Invar
Encyclopedia
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
notable for its uniquely low coefficient of thermal expansion (CTE or α). The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.
It was invented in 1896 by Swiss scientist Charles Édouard Guillaume
Charles Edouard Guillaume
Charles Édouard Guillaume was a Swiss physicist who received the Nobel Prize in Physics in 1920 in recognition of the service he had rendered to precision measurements in physics by his discovery of anomalies in nickel steel alloys.Guillaume is known for his discovery of nickel-steel alloys he...
. He received the Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
in 1920 for this discovery, which enabled improvements in scientific instruments.
Properties and applications
Like other nickelNickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
/iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
compositions, Invar is a solid solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
; that is, it is a single-phase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
.
Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between of about 1.2 × 10−6 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
−1 (1.2 ppm
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
/°C). Extra-pure grades (<0.1% Co
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
) can readily produce values as low as 0.62–0.65 ppm/°C. Some formulations display negative thermal expansion
Negative thermal expansion
Negative Thermal Expansion is a physicochemical process in which some materials contract upon heating rather than expanding as most materials do. Materials which undergo this unusual process have a range of potential engineering, photonic, electronic, and structural applications...
(NTE) characteristics. Though it displays high dimensional stability over a range of temperatures, it does have a propensity to creep
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
.
Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, television shadow-mask
Shadow mask
The shadow mask is one of two major technologies used to manufacture cathode ray tube televisions and computer displays that produce color images. The other approach is aperture grille, better known by its trade name, Trinitron. All early color televisions and the majority of CRT computer monitors...
frames, valves in motors, and antimagnetic watch
Antimagnetic watch
Anti-magnetic watches are those that are able to run with minimal deviation when exposed to a certain magnetic field. The International Organization for Standardization issued a standard for magnetic resistant watches, which many countries have adopted.-ISO 764 magnetic resistant watches...
es. In land surveying
Surveying
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
, when first-order (high-precision) elevation leveling is to be performed, the leveling rods used are made of Invar, instead of wood, fiberglass, or other metals.
There are variations of the original Invar material that have slightly different coefficient of thermal expansion such as:
- Inovco, which is Fe-33Ni-4.5Co and has an α of 0.55 ppm/°C (from 20–100 °C).
- FeNi42 (for example NILO alloy 42), has a nickel content of 42% and which matches that of siliconSiliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
and therefore is widely used as lead frame material for electronic components, integrated circuits, etc. - FeNiCo alloys—named KovarKovarKovar is a nickel-cobalt ferrous alloy designed to be compatible with the thermal expansion characteristics of borosilicate glass in order to allow direct mechanical connections over a range of temperatures...
or Dilver P—that have the same expansion behaviour as borosilicate glassBorosilicate glassBorosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
, and because of that are used for optical parts in a wide range of temperatures and applications, such as satelliteSatelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s.
Explanation of anomalous properties
A detailed explanation of Invar’s anomalously low CTE has proven elusive for physicists.All the iron-rich face centered cubic Fe-Ni alloys show Invar anomalies in their measured thermal and magnetic properties that evolve continuously in intensity with varying alloy composition. Scientists had once proposed that Invar’s behavior was a direct consequence of a high-magnetic-moment to low-magnetic-moment transition occurring in the face centered cubic Fe-Ni series (and that gives rise to the mineral antitaenite
Antitaenite
Antitaenite is a meteoritic metal alloy mineral composed of iron and nickel, 20-40% Ni that has a face centered cubic crystal structure. Its existence as a new mineral species occurring in both iron meteorites and in chondrites was first recognized in 1995. There are three other known Fe-Ni...
), however was proven incorrect. Instead, it appears that the low-moment/high-moment transition is preceded by a high-magnetic-moment frustrated ferromagnetic state in which the Fe-Fe magnetic exchange bonds have a large magneto-volume effect of the right sign and magnitude to create the observed thermal expansion anomaly.
External links
- What is Invar?, Antica Orologeria Lamberlan, retrieved Aug. 11, 2007. Properties of Invar, by Italian antique clock repair firm
- Invar, Metalworking FAQ, Univ. of Wyoming, retrieved Aug. 11, 2007. Metallurgist Jim Kirkpatrick's notes on different Invar alloys, handling and sources.