Invar Properties & Low Coefficient of Thermal Expansion


36 ALLOY Invar Properties & Low Coefficient of Thermal Expansion

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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). 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. 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 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.

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