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What are alloys?

Alloys are metallic materials consisting of two or more elements combined in such a way that they cannot be readily separated by physical means. More than 90% of metals used are in the form of alloys. They represent an enormous family of engineering materials that provide a wide range of products with useful properties.

Each alloy is distinct from its components, and the properties of each alloy are distinct. Indeed, the purpose in forming an alloy is to provide a metallic substance with physical, mechanical and/or chemical properties and characteristics that are different from those of its components. Moreover, these properties are influenced by the manner in which the alloy is formed and treated. The physical and chemical properties of an alloy can be modified by heat treatment and mechanical working. In most cases, the alloy is chemically more stable than the component elements, so that alloys are designed for specific resistance to actions such as corrosion, wear, fatigue and temperature. Other alloys are made to impart magnetic or electrical properties, strength, formability, etc.
Examples of the properties of alloys: 
  • steel is an alloy of iron with carbon and, usually, small amounts of a number of other elements, each of which imparts some unique characteristic to steel;
  • stainless steel alloys are a combination of iron, chromium and nickel frequently modified by the presence of other elements. This family of alloys is particularly resistant to corrosion, in contrast to the rusting phenomenon that consumes ordinary steel;
  • beryllium-copper alloys are stronger and have higher electrical conductivity that other copper alloys;
  • gallium arsenide is a superconducting alloy used in laser-beam technology;
  • superalloys of nickel and cobalt are used in aircraft engines due to their corrosion- and heat-resistance;
  • aluminum with small amounts of silicon, iron, copper, manganese, magnesium and zinc provides an alloy specifically designed for the manufacture of beverage cans;
  • pewter is an alloy of tin with minor amounts of antimony and copper;
  • copper with some zinc makes brass for a variety of fittings, and copper with tin forms bronze for plumbing fixtures;
  • 18-carat gold is 75% gold, with the balance made up of nickel, copper and zinc.
 
About the author: Dr. Mats Hillert is Emeritus Professor of Physical Metallurgy, KTH (Royal Institute of Technology), Stockholm, Sweden. He has authored about 250 papers, mainly on thermodynamics and phase transformations of alloys, and one textbook on thermodynamics.

Published in the ICME Newsletter, Vol. 5 No. 4 (1997)

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