Invar is an alloy added by Thermal Foundation. It consists of Iron and Ferrous Metal in a 2:1 ratio. The alloy is known for how little it suffers from thermal expansion.
Invar can also be made using Ingots instead of Pulverized metals in an Induction Smelter.
Invar can be used to craft tools and armor. It is also a common ingredient for crafting the second tier of machines and devices in Thermal Expansion.