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Invar, also known generically as FeNi36 (64FeNi in the US), is a
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...
notable for its uniquely low
coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...
(CTE or α). The name ''Invar'' comes from the word ''invariable'', referring to its relative lack of expansion or contraction with temperature changes. The discovery of the alloy was made in 1895 by Swiss physicist
Charles Édouard Guillaume Charles Édouard Guillaume (15 February 1861, in Fleurier, Switzerland – 13 May 1938, in Sèvres, France) was a Swiss physicist who received the Nobel Prize in Physics in 1920 in recognition of the service he had rendered to precision measuremen ...
for which he received the
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
in 1920. It enabled improvements in scientific instruments.


Properties

Like other nickel/iron compositions, Invar is a
solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...
; that is, it is a single- phase
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...
. In one commercial version it consists of approximately 36% nickel and 64% iron. The invar range was described by Westinghouse scientists in 1961 as "30–45 atom per cent nickel". Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between 20 °C and 100 °C) of about 1.2 × 10−6  K−1 (1.2  ppm/°C), while ordinary steels have values of around 11–15 ppm/°C. Extra-pure grades (<0.1% Co) can readily produce values as low as 0.62–0.65 ppm/°C. Some formulations display
negative thermal expansion Negative thermal expansion (NTE) is an unusual physicochemical process in which some materials contract upon heating, rather than expand as most other materials do. The most well-known material with NTE is water at 0~4 °C. Water's NTE is the ...
(NTE) characteristics. Though it displays high dimensional stability over a range of temperatures, it does have a propensity to
creep Creep, Creeps or CREEP may refer to: People * Creep, a creepy person Politics * Committee for the Re-Election of the President (CRP), mockingly abbreviated as CREEP, an fundraising organization for Richard Nixon's 1972 re-election campaign Art ...
.


Applications

Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, television shadow-mask frames, valves in engines and large aerostructure molds. One of its first applications was in watch
balance wheel A balance wheel, or balance, is the timekeeping device used in mechanical watches and small clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a ...
s and
pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward th ...
rods for precision
regulator clock A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator: It swings back and forth in a precise time interval dependent on ...
s. At the time it was invented the
pendulum clock A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator: It swings back and forth in a precise time interval dependent on i ...
was the world's most precise timekeeper, and the limit to timekeeping accuracy was due to thermal variations in length of clock pendulums. The
Riefler regulator clock The Riefler escapement is a mechanical escapement for precision pendulum clocks invented and patented by German instrument maker Sigmund Riefler in 1889. It was used in the astronomical regulator clocks made by his German firm Clemens Riefler ...
developed in 1898 by Clemens Riefler, the first clock to use an Invar pendulum, had an accuracy of 10 milliseconds per day, and served as the primary time standard in naval observatories and for national time services until the 1930s. In
land surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ...
, when first-order (high-precision) elevation leveling is to be performed, the
level staff {{short description, Graduated rod used to measure differences between heights A level staff, also called levelling rod, is a graduated wooden or aluminium rod, used with a levelling instrument to determine the difference in height between points ...
(leveling rod) used is made of Invar, instead of wood, fiberglass, or other metals. Invar struts were used in some pistons to limit their thermal expansion inside their cylinders. In the manufacture of large
composite material A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...
structures for
aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and ast ...
carbon fibre Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
layup mold A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...
s, Invar is used to facilitate the manufacture of parts to extremely tight tolerances. In the astronomical field, Invar is used as the structural components that support dimension-sensitive optics of astronomical telescopes. Superior dimensional stability of Invar allows the astronomical telescopes to significantly improve the observation precision and accuracy.


Variations

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), which has a nickel content of 42% and , is widely used as lead frame material for electronic components, integrated circuits, etc. *FeNiCo alloys—named
Kovar Kovar (trademark of CRS Holdings, inc., Delaware) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K betwe ...
or Dilver P—that have the same expansion behaviour as
borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
, and because of that are used for optical parts in a wide range of temperatures and applications, such as
satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
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 is a meteoritic metal alloy mineral composed of iron (Fe) and 20–40% nickel (Ni), (and traces of other elements) that has a face centered cubic crystal structure. There are three known Fe-Ni meteoritic minerals: kamacite, taenite, ...
); however, this theory 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. Wang et al. considered the statistical mixture between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs) in with the free energies of FM and SFCs predicted from first-principles calculations and were able to predict the temperature ranges of negative thermal expansion under various pressures. It was shown that all individual FM and SFCs have positive thermal expansion, and the negative thermal expansion originates from the increasing populations of SFCs with smaller volumes than that of FM.


See also

*
Constantan Constantan is a proprietary name for a copper– nickel alloy also known as Eureka, Advance, and Ferry. It usually consists of 55% copper and 45% nickel. Its main feature is the low thermal variation of its resistivity, which is consta ...
and Manganin, alloys with relatively constant electrical resistivity * Elinvar, alloy with relatively constant elasticity over a range of temperatures *
Sitall Sitall (also known as Sitall CO-115M or Astrositall) is a crystalline glass-ceramic with ultra-low coefficient of thermal expansion (CTE). It was originally manufactured in the former Soviet Union and was used in the making of primary mirrors for t ...
and
Zerodur Zerodur (notation of the manufacturer: ZERODUR®), registered trademarkof Schott AG, is a lithium-aluminosilicate glass-ceramic produced by Schott AG since 1968. It has been used for a number of very large telescope mirrors including GTC, Keck ...
, ceramic materials with a relatively low thermal expansion *
Borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
and Ultra low expansion glass, low expansion glasses resistant to thermal shock


References

{{reflist, 30em Ferrous alloys Nickel alloys Surveying instruments Low thermal expansion materials