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An alloy is an admixture of
metal A metal (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mi ...

metal
s, or a metal combined with one or more other
element Element may refer to: Science * Chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all ...
s. For example, combining the metallic elements
gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In a pure form, it is a brightness, bright, slightly reddish yel ...

gold
and
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

copper
produces
red gold Pure gold is slightly reddish yellow in color, but colored gold in various other colors can be produced. Colored golds can be classified in three groups: * Alloys with silver and copper in various proportions, producing white, yellow, green and re ...
, gold and
silver Silver is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

silver
becomes
white gold Pure gold is slightly reddish yellow in color, but colored gold in various other colors can be produced. Colored golds can be classified in three groups: * Alloys with silver and copper in various proportions, producing white, yellow, green and re ...

white gold
, and silver combined with copper produces
sterling silver Sterling silver is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous a ...
. Combining
iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), forming much of Earth's and . It is the fourth most common . In its metallic state, iron ...

iron
with non-metallic
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
or
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

silicon
produces alloys called
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

steel
or
silicon steel upright=1.35, Polycrystalline structure of electrical steel after coating has been removed. Electrical steel (lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is an iron Iron () is a chemical elemen ...
. The resulting mixture forms a substance with properties that often differ from those of the pure metals, such as increased strength or hardness. Unlike other substances that may contain metallic bases but do not behave as metals, such as
aluminium oxide Aluminium oxide is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by ...

aluminium oxide
(sapphire),
beryllium aluminium silicate Beryl ( ) is a mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it ...
(emerald) or
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salt (chemistry), salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With Molar mass, molar ...
(salt), an alloy will retain all the properties of a metal in the resulting material, such as
electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current An electric current is a stream of charged particles, suc ...
,
ductility Ductility is a mechanical property commonly described as a material's amenability to drawing Drawing is a form of visual art The visual arts are art forms such as painting Painting is the practice of applying paint, pigment, c ...

ductility
, opacity, and
luster Lustre or Luster may refer to: Places * Luster, Norway, a municipality in Vestlandet, Norway ** Luster (village), a village in the municipality of Luster * Lustre, Montana, an unincorporated community in the United States Entertainment * Luste ...
. 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
titanium Titanium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

titanium
alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel,
solder Solder (, or in North America ) is a fusible metal alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polish ...

solder
,
brass Brass is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appear ...

brass
,
pewter Pewter () is a malleable Ductility is a mechanical property commonly described as a material's amenability to drawing Drawing is a form of visual art in which an artist uses instruments to mark paper Paper is a thin sheet material ...
,
duralumin Fire-damaged Duralumin cross brace from the Zeppelin airship LZ 129 Hindenburg, ''Hindenburg'' (DLZ129) salvaged from its crash site at Lakehurst Naval Air Station, New Jersey, NJ on May 6, 1937 Duralumin (also called duraluminum, duraluminium, dur ...
,
bronze Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (such as aluminum, manganese, nickel or zinc) and sometimes non-metals or metalloids such as arsenic, phosphorus or ...

bronze
, and amalgams. An alloy may be a
solid solution A solid solution describes a family of materials which have a range of compositions (e.g. AxB1−x) and a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" is used to desc ...

solid solution
of metal elements (a single phase, where all metallic grains (crystals) are of the same composition) or a
mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...

mixture
of metallic phases (two or more solutions, forming a
microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymer ...
of different crystals within the metal).
Intermetallic An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic Metallic may be a reference to: *Metal A metal (from Ancient Greek, Greek μ ...
compounds are alloys with a defined
stoichiometry Stoichiometry refers to the relationship between the quantities of reactant 200px, Reactants, such as sulfur (''pictured''), are the starting materials that are used in chemical reactions. A reagent is a substance or compound added to a sy ...
and crystal structure.
Zintl phaseIn chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo ...
s are also sometimes considered alloys depending on bond types (see Van Arkel–Ketelaar triangle for information on classifying bonding in binary compounds). Alloys are defined by a
metallic bonding Metallic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of att ...

metallic bonding
character. The alloy constituents are usually measured by mass percentage for practical applications, and in atomic fraction 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
intermetallic An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic Metallic may be a reference to: *Metal A metal (from Ancient Greek, Greek μ ...
.


