Invar, also known as FeNi36 in its generic form, is a nickel-iron alloy with a very low coefficient of thermal expansion. The term Invar derives from the word invariable, which refers to its lack of temperature-related expansion or contraction.
Below their Curie temperature, these alloys have a very low expansivity (the temperature below which they are ferromagnetic). The “Invar Effect” is a low thermal expansivity anomaly caused by spontaneous volume magnetostriction, in which lattice distortion counteracts normal lattice thermal expansivity. Following are some properties of invar 36:
Properties of Invar 36
Invar is a single-phase alloy that is a solid solution. It contains around 36 percent nickel and 64 percent iron in one commercial variant. In 1961, Westinghouse scientists defined the invar range as “30–45 atom percent nickel.”
The coefficient of thermal expansion (denoted, and measured between 20 °C and 100 °C) of common Invar grades is approximately 1.2 106 K1 (1.2 ppm/°C), whereas conventional steels have values of roughly 11–15 ppm/°C. Values as low as 0.62–0.65 ppm/°C can be achieved with extra-pure grades (0.1 percent Co).
Applications of Invar 36
It didn’t take long after the 36 percent nickel and its companion nickel-iron alloys were found to identify applications that may benefit from its low thermal expansivity. Early uses included surveying tapes and wires, as well as pendulums for grandfather clocks.
Newer uses include structural components in precision laser and optical measurement systems, as well as waveguide tubes made of nickel-iron alloys. These alloys have been utilized in microscopes, telescope support systems, and a range of scientific equipment that need mounted lenses.
The aerospace industry has employed 36 percent nickel-balanced iron alloys for composite molds. Molds for new generation airplanes, in particular, need 36% nickel-balance iron alloys to maintain tight dimensional tolerances while advanced composites cure at fairly high temperatures. In reality, the “Invar-Effect” family of alloys is helping to advance current science by being used in orbiting satellites, lasers, ring laser gyroscopes, and a variety of other high-tech applications.
In such complex devices, an alloy with near-constant dimensions and long-term dimensional stability, such as Invar 36, is critical. Alternative alloys with low CTE, like Kovar, are also feasible candidates for LiDAR sensor casting. When compared to machining LiDAR components from solid, investment casting is a more cost-effective option.
Invar 36 Family
The Invar family of alloys are all nickel-iron or nickel-iron-cobalt alloys with a face-centered cubic crystal structure. When nickel concentration rises above 36%, thermal expansivity and Curie temperature rise with it. Curie temperature rises from 280°C (536°F) for 36% nickel to over 565°C (1050°F) for 50% nickel.
Other Invar alloys, on the other hand, maybe better suited to particular purposes. The temperature range for the intended application, as well as the coefficient of expansion needed over that range, should all be considered when choosing an alloy.
Who Should Use Invar 36?
Customers with severe temperature requirements will most likely benefit from adopting Invar 36. Unfortunately, many people who are currently machining from solid are unaware that casting net-shape is an alternative. Invar 36 is often used in measuring instruments, precision mechanical systems, laser components, thermostat rods, meters, and components that transfer liquid gases in today’s world.
Advantages of Invar 36
The ability of Invar 36 to hold dimensions at cryogenic temperatures is its most obvious advantage. Apart from that, Invar 36 resembles steel in appearance and feel. It’s also exceptionally weldable and machinable. Customers can have Invar made with customized chemistries to meet their specific strength and hardness requirements.