The Elemental Compositions of Metal Alloys Aug 15

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Today’s post looks at an aspect of chemistry we come across every day: alloys. Alloys make up parts of buildings, transport, coins, and plenty of other objects in our daily lives. But what are the different alloys we use made up of, and why do we use them instead of elemental metals? The graphic answers the first of these questions, and in the post we’ll try and answer the second.

First, a little on what alloys are, for anyone unfamiliar with the term. Alloys are a mixture of elements, where at least one of the elements is a metal. There are over 80 metals in the periodic table of elements, and we can mix selections of these different metals in varying proportions, sometimes with non-metals too, to create alloys. Note the use of the word mixture: in the vast majority of cases, alloys are simply intermixed elements, rather than elements that are chemically bonded together.

Alloys can be simply classified in terms of their atomic arrangements. In cases where the two elements being mixed to make the alloy have similar atom sizes, atoms of the second element can simply take the place of atoms of the first element in the structure. These types of alloys are called substitution alloys. On the other hand, if the atoms of the second element are much smaller, they can slot into the gaps between atoms of the first element. These alloys are known as interstitial alloys. Alloys can be made in a number of ways, but they are primarily fashioned by mixing together the molten components.

Types of Alloys

There are a great range of alloys; the main graphic illustrates just a small selection of those that we use in a range of applications. But why use them in the first place when there are so many different metals with varying properties in the periodic table? Whilst metallic elements may have desirable properties, unfortunately they rarely have them in convenient combinations. Gold is shiny and, well, golden, but it’s also quite soft, meaning if you try and make a ring from pure gold, it’ll deform easily. Iron is present in many buildings, but on its own it too is a little on the soft side, and also has a tendency to rust when exposed to damp air.

Making alloys is essentially a way for us to ‘tweak’ the properties of a metal, to make them closer to the ideal properties we want for a particular purpose. Alloying gold with copper or silver makes it harder, whilst alloying iron with carbon and a selection of other metals accomplishes a similar effect, and also helps prevent it rusting. The softness of most pure metals stems from the fact that the layers of atoms that make up their structure can easily slide over each other. As alloys contain a mixture of atom sizes, as a consequence of the mixture of elements that make them up, it makes it harder for the layers to slide over each other, as the regular arrangement of atoms is distorted.

There can be other reasons for using alloys as well as properties, of course. Coins may have been made out of gold in the distant past, but the majority of coin denominations would contain a much higher value of gold than their face value if we were to make them from gold today. Even copper, a former coin-making favourite, is more valuable than the pennies it was once used to fashion. Prior to 1992 in the UK, 1 pence coins consisted of an alloy containing 97% copper. Today, they’re copper-plated, with cheaper steel forming the core of the coin. There’s more on the alloys used in British coins here.

Alloys can also have properties impossible with pure metals alone. The alloy nitinol, formed from the mixing of nickel and titanium, is what’s known as a shape memory alloy. These alloys are capable of ‘remembering’ their original shape, and when heated to a certain temperature, are capable of reverting to this shape. It’s used in some glasses frames, and also in more novelty applications such as bendable spoons.

Pure metals are, in fact, much less commonly used than alloys, since the altered properties that the range of alloys afford are much more useful to us. The majority of metal objects you encounter are likely to be alloys of some description – ones that chemists and materials scientists will have been responsible for engineering. Proof, if more were needed, that chemistry is a pervasive force in modern society!

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