Editing Alloy (section)

== Theory ==

Alloying a metal is done by combining it with one or more other elements. The most common and oldest alloying process is performed by heating the base metal beyond its [[melting point]] and then dissolving the solutes into the molten liquid, which may be possible even if the melting point of the solute is far greater than that of the base. For example, in its liquid state, titanium is a very strong solvent capable of dissolving most metals and elements. In addition, it readily absorbs gases like oxygen and burns in the presence of nitrogen. This increases the chance of contamination from any contacting surface, and so must be melted in vacuum induction-heating and special, water-cooled, copper [[crucible]]s.<ref>''Metals Handbook: Properties and selection'' By ASM International – ASM International 1978 Page 407</ref> However, some metals and solutes, such as iron and carbon, have very high melting-points and were impossible for ancient people to melt. Thus, alloying (in particular, interstitial alloying) may also be performed with one or more constituents in a gaseous state, such as found in a [[blast furnace]] to make pig iron (liquid-gas), [[nitriding]], [[carbonitriding]] or other forms of [[case hardening]] (solid-gas), or the [[cementation process]] used to make [[blister steel]] (solid-gas). It may also be done with one, more, or all of the constituents in the solid state, such as found in ancient methods of [[pattern welding]] (solid-solid), [[shear steel]] (solid-solid), or [[crucible steel]] production (solid-liquid), mixing the elements via solid-state [[diffusion]].

By adding another element to a metal, differences in the size of the atoms create internal stresses in the lattice of the metallic crystals; stresses that often enhance its properties. For example, the combination of carbon with iron produces steel, which is stronger than iron, its primary element. The [[electrical conductivity|electrical]] and [[thermal conductivity]] of alloys is usually lower than that of the pure metals. The physical properties, such as [[density]], [[Reactivity (chemistry)|reactivity]], [[Young's modulus]] of an alloy may not differ greatly from those of its base element, but engineering properties such as [[tensile strength]],<ref>Mills, Adelbert Phillo (1922) ''Materials of Construction: Their Manufacture and Properties'', John Wiley & sons, inc, originally published by the University of Wisconsin, Madison</ref> ductility, and [[shear strength]] may be substantially different from those of the constituent materials. This is sometimes a result of the sizes of the [[atom]]s in the alloy, because 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 are present. For example, impurities in semiconducting [[ferromagnetic]] alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura.<ref>{{cite journal|last1=Hogan|first1=C.|title=Density of States of an Insulating Ferromagnetic Alloy|journal=Physical Review|volume=188|issue=2|pages=870–874|year=1969|doi=10.1103/PhysRev.188.870|bibcode=1969PhRv..188..870H}}</ref><ref>{{cite journal|last1=Zhang|first1=X.|last2=Suhl|first2=H.|title=Spin-wave-related period doublings and chaos under transverse pumping|journal=Physical Review A|volume=32|pages=2530–2533|year=1985|doi=10.1103/PhysRevA.32.2530|pmid=9896377|issue=4|bibcode=1985PhRvA..32.2530Z}}</ref>

Unlike pure metals, most alloys do not have a single [[melting point]], but a melting range during which the material is a mixture of [[solid]] and [[liquid]] phases (a slush). The temperature at which melting begins is called the [[solidus (chemistry)|solidus]], and the temperature when melting is just complete is called the [[liquidus]]. For many alloys there is a particular alloy proportion (in some cases more than one), called either a [[eutectic]] mixture or a peritectic composition, which gives the alloy a unique and low melting point, and no liquid/solid slush transition.

=== Heat treatment ===
[[file:IronAlfa&IronGamma.svg|thumb|left|[[Allotropes of iron]], ([[alpha iron]] and [[gamma iron]]) showing the differences in atomic arrangement]]
[[file:Photomicrograph of annealed and quenched steel, from 1911 Britannica plates 11 and 14.jpg|thumb|Photomicrographs of steel. Top photo: [[annealing (metallurgy)|Annealed]] (slowly cooled) steel forms a heterogeneous, lamellar microstructure called [[pearlite]], consisting of the phases [[cementite]] (light) and [[Ferrite (magnet)|ferrite]] (dark). Bottom photo: [[Quenched]] (quickly cooled) steel forms a single phase called [[martensite]], in which the carbon remains trapped within the crystals, creating internal stresses]]

Alloying elements are added to a base metal, to induce [[hardness]], [[toughness]], ductility, or other desired properties. Most metals and alloys can be [[work hardened]] by creating defects in their crystal structure. These defects are created during [[plastic deformation]] by hammering, bending, extruding, et cetera, and are permanent unless the metal is [[recrystallization (metallurgy)|recrystallized]]. Otherwise, some alloys can also have their properties altered by [[heat treatment]]. Nearly all metals can be softened by [[annealing (metallurgy)|annealing]], which recrystallizes the alloy and repairs the defects, but not as many can be hardened by controlled heating and cooling. Many alloys of aluminium, copper, [[magnesium]], titanium, and nickel can be strengthened to some degree by some method of heat treatment, but few respond to this to the same degree as does steel.<ref name="Jon L. Dossett Page 1-14"/>

