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An alloy is a of of which at least one is a . Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, , opacity, and luster, but may have properties that differ from those of the pure metals, such as increased strength or hardness. In some cases, an alloy may reduce the overall cost of the material while preserving important properties. In other cases, the mixture imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength.

Alloys are defined by a character.Callister, W.D. "Materials Science and Engineering: An Introduction" 2007, 7th edition, John Wiley and Sons, Inc. New York, Section 4.3 and Chapter 9. The alloy constituents are usually measured by mass percentage for practical applications, and in for basic science studies. Alloys are usually classified as substitutional or interstitial alloys, depending on the atomic arrangement that forms the alloy. They can be further classified as homogeneous (consisting of a single phase), or heterogeneous (consisting of two or more phases) or . An alloy may be a of metal elements (a single phase, where all metallic grains (crystals) are of the same composition) or a of metallic phases (two or more solutions, forming a of different crystals within the metal).

Examples of alloys include ( and ) (gold and ), (silver and copper), or ( with non-metallic or respectively), , , , , , and amalgams.

Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools.

An alloy is a mixture of , which forms an impure substance (admixture) that retains the characteristics of a . 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 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 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 (), such as , can be altered by alloying it with another soft metal, such as . Although both metals are very soft and , the resulting will have much greater strength. Adding a small amount of non-metallic to trades its great ductility for the greater strength of an alloy called . Due to its very-high strength, but still substantial , and its ability to be greatly altered by , steel is one of the most useful and common alloys in modern use. By adding to steel, its resistance to can be enhanced, creating , while adding will alter its electrical characteristics, producing .

Like oil and water, a molten metal may not always mix with another element. For example, pure iron is almost completely with copper. Even when the constituents are soluble, each will usually have a 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 state, they may not always be soluble in the state. If the metals remain soluble when solid, the alloy forms a , becoming a homogeneous structure consisting of identical crystals, called a 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 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 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.

Some alloys, such as —an alloy of and —occur naturally. Meteorites are sometimes made of naturally occurring alloys of iron and , but are not native to the Earth. One of the first alloys made by humans was , which is a mixture of the metals and . 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 () during the manufacture of iron. Other ancient alloys include , and . In the modern age, steel can be created in many forms. can be made by varying only the carbon content, producing soft alloys like or hard alloys like . can be made by adding other elements, such as , , or , resulting in alloys such as or . Small amounts of are usually alloyed with most modern steels because of its ability to remove unwanted impurities, like , and , which can have detrimental effects on the alloy. However, most alloys were not created until the 1900s, such as various aluminium, , , and . Some modern , such as , , and , 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 , which is very brittle, creating weak spots in the steel.

(2022). 9781615030569, ASM International. .
, and 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 ; especially at higher temperatures encountered during alloying. Great care is often taken during the alloying process to remove excess impurities, using fluxes, chemical additives, or other methods of extractive metallurgy.Davis, Joseph R. (1993) ASM Specialty Handbook: Aluminum and Aluminum Alloys. ASM International. p. 211. .

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 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, 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 . Metals Handbook: Properties and selection By ASM International – ASM International 1978 Page 407 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 to make pig iron (liquid-gas), , or other forms of (solid-gas), or the cementation process used to make (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 (solid-solid), (solid-solid), or production (solid-liquid), mixing the elements via solid-state .

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 , which is stronger than , its primary element. The electrical and thermal conductivity of alloys is usually lower than that of the pure metals. The physical properties, such as , reactivity, Young's modulus of an alloy may not differ greatly from those of its base element, but engineering properties such as ,Mills, Adelbert Phillo (1922) Materials of Construction: Their Manufacture and Properties, John Wiley & sons, inc, originally published by the University of Wisconsin, Madison ductility, and may be substantially different from those of the constituent materials. This is sometimes a result of the sizes of the 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 alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura.

