Invar®, also known generically as FeNi36 (64FeNi in the US), is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). It was invented in 1896 by Swiss scientist Charles Édouard Guillaume. He received the Nobel Prize in Physics in 1920 for this discovery, which shows the importance of this alloy in scientific instruments. Invar® is a registered trademark of ArcelorMittal - Stainless & Nickel Alloys, formerly known as Imphy Alloys (US Trademark #63970). Like other nickel/iron compositions, Invar® is a solid solution; that is, it is a single-phase alloy — similar to a dilution of common table salt mixed into water. "Invar®" refers to invariable; that is, it will not react to thermal expansion.cite book | last = Davis | first = Joseph R. | authorlink = | coauthors = | title = Alloying: Understanding the Basics | publisher = ASM International | date = | location = | pages = 587-589 | url = | doi = | id = | isbn = 0871707446 ]

Common grades of Invar® have an α (20–100 °C) of about 1.2 × 10–6 K–1 (1.2 ppm/°C). However, 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 (NTE) characteristics. It is used in precision instruments such as clocks, physics laboratory devices, seismic creep gauges, shadow-mask frames, [Nickel Institute: " [ Nickel & Its Uses] ] valves in motors, and antimagnetic watches, etc. However, it has a propensity to creep. In Land Surveying, 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 Fe-33Ni-4.5Co and has an α (20–100 °C) of 0.55 ppm/°C.
* FeNi42 (for example NILO alloy 42), has a nickel content of 42% and α ≈ 5.3 ppm/°C which matches that of silicon and therefore is widely used as lead frame material for electronic components, integrated circuits, etc.
* FeNiCo alloys — named Kovar or Dilver P — that have the same expansion behaviour as borosilicate glass, and because of that are used for optical parts in a wide range of temperatures and applications, such as satellites.

ource of Invar®’s CTE 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), however this has now been shown to be incorrect. [ K. Lagarec, D.G. Rancourt, S.K. Bose, B. Sanyal, and R.A. Dunlap. Observation of a composition-controlled high-moment/low-moment transition in the face centered cubic Fe-Ni system: Invar® effect is an expansion, not a contraction. "Journal of Magnetism and Magnetic Materials" 236 (2001) 107-130.] 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. [ D.G. Rancourt and M.-Z. Dang. Relation between anomalous magneto-volume behaviour and magnetic frustration in Invar alloys. "Physical Review B" 54 (1996) 12225-12231.]


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.

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  • invar — invar …   Dictionnaire des rimes

  • invar — INVÁR s.n. Oţel aliat cu mult nichel, caracterizat printr o dilatare foarte mică şi folosit la fabricarea instrumentelor şi a aparatelor de precizie. – Din fr. invar. Trimis de valeriu, 21.07.2003. Sursa: DEX 98  invár s. n. Trimis de siveco, 10 …   Dicționar Român

  • invar — [ ɛ̃var ] n. m. • 1904; marque déposée , abrév. de invariable ♦ Techn. Acier au nickel, de dilatation très faible. ● Invar nom masculin (nom déposé [abréviation de invariable]) Alliage de fer à 36 % de nickel, dont le coefficient de dilatation… …   Encyclopédie Universelle

  • Invar — Invar, Verbindung von etwa 36% Nickel und 64% Stahl. Invar hat einen sehr geringen Ausdehnungswert, ändert also seine Ausdehnung bei Aenderung des Wärmezustandes nur sehr wenig, daher die Bezeichnung. Benoît und Guillaume fanden diese Verbindung… …   Lexikon der gesamten Technik

  • invar — (Del fr. invar, y este acort. de invariable, marca reg.). m. Aleación de hierro y níquel que, por su escaso coeficiente de dilatación, se emplea para instrumentos de medida y aparatos de precisión …   Diccionario de la lengua española

  • Invar — Invar, eine Nickelstahllegierung von sehr geringer Ausdehnung durch die Wärme, dient zu Meßinstrumenten …   Meyers Großes Konversations-Lexikon

  • invar — ìnvār m <G invára> DEFINICIJA legura željeza i nikla, s vrlo malim istezanjem; upotrebljava se u izradi preciznih instrumenata ETIMOLOGIJA tvorničko ime proizvoda© …   Hrvatski jezični portal

  • Invar — [in′vär΄] [abbrev. < INVARIABLE] trademark for a steel alloy containing approximately 36% nickel, used for making precision instruments and thermostatic elements because of its low coefficient of thermal expansion n. [i ] this substance …   English World dictionary

  • Invar® — /inˈvär or in värˈ/ noun An alloy of iron and nickel that expands only slightly when heated, used in the making of scientific instruments ORIGIN: From ↑invariable …   Useful english dictionary

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