Invar

Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel–iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name Invar comes from the word invariable, referring to its relative lack of expansion or contraction with temperature changes.

It was invented in 1896 by Swiss physicist Charles Édouard Guillaume. He received the Nobel Prize in Physics in 1920 for this discovery, which enabled improvements in scientific instruments.

Like other nickel/iron compositions, Invar is a solid solution; that is, it is a single-phase alloy, consisting of around 36% nickel and 64% iron.

Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between 20 °C and 100 °C) of about 1.2 × 10⁻⁶ K⁻¹ (1.2 ppm/°C), while ordinary steels have values of around 11–15 ppm. 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. Though it displays high dimensional stability over a range of temperatures, it does have a propensity to creep.

Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, television shadow-mask frames, valves in engines, and antimagnetic watches. In land surveying, when first-order (high-precision) elevation leveling is to be performed, the level staff (leveling rod) used is made of Invar, instead of wood, fiberglass, or other metals. Invar struts were used in some pistons to limit their thermal expansion inside their cylinders.

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