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

Heat treatment
Alloying elements are added to a base metal, to induce , , , or other desired properties. Most metals and alloys can be 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 recrystallized. Otherwise, some alloys can also have their properties altered by . Nearly all metals can be softened by annealing, which recrystallizes the alloy and repairs the defects, but not as many can be hardened by controlled heating and cooling. Many alloys of , , , , and can be strengthened to some degree by some method of heat treatment, but few respond to this to the same degree as does .

The base metal iron of the iron-carbon alloy known as steel, undergoes a change in the arrangement () of the atoms of its crystal matrix at a certain temperature (usually between and , depending on carbon content). This allows the smaller carbon atoms to enter the interstices of the iron crystal. When this happens, the carbon atoms are said to be in in the iron, forming a particular single, homogeneous, crystalline phase called . 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 with the iron, and will be forced to precipitate out of solution, into a more concentrated form of iron carbide (Fe3C) 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 (or ), and pure 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 (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 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.

In 1906, precipitation hardening alloys were discovered by . Precipitation hardening alloys, such as certain alloys of , , and copper, are heat-treatable alloys that soften when (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 , 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, was one of the first "age hardening" alloys used, becoming the primary building material for the first , and was soon followed by many others. Metallurgy for the Non-Metallurgist by Harry Chandler – ASM International 1998 Page 1—3 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 .Jacobs, M.H. Precipitation Hardnening . University of Birmingham. TALAT Lecture 1204.

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 , 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 sites 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.

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.Dossett, Jon L. and Boyer, Howard E. (2006) Practical heat treating. ASM International. pp. 1–14. .

History and examples

Meteoric iron
The use of alloys by humans started with the use of , a naturally occurring alloy of and . It is the main constituent of . 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 .Buchwald, pp. 13–22

Bronze and brass
Iron is usually found as on Earth, except for one deposit of in , which was used by the .Buchwald, pp. 35–37 Native , however, was found worldwide, along with , , and , 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. Around 10,000 years ago in the highlands of (Turkey), humans learned to metals such as copper and from . Around 2500 BC, people began alloying the two metals to form , which was much harder than its ingredients. Tin was rare, however, being found mostly in Great Britain. In the Middle East, people began alloying copper with to form .Buchwald, pp. 39–41 Ancient civilizations took into account the mixture and the various properties it produced, such as , and , under various conditions of and , developing much of the information contained in modern . For example, arrowheads from the Chinese (around 200 BC) were often constructed with a hard bronze-head, but a softer bronze-tang, combining the alloys to prevent both dulling and breaking during use. Emperor's Ghost Army . November 2014

Mercury has been smelted from 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 temperature). Amalgams have been used since 200 BC in China for objects such as and with precious metals. 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.Rapp, George (2009) Archaeomineralogy . Springer. p. 180. Mercury was often used in mining, to extract precious metals like gold and silver from their ores.Miskimin, Harry A. (1977) The economy of later Renaissance Europe, 1460–1600 . Cambridge University Press. p. 31. .

Precious metals
Many ancient civilizations alloyed metals for purely aesthetic purposes. In ancient and , gold was often alloyed with copper to produce red-gold, or iron to produce a bright burgundy-gold. Gold was often found alloyed with silver or other metals to produce various types of . These metals were also used to strengthen each other, for more practical purposes. Copper was often added to silver to make , increasing its strength for use in dishes, silverware, and other practical items. Quite often, precious metals were alloyed with less valuable substances as a means to deceive buyers.Nicholson, Paul T. and Shaw, Ian (2000) Ancient Egyptian materials and technology . Cambridge University Press. pp. 164–167. . Around 250 BC, was commissioned by the King of Syracuse 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.Kay, Melvyn (2008) Practical Hydraulics . Taylor and Francis. p. 45. .

