Duralumin (also written Dural, duraluminum or duraluminium) is an alloy of aluminum (sorry, Commonwealthers, I'm sticking to one spelling from here on out). Aluminum is an excellent material for applications requiring light weight and comparatively high strength, but there's one problem - aluminum on its own is relatively soft. It can be bent fairly easily. This is a problem when using it for structural components.
It was discovered in 1903, when a German metallurgist noted that if an aluminum sheet contained copper (around 4% in his experiments) and was left to itself after being quenched during manufacturing, its hardness would slowly increase over a week or so to result in a much stronger substance. Eventually, after experimentation, Duralumin was introduced, containing copper, manganese, silicon and iron along with the base aluminum.
The primary disadvantage of the addition of copper to the alloy is that the copper makes it sensitive to corrosion, as the copper oxidizes. Pure aluminum oxidizes quite quickly, and in doing so it forms a surface layer of aluminum oxide that protects the interior of the metal from further corrosion - the oxidation layer is an air and waterproof boundary. As a result, Duralumin sheets that are expected to suffer exposure to the elements are usually coated with a layer of pure aluminum. The industry term for this is 'AlClad' and applies to a whole range of various products consisting of slight variations on the original alloy and on the thickness of the coating. These alloys (now standardized internationally and referred to by four-digit composition number) are used for everything from aircraft and spacecraft skin (see aircraft aluminum) to high-performance engine parts and structural members for vehicles and buildings.