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, as its name suggests white gold is not yellow but appears a silvery colour. The addition of platinum and/or palladium makes this alloy of gold hard and scratch resistant.
Pure gold is referred to as 24 carat gold, 18 carat gold is 75% gold with other metals mixed in, 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.

gold rings

Explaining the properties of alloys

The diagram below illustrates why adding other elements, either metals or non-metals to an existing metal 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 structure are able to move and slide over each other if a large enough force is applied. Anything which stops these layers of metal atoms sliding 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 substitutional alloy. 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 and also make it much harder.

alloy types

If the added atom is much smaller than the existing metal atoms then it will fit into the gaps between the metal atoms, this is called an interstitial alloy. These small atoms can bond to the metal atoms and prevent them from moving. 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 an 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 atoms. The addition of the metal chromium increases the strength and corrosion resistance of the alloy.

Common alloys

Brass

brass tubaEveryone 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 subsitutional 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 such as locks, hinges, electrical plugs, gears, outdoor taps and fittings, especially on boats due to its cosrrosion resistance properties. Brass is also widely 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 bacause 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

Bronze is an alloy of copper and tin. Copper is the main metal with a % 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 sculptues still survive today due it its good resistance to corrosion.

Steel

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.
  1. The simplest and cheapest steels are called carbon steels or mild steels. These are alloys of iron with small amounts of the non-metal carbon (between 0.03-0.5%) added. These low carbon steels are malleable, ductile and fairly strong in tension. These steels are used to make bodies for fridges, washing machines, cars, buses, rails, pipes and it is widely used in buildings and construction.
  2. High carbon steel, have a higher carbon content, typically between 0.5-0.8% carbon but it can be as high as 2%. These high carbon steel are much stronger than low carbon steels but are more brittle, these are used to make cutting tools, knives, saw blades and chains.
  3. Alloy steels are steels where other metals, mainly transition metals such as nickel, chromium, titanium, niobium, tungsten and vanadium are added. These alloy steels are expensive but have excellent corrosion resistance, strength and hardness. Stainless steel is an alloy steel, it contains iron, carbon, chromium nad nickel. It has a high resistance to corrosion and is used to make such things as: cooking utensils, knives, forkes and car exhaust systems.

Aluminium alloys

Aluminium alloys are used extensively in aircraft construction due to their low weight and corrosion resistance properties. Aluminium alloys are also commonly used in bike frames. Aluminium may be alloyed with metals including copper, manganese, chromium, magnesium. The exact metals used will depend on the final use of the aluminium alloy.

Key points

Practice questions

Check your understanding - Questions on alloys.

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