The term covers a variety of alloys consisting primarily of tin. As a pure metal, tin is much too soft to use for most practical purposes. However, during the , tin was a rare metal in many parts of Europe and the Mediterranean, so it was often valued higher than gold. To make jewellery, cutlery, or other objects from tin, workers usually alloyed it with other metals to increase strength and hardness. These metals were typically , , or copper. These solutes were sometimes added individually in varying amounts, or added together, making a wide variety of objects, ranging from practical items such as dishes, surgical tools, candlesticks or funnels, to decorative items like ear rings and hair clips.

The earliest examples of pewter come from ancient Egypt, around 1450 BC. The use of pewter was widespread across Europe, from France to Norway and Britain (where most of the ancient tin was mined) to the Near East.Hull, Charles (1992) Pewter. Shire Publications. pp. 3–4; The alloy was also used in China and the Far East, arriving in Japan around 800 AD, where it was used for making objects like ceremonial vessels, tea canisters, or chalices used in shrines.Brinkley, Frank (1904) Japan and China: Japan, its history, arts, and literature. Oxford University. p. 317

The first known smelting of iron began in , around 1800 BC. Called the , it produced very soft but . By 800 BC, iron-making technology had spread to Europe, arriving in Japan around 700 AD. , a very hard but brittle alloy of iron and , was being produced in China as early as 1200 BC, but did not arrive in Europe until the Middle Ages. Pig iron has a lower melting point than iron, and was used for making . However, these metals found little practical use until the introduction of around 300 BC. These steels were of poor quality, and the introduction of , around the 1st century AD, sought to balance the extreme properties of the alloys by laminating them, to create a tougher metal. Around 700 AD, the Japanese began folding bloomery-steel and cast-iron in alternating layers to increase the strength of their swords, using clay fluxes to remove and impurities. This method of Japanese swordsmithing produced one of the purest steel-alloys of the ancient world.Smith, Cyril (1960) History of metallography. MIT Press. pp. 2–4. .

While the use of iron started to become more widespread around 1200 BC, mainly because of interruptions in the trade routes for tin, the metal was much softer than bronze. However, very small amounts of , (an alloy of iron and around 1% carbon), was always a byproduct of the bloomery process. The ability to modify the hardness of steel by had been known since 1100 BC, and the rare material was valued for the manufacture of tools and weapons. Because the ancients could not produce temperatures high enough to melt iron fully, the production of steel in decent quantities did not occur until the introduction of during the Middle Ages. This method introduced carbon by heating wrought iron in charcoal for long periods of time, but the absorption of carbon in this manner is extremely slow thus the penetration was not very deep, so the alloy was not homogeneous. In 1740, Benjamin Huntsman began melting blister steel in a crucible to even out the carbon content, creating the first process for the mass production of . Huntsman's process was used for manufacturing tool steel until the early 1900s.Roberts, George Adam; Krauss, George; Kennedy, Richard and Kennedy, Richard L. (1998) Tool steels . ASM International. pp. 2–3. .

The introduction of the blast furnace to Europe in the Middle Ages meant that people could produce in much higher volumes than wrought iron. Because pig iron could be melted, people began to develop processes to reduce carbon in pig iron to create steel. Puddling had been used in China since the first century, and was introduced in Europe during the 1700s, where molten pig iron was stirred while exposed to the air, to remove the carbon by . In 1858, developed a process of steel-making by blowing hot air through liquid pig iron to reduce the carbon content. The led to the first large scale manufacture of steel.

Steel is an alloy of iron and carbon, but the term usually only refers to steels that contain other elements— like , , or —in amounts sufficient to alter the properties of the base steel. Since ancient times, when steel was used primarily for tools and weapons, the methods of producing and working the metal were often closely guarded secrets. Even long after the Age of reason, the steel industry was very competitive and manufacturers went through great lengths to keep their processes confidential, resisting any attempts to scientifically analyze the material for fear it would reveal their methods. For example, the people of , a center of steel production in England, were known to routinely bar visitors and tourists from entering town to deter industrial espionage. Thus, almost no metallurgical information existed about steel until 1860. Because of this lack of understanding, steel was not generally considered an alloy until the decades between 1930 and 1970 (primarily due to the work of scientists like William Chandler Roberts-Austen, , and ), so "alloy steel" became the popular term for ternary and quaternary steel-alloys. Sheffield Steel and America: A Century of Commercial and Technological Independence By Geoffrey Tweedale – Cambridge University Press 1987 Page 57—62 Experimental Techniques in Materials and Mechanics By C. Suryanarayana – CRC Press 2011 p. 202

