An alloy is a substance made by combining together two or more elements where the primary element is a metal. Most alloys form by melting the elements together. Upon cooling, an alloy crystallizes into a solid, intermetallic compound, or mixture that cannot be separated using a physical method. Although an alloy may contain metalloids or nonmetals, it displays the properties of a metal.
The primary metal in an alloy is called its base, solvent, or matrix. Secondary elements are called solutes. Undesirable elements are called impurities. If the alloy consists of only two elements, the result is a binary alloy. If three elements, the result is a ternary alloy. Varying the percentage of elements creates binary systems, ternary systems, quaternary systems, and so on.
Examples of Alloys
Familiar examples of alloys include brass, bronze, stainless steel, 14k gold, sterling silver, and cast iron.
- Alnico: Alnico contains at least 50% iron, with aluminum, nickel, cobalt, and other metals. It is used in electric guitar pickups and speaker magnets.
- Amalgam: An amalgam is a mercury alloy. Because pure mercury is a liquid element, amalgams tend to be paste-like. Mercury also has a high vapor pressure, so sometimes an amalgam is heated to drive off the mercury, leaving the other components.
- Brass: Brass is an alloy of copper with zinc and sometimes other elements. Because it is hard and tough, brass finds use in machined parts and plumbing fixtures.
- Bronze: Bronze is an alloy of copper and tin, sometimes with other elements. Bronze finds use in statues and some musical instruments.
- Cast Iron: Cast iron is an example of an alloy containing a nonmetal. It is iron with at least 2% carbon.
- Electrum: Electrum is a naturally-occurring alloy of silver and gold.
- 14k Gold: 14k gold is 58.5% gold, usually with silver, copper, and zinc. Alloying gold makes it harder and stronger.
- 18k Gold: 18k gold is 75% gold, usually with copper, nickel, or zinc. The alloy retains the color and luster of gold, but is harder and stronger than the pure element.
- Meteoritic Iron: Meteorites have variable composition, but some are natural alloys of iron and nickel.
- Nitinol: Nitinol is 50-55% nickel with 45-50% titanium. It is a shape-memory alloy used in eyeglass frames, medical items, and temperature switches.
- Pewter: Pewter is a tin alloy. The other elements may be copper, antimony, or lead. Pewter is stronger than pure tin, malleable, and resists crumbling at low temperatures.
- Sterling Silver: Sterling silver is 92.5% silver, usually with copper, but sometimes with other elements. Alloying silver makes it harder and more durable, but also prone to tarnish.
How Alloys Are Made
Two methods lead to alloy formation. These methods may be combined to form a third type of alloy.
- Substitutional alloy – A substitutional alloy forms when one atom exchanges with another atom of comparable size. Brass and bronze are examples of substitutional alloys. Tin or zinc, respectively, replace some of the copper atoms in the crystal lattice.
- Interstitial alloy – An interstitial alloy forms when smaller atoms become trapped within the crystal lattice of larger atoms. Steel is an example of an interstitial alloy. Carbon atoms fit into the interstices of the iron crystal matrix.
Some alloys form from a combination of atom exchange and interstitial mechanisms. For example, stainless steel has carbon atoms in its interstices, plus nickel and chromium atoms replace some carbon atoms.
By design, alloys have chemical and physical properties that are superior for an application than a pure element. So, over 90% of metals in commercial use are alloys. Alloys improve over pure elements in terms of corrosion resistance, heat resistance, hardness, machinability, improved wear, or special electrical or magnetic properties. Sometimes the improvement simply reflects cost-effectiveness, where an alloy retains the key properties of a pure metal but is less expensive.
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