Iron ores, extraction, steel and alloys

An alloy is a metal mixed with other elements, usually other metals but sometimes a non-metal. Steel is iron with carbon; ferrochrome, ferromanganese, ferrovanadium and ferronickel are iron with another metal. In industry alloys are made by melting the parts together and cooling slowly, or by depositing metals together from a solution. Three types depend on atom size and chemistry. In an interstitial alloy small atoms, such as carbon or nitrogen, sit in the gaps between large metal atoms; the layers of the host can no longer slide so easily and the metal is harder. Steel is the main example. In a substitutional alloy atoms of nearly the same size and chemistry replace one another: chromium (about 128 pm) in iron (about 126 pm) gives stainless steel, and zinc in copper gives brass. An intermetallic compound is a real compound with a fixed ratio that does not follow the usual valence rules, such as Au₂Pb or Fe₃C (iron carbide). Duralumin is an aluminium alloy, used where low mass and strength are both needed.