Introduction

An alloy is a mixture of
chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, eleme ...
s, which forms an impure substance (admixture) that retains the characteristics of a
metal A metal (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mi ...

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 Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag Inclusion (mineral), inclusions (up to 2% by weight), which gives it a ...
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 Solubility is the property of a solid, liquid or gaseous chemical substance called ''solution, solute'' to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the Physical property, physical an ...

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 Ductility is a mechanical property commonly described as a material's amenability to drawing Drawing is a form of visual art in which an artist uses instruments to mark paper Paper is a thin sheet material produced by mechanically a ...
), such as
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
, can be altered by alloying it with another soft metal, such as
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

copper
. Although both metals are very soft and
ductile Ductility is a mechanical property commonly described as a material's amenability to drawing Drawing is a form of visual art in which an artist uses instruments to mark paper Paper is a thin sheet material produced by mechanically a ...

ductile
, the resulting
aluminium alloy Aluminium alloys (or aluminum alloys; see spelling differences) are alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly pr ...
will have much greater
strength Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human capability *A common attrib ...
. Adding a small amount of non-metallic
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
to
iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), forming much of Earth's and . It is the fourth most common . In its metallic state, iron ...

iron
trades its great ductility for the greater strength of an alloy called
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

steel
. Due to its very-high strength, but still substantial
toughness In materials science The Interdisciplinarity, interdisciplinary field of materials science, also commonly termed materials science and engineering, covers the design and discovery of new materials, particularly solids. The intellectual origin ...
, and its ability to be greatly altered by
heat treatment ''Heat Treatment'' is the second album by English singer-songwriter Graham Parker and his band The Rumour, released in 1976. A close follow-up to Parker's debut album ''Howlin' Wind'', ''Heat Treatment'' was well received by critics and contains ...
, steel is one of the most useful and common alloys in modern use. By adding
chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

chromium
to steel, its resistance to
corrosion Corrosion is a that converts a refined metal into a more chemically stable form such as , , or . It is the gradual destruction of materials (usually a ) by chemical and/or electrochemical reaction with their environment. is the field dedica ...

corrosion
can be enhanced, creating
stainless steel Stainless steel is a group of ferrous alloys that contain a minimum of approximately 11% chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical ele ...
, while adding
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

silicon
will alter its electrical characteristics, producing
silicon steel upright=1.35, Polycrystalline structure of electrical steel after coating has been removed. Electrical steel (lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is an iron Iron () is a chemical elemen ...
. Like oil and water, a molten metal may not always mix with another element. For example, pure iron is almost completely
insoluble In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atom ...
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
liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material density is constant within a fluid par ...

liquid
state, they may not always be soluble in the
solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied ...

solid
state. If the metals remain soluble when solid, the alloy forms a
solid solution A solid solution describes a family of materials which have a range of compositions (e.g. AxB1−x) and a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" is used to desc ...

solid solution
, becoming a homogeneous structure consisting of identical crystals, called a
phase Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
. 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 Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymer ...
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 Supersaturation occurs with a chemical solution when the concentration of a solute exceeds the concentration specified by the value equilibrium solubility. Most commonly the term is applied to a solution of a solid in a liquid. A supersaturated sol ...
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
electrum Brooch with a griffin protome, from the necropolis of Kameiros, Rhodes, (Louvre">Kameiros.html" ;"title="protome, from the necropolis of Kameiros">protome, from the necropolis of Kameiros, Rhodes, (Louvre) Electrum is a naturally occurri ...

electrum
—an alloy of
silver Silver is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

silver
and
gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In a pure form, it is a brightness, bright, slightly reddish yel ...

gold
—occur naturally. Meteorites are sometimes made of naturally occurring alloys of iron and
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

nickel
, but are not native to the Earth. One of the first alloys made by humans was
bronze Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (such as aluminum, manganese, nickel or zinc) and sometimes non-metals or metalloids such as arsenic, phosphorus or ...

