Alloy

Alloy, material composed of two or more metals, or of a metal combined with a smaller quantity of some non-metal. Most familiar metals are not of very high purity: impurity atoms are added intentionally, thus forming an alloy. Alloying is used in metals to improve their mechanical strength, electrical and magnetic properties, resistance to corrosion, and many other characteristics. For example, pure silver is very soft, but the addition of 7.5 per cent copper to form the alloy sterling silver produces a significant increase in its hardness and thus in its usefulness. Again, the addition of chromium to steel produces a type of stainless steel, which does not rust (see Iron and Steel Manufacture: Stainless Steels).

Bronze (consisting of copper alloyed with, predominantly, tin) and brass (copper alloyed mainly with zinc) were the first major metals to be worked. Iron, alloyed with carbon in varying quantities, grew to prominence later. Historically, most alloys have been prepared from the molten materials, but today powder metallurgy is also often used. Another modern alloying technique is ion implantation, which has been adapted from a process used to produce microchips: beams of ions of carbon, nitrogen and other elements are fired into selected metals in a vacuum chamber to produce a strong, thin layer of alloy on the metal surface. Bombarding titanium with nitrogen, for example, can produce a superior alloy for prosthetic implants. It is estimated that there are some 40,000 metallic alloys in existence, and their development and use constitutes the major part of metallurgy.