After Benjamin Huntsman developed his in 1740, he began experimenting with the addition of elements like (in the form of a high-manganese pig-iron called ), which helped remove impurities such as phosphorus and oxygen; a process adopted by Bessemer and still used in modern steels (albeit in concentrations low enough to still be considered carbon steel). Tool Steels, 5th Edition By George Adam Roberts, Richard Kennedy, G. Krauss – ASM International 1998 p. 4 Afterward, many people began experimenting with various alloys of steel without much success. However, in 1882, , being a pioneer in steel metallurgy, took an interest and produced a steel alloy containing around 12% manganese. Called , it exhibited extreme hardness and toughness, becoming the first commercially viable alloy-steel.

(2022). 9781615031467, ASM International. .
Afterward, he created , launching the search for other possible alloys of steel. Sheffield Steel and America: A Century of Commercial and Technological Independence By Geoffrey Tweedale – Cambridge University Press 1987 pp. 57—62

Robert Forester Mushet found that by adding to steel it could produce a very hard edge that would resist losing its hardness at high temperatures. "R. Mushet's special steel" (RMS) became the first . Sheffield Steel and America: A Century of Commercial and Technological Independence By Geoffrey Tweedale – Cambridge University Press 1987 pp. 66—68 Mushet's steel was quickly replaced by steel, developed by Taylor and White in 1900, in which they doubled the tungsten content and added small amounts of chromium and vanadium, producing a superior steel for use in lathes and machining tools. In 1903, the used a chromium-nickel steel to make the crankshaft for their airplane engine, while in 1908 began using vanadium steels for parts like crankshafts and valves in his Model T Ford, due to their higher strength and resistance to high temperatures. Metallurgy for the Non-Metallurgist by Harry Chandler – ASM International 1998 Page 3—5 In 1912, the Krupp Ironworks in Germany developed a rust-resistant steel by adding 21% and 7% , producing the first . Sheffield Steel and America: A Century of Commercial and Technological Independence By Geoffrey Tweedale – Cambridge University Press 1987 p. 75

Due to their high reactivity, most metals were not discovered until the 19th century. A method for extracting aluminium from was proposed by in 1807, using an . Although his attempts were unsuccessful, by 1855 the first sales of pure aluminium reached the market. However, as extractive metallurgy was still in its infancy, most aluminium extraction-processes produced unintended alloys contaminated with other elements found in the ore; the most abundant of which was copper. These aluminium-copper alloys (at the time termed "aluminum bronze") preceded pure aluminium, offering greater strength and hardness over the soft, pure metal, and to a slight degree were found to be heat treatable. Aluminium: Its History, Occurrence, Properties, Metallurgy and Applications by Joseph William Richards – Henry Cairy Baird & Co 1887 Page 25—42 However, due to their softness and limited hardenability these alloys found little practical use, and were more of a novelty, until the used an aluminium alloy to construct the first airplane engine in 1903. During the time between 1865 and 1910, processes for extracting many other metals were discovered, such as chromium, vanadium, tungsten, , , and , and various alloys were developed. Metallurgy: 1863–1963 by W.H. Dennis – Routledge 2017

Prior to 1910, research mainly consisted of private individuals tinkering in their own laboratories. However, as the aircraft and automotive industries began growing, research into alloys became an industrial effort in the years following 1910, as new were developed for pistons and in cars, and for levers and knobs, and aluminium alloys developed for and were put into use.

See also

  • (2022). 9788773043080, Det Kongelige Danske Videnskabernes Selskab.

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