bronze
, which is a mixture of the metals
tin Tin is a with the Sn (from la, ) and  50. Tin is a silvery-colored metal that characteristically has a faint yellow hue. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent ...

tin
and
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

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 Smelting is a process of applying heat to ore ore – psilomelane (size: 6.7 × 5.8 × 5.1 cm) ore – galena and anglesite (size: 4.8 × 4.0 × 3.0 cm) ore (size: 7.5 × 6.1 × 4.1 cm) File:OreCartPachuca.JPG, upMinecart on ...
) during the manufacture of iron. Other ancient alloys include
pewter Pewter () is a malleable Ductility is a mechanical property commonly described as a material's amenability to drawing Drawing is a form of visual art in which an artist uses instruments to mark paper Paper is a thin sheet material ...
,
brass Brass is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appear ...

brass
and
pig iron Pig iron, also known as crude iron, is an intermediate product Intermediate goods, producer goods or semi-finished products are goods In economics Economics () is the social science that studies how people interact with value; in parti ...

pig iron
. In the modern age, steel can be created in many forms.
Carbon steel Carbon steel is a steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fract ...
can be made by varying only the carbon content, producing soft alloys like
mild steel Carbon steel is a steel with carbon content from about 0.05 up to 3.8 per cent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...
or hard alloys like
spring steel Spring steel is a name given to a wide range of steel Steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength of materials, strength and fracture toughness, fracture resistance compared to iron. Ma ...
.
Alloy steel Alloy steel is steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, s ...
s can be made by adding other elements, such as
chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

chromium
,
molybdenum Molybdenum is a chemical element with the Symbol (chemistry), symbol Mo and atomic number 42. The name is from New Latin, Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ores. Moly ...

molybdenum
,
vanadium Vanadium is a chemical element with the Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...

vanadium
or
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

nickel
, resulting in alloys such as
high-speed steel High-speed steel (HSS or HS) is a subset of tool steel Tool steel is any of various carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 3.8 per cent by weight. The definition of carbon steel from the American Iron ...
or
tool steel Tool steel is any of various carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 3.8 per cent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content ...
. Small amounts of
manganese Manganese is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

manganese
are usually alloyed with most modern steels because of its ability to remove unwanted impurities, like
phosphorus Phosphorus is a chemical element with the Symbol (chemistry), symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly Reactivity (chemistry), reactive, phosphor ...

phosphorus
,
sulfur Sulfur (in nontechnical British English: sulphur) is a with the  S and  16. It is , and lic. Under , sulfur atoms form cyclic octatomic molecules with a chemical formula . Elemental sulfur is a bright yellow, line solid at . Sul ...

sulfur
and
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, which can have detrimental effects on the alloy. However, most alloys were not created until the 1900s, such as various aluminium,
titanium Titanium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...
,
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...
, and
magnesium alloy thumbnail, Figure 1: Number of scientific articles with terms ''AZ91'' or ''AZ31'' in the abstract. Magnesium alloys are mixtures of magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic ...
s. Some modern
superalloy Nickel superalloy jet engine (RB199) turbine blade A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Several key characteristics of a superalloy are excellent mechanical str ...
s, such as
incoloyIncoloy refers to a range of superalloys Nickel superalloy jet engine ( RB199) turbine blade A superalloy, or high-performance alloy, is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''m ...
,
inconel Inconel is a registered trademark of Special Metals Corporation for a family of austenitic nickel-chromium-based superalloys. Inconel alloys are oxidation-corrosion resistance, corrosion-resistant materials well suited for service in extreme envi ...
, and hastelloy, may consist of a multitude of different elements.


