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 lack of expansion or contraction with temperature changes.
Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between 20–100 °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 motors, and antimagnetic watches. 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. Invar struts were used in some pistons to limit their thermal expansion inside their cylinders.
There are variations of the original Invar material that have slightly different coefficient of thermal expansion such as:
- Inovco, which is Fe–33Ni–4.5Co and has an α of 0.55 ppm/°C (from 20–100 °C).
- FeNi42 (for example NILO alloy 42), which has a nickel content of 42% and α ≈ 5.3 ppm/°C, 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.
Explanation of anomalous 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 theory was proven 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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- What is Invar?, Antica Orologeria Lamberlan, retrieved 11 Aug 2007. Properties of Invar, by Italian antique clock repair firm