Invar

The coefficient of thermal expansion of nickel/iron alloys is plotted here against the nickel percentage (on a mass basis) in the alloy. The sharp minimum occurs at the Invar ratio of 36% Ni.

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), however FeNi36 is also manufactured by Japanese companies. 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. The name "Invar" comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.^[1]

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 where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, television shadow-mask frames,^[2] valves in motors, and antimagnetic watches. 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.

Source of Invar’s thermal 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.^[3] 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.^[4]

See also

• Elinvar

References

1. ^ Davis, Joseph R.. Alloying: Understanding the Basics. ASM International. pp. 587–589. ISBN 0871707446. 
2. ^ Nickel Institute: Nickel & Its Uses
3. ^ 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.
4. ^ 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.