Terminology

As a noun, the term alloy is used to describe a mixture of atoms in which the primary constituent is a metal. When used as a verb, the term refers to the act of mixing a metal with other elements. The primary metal is called the ''base'', the ''matrix'', or the ''
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

solvent
''. The secondary constituents are often called ''solutes''. If there is a mixture of only two types of atoms (not counting impurities) such as a
copper-nickel Cupronickel or copper-nickel (CuNi) is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured ...
alloy, then it is called a ''binary alloy.'' If there are three types of atoms forming the mixture, such as iron, nickel and chromium, then it is called a ''ternary alloy.'' An alloy with four constituents is a ''quaternary alloy,'' while a five-part alloy is termed a ''quinary alloy.'' Because the percentage of each constituent can be varied, with any mixture the entire range of possible variations is called a ''system''. In this respect, all of the various forms of an alloy containing only two constituents, like iron and carbon, is called a ''binary system,'' while all of the alloy combinations possible with a ternary alloy, such as alloys of iron, carbon and chromium, is called a ''ternary system''. 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 sulfideIron sulfide or Iron sulphide can refer to range of chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element ...
, which is very brittle, creating weak spots in the steel.
Lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consist ...

Lithium
,
sodium Sodium is a with the  Na (from Latin ''natrium'') and  11. It is a soft, silvery-white, highly reactive . Sodium is an , being in of the periodic table. Its only stable is 23Na. The free metal does not occur in nature, and must be ...

sodium
and
calcium Calcium is a with the Ca and 20. As an , calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues and . It is the fifth most abun ...

calcium
are common impurities in aluminium alloys, which can have adverse effects on the structural integrity of castings. Conversely, otherwise pure-metals that simply 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 of rutile Rutile is a mineral composed primarily of titanium dioxide (TiO2), and is the most common natural form of TiO2. Other rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest re ...
s; 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 Extractive metallurgy is a branch of metallurgical engineering Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metal A metal (from Ancient Greek, Greek μέταλλον ''mé ...
. 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 The fineness of a precious metal object (coin, bar, jewelry, etc.) represents the weight of ''fine metal'' therein, in proportion to the total weight which includes alloying base metals and any impurity, impurities. Alloy metals are added to increa ...
gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In a pure form, it is a brightness, bright, slightly reddish yel ...

gold
is an alloy of gold with other elements. Similarly, the
silver Silver is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

silver
used in
jewelry Jewellery or jewelry consists of decorative items worn for personal adornment, such as brooches, ring (jewellery), rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a wester ...

jewelry
and the
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

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 Aluminium alloys (or aluminum alloys; see spelling differences) are alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly pr ...
are commonly referred to as simply "
alloy wheel In the automotive industry, alloy wheels are wheels that are made from an alloy of aluminium or magnesium. Alloys are mixtures of a metal and other elements. They generally provide greater strength over pure metals, which are usually much softer ...
s", although in point of fact steels and most other metals in practical use are also alloys. Steel is such a common alloy that many items made from it, like
wheel File:Roue primitive.png, An early wheel made of a solid piece of wood A wheel is a circular component that is intended to rotate on an axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the ...

wheel
s,
barrel A barrel or cask is a hollow cylindrical A cylinder (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Eu ...

barrel
s, or
girder A girder () is a support beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Particle beam, a stream of charged or neutral particles **Charged particle ...
s, are simply referred to by the name of the item, assuming it is made of steel. When made from other materials, they are typically specified as such, (i.e.: "bronze wheel", "plastic barrel", or "wood girder").


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 The melting point (or, rarely, liquefaction point) of a substance is the at which it changes from to . At the melting point the solid and liquid phase exist in . The melting point of a substance depends on and is usually specified at a such ...

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 Titanium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

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 A modern crucible used in the production of silicon ingots via the Czochralski process">400x400px A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibl ...

crucible
s. 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 A blast furnace is a type of metallurgical Metallurgy is a domain of Materials science, materials science and engineering that studies the physical and chemical behavior of metallic Chemical element, elements, their Inter-metallic alloy, inter-m ...
to make pig iron (liquid-gas),
nitriding Nitriding is a heat treating Heat treating (or heat treatment) is a group of industrial, thermal and metalworking, metalworking processes used to alter the physical property, physical, and sometimes chemical property, chemical, properties of a m ...
, carbonitriding or other forms of
case hardening Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal at the surface. For iron or steel with low carbon ...
(solid-gas), or the
cementation process The cementation process is an obsolete technology Technology ("science of craft", from Ancient Greek, Greek , ''techne'', "art, skill, cunning of hand"; and , ''wikt:-logia, -logia'') is the sum of Art techniques and materials, techniques, s ...
used to make
blister steel The cementation process is an obsolete Obsolescence is the state of being which occurs when an object, service, or practice is no longer maintained, required, or degraded even though it may still be in good working order. The international stan ...
(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 Pattern welding is the practice in sword A sword is a Edged and bladed weapons, bladed melee weapon intended for cutting or thrusting that is longer than a knife or dagger, consisting of a long blade attached to a hilt. The precise definition ...
(solid-solid), shear steel (solid-solid), or
crucible steel Crucible steel is steel Steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength of materials, strength and fracture toughness, fracture resistance compared to iron. Many other elements may be pre ...
production (solid-liquid), mixing the elements via solid-state
diffusion File:DiffusionMicroMacro.gif, 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diff ...

