Editing Alloy (section)

=== 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>