The base metal iron of the iron-carbon alloy known as steel, undergoes a change in the arrangement ([[allotropy]]) of the atoms of its crystal matrix at a certain temperature (usually between {{convert|1500|F|C|order=flip}} and {{convert|1600|F|C|order=flip}}, depending on carbon content). This allows the smaller carbon atoms to enter the interstices of the iron crystal. When this [[diffusion]] happens, the carbon atoms are said to be in ''solution'' in the iron, forming a particular single, homogeneous, crystalline phase called [[austenite]]. If the steel is cooled slowly, the carbon can diffuse out of the iron and it will gradually revert to its low temperature allotrope. During slow cooling, the carbon atoms will no longer be as [[soluble]] with the iron, and will be forced to [[precipitate]] out of solution, [[nucleating]] into a more concentrated form of iron carbide (Fe<sub>3</sub>C) in the spaces between the pure iron crystals. The steel then becomes heterogeneous, as it is formed of two phases, the iron-carbon phase called [[cementite]] (or [[carbide]]), and pure iron [[Allotropes of iron|ferrite]]. Such a heat treatment produces a steel that is rather soft. If the steel is cooled quickly, however, the carbon atoms will not have time to diffuse and precipitate out as carbide, but will be trapped within the iron crystals. When rapidly cooled, a [[diffusionless transformation|diffusionless (martensite) transformation]] occurs, in which the carbon atoms become trapped in solution. This causes the iron crystals to deform as the crystal structure tries to change to its low temperature state, leaving those crystals very hard but much less ductile (more brittle).

While the high strength of steel results when diffusion and precipitation is prevented (forming martensite), most heat-treatable alloys are [[precipitation hardening]] alloys, that depend on the diffusion of alloying elements to achieve their strength. When heated to form a solution and then cooled quickly, these alloys become much softer than normal, during the diffusionless transformation, but then harden as they age. The solutes in these alloys will precipitate over time, forming [[intermetallic]] phases, which are difficult to discern from the base metal. Unlike steel, in which the solid solution separates into different crystal phases (carbide and ferrite), precipitation hardening alloys form different phases within the same crystal. These intermetallic alloys appear homogeneous in crystal structure, but tend to behave heterogeneously, becoming hard and somewhat brittle.<ref name="Jon L. Dossett Page 1-14"/>

In 1906, [[precipitation hardening]] alloys were discovered by [[Alfred Wilm]]. Precipitation hardening alloys, such as certain alloys of aluminium, titanium, and copper, are heat-treatable alloys that soften when [[quenched]] (cooled quickly), and then harden over time. Wilm had been searching for a way to harden aluminium alloys for use in machine-gun cartridge cases. Knowing that aluminium-copper alloys were heat-treatable to some degree, Wilm tried quenching a ternary alloy of aluminium, copper, and the addition of magnesium, but was initially disappointed with the results. However, when Wilm retested it the next day he discovered that the alloy increased in hardness when left to age at room temperature, and far exceeded his expectations. Although an explanation for the phenomenon was not provided until 1919, [[duralumin]] was one of the first "age hardening" alloys used, becoming the primary building material for the first [[Zeppelin]]s, and was soon followed by many others.<ref>''Metallurgy for the Non-Metallurgist'' by Harry Chandler – ASM International 1998 Page 1–3</ref> Because they often exhibit a combination of high strength and low weight, these alloys became widely used in many forms of industry, including the construction of modern [[aircraft]].<ref>Jacobs, M.H. [http://www.slideshare.net/corematerials/talat-lecture-1204-precipitation-hardening-2318135 Precipitation Hardnening] {{webarchive|url=https://web.archive.org/web/20121202213718/http://www.slideshare.net/corematerials/talat-lecture-1204-precipitation-hardening-2318135 |date=2012-12-02 }}. University of Birmingham. TALAT Lecture 1204. slideshare.net</ref>

=== Mechanisms ===
[[file:Alloy atomic arrangements showing the different types.svg|thumb|Different atomic mechanisms of alloy formation, showing pure metal, substitutional, interstitial, and a combination of the two]]

When a molten metal is mixed with another substance, there are two mechanisms that can cause an alloy to form, called ''atom exchange'' and the ''interstitial mechanism''. The relative size of each element in the mix plays a primary role in determining which mechanism will occur. When the atoms are relatively similar in size, the atom exchange method usually happens, where some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. This is called a ''substitutional alloy''. Examples of substitutional alloys include bronze and brass, in which some of the copper atoms are substituted with either tin or zinc atoms respectively.

In the case of the interstitial mechanism, one atom is usually much smaller than the other and  can not successfully substitute for the other type of atom in the crystals of the base metal. Instead, the smaller atoms become trapped in the [[interstitial site]]s between the atoms of the crystal matrix. This is referred to as an ''interstitial alloy''. Steel is an example of an interstitial alloy, because the very small carbon atoms fit into interstices of the iron matrix.

[[Stainless steel]] is an example of a combination of interstitial and substitutional alloys, because the carbon atoms fit into the interstices, but some of the iron atoms are substituted by nickel and chromium atoms.<ref name="Jon L. Dossett Page 1-14">Dossett, Jon L.; Boyer, Howard E. (2006) ''Practical heat treating''. ASM International. pp. 1–14. {{ISBN|1-61503-110-3}}.</ref>