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 Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

steel
, which is stronger than
iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), forming much of Earth's and . It is the fourth most common . In its metallic state, iron ...

iron
, its primary element. The
electrical Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon in which an object changes its positio ...
and
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal c ...

thermal conductivity
of alloys is usually lower than that of the pure metals. The physical properties, such as
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
, reactivity,
Young's modulus Young's modulus E, the Young modulus, or the in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive of a material when the force is applied lengthwise. It quantifies the relations ...
of an alloy may not differ greatly from those of its base element, but engineering properties such as
tensile strength In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
, ductility, and
shear strength In engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range ...
may be substantially different from those of the constituent materials. This is sometimes a result of the sizes of the
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

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 Ferromagnetism is the basic mechanism by which certain materials (such as iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first tr ...
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 Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (such as aluminum, manganese, nickel or zinc) and sometimes non-metals or metalloids such as arsenic, phosphorus or ...

Bronze
, an alloy of
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

copper
and
tin Tin is a with the Sn (from la, ) and  50. Tin is a silvery-colored metal that characteristically has a faint yellow hue. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent ...

tin
, was the first alloy discovered, during the
prehistoric Prehistory, also known as pre-literary history, is the period of human history Human history, also known as world history, is the description of humanity's past. It is informed by archaeology Archaeology or archeology is the study ...
period now known as the
Bronze Age The Bronze Age is a prehistoric that was characterized by the use of , in some areas , and other early features of urban . The Bronze Age is the second principal period of the , as proposed in modern times by , for classifying and studying a ...
. 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,
bell A bell is a struck idiophone, directly struck idiophone percussion instrument. Most bells have the shape of a hollow cup that when struck vibrates in a single strong strike tone, with its sides forming an efficient resonator. The strike may be m ...

bell
s,
statue A statue is a free-standing in which the realistic, full-length figures of persons or animals are carved or in a durable material such as wood, metal or stone. Typical statues are life-sized or close to life-size; a sculpture that represent ...

statue
s, and bearings.
Brass Brass is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appear ...

Brass
is an alloy made from
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

copper
and
zinc Zinc is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...

zinc
. Unlike pure metals, most alloys do not have a single
melting point The melting point (or, rarely, liquefaction point) of a substance is the at which it changes from to . At the melting point the solid and liquid phase exist in . The melting point of a substance depends on and is usually specified at a such ...

melting point
, but a melting range during which the material is a mixture of
solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied ...

solid
and
liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material density is constant within a fluid par ...

liquid
phases (a slush). The temperature at which melting begins is called the
solidus Solidus (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republi ...
, and the temperature when melting is just complete is called the
liquidus Image:SiO2 Li2O.GIF, thumbnail, 250px, Liquidus temperature curve in the binary glass system SiO2-Li2O, based on 91 published data collected iSciGlass model fit froGlassproperties.com The liquidus temperature, TL or Tliq, specifies the temperature ...
. 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-treatable alloys

Alloying elements are added to a base metal, to induce
hardness Hardness (antonym: softness) is a measure of the resistance to localized induced by either mechanical or . In general, different materials differ in their hardness; for example hard metals such as and are harder than soft metals such as and ...
,
toughness In materials science The Interdisciplinarity, interdisciplinary field of materials science, also commonly termed materials science and engineering, covers the design and discovery of new materials, particularly solids. The intellectual origin ...
,
ductility Ductility is a mechanical property commonly described as a material's amenability to drawing Drawing is a form of visual art The visual arts are art forms such as painting Painting is the practice of applying paint, pigment, c ...

