Invar

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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 worldwide 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 the Acieries 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.