Editing Alloy (section)

== Characteristics ==
[[file:Born bronze - Bronze casts.jpg|thumb|Liquid [[bronze]], being poured into molds during casting]]

An alloy is a mixture of [[chemical element]]s, which forms an impure substance (admixture) that retains the characteristics of a metal. An alloy is distinct from an impure metal in that, with an alloy, the added elements are well controlled to produce desirable properties, while impure metals such as [[wrought iron]] are less controlled, but are often considered useful. Alloys are made by mixing two or more elements, at least one of which is a metal. This is usually called the primary metal or the base metal, and the name of this metal may also be the name of the alloy. The other constituents may or may not be metals but, when mixed with the molten base, they will be [[soluble]] and dissolve into the mixture.
The mechanical properties of alloys will often be quite different from those of its individual constituents. A metal that is normally very soft ([[malleable]]), such as [[aluminium]], can be altered by alloying it with another soft metal, such as [[copper]]. Although both metals are very soft and [[ductile]], the resulting [[aluminium alloy]] will have much greater [[strength of materials|strength]]. Adding a small amount of non-metallic [[carbon]] to [[iron]] trades its great ductility for the greater strength of an alloy called steel. Due to its very-high strength, but still substantial [[toughness]], and its ability to be greatly altered by [[heat treatment]], steel is one of the most useful and common alloys in modern use. By adding [[chromium]] to steel, its resistance to [[corrosion]] can be enhanced, creating [[stainless steel]], while adding [[silicon]] will alter its electrical characteristics, producing [[silicon steel]].

[[file:A brass light.JPG|thumb|left|A [[brass]] lamp]]

Like oil and water, a molten metal may not always mix with another element. For example, pure iron is almost completely [[insoluble]] with copper. Even when the constituents are soluble, each will usually have a [[Saturated solution|saturation point]], beyond which no more of the constituent can be added. Iron, for example, can hold a maximum of 6.67% carbon. Although the elements of an alloy usually must be soluble in the [[liquid]] state, they may not always be soluble in the [[solid]] state. If the metals remain soluble when solid, the alloy forms a [[solid solution]], becoming a homogeneous structure consisting of identical crystals, called a [[phase (matter)|phase]]. If as the mixture cools the constituents become insoluble, they may separate to form two or more different types of crystals, creating a heterogeneous [[microstructure]] of different phases, some with more of one constituent than the other. However, in other alloys, the insoluble elements may not separate until after crystallization occurs. If cooled very quickly, they first crystallize as a homogeneous phase, but they are [[supersaturated]] with the secondary constituents. As time passes, the atoms of these supersaturated alloys can separate from the crystal lattice, becoming more stable, and forming a second phase that serves to reinforce the crystals internally.

[[file:Inconel gate valve--The-Alloy-Valve-Stockist.JPG|thumb|A gate valve, made from [[Inconel]]]]

Some alloys, such as [[electrum]]—an alloy of [[silver]] and [[gold]]—occur naturally. Meteorites are sometimes made of naturally occurring alloys of iron and [[nickel]], but are not native to the Earth. One of the first alloys made by humans was bronze, which is a mixture of the metals [[tin]] and copper. Bronze was an extremely useful alloy to the ancients, because it is much stronger and harder than either of its components. Steel was another common alloy. However, in ancient times, it could only be created as an accidental byproduct from the heating of iron ore in fires ([[smelting]]) during the manufacture of iron. Other ancient alloys include [[pewter]], brass and [[pig iron]]. In the modern age, steel can be created in many forms. [[Carbon steel]] can be made by varying only the carbon content, producing soft alloys like [[mild steel]] or hard alloys like [[spring steel]]. [[Alloy steel]]s can be made by adding other elements, such as [[chromium]], [[molybdenum]], [[vanadium]] or [[nickel]], resulting in alloys such as [[high-speed steel]] or [[tool steel]]. Small amounts of [[manganese]] are usually alloyed with most modern steels because of its ability to remove unwanted impurities, like [[phosphorus]], [[sulfur]] and [[oxygen]], which can have detrimental effects on the alloy. However, most alloys were not created until the 1900s, such as various aluminium, [[titanium alloy|titanium]], [[nickel alloys|nickel]], and [[magnesium alloy]]s. Some modern [[superalloy]]s, such as [[incoloy]], inconel, and [[hastelloy]], may consist of a multitude of different elements.

An alloy is technically an impure metal, but when referring to alloys, the term ''impurities'' usually denotes undesirable elements. Such impurities are introduced from the base metals and alloying elements, but are removed during processing. For instance, sulfur is a common impurity in steel. Sulfur combines readily with iron to form [[iron sulfide]], which is very brittle, creating weak spots in the steel.<ref>{{cite book|author=Verhoeven, John D.|title=Steel Metallurgy for the Non-metallurgist|url=https://books.google.com/books?id=brpx-LtdCLYC&pg=PA56|year=2007|publisher=ASM International|isbn=978-1-61503-056-9|page=56|url-status=live|archive-url=https://web.archive.org/web/20160505065853/https://books.google.com/books?id=brpx-LtdCLYC&pg=PA56|archive-date=2016-05-05}}</ref> [[Lithium]], [[sodium]] and [[calcium]] are common impurities in aluminium alloys, which can have adverse effects on the [[structural integrity]] of castings. Conversely, otherwise pure-metals that contain unwanted impurities are often called "impure metals" and are not usually referred to as alloys. Oxygen, present in the air, readily combines with most metals to form [[metal oxide]]s; especially at higher temperatures encountered during alloying. Great care is often taken during the alloying process to remove excess impurities, using [[flux (metallurgy)|fluxes]], chemical additives, or other methods of [[extractive metallurgy]].<ref>Davis, Joseph R. (1993) ''ASM Specialty Handbook: Aluminum and Aluminum Alloys''. ASM International. p. 211. {{ISBN|978-0-87170-496-2}}.</ref>