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 In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection (engineering) , Deflection is the relative change in external displace ...
by hammering, bending, extruding, et cetera, and are permanent unless the metal is recrystallized. Otherwise, some alloys can also have their properties altered by
heat treatment ''Heat Treatment'' is the second album by English singer-songwriter Graham Parker and his band The Rumour, released in 1976. A close follow-up to Parker's debut album ''Howlin' Wind'', ''Heat Treatment'' was well received by critics and contains ...
. 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
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
,
copper Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

copper
,
magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

magnesium
,
titanium Titanium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

titanium
, and
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

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 Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

steel
. The base metal iron of the iron-carbon alloy known as steel, undergoes a change in the arrangement (
allotropy Allotropy or allotropism () is the property of some chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting o ...
) 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
diffusion File:DiffusionMicroMacro.gif, 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diff ...

diffusion
happens, the carbon atoms are said to be in ''
solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the water the solvent. In chemistry ...

solution
'' in the iron, forming a particular single, homogeneous, crystalline phase called
austenite 250px, Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron I ...
. 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 Solubility is the property of a solid, liquid or gaseous chemical substance called ''solution, solute'' to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the Physical property, physical an ...

soluble
with the iron, and will be forced to
precipitate Precipitation is the process of conversion of a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ever ...
out of solution, nucleating 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
cementite Cementite (or iron carbide) is a Chemical compound, compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. I ...

cementite
(or
carbide . In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms ...

carbide
), 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 Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium Aluminium (aluminum in Ameri ...
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 An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic Metallic may be a reference to: *Metal A metal (from Ancient Greek, Greek μ ...
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.


Substitutional and interstitial alloys

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 Brass is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appear ...

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 spaces between the atoms of the crystal matrix, called the ''interstices''. 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.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 meteoric iron, a naturally occurring alloy of
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

nickel
and
iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), forming much of Earth's and . It is the fourth most common . In its metallic state, iron ...

iron
. It is the main constituent of iron meteorites. 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 cold working, work.


Bronze and brass

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 Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

copper
, however, was found worldwide, along with
silver Silver is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

silver
,
gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In a pure form, it is a brightness, bright, slightly reddish yel ...

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. Around 10,000 years ago in the highlands of Anatolia (Turkey), humans learned to smelting, smelt metals such as copper and
tin Tin is a with the Sn (from la, ) and  50. Tin is a silvery-colored metal that characteristically has a faint yellow hue. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent ...

tin
from ore. Around 2500 BC, people began alloying the two metals to form
bronze Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (such as aluminum, manganese, nickel or zinc) and sometimes non-metals or metalloids such as arsenic, phosphorus or ...

bronze
, 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
zinc Zinc is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...

zinc
to form
brass Brass is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appear ...

brass
. Ancient civilizations took into account the mixture and the various properties it produced, such as
hardness Hardness (antonym: softness) is a measure of the resistance to localized induced by either mechanical or . In general, different materials differ in their hardness; for example hard metals such as and are harder than soft metals such as and ...
,
toughness In materials science The Interdisciplinarity, interdisciplinary field of materials science, also commonly termed materials science and engineering, covers the design and discovery of new materials, particularly solids. The intellectual origin ...
and
melting point The melting point (or, rarely, liquefaction point) of a substance is the at which it changes from to . At the melting point the solid and liquid phase exist in . The melting point of a substance depends on and is usually specified at a such ...

melting point
, under various conditions of temperature and work hardening, developing much of the information contained in modern Phase diagram, alloy phase diagrams. For example, arrowheads from the Chinese Qin dynasty (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.


Amalgams

Mercury (element), Mercury has been smelted from cinnabar for thousands of years. Mercury dissolves many metals, such as gold, silver, and tin, to form amalgam (chemistry), amalgams (an alloy in a soft paste or liquid form at ambient temperature). Amalgams have been used since 200 BC in China for gilding objects such as armor and mirrors 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. Mercury was often used in mining, to extract precious metals like gold and silver from their ores.


