Invar 36 is an iron-nickel alloy known for its unique combination of low thermal expansion and high strength. It comprises 36% nickel and 64% iron and contains small amounts of carbon, manganese, silicon, sulfur and phosphorus. Its coefficient of thermal expansion (CTE) ranges from approximately 5 to 7 parts per million (ppm)/°C in the temperature range − 100 °C to 200 °C. Invar 36's stability at temperatures ranging from cryogenic up to 310 °C makes it useful in many applications such as precision instruments, optical devices, Penning traps, etc.

Invar 36 is an iron-nickel alloy exhibiting a near-zero thermal expansion coefficient. Its low thermal expansion makes it ideal for use in applications requiring dimensional stability over a wide temperature range, such as antennae, precision instruments, and optical devices. It also improves stress relief due to its higher creep resistance than other tool steels. Furthermore, Invar 36 has excellent fabrication properties and can be machined into intricate shapes without losing any beneficial physical properties. Its strength and corrosion resistance make it suitable for many industrial applications, such as chemical processing and nuclear power plants.