Alloy

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Steel is a metal alloy whose major component is iron, with carbon content between 0.02% and 2.14% by mass.

An alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal (heat treatment) history. Alloys usually have different properties from those of the component elements.

Alloys' constituents are usually measured by mass.

Theory

Alloying one metal with other metal(s) or non metal(s) often enhances its properties. For example, steel is stronger than iron, its primary element. The physical properties, such as density, reactivity, Young's modulus, and electrical and thermal conductivity, of an alloy may not differ greatly from those of its elements, but engineering properties, such as tensile strength and shear strength may be substantially different from those of the constituent materials. This is sometimes due to the sizes of the atoms in the alloy, since larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy resist deformation. Sometimes alloys may exhibit marked differences in behavior even when small amounts of one element occur. For example, impurities in semi-conducting ferromagnetic alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura. Some alloys are made by melting and mixing two or more metals. Bronze, an alloy of copper and tin, was the first alloy discovered, during the prehistoric period now known as the bronze age; it was harder than pure copper and originally used to make tools and weapons, but was later superseded by metals and alloys with better properties. In later times bronze has been used for ornaments, bells, statues, and bearings. Brass is an alloy made from copper and zinc.

Unlike pure metals, most alloys do not have a single melting point, but a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and the temperature when melting is just complete is called the liquidus. However, for most alloys there is a particular proportion of constituents (in rare cases two)—the eutectic mixture—which gives the alloy a unique melting point.

Terminology

In practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 karat gold is an alloy of gold with other elements. Similarly, the silver used in jewelry and the aluminium used as a structural building material are also alloys.

The term "alloy" is sometimes used in everyday speech as a synonym for a particular alloy. For example, automobile wheels made of an aluminium alloy are commonly referred to as simply "alloy wheels", although in point of fact steels and most other metals in practical use are also alloys.

History

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Bronze axe 1100 BCE

The use of alloys by humans started with the use of meteoric iron, a naturally occurring alloy of nickel and iron. As no metallurgic processes were used to separate iron from nickel, the alloy was used as it was. Meteoric iron could be forged from a red heat to make objects such as tools, weapons, and nails. In many cultures it was shaped by cold hammering into knives and arrowheads. They were often used as anvils. Meteoric iron was very rare and valuable, and difficult for ancient people to work.

Iron is usually found as iron ore on Earth, except for one deposit of native iron in Greenland, which was used by the Inuit people. Native copper, however, was found worldwide, along with silver, gold and platinum, which were also used to make tools, jewelry, and other objects since Neolithic times. Copper was the hardest of these metals, and the most widely distributed. It became one of the most important metals to the ancients. Eventually, humans learned to smelt metals such as copper and tin from ore, and, around 2500 B.C, began alloying the two metals to form bronze, which is much harder than its ingredients. Tin was rare, however, being found mostly in Great Britain. In the Middle East, people began alloying copper with zinc to form brass. Ancient civilizations made use of the information contained in modern alloy constitution diagrams, taking into account the mixture and the various properties it produced, such as hardness, toughness and melting point, under various conditions of temperature and work hardening.

The first known smelting of iron began in Anatolia, around 1800 B.C. Called the bloomery process, it produced very soft but ductile wrought iron and, by 800 B.C., the technology had spread to Europe. Pig iron, a very hard but brittle alloy of iron and carbon, was being produced in China as early as 1200 B.C., but did not arrive in Europe until the middle ages. These metals found little practical use until the introduction of crucible steel around 300 B.C. These steels were of poor quality, and the introduction of pattern welding, around the first century A.D., sought to balance the extreme properties of the alloys by laminating them, to create a tougher metal.

Mercury had been smelted from cinnabar for thousands of years. Mercury dissolves many metals, such as gold, silver, and tin, to form amalgams, (an alloy in a soft, paste, or liquid form at ambient temperture). Amalgams have been used since 200 B.C. in China for plating objects with precious metals, called gilding, such as armor and mirrors. The ancient Romans often used mercury-tin amalgams for gilding their armor. The amalgam was applied as a paste and then heated until the mercury vaporized, leaving the gold, silver, or tin behind. Mercury was often used in mining, to extract precious metals like gold and silver from their ores.

Many ancient civilizations alloyed metals for purely aesthetic purposes. In ancient Egypt and Mycenae, copper was often alloyed with gold to produce red-gold, or iron to produce a bright burgandy-gold. Silver was often found alloyed with gold. These metals were also used to strengthen each other, for more pratical purposes. Quite often, precious metals were alloyed with less valuable substances as a means to deceive buyers. Around 250 B.C., Archimedes was commisssioned by the king to find a way to check the purity of the gold in a crown, leading to the famous bath-house shouting of "Eureka!" upon the discovery of Archimedes principle.

See also

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• List of alloys
• Intermetallics
• Heat treatment
• CALPHAD (method)

External links

• Surface Alloys
• Template:Wikisource1911Enc Citation

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