Alloys are a mixture of two or more metals or minerals. Metal alloys have different structural and behavioural characteristics than pure metals. Typically, alloys of metals produce a harder product than their individual elements, such as bronze, which is a mixture of copper and tin. Alloying a metal also gives it a different appearance.
Structure of Metals
Metal atoms are arranged in groups of crystals that are packed together. Between these crystalline groups are gaps or boundaries. Metals can be bent or deformed, because atoms in the crystalline groups will slide and roll over each other when stress is placed on the metal. The sizes of the crystal grains (and the metal's physical characteristics) can be changed by subjecting the metal to heat, hammering and controlled cooling.
The more boundaries between crystal groups, the harder the metal. This is because the numerous boundaries lead to an irregular atomic structure, preventing atoms from rolling or sliding over each other as easily. As a result, the metal cannot be bent or deformed. A downside to a metal's hardness is brittleness, because the boundary gaps are also stress points in the metal.
History of Alloys
The first use of alloys was around 2500 B.C. by the ancient Sumerians. They discovered that the blending of copper and tin produced bronze, an alloy that was stronger than copper and did not lose its edge during use. Other alloys were soon discovered, such as steel, which is iron smelted at a high temperature with charcoal to incorporate carbon atoms into its crystalline structure.
Most metals do not exist in a pure form but as ores mixed with other minerals in rock. The ore is mined, crushed and then heated in a furnace until it is molten. Another mineral is added to the liquid metal to chemically react with impurities and precipitate them to the surface, where they are skimmed off. Other refining may follow. Different metals are then added to create the alloy, which is poured into casts for further tempering or machined into the final product.
Alloys provide additional strength to a metal's structure because of the differing sizes of atoms in the alloy. Brass is harder than its component elements of copper and zinc because their different atomic sizes prevent atoms from moving over each other when the alloy is subjected to stress; this increases its hardness. Other common alloys are bronze (copper and tin), and steel (iron and carbon). Precious metals are mixed to enhance appearance. Gold jewellery is typically an alloy of gold, silver and copper.
Uses of Alloys
Alloys serve a variety of functions. Titanium-aluminium alloys have applications for light, hardened frames in structures or machines. Nickel and chrome alloys serve as corrosion-resistant parts for electronics or as a protective coating. Different grades of steel have varying amounts of carbon to provide a mix of strength and flexibility, depending on how the steel will be used.