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Invar, also called FeNi, is an alloy of iron (64%) and nickel (36%) with some carbon and chromium. This alloy is known for its unique properties of controlled coefficient of thermal expansion (CTE). It was invented in 1896 by a Swiss Charles Edouard Guillaume, who later received the Nobel Prize in Physics in 1920.

Due to its low coefficient of thermal expansion at room temperature (about 10^-6 K^-1 in length; some formulations have negative thermal expansion, NTE) it is used in precision instruments (clocks, physics laboratory devices, seismic creep gauges, shadow-mask frames,[1] valves in motors, antimagnetic watches, etc.) However, it has a propensity to creep.

Although Invar is today a widely used material in many industries and applications, this is a particular trademark of a French company named Imphy Alloys: this company originates from Aciéries d’Imphy (a small city near Nevers, France) where the alloy was initially industrialised after its invention. The generic reference for Invar® is FeNi36.

There are variations of the original Invar material that have slightly different coefficient of thermal expansion such as:

• FeNi42, which matches the CTE 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 glass, and because of that are used for optical parts in a wide range of temperatures and applications, such as satellites.

Physicists consider true ideal Invar to be the pure bi-metallic alloy with stoichiometry Fe₆₅Ni₃₅ that has the face centered cubic crystal structure, since this laboratory alloy has all the essential properties of the commercial varieties. The Invar problem of physics has been to discover the microscopic mechanism that gives Invar its exceptional thermal properties. It was established early on that Invar behaviour is directly related to and dependent upon the alloy's ferromagnetism however the exact mechanism has been the subject of much research and debate, including several international conferences on the subject. The dominant theories of Invar were recently critically reviewed.^[1]

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. Recently it was advanced that Invar behaviour 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.^[2] 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.^[3]

1. ^ D.G. Rancourt. Invar behaviour in Fe-Ni alloys is predominantly a local moment effect arising from the magnetic exchange interactions between high moments. Phase Transitions 75 (2001) 201-209.
2. ^ 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.
3. ^ 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.

4. Beranger, G., Duffaut, F., Morlet, J., Tiers, Jean-Francois, A Hundred Years after the Discovery of Invar®…The Iron-Nickel Alloys, Intercept Limited, Andover, UK, 1996.

• http://www.imphyalloys.com/
• http://orologeria.com/english/magazine/magazine3.htm