A alloy is a combination of a Métal with one or more other elements.

A pure Métal has relatively weak mechanical characteristics. The fact of adding other elements makes it possible “to harden” (to increase the mechanical characteristics) and/or to modify the chemical characteristics of metals (in particular their behavior with the Corrosion), or to improve of other characteristics (simplicity of implementation: flow for example).

Principal metal is called the base metal or “bases”. The elements added voluntarily are called the elements of alloy (or of addition) and the elements nondesired are called the impurities.

The alloy elements are generally metals but they can also be other chemical elements (examples: the Carbon in the Steel or the cast iron, silicon in aluminum,…).

Generally, when the alloy element is not a metal, its proportion remains weak. The carbon concentration is lower than 2% in mass for steel and lower than 6% in mass for the cast iron, whereas it is possible to make an alloy Cuivre Zinc (Laiton) with 50% of each element.

An alloy can be natural, it is rare but that exists, for example electrum alloy of gold and money natives used in prehistory and antiquity: Varna, Anatolia, Ur, Egypt etc

Binary alloying

Binary alloying with only one phase

A homogeneous alloy consists of only one homogeneous phase solid. To obtain a homogeneous alloy, it is necessary that there is total miscibility between the alloy elements. There are two possibilities:

* the two alloy elements are soluble one in another whatever their respective proportions.
*la concentration of the alloy element is lower than the limit of solubility.

Conditions to obtain a homogeneous alloy with total solubility in a solid state (rule of Hume-Rothery):

  • the elements constituting alloy must be isomorphous: even crystallographic structure (Cubic with centered faces with cubic with centered faces, of a Cubic centered with cubic centered, Hexagonal compact with hexagonal compact).
  • the Atome S must have a close size: their rays should not differ of more than 15%. In the contrary case solubility will be very limited.
  • the valence of the two elements must be identical.
Example: the Bismuth and the Antimony

The antimony and bismuth alloys form a solid solution (in lower part of the solidus) in all the cases of figure and at all the temperatures. The Diagramme of phase is a diagram with a spindle. Some other couples of which miscibility is good and makes it possible to obtain homogeneous solid solutions at all temperatures: Copper - Nickel, copper palladium, Money - Gold, money - palladium, Molybdenum - Vanadium, molydène- Tungsten etc

Certain soluble binary alloyings present to the lowest temperatures a defect of solubility. It appears on the diagram of phase a sector where two phases cohabit, the first consisting of a solid solution saturated with B in has, the second conversely has in B. It is the case for example system copper-nickel which presents in lower part 322°C a zone with two phases 1 and 2.

Structure of an alloy

Homogeneous alloy

A homogeneous alloy can be ordered (the atoms of various natures follow a strict alternation) or disordered (the various atoms occupy of the random places).

In general, there is a majority metal, other metals are called “elements of alloy”. The atoms of the alloy elements can take the place of the atoms of majority metal, one speaks then about “substitution”; they can also slip between the atoms of majority alloy, one speaks about “insertion”. When a metal is present in low content in an alloy, one often speaks about solid Solution.:: * The dogs are animals

Heterogeneous alloy

When the content of alloy element increases, one can have formation of two phases: a phase containing few alloy elements, and a phase with strong content of alloy elements. The Cristallite S with strong content are called “precipitated”.

The Précipité S are often ordered alloys, which one calls “intermetallic”. The intermetallic ones thus formed are studied sometimes thereafter as clean alloys, like a new material, and one tries to produce some as such and either as precipitates.

Examples of alloys

Principal alloys

Alloys of Iron

* cast iron: Iron + Carbon (with more than 2,1% and up to 6.7% in carbon mass)

* Steel: iron + carbon (with less than 2,8% in carbon mass)
* Stainless steel: iron + nickel+ Chromium, and sometimes, Molybdenum, Vanadium
cuani (2.7 OF cast iron and 2.9 of nickel)

Alloys of copper

* Bronze: Copper + tin; the Airain is the old name of bronze

* Laiton: copper + Zinc
* Dolomite: copper + rockwool + nickel
* Ebonite: copper + iron + chromium + calcium

Alloys of Aluminum

They are also called light alloys

For more detail to see the two articles below:

* Alloys of aluminum for welding
* Alloys of aluminum for foundry

Less known alloys

  • Alloy lead-tin: for the welding

  • Amalgam: mercury + another metal, for example Gold or coppers; the term also indicates a mixture of metals used for the dental care (“leading”)
  • Maillechort: copper + zinc + nickel
  • Monel ® (trade name): nickel + copper
  • Babbit metal: tin or Lead + Antimony
  • Zamak: zinc + aluminum + magnesium + copper

Alloys for specific applications

  • Ferrotitaniums: iron + 25 with Ti 70%m + 4 with aluminum 10%m

  • TA6V: Titanium + 6%m Aluminum + 4%m vanadium, very much used in the aircraft industry.
    Nota: usual designation French TA6V is based on old standard NF has 02-004 cancelled today, its chemical designation is Ti Al 6 V.
  • MCrAl: metal + chromium + aluminum + sometimes of the Yttrium (MCrAlY), alloys considered for their good mechanical resistance and resistance to the Corrosion at high temperature)
    • FeCrAl: iron + chromium + aluminum
    • NiCrAl: nickel + chromium + aluminum
  • superalloys at base nickel (for example the Inconel S): good mechanical resistance and corrosion resistance at high Intermetallic temperature
  • S: ordered alloys, respecting a stoichiometry specifies (but variations with stoichiometry are tolerated)
    • NiAl β: 50%at nickel + 50%at aluminum
    • B2 Ferroaluminium: 50%at iron + 50%at aluminum
    • TiAl: 50%at titanium + 50%at aluminum

  • alloys presenting of low dilation coefficients (Inventor Charles Edouard Guillaume)

    • Invar (nickel 36%, manganese 0,4%, carbon 0,1%, iron 63,5%)
    • elinvar (Nivarox, Métélinvar, Isoval) (nickel, chromium, iron)

  • FeNiCo : the Kovar, an alloy Fe/Nickel/Cobalt intended for sealing glass/metal or ceramics/metal

  • the Memory-shape alloys:

    • NiTi : 50% Nickel + 50% titanium
    • CuZnAl: 70% Copper + 25% Zinc + 5% aluminum
    • CuAlNi: 82% copper + 12% aluminum + 4% nickel
    • CuALBe: copper + aluminum + Béryllium

Mythical alloys

See also

alloy

Simple: Alloy

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