Precious-metal alloys

Many ancient civilizations alloyed metals for purely aesthetic purposes. In ancient Egypt and Mycenae, 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 colored gold. These metals were also used to strengthen each other, for more practical purposes. Copper was often added to silver to make
sterling silver Sterling silver is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous a ...
, 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. Around 250 BC, Archimedes was commissioned by the King of Syracuse, Sicily, 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.


Pewter

The term
pewter Pewter () is a malleable Ductility is a mechanical property commonly described as a material's amenability to drawing Drawing is a form of visual art in which an artist uses instruments to mark paper Paper is a thin sheet material ...
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
Bronze Age The Bronze Age is a prehistoric that was characterized by the use of , in some areas , and other early features of urban . The Bronze Age is the second principal period of the , as proposed in modern times by , for classifying and studying a ...
, 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 lead, antimony, bismuth 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. 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 shinto shrines.


Steel and pig iron

The first known smelting of iron began in Anatolia, around 1800 BC. Called the bloomery, bloomery process, it produced very soft but ductile
wrought iron Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag Inclusion (mineral), inclusions (up to 2% by weight), which gives it a ...
. By 800 BC, iron-making technology had spread to Europe, arriving in Japan around 700 AD. Pig iron, a very hard but brittle alloy of iron and
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
, was being produced in History of China#Shang Dynasty (c. 1700–1046 BC), 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 cast-iron. However, these metals found little practical use until the introduction of
crucible steel Crucible steel is steel Steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength of materials, strength and fracture toughness, fracture resistance compared to iron. Many other elements may be pre ...
around 300 BC. These steels were of poor quality, and the introduction of
pattern welding Pattern welding is the practice in sword A sword is a Edged and bladed weapons, bladed melee weapon intended for cutting or thrusting that is longer than a knife or dagger, consisting of a long blade attached to a hilt. The precise definition ...
, 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 slag 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
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

steel
, (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
heat treatment ''Heat Treatment'' is the second album by English singer-songwriter Graham Parker and his band The Rumour, released in 1976. A close follow-up to Parker's debut album ''Howlin' Wind'', ''Heat Treatment'' was well received by critics and contains ...
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
blister steel The cementation process is an obsolete Obsolescence is the state of being which occurs when an object, service, or practice is no longer maintained, required, or degraded even though it may still be in good working order. The international stan ...
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
tool steel Tool steel is any of various carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 3.8 per cent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content ...
. 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
pig iron Pig iron, also known as crude iron, is an intermediate product Intermediate goods, producer goods or semi-finished products are goods In economics Economics () is the social science that studies how people interact with value; in parti ...

pig iron
in much higher volumes than wrought iron. Because pig iron could be melted, people began to develop processes to reduce carbon in
liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material density is constant within a fluid par ...

liquid
pig iron to create steel. Puddling (metallurgy), 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 oxidation. In 1858, Henry Bessemer developed a process of steel-making by blowing hot air through liquid pig iron to reduce the carbon content. The Bessemer process led to the first large scale manufacture of steel.


Alloy steels

Steel is an alloy of iron and carbon, but the term ''alloy steel'' usually only refers to steels that contain other elements— like
vanadium Vanadium is a chemical element with the Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...

vanadium
,
molybdenum Molybdenum is a chemical element with the Symbol (chemistry), symbol Mo and atomic number 42. The name is from New Latin, Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ores. Moly ...

molybdenum
, or cobalt—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 Sheffield, 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, Adolf Martens, and Edgar Bain), so "alloy steel" became the popular term for ternary and quaternary steel-alloys. After Benjamin Huntsman developed his
crucible steel Crucible steel is steel Steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength of materials, strength and fracture toughness, fracture resistance compared to iron. Many other elements may be pre ...
in 1740, he began experimenting with the addition of elements like
manganese Manganese is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

