Alloys

Almost nothing made of metal in your everyday life is a single pure metal. The bike frame, the cutlery in the drawer, the coin in your pocket: there is a mixture inside all of them. The reason is simple: pure metals are usually too soft. A ring of pure gold dents quickly in daily use, and pure iron gives way under load. But melt a second element into it and you get a material that carries a whole bridge. Such metal mixtures are called alloys.

What an alloy is

An alloy forms when you melt a metal together with at least one further element and let the melt solidify as one. The atoms of the ingredients spread so finely into one another that a single, uniform material is left: looking at a brass knob, you cannot tell that copper and zinc are inside. Three alloys cross your path all the time: bronze from copper and tin, brass from copper and zinc, steel from iron and less than 2% carbon. The difference from coating matters: a galvanised screw only carries a thin layer of zinc on the outside, while an alloy is mixed all the way through.

Why alloys are harder

In a pure metal all atoms are the same size and lie in neat layers, almost like balls packed in a box. When you bend the metal these layers slide past each other, and that is exactly why pure copper and pure gold are so soft. Now mix in atoms of another kind that are a little bigger or smaller, and they disturb the neat layers. The sliding jams, and the material becomes harder and stronger. That is why bronze is much harder than the copper it mostly consists of. People in the Bronze Age already cast tools and blades from it, long before anyone could work iron.

More than just hard

Hardness is not the only reason for alloys. Stainless steel is steel with at least 10.5% chromium: the chromium builds an invisible, ultra thin protective layer on the surface, which is why the kitchen sink does not rust. Solder, an alloy of tin with a little silver and copper, melts at around 220 °C. That lets you join components on a circuit board without the copper tracks underneath going soft, because copper only melts above 1000 °C. So for every job there is a matching mixture: harder, tougher, rust free or easier to melt.