Low thermal expansion alloys of iron-nickel-cobalt where a martensitic phase transformation is balanced against the thermal expansion coefficient to give a near zero thermal expansion coefficient over a range of temperatures.

Invar, also called FeNi, is an alloy of iron (64%) and nickel (36%) with some carbon and chromium. This alloy is known for its unique properties of controlled coefficient of thermal expansion (CTE). It was invented in 1896 by a Swiss Charles Edouard Guillaume, who later received the Nobel Prize in Physics in 1920.