Invar n : an alloy of iron and nickel having a low coefficient of thermal expansion; used in tuning forks and measuring tapes and other instruments

Invar, also known generically 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 Edouard Guillaume. He received the Nobel Prize in Physics in 1920 for this discovery, which shows the importance of this alloy in scientific instruments. 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.

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, valves in motors, and 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.

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 glass, and because of that are used for optical parts in a wide range of temperatures and applications, such as satellites.

Source 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. 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.

References

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.
• Imphy Alloys Commercial producer of Invar.