manganese
(in the form of a high-manganese pig-iron called ''spiegeleisen''), 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). Afterward, many people began experimenting with various alloys of steel without much success. However, in 1882, Robert Hadfield, being a pioneer in steel metallurgy, took an interest and produced a steel alloy containing around 12% manganese. Called mangalloy, it exhibited extreme hardness and toughness, becoming the first commercially viable alloy-steel. Afterward, he created
silicon steel upright=1.35, Polycrystalline structure of electrical steel after coating has been removed. Electrical steel (lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is an iron Iron () is a chemical elemen ...
, launching the search for other possible alloys of steel. Robert Forester Mushet found that by adding tungsten 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
high-speed steel High-speed steel (HSS or HS) is a subset of tool steel Tool steel is any of various carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 3.8 per cent by weight. The definition of carbon steel from the American Iron ...
. Mushet's steel was quickly replaced by tungsten carbide 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 Wright brothers used a chromium-nickel steel to make the crankshaft for their airplane engine, while in 1908 Henry Ford 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. In 1912, the Krupp Ironworks in Germany developed a rust-resistant steel by adding 21%
chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

chromium
and 7%
nickel Nickel is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

nickel
, producing the first
stainless steel Stainless steel is a group of ferrous alloys that contain a minimum of approximately 11% chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical ele ...
.


Aluminium and other nonferrous-alloys

Nonferrous alloys contain no appreciable amounts of iron. The first alloys, bronze and brass, were used for thousands of years, along with lead alloys, pewter and others—but these were all made from metals that were fairly non-reactive and could be smelted over open flames. In the 18th century, Antoine Lavoisier helped to establish the oxygen theory of combustion, displacing the defunct phlogiston theory that had ruled since the late Middle Ages. The oxygen theory helped correctly explain the phenomenon of things like oxidation of metals (i.e., rust) and how rocky ores transform into metals when heated. Lavoisier predicted that many of the earths, salts, and alkalis—for example in alum, a salt used since antiquity—contained metallic bases that were too reactive to oxygen to smelt by the usual methods. His work eventually led to the periodic table of elements, which helped confirm the existence of these "missing metals." Due to their high reactivity, most metals were not discovered until the 19th century. A method for extracting aluminium from bauxite was proposed by Humphry Davy in 1807, using an electric arc. Although his attempts were unsuccessful, by 1855 the first sales of pure aluminium reached the market. However, as
extractive metallurgy Extractive metallurgy is a branch of metallurgical engineering Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metal A metal (from Ancient Greek, Greek μέταλλον ''mé ...
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. However, due to their softness and limited hardenability these alloys found little practical use, and were more of a novelty, until the Wright brothers 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, iridium, cobalt, and
molybdenum Molybdenum is a chemical element with the Symbol (chemistry), symbol Mo and atomic number 42. The name is from New Latin, Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ores. Moly ...

molybdenum
, and various alloys were developed. 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
magnesium alloy thumbnail, Figure 1: Number of scientific articles with terms ''AZ91'' or ''AZ31'' in the abstract. Magnesium alloys are mixtures of magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic ...
s were developed for pistons and alloy wheel, wheels in cars, and pot metal for levers and knobs, and aluminium alloys developed for airframes and aircraft skins were put into use.


Precipitation-hardening alloys

In 1906,
precipitation hardening Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium Aluminium (aluminum in Ameri ...
alloys were discovered by Alfred Wilm. Precipitation hardening alloys, such as certain alloys of
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
,
titanium Titanium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

titanium
, and copper, are heat-treatable alloys that soften when quenching, 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 Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

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 Fire-damaged Duralumin cross brace from the Zeppelin airship LZ 129 Hindenburg, ''Hindenburg'' (DLZ129) salvaged from its crash site at Lakehurst Naval Air Station, New Jersey, NJ on May 6, 1937 Duralumin (also called duraluminum, duraluminium, dur ...
was one of the first "age hardening" alloys used, becoming the primary building material for the first Zeppelins, and was soon followed by many others. 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.Jacobs, M.H
Precipitation Hardnening
. University of Birmingham. TALAT Lecture 1204. slideshare.net


See also

* Alloy broadening * CALPHAD * Ideal mixture * List of alloys


References


Bibliography

*


External links

* * {{Authority control Alloys, Metallurgy Chemistry