alloy

Metal blended with some other metallic or non-metallic substance to give it special qualities, such as resistance to corrosion, greater hardness, or tensile strength. The atoms in a metal are held together by the metallic bond. In a pure metal the atoms are all the same size and can slip over each other if a force is applied. In an alloy, the presence of different sized atoms prevents such dislocations from weakening the metal. Useful alloys include bronze, brass, cupronickel, duralumin, German silver, gunmetal, pewter, solder, steel, and stainless steel.

Among the oldest alloys is bronze (mainly an alloy of copper and tin), the widespread use of which ushered in the Bronze Age. Complex alloys are now common; for example, in dentistry, where a cheaper alternative to gold is made of chromium, cobalt, molybdenum, and titanium. Among the most recent alloys are superplastics: alloys that can stretch to double their length at specific temperatures, permitting, for example, their injection into moulds as easily as plastic.

Alloys are mixtures, usually made by melting the metals together. (Certain elements which will not melt together, for example copper and graphite, may be combined using techniques of powder metallurgy.) Before adding the alloying element to the principal metal in the molten state it is necessary to ensure that it is free from oxygen, which would otherwise react with the alloying element, reducing the amount which would be dissolved and so causing an error in the composition. For this purpose a deoxidizer is added; this is often another metal.

Master alloys or foundry alloys are compositions made only for the purpose of melting with other metals to form alloys. They are used to overcome the problems of alloying metals of widely differing melting points, or to facilitate closer control over the final composition, or as deoxidizers.

Shape memory alloys are imprinted with a shape so that even after distortion, a threshold temperature will bring about a return to the original shape. Nitinol, an alloy of titanium and nickel, is an example.